HistoryA power transformer developed by Lucien Gaulard and John Dixon Gibbs was demonstrated in London in 1881, and attracted the interest of Westinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Many of their designs were adapted to the particular laws governing electrical distribution in the UK.

In 1882, 1884, and 1885 Gaulard and Gibbs applied for patents on their transformer; however, these were overturned due to prior arts of Nikola Tesla and actions initiated by Sebastian Ziani de Ferranti.

Ferranti went into this business in 1882 when he set up shop in London designing various electrical devices. Ferranti bet on the success of alternating current power distribution early on, and was one of the few experts in this system in the UK. In 1887 the London Electric Supply Corporation (LESCo) hired Ferranti for the design of their power station at Deptford. He designed the building, the generating plant and the distribution system. On its completion in 1891 it was the first truly modern power station, supplying high-voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world. Many homes all over the world still have electric meters with the Ferranti AC patent stamped on them.

William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early transformer. The AC power system used today developed rapidly after 1886, and includes key concepts by Nikola Tesla, who subsequently sold his patent to George Westinghouse. Lucien Gaulard, John Dixon Gibbs, Carl Wilhelm Siemens and others contributed subsequently to this field. AC systems overcame the limitations of the direct current system used by Thomas Edison to distribute electricity efficiently over long distances even though Edison attempted to discredit alternating current as too dangerous during the War of Currents.

The first commercial power plant in the United States using three-phase alternating current was at the Mill Creek hydroelectric plant near Redlands, California in 1893 designed by Almirian Decker. Decker's design incorporated 10,000-volt three-phase transmission and established the standards for the complete system of generation, transmission and motors used today.

The Jaruga power plant in Croatia was set in operation on 28 August 1895, [1]. It was completed three days after the Niagara Falls plant, becoming the second commercial hydro power plant in the world. The two generators (42 Hz, 550 kW each) and the transformers were produced and installed by the Hungarian company Ganz. The transmission line from the power plant to the City of Šibenik was 11.5 kilometers (7.1 mi) long on wooden towers, and the municipal distribution grid 3000 V/110 V included six transforming stations.

Alternating current circuit theory evolved rapidly in the latter part of the 19th and early 20th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, James Clerk Maxwell, Oliver Heaviside, and many others. Calculations in unbalanced three-phase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in 1918.

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Electric shockAn electric shock can occur upon contact of a human body with any source of voltage high enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is thought to be about 1 milliampere (mA). The current may cause tissue damage or fibrillation if it is sufficiently high. Death caused by an electric shock is referred to as electrocution. Generally, currents approaching 100 mA are lethal if they pass through sensitive portions of the body. Electric shock is a jarring, shaking sensation you receive from contact with electricity. You usually feel like you have received a sudden blow. If the voltage and resulting current are sufficiently high, you may become unconscious. Severe burns may appear on your skin at the place of contact; muscular spasms may occur, perhaps causing you to clasp the apparatus or wire which caused the shock and be unable to turn it loose.

Shock effects

Psychological

The perception of electric shock can be different depending on the voltage, duration, current, path taken, frequency, etc. Current entering the hand has a threshold of perception of about 5 to 10 mA (milliampere) for DC and about 1 to 10 mA for AC at 60 Hz. Shock perception declines with increasing frequency, ultimately disappearing at frequencies above 15 to 20 kHz.

Burns

Heating due to resistance can cause extensive and deep burns. Voltage levels of 500 to 1000 volts tend to cause internal burns due to the large energy (which is proportional to the duration multiplied by the square of the current multiplied by resistance) available from the source. Damage due to current is through tissue heating.

Ventricular fibrillation

A low-voltage (110 or 230 V), 50 or 60-Hz AC current through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60 mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other kind of electrode), a much lower current of less than 1 mA, (AC or DC) can cause fibrillation. If not immediately treated by defibrillation, fibrillations are usually lethal because all the heart muscle cells move independently. Above 200 mA, muscle contractions are so strong that the heart muscles cannot move at all.

Neurological effects

Current can cause interference with nervous control, especially over the heart and lungs. Repeated or severe electric shock which does not lead to death has been shown to cause neuropathy.

When the current path is through the head, it appears that, with sufficient current, loss of consciousness almost always occurs swiftly. (This is borne out by some limited self-experimentation by early designers of the electric chair and by research from the field of animal husbandry, where electric stunning has been extensively studied).

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Arc-flash hazards

Approximately 80% of all injuries and fatalities caused by electrical incidents are not caused by electric shock, but by the intense heat, light, and pressure wave (blast) caused by electrical faults. The arc flash in an electrical fault produces the same type of light radiation from which electric welders protect themselves using face shields with dark glass, heavy leather gloves, and full-coverage clothing. The heat produced may cause severe burns, especially on unprotected flesh. The blast produced by vaporizing metallic components can break bones and irreparably damage internal organs. The degree of hazard present at a particular location can be determined by a detailed analysis of the electrical system, and appropriate protection worn if the electrical work must be performed with the electricity on.

Issues affecting lethality

Other issues affecting lethality are frequency, which is an issue in causing cardiac arrest or muscular spasms, and pathway—if the current passes through the chest or head there is an increased chance of death. From a main circuit or power distribution panel the damage is more likely to be internal, leading to cardiac arrest.

The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-magnitude AC tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain. Generally, higher frequency AC current tends to run along the skin rather than penetrating and touching vital organs such as the heart. While there will be severe burn damage at higher voltages, it is normally not fatal.

It is sometimes suggested that human lethality is most common with alternating current at 100–250 volts; however, death has occurred below this range, with supplies as low as 32 volts. Danger increase dramatically with voltages over 250 volts. Shocks above 3300 volts are often fatal, with those above 11000 volts being usually fatal.

Electrical discharge from lightning tends to travel over the surface of the body causing burns and may cause respiratory arrest.

Skin Resistance

The voltage necessary for electrocution depends on the current through the body and the duration of the current. Using Ohm's law, Voltage = Current × Resistance, we see that the current drawn depends on the resistance of the body. The resistance of our skin varies from person to person and fluctuates between different times of day. In general, dry skin is a poor conductor that may have a resistance of around 100,000 Ω, while broken or wet skin may have a resistance of around 1,000 Ω.

Point of entry Macro shock: Current across intact skin and through the body. Current from arm to arm, or

between an arm and a foot, is likely to traverse the heart, therefore it is much more dangerous

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than current between a leg and the ground. This type of shock by definition must pass into the body through the skin.

Micro shock: Direct current path to the heart tissue, the shock is required to be administered from inside the skin, i.e. a malfunctioning pacemaker, or ungrounded catheter etc.

Electrocution Statistics

There were 550 electrocutions in the US in 1993, which translates to 2.1 deaths per million inhabitants. At that time, the incidence of electrocutions was decreasing. Electrocutions in the workplace make up the majority of these fatalities. From 1980–1992, an average of 411 workers were killed each year by electrocution.

Rescue and Care of Shock Victims

The following procedures are recommended for rescue and care of electric shock victims: Remove the victim from electrical contact at once, but DO NOT endanger yourself. You can do this by:

Throwing the switch if it is nearby Cutting the cable or wires to the apparatus, using an ax with a wooden handle while

taking care to protect your eyes from the flash when the wires are severed Using a dry stick, rope, belt, coat, blanket, shirt or any other nonconductor of

electricity, to drag or push the victim to safety Determine whether the victim is breathing. If the victim is not breathing, you must apply artificial ventilation (respiration) without delay, even though the victim may appear to be lifeless.

Do not stop artificial respiration until medical authority pronounces the victim dead.

Lay the victim face up. The feet should be about 12 inches higher than the head. Chest or head injuries require the head to be slightly elevated. If there is vomiting or if facial injuries have occurred which cause bleeding into the throat, the victim should be placed on the stomach with the head turned to one side and 6 to 12 inches lower than the feet. Keep the victim warm. The injured person's body heat must be conserved. Keep the victim covered with one or more blankets, depending on the weather and the person's exposure to the elements. Artificial means of warming, such as hot water bottles should not be used. Drugs, food, and liquids should not be administered if medical attention will be available within a short time. If necessary, liquids may be administered. Small amounts of warm salt water, tea or coffee should be used. Alcohol, opiates, and other depressant substances must never be administered. Send for medical personnel (a doctor if available) at once, but do NOT under any circumstances leave the victim until medical help arrives.

Safety Precautions for Preventing Electric Shock

You must observe the following safety precautions when working on electrical equipment: Never work alone. Another person may save your life if you receive an electric shock. Work on energized circuits ONLY WHEN ABSOLUTELY NECESSARY.

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Power should be tagged out, using approved tag out procedures, at the nearest source of electricity.

Stand on an approved insulating material, such as a rubber mat. Discharge power capacitors before working on de-energized equipment. Remember, a

capacitor is an electrical power storage device. When you must work on an energized circuit, wear rubber gloves and cover as much

of your body as practical with an insulating material (Such as shirt sleeves). This is especially important when you are working in a warm

space where sweating may occur. De-energize equipment prior to hooking up or removing test equipment. Work with only one hand inside the equipment. Keep the other hand clear of all

obstacles that may provide a path, such as a ground, for current to flow. Wear safety goggles. Sparks could damage your eyes, as could the cooling liquids in

some components such as transformers should they overheat and explode. Keep a cool head and think about the possible consequences before performing any

action. Carelessness is the cause of most accidents. Remember the best technician is NOT necessarily the fastest one, but the one who will be on the job tomorrow.

Deliberate uses

Electroconvulsive therapy

Electric shock is also used as a medical therapy, under carefully controlled conditions:

Electroconvulsive therapy or ECT is a psychiatric therapy for mental illness. The objective of the therapy is to induce a seizure for therapeutic effect. There is no sensation of shock because the patient is anesthetized. The therapy was originally conceived of after it was observed that depressed patients who also suffered from epilepsy experienced some remission after a spontaneous seizure. The first attempts at deliberately inducing seizure as therapy used not electricity but chemicals; however electricity provided finer control for delivering the minimum stimulus needed. Ideally some other method of inducing seizure would be used, as the electricity may be associated with some of the negative side effects of ECT including amnesia. ECT is generally administered three times a week for about 8-12 treatments.

As a treatment for fibrillation or irregular heart rhythms: see defibrillator and cardioversion. As a method of pain relief: see Transcutaneous Electrical Nerve Stimulator (more commonly

referred to as a TENS unit). As an aversive punishment for conditioning of mentally handicapped patients with severe

behavioral issues. This method is highly controversial and is employed at only one institution in the United States, the Judge Rotenberg Educational Center. The institute also uses electric shock punishments on non-handicapped children with behavioral problems. Whether this constitutes legitimate medical treatment versus abusive discipline is the subject of ongoing litigation.

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Torture

Electric shocks have been used as a method of torture, since the received voltage and amperage can be controlled with precision and used to cause pain while avoiding obvious evidence on the victim's body. Such torture usually uses electrodes attached to parts of the victim's body. Another method of electrical torture is stunning with an electroshock gun such as a cattle prod or a taser (provided a sufficiently high voltage and non-lethal current is used in the former case).

The US Army is known to have used electrical torture during World War II. An extensive fictional depiction of such torture is included in the 1966 book The Secret of Santa Vittoria by Robert Crichton. During the Vietnam War, electric shock torture is said to have been used by both the Americans and Vietnamese. A scene of electrical torture in the American Deep South is included in the 1980 Robert Redford film Brubaker. Amnesty International published an official statement that Russian military forces in Chechnya tortured local women with electric shocks by connecting electric wires to their bra straps. Examples in popular modern culture are the electric torture of Martin Riggs in Lethal Weapon and John Rambo in Rambo: First Blood Part II. Japanese serial killer Futoshi Matsunaga used electric shocks for controlling his victims.[8] In the 2008 movie Taken electical torture was used as well.

Advocates for the mentally ill and some psychiatrists such as Thomas Szasz have asserted that electroconvulsive therapy is torture when used without a bona fide medical benefit against recalcitrant or non-responsive patients. See above for ECT as medical therapy. These same arguments and oppositions apply to the use of extremely painful shocks as punishment for behavior modification, a practice that is openly used only at the Judge Rotenberg Institute.

Capital punishment

Electric shock delivered by an electric chair is sometimes used as an official means of capital punishment in the United States, although its use has become rare in recent times. Although the electric chair was at one time considered a more humane and modern execution method than hanging, shooting, poison gassing, the guillotine, etc., it has now been replaced in countries which practice capital punishment by lethal injections. Modern reporting has claimed that it sometimes takes several shocks to be lethal, and that the condemned person may actually catch fire before the process is complete. The brain is always severely damaged and inactivated.

Other than in parts of the United States, only the Philippines reportedly has used this method, and only for a few years. It remains a legal means of execution in 10 states of the USA.

References collected from:-

1. ^ http://hypertextbook.com/facts/2000/JackHsu.shtml 2. ^ http://www.grandin.com/humane/elec.stun.html 3. ^ "Industry Backs IEEE-NFPA Arc Flash Testing Program With Initial Donations Of $1.25 Million". IEEE. 14 July 2006.

http://standards.ieee.org/announcements/pr_FINArc.html. Retrieved 2008-01-01. 4. ^ a b "Publication No. 98-131: Worker Deaths by Electrocution". National Institute for Occupational Safety and Health.

http://www.cdc.gov/niosh/docs/98-131/overview.html. Retrieved 2008-08-16. 5. ^ Folliot, Dominigue (1998). "Electricity: Physiological Effects". Encyclopaedia of Occupational Health and Safety, Fourth

Edition. http://www.ilo.org/encyclopedia/?doc&nd=857100207&nh=0. Retrieved 2006-09-04. 6. ^ "Torture, American style: The surprising force behind torture: democracies". Boston Globe. 2007-12-16.

http://www.boston.com/bostonglobe/ideas/articles/2007/12/16/torture_american_style/. Retrieved 2008-01-01. 7. ^ Russian Federation Preliminary briefing to the UN Committee against Torture 1 April 2006, statement by Amnesty

International

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Electrical breakdown

The term electrical breakdown has several similar but distinctly different meanings. The term can apply to the failure of an electric circuit. Alternately, it may refer to a rapid reduction in the resistance of an electrical insulator that can lead to a spark jumping around or through the insulator. This may be a momentary event (as in an electrostatic discharge), or may lead to a continuous arc discharge if protective devices fail to interrupt the current in a high power circuit.

Electrical system failureThe most common meaning is related to automobiles and is the failure of an electric circuit or associated device resulting in a loss of vehicle function (a breakdown). Common problems include battery discharge, alternator failure, broken wires, blown fuses, etc.

Failure of electrical insulationThe second meaning of the term is more specifically a reference to the breakdown of the insulation of an electrical wire or other electrical component. Such breakdown usually results in a short circuit or a blown fuse. This occurs at the breakdown voltage. Actual insulation breakdown is more generally found in high-voltage applications, where it sometimes causes the opening of a protective circuit breaker. Electrical breakdown is often associated with the failure of solid or liquid insulating materials used inside high voltage transformers or capacitors in the electricity distribution grid. Electrical breakdown can also occur across the strings of insulators that suspend overhead power lines, within underground power cables, or lines arcing to nearby branches of trees. Under sufficient electrical stress, electrical breakdown can occur within solids, liquids, gases or vacuum. However, the specific breakdown mechanisms are significantly different for each, particularly in different kinds of dielectric medium. All this leads to catastrophic failure of the instruments.

Disruptive devicesA disruptive device is a device that has a dielectric, whereupon being stressed beyond its dielectric strength, has an electrical breakdown. This results in the sudden transition of part of the dielectric material from an insulating state to a highly conductive state. This transition is characterized by the formation of an electric spark, and possibly an electric arc through the material. If this occurs within a solid dielectric, physical and chemical changes along the path of the discharge will cause permanent degradation and significant reduction in the material's dielectric strength. A spark gap is a type of disruptive device that uses a gas or fluid dielectric between spaced electrodes. Unlike solid dielectrics, liquid or gaseous dielectrics can usually recover their full dielectric strength once current flow (through the plasma in the gap) has been externally interrupted.

MechanismElectrical breakdown occurs within a gas (or mixture of gases, such as air) when the dielectric strength of the gas is exceeded. Regions of high electrical stress can cause nearby gas to partially ionize and begin conducting. This is done deliberately in low pressure discharges such as in fluorescent lights (see also Electrostatic Discharge) or in an electrostatic precipitator.

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Partial electrical breakdown of the air causes the "fresh air" smell of ozone during thunderstorms or around high-voltage equipment. Although air is normally an excellent insulator, when stressed by a sufficiently high voltage (an electric field strength of about 3 x 106V/m, air can begin to break down, becoming partially conductive. If the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or arc that bridges the entire gap. While the small sparks generated by static electricity may barely be audible, larger sparks are often accompanied by a loud snap or bang. Lightning is an example of an immense spark that can be many miles long. The color of the spark depends upon the gases that make up the gaseous media. If a fuse or circuit breaker fails to interrupt the current through a spark in a power circuit, current may continue, forming a very hot electric arc. The color of an arc depends primarily upon the conductor materials (as they are vaporized and mix within the hot plasma in the arc). Although sparks and arcs are usually undesirable, they can be useful in everyday applications such as spark plugs for gasoline engines, electrical welding of metals, or for metal melting in an electric arc furnace.

Voltage-Current relation

Voltage-current relation before breakdown

Before breakdown, there is a non-linear relation between voltage and current as shown in figure. In region 1, there are free ions that can be accelerated by the field and induce a current. These will be saturated after a certain voltage and give a constant current, region 2. Region 3 and 4 are caused by ion avalanche as explained by the Townsend discharge mechanism.

Corona breakdown

Partial breakdown of the air occurs as a corona discharge on high voltage conductors at points with the highest electrical stress. As the dielectric strength of the material surrounding the conductor determines the maximum strength of the electric field the surrounding material can tolerate before becoming conductive, conductors that consist of sharp points, or balls with small radii, are more prone to causing dielectric breakdown. Corona is sometimes seen as a bluish glow around high voltage wires and heard as a sizzling sound along high voltage power lines. Corona also generates radio frequency noise that can also be heard as 'static' or buzzing on radio receivers. Corona can also occur naturally at high points (such as church spires, treetops, or ship masts) during thunderstorms as St. Elmo's Fire. Although corona discharge is usually undesirable, until recently it was essential in the operation of photocopiers (Xerography) and laser printers. Many modern copiers and laser printers now charge the photoconductor drum with an electrically conductive roller, reducing undesirable indoor ozone pollution. Additionally, lightning rods use corona discharge to create conductive paths in the air that point towards the rod, deflecting potentially-damaging lightning away from buildings and other structures.[2]Corona discharge ozone generators have been used for more than 30 years in the water purification process. Ozone is a toxic gas, even more potent than chlorine. In a typical drinking water treatment plant, the ozone gas is dissolved into the filtered water to kill bacteria and viruses. Ozone also removes the bad odors and taste from the water. The main advantage of ozone is that the overdose (residual) decomposes to gaseous oxygen well before the water reaches the consumer. This is in contrast with chlorine which stays in the water and can be tasted by the consumer.

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Corona discharges are also used to modify the surface properties of many polymers. An example is the corona treatment of plastic materials which allows paint or ink to adhere properly.

Transmission, distribution, and domestic power supplyAC voltage may be increased or decreased with a transformer. Use of a higher voltage leads to significantly more efficient transmission of power. The power losses in a conductor are a product of the square of the current and the resistance of the conductor, described by the formula P = I2R.

Since the power transmitted is equal to the product of the current and the voltage (assuming no phase difference), the same amount of power can be transmitted with a lower current by increasing the voltage. Therefore it is advantageous when transmitting large amounts of power to distribute the power with high voltages (often hundreds of kilovolts).

High voltage transmission lines deliver power from electric generation plants over long distances using alternating current. These lines are located in eastern Utah.

However, high voltages also have disadvantages, the main one being the increased insulation required, and generally increased difficulty in their safe handling. In a power plant, power is generated at a convenient voltage for the design of a generator, and then stepped up to a high voltage for transmission. Near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary depending on the country and size of load, but generally motors and lighting are built to use up to a few hundred volts between phases.

The utilization voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world.

Modern high-voltage, direct-current electric power transmission systems contrast with the more common alternating-current systems as a means for the efficient bulk transmission of electrical power over long distances. HVDC systems, however, tend to be more expensive and less efficient over shorter distances than transformers. Transmission with high voltage direct current was not feasible when Edison, Westinghouse and Tesla were designing their power systems, since there was then no way to economically convert AC power to DC and back again at the necessary voltages.

Three-phase electrical generation is very common. Three separate coils in the generator stator are physically offset by an angle of 120° to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other.

If the load on a three-phase system is balanced equally among the phases, no current flows through the neutral point. Even in the worst-case unbalanced (linear) load, the neutral current will not exceed the highest of the phase currents. Non-linear loads (e.g. computers) may require an oversized neutral bus and neutral conductor in the upstream distribution panel to handle harmonics. Harmonics can cause neutral conductor current levels to exceed that of one or all phase conductors.

For three-phase at utilization voltages a four-wire system is often used. When stepping down three-phase, a transformer with a Delta (3-wire) primary and a Star (4-wire, center-earthed) secondary is often used so there is no need for a neutral on the supply side.

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For smaller customers (just how small varies by country and age of the installation) only a single phase and the neutral or two phases and the neutral are taken to the property. For larger installations all three phases and the neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off.

Three-wire single phase systems, with a single center-tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America. This arrangement is sometimes incorrectly referred to as "two phase". A similar method is used for a different reason on construction sites in the UK. Small power tools and lighting are supposed to be supplied by a local center-tapped transformer with a voltage of 55 V between each power conductor and earth. This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage of 110 V between the two conductors for running the tools.

A third wire, called the bond (or earth) wire, is often connected between non-current-carrying metal enclosures and earth ground. This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all non-current-carrying metal parts into one complete system ensures there is always a low electrical impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current, causing the over current protection device (breakers, fuses) to trip or burn out as quickly as possible, bringing the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the Neutral/Identified conductor if present.

Alternating current (AC)In alternating current (AC, also ac) the movement (or flow) of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again. In direct current (DC), the movement (or flow) of electric charge is only in one direction.

Used generically, AC refers to the form in which electricity is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave, however in certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.

AC power supply frequenciesThe frequency of the electrical system varies by country; most electric power is generated at either 50 or 60 Hz.. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably Japan.

A low frequency eases the design of low speed electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways, but also causes a noticeable flicker in incandescent lighting and an objectionable flicker in fluorescent lamps. 16⅔ Hz power is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland. The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency. The original Niagara Falls generators

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were built to produce 25 Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate (although with noticeable flicker); most of the 25 Hz residential and commercial customers for Niagara Falls power were converted to 60 Hz by the late 1950s, although some 25 Hz industrial customers still existed as of the start of the 21st century.

Off-shore, military, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds.

Effects at high frequenciesA direct, constant current flows uniformly throughout the cross-section of the (uniform) wire that carries it. With alternating current of any frequency, the current is forced towards the outer surface of the wire, and away from the center. This is because an electric charge which accelerates (as is the case of an alternating current) radiates electromagnetic waves, and materials of high conductivity (the metal which makes up the wire) do not allow propagation of electromagnetic waves. This phenomenon is called skin effect.

At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for high power transmission (50–60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost.

Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area in which the current actually flows. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I2R loss).

Techniques for reducing AC resistance

For low to medium frequencies, conductors can be divided into stranded wires, each insulated from one other, and the relative positions of individual strands specially arranged within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current flow throughout the total cross section of the stranded conductors. Litz wire is used for making high-Q inductors, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher radio frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.

Techniques for reducing radiation loss

As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated represents a loss. Depending on the frequency, different techniques are used to minimize the loss due to radiation.

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Twisted pairs

At frequencies up to about 1 GHz, pairs of wires are twisted together in a cable, forming a twisted pair. This reduces losses from electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signaling system, so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively cancelled by radiation from the other wire, resulting in almost no radiation loss.

Coaxial cables

At frequencies above 1 GHz, unshielded wires of practical dimensions lose too much energy to radiation, so coaxial cables are used instead. A coaxial cable has a conductive wire inside a conductive tube, separated by a dielectric layer. The current flowing on the inner conductor is equal and opposite to the current flowing on the inner surface of the tube. The electromagnetic field is thus completely contained within the tube, and (ideally) no energy is radiation or coupling outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 20 GHz. For microwave frequencies greater than 20 GHz, the losses (due mainly to the dissipation factor of the dielectric) become too large, making waveguides a more efficient medium for transmitting energy.

Waveguides

Waveguides are similar to coax cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an electric current, but rather by means of a guided electromagnetic field. Although surface currents do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide.

Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so they are only feasible at microwave frequencies. In addition to this mechanical feasibility, electrical resistance of the non-ideal metals forming the walls of the waveguide cause dissipation of power (surface currents flowing on lossy conductors dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.

Fiber optics

At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which is a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.

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Short circuit

A short circuit (sometimes abbreviated to short or s/c) in an electrical circuit is one that allows a current to travel along a different path from the one originally intended. The electrical opposite of a short circuit is an "open circuit", which is an infinite resistance between two nodes. It is common to misuse "short circuit" to describe any electrical malfunction, regardless of the actual problem.

DefinitionA short circuit is an abnormal low-resistance connection between two nodes of an electrical circuit that are meant to be at different voltages. This results in an excessive electric current (overcurrent) limited only by the Thevenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors.

In circuit analysis, the term short circuit is used by analogy to designate a zero-impedance connection between two nodes. This forces the two nodes to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop across the short. In simple circuit analysis, wires are considered to be shorts. In real circuits, the result is a connection of nearly zero impedance, and almost no resistance. In such a case, the current drawn is limited by the rest of the circuit.

ExamplesAn easy way to create a short circuit is to connect the positive and negative terminals of a battery together with a low-resistance conductor, like a wire. With low resistance in the connection, a high current exists, causing the cell to deliver a large amount of energy in a short time.

In electrical devices, unintentional short circuits are usually caused when a wire's insulation breaks down, or when another conducting material is introduced, allowing charge to flow along a different path than the one intended.

A large current through a battery (also called a cell) can cause the rapid buildup of heat, potentially resulting in an explosion or the release of hydrogen gas and electrolyte, which can burn tissue and may be either an acid or a base. Overloaded wires can also overheat, sometimes causing damage to the wire's insulation, or a fire. High current conditions may also occur with electric motor loads under stalled conditions, such as when the impeller of an electrically driven pump is jammed by debris; this is not a short, though it may have some similar effects.

In mains circuits, short circuits are most likely to occur between two phases, between a phase and neutral or between a phase and earth (ground). Such short circuits are likely to result in a very high current and therefore quickly trigger an over current protection device. However, it is possible for short circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated. To help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage

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reactance. The leakage reactance (usually about 5 to 10% of the full load impedance) helps limit both the magnitude and rate of rise of the fault current.

A short circuit may lead to formation of an electric arc. The arc, a channel of hot ionized plasma, is highly conductive and can persist even after significant amount of original material of the conductors was evaporated. Surface erosion is a typical sign of electric arc damage. Even short arcs can remove significant amount of materials from the electrodes.

Mathematics of AC voltages

A sine wave, over one cycle (360°). The dashed line represents the root mean square (RMS) value at about 0.707. Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:

, Where

is the peak voltage (unit: volt), is the angular frequency (unit: radians per second)

o The angular frequency is related to the physical frequency, (unit = hertz), which represents the number of cycles per second , by the equation .

is the time (unit: second).

The peak-to-peak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of sin(x) is +1 and the minimum value is −1, an AC voltage swings between + Vpeak and − Vpeak. The peak-to-peak voltage, usually written as Vpp or VP − P, is therefore Vpeak − ( − Vpeak) = 2Vpeak.

Power and root mean square

The relationship between voltage and the power delivered is

where R represents a load resistance.

Rather than using instantaneous power, p(t), it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value, written as Vrms, because

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For a sinusoidal voltage:

The factor is called the crest factor, which varies for different waveforms.

For a triangle wave form centered about zero

For a square wave form centered about zero

Example

To illustrate these concepts, consider a 230 V AC mains supply used in many countries around the world. It is so called because its root mean square value is 230 V. This means that the time-averaged power delivered is equivalent to the power delivered by a DC voltage of 230 V. To determine the peak voltage (amplitude), we can rearrange the above equation to:

For our 230 V AC, the peak voltage Vpeak is therefore , which is about 325 V. The peak-to-peak value of the 230 V AC is double that, at about 650 V.

Note that some countries use a frequency of 50 Hz, while others use a frequency of 60 Hz. The calculation to convert from RMS voltage to peak voltage is independent of the frequency.

Direct current (DC)Direct current (DC) is the undirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was G alvanic current.

Types of direct current.

Direct current may be obtained from an alternating current supply by use of a current-switching arrangement called a rectifier, which contains electronic elements (usually) or electromechanical elements (historically) that allow current to flow only in one direction. Direct current may be made into alternating current with an inverter or a motor-generator set.

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The first commercial electric power transmission (developed by Thomas Edison in the late nineteenth century) used direct current. Because of the advantage of alternating current over direct current in transforming and transmission, electric power distribution today is nearly all alternating current. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current. See War of Currents.

Direct current is used to charge batteries, and in nearly all electronic systems as the power supply. Very large quantities of direct-current power are used in production of aluminum and other electrochemical processes. Direct current is used for some railway propulsion, especially in urban areas. High voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.

Various definitionsWithin electrical engineering, the term that use only one polarity of voltage or current, and to refer to the constant, frequency, or slowly varying local mean value of a voltage or current. For example, the voltage across a DC voltage source is constant as is the current through a DC current source. The DC solution of an electric circuit is the solution where all voltages and currents are constant. It can be shown that any stationary voltage or current waveform can be decomposed into a sum of a DC component and a zero-mean time-varying component; the DC component is defined to be the expected value or the average value of the voltage or current over all time.

Although DC stands for "Direct Current", DC sometimes refers to "constant polarity." With this definition, DC voltages can vary in time, such as the raw output of a rectifier or the fluctuating voice signal on a telephone line.

Some forms of DC (such as that produced by a voltage regulator) have almost no variations in voltage, but may still have variations in output power and current.

ApplicationsDirect-current installations usually have different types of sockets, switches, and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is usually important with a direct-current appliance not to reverse polarity unless the device has a diode bridge to correct for this (most battery-powered devices do not).

This symbol is found on many electronic devices that either require or produce direct current.

DC is commonly found in all low-voltage applications, especially where these are powered by batteries, which can produce only DC, or solar power systems. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells (mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts) also produce only DC.

Many telephones connect to a twisted pair of wires, and internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone).

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Telephone exchange communication equipment, such as DSLAM, uses standard -48V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the battery bank. This is done to prevent electrolysis depositions.

Electrical impedanceElectromagnetism

Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angles. The symbol for impedance is usually and it may be represented by writing its magnitude and phase in the form

. However, complex number representation is more powerful for circuit analysis purposes. The term impedance was coined by Oliver Heaviside in July 1886. Arthur Kennelly was the first to represent impedance with complex numbers in 1893. Impedance is defined as the frequency domain ratio of the voltage to the current. In other words, it is voltage–current ratio for a single complex exponential at a particular frequency ω. In general, impedance will be a complex number, but this complex number has the same units as resistance, for which the SI unit is the ohm. For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular,

The magnitude of the complex impedance is ratio of the voltage amplitude to the current amplitude.

The phase of the complex impedance is the phase shift by which the current is ahead of the voltage.

The reciprocal of impedance is admittance (i.e., admittance is the current-to-voltage ratio, and it conventionally carries mho or Siemens units).

Complex impedanceImpedance is represented as a complex quantity and the term complex impedance may be used interchangeably; the polar form conveniently captures both magnitude and phase characteristics,

where the magnitude represents the ratio of the voltage difference amplitude to the current amplitude, while the argument gives the phase difference between voltage and current and is the imaginary unit. In Cartesian form,

where the real part of impedance is the resistance and the imaginary part is the reactance .

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Where it is required to add or subtract impedances the cartesian form is more convenient, but when quantities are multiplied or divided the calculation becomes simpler if the polar form is used. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation. Conversion between the forms follows the normal conversion rules of complex numbers.

Ohm's lawAn AC supply applying a voltage , across a load , driving a current .

The meaning of electrical impedance can be understood by substituting it into Ohm's law.

The magnitude of the impedance acts just like resistance, giving the drop in voltage amplitude across an impedance for a given current . The phase factor tells us that the current lags the voltage by a phase of (i.e. in the time domain, the current signal is shifted

to the right with respect to the voltage signal).

Just as impedance extends Ohm's law to cover AC circuits, other results from DC circuit analysis such as voltage division, current division, Thevenin's theorem, and Norton's theorem, can also be extended to AC circuits by replacing resistance with impedance.

Complex voltage and currentGeneralized impedances in a circuit can be drawn with the same symbol as a resistor (US ANSI or DIN Euro) or with a labeled box.

In order to simplify calculations, sinusoidal voltage and current waves are commonly represented as complex-valued functions of time denoted as and .

Impedance is defined as the ratio of these quantities.

Substituting these into Ohm's law we have

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Noting that this must hold for all t, we may equate the magnitudes and phases to obtain

The magnitude equation is the familiar Ohm's law applied to the voltage and current amplitudes, while the second equation defines the phase relationship.

Validity of complex representation

This representation using complex exponentials may be justified by noting that (by Euler's formula):

i.e. a real-valued sinusoidal function (which may represent our voltage or current waveform) may be broken into two complex-valued functions. By the principle of superposition, we may analyze the behavior of the sinusoid on the left-hand side by analyzing the behavior of the two complex terms on the right-hand side. Given the symmetry, we only need to perform the analysis for one right-hand term; the results will be identical for the other. At the end of any calculation, we may return to real-valued sinusoids by further noting that

In other words, we simply take the real part of the result.

Phasors

A phasor is a constant complex number, usually expressed in exponential form, representing the complex amplitude (magnitude and phase) of a sinusoidal function of time. Phasors are used by electrical engineers to simplify computations involving sinusoids, where they can often reduce a differential equation problem to an algebraic one.

The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element, as determined by the relative amplitudes and phases of the voltage and current. This is identical to the definition from Ohm's law given above, recognising that the factors of cancel.

Device examples

The phase angles in the equations for the impedance of inductors and capacitors indicate that the voltage across a capacitor lags the current through it by a phase of π / 2, while the voltage across an inductor leads the current through it by π / 2. The identical voltage and current amplitudes tell us that the magnitude of the impedance is equal to one.

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The impedance of an ideal resistor is purely real and is referred to as a resistive impedance:

Ideal inductors and capacitors have a purely imaginary reactive impedance:

Note the following identities for the imaginary unit and its reciprocal:

Thus we can rewrite the inductor and capacitor impedance equations in polar form:

The magnitude tells us the change in voltage amplitude for a given current amplitude through our impedance, while the exponential factors give the phase relationship.

Deriving the device specific impedances

What follows below is a derivation of impedance for each of the three basic circuit elements, the resistor, the capacitor, and the inductor. Although the idea can be extended to define the relationship between the voltage and current of any arbitrary signal, these derivations will assume sinusoidal signals, since any arbitrary signal can be approximated as a sum of sinusoids through Fourier Analysis.

Resistor

For a resistor, we have the relation:

This is simply a statement of Ohm's Law.

Considering the voltage signal to be

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it follows that

This tells us that the ratio of AC voltage amplitude to AC current amplitude across a resistor is , and that the AC voltage leads the AC current across a resistor by 0 degrees.

This result is commonly expressed as

Capacitor

For a capacitor, we have the relation:

Considering the voltage signal to be

it follows that

And thus

This tells us that the ratio of AC voltage amplitude to AC current amplitude across a capacitor is , and that the AC voltage leads the AC current across a capacitor by -90 degrees (or the AC current leads the AC voltage across a capacitor by 90 degrees).

This result is commonly expressed in polar form, as

or, by applying Euler's formula, as

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Inductor

For the inductor, we have the relation:

This time, considering the current signal to be

It follows that

And thus

This tells us that the ratio of AC voltage amplitude to AC current amplitude across an inductor is , and that the AC voltage leads the AC current across an inductor by 90 degrees.

This result is commonly expressed in polar form, as

Or, more simply, using Euler's formula, as

Resistance vs. Reactance

It is important to realize that resistance and reactance are not individually significant; together they determine the magnitude and phase of the impedance, through the following relations:

In many applications the relative phase of the voltage and current is not critical so only the magnitude of the impedance is significant.

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Resistance

Resistance is the real part of impedance; a device with purely resistive impedance exhibits no phase shift between the voltage and current.

Reactance

Reactance is the imaginary part of the impedance; a component with a finite reactance induces a phase shift between the voltage across it and the current through it.

A reactive component is distinguished by the fact that the sinusoidal voltage across the component is in quadrate with the sinusoidal current through the component. This implies that the component alternately absorbs energy from the circuit and then returns energy to the circuit. A pure reactance will not dissipate any power.

Capacitive reactance

A capacitor has purely reactive impedance which is inversely proportional to the signal frequency. A capacitor consists of two conductors separated by an insulator, also known as a dielectric.

At low frequencies a capacitor is open circuit, as no charge flows in the dielectric. A DC voltage applied across a capacitor causes charge to accumulate on one side; the electric field due to the accumulated charge is the source of the opposition to the current. When the potential associated with the charge exactly balances the applied voltage, the current goes to zero.

Driven by an AC supply, a capacitor will only accumulate a limited amount of charge before the potential difference changes sign and the charge dissipates. The higher the frequency, the less charge will accumulate and the smaller the opposition to the current.

Inductive reactance

An inductor has purely reactive impedance which is proportional to the signal frequency. An inductor consists of a coiled conductor. Faraday's law of electromagnetic induction gives the back emf (voltage opposing current) due to a rate-of-change of magnetic field through a current loop.

For an inductor consisting of a coil with loops this gives.

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The back-emf is the source of the opposition to current flow. A constant direct current has a zero rate-of-change, and sees an inductor as a short-circuit (it is typically made from a material with a low resistivity). An alternating current has a time rate-of-change that is proportional to frequency and so the inductive reactance is proportional to frequency.

Combining impedancesThe total impedance of many simple networks of components can be calculated using the rules for combining impedances in series and parallel. The rules are identical to those used for combining resistances, except that the numbers in general will be complex numbers. In the general case however, equivalent impedance transforms in addition to series and parallel will be required.

Series combination

For components connected in series, the current through each circuit element is the same; the ratio of voltages across any two elements is the inverse ratio of their impedances.

Parallel combination

For components connected in parallel, the voltage across each circuit element is the same; the ratio of currents through any two elements is the inverse ratio of their impedances.

The equivalent impedance can be calculated in terms of the equivalent resistance and reactance .[9]

Measuring ImpedanceAccording to Ohm’s law the impedance of a device can be calculated by complex division of the voltage and current. The impedance of the device can be calculated by applying a sinusoidal voltage to the device in series with a resistor, and measuring the voltage across the resistor and across the device. Performing this measurement by sweeping the frequencies of the applied signal provides the impedance phase and magnitude.[10]

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Impulse impedance spectroscopy

The use of an impulse response may be used in combination with the fast Fourier transform (FFT) to rapidly measure the electrical impedance of various electrical devices. The technique compares well to other methodologies such as network and impedance analyzers while providing additional versatility in the electrical impedance measurement. The technique is theoretically simple, easy to implement and completed with ordinary laboratory instrumentation for minimal cost.[10]

Variable impedanceIn general, neither impedance nor admittance can be time varying as they are defined for complex exponentials for –∞ < t < +∞. If the complex exponential voltage–current ratio changes over time or amplitude, the circuit element cannot be described using the frequency domain. However, many systems (e.g., varicaps that are used in radio tuners) may exhibit non-linear or time-varying voltage–current ratios that appear to be LTI for small signals over small observation windows; hence, they can be roughly described as having a time-varying impedance. That is, this description is an approximation; over large signal swings or observation windows, the voltage–current relationship is non-LTI and cannot be described by impedance.

References Collected from:-

1. Oliver Heaviside, The Electrician, p. 212, 23rd July 1886 reprinted as Electrical Papers, p64, AMS Bookstore, ISBN 0821834657

2. AC Ohm's law , Hyperphysics 3. Horowitz, Paul; Hill, Winfield (1989). "1". The Art of Electronics. Cambridge University Press. pp. 32–33. ISBN 0-521-

37095-7. 4. Capacitor/inductor phase relationships , Yokogawa 5. Complex impedance , Hyperphysics 6. Horowitz, Paul; Hill, Winfield (1989). "1". The Art of Electronics. Cambridge University Press. pp. 31–32. ISBN 0-521-

37095-7. 7. Lewis Jr., George; George K. Lewis Sr. and William Olbricht (August 2008). "Cost-effective broad-band electrical

impedance spectroscopy measurement circuit and signal analysis for piezo-materials and ultrasound transducers".

Electrical load

If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. (The term 'load' may also refer to the power consumed by a circuit; that topic is not discussed here.)

Load affects the performance of circuits that output voltages or currents, such as sensors, voltage sources, and amplifiers. A household's power outlets provide an easy example: they are a voltage source, outputting 120 V AC for example (in USA), with the household's appliances collectively making up the load. When a power-hungry appliance switches on, it dramatically reduces the load impedance, causing the output voltage to drop. This drop is easily observed; for instance, turning on a vacuum cleaner dims the lights.

A more technical approach(Two side notes on generality, for advanced readers: This discussion will disregard nonlinearity. It will also use simple resistances, but they can be readily generalized to impedances for AC analysis.)

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When discussing the effect of load on a circuit, it is helpful to disregard the circuit's actual design and consider only the Thévenin equivalent. (The Norton equivalent works just as well, but this discussion will use the Thévenin form.) The Thévenin equivalent of a circuit looks like this:

The circuit is represented by an ideal voltage source Vs in series with an internal resistance Rs.

With no load (open-circuited terminals), all of VS falls across the output; the output voltage is VS. However, the circuit will behave differently if a load is added. We would like to ignore the details of the load circuit, as we did for the power supply, and represent it as simply as possible. If we use an input resistance to represent the load, the complete circuit looks like this:

The input resistance of the load stands in series with Rs.

Whereas the voltage source by itself was an open circuit, adding the load makes a closed circuit and allows current to flow. This current places a voltage drop across RS, so the voltage at the output terminal is no longer VS. The output voltage can be determined by the voltage division rule:

Volt

The volt (symbol: V) is the SI derived unit of electromotive force, commonly called "voltage".[1] It is also the unit for the related but slightly different quantity electric potential difference (also called "electrostatic potential difference"). It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery (see Baghdad Battery).

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DefinitionThe volt is defined as the value of the voltage across a conductor when a current of one ampere dissipates one watt of power in the conductor. [2] It can be written in terms of SI base units as: m2 · kg · s−3 · A−1. It is also equal to one joule of energy per coulomb of charge, J/C.

Josephson junction definition

Since 1990 the volt has been maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as:

K{J-90} = 0.4835979 GHz/µV.

Water flow analogyIn the water flow analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage difference is likened to water pressure difference – the difference determines how quickly the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time (volumetric flow rate). The flow rate is determined by the width of the pipe (analogous to electrical resistance), and the pressure difference between the front end of the pipe and the exit is analogous to voltage. The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (amperes x volts = watts).

The relationship between voltage and current (in ohmic devices) is defined by Ohm's Law.

A multimeter can be used to measure the voltage between two positions.

History of the voltIn 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress,

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now the International Electro technical Commission (IEC), approved the volt as the unit for electromotive force. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power.

The international volt was defined in 1893 as 1/1.434 of the emf of a Clark cell. This definition was abandoned in 1908 in favor of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948.

Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called standard cells. The United States used a design called the Weston cell from 1905 to 1972.

This SI unit is named after Alessandro Volta. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (V). When an SI unit is spelled out in English, it should always begin with a lowercase letter (volt), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

—Based on The International System of Units, section 5.2.

References

1. ̂ "SI Brochure, Table 3 (Section 2.2.2)". BIPM. 2006. http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html. Retrieved 2007-07-29.

2. ̂ BIPM SI Brochure: Appendix 1, p. 144 3. ̂ Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484

Electric current

Electric current can mean, depending on the context, a flow of electric charge (a phenomenon) or the rate of flow of electric charge (a quantity).[1] The electric charge that flows is carried by, for example, mobile electrons in a conductor, ions in an electrolyte or both in a plasma.[2]

The SI unit for rate of flow of electric charge is the ampere. Electric current is measured using an ammeter.[1]

Physics

Electric current through various media

Metals

A solid conductive metal contains mobile, or free, electrons. These electrons are bound to the metal lattice but not to any individual atom. Even with no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there is zero net current within the metal. Given a plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is on average equal to the number passing in the opposite direction.

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As George Gamow put in his science popularizing book, One, Two, Three...Infinity (1947), "The metallic substances differ from all other materials by the fact that the outer shells of their atoms are bound rather loosely, and often let one of their electrons go free. Thus the interior of a metal is filled up with a large number of unattached electrons that travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric force applied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call an electric current."

When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electrons are therefore the current carrier in a typical solid conductor. For an electric current of 1 ampere, 1 coulomb of electric charge (which consists of about 6.242 × 1018 electrons) drifts every second through any plane through which the conductor passes.

For a steady flow, the current I in amperes can be calculated with the following equation:

where

Q is the electric charge in coulombs transferred t is the time in seconds

More generally, electric current can be represented as the time rate of change of charge, or

.

Other media

In metallic solids, electricity flows by means of electrons, from higher to lower electrical potential. In other media, any stream of charged objects may constitute an electric current. In a vacuum, a beam of ions or electrons may be formed. In other conductive materials, the electric current is due to the flow of both positively and negatively charged particles at the same time. In still others, the current is entirely due to positive charge flow. For example, the electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. In a common lead-acid electrochemical cell, electric currents are composed of positive hydrogen ions (protons) flowing in one direction, and negative sulfate ions flowing in the other. Electric currents in sparks or plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, the electric current is entirely composed of flowing ions. In a semiconductor it is sometimes useful to think of the current as due to the flow of positive "holes" (the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron). This is the case in a p-type semiconductor.

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Current density

Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units, the current density is measured in amperes per square meter.

.

where

I is current in the conductor J is the current density, and, A is the cross-sectional area.

The dot product of two vector quantity signifies that electric current is a scalar.

Drift speed

The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. In order for there to be a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation:

I = nAvQ

where

I is the electric current n is number of charged particles per unit volume (or charge carrier density) A is the cross-sectional area of the conductor v is the drift velocity, and Q is the charge on each particle.

Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines at about a tenth of the speed of light.

Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwell's Equations, and is therefore many times faster than the drift velocity of the electrons. For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance.

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The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size.

The nature of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. The low drift velocity of charge carriers is analogous to air motion; in other words, winds. The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas; while the random motion of charges is analogous to heat - the thermal velocity of randomly vibrating gas particles.

Electromagnetism

According to Ampère's law, an electric current produces a magnetic field.

Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.

Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. Devices used for this include Hall effect sensors, current clamps, current transformers, and Rogowski coils.

The theory of Special Relativity allows one to transform the magnetic field into a static electric field for an observer moving at the same speed as the charge in the diagram. The amount of current is particular to a reference frame (who is measuring the current or charge velocity).

Ohm's law

Ohm's law states that the potential difference across a (ideal) resistor (or other ohmic device) is directly proportional to the current flowing in the resistor, i.e

V = IR

where

I is the current, measured in amperes V is the potential difference measured in volts R is the resistance measured in ohms

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Conventions

Current flow

Diagram showing conventional current notation and the flow of electrons. Electric charge moves from the positive side of the power source to the negative, while electrons move from anode to cathode.

A flow of positive charge gives the same electric current as an opposite flow of negative charge. Thus, opposite flows of opposite charges contribute to a single electric current. For this reason, the polarity of the flowing charges can usually be ignored during measurements. All the flowing charges are assumed to have positive polarity, and this flow is called conventional current. Conventional current represents the net effect of the current flow, without regard to the sign of the charge of the objects carrying the current.

In solid metals such as wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron motion in a metal is in the direction opposite to that of conventional (or electric) current.

Reference direction

When solving electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction. When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction.

OccurrencesNatural examples include lightning and the solar wind, the source of the polar auroras (the aurora borealis and aurora australis). The artificial form of electric current is the flow of conduction electrons in metal wires, such as the overhead power lines that deliver electrical energy across long distances and the smaller wires within electrical and electronic equipment. In electronics, other forms of electric current include the flow of electrons through resistors or through the vacuum in a vacuum tube, the flow of ions inside a battery or a neuron, and the flow of holes within a semiconductor.

Electrical safetyThe most obvious hazard is electrical shock, where a current passes through part of the body. It is the amount of current passing through the body that determines the effect, and this depends on the

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nature of the contact, the condition of the body part, the current path through the body and the voltage of the source. While a very small amount can cause a slight tingle, too much can cause severe burns if it passes through the skin or even cardiac arrest if enough passes through the heart. The effect also varies considerably from individual to individual.[citation needed]

Accidental electric heating can also be dangerous. An overloaded power cable is a frequent cause of fire. A battery as small as an AA cell placed in a pocket with metal coins can lead to a short circuit heating the battery and the coins which may inflict burns. NiCad, NiMh cells, and lithium batteries are particularly risky because they can deliver a very high current due to their low internal resistance.

References

1. ^ a b Lakatos, John; Oenoki, Keiji; Judez, Hector; Oenoki, Kazushi; Hyun Kyu Cho (March 1998). "Learn Physics Today!". Lima, Peru: Colegio Franklin D. Roosevelt. http://library.thinkquest.org/10796/ch13/ch13.htm. Retrieved 2009-03-10.

2. ^ Anthony C. Fischer-Cripps (2004). The electronics companion. CRC Press. p. 13. ISBN 9780750310123. http://books.google.com/books?id=3SsYctmvZkoC&pg=PA13&dq=positive-ions+carrier+current+charge+electrons&lr=&as_brr=3&as_pt=ALLTYPES&ei=RbfWSZnELJeSkASZvbi-Bg.

Relays

A relay is an electrically operated switch. Electric current through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double-throw (changeover) switches.

Basic design and operation

A simple electromagnetic relay, such as the one taken from a car in the first picture, is an adaptation of an electromagnet. It consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energised there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the Printed Circuit Board (PCB) via the yoke, which is soldered to the PCB.When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energised, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to circuit components. Some automotive relays already include that diode inside the relay

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case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.[1]By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an opt coupler can be used which is a light-emitting diode (LED) coupled with a photo transistor.

Types of relay

Latching relayA latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or "stay" relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remanent core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. A remanent core latching relay requires a current pulse of opposite polarity to make it change state.Reed relayA reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which protects the contacts against atmospheric corrosion. The contacts are closed by a magnetic field generated when current passes through a coil around the glass tube. Reed relays are capable of faster switching speeds than larger types of relays, but have low switch current and voltage ratings. See also reed switch.Mercury-wetted relayA mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) because of their low contact resistance, or for high-speed counting and timing applications where the mercury eliminates contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment. See also mercury switch.Polarized relayA Polarized Relay placed the armature between the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion. The poles were on screws, so a technician could first adjust them for maximum sensitivity and then apply a bias spring to set the critical current that would operate the relay.Machine tool relayA machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally-open to normally-closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel. Although such relays once were the backbone of automation in such industries as

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automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications.

Contactor relay

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. Continuous current ratings for common contactors range from 10 amps to several hundred amps. High-current contacts are made with alloys containing silver. The unavoidable arcing causes the contacts to oxidize and silver oxide is still a good conductor. Such devices are often used for motor starters. A motor starter is a contactor with overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized. Contactor relays can be extremely loud to operate, making them unfit for use where noise is a chief concern.Solid-state relayA solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across it. This voltage drop limited the amount of current a given SSR could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 Amperes, have become commercially available. Compared to electromagnetic relays, they may be falsely triggered by transients.Solid state contactor relayA solid state contactor is a very heavy-duty solid state relay, including the necessary heat sink, used for switching electric heaters, small electric motors and lighting loads; where frequent on/off cycles are required. There are no moving parts to wear out and there is no contact bounce due to vibration. They are activated by AC control signals or DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls.Buchholz relayA Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil.Forced-guided contacts relayA forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays".Overload protection relayOne type of electric motor overload protection relay is operated by a heating element in series with the electric motor. The heat generated by the motor current operates a bi-metal strip or melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.Pole and throwCircuit symbols of relays. "C" denotes the common terminal in SPDT and DPDT types.

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Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways:Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact. Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact. Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact ("break before make"). If this type of contact utilizes a "make before break" functionality, then it is called a Form D contact. The following designations are commonly encountered:• SPST - Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity. • SPDT - Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total. • DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each). • DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil. • The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example 4PDT indicates a four pole double throw relay (with 14 terminals).

Applications

Relays are used to and for:• Control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers, • Control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile, • Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays), • Isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy, • Logic functions. For example, the boolean AND function is realised by connecting normally open relay contacts in series, the OR function by connecting normally open contacts in parallel. The change-over or Form C contacts perform the XOR (exclusive or) function. Similar functions for NAND and NOR are accomplished using normally closed contacts. The Ladder programming language is often used for designing relay logic networks. o Early computing. Before vacuum tubes and transistors, relays were used as logical elements in digital computers. See ARRA (computer), Harvard Mark II, Zuse Z2, and Zuse Z3.

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o Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear radiation, they are widely used in safety-critical logic, such as the control panels of radioactive waste-handling machinery. • Time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed.

Relay application considerations

Selection of an appropriate relay for a particular application requires evaluation of many different factors:• Number and type of contacts - normally open, normally closed, (double-throw) • Contact sequence - "Make before Break" or "Break before Make". For example, the old style telephone exchanges required Make-before-break so that the connection didn't get dropped while dialing the number. • Rating of contacts - small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current • Voltage rating of contacts - typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15 000 V • Coil voltage - machine-tool relays usually 24 VAC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few milliamperes • Coil current - Usually in the range of 40 - 200 mA for 0 - 24 VDC coils.[2] • Package/enclosure - open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil and splash resistant, washable for printed circuit board assembly • Assembly - Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning agents. Which is removed once assembly is complete? • Mounting - sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment • Switching time - where high speed is required • "Dry" contacts - when switching very low level signals, special contact materials may be needed such as gold-plated contacts • Contact protection - suppress arcing in very inductive circuits • Coil protection - suppress the surge voltage produced when switching the coil current • Isolation between coil circuit and contacts • Aerospace or radiation-resistant testing, special quality assurance • Expected mechanical loads due to acceleration - some relays used in aerospace applications are designed to function in shock loads of 50 g or more • Accessories such as timers, auxiliary contacts, pilot lamps, test buttons • Regulatory approvals • Stray magnetic linkage between coils of adjacent relays on a printed circuit board.

Protective relay

A protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault was

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found. Unlike switching type relays with fixed and usually ill-defined operating voltage thresholds and operating times, protective relays had well-established, selectable, time/current (or other operating parameter) curves. Such relays were very elaborate, using arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay style contacts, and phase-shifting networks to allow the relay to respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency, and even distance relays that would trip for faults up to a certain distance away from a substation but not beyond that point. An important transmission line or generator unit would have had cubicles dedicated to protection, with a score of individual electromechanical devices. The various protective functions available on a given relay are denoted by standard ANSI Device Numbers. For example, a relay including function 51 would be a timed over current protective relay. These protective relays provide various types of electrical protection by detecting abnormal conditions and isolating them from the rest of the electrical system by circuit breaker operation. Such relays may be located at the service entrance or at major load centers. Design and theory of these protective devices is an important part of the education of an electrical engineer who specializes in power systems. Today these devices are nearly entirely replaced (in new designs) with microprocessor-based instruments (numerical relays) that emulate their electromechanical ancestors with great precision and convenience in application. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays. However, due to their very long life span, tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world.

Over current relay

An "Over current Relay" is a type of protective relay which operates when the load current exceeds a preset value. The ANSI Device Designation Number is 50 for an Instantaneous Over Current (IOC), 51 for a Time Over Current (TOC). In a typical application the over current relay is used for over current protection, connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize a trip coil in a Circuit Breaker and trip (open) the Circuit Breaker.Induction disc over current relayThese robust and reliable electromagnetic relays use the induction principle discovered by Ferraris in the late 19th century. The magnetic system in induction disc over current relays is designed to detect over currents in a power system and operate with a pre determined time delay when certain over current limits have been reached. In order to operate, the magnetic system in the relays produces rotational torque that acts on a metal disc to make contact, according to the following basic current/torque equation:T = K x φ1 x φ2 SinθWhereK – is a constantφ1 and φ2 are the two fluxesθ is the phase angle between the fluxesThe relay's primary winding is supplied from the power systems current transformer via a plug bridge, which is also commonly known as the plug setting multiplier (psm). The variations in the current setting are usually seven equally spaced tappings or operating bands that determine the relays sensitivity. The primary winding is located on the upper electromagnet. The secondary winding has connections on the upper electromagnet that are energized from the primary winding and connected to the lower electromagnet. Once the upper and lower electromagnets are energized they produce eddy currents that are induced onto the metal disc and flow through the flux paths. This relationship

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of eddy currents and fluxes creates rotational torque proportional to the input current of the primary winding, due to the two flux paths been out of phase by 90º.Therefore in an over current condition a value of current will be reached that overcomes the control spring pressure on the spindle and the breaking magnet causing the metal disc to rotate moving towards the fixed contact. This initial movement of the disc is also held off to a critical positive value of current by small slots that are often cut into the side of the disc. The time taken for rotation to make the contacts is not only dependent on current but also the spindle backstop position, known as the time multiplier (tm). The time multiplier is divided into 10 linear divisions of the full rotation time.Providing the relay is free from dirt, the metal disc and the spindle with its contact will reach the fixed contact, thus sending a signal to trip and isolate the circuit, within its designed time and current specifications. Drop off current of the relay is much lower than its operating value, and once reached the relay will be reset in a reverse motion by the pressure of the control spring governed by the braking magnet.

Distance relay

The most common form of protection on high voltage transmission systems is distance relay protection. Power lines have set impedance per kilometer and using this value and comparing voltage and current the distance to a fault can be determined. The ANSI standard device number for a distance relay is 21. The main types of distance relay protection schemes are:-• Three step distance protection • Switched distance protection • Accelerated or permissive inter trip protection • Blocked distance protection In three step distance protection, the relays are separated into three separate zones of impedance measurement to accommodate for over reach and under reach conditions. Zone 1 is instantaneous in operation and has a purposely set under reach of 80% of the total line length to avoid operation for the next line. This is due to measurements of impedance of lines not being entirely accurate, errors in voltage and current transformers and relay tolerances. These errors can be up to ±20% of the line impedance, hence the zones 80% reach. Zone 2 covers the last 20% of the feeder line length and provides backup to the next line by having a slight over reach. To prevent mal-operation the zone has a 0.5 second time delay. Zone 3 provides backup for the next line and has a time delay of 1 second to grade with zone 2 protection of the next line.

Motor protection relay

AC motors need over current protection against short circuits from external faults in connecting cables and from internal faults in motor windings. In addition, motors are thermally rated and limited, and protective relays must be applied to prevent overheating during operating conditions where no fault is present. References1. ^ Mason, C. R., Art & Science of Protective Relaying, Chapter 2, GE Consumer & Electrical [1], 2. ^ "Omron: Power PCB Relay G2R". http://www.components.omron.com/components/web/pdflib.nsf/0/109B19860C4214F385257201007DD570/$file/G2R_0607.pdf. 090103 components.omron.com 3. ^ Zocholl, Stan (2003). AC Motor Protection. Schweitzer Engineering Laboratories,Inc.. ISBN 0972502610, 978-0972502610. • Gurevich, Vladimir (2005). Electrical Relays: Principles and Applications. London - New York: CRC Press. • Westinghouse Corporation (1976). Applied Protective Relaying. Library of Congress card no. 76-8060.

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Electrical substationAn electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps.

A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution. The word substation comes from the days before the distribution system became a grid. The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station.

Elements of a substationSubstations generally have switching, protection and control equipment and one or more transformers. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use re-closer circuit breakers or fuses for protection of distribution circuits. Substations do not usually have generators, although a power plant may have a substation nearby. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.

Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the network. Earth faults at a substation can cause a ground potential rise leading to a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution.

One-line diagramA typical one-line diagram with annotated power flows. Red boxes represent circuit breakers, grey lines represent three-phase bus and interconnecting conductors, the orange circle represents a electric generator, the green spiral is an inductor, and the three overlapping blue circles represent a double-wound transformer with a tertiary winding.

In power engineering, a one-line diagram or single-line diagram is a simplified notation for representing a three-phase power system.[1] The one-line diagram has its largest application in power flow studies. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic

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symbols.[1] Instead of representing each of three phases with a separate line or terminal, only one conductor is represented. It is a form of block diagram graphically depicting the paths for power flow between entities of the system. Elements on the diagram do not represent the physical size or location of the electrical equipment, but it is a common convention to organize the diagram with the same left-to-right, top-to-bottom sequence as the switchgear or other apparatus represented.

The theory of three-phase power systems tells us that as long as the loads on each of the three phases are balanced, we can consider each phase separately.[2] In power engineering, this assumption is usually true (although an important exception is the asymmetric fault), and to consider all three phases requires more effort with very little potential advantage.[3]

A one-line diagram is usually used along with other notational simplifications, such as the per-unit system.

A secondary advantage to using a one-line diagram is that the simpler diagram leaves more space for non-electrical, such as economic, information to be included.

References

1. ^ a b McAvinew, Thomas; Mulley, Raymond (2004), Control System Documentation, ISA, p. 165, ISBN 1556178964 2. ^ Electrical Power Systems, Pergamon, 1978, p. 4, ISBN 0-08-021729-X 3. ^ Tleis, Nasser (2008), Power System Modelling and Fault Analysis, Elsevier, p. 28, ISBN 978-0-7506-8074-5

Transmission substationA transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control devices such as capacitors, reactors or Static VAr Compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current transformers, relays and SCADA systems). Modern substations may be implemented using International Standards such as IEC61850.

Distribution substationA distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the high-voltage main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a value suitable for local distribution.

The input for a distribution substation is typically at least two transmission or sub-transmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.

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The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.

Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

Collector substationIn distributed generation projects such as a wind farm, a collector substation may be required. It somewhat resembles a distribution substation although power flow is in the opposite direction, from many wind turbines up into the transmission grid. Usually for economy of construction the collector system operates around 35 kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation also provides power factor correction, metering and control of the wind farm.

Collector substations also exist, when there are in a certain area multiple thermal or hydroelectric power plants of comparable output power are in close proximity. In these cases the collector substation uses as these plants have a higher output than wind or solar power plants higher voltages, often even the highest voltage of the grid. Examples for such substations are Brauweiler in Germany and Hradec in Czech, where power of lignite fired power plants nearby is collected.

It is also possible that a collector substation has only one voltage level and no transformers. In this cases the only function of the substation are switching actions for distributing the power. Such substations are called Switching Stations.

DesignThe main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost. The design should also allow easy expansion of the station, if required.

Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers. Where land is costly, such as in urban areas, gas insulated switchgear may save money overall. The site must have room for expansion due to load growth or planned transmission additions. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects. Grounding (earthing) and ground potential rise must be calculated to protect passers-by during a short-circuit in the transmission system. And of course, the substation site must be reasonably central to the distribution area to be served.

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LayoutThe first step in planning a substation layout is the preparation of a one-line diagram which shows in simplified form the switching and protection arrangement required, as well as the incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by many electrical utilities to prepare one-line diagrams with principal elements (lines, switches, circuit breakers, transformers) arranged on the page similarly to the way the apparatus would be laid out in the actual station.

Incoming lines will almost always have a disconnect switch and a circuit breaker. In some cases, the lines will not have both; with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off. When a large fault current flows through the circuit breaker, this may be detected through the use of current transformers. The magnitude of the current transformer outputs may be used to 'trip' the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the rest of the system to continue operating with minimal impact. Both switches and circuit breakers may be operated locally (within the substation) or remotely from a supervisory control center.

Once past the switching components, the lines of a given voltage connect to one or more buses. These are sets of bus bars, usually in multiples of three, since three-phase electrical power distribution is largely universal around the world.

The arrangement of switches, circuit breakers and buses used affects the cost and reliability of the substation. For important substations a ring bus, double bus, or so-called "breaker and a half" setup can be used, so that the failure of any one circuit breaker does not interrupt power to branch circuits for more than a brief time, and so that parts of the substation may be de-energized for maintenance and repairs. Substations feeding only a single industrial load may have minimal switching provisions, especially for small installations.

Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a 'short circuit').

Along with this, a substation always has control circuitry needed to command the various breakers to open in case of the failure of some component.

Switching functionAn important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".

A transmission line or other component may need to be de-energized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer.

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To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.

Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.

There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.

RailwaysElectrified railways also use substations which may also include rectifier equipment to change alternating current from the utility power distribution network to direct current for use by traction motors.

What is earthing?The whole of the world may be considered as a vast conductor which is at reference (zero) potential. In the UK we refer to this as 'earth' whilst in the USA it is called 'ground'. People are usually more or less in contact with earth, so if other parts which are open to touch become charged at a different voltage from earth a shock hazard exists (see {3.4}). The process of earthing is to connect all these parts which could become charged to the general mass of earth, to provide a path for fault currents and to hold the parts as close as possible to earth potential. In simple theory this will prevent a potential difference between earth and earthed parts, as well as permitting the flow of fault current which will cause the operation of the protective systems.

The standard method of tying the electrical supply system to earth is to make a direct connection between the two. This is usually carried out at the supply transformer, where the neutral conductor (often the star point of a three-phase supply) is connected to earth using an earth electrode or the metal sheath and armoring of a buried cable. {Figure 5.1} shows such a connection. Lightning conductor systems must be bonded to the installation earth with a conductor no larger in cross-sectional area than that of the earthing conductor.

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Three-phase delta/star transformer showing earthing arrangements

Testing insulation resistanceA low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This current could cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation. Thus, the resistance between poles or to earth must never be less than half of one meg ohm (0.5 M Ohms) for the usual supply voltages. In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of the insulation, so a direct voltage is used to prevent reactance from being included in the measurement. Insulation will sometimes have high resistance when low potential differences apply across it, but will break down and offer low resistance when a higher voltage is applied. For this reason, the high levels of test voltage shown in {Table 8.8} are necessary. {8.7.1} gives test instrument requirements.

Before commencing the test it is important that:

1. Electronic equipment which could be damaged by the application of the high test voltage should be disconnected. Included in this category are electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers, switches associated with passive infrared detectors (PIRs), RCDs with electronic operation etc. An alternative to disconnection is to ensure that phase and neutral are connected together before an insulation test is made between them and earth.

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2. Capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.

Table 8.8 - Required test voltages and minimum resistance

Nominal circuit voltage Test voltage (V)

Minimum insulation resistance (M Ohms)

Extra-low voltage circuits supplied from a safety transformer 250 0.25

Up to 500 V except for above 500 0.5

Above 500 V up to 1000 V 1000 1.0

The insulation resistance tester must be capable of maintaining the required voltage when providing a steady state of current of 1mA.

Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using a voltage which is not likely to result in damage. The result must conform to that specified in the British Standard concerned, or be at least 0.5 M Ohms if there is no Standard. The test to earth {Fig 8.10} must be carried out on the complete installation with the main switch off, with phase and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuit breakers closed and all circuit switches closed. Where two-way switching is wired, only one of the two strapper wires will be tested. To test the other, both two-way switches should be

Fig - Insulation test to earth

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Fig - Insulation tests between poles

Operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms should be achieved, see {Fig 8.10}. In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower. If this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.

The tests to earth {Fig 8.10} and between poles {Fig 8.11} must be carried Out as indicated, with a minimum acceptable value for each test of 0.5 M Ohms. However, where a reading of less than 2 M Ohms is recorded for an individual circuit, (the minimum value required by the Health and Safety Executive), there is the possibility of defective insulation, and remedial work may be necessary. A test result of 2 M Ohms may sometimes be unsatisfactory. If such a reading is the result of a re-test, it is necessary to consult the data from previous tests to identify deterioration. A visual inspection of cables to determine their condition is necessary during periodic tests; perished insulation may not always give low insulation readings

As indicated above, tests on SELV and PELV circuits are carried out at 250 V. However tests between these circuits and the live conductors of other circuits must be made at 500 V. Tests to earth for PELV circuits are at 250 V, whilst FELV circuits are tested as LV circuits at 500 V. Readings of less than 5 M will require further investigation.

Ground and neutralSince the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.

DefinitionsGround or earth in a mains (AC power) electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment (the terms "ground" (North American practice) and "earth" (most other English-speaking countries) are used synonymously here). Normally a grounding conductor does not carry current. Neutral is a circuit conductor that may carry current in normal operation, and which is usually connected to earth.

In a poly phase or three-wire (single-phase) AC system, the neutral conductor is intended to have similar voltages to each of the other circuit conductors. By this definition, a circuit must have at least three wires for one to serve as a neutral.

In the electrical trade, the conductor of a 2-wire circuit that is connected to the supply neutral point and earth ground is also referred to as the "neutral". This is formally described in the US and Canadian electrical codes as the "identified" circuit conductor. [1]

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The NEC and Canadian electrical code only define neutral as the first of these. In North American use, the second definition is used in less formal language but not in official specifications. In the UK the IET definition of a neutral conductor is one connected to the supply system neutral point, which includes both these uses.

All neutral wires of the same electrical system should have the same electrical potential, because they are all connected together through the system ground. Neutral conductors are usually insulated for the same voltage as the live conductors, with interesting exceptions [2].

CircuitryNeutral wires are usually connected together at a neutral bus (bar) within panel boards or switchboards, and are "bonded" to earth ground at either the electrical service entrance, or at transformers within the system. For electrical installations with three-wire single phase service, the neutral point of the system is at the center-tap on the secondary side of the service transformer. For larger electrical installations, such as those with poly phase service, the neutral point is usually at the common connection on the secondary side of delta/wye connected transformers. Other arrangements of poly phase transformers may result in no neutral point, and no neutral conductors.

Wiring colorsThe insulation of a neutral wire is colored blue in the EU including the UK, but white or grey in the USA. For large diameter wires or "mains", the insulation of neutral conductors may be colored black, as are also the phase or hot conductors, but are distinctively designated by applying colored tape -- again blue in the EU (including the UK), and white or grey in the USA.

Earthing systemsThe names for the following methods of earthing are those defined by IEC standards, which are used in Europe and many other regions. For a more detailed explanation, see earthing systems. Different terminology is used in North America, but the basic principles should be the same everywhere.

Different systems are used to minimize the voltage difference between neutral and local earth ground. In some systems, the neutral and earth join together at the service intake (TN-C-S); in others, they run completely separately back to the transformer neutral terminal (TN-S), and in others they are kept completely separate with the house earth having its own rod and the neutral being rodded down to earth within the distribution network (TT). In a few cases, they are combined in house wiring (TN-C), but the dangers of broken neutrals (see below) and the cost of the special cables needed to mitigate this mean that it is rarely done nowadays.

Combining neutral with earthVoltages created in grounding (earthing) conductors by currents flowing in the supply utility neutral conductors can be troublesome. For example, special measures may be required in barns used for milking dairy cattle. Very small differential voltages, not usually perceptible to humans, may cause low milk yield, or even mastitis (inflammation of the udder). So-called "tingle voltage filters" may be required in the electrical distribution system for a milking parlor.

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Connecting the neutral to the equipment case provides some protection against faults/shorts, but may produce a dangerous voltage on the case if the neutral connection is broken.

Combined neutral and ground conductors are commonly used in electricity supply companies' wiring and occasionally for fixed wiring in buildings and for some specialist applications where there is little choice like railways and trams. Since normal circuit currents in the neutral conductor can lead to objectionable or dangerous differences between local earth potential and the neutral and to protect against neutral breakages, special precautions such as frequent rodding down to earth, use of cables where the combined neutral and earth completely surrounds the phase conductor(s), and thicker than normal equipotential bonding must be considered to ensure the system is safe.

Fixed appliances on three-wire circuits

In the USA, the cases of some ovens and clothes dryers were grounded through their neutral wires as a measure to conserve copper during the Second World War. This practice was removed from the NEC in the 1996 edition, but existing installations may still allow the case of such appliances to be connected to the neutral conductor for grounding. Note that the NEC may be amended by local regulations in each state and city. This practice arose from the three wire system used to supply both 120 volt and 240 volt loads. Because ovens and dryers have components that use both 120 and 240 volts there is often some current on the neutral wire. This differs from the protective grounding wire, which only carries current under fault conditions. Using the neutral conductor for grounding the equipment enclosure was considered safe since the devices were permanently wired to the supply and so the neutral was unlikely to be broken without also breaking both supply conductors. Also, the unbalanced current due to lamps and small motors in the appliance was small compared to the rating of the conductors and therefore unlikely to cause a large voltage drop in the neutral conductor.

Portable appliances

In North American practice small portable equipment connected by a cord set may have only two conductors in the attachment plug. A polarized plug is used to maintain the identity of the neutral conductor into the appliance but it is never used as a chassis/case ground. The small cords to lamps, etc., often have one or more ridges or embedded strings to identify the neutral conductor, or may be identified by color. Portable appliances never rely on using the neutral conductor for case grounding.

In places where the design of the plug and socket cannot ensure that a system neutral conductor is connected to particular terminals of the device, portable appliances must be designed as if both poles of each circuit may reach full voltage with respect to ground.

References collected from:-

[1] Rick Gilmour et al., editor, Canadian Electrical Code Part I, Nineteenth Edition, C22.1-02 Safety Standard for Electrical Installations, Canadian Standards Association, Toronto, Ontario Canada (2002) ISBN 1-55324-690-X

[2] NFPA 70, National Electrical Code 2002, National Fire Protection Association, Inc., Quincy, Massachusetts USA, (2002). no ISBN

[3] IEE Wiring Regulations Regulations for Electrical Installations Fifteenth Edition 1981, The Institution of Electrical Engineers, (1981) Hitchin, Herts. United Kingdom

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Sulfur hexafluoride (SF6)

Sulfur hexafluoride

IUPAC name Sulfur hexafluoride

Other namesSulfur fluorideElagasEsaflon

IdentifiersCAS number 2551-62-4 UN number 1080RTECS number WS4900000SMILESPropertiesMolecular formula SF6

Molar mass 146.06 g/molAppearance colorless, odorless gas

Density6.164 g/L (gas, 1 bar: ~5.1 times denser than air)1.329 g/ml (liquid, 25 °C)2.510 g/cm3 (solid, −50.8 °C)

Melting point-50.7 °C (triple point)

Boiling point −64 °C (209 K) (subl.)decomp. at ca. 500 °C (773 K)

Solubility in water slightly solubleSolubility in ethanol solubleStructureCrystal structure Orthorhombic, oP28Space group Pnma, No. 62Coordinationgeometry octahedral (Oh)

Dipole moment 0HazardsMSDS External MSDSEU Index Not listed

Main hazards Asphyxiate in high concentrations, no odor warning

Related compounds

Other cations Selenium hexafluorideTellurium hexafluoride

Related sulfur fluorides

Disulfur difluorideSulfur difluorideSulfur tetrafluorideDisulfur decafluoride

Related compounds Sulfuryl fluoride

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Sulfur hexafluoride (SF6) is an inorganic, colorless, odorless, non-toxic and non-flammable gas (under standard conditions). SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in nonpolar organic solvents. It is generally transported as a liquefied compressed gas. It has a density of 6.13 g/L at sea level conditions, which is considerably higher than the density of air.

Synthesis and chemistrySF6 can be prepared from the elements through exposure of S8 to F2. This is also the method used by the discoverers Henri Moissan and Paul Lebeau in 1901. Some other sulfur fluorides are co generated, but these are removed by heating the mixture to disproportionate any S2F10 (which is highly toxic, unlike SF6) and then scrubbing the product with NaOH to destroy remaining SF4.

There is virtually no reaction chemistry for SF6. It does not react with molten sodium, but reacts exothermically with lithium.

Starting from SF4, one can prepare SF5Cl, which is structurally related to SF6. The monochloride is, however, a strong oxidant and readily hydrolyzed to sulfate.

ApplicationsOf the 8,000 tons of SF6 produced per year, most (6,000 tons) is used as a gaseous dielectric medium in the electrical industry, an inert gas for the casting of magnesium, and as an inert filling for windows.

Dielectric medium

SF6 is used in the electrical industry as a gaseous dielectric medium for high-voltage (35 kV and above) circuit breakers, switchgear, and other electrical equipment, often replacing oil filled circuit breakers (OCBs) that can contain harmful PCBs. SF6 gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room. Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment. Vacuum circuit breakers (VCBs) are displacing SF6

breakers in industry as they are safer and require less maintenance. Although most of the decomposition products tend to quickly re-form SF6, arcing or corona can produce disulfur decafluoride (S2F10), a highly toxic gas, with toxicity similar to phosgene. S2F10 was considered a potential chemical warfare agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure.

SF6 is also commonly encountered as a high voltage dielectric in the high voltage supplies of particle accelerators, such as Van de Graaff generators and Pelletrons and high voltage transmission electron microscopes.

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Medical use

Because SF6 is relatively slowly absorbed by the bloodstream, it is used to provide a long-term tamponade or plug of a retinal hole in retinal detachment repair operations.

In a further medical application, SF6 is employed as a contrast agent for ultrasound imaging. Sulfur hexafluoride microbubbles are administered in solution through injection into a peripheral vein. These microbubbles enhance the visibility of blood vessels to ultrasound. This application has been utilized to examine the vascularity of tumors amongst other things.

Tracer compound

Sulfur hexafluoride was the tracer gas used in the first roadway air dispersion model calibration; this research program was sponsored by the U.S. Environmental Protection Agency and conducted in Sunnyvale, California on U.S. Highway 101.[1] Gaseous SF6 is an ongoing commonly used tracer gas for use in short-term experiments of ventilation efficiency in buildings and indoor enclosures, and for determining infiltration rates. Two major factors recommend its use: Its concentration can be measured with satisfactory accuracy at very low concentrations, and the Earth's atmosphere has a negligible concentration of SF6.

Sulfur hexafluoride was used as a harmless test gas in an experiment at St John's Wood tube station in London, United Kingdom on 25 March 2007.[2] The gas was released throughout the station, and monitored as it drifted around. The purpose of the experiment, which had been announced earlier in March by the Secretary of State for Transport Douglas Alexander, was to investigate how toxic gas might spread throughout London Underground stations and buildings during a terrorist attack.

It has been used successfully as a tracer in oceanography to study diapycnal mixing and air-sea gas exchange.

Other uses

Sulfur hexafluoride is also used as a reagent for creating thrust in a closed Rankine cycle propulsion system, reacting with solid lithium as used in the United States Navy's Mark 50 torpedo.

SF6 plasma is also used in the semiconductor industry as an etchant.

The magnesium industry uses large amounts of SF6 as inert gas to fill casting forms.

Sulfur Hexafluoride is also used to pressurize the waveguide in radar systems.

Greenhouse gasAccording to the Intergovernmental Panel on Climate Change, SF6 is the most potent greenhouse gas that it has evaluated, with a global warming potential of 22,800[3] times that of CO2 when compared over a 100 year period. However, due to its high density relative to air, SF6 flows to the bottom of the atmosphere which limits its ability to heat the atmosphere. SF6 is very stable (for countries reporting their emissions to the UNFCCC, a GWP of 23,900 for SF6 was suggested at the third Conference of the Parties: GWP used in Kyoto protocol).[4] Its mixing ratio in the atmosphere is

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lower than that of CO2 about 6.5 parts per trillion (ppt) in 2008 versus 380 ppm of carbon dioxide, but has steadily increased (from a figure of 4.0 parts per trillion in the late 1990s).[5] Its atmospheric lifetime is 3200 years.

Physiological effects and precautionsAnother effect is the gas's ability to alter vocal sound waves. The gas can be inhaled in a small, safe amount and cause the breather's voice to sound very deep. This is due to the gas density. Unlike helium, which is much less dense than air, SF6 is approximately 5 times denser than air, and the velocity of sound through the gas is 0.44 times the speed of sound in air. Inhalation of SF6 causes a lowering of the timbre, or frequency of the formants, of the vocal tract, by contrast with inhalation of helium, which raises it.

This was demonstrated by Adam Savage on the Myth busters’ television program on 3 September 2008 (along with an inhalation of helium, to show higher pitched sound).

Although inhaling SF6 can be a novel amusement, the practice can be dangerous because, like other inert gases, it displaces not only the oxygen needed for life, but also the CO2 that is the primary trigger of the breathing reflex. In general, dense, odorless gases in confined areas present the hazard of suffocation. A myth exists that SF6 is too heavy for the lungs to expel unassisted, and that after inhaling SF6, it is necessary to bend over completely at the waist to allow the excess gas to "spill" out of the body.

Other PropertiesThermal Conductivity at STP (101.3 kPa and 0 °C) = 12.058 mW/(m.K) Heat capacity at constant pressure (Cp) (101.3 kPa and 21 °C) = 0.097 kJ/(mol.K)

References collected from:-

1. C.Michael Hogan, Leda C. Patmore, Richard Venti, Gary Latshaw et al. (1973) Calibration of a line source model for air pollutant dispersal from roadways, ESL Inc., U.S. Environmental Protection Agency Technology Series, Government Printing Office, Washington, DC

2. "'Poison gas' test on Underground". BBC News. 25 March 2007. http://news.bbc.co.uk/1/hi/england/london/6492501.stm. Retrieved 2007-03-31.

3. Intergovernmental Panel on Climate Change, Working Group 1, Climate Change 2007, Chapter 2. 4. "Climate Change 2001: Working Group I: The Scientific Basis". Intergovernmental Panel on Climate Change. 2001.

http://www.grida.no/climate/ipcc_tar/wg1/248.htm. Retrieved 2007-03-31. 5. "NOAA ESRL GMD Carbon Cycle - Interactive Atmospheric Data Visualisation". US National Oceanic and Atmospheric

Administration. 21 April 2008. http://www.esrl.noaa.gov/gmd/ccgg/iadv/. Retrieved 2008-04-21. 6. "Physics in speech". phys.unsw.edu.au.. http://www.phys.unsw.edu.au/PHYSICS_!/SPEECH_HELIUM/speech.html.

Retrieved 2008-07-20. 7. "Mythbusters - Fun With Gas". YouTube. 2008-06-02. http://www.youtube.com/watch?v=d-XbjFn3aqE. Retrieved 2008-

09-09. 8. Air Liquide Gas Encyclopedia Sulfur hexafluoride". http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=34.

Retrieved 2008-10-26.

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Nitrogen (N2)

Nitrogen (pronounced /ˈnaɪtrədʒɨn/) is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674µ. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.

Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The extremely strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in converting the N2 into useful compounds, and releasing large amounts of energy when these compounds burn or decay back into nitrogen gas.

The element nitrogen was discovered by Daniel Rutherford, a Scottish physician, in 1772. Nitrogen occurs in all living organisms. It is a constituent element of amino acids and thus of proteins, and of nucleic acids (DNA and RNA). It resides in the chemical structure of almost all neurotransmitters, and is a defining component of alkaloids, biological molecules produced by many organisms.

HistoryNitrogen (Latin nitrogenium, where nitrum (from Greek nitron) means "saltpetre" (see nitre), and genes means "forming") is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or fixed air. That there was a fraction of air that did not support combustion was well known to the late 18th century chemist. Nitrogen was also

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7carbon ← nitrogen → oxygen

-↑N↓P

Periodic table

GeneralName, symbol, number nitrogen, N, 7

Element category nonmetal

Group, period, block 15, 2, p

Appearance colorless gas

Standard atomic weight 14.0067 (2)  g·mol−1

Electron configuration 1s2 2s2 2p3

Electrons per shell 2, 5 (Image)

Physical propertiesPhase gas

Density (0 °C, 101.325 kPa)1.251 g/L

Melting point 63.153 K(-210.00 °C, -346.00 °F)

Boiling point 77.36 K(-195.79 °C, -320.3342 °F)

Triple point 63.1526 K (-210°C), 12.53 kPa

Critical point 126.19 K, 3.3978 MPa

Heat of fusion (N2) 0.72 kJ·mol−1

Heat of vaporization (N2) 5.56 kJ·mol−1

Specific heat capacity (25 °C) (N2)29.124 J·mol−1·K−1

Vapor pressureP/Pa 1 10 100 1 k 10 k 100 k

at T/K 37 41 46 53 62 77

Atomic propertiesCrystal structure hexagonal

Oxidation states 5, 4, 3, 2, 1,[1], -1, -3(strongly acidic oxide)

studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephetic air" or azote, from the Greek word αζωτος (azotos) meaning "lifeless". Animals died in it, and it was the principal component of air in which animals had suffocated and flames had burned to extinction. Lavoisier's name for nitrogen still remains in the common names of many compounds, such as hydrazine and compounds of the azide ion. Compounds of nitrogen were known in the Middle Ages. The alchemists knew nitric acid as aqua fortis (strong water). The mixture of nitric and hydrochloric acids was known as aqua regia (royal water), celebrated for its ability to dissolve gold (the king of metals). The earliest military, industrial and agricultural applications of nitrogen compounds involved uses of saltpeter (sodium nitrate or potassium nitrate), notably in gunpowder, and much later, as fertilizer.

PropertiesNitrogen is a nonmetal, with an electro negativity of 3.04. It has five electrons in its outer shell and is therefore trivalent in most compounds. The triple bond in molecular nitrogen (N2) is the strongest in nature. The resulting difficulty of converting (N2) into other compounds, and the ease (and associated high energy release) of converting nitrogen compounds into elemental N2, have dominated the role of nitrogen in both nature and human economic activities.

At atmospheric pressure molecular nitrogen condenses (liquefies) at 77 K (−195.8 °C) and freezes at 63 K (−210.0 °C) into the beta hexagonal close-packed crystal allotropic form. Below 35.4 K (−237.6 °C) nitrogen assumes the alpha cubic crystal allotropic form. Liquid nitrogen, a fluid resembling water, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808 g/mL), is a common cryogen.

Unstable allotropes of nitrogen consisting of more than two nitrogen atoms have been produced in the laboratory, like N3 and N4.[1] Under extremely high pressures (1.1 million atm) and high temperatures (2000 K), as produced under diamond anvil conditions, nitrogen polymerizes into the single bonded diamond crystal structure, an allotrope nicknamed "nitrogen diamond."[2]

Isotopes

There are two stable isotopes of nitrogen: 14N and 15N. By far the most common is 14N (99.634%), which is produced in the CNO cycle in stars. Of the ten isotopes produced synthetically, 13N has a half life of ten minutes and the remaining isotopes have half lives on the order of seconds or less. Biologically-mediated reactions (e.g., assimilation, nitrification, and gentrification) strongly control nitrogen dynamics in the soil. These reactions typically result in 15N enrichment of the substrate and depletion of the product.

0.73% of the molecular nitrogen in Earth's atmosphere is comprised of the isotopologue 14N15N and almost all the rest is 14N2.

Radioisotope 16N is the dominant radionuclide in the coolant of pressurized water reactors during normal operation. It is produced from 16O (in water) via (n,p) reaction. It has a short half-life of about 7.1 s, but during its decay back to 16O produces high-energy gamma radiation (5 to 7 MeV). Because of this, the access to the primary coolant piping must be restricted during reactor power operation. N is one of the main means used to immediately detect even small leaks from the primary coolant to the secondary steam cycle.

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Electromagnetic spectrum

Molecular nitrogen (14N2) is largely transparent to infrared and visible radiation because it is a homo nuclear molecule and thus has no dipole moment to couple to electromagnetic radiation at these wavelengths. Significant absorption occurs at extreme ultraviolet wavelengths, beginning around 100 nanometers. This is associated with electronic transitions in the molecule to states in which charge is not distributed evenly between nitrogen atoms. Nitrogen absorption leads to significant absorption of ultraviolet radiation in the Earth's upper atmosphere as well as in the atmospheres of other planetary bodies. For similar reasons, pure molecular nitrogen lasers typically emit light in the ultraviolet range.

Nitrogen also makes a contribution to visible air glow from the Earth's upper atmosphere, through electron impact excitation followed by emission. This visible blue air glow (seen in the polar aurora and in the re-entry glow of returning spacecraft) typically results not from molecular nitrogen, but rather from free nitrogen atoms combining with oxygen to form nitric oxide (NO).

Reactions

Structure of [Ru(NH3)5(N2)]2+.

Nitrogen is generally un reactive at standard temperature and pressure. N2 reacts spontaneously with few reagents, being resilient to acids and bases as well as oxidants and most reductants. When nitrogen reacts spontaneously with a reagent, the net transformation is often called nitrogen fixation.

Nitrogen reacts with elemental lithium at STP. Lithium burns in an atmosphere of N2 to give lithium nitride:

6 Li + N2 → 2 Li3N

Magnesium also burns in nitrogen, forming magnesium nitride.

3 Mg + N2 → Mg3N2

N2 forms a variety of adducts with transition metals. The first example of a dinitrogen complex is [Ru(NH3)5(N2)]2+ (see figure at right). Such compounds are now numerous, other examples include IrCl(N2)(PPh3)2, W(N2)2(Ph2CH2CH2PPh2)2, and [(η5-C5Me4H)2Zr]2(μ2,η²,η²-N2). These complexes illustrate how N2 might bind to the metal(s) in nitrogenase and the catalyst for the Haber-Bosch Process. A catalytic process to reduce N2 to ammonia with the use of a molybdenum complex in the presence of a proton source was published in 2005.[4] (see nitrogen fixation)

The starting point for industrial production of nitrogen compounds is the Haber-Bosch process, in which nitrogen is fixed by reacting N2 and H2 over a ferric oxide (Fe3O4) catalyst at about 500 °C and 200 atmospheres pressure. Biological nitrogen fixation in free-living cyan bacteria and in the root nodules of plants also produces ammonia from molecular nitrogen. The reaction, which is the source of the bulk of nitrogen in the biosphere, is catalyzed by the nitrogenase enzyme complex which contains Fe and Mo atoms, using energy derived from hydrolysis of adenosine triphosphate (ATP) into adenosine diphosphate and inorganic phosphate (−20.5 kJ/mol).

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OccurrenceNitrogen is the largest single constituent of the Earth's atmosphere (78.082% by volume of dry air, 75.3% by weight in dry air). It is created by fusion processes in stars, and is estimated to be the 7th most abundant chemical element by mass in the universe.

Molecular nitrogen and nitrogen compounds have been detected in interstellar space by astronomers using the Far Ultraviolet Spectroscopic Explorer.[6] Molecular nitrogen is a major constituent of the Saturnian’s moon Titan's thick atmosphere, and occurs in trace amounts in other planetary atmospheres.

Nitrogen is present in all living organisms, in proteins, nucleic acids and other molecules. It typically makes up around 4% of the dry weight of plant matter, and around 3% of the weight of the human body. It is a large component of animal waste (for example, guano), usually in the form of urea, uric acid, ammonium compounds and derivatives of these nitrogenous products, which are essential nutrients for all plants that are unable to fix atmospheric nitrogen.

Nitrogen occurs naturally in a number of minerals, such as saltpeter (potassium nitrate), Chile saltpeter (sodium nitrate) and sal ammoniac (ammonium chloride). Most of these are relatively uncommon, partly because of the minerals' ready solubility in water. See also Nitrate minerals and Ammonium minerals.

CompoundsThe main neutral hydride of nitrogen is ammonia (NH3), although hydrazine (N2H4) is also commonly used. Ammonia is more basic than water by 6 orders of magnitude. In solution ammonia forms the ammonium ion (NH4

+). Liquid ammonia (b.p. 240 K) is amphiprotic (displaying either Brønsted-Lowry acidic or basic character) and forms ammonium and the less common amide ions (NH2

-); both amides and nitride (N3-) salts are known, but decompose in water. Singly, doubly, triply and quadruple substituted alkyl compounds of ammonia are called amines (four substitutions, to form commercially and biologically important quaternary amines, results in a positively charged nitrogen, and thus a water-soluble, or at least amphiphilic, compound). Larger chains, rings and structures of nitrogen hydrides are also known, but are generally unstable. N2

2+ is another polyatomic cation as in hydrazine.

Other classes of nitrogen anions (negatively charged ions) are the poisonous azides (N3-), which are

linear and isoelectronic to carbon dioxide, but which bind to important iron-containing enzymes in the body in a manner more resembling cyanide. Another molecule of the same structure is the colorless and relatively inert anesthetic gas Nitrous oxide (dinitrogen monoxide, N2O), also known as laughing gas. This is one of a variety of nitrogen oxides that form a family often abbreviated as NOx. Nitric oxide (nitrogen monoxide, NO), is a natural free radical used in signal transduction in both plants and animals, for example in vasodilation by causing the smooth muscle of blood vessels to relax. The reddish and poisonous nitrogen dioxide NO2 contains an unpaired electron and is an important component of smog. Nitrogen molecules containing unpaired electrons show an understandable tendency to dimerize (thus pairing the electrons), and are generally highly reactive. The corresponding acids are nitrous HNO2 and nitric acid HNO3, with the corresponding salts called nitrites and nitrates.

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The higher oxides dinitrogen trioxide N2O3, dinitrogen tetroxide N2O4 (DTO) and dinitrogen pentoxide N2O5, are fairly unstable and explosive, a consequence of the chemical stability of N2. DTO is one of the most important oxidisers of rocket fuels, used to oxidise hydrazine in the Titan rocket and in the recent NASA MESSENGER probe to Mercury. DTO is an intermediate in the manufacture of nitric acid HNO3, one of the few acids stronger than hydronium and a fairly strong oxidizing agent.

Nitrogen is notable for the range of explosively unstable compounds that it can produce. Nitrogen triiodide NI3 is an extremely sensitive contact explosive. Nitrocellulose, produced by nitration of cellulose with nitric acid, is also known as guncotton. Nitroglycerin, made by nitration of glycerin, is the dangerously unstable explosive ingredient of dynamite. The comparatively stable, but more powerful explosive trinitrotoluene (TNT) is the standard explosive against which the power of nuclear explosions are measured.

Nitrogen can also be found in organic compounds. Common nitrogen functional groups include: amines, amides, nitro groups, imines, and enamines. The amount of nitrogen in a chemical substance can be determined by the Kjeldahl method.

ApplicationsA computer rendering of the nitrogen molecule, N2.

Nitrogen gas is an industrial gas produced by the fractional distillation of liquid air, or by mechanical means using gaseous air (i.e. pressurised reverse osmosis membrane or Pressure swing adsorption). Commercial nitrogen is often a byproduct of air-processing for industrial concentration of oxygen for steelmaking and other purposes. When supplied compressed in cylinders it is often referred to as OFN (oxygen-free nitrogen)[citation needed].

Nitrogen gas has a wide variety of applications, including serving as an inert replacement for air where oxidation is undesirable;

To preserve the freshness of packaged or bulk foods (by delaying rancidity and other forms of oxidative damage)

In ordinary incandescent light bulbs as an inexpensive alternative to argon[citation needed] On top of liquid explosives as a safety measure The production of electronic parts such as transistors, diodes, and integrated circuits Dried and pressurized, as a dielectric gas for high voltage equipment The manufacturing of stainless steel. Use in military aircraft fuel systems to reduce fire hazard, (see inerting system_ Filling automotive and aircraft tires[8] due to its inertness and lack of moisture or oxidative

qualities, as opposed to air, though this is not necessary for consumer automobiles.

Nitrogen molecules are less likely to escape from the inside of a tire compared with the traditional air mixture used Air consists mostly of nitrogen and oxygen. Nitrogen molecules have a larger effective diameter than oxygen molecules and therefore diffuse through porous substances more slowly.

Molecular nitrogen, a diatomic gas, is apt to dimerize into a linear four nitrogen long polymer (NSubscript text4)[This is an important phenomenon for understanding high-voltage nitrogen dielectric

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switches because the process of polymerization can continue to lengthen the molecule to still longer lengths in the presence of an intense electric field. A nitrogen polymer fog can thereby be created The second viral coefficient of nitrogen also shows this effect as the compressibility of nitrogen gas is changed by the dimerization process at moderate and low temperatures.[

Nitrogen is commonly used during sample preparation procedures for chemical analysis. Specifically, it is used as a means of concentrating and reducing the volume of liquid samples. Directing a pressurized stream of nitrogen gas perpendicular to the surface of the liquid allows the solvent to evaporate while leaving the solute(s) and un-evaporated solvent behind.

Nitrogen tanks are also replacing carbon dioxide as the main power source for paintball guns. The downside is that nitrogen must be kept at higher pressure than CO2, making N2 tanks heavier and more expensive.

Nitrogenated beer

A further example of its versatility is its use as a preferred alternative to carbon dioxide to pressurize kegs of some beers, particularly stouts and British ales, due to the smaller bubbles it produces, which make the dispensed beer smoother and headier. A modern application of a pressure sensitive nitrogen capsule known commonly as a "widget" now allows nitrogen charged beers to be packaged in cans and bottles.

Liquid nitrogen

Liquid nitrogen is a cryogenic liquid. At atmospheric pressure, it boils at −195.8 °C. When insulated in proper containers such as Dewar flasks, it can be transported without much evaporative loss.

Like dry ice, the main use of liquid nitrogen is as a refrigerant. Among other things, it is used in the cry preservation of blood, reproductive cells (sperm and egg), and other biological samples and materials. It is used in cold traps for certain laboratory equipment and to cool x-ray detectors . It has also been used to cool central processing units and other devices in computers which are over clocked, and which produce more heat than during normal operation.

Applications of nitrogen compounds

Molecular nitrogen (N2) in the atmosphere is relatively non-reactive due to its strong bond, and N2

plays an inert role in the human body, being neither produced nor destroyed. In nature, nitrogen is converted into biologically (and industrially) useful compounds by lightning, and by some living organisms, notably certain bacteria (i.e. nitrogen fixing bacteria – see Biological role below). Molecular nitrogen is released into the atmosphere in the process of decay, in dead plant and animal tissues.

The ability to combine or fix molecular nitrogen is a key feature of modern industrial chemistry, where nitrogen and natural gas are converted into ammonia via the Haber process. Ammonia, in turn, can be used directly (primarily as a fertilizer, and in the synthesis of nitrated fertilizers), or as a precursor of many other important materials including explosives, largely via the production of nitric acid by the Ostwald process.

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The organic and inorganic salts of nitric acid have been important historically as convenient stores of chemical energy. They include important compounds such as potassium nitrate (or saltpeter used in gunpowder) and ammonium nitrate, an important fertilizer and explosive (see ANFO). Various other nitrated organic compounds, such as nitroglycerin and trinitrotoluene, and nitrocellulose, are used as explosives and propellants for modern firearms. Nitric acid is used as an oxidizing agent in liquid fueled rockets. Hydrazine and hydrazine derivatives find use as rocket fuels and monopropellants. In most of these compounds, the basic instability and tendency to burn or explode is derived from the fact that nitrogen is present as an oxide, and not as the far more stable nitrogen molecule (N2) which is a product of the compounds' thermal decomposition. When nitrates burn or explode, the formation of the powerful triple bond in the N2 which results, produces most of the energy of the reaction.

Nitrogen is a constituent of molecules in every major drug class in pharmacology and medicine. Nitrous oxide (N2O) was discovered early in the 19th century to be a partial anesthetic, though it was not used as a surgical anesthetic until later. Called "laughing gas", it was found capable of inducing a state of social disinhibition resembling drunkenness. Other notable nitrogen-containing drugs are drugs derived from plant alkaloids, such as morphine (there exist many alkaloids known to have pharmacological effects; in some cases they appear natural chemical defences of plants against predation). Nitrogen containing drugs include all of the major classes of antibiotics, and organic nitrate drugs like nitroglycerin and nitroprusside which regulate blood pressure and heart action by mimicking the action of nitric oxide.

Biological roleNitrogen is an essential building block of both amino acids and nucleic acids, essential to life on Earth.

Elemental nitrogen in the atmosphere cannot be used directly by either plants or animals, and must convert to a reduced (or 'fixed') state in order to be useful for higher plants and animals. Precipitation often contains substantial quantities of ammonium and nitrate, thought[who?] to result from nitrogen fixation by lightning and other atmospheric electric phenomena. However, because ammonium is preferentially retained by the forest canopy relative to atmospheric nitrate, most fixed nitrogen that reaches the soil surface under trees as nitrate. Soil nitrate is preferentially assimilated by these tree roots relative to soil ammonium.

Specific bacteria (e.g. Rhizobium trifolium) possess nitrogenase enzymes which can fix atmospheric nitrogen (see nitrogen fixation) into a form (ammonium ion) that is chemically useful to higher organisms. This process requires a large amount of energy and anoxic conditions. Such bacteria may live freely in soil (e.g. Azotobacter) but normally exist in a symbiotic relationshipin the root nodules of leguminous plants (e.g. clover, Trifolium, or soybean plant, Glycine max). Nitrogen-fixing bacteria are also symbiotic with a number of unrelated plant species such as alders (Alnus) spp., lichens (Casuarina), Myrica, liverworts, and Gunnera.

As part of the symbiotic relationship, the plant converts the 'fixed' ammonium ion to nitrogen oxides and amino acids to form proteins and other molecules, (e.g. alkaloids). In return for the 'fixed' nitrogen, the plant secretes sugars to the symbiotic bacteria.

Some plants are able to assimilate nitrogen directly in the form of nitrates which may be present in soil from natural mineral deposits, artificial fertilizers, animal waste, or organic decay (as the product of bacteria, but not bacteria specifically associated with the plant). Nitrates absorbed in this

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fashion are converted to nitrites by the enzyme [[nitrate reductase]], and then converted to ammonia by another enzyme called [[nitrite reductase]].

Nitrogen compounds are basic building blocks in animal biology as well. Animals use nitrogen-containing amino acids from plant sources, as starting materials for all nitrogen-compound animal biochemistry, including the manufacture of proteins and nucleic acids. Plant-feeding insects are dependent on nitrogen in their diet, such that varying the amount of nitrogen fertilizer applied to a plant can affect the reproduction rate of insects feeding on fertilized plants.[14]

Soluble nitrate is an important limiting factor in the growth of certain bacteria in ocean waters [In many places in the world, artificial fertilizers applied to crop-lands to increase yields result in run-off delivery of soluble nitrogen to oceans at river mouths[.This process can result in eutrophication of the water, as nitrogen-driven bacterial growth depletes water oxygen to the point that all higher organisms die. Well-known "dead zone" areas in the U.S. Gulf Coast and the Black Sea are due to this important polluting process.

Many saltwater fish manufacture large amounts of trim ethylamine oxide to protect them from the high osmotic effects of their environment (conversion of this compound to dim ethylamine is responsible for the early odor in unfresh saltwater fish [15]. In animals, free radical nitric oxide (NO) (derived from an amino acid), serves as an important regulatory molecule for circulation].

Animal metabolism of NO results in production of nitrite[.Animal metabolism of nitrogen in proteins generally results in excretion of urea, while animal metabolism of nucleic acids results in excretion of urea and uric acid[.The characteristic odor of animal flesh decay is caused by the creation of long-chain, nitrogen-containing amines, such as putrescine and cadaverine.

Decay of organisms and their waste products may produce small amounts of nitrate but most decay eventually returns nitrogen content to the atmosphere[citation needed], as molecular nitrogen . The circulation of nitrogen from atmosphere to organic compounds and back is referred to as the nitrogen cycle.

SafetyRapid release of nitrogen gas into an enclosed space can displace oxygen, and therefore represents an asphyxiation hazard. This may happen with few warning symptoms, since the human carotid body is a relatively slow and a poor low-oxygen (hypoxia) sensing system. [16] An example occurred shortly before the launch of the first Space Shuttle mission in 1981, when two technicians lost consciousness and died after they walked into a space located in the Shuttle's Mobile Launcher Platform that was pressurized with pure nitrogen as a precaution against fire. The technicians would have been able to exit the room if they had experienced early symptoms from nitrogen-breathing.

When inhaled at high partial pressures (more than about 4 bar, encountered at depths below about 30 m in scuba diving) nitrogen begins to act as an anesthetic agent. It can cause nitrogen narcosis, a temporary semi-anesthetized state of mental impairment similar to that caused by nitrous oxide.

Nitrogen also dissolves in the bloodstream and body fats. Rapid decompression (particularly in the case of divers ascending too quickly, or astronauts decompressing too quickly from cabin pressure to spacesuit pressure) can lead to a potentially fatal condition called decompression sickness (formerly known as caisson sickness or more commonly, the "bends"), when nitrogen bubbles form in the

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bloodstream, nerves, joints, and other sensitive or vital areas.[19][20] Other "inert" gases (those gases other than carbon dioxide and oxygen) cause the same effects from bubbles composed of them, so replacement of nitrogen in breathing gases may prevent nitrogen narcosis, but does not prevent decompression sickness.[21]

Direct skin contact with liquid nitrogen will eventually cause severe frostbite (cryogenic burns). This may happen almost instantly on contact, depending on the form of liquid nitrogen. Bulk liquid nitrogen causes less rapid freezing than a spray of nitrogen mist (such as is used to freeze certain skin growths in the practice of dermatology). The extra surface area provided by nitrogen-soaked materials is also important, with soaked clothing or cotton causing far more rapid damage than a spill of direct liquid to skin. Full "contact" between naked skin and large droplets or pools of undisturbed liquid nitrogen may be prevented for a few seconds by a layer of insulating gas from the Leidenfrost effect. However, liquid nitrogen applied to skin in mists, and on fabrics, bypasses this effect.

ElectrolyteIn chemistry, an electrolyte is any substance containing free ions that behaves as an electrically conductive medium, usually when in a solution. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible.

Principles

Electrolytes commonly exist as solutions of acids, bases or salts. Furthermore, some gases may act as electrolytes under conditions of high temperature or low pressure. Electrolyte solutions can also result from the dissolution of some biological (e.g., DNA, polypeptides) and synthetic polymers (e.g., polystyrene sulfonate), termed polyelectrolyte, which contain multiple charged moieties.

Electrolyte solutions are normally formed when a salt is placed into a solvent such as water and the individual components dissociate due to the thermodynamic interactions between solvent and solute molecules, in a process called solvation. For example, when table salt, NaCl, is placed in water, the salt (a solid) dissolves into its component elements, according to the dissociation reaction

NaCl(s) → Na+(aq) + Cl−

(aq).

It is also possible for substances to react with water when they are added to it, producing ions, e.g., carbon dioxide gas dissolves in water to produce a solution which contains hydronium, carbonate, and hydrogen carbonate ions.

Note that molten salts can be electrolytes as well. For instance, when sodium chloride is molten, the liquid conducts electricity.

An electrolyte in a solution may be described as concentrated if it has a high concentration of ions, or dilute if it has a low concentration. If a high proportion of the solute dissociates to form free ions, the electrolyte is strong; if most of the solute does not dissociate, the electrolyte is weak. The properties of electrolytes may be exploited using electrolysis to extract constituent elements and compounds contained within the solution.

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Physiological importance

In physiology, the primary ions of electrolytes are sodium(Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), hydrogen phosphate (HPO4

2−), and hydrogen carbonate (HCO3−).

The electric charge symbols of plus (+) and minus (−) indicate that the substance in question is ionic in nature and has an imbalanced distribution of electrons, which is the result of chemical dissociation.

All known higher life forms require a subtle and complex electrolyte balance between the intracellular and extra cellular milieu. In particular, the maintenance of precise osmotic gradients of electrolytes is important. Such gradients affect and regulate the hydration of the body, blood pH, and are critical for nerve and muscle function. Various mechanisms exist in living species that keep the concentrations of different electrolytes under tight control.

Both muscle tissue and neurons are considered electric tissues of the body. Muscles and neurons are activated by electrolyte activity between the extra cellular fluid or interstitial fluid, and intracellular fluid. Electrolytes may enter or leave the cell membrane through specialized protein structures embedded in the plasma membrane called ion channels. For example, muscle contraction is dependent upon the presence of calcium (Ca2+), sodium (Na+), and potassium (K+). Without sufficient levels of these key electrolytes, muscle weakness or severe muscle contractions may occur.

Electrolyte balance is maintained by oral, or in emergencies, intravenous (IV) intake of electrolyte-containing substances, and is regulated by hormones, generally with the kidneys flushing out excess levels. In humans, electrolyte homeostasis is regulated by hormones such as antidiuretic hormone, aldosterone and parathyroid hormone. Serious electrolyte disturbances, such as dehydration and over hydration, may lead to cardiac and neurological complications and, unless they are rapidly resolved, will result in a medical emergency.

Measurement

Measurement of electrolytes is a commonly performed diagnostic procedure, performed via blood testing with ion selective electrodes or urinalysis by medical technologists. The interpretation of these values is somewhat meaningless without analysis of the clinical history and is often impossible without parallel measurement of renal function. Electrolytes measured most often are sodium and potassium. Chloride levels are rarely measured except for arterial blood gas interpretation since they are inherently linked to sodium levels. One important test conducted on urine is the specific gravity test to determine the occurrence of electrolyte imbalance.

Sports drinks

Electrolytes are commonly found in sports drinks. In oral re hydration therapy, electrolyte drinks containing sodium and potassium salts replenish the body's water and electrolyte levels after dehydration caused by exercise, diaphoresis, diarrhea, vomiting, intoxication or starvation. Athletes exercising in extreme conditions (for three or more hours continuously e.g. marathon or triathalon) who do not consume electrolytes, risk dehydration (or hyponatremia).

Because sports drinks typically contain high levels of sugar, they are not recommended for regular use by children. Water is considered the only essential beverage for children during exercise.

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Medicinal rehydration sachets and drinks are available to replace the key electrolyte ions lost during diarrhea and other gastro-intestinal distresses. Dentists recommend that regular consumers of sports drinks observe precautions against tooth decay.[

A simple electrolyte drink can be home-made by using the correct proportions of water, sugar, salt, salt substitute for potassium, and baking soda.[1] However, effective electrolyte replacements should include all electrolytes required by the body, including sodium chloride, potassium, magnesium, and calcium that can be either obtained in a sports drink or a solid electrolyte capsule.[2]

Electrochemistry

When electrodes are placed in an electrolyte and a voltage is applied, the electrolyte will conduct electricity. Lone electrons normally cannot pass through the electrolyte; instead, a chemical reaction occurs at the cathode consuming electrons from the cathode, and another reaction occurs at the anode producing electrons to be taken up by the anode. As a result, a negative charge cloud develops in the electrolyte around the cathode, and a positive charge develops around the anode. The ions in the electrolyte move to neutralize these charges so that the reactions can continue and the electrons can keep flowing.

For example, in a solution of ordinary salt (sodium chloride, NaCl) in water, the cathode reaction will be

2H2O + 2e− → 2OH− + H2

and hydrogen gas will bubble up; the anode reaction is

2H2O → O2 + 4H+ + 4e−

And oxygen gas will be liberated. The positively charged sodium ions Na+ will react towards the cathode neutralizing the negative charge of OH− there, and the negatively charged chlorine ions Cl−

will react towards the anode neutralizing the positive charge of H+ there. Without the ions from the electrolyte, the charges around the electrode would slow down continued electron flow; diffusion of H+ and OH− through water to the other electrode takes longer than movement of the much more prevalent salt ions.

In other systems, the electrode reactions can involve the metals of the electrodes as well as the ions of the electrolyte.

Electrolytic conductors are used in electronic devices where the chemical reaction at a metal/electrolyte interface yields useful effects.

In batteries, two metals with different electron affinities are used as electrodes; electrons flow from one electrode to the other outside of the battery, while inside the battery the circuit is closed by the electrolyte's ions. Here the electrode reactions convert chemical energy to electrical energy.

In some fuel cells, a solid electrolyte or proton conductor connects the plates electrically while keeping the hydrogen and oxygen fuel gases separated.

In electroplating tanks, the electrolyte simultaneously deposits metal onto the object to be plated, and electrically connects that object in the circuit.

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In operation-hours gauges, two thin columns of mercury are separated by a small electrolyte-filled gap, and, as charge is passed through the device, the metal dissolves on one side and plates out on the other, causing the visible gap to slowly move along.

In electrolytic capacitors the chemical effect is used to produce an extremely thin 'dielectric' or insulating coating, while the electrolyte layer behaves as one capacitor plate.

In some hygrometers the humidity of air is sensed by measuring the conductivity of a nearly dry electrolyte.

Hot, softened glass is an electrolytic conductor, and some glass manufacturers keep the glass molten by passing a large current through it.

Dry electrolyte

Dry electrolytes are essentially gels in a flexible lattice framework.

References collected from:-

1. http://www.webmd.com/hw/health_guide_atoz/str2254.asp?navbar=hw86827 2. http://www.runnersweb.com/running/rw_news_frameset.html?http://www.runnersweb.com/running/news/

rw_news_20060612_ERB_Electrolytes.html 3. http://www.evworld.com/article.cfm?storyid=933

Galvanic cells

Non-rechargeable:primary cells

Alkaline battery | Aluminium battery | Bunsen cell | Chromic acid cell | Clark cell | Daniell cell | Dry cell | Grove cell | Leclanché cell | Lithium battery | Mercury battery | Nickel oxyhydroxide battery | Silver-oxide battery | Weston cell | Zamboni pile | Zinc-air battery | Zinc-carbon battery

Rechargeable:secondary cells

Air-fueled lithium-ion battery | Lead-acid battery | Lithium-ion battery | Lithium-ion polymer battery | Lithium iron phosphate battery | Lithium sulfur battery | Lithium-titanate battery | Nickel-cadmium battery | Nickel hydrogen battery | Nickel-iron battery | Nickel-metal hydride battery | Nickel-zinc battery | Rechargeable alkaline battery | Sodium-sulfur battery | Vanadium redox battery | Zinc-bromine battery

Kinds of cells Battery | Concentration cell | Flow battery | Fuel cell | Trough battery | Voltaic pile

Parts of cells Anode | Catalyst | Cathode | Electrolyte | Half cell | Ions | Salt bridge | Semipermeable membrane

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Anode

Diagram of a zinc anode in a galvanic cell.

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID (Anode Current Into Device). Electrons flow in the opposite direction to the electric current (flow of hypothetical positive charge)

A widespread misconception is that anode polarity is always positive. This is often incorrectly inferred from the correct fact that in all electrochemical devices negatively charged anions move towards the anode (hence their name) and/or positively charged cautions move

away from it. In fact anode polarity depends on the device type, and sometimes even in which mode it operates, as per the above electric current direction-based universal definition. Consequently, as can be seen from the following examples, in a device which consumes power the anode is positive, and in a device which provides power the anode is negative:

In a discharging battery or galvanic cell (diagram at right) the anode is the negative terminal, where the hypothetic charges constituting a conventional current flow in, and electrons out. Since this inwards current is carried externally by electrons moving outwards, the negative charge moving one way amounts to positive charge flowing into the electrolyte from the anode, i.e., away (surprisingly) from the more negative electrode and towards the more positive one (chemical energy is responsible for this "uphill" motion). If the anode is composed of a metal, electrons which it gives up to the external circuit must be accompanied by metal atoms missing those electrons (cautions) moving away from the electrode and into the electrolyte.

In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; In other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.

In a diode, it is the positive terminal at the tail of the arrow symbol, where current flows into the device. Note electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as zener diodes or solar cells where the current of interest is the reverse current.

In a cathode ray tube, it is the positive terminal where electrons flow out, i.e., where positive electric current flows in.

An electrode through which current flows the other way (out of the device) is termed a cathode.

EtymologyThe word was coined in 1834 from the Greek ἄνοδος (anodos), 'way up', by William Whewell, who had been consulted by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis. In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in a direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears

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to move", the anode is where the current enters the electrolyte, on the East side: "ano upwards, odos a way ; the way which the sun rises" ( reprinted in).

The use of 'East' to mean the 'in' direction (actually 'in' → 'East' → 'sunrise' → 'up') may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term "eisode" (the doorway where the current enters). His motivation for changing it to something meaning 'the East electrode' (other candidates had been "eastode", "oriode" and "anatolode") was to make it immune to a possible later change in the direction convention for current, whose exact nature was not known at the time. The reference he used to this effect was the Earth's magnetic field direction, which at that time was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. This made the internal current East to West as previously mentioned, but in the event of a later convention change it would have become West to East, so that the East electrode would not have been the 'way in' any more. Therefore "eisode" would have become inappropriate, whereas "anode" meaning 'East electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the anode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "anode" term is based is subject to reversals whereas the current direction convention on which the "eisode" term was based has no reason to change in the future.

Since the later discovery of the electron, an easier to remember, and more durably correct technically although historically false, etymology has been suggested: anode, from the Greek anodos, 'way up', 'the way (up) out of the cell (or other device) for electrons'.

Flow of electronsThe flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes, where electrode naming always assumes current flows in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode or solar cell).

Electrolytic anodeIn electrochemistry, the anode is where oxidation occurs, and is the positive polarity contact in an electrolytic cell. At the anode, anions (negative ions) are forced by the electrical potential to react chemically and give off electrons (oxidation) which then flow up and into the driving circuit.

This process is widely used in metals refining. For example, in copper refining, copper anodes, an intermediate product from the furnaces, are electrolyzed in an appropriate solution (such as sulphuric acid) to yield high purity (99.99%) cathodes. Copper cathodes produced using this method is also described as electrolytic copper.

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Battery or galvanic cell anodeIn a battery or galvanic cell, the anode is the negative electrode from which electrons flow out towards the external part of the circuit. Internally the positively charged cations are flowing away from the anode (even though it is negative and therefore would be expected to attract them, this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems); but, external to the cell in the circuit, electrons are being pushed out through the negative contact and thus through the circuit by the voltage potential as would be expected. Note: in a galvanic cell, contrary to what occurs in an electrolytic cell, no anions flow to the anode, the internal current being entirely accounted for by the cations flowing away from it (cf drawing).

In the United States, many battery manufacturers regard the positive electrode as the anode, particularly in their technical literature. Though technically incorrect, it does resolve the problem of which electrode is the anode in a secondary (or rechargeable) cell. Using the traditional definition, the anode switches ends between charge and discharge cycles.

Vacuum tube anodeIn electronic vacuum devices such as a cathode ray tube, the anode is the positively charged electron collector. In a tube, the anode is a charged positive plate that collects the electrons emitted by the cathode through electric attraction. It also accelerates the flow of these electrons.

Diode anodeIn a semiconductor diode, the anode is the P-doped layer which initially supplies holes to the junction. In the junction region, the holes supplied by the anode combine with electrons supplied from the N-doped region, creating a depleted zone. As the P-doped layer supplies holes to the depleted region, negative dope ions are left behind in the P-doped layer ('P' for positive charge-carrier ions). This creates a base negative charge on the anode. When a positive voltage is applied to anode of the diode from the circuit, more holes are able to be transferred to the depleted region, and this causes the diode to become conductive, allowing current to flow through the circuit. The terms anode and cathode should not be applied to a zener diode, since it allows flow in either direction, depending on the polarity of the applied potential (i.e. voltage).

Sacrificial anodeIn cathodic protection, a metal anode that is more reactive to the corrosive environment of the system to be protected is electrically linked to the protected system, and partially corrodes or dissolves, which protects the metal of the system it is connected to. As an example, an iron or steel ship's hull may be protected by a zinc sacrificial anode, which will dissolve into the seawater and prevent the hull from being corroded. Sacrificial anodes are particularly needed for systems where a static charge is generated by the action of flowing liquids, such as pipelines and watercraft.

A less obvious example of this type of protection is the process of galvanizing iron (though the name of the process provides the essential clue). This process coats iron structures (such as fencing) with a coating of zinc metal. As long as the zinc remains intact, the iron is protected from the effects of

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corrosion. Inevitably, the zinc coating becomes breached, either by cracking or physical damage. Once this occurs, corrosive elements act as an electrolyte and the zinc/iron combination as electrodes. The resultant current flow ensures that the zinc coating is sacrificed but that the base iron does not corrode. Such a coating can potentially protect an iron structure for a few decades, but once the protecting coating is consumed, the iron rapidly corrodes.

At least one anode is found in tank-type water heaters. The anode should be removed and checked after 5 years (sooner if there is a sodium based water softner inline), and replaced if 15 cm (6 inches) or more of bare wire is showing. This will greatly extend the life of the tank.

Related antonymThe opposite of an anode is a cathode. When the current through the device is reversed, the electrodes switch functions, so anode becomes cathode, while cathode becomes anode, as long as the reversed current is applied, with the exception of diodes where electrode naming is always based on the forward current direction.

References collected from:-

1. ^ Ross, S, Faraday Consults the Scholars: The Origins of the Terms of Electrochemistry in Notes and Records of the Royal Society of London (1938-1996), Volume 16, Number 2 / 1961, Pages: 187 - 220, [1] consulted 2006-12-22

2. ^ Faraday, Michael, Experimental Researches in Electricity. Seventh Series, Philosophical Transactions of the Royal Society of London (1776-1886), Volume 124, 01 Jan 1834, Page 77, [2] consulted 2006-12-27 (in which Faraday introduces the words electrode, anode, cathode, anion, cation, electrolyte, electrolyze)

3. ^ Faraday, Michael, Experimental Researches in Electricity, Volume 1, 1849, reprint of series 1 to 14, freely accessible Gutenberg.org transcript [3] consulted 2007-01-11

Cathode

Diagram of a copper cathode in a galvanic cell.

A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD (Cathode Current Departs).

A widespread misconception is that cathode polarity is always negative. This is often incorrectly inferred from the correct fact that in all electrochemical devices positively charged cations move towards the cathode (whence their name) and/or negatively charged anions move away from it. In fact cathode polarity depends on the device

type, and can even vary according to the operating mode, as per the above electric current direction-based universal definition. Consequently, as can be seen from the following examples, in a device which consumes power the cathode is negative, and in a device which provides power the cathode is positive:

In a discharging battery or a galvanic cell the cathode is the positive terminal since that is where the current flows out of the device (see drawing). This outward current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this "uphill" motion). It is continued externally by electrons moving

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inwards, negative charge moving one way constituting positive current flowing the other way.

In a recharging battery, or an electrolytic cell, the cathode is the negative terminal, which sends current back to the external generator.

In a diode, it is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device. Note electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as zener diodes or solar cells where the current of interest is the reverse current.

In a cathode ray tube, it is the negative terminal where electrons flow in from the wiring and leave for the tube's near vacuum, constituting a positive current flowing out of the device.

An electrode through which current flows the other way (into the device) is termed an anode.

EtymologyThe word was coined in 1834 from the Greek κάθοδος (kathodos), 'descent' or 'way down', by William Whewell, who had been consulted[1] by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis. In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in a direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears to move", the cathode is where the current leaves the electrolyte, on the West side: "kata downwards, `odos a way ; the way which the sun sets" ([2], reprinted in [3]).

The use of 'West' to mean the 'out' direction (actually 'out' → 'West' → 'sunset' → 'down') may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term "exode" (the doorway where the current exits). His motivation for changing it to something meaning 'the West electrode' (other candidates had been "westode", "occiode" and "dysiode") was to make it immune to a possible later change in the direction convention for current, whose exact nature was not known at the time. The reference he used to this effect was the Earth's magnetic field direction, which at that time was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. This made the internal current East to West as previously mentioned, but in the event of a later convention change it would have become West to East, so that the West electrode would not have been the 'way out' any more. Therefore "exode" would have become inappropriate, whereas "cathode" meaning 'West electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the cathode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "cathode" term is based is subject to reversals whereas the current direction convention on which the "exode" term was based has no reason to change in the future.

Since the later discovery of the electron, an easier to remember, and more durably technically correct (although historically false), etymology has been suggested: cathode, from the Greek kathodos, 'way down', 'the way (down) into the cell (or other device) for electrons'.

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Flow of electronsThe flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes where electrode naming always assumes current flows in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode or solar cell).

Chemistry cathodeIn chemistry, a cathode is the electrode of an electrochemical cell at which reduction occurs. The cathode can be negative as when the cell is electrolytic (where electrical energy provided to the cell is being used to decompose chemical compounds); or positive as when the cell is galvanic (where chemical reactions are used to generate electrical energy). The cathode supplies electrons to the positively charged cations which flow to it from the electrolyte (even if the cell is galvanic, i.e., when the cathode is positive and therefore would be expected to repel the positively charged cations; this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems in a galvanic cell).

Electrolytic cell

In an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions. When discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more "cathodic" with respect to the more easily reduced reagent.

Galvanic cell

In a galvanic cell, the cathode is where the positive pole is connected to allow the circuit to be completed: as the anode of the galvanic cell gives off electrons, they return from the circuit into the cell through the cathode.

Electroplating metal cathode

When metal ions are reduced from ionic solution, they form a pure metal surface on the cathode. Items to be plated with pure metal are attached to and become part of the cathode in the electrolytic solution.

Electronics and physics cathodeIn physics or electronics, a cathode is an electrode that emits electrons into the device.

Vacuum tubes

In a vacuum tube or electronic vacuum system, the cathode emits free electrons. Electrons are extracted from metal electrodes either by heating the electrode, causing thermionic emission, or by applying a strong electric field and causing field electron emission. Electrons can also be emitted

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from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it. This effect is called photoelectric emission.

Cold cathodes and hot cathodes

Cathodes used for field electron emission in vacuum tubes are called cold cathodes. Heated electrodes or hot cathodes, frequently called filaments, are much more common. Most radios and television sets prior to the 1970s used filament-heated-cathode electron tubes for signal selection and processing; to this day, a hot cathode forms the source of the electron beam(s) in cathode ray tubes in many television sets and computer monitors. Hot electron emitters are also used as the electrodes in fluorescent lamps and in the source tubes of X-ray machines.

Diodes

In a semiconductor diode, the cathode is the N–doped layer of the PN junction with a high density of free electrons as a result of doping, and an equal density of fixed positive charges, which are the dopants that have been thermally ionized. In the anode, the converse applies: It features a high density of free "holes" and consequently fixed negative dopants which have captured an electron (hence the origin of the holes). When P and N-doped layers are placed in contact, diffusion ensures that electrons flow from high to low density areas: That is, from the N to the P side. They leave behind the fixed positively charged dopants near the junction. Similarly, holes diffuse from P to N leaving behind fixed negative ionised dopants near the junction. These layers of fixed positive and negative charges, collectively known as the depletion layer because they are depleted of free electrons and holes. The depletion layer at the junction is at the origin of the diode's rectifying properties. This is due to the resulting internal field and corresponding potential barrier which inhibit current flow in reverse applied bias which increases the internal depletion layer field. Conversely, they allow it in forwards applied bias where the applied bias reduces the built in potential barrier.

Electrons which diffuse from the cathode into the P-doped layer, or anode, become what is termed "minority carriers" and tend to recombine there with the majority carriers, which are holes, on a timescale characteristic of the material which is the p-type minority carrier lifetime. Similarly, holes diffusing into the N-doped layer become minority carriers and tend to recombine with electrons. In equilibrium, with no applied bias, thermally assisted diffusion of electrons and holes in opposite directions across the depletion layer ensure a zero net current with electrons flowing from cathode to anode and recombining, and holes flowing from anode to cathode across the junction or depletion layer and recombining. Like a typical diode, there is a fixed anode and cathode in a zener diode, but it will conduct current in the reverse direction (electrons from anode to cathode) if its breakdown or Zener voltage is exceeded.

References collected from:-

1. ^ Ross, S, Faraday Consults the Scholars: The Origins of the Terms of Electrochemistry in Notes and Records of the Royal Society of London (1938-1996), Volume 16, Number 2 / 1961, Pages: 187 - 220, [1] consulted 2006-12-22

2. ^ Faraday, Michael, Experimental Researches in Electricity. Seventh Series, Philosophical Transactions of the Royal Society of London (1776-1886), Volume 124, 01 Jan 1834, Page 77, [2] consulted 2006-12-27 (in which Faraday introduces the words electrode, anode, cathode, anion, cation, electrolyte, electrolyze)

3. ^ Faraday, Michael, Experimental Researches in Electricity, Volume 1, 1849, reprint of series 1 to 14, freely accessible Gutenberg.org transcript [3] consulted 2007-01-11

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Battery (electricity)

Various batteries (clockwise from bottom left): two 9-volt PP3, two AA, one D, one handheld ham radio battery, one cordless phone battery, one camcorder battery, one C, two AAA.

In electronics, a battery or voltaic cell is a combination of many electrochemical Galvanic cells of identical type to store chemical energy and to deliver higher voltage or higher current than with single cells.

The battery cells create a voltage difference between the terminals of each cell and hence to its combination in battery. When an external electrical circuit is connected to the battery, then the battery drives electrons through the circuit and the electrical circuit is complete powering the device attached. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial applications, and is now a multi-billion dollar industry.

HistoryThe name "battery" was coined by Benjamin Franklin for an arrangement of multiple Leyden jars (an early type of capacitor) after a battery of cannon.[1] Strictly, a battery is a collection of two or more cells, but in popular usage battery often refers to a single electrical cell.[2]

An early form of electrochemical battery called the Baghdad Battery may have been used in antiquity.[3] However, the modern development of batteries started with the Voltaic pile, invented by the Italian physicist Alessandro Volta in 1800.[4]

In 1780 the Italian anatomist and physiologist Luigi Galvani noticed that dissected frog's legs would twitch when struck by a spark from a Leyden jar, an external source of electricity.[5] In 1786 he noticed that twitching would occur during lightning storms.[6] After many years Galvani learned how to produce twitching without using any external source of electricity. In 1791 he published a report on "animal electricity."[7] He created an electric circuit consisting of the frog's leg (FL) and two different metals A and B, each metal touching the frog's leg and each other, thus producing the circuit A-FL-B-A-FL-B...etc. In modern terms, the frog's leg served as both the electrolyte and the sensor, and the metals served as electrodes. He noticed that even though the frog was dead, its legs would twitch when he touched them with the metals.

Within a year, Volta realized the frog's moist tissues could be replaced by cardboard soaked in salt water, and the frog's muscular response could be replaced by another form of electrical detection. He already had studied the electrostatic phenomenon of capacitance, which required measurements of electric charge and of electrical potential ("tension"). Building on this experience, Volta was able to detect electric current through his system, also called a Galvanic cell. The terminal voltage of a cell that is not discharging is called its electromotive force (emf), and has the same unit as electrical potential, named (voltage) and measured in volts, in honor of Volta. In 1800, Volta invented the battery by placing many voltaic cells in series, literally piling them one above the other. This Voltaic pile gave a greatly enhanced net emf for the combination,[8] with a voltage of about 50 volts for a 32-cell pile.[9] In many parts of Europe batteries continue to be called piles.[10][11]

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Volta did not appreciate that the voltage was due to chemical reactions. He thought that his cells were an inexhaustible source of energy,[12] and that the associated chemical effects (e.g. corrosion) were a mere nuisance, rather than an unavoidable consequence of their operation, as Michael Faraday showed in 1834.[13] According to Faraday, cations (positively charged ions) are attracted to the cathode,[14] and anions (negatively charged ions) are attracted to the anode.[15]

Although early batteries were of great value for experimental purposes, in practice their voltages fluctuated and they could not provide a large current for a sustained period. Later, starting with the Daniell cell in 1836, batteries provided more reliable currents and were adopted by industry for use in stationary devices, particularly in telegraph networks where they were the only practical source of electricity, since electrical distribution networks did not then exist.[16] These wet cells used liquid electrolytes, which were prone to leakage and spillage if not handled correctly. Many used glass jars to hold their components, which made them fragile. These characteristics made wet cells unsuitable for portable appliances. Near the end of the nineteenth century, the invention of dry cell batteries, which replaced the liquid electrolyte with a paste, made portable electrical devices practical.[17]

Since then, batteries have gained popularity as they became portable and useful for a variety of purposes.[18]

Battery industryAccording to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales each year, with 6% annual growth.

How batteries work

A voltaic cell for demonstration purposes. In this example the two half-cells are linked by a salt bridge separator that permits the transfer of ions, but not water molecules.

A battery is a device that converts chemical energy directly to electrical energy. It consists of a number of voltaic cells; each voltaic cell consists of two half cells connected in series by a conductive electrolyte containing anions and cations. One half-cell includes electrolyte and the electrode to which anions (negatively-charged ions) migrate, i.e. the anode or negative electrode; the other half-cell includes

electrolyte and the electrode to which cations (positively-charged ions) migrate, i.e. the cathode or positive electrode. In the redox reaction that powers the battery, reduction (addition of electrons) occurs to cations at the cathode, while oxidation (removal of electrons) occurs to anions at the anode.[22] The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either solid or liquid. Many cells use two half-cells with different electrolytes. In that case each half-cell is enclosed in a container, and a separator that is porous to ions but not the bulk of the electrolytes prevents mixing.

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Each half cell has an electromotive force (or emf), determined by its ability to drive electric current from the interior to the exterior of the cell. The net emf of the cell is the difference between the emfs of its half-cells, as first recognized by Volta.[9] Therefore, if the electrodes have emfs and , then the net emf is ; in other words, the net emf is the difference between the reduction potentials of the half-reactions.[24]

The electrical driving force or across the terminals of a cell is known as the terminal voltage (difference) and is measured in volts. The terminal voltage of a cell that is neither charging nor discharging is called the open-circuit voltage and equals the emf of the cell. Because of internal resistance, the terminal voltage of a cell that is discharging is smaller in magnitude than the open-circuit voltage and the terminal voltage of a cell that is charging exceeds the open-circuit voltage. An ideal cell has negligible internal resistance, so it would maintain a constant terminal voltage of until exhausted, then dropping to zero. If such a cell maintained 1.5 volts and stored a charge of one Coulomb then on complete discharge it would perform 1.5 Joule of work.[25] In actual cells, the internal resistance increases under discharge, and the open circuit voltage also decreases under discharge. If the voltage and resistance are plotted against time, the resulting graphs typically are a curve; the shape of the curve varies according to the chemistry and internal arrangement employed.

As stated above, the voltage developed across a cell's terminals depends on the energy release of the chemical reactions of its electrodes and electrolyte. Alkaline and carbon-zinc cells have different chemistries but approximately the same emf of 1.5 volts; likewise NiCd and NiMH cells have different chemistries, but approximately the same emf of 1.2 volts.[29] On the other hand the high electrochemical potential changes in the reactions of lithium compounds give lithium cells emfs of 3 volts or more.[30]

Categories and types of batteriesFrom top to bottom: SR41/AG3, SR44/AG13 (button cells), a 9-volt PP3 battery, an AAA cell, an AA cell, a C cell, a D Cell, and a large 3R12. (Ruler in centimeters.)

Batteries are classified into two broad categories, each type with advantages and disadvantages.

Primary batteries irreversibly (within limits of practicality) transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the battery by electrical means.

Secondary batteries can be recharged; that is, they can have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.

Historically, some types of primary batteries used, for example, for telegraph circuits, were restored to operation by replacing the components of the battery consumed by the chemical reaction. Secondary batteries are not indefinitely rechargeable due to dissipation of the active materials, loss of electrolyte and internal corrosion.

Primary batteries

Primary batteries can produce current immediately on assembly. Disposable batteries, also called primary cells, are intended to be used once and discarded. These are most commonly used in portable devices that have low current drain, are only used intermittently, or are used well away from

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an alternative power source, such as in alarm and communication circuits where other electric power is only intermittently available. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms. Battery manufacturers recommend against attempting recharging primary cells.

Common types of disposable batteries include zinc-carbon batteries and alkaline batteries. Generally, these have higher energy densities than rechargeable batteries, but disposable batteries do not fare well under high-drain applications with loads under 75 ohms (75 Ω).

Secondary batteries

Secondary batteries must be charged before use; they are usually assembled with active materials in the discharged state. Rechargeable batteries or secondary cells can be recharged by applying electrical current, which reverses the chemical reactions that occur during its use. Devices to supply the appropriate current are called chargers or recharges.

The oldest form of rechargeable battery is the lead-acid battery. This battery is notable in that it contains a liquid in an unsealed container, requiring that the battery be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging. The lead-acid battery is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where a large capacity (over approximately 10Ah) is required or where the weight and ease of handling are not concerns.

A common form of the lead-acid battery is the modern car battery, which can generally deliver a peak current of 450 amperes. An improved type of liquid electrolyte battery is the sealed valve regulated lead acid (VRLA) battery, popular in the automotive industry as a replacement for the lead-acid wet cell. The VRLA battery uses an immobilized sulfuric acid electrolyte, reducing the chance of leakage and extending shelf life. VRLA batteries have the electrolyte immobilized, usually by one of two means:

Gel batteries (or "gel cell") contain a semi-solid electrolyte to prevent spillage. Absorbed Glass Mat (AGM) batteries absorb the electrolyte in a special fiberglass matting

Other portable rechargeable batteries include several "dry cell" types, which are sealed units and are therefore useful in appliances such as mobile phones and laptop computers. Cells of this type (in order of increasing power density and cost) include nickel-cadmium (NiCd), nickel metal hydride (NiMH) and lithium-ion (Li-ion) cells. By far, Li-ion has the highest share of the dry cell rechargeable market. Meanwhile, NiMH has replaced NiCd in most applications due to its higher capacity, but NiCd remains in use in power tools, two-way radios, and medical equipment.

Recent developments include batteries with embedded functionality such as USBCELL, with a built-in charger and USB connector within the AA format, enabling the battery to be charged by plugging into a USB port without a charger,[41] and low self-discharge (LSD) mix chemistries such as Hybrio, ReCyko, and Eneloop, where cells are precharged prior to shipping.

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Battery cell types

There are many general types of electrochemical cells, according to chemical processes applied and design chosen. The variation includes galvanic cells, electrolytic cells, fuel cells, flow cells and voltaic piles.

Wet cell

A wet cell battery has a liquid electrolyte. Other names are flooded cell since the liquid covers all internal parts, or vented cell since gases produced during operation can escape to the air. Wet cells were a precursor to dry cells and are commonly used as a learning tool for electrochemistry. It is often built with common laboratory supplies, like beakers, for demonstrations of how electrochemical cells work. A particular type of wet cell known as a concentration cell is important in understanding corrosion. Wet cells may be primary cells (non-rechargeable) or secondary cells (rechargeable). Originally all practical primary batteries such as the Daniel cell were built as open-topped glass jar wet cells. Other primary wet cells are the Leclanche cell, Grove cell, Bunsen cell, Chromic acid cell, Clark cell and Weston cell. The Leclanche cell chemistry was adapted to the first dry cells.

Wet cells are still used in automobile batteries and in industry for standby power for switchgear, telecommunication or large uninterruptible power supplies, but in many places batteries with gel cells have been used instead. These applications commonly use lead-acid or nickel-cadmium cells.

Dry cell

A dry cell has the electrolyte immobilized as a paste, with only enough moisture in the paste to allow current to flow. Compared to a wet cell, the battery can be operated in any random position, and will not spill its electrolyte if inverted.

Molten salt

A molten salt battery is a primary or secondary battery that uses a molten salt as its electrolyte. Their energy density and power density makes them potentially useful for electric vehicles, but they must be carefully insulated to retain heat.

Reserve

A reserve battery can be stored for a long period of time and is activated when its internal parts (usually electrolyte) are assembled. For example, a battery for an electronic fuse might be activated by the impact of firing a gun, breaking a capsule of electrolyte to activate the battery and power the fuse’s circuits. Reserve batteries are usually designed for a short service life (seconds or minutes) after long storage (years).

Battery cell performance

A battery's characteristics may vary over load cycle, charge cycle and over life time due to many factors including internal chemistry, current drain and temperature.

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Battery capacity and dischargingA device to check battery voltage.

The more electrolyte and electrode material there is in the cell, the greater the capacity of the cell. Thus a small cell has less capacity than a larger cell, given the same chemistry (e.g. alkaline cells), though they develop the same open-circuit voltage.

Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude

of the current (which may vary with time), the allowable terminal voltage of the battery, temperature and other factors. The available capacity of a battery depends upon the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected.

The battery capacity that battery manufacturers print on a battery is usually the product of 20 hours multiplied by the maximum constant current that a new battery can supply for 20 hours at 68 F° (20 C°), down to a predetermined terminal voltage per cell. A battery rated at 100 A·h will deliver 5 A over a 20 hour period at room temperature. However, if it is instead discharged at 50 A, it will have a lower apparent capacity.

The relationship between current, discharge time, and capacity for a lead acid battery is approximated (over a certain range of current values) by Peukert's law:

Where

QP is the capacity when discharged at a rate of 1 amp. I is the current drawn from battery (A). t is the amount of time (in hours) that a battery can sustain. k is a constant around 1.3.

For low values of I internal self-discharge must be included.

In practical batteries, internal energy losses, and limited rate of diffusion of ions through the electrolyte, cause the efficiency of a battery to vary at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates, but if the rate is too low, it will self-discharge during the long time of operation, again lowering its efficiency.

Installing batteries with different A·h ratings will not affect the operation of a device rated for a specific voltage unless the load limits of the battery are exceeded. High-drain loads like digital cameras can result in lower actual energy, most notably for alkaline batteries. For example, a battery rated at 2000 mA·h would not sustain a current of 1 A for the full two hours, if it had been rated at a 10-hour or 20-hour discharge.

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Fastest charging, largest, and lightest batteries

Lithium iron phosphate (LiFePO4) batteries are the fastest charging and discharging, next to supercapacitors. The world's largest battery is in Fairbanks, Alaska, composed of Ni-Cd cells. Sodium-sulfur batteries are being used to store wind power. Lithium-sulfur batteries have been used on the longest and highest solar powered flight. The speed of recharging for lithium-ion batteries may be increased by manipulation.

Battery lifetime

Life of primary batteries

Even if never taken out of the original package, disposable (or "primary") batteries can lose 8 to 20 percent of their original charge every year at a temperature of about 20°–30°C. This is known as the "self discharge" rate and is due to non-current-producing "side" chemical reactions, which occur within the cell even if no load is applied to it. The rate of the side reactions is reduced if the batteries are stored at low temperature, although some batteries can be damaged by freezing. High or low temperatures may reduce battery performance. This will affect the initial voltage of the battery. For an AA alkaline battery this initial voltage is approximately normally distributed around 1.6 volts.

Typical alkaline battery sizes and capacities (at lowest discharge rates, to 0.8V/cell)

Size   ANSI/NEDA  

IEC  

Capacity (mA·h)   Voltage  Energy,

theoretical (J)  

Mass (g)  

Height (mm)  

Diameter (mm)  

Length (mm)  

Width (mm)  

AAAA 25A LR8D425 625 1.5 3375 6.5 42.5 8.3 cylinder   ---J 1412A 4LR61 625 6 13500 30 48.5 prismatic 35.6 9.189V 1604A 6LR61 625 9 20250 45.6 48.5 prismatic 26.5 17.5N 910A LR1 1000 1.5 5400 9 30.2 12 cylinder   ---AAA 24A LR03 1250 1.5 6750 11.5 44.5 10.5 cylinder   ---AA 15A LR6 2890 1.5 15390 23 50.5 14.5 cylinder   ---C 14A LR14 8350 1.5 45090 66.2 50 26.2 cylinder   ---D 13A LR20 20500 1.5 110700 148 61.5 34.2 cylinder   ---Lantern 915A 4R25Y 26000 6 561600 885 112 prismatic 68.2 68.2Lantern 908A 4LR25X 26000 6 561600 885 115 prismatic 68.2 68.2Lantern 918A 4LR25-2 52000 6 1123200 1900 127 prismatic 136.5 73

Discharging performance of all batteries drops at low temperature.

Life of rechargeable batteriesRechargeable batteries

Rechargeable batteries traditionally self-discharge more rapidly than disposable alkaline batteries, especially nickel-based batteries; a freshly charged NiCd loses 10% of its charge in the first 24 hours, and thereafter discharges at a rate of about 10% a month. However, modern lithium designs have reduced the self-discharge rate to a relatively low level (but still poorer than for primary

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batteries). Most nickel-based batteries are partially discharged when purchased, and must be charged before first use.

Although rechargeable batteries may be refreshed by charging, they still suffer degradation through usage. Low-capacity nickel metal hydride (NiMH) batteries (1700-2000 mA·h) can be charged for about 1000 cycles, whereas high capacity NiMH batteries (above 2500 mA·h) can be charged for about 500 cycles. Nickel cadmium (NiCd) batteries tend to be rated for 1,000 cycles before their internal resistance increases beyond usable values. Normally a fast charge, rather than a slow overnight charge, will result in a shorter battery lifespan. However, if the overnight charger is not "smart" and cannot detect when the battery is fully charged, then overcharging is likely, which will damage the battery. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material falls off the electrodes. NiCd batteries suffer the drawback that they should be fully discharged before recharge. Without full discharge, crystals may build up on the electrodes, thus decreasing the active surface area and increasing internal resistance. This decreases battery capacity and causes the "memory effect". These electrode crystals can also penetrate the electrolyte separator, thereby causing shorts. NiMH, although similar in chemistry, does not suffer from memory effect to quite this extent. When a battery reaches the end of its lifetime, it will not suddenly lose all of its capacity; rather, its capacity will gradually decrease.

Automotive lead-acid rechargeable batteries have a much harder life. Because of vibration, shock, heat, cold, and sulfation of their lead plates, few automotive batteries last beyond six years of regular use.[64] Automotive starting batteries have many thin plates to provide as much current as possible in a reasonably small package. In general, the thicker the plates, the longer the life of the battery. [63]

Typically they are only drained a small amount before recharge. Care should be taken to avoid deep discharging a starting battery, since each charge and discharge cycle causes active material to be shed from the plates.

"Deep-cycle" lead-acid batteries such as those used in electric golf carts have much thicker plates to aid their longevity.[65] The main benefit of the lead-acid battery is its low cost; the main drawbacks are its large size and weight for a given capacity and voltage.[63] Lead-acid batteries should never be discharged to below 20% of their full capacity,[66] because internal resistance will cause heat and damage when they are recharged. Deep-cycle lead-acid systems often use a low-charge warning light or a low-charge power cut-off switch to prevent the type of damage that will shorten the battery's life.

Extending battery life

Battery life can be extended by storing the batteries at a low temperature, as in a refrigerator or freezer, because the chemical reactions in the batteries are slower. Such storage can extend the life of alkaline batteries by ~5%; while the charge of rechargeable batteries can be extended from a few days up to several months. In order to reach their maximum voltage, batteries must be returned to room temperature; discharging an alkaline battery at 250 mAh at 0°C is only half as efficient as it is at 20°C. As a result, alkaline battery manufacturers like Duracell do not recommend refrigerating or freezing batteries.

Prolonging life in multiple cells through cell balancing

Analog front ends that balances cells and eliminate mismatches of cells in series or parallel combination significantly improve battery efficiency and increase the overall pack capacity. As the

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number of cells and load currents increase, the potential for mismatch also increases. There are two kinds of mismatch in the pack: State-of-Charge (SOC) and capacity/energy (C/E) mismatch. Though the SOC mismatch is more common, each problem limits the pack capacity (mAh) to the capacity of the weakest cell.

Cell balancing principle

Battery pack cells are balanced when all the cells in the battery pack meet two conditions:

1. If all cells have the same capacity, then they are balanced when they have the same State of Charge (SOC.) In this case, the Open Circuit Voltage (OCV) is a good measure of the SOC. If, in an out of balance pack, all cells can be differentially charged to full capacity (balanced), then they will subsequently cycle normally without any additional adjustments. This is mostly a one shot fix.

2. If the cells have different capacities, they are also considered balanced when the SOC is the same. But, since SOC is a relative measure, the absolute amount of capacity for each cell is different. To keep the cells with different capacities at the same SOC, cell balancing must provide differential amounts of current to cells in the series string during both charge and discharge on every cycle.

Cell balancing electronics

A battery pack requires additional components and circuitry to achieve cell balancing. Cell balancing is defined as the application of differential currents to individual cells (or combinations of cells) in a series string. Normally, of course, cells in a series string receive identical currents. A battery pack requires additional components and circuitry to achieve cell balancing. However, the use of a fully integrated analog front end for cell balancing reduces the required external components to just balancing resistors. It is important to recognize that the cell mismatch results more from limitations in process control and inspection than from variations inherent in the Lithium Ion chemistry. The use of a fully integrated analog front end for cell balancing can improve the performance of series connected Li-ion Cells by addressing both SOC and C/E issues. SOC mismatch can be remedied by balancing the cell during an initial conditioning period and subsequently only during the charge phase. C/E mismatch remedies are more difficult to implement and harder to measure and require balancing during both charge and discharge periods.

This type of solution eliminates the quantity of external components, as for discrete capacitors, diodes and most other resistors to achieve balance.

Hazards

Explosion

A battery explosion is caused by the misuse or malfunction of a battery, such as attempting to recharge a primary (non-rechargeable) battery, or short circuiting a battery.[71] With car batteries, explosions are most likely to occur when a short circuit generates very large currents. In addition, car batteries liberate hydrogen when they are overcharged (because of electrolysis of the water in the electrolyte). Normally the amount of overcharging is very small, as is the amount of explosive gas developed, and the gas dissipates quickly. However, when "jumping" a car battery, the high current

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can cause the rapid release of large volumes of hydrogen, which can be ignited by a nearby spark (for example, when removing the jumper cables).

When a battery is recharged at an excessive rate, an explosive gas mixture of hydrogen and oxygen may be produced faster than it can escape from within the walls of the battery, leading to pressure build-up and the possibility of the battery case bursting. In extreme cases, the battery acid may spray violently from the casing of the battery and cause injury. Overcharging—that is, attempting to charge a battery beyond its electrical capacity—can also lead to a battery explosion, leakage, or irreversible damage to the battery. It may also cause damage to the charger or device in which the overcharged battery is later used. Additionally, disposing of a battery in fire may cause an explosion as steam builds up within the sealed case of the battery.

Leakage

One style of disposable battery uses a zinc "can" as both a reactant and as the container to hold the other reagents. If this kind of battery is run all the way down, or if it is recharged after running down too far, the reagents can emerge through the cardboard and plastic that forms the remainder of the container. The active chemicals can then corrode or otherwise destroy the equipment that they were inserted into.

Many battery chemicals are corrosive or poisonous or both. If leakage occurs, either spontaneously or through accident, the chemicals released may be dangerous.

Environmental concerns

The widespread use of batteries has created many environmental concerns, such as toxic metal pollution. Battery manufacture consumes resources and often involves hazardous chemicals. Used batteries also contribute to electronic waste. Some areas now have battery recycling services available to recover some of the materials from used batteries. Batteries may be harmful or fatal if swallowed. Recycling or proper disposal prevents dangerous elements (such as lead, mercury, and cadmium) found in some types of batteries from entering the environment. In the United States, Americans purchase nearly three billion batteries annually, and about 179,000 tons of those end up in landfills across the country.

In the United States, the Mercury-Containing and Rechargeable Battery Management Act of 1996 banned the sale of mercury-containing batteries (except small button cell batteries), enacted uniform labeling requirements for rechargeable batteries, and required that rechargeable batteries be easily removable. California and New York City prohibit the disposal of rechargeable batteries in solid waste, and along with Maine require recycling of cell phones. The rechargeable battery industry has nationwide recycling programs in the United States and Canada, with dropoff points at local retailers.

The Battery Directive of the European Union has similar requirements, in addition to requiring increased recycling of batteries, and promoting research on improved battery recycling methods.

Battery chemistryOlder batteries were mostly based on rechargeable lead-acid or non-rechargeable alkaline chemistries, with nominal voltages in increments of 2.10 - 2.13 and 1.5 Volts respectively, each

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representing one individual electrochemical cell. New special battery chemistries have strained older naming conventions. Rechargeable NiCd (Nickel Cadmium) and NiMH (Nickel Metal Hydride) typically output 1.25 V per cell. Some devices may not operate properly with these cells, given the 16% reduction in voltage, but most modern ones handle them well. Conversely, lithium-ion rechargeable batteries output 3.7 V per cell, 23% higher than a pair of alkaline cells (3 V), which they are often designed to replace. Non-rechargeable lithium-chemistry batteries, which provide exceptionally high energy density, produce about 1.5 V per cell and are thus similar to alkaline batteries. Many new battery sizes refer to both the batteries' size and chemistry, while older names do not. For a more complete list see battery types. This summary is only for types relating to battery "sizes".

Primary battery chemistries

Chemistry  

Cell

Voltage  

Energy Density [MJ/kg]   Elaboration  

Zinc–carbon 1.5 0.13 Inexpensive.Zinc chloride 1.5 Also known as "heavy duty", inexpensive.alkaline(zinc–manganese dioxide) 1.5 0.4-0.59 Moderate energy density.

Good for high and low drain uses.oxy nickel hydroxide(zinc-manganese dioxide/oxy nickel hydroxide)

1.7 Moderate energy density.Good for high drain uses

Lithium(lithium–copper oxide)Li–CuO

1.7No longer manufactured.Replaced by silver oxide (IEC-type "SR") batteries.

Lithium(lithium–iron disulfide)LiFeS2

1.5 Expensive.Used in 'plus' or 'extra' batteries.

Lithium(lithium–manganese dioxide)LiMnO2

3.0 0.83-1.01

Expensive.Only used in high-drain devices or for long shelf life due to very low rate of self discharge.'Lithium' alone usually refers to this type of chemistry.

Mercury oxide 1.35High drain and constant voltage.Banned in most countries because of health concerns.

Zinc–air 1.35–1.65 1.59 Mostly used in hearing aids.

Silver oxide (silver-zinc) 1.55 0.47 Very expensive.Only used commercially in 'button' cells.

Rechargeable battery chemistries

Chemistry  Cell

Voltage  

Energy density[MJ/kg]   Comments  

NiCd 1.2 0.14 Inexpensive.

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High/low drain, moderate energy density.Can withstand very high discharge rates with virtually no loss of capacity.Moderate rate of self discharge.Reputed to suffer from memory effect (which is alleged to cause early failure).Environmental hazard due to Cadmium - use now virtually prohibited in Europe.

Lead Acid 2.1 0.14

Moderately expensive.Moderate energy density.Moderate rate of self discharge.Higher discharge rates result in considerable loss of capacity.Does not suffer from memory effect.Environmental hazard due to Lead.Common use - Automobile batteries

NiMH 1.2 0.36

Inexpensive.Not usable in higher drain devices.Traditional chemistry has high energy density, but also a high rate of self-discharge.Newer chemistry has low self-discharge rate, but also a ~25% lower energy density.Very heavy. Used in some cars.

Lithium ion 3.6 0.46

Very expensive.Very high energy density.Not usually available in "common" battery sizes (but see RCR-V3 for a counter-example).Very common in laptop computers, moderate to high-end digital cameras and camcorders, and cellphones.Very low rate of self discharge.Volatile: Chance of explosion if short circuited, allowed to overheat, or not manufactured with rigorous quality standards.

Homemade cellsAlmost any liquid or moist object that has enough ions to be electrically conductive can serve as the electrolyte for a cell. As a novelty or science demonstration, it is possible to insert two electrodes made of different metals into a lemon, potato, etc. and generate small amounts of electricity. "Two-potato clocks" are also widely available in hobby and toy stores; they consist of a pair of cells, each consisting of a potato (lemon, et cetera) with two electrodes inserted into it, wired in series to form a battery with enough voltage to power a digital clock. Homemade cells of this kind are of no real practical use, because they produce far less current—and cost far more per unit of energy generated—than commercial cells, due to the need for frequent replacement of the fruit or vegetable. In addition, one can make a voltaic pile from two coins (such as a nickel and a penny) and a piece of paper towel dipped in salt water. Such a pile would make very little voltage itself, but when many of them are stacked together in series, they can replace normal batteries for a short amount of time.

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Sony has developed a biologically friendly battery that generates electricity from sugar in a way that is similar to the processes observed in living organisms. The battery generates electricity through the use of enzymes that break down carbohydrates, which are essentially sugar.

Lead acid cells can easily be manufactured at home, but a tedious charge/discharge cycle is needed to 'form' the plates. This is a process whereby lead sulfate forms on the plates, and during charge is converted to lead dioxide (positive plate) and pure lead (negative plate). Repeating this process results in a microscopically rough surface, with far greater surface area being exposed. This increases the current the cell can deliver.

Daniell cells are also easy to make at home. Aluminum-air batteries can also be produced with high purity aluminum. Aluminum foil batteries will produce some electricity, but they are not very efficient, in part because a significant amount of hydrogen gas is produced.

References collected from:-

1. ^ Bellis, Mary. History of the Electric Battery. About.com. Retrieved 11 August 2008. 2. ^ "battery" (def. 4b), Merriam-Webster Online Dictionary (2009). Retrieved 25 May 2009. 3. ^ Corder, Gregory W. Using an Unconventional History of the Battery to Engage Students and Explore the Importance of

Evidence (PDF). Virginia Journal of Science Education, Vol. 1, No. 1. Retrieved 7 August 2008. 4. ^ Bellis, Mary. Alessandro Volta - Biography of Alessandro Volta - Stored Electricity and the First Battery. About.com.

Retrieved 7 August 2008. 5. ^ Asimov, Isaac. [1] Retrieved 3 May 2009. 6. ^ Encyclopedia Britannica. [2] Retrieved 3 May 2009. 7. ^ Bernardi, Walter. The Controversy on Animal Electricity in Eighteenth-Century Italy: Galvani, Volta and Others. Pavia

Project Physics. Retrieved 21 May 2008. 8. ^ Weinberg, Willie. Volta: A pioneer in Electrochemistry. The Italian-American Web Site of New York. Retrieved 19 March

2007. 9. ^ a b Saslow 338. 10. ^ "pila" (def. 1), Pocket Oxford Spanish Dictionary (2005). WordReference.com. Retrieved 6 August 2008. 11. ^ "pile" (def. 2.2), Pocket Oxford-Hachette French Dictionary (2005). WordReference.com. Retrieved 6 August 2008. 12. ^ Stinner, Arthur. Alessandro Volta and Luigi Galvani (PDF). Retrieved 11 August 2008. 13. ^ Electric Battery History - Invention of the Electric Battery. The Great Idea Finder. Retrieved 11 August 2008. 14. ^ "cation". Dictionary.com. Originally published in Webster's Revised Unabridged Dictionary. Retrieved 3 February 2009. 15. ^ "anion". Dictionary.com. Originally published in Webster's Revised Unabridged Dictionary. Retrieved 3 February 2009. 16. ^ Battery History, Technology, Applications and Development. MPower Solutions Ltd. Retrieved 19 March 2007. 17. ^ History of the Battery. American Chemical Society. Retrieved 3 February 2009. 18. ^ Batteries: History, Present, and Future of Battery Technology. ExtremeTech. Retrieved 10 September 2007. 19. ^ Power Shift: DFJ on the lookout for more power source investments. Draper Fisher Jurvetson. Retrieved 20 November

2005. 20. ^ a b c Buchmann, Isidor. Battery statistics. Battery University. Retrieved 11 August 2008. 21. ^ "battery" (def. 6), The Random House Dictionary of the English Language, the Unabridged Edition (2nd edition), 1996

ed. 22. ^ Dingrando 665. 23. ^ BBC- Rough Science Library. Retrieved 28 March 2007. 24. ^ Dingrando 666. 25. ^ a b Knight 943. 26. ^ a b Knight 976. 27. ^ Terminal Voltage - Tiscali Reference. Originally from Hutchinson Encyclopaedia. Retrieved 7 April 2007. 28. ^ Buchmann, Isidor. How does the internal battery resistance affect performance?. Battery University. Retrieved 14 August

2008. 29. ^ Dingrando 674. 30. ^ Dingrando 677. 31. ^ a b c Buchmann, Isidor. Will secondary batteries replace primaries?. Battery University. Retrieved 6 January 2008. 32. ^ Dingrando 675. 33. ^ Fink, Ch. 11, Sec. "Batteries and Fuel Cells." 34. ^ Franklin Leonard Pope, Modern Practice of the Electric Telegraph 15th Edition, D. Van Nostrand Company, New York,

1899 , pages 7-11. Available on the Internet Archive 35. ^ a b Duracell: Battery Care. Retrieved 10 August 2008. 36. ^ a b Alkaline Manganese Dioxide Handbook and Application Manual (PDF). Energizer. Retrieved 25 August 2008. 37. ^ Buchmann, Isidor. Can the lead-acid battery compete in modern times?. Battery University. Retrieved 2 September 2007.

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38. ^ Vector VEC012APM Jump Starter (450 Amp). Amazon. Retrieved 26 August 2008. 39. ^ Dynasty VRLA Batteries and Their Application. C&D Technologies, Inc. Retrieved 26 August 2008. 40. ^ What's the best battery?. Battery University. Retrieved 26 August 2008. 41. ^ USBCELL - Revolutionary rechargeable USB battery that can charge from any USB port. Retrieved 6 November 2007. 42. ^ Long Life Batteries You Can Recharge - Hybrio. Retrieved 6 January 2008. 43. ^ GP ReCyko. Retrieved 6 January 2008. 44. ^ SANYO Presents 'eneloop' : A New Battery in place of Dry Cell Battery for the 21st Century. Retrieved 6 January 2008. 45. ^ "Spotlight on Photovoltaics & Fuel Cells: A Web-based Study & Comparison" (PDF). pp. 1-2.

http://www.pspb.org/e21/media/Compare_pvfc_v108_TN.pdf. Retrieved 2007-03-14. 46. ^ a b Battery Knowledge - AA Portable Power Corp.. Retrieved 16 April 2007. 47. ^ Battery Capacity - Techlib. Retrieved 10 April 2007. 48. ^ a b Buchmann, Isidor. Discharge methods. Battery University. Retrieved 14 August 2008. 49. ^ Kang, B. and Ceder, G. (2009) "Battery materials for ultrafast charging and discharging" Nature 458: 190-3. 1:00-6:50

(audio) 50. ^ Conway, E. (2 September 2008) "World's biggest battery switched on in Alaska" Telegraph.co.uk 51. ^ Biello, D. (December 22, 2008) "Storing the Breeze: New Battery Might Make Wind Power More Reliable" Scientific

American 52. ^ Amos, J. (24 August 2008) "Solar plane makes record flight" BBC News 53. ^ Increasing recharge speed of lithium-ion batteries 54. ^ Self discharge of batteries - Corrosion Doctors. Retrieved 9 September 2007. 55. ^ Alkaline Technical Information. Energizer. Retrieved 11 July 2007. 56. ^ Discharging at high and low temperature 57. ^ a b Buchmann, Isidor. Non-Correctable Battery Problems. Battery University. Retrieved 3 February 2009. 58. ^ Energizer Rechargeable Batteries and Chargers: Frequently Asked Questions. Energizer. Retrieved 3 February 2009. 59. ^ a b Rechargeable battery Tips - NIMH Technology Information. Retrieved 10 August 2007. 60. ^ battery myths vs battery facts - free information to help you learn the difference. Retrieved 10 August 2007. 61. ^ What does ‘memory effect’ mean?. Retrieved 10 August 2007. 62. ^ Battery Life and How To Improve It 63. ^ a b c Buchmann, Isidor. Can the lead-acid battery compete in modern times? Battery University. Retrieved 3 February 2009. 64. ^ Rich, Vincent (1994). The International Lead Trade. Cambridge: Woodhead. 129. 65. ^ Deep Cycle Battery FAQ. Northern Arizona Wind & Sun. Retrieved 3 February 2009. 66. ^ Car and Deep Cycle Battery FAQ. Rainbow Power Company. Retrieved 3 February 2009. 67. ^ Deep cycle battery guide. Energy Matters. Retrieved 3 February 2009. 68. ^ Ask Yahoo: Does putting batteries in the freezer make them last longer?. Retrieved 7 March 2007. 69. ^ AN1333 70. ^ Energizer.com - Learning Center - Energizer and the Environment. Retrieved 17 December 2007. 71. ^ a b Battery dont's - Global-Batteries. Retrieved 20 August 2007. 72. ^ Batteries - Product Stewardship. EPA. Retrieved 11 September 2007. 73. ^ Battery Recycling » Earth 911. Retrieved 9 September 2007. 74. ^ Product Safety DataSheet - Energizer (PDF, p. 2). Retrieved 9 September 2007. 75. ^ "San Francisco Supervisor Takes Aim at Toxic Battery Waste". Environmental News Network (11 July 2001). 76. ^ http://www.epa.gov/epawaste/laws-regs/state/policy/p1104.pdf 77. ^ a b http://www.rbrc.org/consumer/howitallworks_faq.shtml?PHPSESSID=ad1e142bcdd99cd67418f2171794d892 78. ^ Disposal of spent batteries and accumulators. European Union. Retrieved 27 July 2009. 79. ^ Excludes the mass of the air oxidizer. 80. ^ ushistory.org: The Lemon Battery. Accessed 10 April 2007. 81. ^ ZOOM . activities . phenom . Potato Battery. Accessed 10 April 2007. 82. ^ Two-Potato Clock - Science Kit and Boreal Laboratories. Accessed 10 April 2007. 83. ^ Howstuffworks "Battery Experiments: Voltaic Pile". Accessed 10 April 2007. 84. ^ Sony Develops A Bio Battery Powered By Sugar. Accessed 24 August 2007.

CorrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is commonly known as rusting. This type of damage typically produces oxide(s) and/or salt(s) of the original metal. Corrosion can also refer to other materials than metals, such as ceramics or polymers. Although in this context, the term degradation is more common. ost structural alloys corrode merely from exposure to moisture in the

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air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration. While some efforts to reduce corrosion merely redirect the damage into less visible, less predictable forms, controlled corrosion treatments such as passivation and chromate-conversion will increase a material's corrosion resistance.

Galvanic corrosionGalvanic corrosion occurs when two different metals electrically contact each other and are immersed in an electrolyte. In order for galvanic corrosion to occur, an electrically conductive path and an ionically conductive path are necessary. This affects a galvanic couple where the more active metal corrodes at an accelerated rate and the more noble metal corrodes at a retarded rate. When immersed, neither metal would normally corrode as quickly without the electrically conductive connection (usually via a wire or direct contact). Galvanic corrosion is often utilized in sacrificial anodes. What type of metal(s) to use is readily determined by following the galvanic series. For example, zinc is often used as a sacrificial anode for steel structures, such as pipelines or docked naval ships. Galvanic corrosion is of major interest to the marine industry and also anywhere water can contact pipes or metal structures.

Factors such as relative size of anode (smaller is generally less desirable), types of metal, and operating conditions (temperature, humidity, salinity, etc.) will affect galvanic corrosion. The surface area ratio of the anode and cathode will directly affect the corrosion rates of the materials.

Galvanic series

In a given sea environment (one standard medium is aerated, room-temperature seawater), one metal will be either more noble or more active than the next, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electron gas, so that the tug-of-war at each surface is translated into a competition for free electrons between the two materials. The noble metal will tend to take electrons from the active one, while the electrolyte hosts a flow of ions in the same direction. The resulting mass flow or electrical current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a galvanic series, and can be a very useful in predicting and understanding corrosions.

Resistance to corrosionSome metals are more intrinsically resistant to corrosion than others, either due to the fundamental nature of the electrochemical processes involved or due to the details of how reaction products form. For some examples, see galvanic series. If a more susceptible material is used, many techniques can be applied during an item's manufacture and use to protect its materials from damage.

Intrinsic chemistry

The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Any corrosion products of gold or platinum tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form on Earth, and is a large part of their intrinsic value. More common "base" metals can only be protected by more temporary means.

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Some metals have naturally slow reaction kinetics, even though their corrosion is thermodynamically favorable. These include such metals as zinc, magnesium, and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate. An extreme example is graphite, which releases large amounts of energy upon oxidation, but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions.

Passivation

Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation. When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon is known as passivation (rust, for example, usually grows to be much thicker, and so is not considered passivation, because this mixed oxidized layer is not protective). While this effect is in some sense a property of the material, it serves as an indirect kinetic barrier: the reaction is often quite rapid unless and until an impermeable layer forms. Passivation in air and water at moderate pH is seen in such materials as aluminum, stainless steel, titanium, and silicon.

These conditions required for passivation are specific to the material. The effect of pH is recorded using Pourbaix diagrams, but many other factors are influential. Some conditions that inhibit passivation include: high pH for aluminum, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and fluoride ions for silicon. On the other hand, sometimes unusual conditions can bring on passivation in materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms.

Corrosion in passivated materialsPassivation is extremely useful in alleviating corrosion damage, but care must be taken not to trust it too thoroughly. Even a high-quality alloy will corrode if its ability to form a passivating film is hindered. Because the resulting modes of corrosion are more exotic and their immediate results are less visible than rust and other bulk corrosion, they often escape notice and cause problems among those who are not familiar with them.

Pitting corrosion

Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which compete as anions, can interfere with a given alloy's ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an auto-catalytic process. In extreme cases, the sharp tips of extremely long and narrow can cause stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in passivated alloys, but it can be prevented by control of the alloy's environment,

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which often includes ensuring that the material is exposed to oxygen uniformly (i.e., eliminating crevices)

Weld decay and knifeline attack

Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a minor alloying component (Chromium, typically only 18%). Due to the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries near welds) in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name applies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable1.

Crevice corrosion

Crevice corrosion is a localized form of corrosion occurring in spaces to which the access of the working fluid from the environment is limited and a concentration cell, areas with different oxygen concentration, will take place with consequent high corrosion rate . These spaces are generally called crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles.

Microbial corrosionMicrobial corrosion, or bacterial corrosion, is a corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and non-metallic materials, in both the presence and lack of oxygen. Sulfate-reducing bacteria are common in lack of oxygen; they produce hydrogen sulfide, causing sulfide stress cracking. In presence of oxygen, some bacteria directly oxidize iron to iron oxides and hydroxides, other bacteria oxidize sulfur and produce sulfuric acid causing biogenic sulfide corrosion. Concentration cells can form in the deposits of corrosion products, causing and enhancing galvanic corrosion.

High temperature corrosionHigh temperature corrosion is chemical deterioration of a material (typically a metal) under very high temperature conditions. This non-galvanic form of corrosion can occur when a metal is subject to a high temperature atmosphere containing oxygen, sulfur or other compounds capable of oxidising (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.

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The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions. Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.

Applied coatingsGalvanization

Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper, tougher, and/or easier-to-process) structural material. Aside from cosmetic and manufacturing issues, there are tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with a more active metal such as zinc or cadmium.

Reactive coatings

If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to it. These form an electrically insulating and/or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed. Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous for their mineral deposits), chromates, phosphates, and a wide range of specially-designed chemicals that resemble surfactants (i.e. long-chain organic molecules with ionic end groups).

Anodization

Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.

Controlled Permeability Formwork

Controlled Permeability Formwork (CPF) is a method of preventing the corrosion of reinforcement by naturally enhancing the durability of the cover during concrete placement, . CPF has been used in environments to combat the effects of Carbonation, chlorides, frost and abrasion.

Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

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It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide or platinum coated titanium or niobium coated rod and wires.

Corrosion in nonmetalsMost ceramic materials are almost entirely immune to corrosion. The strong ionic and/or covalent bonds that hold them together leave very little free chemical energy in the structure; they can be thought of as already corroded. When corrosion does occur, it is almost always a simple dissolution of the material or chemical reaction, rather than an electrochemical process. A common example of corrosion protection in ceramics is the lime added to soda-lime glass to reduce its solubility in water; though it is not nearly as soluble as pure sodium silicate, normal glass does form sub-microscopic flaws when exposed to moisture. Due to its brittleness, such flaws cause a dramatic reduction in the strength of a glass object during its first few hours at room temperature.

Polymer degradation is due to a wide array of complex and often poorly-understood physiochemical processes. These are strikingly different from the other processes discussed here, and so the term "corrosion" is only applied to them in a loose sense of the word. Because of their large molecular weight, very little entropy can be gained by mixing a given mass of polymer with another substance, making them generally quite difficult to dissolve. While dissolution is a problem in some polymer applications, it is relatively simple to design against. A more common and related problem is swelling, where small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible vinyl) are intentionally swelled with plasticizers, which can be leached out of the structure, causing brittleness or other undesirable changes. The most common form of degradation, however, is a decrease in polymer chain length. Mechanisms which break polymer chains are familiar to biologists because of their effect on DNA: ionizing radiation (most commonly ultraviolet light), free radicals, and oxidizers such as oxygen, ozone, and chlorine. Additives can slow these process very effectively, and can be as simple as a UV-absorbing pigment (i.e., titanium dioxide or carbon black). Plastic shopping bags often do not include these additives so that they break down more easily as litter.

Corrosion of glassesThe corrosion of silicate glasses in aqueous solutions is governed by two mechanisms: diffusion-controlled leaching (ion exchange) and glass network hydrolytic dissolution[1]. Both corrosion

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mechanisms strongly depend on the pH of contacting solution: the rate of ion exchange decreases with pH as 10-0.5pH whereas the rate of hydrolytic dissolution increases with pH as 100.5pH [2]

Mathematically, corrosion rates of glasses are characterized by normalized corrosion rates of elements NRi (g/cm2 d) which are determined as the ratio of total amount of released species into the water Mi (g) to the water-contacting surface area S (cm2), time of contact t (days) and weight fraction content of the element in the glass fi:

.

The overall corrosion rate is a sum of contributions from both mechanisms (leaching + dissolution) NRi=Nrxi+NRh. Diffusion-controlled leaching (ion exchange) is characteristic of the initial phase of corrosion and involves replacement of alkali ions in the glass by a hydronium (H3O+) ion from the solution. It causes an ion-selective depletion of near surface layers of glasses and gives an inverse square root dependence of corrosion rate with exposure time. The diffusion controlled normalized leaching rate of cations from glasses (g/cm2 d) is given by:

,

where t is time, Di is the i-th cation effective diffusion coefficient (cm2/d), which depends on pH of contacting water as Di = Di0·10-pH, and ρ is the density of the glass (g/cm3).

Glass network dissolution is characteristic of the later phases of corrosion and causes a congruent release of ions into the water solution at a time-independent rate in dilute solutions (g/cm2 d):

NRh = ρrh,

Where rh is the stationary hydrolysis (dissolution) rate of the glass (cm/d). In closed systems the consumption of protons from the aqueous phase increases the pH and causes a fast transition to hydrolysis. However further silica saturation of solution impedes hydrolysis and causes the glass to return to an ion-exchange, e.g. diffusion-controlled regime of corrosion.

In typical natural conditions normalized corrosion rates of silicate glasses are very low and are of the order of 10-7 - 10-5 g/cm2 d. The very high durability of silicate glasses in water makes them suitable for hazardous and nuclear waste immobilization.

Glass corrosion tests

There exist numerous standardized procedures for measuring the corrosion (also called chemical durability) of glasses in neutral, basic, and acidic environments, under simulated environmental conditions, in simulated body fluid, at high temperature and pressure[5], and under other conditions.

In the standard procedure ISO 719[6] a test of the extraction of water soluble basic compounds under neutral conditions is described: 2 g glass, particle size 300-500 μm, is kept for 60 min in 50 ml de-ionized water of grade 2 at 98°C. 25 ml of the obtained solution is titrated against 0.01 mol/l HCl

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solution. The volume of HCl needed for neutralization is recorded and classified following the values in the table below.

0.01M HCl needed to neutralizeextracted basic oxides, ml

Extracted Na2Oequivalent, μg Hydrolytic class

to 0.1 to 31 1above 0.1 to 0.2 above 31 to 62 2above 0.2 to 0.85 above 62 to 264 3above 0.85 to 2.0 above 264 to 620 4above 2.0 to 3.5 above 620 to 1085 5above 3.5 above 1085 >5

References collected from:-

1. ^ A.K. Varshneya. Fundamentals of inorganic glasses. Society of Glass Technology, Sheffield, 682pp. (2006). 2. ^ M.I. Ojovan, W.E. Lee. New Developments in Glassy Nuclear Wasteforms. Nova Science Publishers, New York, 136pp.

(2007). 3. ^ Corrosion of Glass, Ceramics and Ceramic Superconductors. Edited by: D.E. Clark, B.K. Zoitos, William Andrew

Publishing/Noyes, 672pp. (1992). 4. ^ Calculation of the Chemical Durability (Hydrolytic Class) of Glasses 5. ^ International Organization for Standardization, Procedure 719 (1985) 6. Jones, Denny (1996). Principles and Prevention of Corrosion (2nd edition ed.). Upper Saddle River, New Jersey: Prentice

Hall. ISBN 0-13-359993-0. 7. Working Safely with Corrosive Chemicals

Sulfation

Sulfation refers to the process whereby a lead-acid battery (such as a car battery) loses its ability to hold a charge after it is kept in a discharged state too long due to the crystallization of lead sulfate.

Lead-acid batteries generate electricity through a double sulfate chemical reaction. Lead and lead dioxide, which are the active materials on the battery's plates, react with sulfuric acid in the electrolyte to form lead sulfate. When formed, the lead sulfate is in a finely divided, amorphous form, which is easily converted back to lead, lead oxide and sulfuric acid when the battery is recharged.

Over time, lead sulfate converts to the more stable crystalline form, coating the battery's plates. Crystalline lead sulfate does not conduct electricity and cannot be converted back into lead and lead oxide under normal charging conditions. As batteries are "cycled" through numerous discharge and charge sequences, lead sulfate that forms during normal discharge is slowly converted to a very stable crystalline form. This process is known as sulfation.

Sulfation is a natural, normal process that occurs in all lead-acid batteries during normal operation. Sulfation clogs grids, impedes recharging and ultimately can expand and crack the plates as it accumulates, destroying the battery. Crystalline lead sulfate is resistant to normal charging current, and does not re-dissolve completely. Thus, not all the lead is returned to the battery plates, and the amount of usable active material necessary for electricity generation declines over time. In addition, the sulfate portion (of the lead sulfate) is not returned to the electrolyte as sulfuric acid. Sulfating also affects the charging cycle, resulting in longer charging times, less efficient and incomplete charging, excessive heat generation (higher battery temperatures). Higher battery temperatures cause longer cool-down times and can accelerate corrosion.

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The process can often be at least partially prevented and/or reversed by devices known as desulfators, which repeatedly send short but powerful current surges through the damaged battery. Over time, this procedure tends to break down and dissolve the sulfate crystals, restoring some of the battery's capacity. [1]

Sulfation in proteinsBiochemical sulfation is a phase II enzyme reaction. This biotransformation process uses its cosubstrate 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to transfer sulfonate to a xenobiotic. Most of the time this is effective in rendering the xenobiotic pharmacological and toxicological less active, but sometimes it plays a role in the activation of xenobiotics (e.g. aromatic amines, methyl substituted polycyclic aromatic hydrocarbons).

Sulfation can also be posttranslational modification of protein. The target amino acid is tyrosine. It is called tyrosine sulfation.

Another use of the term 'sulfation' is in the creation of the sulfated glycosaminoglycans. Here, the sulfate group is being added either via oxygen or nitrogen: O-sulfation, N-sulfation.

References collected from:-

1. ^ Lead-acid Battery Reconditioning Tehnique

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Sulfuric acid

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Sulfuric acid

Other names Oil of vitriol

IdentifiersCAS number 7664-93-9 EC number 231-639-5UN number 1830RTECS number WS5600000PropertiesMolecular formula H2SO4

Molar mass 98.08 g/molAppearance clear, colorless, odorless liquidDensity 1.84 g/cm3, liquid

Melting point10 °C, 283 K, 50 °F

Boiling point337 °C, 610 K, 639 °F

Solubility in water miscibleAcidity (pKa) −3Viscosity 26.7 cP (20 °C)HazardsMSDS External MSDSEU Index 016-020-00-8EU classification Corrosive (C)R-phrases R35S-phrases (S1/2), S26, S30, S45

NFPA 704

032WFlash point Non-flammableRelated compounds

Related strong acidsSelenic acidHydrochloric acidNitric acid

Related compounds

Sulfurous acidPeroxymonosulfuric acidSulfur trioxideOleum

Supplementary data pageStructure andproperties n, εr, etc.

Thermodynamicdata

Phase behaviourSolid, liquid, gas

Spectral data UV, IR, NMR, MS

Sulfuric (or sulphuric) acid, H2SO4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tones, with an approximate value of US$8 billion. Principal uses include lead-acid batteries for cars and other vehicles, ore processing, fertilizer manufacturing, oil refining, wastewater processing, and chemical synthesis.

OccurrencePure (undiluted) sulfuric acid is not encountered naturally on Earth, due to its great affinity for water. Apart from that, sulfuric acid is a constituent of acid rain, which is formed by atmospheric oxidation of sulfur dioxide in the presence of water - i.e., oxidation of sulfurous acid. Sulfur dioxide is the main byproduct produced when sulfur-containing fuels such as coal or oil are burned.

Sulfuric acid is formed naturally by the oxidation of sulfide minerals, such as iron sulfide. The resulting water can be highly acidic and is called acid mine drainage (AMD) or acid rock drainage (ARD). This acidic water is capable of dissolving metals present in sulfide ores, which results in brightly-colored, toxic streams. The oxidation of iron sulfide pyrite by molecular oxygen produces iron (II), or Fe2+:

2 FeS2 + 7 O2 + 2 H2O → 2 Fe2+ + 4 SO2−4 + 4 H+

The Fe2+ can be further oxidized to Fe3+:

4 Fe2+ + O2 + 4 H+ → 4 Fe3+ + 2 H2O

The Fe3+ produced can be precipitated as the hydroxide or hydrous oxide:

Fe3+ + 3 H2O → Fe(OH)3 + 3 H+

The iron (III) ion ("ferric iron") can also oxidize pyrite. When iron(III) oxidation of pyrite occurs, the process can become rapid. pH values below zero have been measured in ARD produced by this process.

ARD can also produce sulfuric acid at a slower rate, so that the Acid Neutralization Capacity (ANC) of the aquifer can neutralize the produced acid. In such cases, the Total dissolved solids (TDS) concentration of the water can be increased from the dissolution of minerals from the acid-neutralization reaction with the minerals.

Extraterrestrial sulfuric acid

Atmosphere of Venus

Sulfuric acid is produced in the upper atmosphere of Venus by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapor. Ultraviolet photons of wavelengths less than 169 nm can photo dissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive. When it reacts with sulfur dioxide, a trace component of the Venusians atmosphere, the result is sulfur trioxide, which can combine with water vapor, another trace component of Venus's atmosphere, to yield sulfuric acid.

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CO2 → CO + O SO2 + O → SO3 SO3 + H2O → H2SO4

In the upper, cooler portions of Venus's atmosphere, sulfuric acid exists as a liquid, and thick sulfuric acid clouds completely obscure the planet's surface when viewed from above. The main cloud layer extends from 45–70 km above the planet's surface, with thinner hazes extending as low as 30 and as high as 90 km above the surface.

The permanent Venusians clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain.

The atmosphere exhibits a sulfuric acid cycle. As sulfuric acid rain droplets fall down through the hotter layers of the atmosphere's temperature gradient, they are heated up and release water vapor, becoming more and more concentrated. When they reach temperatures above 300°C, sulfuric acid begins to decompose into sulfur trioxide and water, both in the gas phase. Sulfur trioxide is highly reactive and dissociates into sulfur dioxide and atomic oxygen, which oxidizes traces of carbon monoxide to form carbon dioxide.

Sulfur dioxide and water vapor rise on convection currents from the mid-level atmospheric layers to higher altitudes, where they will be transformed again into sulfuric acid, and the cycle repeats.

On Europa's icy surface

Infrared spectra from NASA's Galileo mission show distinct absorptions on Jupiter's moon Europa that have been attributed to one or more sulfuric acid hydrates. The interpretation of the spectra is somewhat controversial. Some planetary scientists prefer to assign the spectral features to the sulfate ion, perhaps as part of one or more minerals on Europa's surface.

ManufactureSulfuric acid is produced from sulfur, oxygen and water via the conventional contact process (DCDA) or the wet sulfuric acid process (WSA).

Contact process (DCDA)

In the first step, sulfur is burned to produce sulfur dioxide.

S (s) + O2 (g) → SO2 (g)

This is then oxidized to sulfur trioxide using oxygen in the presence of a vanadium(V) oxide catalyst.

2 SO2 + O2 (g) → 2 SO3 (g) (in presence of V2O5)

The sulfur trioxide is absorbed into 97-98% H2SO4 to form oleum (H2S2O7), also known as fuming sulfuric acid. The oleum is then diluted with water to form concentrated sulfuric acid.

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H2SO4 (l) + SO3 → H2S2O7 (l) H2S2O7 (l) + H2O(l) → 2 H2SO4 (l)

Note that directly dissolving SO3 in water is not practical due to the highly exothermic nature of the reaction between sulfur trioxide and water. The reaction forms a corrosive aerosol that is very difficult to separate, instead of a liquid.

SO3 (g) + H2O (l) → H2SO4 (l)

Wet sulfuric acid process (WSA)

In the first step, sulfur is burned to produce sulfur dioxide:

S (s) + O2 (g) → SO2 (g)

or, alternatively, hydrogen sulfide (H2S) gas is incinerated to SO2 gas:

H2S + 3⁄2O2 → H2O + SO2 (−518 kJ/mol)

This is then oxidized to sulfur trioxide using oxygen with vanadium(V) oxide as catalyst.

2 SO2 + O2 → 2 SO3 (−99 kJ/mol)

The sulfur trioxide is hydrated into sulfuric acid H2SO4:

SO3 + H2O → H2SO4(g) (−101 kJ/mol)

The last step is the condensation of the sulfic acid to liquid 97-98% H2SO4:

H2SO4(g) → H2SO4(l) (−69 kJ/mol)

Other methods

Another method is the less well-known metabisulfite method, in which metabisulfite in placed at the bottom of a beaker, and 12.6 molar concentrations hydrochloric acid is added. The resulting gas is bubbled through nitric acid, which will release brown/red vapors. The completion of the reaction is indicated by the ceasing of the fumes. This method does not produce an inseparable mist, which is quite convenient. Prior to 1900, most sulfuric acid was manufactured by the chamber process.

Physical properties

Forms of sulfuric acid

Although nearly 100% sulfuric acid can be made, this loses SO3 at the boiling point to produce 98.3% acid. The 98% grade is more stable in storage, and is the usual form of what is described as concentrated sulfuric acid. Other concentrations are used for different purposes. Some common concentrations are

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10%, dilute sulfuric acid for laboratory use, (1.1 molar), 33.53%, battery acid (used in lead-acid batteries), (4.2 molar), 62.18%, chamber or fertilizer acid, (9.6 molar), 73.61%, tower or Glover acid, ( 12.3 molar) 97%, concentrated acid, (Approx. 18 molar).

Different purities are also available. Technical grade H2SO4 is impure and often colored, but is suitable for making fertilizer. Pure grades such as United States Pharmacopoeia (USP) grade are used for making pharmaceuticals and dyestuffs.

When high concentrations of SO3 gas are added to sulfuric acid, H2S2O7, called pyrosulfuric acid, fuming sulfuric acid or oleum or, less commonly, Nordhausen acid is formed. Concentrations of oleum are either expressed in terms of % SO3 (called % oleum) or as % H2SO4 (the amount made if H2O were added); common concentrations are 40% oleum (109% H2SO4) and 65% oleum (114.6% H2SO4). Pure H2S2O7 is a solid with melting point 36°C.

Pure sulfuric acid is an oily clear liquid and this explains the old name of the acid ('oil of vitriol').

Polarity and conductivity

Anhydrous H2SO4 is a very polar liquid, having a dielectric constant of around 100. It has a high electrical conductivity, caused by dissociation through protonating itself, a process known as auto protolysis.

2 H2SO4 H3SO+4 +HSO−4

The equilibrium constant for the auto protolysis is

Kap(25°C)= [H3SO+4][HSO−4] = 2.7×10−4.

The comparable equilibrium constant for water, Kw is 10−14, a factor of 1010 (10 billion) smaller.

In spite of the viscosity of the acid, the effective conductivities of the H3SO+4 and HSO−4 ions are high due to an intra-molecular proton-switch mechanism (analogous to the Grotthuss mechanism in water), making sulfuric acid a good conductor. It is also an excellent solvent for many reactions. The equilibrium is actually more complex than shown above; 100% H2SO4 contains the following species at equilibrium (figures shown as milli moles per kilogram of solvent): HSO−4 (15.0), H3SO+4 (11.3), H3O+ (8.0), HS2O−7 (4.4), H2S2O7 (3.6), H2O (0.1).[3]

Chemical properties

Reaction with water

The hydration reaction of sulfuric acid is highly exothermic. One should always add the acid to the water rather than the water to the acid, because of the relative densities of these two liquids. Water is less dense than sulfuric acid, and will tend to float on top of it. Thus, if water is added to the concentrated sulfuric acid, it can boil and splatter dangerously. This reaction is best thought of as the formation of hydronium ions:

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H2SO4 + H2O → H3O+ + HSO−4 HSO−4 + H2O → H3O+ + SO2−4

Because the hydration of sulfuric acid is thermodynamically favorable, sulfuric acid is an excellent dehydrating agent, and is used to prepare many dried fruits. The affinity of sulfuric acid for water is sufficiently strong that it will remove hydrogen and oxygen atoms from other compounds; for example, mixing starch (C6H12O6)n and concentrated sulfuric acid will give elemental carbon and water which is absorbed by the sulfuric acid (which becomes slightly diluted):

(C6H12O6)n → 6n C + 6n H2O

The effect of this can be seen when concentrated sulfuric acid is spilled on paper; the cellulose reacts to give a burnt appearance, the carbon appears much as soot would in a fire. A more dramatic reaction occurs when sulfuric acid is added to a tablespoon of white sugar in a beaker; a rigid column of black, porous carbon will quickly emerge. The carbon will smell strongly of caramel.

Other reactions

As an acid, sulfuric acid reacts with most bases to give the corresponding sulfate. For example, the blue copper salt copper(II) sulfate, commonly used for electroplating and as a fungicide, is prepared by the reaction of copper(II) oxide with sulfuric acid:

CuO + H2SO4 → CuSO4 + H2O

Sulfuric acid can also be used to displace weaker acids from their salts. Reaction with sodium acetate, for example, displaces acetic acid, CH3COOH, and forms sodium bisulfate:

H2SO4 + CH3COONa → NaHSO4 + CH3COOH Similarly, reacting sulfuric acid with potassium nitrate can be used to produce nitric acid and a precipitate of potassium bisulfate. When combined with nitric acid, sulfuric acid acts both as an acid and a dehydrating agent, forming the nitronium ion NO+2, which is important in nitration reactions involving electrophilic aromatic substitution. This type of reaction, where protonation occurs on an oxygen atom, is important in many organic chemistry reactions, such as Fischer esterification and dehydration of alcohols.

Sulfuric acid reacts with most metals via a single displacement reaction to produce hydrogen gas and the metal sulfate. Dilute H2SO4 attacks iron, aluminium, zinc, manganese, magnesium and nickel, but reactions with tin and copper require the acid to be hot and concentrated. Lead and tungsten, however, are resistant to sulfuric acid. The reaction with iron shown below is typical for most of these metals, but the reaction with tin produces sulfur dioxide rather than hydrogen.

Fe(s) + H2SO4(aq) → H2(g) + FeSO4(aq) Sn(s) + 2 H2SO4 (aq) → SnSO4(aq) + 2 H2O(l) + SO2(g)

These reactions may be taken as typical: the hot concentrated acid generally acts as an oxidising agent whereas the dilute acid acts a typical acid. Hence hot concentrated acid reacts with tin, zinc and copper to produce the salt, water and sulfur dioxide, whereas the dilute acid reacts with metals high in the reactivity series (such as Zn) to produce a salt and hydrogen. This is explained more fully in 'A New Certificate Chemistry' by Holderness and Lambert.

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Sulfuric acid undergoes electrophilic aromatic substitution with aromatic compounds to give the corresponding sulfonic acids:[4]

UsesSulfuric acid is a very important commodity chemical, and indeed, a nation's sulfuric acid production is a good indicator of its industrial strength.[5] The major use (60% of total production worldwide) for sulfuric acid is in the "wet method" for the production of phosphoric acid, used for manufacture of phosphate fertilizers as well as trisodium phosphate for detergents. In this method, phosphate rock is used, and more than 100 million tonnes are processed annually. This raw material is shown below as fluorapatite, though the exact composition may vary. This is treated with 93% sulfuric acid to produce calcium sulfate, hydrogen fluoride (HF) and phosphoric acid. The HF is removed as hydrofluoric acid. The overall process can be represented as:

Ca5F(PO4)3 + 5 H2SO4 + 10 H2O → 5 CaSO4·2H2O + HF + 3 H3PO4

Sulfuric acid is used in large quantities by the iron and steelmaking industry to remove oxidation, rust and scale from rolled sheet and billets prior to sale to the automobile and white goods industry. Used acid is often recycled using a Spent Acid Regeneration (SAR) plant. These plants combust spent acid with natural gas, refinery gas, fuel oil or other fuel sources. This combustion process produces gaseous sulfur dioxide (SO2) and sulfur trioxide (SO3) which are then used to manufacture "new" sulfuric acid. SAR plants are common additions to metal smelting plants, oil refineries, and other industries where sulfuric acid is consumed in bulk, as operating a SAR plant is much cheaper than the recurring costs of spent acid disposal and new acid purchases.

Ammonium sulfate, an important nitrogen fertilizer, is most commonly produced as a byproduct from coking plants supplying the iron and steel making plants. Reacting the ammonia produced in the thermal decomposition of coal with waste sulfuric acid allows the ammonia to be crystallized out as a salt (often brown because of iron contamination) and sold into the agro-chemicals industry.

Another important use for sulfuric acid is for the manufacture of aluminum sulfate, also known as paper maker's alum. This can react with small amounts of soap on paper pulp fibers to give gelatinous aluminum carboxylates, which help to coagulate the pulp fibers into a hard paper surface. It is also used for making aluminum hydroxide, which is used at water treatment plants to filter out impurities, as well as to improve the taste of the water. Aluminum sulfate is made by reacting bauxite with sulfuric acid:

Al2O3 + 3 H2SO4 → Al2(SO4)3 + 3 H2O

Sulfuric acid is used for a variety of other purposes in the chemical industry. For example, it is the usual acid catalyst for the conversion of cyclohexanoneoxime to caprolactam, used for making nylon. It is used for making hydrochloric acid from salt via the Mannheim process. Much H2SO4 is

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used in petroleum refining, for example as a catalyst for the reaction of isobutane with isobutylene to give isooctane, a compound that raises the octane rating of gasoline (petrol). Sulfuric acid is also important in the manufacture of dyestuffs solutions and is the "acid" in lead-acid (car) batteries.

Sulfuric acid is also used as a general dehydrating agent in its concentrated form. See Reaction with water.

Sulfur-iodine cycle

The sulfur-iodine cycle is a series of thermo-chemical processes used to obtain hydrogen. It consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen.

2 H2SO4 → 2 SO2 + 2 H2O + O2 (830 °C)I2 + SO2 + 2 H2O → 2 HI + H2SO4 (120 °C)2 HI → I2 + H2 (320 °C)

The sulfur and iodine compounds are recovered and reused, hence the consideration of the process as a cycle. This process is endothermic and must occur at high temperatures, so energy in the form of heat has to be supplied.

The sulfur-iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economy. It does not require hydrocarbons like current methods of steam reforming.

The sulfur-iodine cycle is currently being researched as a feasible method of obtaining hydrogen, but the concentrated, corrosive acid at high temperatures poses currently insurmountable safety hazards if the process were built on a large scale.

HistoryJohn Dalton's 1808 sulfuric acid molecule shows a central sulfur atom bonded to three oxygen atoms.

The discovery of sulfuric acid is credited to the 8th century Muslim chemist and alchemist, Jabir ibn Hayyan (Geber). The acid was later studied by 9th century Persian physician and alchemist Ibn Zakariya al-Razi (Rhazes), who obtained the substance by dry distillation of minerals including iron(II) sulfate heptahydrate, FeSO4·7H2O, and copper(II) sulfate pentahydrate, CuSO4·5H2O. When heated, these compounds decompose to iron(II) oxide and copper(II) oxide, respectively, giving off water and sulfur trioxide, which combine to produce a dilute solution of sulfuric acid. This method was popularized in Europe through translations of Arabic and Persian treatises, as well as books by European alchemists, such as the 13th-century German Albertus Magnus.

Sulfuric acid was known to medieval European alchemists as oil of vitriol, spirit of vitriol, or simply vitriol, among other names. The word vitriol derives from the Latin vitreus, 'glass', referring to the glassy appearance of the hydrated sulfate salts, which also carried the name vitriol. Salts called by this name included copper(II) sulfate (blue vitriol, or rarely Roman vitriol), zinc sulfate (white vitriol), iron(II) sulfate (green vitriol), iron(III) sulfate (vitriol of Mars), and cobalt(II) sulfate (red vitriol). However, Red Vitriol is also a 20th century technical name for a grade of sulfuric acid.

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Vitriol was widely considered the most important alchemical substance, intended to be used as a philosopher's stone. Highly purified vitriol was used as a medium for reacting other substances. This was largely because the acid does not react with gold, production of which was often the final goal of alchemical processes. The importance of vitriol to alchemy is highlighted in the alchemical motto, Visita Interiora Terrae Rectificando Invenies Occultum Lapidem which is a backronym meaning ('Visit the interior of the earth and rectifying (i.e. purifying) you will find the hidden/secret stone'), found in L'Azoth des Philosophes by the 15th Century alchemist Basilius Valentinus, .

In the 17th century, the German-Dutch chemist Johann Glauber prepared sulfuric acid by burning sulfur together with saltpeter (potassium nitrate, KNO3), in the presence of steam. As saltpeter decomposes, it oxidizes the sulfur to SO3, which combines with water to produce sulfuric acid. In 1736, Joshua Ward, a London pharmacist, used this method to begin the first large-scale production of sulfuric acid.

In 1746 in Birmingham, John Roebuck adapted this method to produce sulfuric acid in lead-lined chambers, which were stronger, less expensive, and could be made larger than the previously used glass containers. This lead chamber process allowed the effective industrialization of sulfuric acid production. After several refinements, this method remained the standard for sulfuric acid production for almost two centuries.

Sulfuric acid created by John Roebuck's process only approached a 35–40% concentration. Later refinements to the lead-chamber process by French chemist Joseph-Louis Gay-Lussac and British chemist John Glover improved the yield to 78%.However, the manufacture of some dyes and other chemical processes require a more concentrated product. Throughout the 18th century, this could only be made by dry distilling minerals in a technique similar to the original alchemical processes. Pyrite (iron disulfide, FeS2) was heated in air to yield iron (II) sulfate, FeSO4, which was oxidized by further heating in air to form iron(III) sulfate, Fe2(SO4)3, which, when heated to 480 °C, decomposed to iron(III) oxide and sulfur trioxide, which could be passed through water to yield sulfuric acid in any concentration. However, the expense of this process prevented the large-scale use of concentrated sulfuric acid.

In 1831, British vinegar merchant Peregrine Phillips patented the contact process, which was a far more economical process for producing sulfur trioxide and concentrated sulfuric acid. Today, nearly all of the world's sulfuric acid is produced using this method.

Safety

Laboratory hazardsDrops of 98% sulfuric acid char a piece of tissue paper instantly

The corrosive properties of sulfuric acid are accentuated by its highly exothermic reaction with water. Burns from sulfuric acid are potentially more serious than those of comparable strong acids (e.g. hydrochloric acid), as there is additional tissue damage due to dehydration and particularly secondary thermal damage due to the heat liberated by the reaction with water.

The danger is greater with more concentrated preparations of sulfuric acid, but even the normal laboratory "dilute" grade (approximately 1 M, 10%) will char paper by dehydration if left in contact for a sufficient time. Therefore, solutions equal to or stronger than 1.5 M are labeled

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"CORROSIVE", while solutions greater than 0.5 M but less than 1.5 M are labeled "IRRITANT". Fuming sulfuric acid (oleum) is not recommended for use in schools due to it being quite hazardous.

The standard first aid treatment for acid spills on the skin is, as for other corrosive agents, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes to cool the tissue surrounding the acid burn and to prevent secondary damage. Contaminated clothing is removed immediately and the underlying skin washed thoroughly.

Preparation of the diluted acid can also be dangerous due to the heat released in the dilution process. The concentrated acid is always added to water and not the other way round, to take advantage of the relatively high heat capacity of water. Addition of water to concentrated sulfuric acid leads to the dispersal of a sulfuric acid aerosol or worse, an explosion. Preparation of solutions greater than 6 M (35%) in concentration is most dangerous, as the heat produced may be sufficient to boil the diluted acid: efficient mechanical stirring and external cooling (such as an ice bath) are essential.

On a laboratory scale, sulfuric acid is advantageously diluted by pouring the concentrated version onto crushed ice. The ice used is sufficiently chemically pure so as not to interfere with the intended use of the diluted acid.

Industrial hazards

Although sulfuric acid is non-flammable, contact with metals in the event of a spillage can lead to the liberation of hydrogen gas. The dispersal of acid aerosols and gaseous sulfur dioxide is an additional hazard of fires involving sulfuric acid.

Sulfuric acid is not considered toxic besides its obvious corrosive hazard, and the main occupational risks are skin contact leading to burns (see above) and the inhalation of aerosols. Exposure to aerosols at high concentrations leads to immediate and severe irritation of the eyes, respiratory tract and mucous membranes: this ceases rapidly after exposure, although there is a risk of subsequent pulmonary edema if tissue damage has been more severe. At lower concentrations, the most commonly reported symptom of chronic exposure to sulfuric acid aerosols is erosion of the teeth, found in virtually all studies: indications of possible chronic damage to the respiratory tract are inconclusive as of 1997. In the United States, the permissible exposure limit (PEL) for sulfuric acid is fixed at 1 mg/m³: limits in other countries are similar. Interestingly there have been reports of sulfuric acid ingestion leading to vitamin B12 deficiency with subacute combined degeneration. The spinal cord is most often affected in such cases, but the optic nerves may show demyelination, loss of axons and gliosis.

Legal restrictionsInternational commerce of sulfuric acid is controlled under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances, 1988, which lists sulfuric acid under Table II of the convention as a chemical frequently used in the illicit manufacture of narcotic drugs or psychotropic substances.[6]

In the US sulfuric acid is included in List II of the list of essential or precursor chemicals established pursuant to the Chemical Diversion and Trafficking Act. Accordingly, transactions of sulfuric acid—such as sales, transfers, exports from and imports to the United States—are subject to regulation and monitoring by the Drug Enforcement Administration.[7][8][9]

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Vitriol age and use in popular cultureThe use of sulfuric acid as a weapon in crimes of assault, known as "vitriol throwing", is fairly common in Asian countries (and formerly also in the Western world) and has made its way into novels and short stories.

Examples include the Sherlock Holmes short story The Adventure of the Illustrious Client, by Arthur Conan Doyle; The Love of Long Ago, by Guy de Maupassant; and Brighton Rock by Graham Greene. A band, My Vitriol, take their name from its use as a weapon in Brighton Rock. Australian band Bluejuice's 2007 single "Vitriol" enjoyed wide airplay on the Australian national radio network Triple J, being the second most popular track in 2007 and ultimately reaching #11 in the annual Triple J Hottest 100 countdown in 2007. "Vitriol" was the iTunes Single of the week in January 2008 and ranked number 67 in the Australian Rolling Stone Magazine's 100 Best songs of 2007. The video clip for "Vitriol" won best clip at Sunscreen Video Awards in 2007. An episode of Saturday Night Live hosted by Mel Gibson included a parody Western sketch about "Sheriff Josh Acid," who carries a flask of acid instead of a six shooter. The Batman villain Two-Face was disfigured as a result of a vitriol throw. In the George Orwell novel, 1984, the protagonist swears his resistance to Big Brother and, among other things, promises to resort to splashing a child with sulfuric acid to avoid capture.

In the pilot episode of the television show MacGyver, the eponymous protagonist used a chocolate bar to stop a sulfuric acid leak. This concept was later confirmed to work by the television show MythBusters.

George Rodgers VC, died after accidentally drinking a bottle of Vitriol.

References collected from:-

1. T.M. Orlando, T.B. McCord, G.A Grieves, Icarus 177 (2005) 528–533 2. Edward M. Jones, "Chamber Process Manufacture of Sulfuric Acid", Industrial and Engineering Chemistry, Nov 1950, Vol

42, No. 11, pp 2208-10. 3. Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN

0-7506-3365-4 4. F. A. Carey. "Reactions of Arenes. Electrophilic Aromatic Substitution". On-Line Learning Center for Organic Chemistry.

University of Calgary. http://www.chem.ucalgary.ca/courses/351/Carey/Ch12/ch12-4.html. Retrieved 2008-01-27. 5. Chenier, Philip J. Survey of Industrial Chemistry, pp 45-57. John Wiley & Sons, New York, 1987. ISBN. 6. Annex to Form D ("Red List"), 11th Edition, January 2007 (pg. 4). International Narcotics Control Board. Vienna, Austria;

2007. 7. 66 FR 52670—52675. 17 October 2001. 8. 21 CFR 1309 9. 21 USC, Chapter 13 (Controlled Substances Act)

A New Certificate Chemistry by A Holderness and J Lambert, Heinemann 1976.

Institut National de Recherche et de Sécurité. (1997). "Acide sulfurique". Fiche toxicologique n°30, Paris: INRS, 5 pp. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990. Agamanolis DP. Metabolic and toxic disorders. In: Prayson R, editor. Neuropathology: a volume in the foundations in

diagnostic pathology series. Philadelphia: Elsevier/Churchill Livingstone, 2005; 413-315.

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Testing of HT Transformers. Ensure that the electrode air gap and tilt angle are correctly set and visually inspect inside the

treater for signs of arcing. Inspect the insulators to ensure that they are clean and dry. The flexible HT links should be checked for damage and also fully secured.

Switch on the generator and ensure the up to speed and interlocks lamps are illuminated. If when the generator is started or the output power is increased the generator trips, switch off the generator and remove SKT3. This is the output from the generator to the HT transformer.

Once again switch on the generator, and press the start button. Increase the output potentiometer to maximum (with the "load" removed). Note: The digital meter will not increase a great deal, because there is only a small current being pulled through the inverter.

If the generator appears to run, i.e. the circuit breaker does not trip and an audible "whistling" can be heard, then it is safe to say that the generator is working. Provided all the checks for the GT generators have been first completed.

Switch off the generator, and re-connect the output from the generator to the HT transformer. Then disconnect the output from the HT transformer to the electrode(s).

Once again switch on the generator. If the generator runs then the problem lies within the treater. The electrodes may be damaged, or there must be arcing within the insulators. A thorough check on the treater components is therefore required.

If the generator trips, then the problem must lie within the HT transformer. The transformer will have to replaced, as there are no user serviceable parts inside.

It is also possible to measure the inductance and resistance values of the transformer. See the table below for guidance.

Inductance of HT Transformers.

 HT8 HT3 HT9 HT10

  Secondary

o/cSecondary s/c

Secondary o/c

Secondary

s/c

Secondary

s/cSecondary o/c

TAP’S (WRT 1)

mH mH mH mH Turns mH Turns mH

7 8.86 .22 .068 19.4 .131 50 4.96 24 2.25

6 7.18 .21 .058 15.72 .112 45 3.45 20 1.57

5 5.68 .2 .049 12.42 .097 40 2.22 16 1.01

4 4.35 .2 .040 9.52 .079 35 1.46 13 .66

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3 3.20 .2 .032 7.0 .063 30 1.05 11 

2 2.22 .2 .024 4.86 .046 25 0.71 9 

OUTPUT TO GROUND

0.913H .301 s/c 2.0H / 7.5 s/c 503 2.0H / 7.5

503 14

 HT1 HT2 HT4 HT5

  Secondary

o/c

Secondary

o/c

Secondary

o/c

Secondary

o/c

TAP’S (WRT 1) mH Turns mH Turns mH Turns mH Turns

7 22.1 60 39.2 71       

6 17.97 54 33.0 65       

5 14.21 48 27.2 59 3.19 16 5.35 24

4 11.95 44 22.0 53 2.1 13 3.72 20

3 10.9 42 17.2 47 1.51 11 2.38 16

2 9.89 40 13.2 41 1.01 9 1.57 13

OUTPUT TO GROUND

  1725

7.5

 715 2.0H

/ 7.5

503 7.5 2.0H

wrt= With respect to

s/c= Short circuit

o/c= Open circuit

For 2 x HT8’s, halve the o/c mH

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All inductance readings are only guidelines, and should be taken as an average. NOT exact figures.

 ITW Surface Treatment, Blauenstraße 67-69, 79576 Weil am Rhein, Germany.

Tel : +49 (0)7621 7905514 Fax : +49 (0)7621 7905516

Capacitance and Dissipation / Power Factor Test System (C&DF)

This unique capacitance and power / dissipation factor (tan delta) measuring instrument is an accessory unit to the CPC 100, completing it with the ultimate insulation diagnosis solution. Controlled by the CPC 100, the CPC 100 + CP TD1 combination provides fully automated testing and reporting capabilities for the comprehensive testing within one portable system.

The application of innovative measurement techniques and the use of high precision components in the CP TD1 bring laboratory precision with a rugged design into the field of insulation condition testing. The CP TD1 also offers new test methods such as testing with frequency sweeps. A custom-built trolley allows for practical handling on and off-site along with easy and quick breakdown into portable components.

Features and Benefits

CPC 100 + CP TD1 Capacitance and Dissipation / Power Factor Test System

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Excellent line-frequency suppression in the presence of electrical and magnetic interference Field ruggedness with laboratory precision Integrated test voltage generator for voltages up to 12 kV with variable test frequency of 15

to 400 Hz No need for multiple test sets: one system for multiple tests and all test results stored in one

device and in the same format Portable, lightweight (heaviest component: 29 kg / 64 lbs) and easy to handle by a single

person Frequency Sweeps: testing at different frequencies with switch-mode power amplifier

technique. Automated testing and reporting: test plans and parameters can be prepared off-line on the

computer, reports are generated automatically. Multiple options for detailed analysis like trending and graphing of the results with templates

for MS Excel

Applications Insulation condition: capacitance Cp, dissipation factor (tan delta), power factor Power transformer diagnosis Bushing testing Circuit breakers insulation diagnosis Rotating machines (motors & generators) insulation diagnosis

Additional Information

The following Accessories are further enhance the CPC 100 + CP TD1 Power Package:

CP TC12 Oil Test Cell CP CR500 Compensation Reactor

Secondary Testing & Calibration

CMC 356 Protection Relays, Energy Meters, Transducers and PQ Analyzers

In electrical power systems. This secondary injection test sets and measurement devices are the right choice for Utilities, Industry and Railway as well as for Relay and Measurement Device Manufacturers.

Multifunctional Primary Test System for Substation Commissioning and Maintenance

This unique primary test system CPC 100 allows for automated testing of power transformers, current transformers (CTs) and voltage transformers (VTs), and resistance testing. The patented device comes complete with an integrated PC, allowing stand-alone operation. Its software routines test a wide range of substation equipment, and automatically create customizable reports. The CPC

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100's compact design and the innovative software save testing time and minimize transportation costs.

 Features and Benefits

Wide range: up to 800 A (or 2000 A with current booster) and 2000 V with a frequency range of 15-400 Hz

High precision measuring of analog voltages and currents Ohm meters offer ranges from µOhm to kOhm to allow a wide variety of applications Portable, lightweight and easy to handle: less than one fourth of the weight of equivalent

conventional equipment (29 kg / 64 lbs) One device for multiple application replaces need for multiple test sets and saves training

costs User-friendly interface Automated testing: test plans can be prepared ahead of time in office – this saves time in

field testing phase Automatic report generation Prepared for the future: Tests even of unconventional equipment like Rogowski Coils or

current sensors Unit accessible in a network or with direct PC connection via standard internet protocols

Applications

Current Transformer (CT) Testing

Ratio, burden and polarity Phase and magnitude error Excitation curve Winding resistance Secondary burden Dielectric withstand voltage (2kV AC) CT circuit continuity

Voltage Transformer (VT) Testing

Ratio and polarity Phase and magnitude error Secondary burden Dielectric withstand voltage (2 kV AC) VT circuit continuity

Power Transformer Testing

Ratio Winding resistance Tap change testing Excitation current Short circuit impedance Leakage reactance

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Resistance

Contact resistance Winding resistance Ground resistance Measuring of complex impedances

Additional Information

The CPC 100 Basic Unit can be extended in various ways: By adding the CP TD1 it becomes a flexible insulation diagnosis system.  In combination with the CP CU1 coupling unit the CPC 100 becomes a safe and most accurate system for measuring primary power system parameters, such as line and ground impedances, k-Factor and mutual coupling.

Hipot

Hipot is an abbreviation for high potential. Traditionally, Hipot is a term given to a class of electrical safety testing instruments used to verify electrical insulation in finished appliances, cables or other wired assemblies, printed circuit boards, electric motors, and transformers. Under normal conditions, any electrical device will produce a minimal amount of leakage current due to the voltages and internal capacitance present within the product. Yet due to design flaws or other factors, the insulation in a product can break down, resulting in excessive leakage current flow. This failure condition can cause shock or death to anyone that comes into contact with the faulty product.

A Hipot test (also called a Dielectric Withstand test) verifies that the insulation of a product or component is sufficient to protect the operator from electrical shock. In a typical Hipot test, high voltage is applied between a product's current-carrying conductors and its metallic chassis. The resulting current that flows through the insulation, known as leakage current, is monitored by the hipot tester. The theory behind the test is that if a deliberate over-application of test voltage does not cause the insulation to break down, the product will be safe to use under normal operating conditions—hence the name, Dielectric Withstand test. In addition to over-stressing the insulation, the test can also be performed to detect material and workmanship defects, most importantly small gap spacing between current-carrying conductors and earth ground. When a product is operated under normal conditions, environmental factors such as humidity, dirt, vibration, shock and contaminants can close these small gaps and allow current to flow. This condition can create a shock hazard if the defects are not corrected at the factory. No other test can uncover this type of defect as well as the Dielectric Withstand test. Three types of Hipot tests are commonly used. These three tests differ in the amount of voltage applied and the amount (or nature) of acceptable current flow:

Dielectric breakdown Test. The test voltage is increased until the dielectric fails, or breaks down, allowing too much current to flow. The dielectric is often destroyed by this test so this test is used on a random sample basis. This test allows designers to estimate the breakdown voltage of a product's design.

Dielectric Withstand Test. A standard test voltage is applied (below the established Breakdown Voltage) and the resulting leakage current is monitored. The leakage current must be below a preset limit or the test is considered to have failed. This test is non-destructive and is usually required by safety agencies to be performed as a 100% production line test on all products before they leave the factory.

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Insulation Resistance Test. This test is used to provide a quantifiable resistance value for all of a product's insulation. The test voltage is applied in the same fashion as a standard Hipot test, but is specified to be Direct Current (DC). The voltage and measured current value are used to calculate the resistance of the insulation.

Hipot testerA hipot tester is an electronic device used to verify the electrical insulation in a cable, printed circuit board, electric motor, transformer or other wired assembly. A Hipot tester is used to perform a high potential test. Generally a hipot tester consists of:

1. A source of high voltage, 2. A current meter, 3. A switching matrix used to connect the high voltage source and the current meter to all of the

contact points in a cable.

In addition to these parts a hipot tester may also have a microcontroller and a display to automate the testing process and display the testing results. A hipot tester can be very similar to a cable tester and often the two are combined into a single device. A hipot tester is used to verify that circuits that should be insulated are well isolated. It does this by applying a high voltage between the circuits and making sure no current flows. In a typical wired assembly a hipot test should connect all circuits in common to ground. Then, one by one the tester will disconnect a given circuit from ground and connect that circuit to high voltage. The current that flows is monitored to verify that it is low enough.

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High voltage dc field testing

In 1996, the insulated conductor industry determined that dc withstand testing of the plastic (XLPE) insulation systems either in the cable factory as a routine production test or after installation as the higher voltage proof test was detrimental to the life of the insulation and therefore discontinued recommending dc testing. Medium voltage EPR insulating systems are not subject to the same aging characteristics and, therefore, can be dc tested as required in accordance with the tables below.

When an insulated cable arrives on the job site, the recipient should be able to confidently assume it will attain the designed service life. This means it must arrive free of internal discontinuities in the dielectric such as voids or inclusions, as well as freedom from air pockets at the interfaces between the shielding systems and the dielectric’s surfaces. It is, however, the specter of mechanical damage, or substandard splicing and terminating that could cause the engineers responsible for continuity of service to desire a field applied proof test to establish the cable’s serviceability. The time-honored methods of proof testing in the field involve high potential direct current (dc). The advantage of the dc test is obvious. Since the dc potential does not produce harmful discharge as readily as the ac, it can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects. The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance. Field tests should be utilized to assure freedom of electrical weakness in the circuit caused by such things as mechanical damage, unexpected environmental factors, etc. Field tests should not be used to seek out minute internal discontinuities in the dielectric or faulty shielding systems, all of which should have been eliminated at the factory, nor should the dc potential be excessive such that it would initiate punctures in otherwise good insulation.

For low voltage power and control cables it is general practice to use a megger for checking the reliability of the circuit. This consists essentially of measuring the insulation resistance of the circuit to determine whether or not it is high enough for satisfactory operation.

For higher voltage cables, the megger is not usually satisfactory and the use of high voltage testing equipment is more common. Even at the lower voltages, high voltage dc tests are finding increasing favor. The use of high voltage dc has many advantages over other types of testing procedure.

DC field acceptance testing

It is general practice, and obviously empirical, to relate the field test voltage upon installation to the final factory applied dc potentials by using a factor of 80 percent. The final factory-test voltages can be found in the appropriate industry specifications. This means that prior to being connected to other equipment; solid extruded dielectric insulated shielded cables rated 5kV and up may be given a field acceptance test of about 300 volts per mil. The actual test values recommended for the field acceptance test are presented in the table below. If other equipment is connected it may limit the test voltage, and considerably lower levels more compatible with the complete system

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would be in order.

High voltage field acceptance testprior to being placed in service

Rated VoltagePhase to Phase

dc Hi-Pot Test dc Hi-Pot Test

(15 Minutes)

Wall - mils Kv Wall - mils kV

5000       8000       15000       25000       28000       35000       46000       69000

90        115       175       260       280       345       445       650

25 35 55 80 85 100 130 195

115 140 220 320 345 420 580 650

35 45 65 95 100 125 170 195

Note: If the leakage current quickly stabilizes,the duration may be reduced to 10 minutes.

Test limitations

The dc leakage can be affected by external factors such as heat, humidity, windage, and water level if unshielded and in ducts or conduits, and by internal heating if the cable under test had recently been heavily loaded. These factors make comparisons of periodic data obtained under different test conditions very difficult. If other equipment is connected into the cable circuit this makes it even more difficult. In the event hot poured compound filled splices and terminations are involved, testing should not be performed until they have cooled to room temperature. The relays in high voltage dc test equipment are usually set to operate between 5 and 25 milli amperes leakage. In practice, the shape of the leakage curve, assuming constant voltage, is more important than either the absolute leakage current of a “go or no go” withstand test result.

Test Notes

From the standpoint of safety as well as data interpretation, only qualified personnel should run these high voltage tests. After the voltage has been applied and the test level reached, the leakage current may be recorded at one minute intervals. As long as the leakage current decreases or stays steady after it has leveled off, the cable is considered satisfactory. If the leakage current starts to increase, excluding momentary spurts due to supply-circuit disturbances, trouble may be developing and the test may be extended to see if the rising trend continues. The end point is, of course, the ultimate breakdown. This is manifested by an abrupt increase in the magnitude of the leakage current and a decrease in the test voltage. It should result in relay action to “trip” the set off the line, but this

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assumes the equipment has enough power to maintain the test voltage and supply the normal test current. Since the total current required is a function of cable capacity, condition of dielectric, temperature, end leakage and length, the test engineer must be sure that “relay action” actually signifies a local fault, rather than being merely an indication that the voltage had been applied too quickly or one of the other factors contributing to the total current had been the cause.

At the conclusion of each test, the discharge and grounding of the circuit likewise requires the attention of a qualified test engineer to prevent damage to the insulation and injury to personnel.

Maintenance proof testing

It may be justifiable in the case of important circuits to make periodic tests during the life of the installation to determine whether or not there had been significant deterioration due to severe and perhaps unforeseen operational or environmental conditions. The advantage of a scheduled proof test is, of course, that it can frequently “anticipate” a future service failure, and the necessary repair or renewal can be made without a service interruption, usually during a major shutdown.

Furthermore, a dc test failure is seldom burned-out, and visual analysis may disclose the cause and permit corrective action.

As a note of caution, once a complete circuit has been connected and all exposed ends sealed, it is not desirable when maintenance proof-testing to remove these seals, disconnect the conductors, and it is sometimes impossible to provide “ends” with adequate clearance and length of insulation surface to permit high voltage testing even at the levels specified in the following table. Further, there is the danger of mechanically injuring the dielectric during the seal removal and end preparation. This is a major reason why a “megger test” is often used in maintenance checking of the numerous circuits in a power plant.

High voltage maintenance test forcables in service less than five years*

Rated VoltagePhase to Phase

dc Proof Test(5 Minutes) kV

5000...............8000...............15000...............25000...............28000...............35000...............46000...............69000...............

20.................25.................40.................60.................65.................75.................100.................145.................

*Consult manufacturer when cablesare in service over five years.

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Frequency of tests

In the case of power plants, it is customary to schedule desired maintenance proof tests to coincide with planned major shutdowns. It is not necessary or justifiable to check every circuit each year. The following schedule is suggested as a guide.

Frequency of proof testing

Period After Installation Acceptance Test

Class ofService

1stMaintenance

test

2ndMaintenance

Test

Period BetweenSucceeding

Maintenance Test

LightingNormalCritical

No Test3 years

12 - 18 months

No Test8-9 years2-3 years

None5-6 years4 - 5 years

Transformer types

Transformer with two windings and iron core.

Step-down or step-up transformer. The symbol shows which winding has more turns, but not usually the exact ratio.

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Transformer with three windings. The dots show the relative configuration of the windings.

Transformer with electrostatic screen preventing capacitive coupling between the windings.

A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.

Kind of Transformers 1 Power transformers

o 1.1 Laminated core o 1.2 Toroidal o 1.3 Autotransformer o 1.4 Variac o 1.5 Stray field transformer o 1.6 Polyphase transformers o 1.7 Resonant transformers

1.7.1 Constant voltage transformer o 1.8 Ferrite Core

1.8.1 Planar transformer o 1.9 Oil cooled transformer o 1.10 Isolating Transformer

2 Instrument transformers o 2.1 Current transformers o 2.2 Voltage transformers

3 Pulse transformers 4 RF transformers

o 4.1 Air-core transformers o 4.2 Ferrite-core transformers o 4.3 Transmission-line transformers o 4.4 Baluns

5 Audio transformers o 5.1 Loudspeaker transformers o 5.2 Output transformer (valve) o 5.3 Small Signal transformers o 5.4 ' Interstage ' and coupling transformers o 5.5 Cast resin transformers

6 Homemade & Obsolete Transformers o 6.1 Transformer kits o 6.2 100% homemade o 6.3 Hedgehog o 6.4 Variocouplers

7 Not Transformers

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Power transformers

Laminated core

This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics

Widely available in power ratings ranging from mW to MW Insulated laminations minimize eddy current losses Small appliance and electronic transformers may use a split bobbin, giving a high level of

insulation between the windings Rectangular core Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used. Mumetal shields can be fitted to reduce EMI (electromagnetic interference) A screen winding is occasionally used between the 2 power windings Small appliance and electronics transformers may have a thermal cut out built in Occasionally seen in low profile format for use in restricted spaces laminated core made with silicon steel with high permeability

Toroidal

Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring (and thus threaded through the ring during winding), and tape for insulation.

Toroidals compared to EI core transformers:

Lower external magnetic field Smaller for a given power rating Higher cost in most cases, as winding requires more complex & slower equipment Less robust Central fixing is either

o bolt, large metal washers & rubber pads o bolt & potting resin

Over tightening the central fixing bolt may short the windings Greater inrush current at switch-on.

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Autotransformer

An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230-volt supply to drive 115-volt equipment, or reversed to drive 230-volt equipment from 115 volts. Since the current in the windings is lower, the transformer is smaller, lighter cheaper and more efficient. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than an isolating (two-winding) transformer of the same rating. Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33 kV and 66 kV sub-transmission networks.

In practice, transformer losses mean that autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical. This is true of isolated winding transformers too.

Variac

By exposing part of the winding coils of an autotransformer, and making the secondary connection through a sliding carbon brush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for wide voltage adjustment in very small increments.

Stray field transformer

A Stray field transformer has a significant stray field or a (sometimes adjustable) magnetic bypass in its core. It can act as a transformer with inherent current limitation due to its lower tight coupling between the primary and the secondary winding, which is unwanted in much other cases. The output and input currents are low enough to prevent thermal overload under each load condition - even if the secondary is shortened.

Stray field transformers are used for arc welding and high voltage discharge lamps (cold cathode fluorescent lamps, series connected up to 7,5 kV AC working voltage). It acts both as voltage transformer and magnetic ballast.

Polyphase transformers

For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Delta, Delta-Y, Delta-Delta and Y-Y. A vector group indicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth (grounded), the earth connection point is usually the center point of a Y winding. If the secondary is a Delta winding, the ground may be connected to a center tap on one winding (high leg delta) or one phase may be grounded (corner grounded delta). A special purpose polyphase transformer is the zigzag

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transformer. There are many possible configurations that may involve more or fewer than six windings and various tap connections.

Example of Y Y Connection

Resonant transformersA resonant transformer operates at the resonant frequency of one or more of its coils and (usually) an external capacitor. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. When the primary coil is driven by a periodic source of alternating current, such as a square or sawtooth wave at the resonant frequency, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are used to generate high alternating voltages, and the current available can be much larger than that from electrostatic machines such as the Van de Graaff generator or Wimshurst machine.

Examples:

Tesla coil Oudin coil (or Oudin resonator; named after its inventor Paul Oudin) D'Arsonval apparatus Ignition coil or induction coil used in the ignition system of a petrol engine Flyback transformer of a CRT television set or video monitor. Electrical breakdown and insulation testing of high voltage equipment and cables. In the

latter case, the transformer's secondary is resonated with the cable's capacitance.

Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.

Constant voltage transformer

By arranging particular magnetic properties of a transformer core, and installing a ferro-resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage relatively constant for varying primary supply without additional circuitry or manual adjustment. CVA transformers run hotter than standard power transformers, because regulating action depends on core saturation, which reduces efficiency somewhat. The output waveform is heavily distorted unless careful measures are taken to prevent this. Saturating transformers provide a simple rugged method to stabilize an AC power supply.

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Ferrite Core

Ferrite core power transformers are widely used in switched mode power supplies (SMPSUs). The powder core enables high frequency operation, and hence much smaller size to power ratio than laminated iron transformers.

Ferrite transformers are not usable as power transformers at mains frequency.

Planar transformerA planar transformerExploded view: the spiral primary "winding" on one side of the PCB (the spiral secondary "winding" is on the other side of the PCB)

Manufacturers etch spiral patterns on a printed circuit board to form the "windings" of a planar transformer. (Manufacturers literally wind pieces of wire on some core or bobbin to form the windings of other kinds of transformers).

Some planar transformers are commercially sold as discrete components -- the transformer is the only thing on that printed circuit board. Other planar transformers are one of many components on one large printed circuit board.

much thinner than other transformers, for low-profile applications (even when several PCBs are stacked)

almost all use a ferrite planar core

Oil cooled transformer

For large transformers used in power distribution or electrical substations, the core and coils of the transformer are immersed in oil which cools and insulates. Oil circulates through ducts in the coil and around the coil and core assembly, moved by convection. The oil is cooled by the outside of the tank in small ratings, and in larger ratings an air-cooled radiator is used. Where a higher rating is required, or where the transformer is used in a building or underground, oil pumps are used to circulate the oil and an oil-to-water heat exchanger may also be used.[1] Formerly, indoor transformers required to be fire-resistant used PCB liquids; since these are now banned, substitute fire-resistant liquids such as silicone oils are instead used.

Isolating Transformer

Most transformers isolate, meaning the secondary winding is not connected to the primary. But this isn't true of all transformers.

However the term 'isolating transformer' is normally applied to mains transformers providing isolation rather than voltage transformation. They are simply 1:1 laminated core transformers. Extra voltage tappings are sometimes included, but to earn the name 'isolating transformer' it is expected that they will usually be used at 1:1 ratio.Instrument transformers

Current transformers

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Current transformers used in metering equipment for three-phase 400 ampere electricity supply

A current transformer (CT) is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.

Current transformers are often constructed by passing a single primary turn (either an insulated cable or an un-insulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or have several tap points to provide a range of ratios. Care must be taken that the secondary winding is not disconnected from its load while current flows in the primary, as this will produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer.

Specially constructed wideband CTs are also used, usually with an oscilloscope, to measure high frequency waveforms or pulsed currents within pulsed power systems. One type provides a voltage output that is proportional to the measured current; another, called a Rogowski coil, requires an external integrator in order to provide a proportional output.

Voltage transformers

Voltage transformers (VTs) or potential transformers (PTs) are another type of instrument transformer, used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protection relays.

While VTs were formerly used for all voltages greater than 240V primary, modern meters eliminate the need VTs for most secondary service voltages. VTs are typically used in circuits where the system voltage level is above 600 V. Modern meters eliminate the need of VT's since the voltage remains constant and it is measured in the incoming supply.

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A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a relatively constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other high energy pulsed power applications.

To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors.

The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.

Pulse transformers by definition have a duty cycle of less than 1, whatever energy stored in the coil during the pulse must be "dumped" out before the pulse is fired again.

RF transformersThere are several types of transformer used in radio frequency (RF) work. Steel laminations are not suitable for RF.

Air-core transformers

These are used for high frequency work. The lack of a core means very low inductance. Such transformers may be nothing more than a few turns of wire soldered onto a printed circuit board.

Ferrite-core transformers

Widely used in intermediate frequency (IF) stages in super heterodyne radio receivers. These are mostly tuned transformers, containing a threaded ferrite slug that is screwed in or out to adjust IF tuning. The transformers are usually canned for stability and to reduce interference.

Transmission-line transformers

For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.

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The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.

Baluns

Baluns are transformers designed specifically to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to transmission line transformers in construction and operation.

Audio transformersTransformers in a tube amplifier. Output transformers are on the left. The power supply toroidal transformer is on right.

Audio transformers are usually the factors which limit sound quality when used; electronic circuits with wide frequency response and low distortion are relatively simple to design.

Transformers are also used in DI boxes to convert high-impedance instrument signals (e.g. bass guitar) to low impedance signals to enable them to be connected to a microphone input on the mixing console.

A particularly critical component is the output transformer of an audio power amplifier. Valve circuits for quality reproduction have long been produced with no other (inter-stage) audio transformers, but an output transformer is needed to couple the relatively high impedance (up to a few hundred ohms depending upon configuration) of the output valve(s) to the low impedance of a loudspeaker. (The valves can deliver a low current at a high voltage; the speakers require high current at low voltage.) Most solid-state power amplifiers need no output transformer at all.

For good low-frequency response a relatively large iron core is required; high power handling increases the required core size. Good high-frequency response requires carefully designed and implemented windings without excessive leakage inductance or stray capacitance. All this makes for an expensive component.

Early transistor audio power amplifiers often had output transformers, but they were eliminated as designers discovered how to design amplifiers without them.

Loudspeaker transformers

In the same way that transformers are used to create high voltage power transmission circuits that minimize transmission losses, loudspeaker transformers can be used allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than normal loudspeaker voltages. This application is common in industrial public address applications. Such circuits are commonly referred to as constant voltage speaker systems, although the audio waveform is a

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changing voltage. Such systems are also known by other terms such as 25-, 70- and 100-volt speaker systems, referring to the nominal voltage of the loudspeaker line.

At the audio amplifier, a large audio transformer may be used to step-up the low impedance, low-voltage output of the amplifier to the designed line voltage of the loudspeaker circuit. At the distant loudspeaker location, a smaller step-down transformer returns the voltage and impedance to ordinary loudspeaker levels. The loudspeaker transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in discrete steps.

Output transformer (valve)

Valve (tube) amplifiers almost always use an output transformer to match the high load impedance requirement of the valves (several kilohms) to a low impedance speaker.

Small Signal transformers

Moving coil phonograph cartridges produce a very small voltage. In order for this to be amplified with a reasonable signal-noise ratio, a transformer is usually used to convert the voltage to the range of the more common moving-magnet cartridges.

Microphones may also be matched to their load with a small transformer, which is mumetal shielded to minimize noise pickup. These transformers are less widely used today, as transistorized buffers are now cheaper.

'Interstage' and coupling transformers

A use for interstage transformers is in the case of push-pull amplifiers where an inverted signal is required. Here two secondary windings wired in opposite polarities may be used to drive the output devices. These phase splitting transformers are not much used today.

Cast resin transformers

Cast-resin power transformers have been widely used for a long time. These transformers have the advantage of easy installation and improved fire behavior in case of class. This indoor type transformer is totally dry, without cooling oil.

Homemade & Obsolete Transformers

Transformer kits

Transformers may be wound at home using commercial transformer kits, which contain laminations & bobbin. Or ready made transformers may be disassembled and rewound. These approaches are occasionally used by home constructors, but are usually avoided where possible due to the number of hours required to hand wind a transformer.

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100% homemade

It is possible to make the transformer laminations by hand too. Such transformers are encountered at times in 3rd world countries, using laminations cut from scrap sheet steel, paper slips between the laminations, and string to tie the assembly together. The result works, but is usually noisy due to poor clamping of laminations.

Hedgehog

Hedgehog transformers are occasionally encountered in homemade 1920s radios. They are homemade audio interstage coupling transformers. Enameled copper wire is wound round the central half of the length of a bundle of insulated iron wire (eg florists' wire), to make the windings. The ends of the iron wires are then bent around the electrical winding to complete the magnetic circuit, and the whole is wrapped with tape or string to hold it together. These were sometimes used when the cost of a ready made transformer could not be justified. Inductance tends to be on the low side, with consequent loss of bass. With the speakers of the day this was no bad thing.

Variocouplers

Variocouplers are rf transformers with 2 windings and variable coupling between the windings. They were standard equipment in 1920s radio sets.

Pancake coil variocouplers were common in 1920s radios for variable rf coupling. The 2 planar coils were arranged to swing away from each other and for the angle between them to increase to 90 degrees, thus giving wide variation in coupling. No core was used. These were mostly used to control reaction. The pancake structure was a means to minimize stray capacitance. In another design of variocoupler, 2 coils were wound on a 2 circular bands, and housed one inside the other, with provision for rotating the inner coil. Coupling varies as one coil is rotated between 0 and 90 degrees from the other. These had higher stray capacitance than the pancake type.

Not TransformersFinally there are some items often mistaken for transformers, but which are not always transformers.

Wall warts: small power supplies with integral mains plug. These can contain a transformer and other circuitry. Most use a laminated iron transformer, but an increasing number now contain a small SMPSU. These are smaller and much lighter.

Halogen lighting transformers: Toroidal transformers are sometimes used for this task, but most halogen 'transformers' are SMPSUs. (switched mode power supply unit)

References collected from

1. ^ ANSI IEEE Stanard C57.12.00 General Requirements for Liquid-Immersed Distribution, Power and Regulating Transformers, 2000

Transformer

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A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils or "windings". Except for air-core transformers, the conductors are commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the first or "primary" winding creates a varying magnetic field in the core (or cores) of the transformer. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the "secondary" winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will flow from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary to the number of turns in the primary as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

Buchholz relay

In the field of electric power distribution and transmission, a Buchholz relay, also called a gas relay or a sudden pressure relay, is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.

The relay has two different detection modes. On a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float operated switch in the relay is used to initiate an alarm signal. This same switch will also operate on low oil level, such as a slow oil leak.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage. Buchholz relays have a test port to allow the accumulated gas to be withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as overheating or arcing, whereas air found in the relay may only indicate low oil level or a leak.

Buchholz relays have been applied to large power transformers at least since the 1940's. The relay was first developed by Max Buchholz (1875-1956) in 1921

Names like beechwood relay or beech relay are an indication of incorrectly translated German language manuals.

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First steps: experiments with induction coils

What would become the "transformer principle" was revealed in 1831 by Michael Faraday in his demonstration of electromagnetic induction, but without recognition of its future role in manipulating EMF. The first "induction coils" to see wide use were invented by Rev. Nicholas Callan of Maynooth College, Ireland in 1836, one of the first researchers to realize that the more turns the secondary winding has in relation to the primary winding, the larger the increase in EMF. Induction coils evolved from scientists' and inventors' efforts to get higher voltages from batteries. Rather than alternating current (AC), their action relied upon a vibrating "make-and-break" mechanism that regularly interrupted the flow of direct current (DC) from the batteries. Between the 1830s and the 1870s, efforts to build better induction coils, mostly by trial and error, slowly revealed the basic principles of transformers. Efficient, practical designs did not appear until the 1880s,[1] but within a decade the "transformer" would be instrumental in the "War of Currents", and in seeing AC distribution systems triumph over their DC counterparts, a position in which they have remained dominant ever since.[1]

In 1876, Russian engineer Pavel Yablochkov invented a lighting system based on a set of induction coils where the primary windings were connected to a source of alternating current and the secondary windings could be connected to several "electric candles" (arc lamps) of his own design.[2]

[3] The coils used in the system behaved as primitive transformers. The patent claimed the system could "provide separate supply to several lighting fixtures with different luminous intensities from a single source of electric power".

In 1878, the engineers of the Ganz Company in Hungary assigned part of its extensive engineering works to the manufacture of electric lighting apparatus for Austria-Hungary, and by 1883 made over fifty installations. It offered an entire system consisting of both arc and incandescent lamps, generators, and other accessories.[4]

Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron core called a "secondary generator" in London in 1882, then sold the idea to the Westinghouse company in the United States. They also exhibited the invention in Turin, Italy in 1884, where it was adopted for an electric lighting system.

Induction coils with open magnetic circuits are inefficient for transfer of power to loads. Various methods of adjusting the cores or bypassing magnetic flux around part of a coil were developed, since until about 1880 the paradigm for AC power transmission from a high voltage supply to a low voltage load was a series circuit. In practice, several coils with a ratio near 1:1 were connected with their primaries in series to allow use of a high voltage for transmission while presenting a low voltage to the lamps. The inherent flaw in this method was that turning off a single lamp affected all the others on the circuit, and many adjustable coil designs were introduced in an effort to accommodate this problematic characteristic of the series circuit.[6]

First transformersStanley's 1886 transformer, a redesigned commercial version of the earlier Hungarian "ZBD" transformer

Between 1884 and 1885, Hungarian engineers Zipernowsky, Bláthy and Déri from the Ganz company in Budapest created the efficient "ZBD" closed-core model, which were based on the design by Gaulard and Gibbs. (Gaulard and Gibbs designed just an open core model) [7][8] They

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discovered that all former (coreless or open-core) devices were incapable of regulating voltage, and were therefore impracticable. Their joint patent described a transformer with no poles and comprised two versions of it, the "closed-core transformer" and the "shell-core transformer. In the closed-core transformer the iron core is a closed ring around which the two coils are arranged uniformly. In the shell type transformer, the copper induction cables are passed through the core. In both designs, the magnetic flux linking the primary and secondary coils travels (almost entirely) in the iron core, with no intentional path through air. The core consists of iron cables or plates. Based on this invention, it became possible to provide economical and cheap lighting for industry and households."[9]

Zipernowsky, Bláthy and Déri discovered the mathematical formula of transformers: Vs/Vp = Ns/Np. With this formula, transformers became calculable and proportionable. Their patent application made the first use of the word "transformer", a word that had been coined by Ottó Bláthy. George Westinghouse had bought both Gaulard and Gibbs' and the "ZBD" patents in 1885. He entrusted William Stanley with the building of a ZBD-type transformer for commercial use.[11]

Stanley built the core from interlocking E-shaped iron plates. This design was first used commercially in 1886.

Early developments and applications

Russian engineer Mikhail Dolivo-Dobrovolsky developed the first three-phase transformer in 1889. In 1891 Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for generating very high voltages at high frequency. Audio frequency transformers (at the time called repeating coils) were used by the earliest experimenters in the development of the telephone.

Basic principlesThe transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnitude of the applied magnetic field. The changing magnetic flux extends to the secondary coil where a voltage is induced across its ends.

A simplified transformer design is shown to the left. A current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron; this ensures that most of the magnetic field lines produced by the primary current are within the iron and pass through the secondary coil as well as the primary coil.

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

Where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer,[12] the instantaneous voltage across the primary winding equals

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Taking the ratio of the two equations for VS and VP gives the basic equation[13] for stepping up or stepping down the voltage

Ideal power equation

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power.

P incoming = IPVP = P outgoing = ISVS

giving the ideal transformer equation

If the voltage is increased (stepped up) (VS > VP), then the current is decreased (stepped down) (IS < IP) by the same factor. Transformers are efficient so this formula is a reasonable approximation.

The impedance in one circuit is transformed by the square of the turns ratio.[12] For example, if an impedance ZS is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of . This relationship is reciprocal, so that the impedance ZP of the primary circuit appears to the secondary to be .

Detailed operation

The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance.[14] When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core.[14]. The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.

The changing magnetic field induces an electromotive force (EMF) across each winding.[15] Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP

and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF".[16] This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

Practical considerations

Leakage flux

Leakage flux of a transformer

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The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings.[17] Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings.[16] Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss (see "Stray losses" below), but results in inferior voltage regulation, causing the secondary voltage to

fail to be directly proportional to the primary, particularly under heavy load.[17] Transformers are therefore normally designed to have very low leakage inductance.

However, in some applications, leakage can be a desirable property, and long magnetic paths, air gaps, or magnetic bypass shunts may be deliberately introduced to a transformer's design to limit the short-circuit current it will supply.[16] Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon signs; or for safely handling loads that become periodically short-circuited such as electric arc welders.[18] Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a direct current flowing through the windings.

Effect of frequency

The time-derivative term in Faraday's Law shows that the flux in the core is the integral of the applied voltage. Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time.[20] In practice, the flux would rise to the point where magnetic saturation of the core occurred, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current.

Transformer universal EMF equation

If the flux in the core is sinusoidal, the relationship for either winding between its rms Voltage of the winding E, and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:

The EMF of a transformer at a given flux density increases with frequency. [14] By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation, and fewer turns are needed to achieve the same impedance. However properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight.[21]

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current; at lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from over voltage at higher than rated frequency.

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Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient response of the windings to impulse and switching surge voltages.

Energy losses

An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.

Experimental transformers using superconducting windings achieving efficiencies of 99.85%,[23]

While the increase in efficiency is small, when applied to large heavily-loaded transformers the annual savings in energy losses are significant.

A small transformer, such as a plug-in "wall-wart" or power adapter type used for low-power consumer electronics, may be no more than 85% efficient, with considerable loss even when not supplying any load. Though individual power loss is small, the aggregate losses from the very large number of such devices are coming under increased scrutiny.

The losses vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, which encourages development of low-loss transformers (also see energy efficient transformer).

Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss. Losses in the transformer arise from:

Winding resistance Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.

Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.

Eddy currents Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and Inverse Square of the material thickness.

Magnetostriction Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers,[13] and in turn causes losses due to frictional heating in susceptible cores.

Mechanical losses In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations

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within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.[26]

Stray losses Leakage inductance is by itself lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.[27]

Equivalent circuitThe physical limitations of the practical transformer may be brought together as an equivalent circuit model (shown below) built around an ideal lossless transformer.[28] Power loss in the windings is current-dependent and is represented as in-series resistances RP and RS. Flux leakage results in a fraction of the applied voltage dropped without contributing to the mutual coupling, and thus can be modeled as reactance of each leakage inductance XP and XS in series with the perfectly-coupled region.

Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional to the square of the core flux for operation at a given frequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance RC in parallel with the ideal transformer.

A core with finite permeability requires a magnetizing current IM to maintain the mutual flux in the core. The magnetizing current is in phase with the flux; saturation effects cause the relationship between the two to be non-linear, but for simplicity this effect tends to be ignored in most circuit equivalents. With a sinusoidal supply, the core flux lags the induced EMF by 90° and this effect can be modeled as a magnetizing reactance (reactance of an effective inductance) XM in parallel with the core loss component. RC and XM are sometimes together termed the magnetizing branch of the model. If the secondary winding is made open-circuit, the current I0 taken by the magnetizing branch represents the transformer's no-load current.

The secondary impedance RS and XS is frequently moved (or "referred") to the primary side after multiplying the components by the impedance scaling factor.

Transformer equivalent circuit, with secondary impedances referred to the primary side

The resulting model is sometimes termed the "exact equivalent circuit", though it retains a number of approximations, such as an assumption of linearity. Analysis may be simplified by moving the magnetizing branch to the left of the primary impedance, an implicit assumption that the magnetizing current is low, and then summing primary and referred secondary impedances, resulting in so-called equivalent impedance.

The parameters of equivalent circuit of a transformer can be calculated from the results of two transformer tests: open-circuit test and short-circuit test.

Types

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A wide variety of transformer designs are used for different applications, though they share several common features. Important common transformer types include:

Autotransformer

An autotransformer has only a single winding with two end terminals, plus a third at an intermediate tap point. The primary voltage is applied across two of the terminals, and the secondary voltage taken from one of these and the third terminal. The primary and secondary circuits therefore have a number of windings turns in common.[30] Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. An adjustable autotransformer is made by exposing part of the winding coils and making the secondary connection through a sliding brush, giving a variable turns ratio. [31]

Polyphase transformers

For three-phase supplies, a bank of three individual single-phase transformers can be used, or all three phases can be incorporated as a single three-phase transformer. In this case, the magnetic circuits are connected together, the core thus containing a three-phase flow of flux.[32] A number of winding configurations are possible, giving rise to different attributes and phase shifts.[33] One particular polyphase configuration is the zigzag transformer, used for grounding and in the suppression of harmonic currents.[34]

Leakage transformers

A leakage transformer, also called a stray-field transformer, has a significantly higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, which is sometimes adjustable with a set screw. This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. The output and input currents are low enough to prevent thermal overload under all load conditions – even if the secondary is shorted.

Leakage transformers are used for arc welding and high voltage discharge lamps (neon lamps and cold cathode fluorescent lamps, which are series-connected up to 7.5 kV AC). It acts then both as a voltage transformer and as a magnetic ballast.

Other applications are short-circuit-proof extra-low voltage transformers for toys or doorbell installations.

Resonant transformers

A resonant transformer is a kind of the leakage transformer. It uses the leakage inductance of its secondary windings in combination with external capacitors, to create one or more resonant circuits. Resonant transformers such as the Tesla coil can generate very high voltages, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator. One of the applications of the resonant transformer is for the CCFL inverter. Another application of the resonant transformer is to couple between stages of a super heterodyne receiver, where the selectivity of the receiver is provided by tuned transformers in the intermediate-frequency amplifiers.

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Audio transformers

Audio transformers are those specifically designed for use in audio circuits. They can be used to block radio frequency interference or the DC component of an audio signal, to split or combine audio signals, or to provide impedance matching between high and low impedance circuits, such as between a high impedance tube (valve) amplifier output and a low impedance loudspeaker, or between a high impedance instrument output and the low impedance input of a mixing console.

Such transformers were originally designed to connect different telephone systems to one another while keeping their respective power supplies isolated, and are still commonly used to interconnect professional audio systems or system components.

Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those generated by AC current-carrying conductors. "Hum" is a term commonly used to describe unwanted signals originating from the "mains" power supply (typically 50 or 60 Hz). Audio transformers used for low-level signals, such as those from microphones, often included shielding to protect against extraneous magnetically-coupled signals.

Instrument transformers

Instrument transformers are used for measuring voltge,current, power and energy in electrical systems, and for protection and control. Where a voltage or current is too large to be conveniently measured by an instrument, it can be scaled down to a standardized low value. Instrument transformers isolate measurement and control circuitry from the high currents or voltages present on the circuits being measured or controlled.

A current transformer is a transformer designed to provide a current in its secondary coil proportional to the current flowing in its primary coil. Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are used in high-voltage circuits. They are designed to present a negligible load to the supply being measured, to allow protective relay equipment to be operated at a lower voltage, and to have a precise winding ratio for accurate metering.

ClassificationTransformers can be classified in different ways:

By power capacity: from a fraction of a volt-ampere (VA) to over a thousand MVA; By frequency range: power-, audio-, or radio frequency; By voltage class: from a few volts to hundreds of kilovolts; By cooling type: air cooled, oil filled, fan cooled, or water cooled; By application: such as power supply, impedance matching, output voltage and current

stabilizer, or circuit isolation; By end purpose: distribution, rectifier, arc furnace, amplifier output; By winding turns ratio: step-up, step-down, isolating (equal or near-equal ratio), variable.

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CoresLaminated steel cores

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel.[39] The steel has a permeability many times that of free space, and the core thus serves to greatly reduce the magnetizing current, and confine the flux to a path which closely couples the windings.[40] Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy-current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive to construct. Thin laminations are generally used on high frequency transformers, with some types of very thin steel laminations able to operate up to 10 kHz.

Laminating the core greatly reduces eddy-current losses

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I transformer".[41] Such a design tends to exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap.[41] They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

A steel core's remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remaining magnetism is reduced, usually after a few cycles of the applied alternating current. [42]

Over current protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long, overhead power transmission lines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core and operation of transformer protection devices.[43]

Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or amorphous (non-crystalline) metal alloy. The higher initial cost of the core material is offset over the life of the transformer by its lower losses at light load.[44]

Solid cores

Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above main frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic materials called ferrites are common. Some radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

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Toroidal cores

Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite.[45]

A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited power capacity (see "Classification" above).

Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to a megahertz, to reduce losses, physical size, and weight of switch-mode power supplies. A drawback of toroidal transformer construction is the higher cost of windings. As a consequence, toroidal transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.

Air cores

A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings in close proximity to each other, an arrangement termed an "air-core" transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material.[16] The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution.[16] They have however very high bandwidth, and are frequently employed in radio-frequency applications,[47] for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings.

WindingsThe conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn.[19] For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided litz wire to minimize the skin-effect and proximity effect losses. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-

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uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture.

For signal transformers, the windings may be arranged in a way to minimize leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding. Both the primary and secondary windings on power transformers may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio. The taps may be connected to an automatic on-load tap changer for voltage regulation of distribution circuits. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar.

Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to damp or dirty environments, but at increased manufacturing cost.

Coolant

High temperatures will damage the winding insulation. [50] Small transformers do not generate significant heat and are cooled by air circulation and radiation of heat. Power transformers rated up to several hundred kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. In larger transformers, part of the design problem is removal of heat. Some power transformers are immersed in transformer oil that both cools and insulates the windings. The oil is a highly refined mineral oil that remains stable at transformer operating temperature. Indoor liquid-filled transformers must use a non-flammable liquid, or must be located in fire resistant rooms. Air-cooled dry transformers are preferred for indoor applications even at capacity ratings where oil-cooled construction would be more economical, because their cost is offset by the reduced building construction cost.

The oil-filled tank often has radiators through which the oil circulates by natural convection; some large transformers employ forced circulation of the oil by electric pumps, aided by external fans or water-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled transformers may be equipped with Buchholz relays, which detect gas evolved during internal arcing and rapidly de-energize the transformer to avert catastrophic failure.

Polychlorinated biphenyls have properties that once favored their use as a coolant, though concerns over their environmental persistence led to a widespread ban on their use. Today, non-toxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Before 1977, even transformers that

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were nominally filled only with mineral oils may also have been contaminated with polychlorinated biphenyls at 10-20 ppm. Since mineral oil and PCB fluid mix, maintenance equipment used for both PCB and oil-filled transformers could carry over small amounts of PCB, contaminating oil-filled transformers.

Some "dry" transformers (containing no liquid) are enclosed in sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.

Experimental power transformers in the 2 MVA range have been built with superconducting winding which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

Oil transformerLarge transformers for indoor use must either be of the dry type, that is, containing no liquid, or use a less-flammable liquid.

Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicone-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and a higher flash points than mineral oil.

Transformer oilTransformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

ExplanationThe oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of KVA) may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.

Large, high voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor

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before the cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.

Oil filled transformers with a conservator (an oil tank above the transformer) tend to be equipped with Buchholz relays. These are safety devices that detect the build up of gases (such as acetylene) inside the transformer (a side effect of corona or an electric arc in the windings) and switch off the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.

The flash point (min) and pour point (max) are 140 °C and −6 °C respectively. The dielectric strength of new untreated oil is 12 MV/m (RMS) and after treatment it should be >24 MV/m (RMS).

Oil alternativesLarge transformers for indoor use must either be of the dry type, that is, containing no liquid, or use a less-flammable liquid.

Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicone-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and a higher flash points than mineral oil.

References collected from:-

Less and nonflammable liquid-insulated transformers, approval standard class Number 3990, Factory Mutual Research Corporation, 1997.

McShane C.P. (2001) Relative properties of the new combustion-resistant vegetable oil-based dielectric coolants for distribution and power transformers. IEEE Trans. on Industry Applications, Vol.37, No.4, July/August 2001, pp.1132-1139, No. 0093-9994/01, 2001 IEEE.

“The Environmental technology verification program”, U.S. Environmental Protection Agency, Washington, DC, VS-R-02-02, June 2002. [1]

IEEE Guide for loading mineral-oil-immersed transformers, IEEE Standard C57.91-1995, 1996.

Transformer Maintenance.

Patent for a high temperature transformer

Flash point

The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. At this temperature the vapor may cease to burn when the source of ignition is

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removed. A slightly higher temperature, the fire point, is defined as the temperature at which the vapor continues to burn after being ignited. Neither of these parameters is related to the temperatures of the ignition source or of the burning liquid, which are much higher. The flash point is often used as one descriptive characteristic of liquid fuel, but it is also used to describe liquids that are not used intentionally as fuels.

MechanismEvery flammable liquid has a vapor pressure, which is a function of that liquid's temperature. As the temperature increases, the vapor pressure increases. As the vapor pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence, temperature determines the concentration of evaporated flammable liquid in the air.

Each flammable liquid requires a different concentration of its vapor in air to sustain combustion. The flash point of a flammable liquid is the lowest temperature at which there can be enough flammable vapor to ignite, when an ignition source is applied.

Measuring flash pointsThere are two basic types of flash point measurement: open cup and closed cup.

In open cup devices the sample is contained in an open cup (hence the name) which is heated, and at intervals a flame is brought over the surface. The measured flash point will actually vary with the height of the flame above the liquid surface, and at sufficient height the measured flash point temperature will coincide with the fire point. The best known example is the Cleveland Open Cup (COC).

There are two types of Closed cup testers: non-equilibrium, such as Pensky-Martens where the vapors above the liquid are not in temperature equilibrium with the liquid, and equilibrium, such as Small Scale (commonly known as Setaflash) where the vapors are deemed to be in temperature equilibrium with the liquid. In both these types the cups are sealed with a lid through which the ignition source can be introduced. Closed cup testers normally give lower values for the flash point than Open cup (typically 5-10 °C) and are a better approximation to the temperature at which the vapor pressure reaches the lower flammable limit (LFL).

The flash point is an empirical measurement rather than a fundamental physical parameter. The measured value will vary with equipment and test protocol variations, including temperature ramp rate (in automated testers), time allowed for the sample to equilibrate, sample volume and whether the sample is stirred.

Methods for determining the flash point of a liquid are specified in many standards. For example, testing by the Pensky-Martens closed cup method is detailed in ASTM D93, IP34, ISO 2719, DIN 51758, JIS K2265 and AFNOR M07-019. Determination of flash point by the Small Scale closed cup method is detailed in ASTM D3828 and D3278, EN ISO 3679 and 3680, and IP 523 and 524.

Examples of flash points

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Gasoline (petrol) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.

Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature.

Diesel flash points vary between 126°F and 204°F (52°C-96°C/WJ). Jet fuels also vary greatly. Both Jet A and jet A-1 have flash points between 100°F and 150°F (38°C-66°C/WJ), close to that of off the shelf kerosene. However, both Jet B and FP-4 have flash points between -10°F and +30°F (-23°C - -1°C/WJ)

StandardizationFlash points of substances are measured according to standard test methods. These test methods define the apparatus required to carry out the measurement, key test parameters, the procedure for the operator or automated apparatus to follow, and the precision of the test method. Standard test methods are written and controlled by a number of national and international committees and organizations. The three main bodies are the CEN / ISO Joint Working Group on Flash Point (JWG-FP), ASTM D02.8B Flammability Section and the Energy Institute's TMS SC-B-4 Flammability Panel.

Terminals

Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.[57]

A major application of transformers is to increase voltage before transmitting electrical energy over long distances through wires. Wires have resistance and so dissipate electrical energy at a rate proportional to the square of the current through the wire. By transforming electrical power to a high-voltage (and therefore low-current) form for transmission and back again afterwards, transformers enable economic transmission of power over long distances. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer. Transformers are also used extensively in electronic products to step down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record player s to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. Transformers are also used

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when it is necessary to couple a differential-mode signal to a ground-referenced signal, and for between external cables and internal circuits.

Onsite Transformer Testing

Turn Ratio Test using a Turns Ratio meter (and not 2 – multimeters)2. Transformer Winding Resistance.3. Hipot Test (Over voltage Test)4. Polarization Index Test5. No Load (Magnetization Test)6. Short Circuit or load test7. Frequency Response Test8. Oil Sample Analysis for Gas, water and Dielectric breakdown test9. Tan Delta Test10. Tap changer / Diverter Continuity Test11. Buchholz Relay test12. Temperature indicator calibration13. Earth Continuity

Tap (transformer)

A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. By this means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism.

Voltage considerationsIf only one tap changer is required, tap points are usually made on the high voltage, or low current, side of the winding in order to minimize the current handling requirements of the contacts. However, a transformer may include a tap changer on each winding if there are advantages to doing so. For example, in power distribution networks, a large transformer may have an off-load tap changer on the primary winding and an on-load tap changer on the secondary winding. To minimize the number of windings and thus reduce the physical size of a transformer, a 'reversing' winding may be used, which is a portion of the main winding able to be connected in its opposite direction and thus oppose the voltage. Insulation requirements place the tap points at the low voltage end of the winding. This is near the star point in a star connected winding. In delta connected windings, the tappings are usually at the center of the winding. In an autotransformer, the taps are usually made between the series and common windings, or as a series 'buck-boost' section of the common winding.

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Power Transformers form a very important and critical part of the Electrical Reticulation. Transformers are tested in the factory but are then transported to site. Before switch-on it is advisable to test these transformers. After maintenance or a fault, thorough testing is recommended before switch-on.

Tap changing

Off-circuit designs (DETC)

In low power, low voltage transformers, the tap point can take the form of a connection terminal, requiring a power lead to be disconnected by hand and connected to the new terminal. Alternatively, the process may be assisted by means of a rotary or slider switch.

Since the different tap points are at different voltages, the two connections can not be made simultaneously, as this would short-circuit a number of turns in the winding and produce excessive circulating current. Consequently, the power to the device must be interrupted during the switchover event. Off-circuit or de-energized tap changing (DETC) is sometimes employed in high voltage transformer designs, although for regular use, it is only applicable to installations in which the loss of supply can be tolerated. In power distribution networks, transformers commonly include an off-circuit tap changer on the primary winding to accommodate system variations within a narrow band around the nominal rating. The tap changer will often be set just once, at the time of installation, although it may be changed later during a scheduled outage in order to accommodate a long-term change in the system voltage profile.

On-load designs

A mechanical on-load tap changer (OLTC), also known as under- load tap changer (ULTC) design, changing back and forth between tap positions 2 and 3

For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex on-load tap-changing (OLTC, sometimes LTC) mechanism. On-load tap changers may be generally classified as mechanical, electronically assisted, or fully electronic.

Mechanical tap changers

A mechanical tap changer physically makes the new connection before releasing the old using multiple tap selector switches, but avoids creating high circulating currents by using a diverter switch to temporarily place large diverter impedance in series with the short-circuited turns. This technique overcomes the problems with open or short circuit taps. In a resistance type tap changer, the changeover must be made rapidly to avoid overheating of the diverter. A reactance type tap changer uses a dedicated preventive autotransformer winding to function as the diverter impedance, and a reactance type tap changer is usually designed to sustain off-tap loading indefinitely.

In a typical diverter switch powerful springs are tensioned by a low power motor (motor drive unit (MDU)), and then rapidly released to effect the tap changing operation. To reduce arcing at the contacts, the tap changer operates in a chamber filled with insulating transformer oil, or inside an SF6 vessel. Reactance-type tap changers, when operating in oil, must allow for with the additional inductive flyback generated by the autotransformer and commonly include a vacuum bottle in

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parallel with the diverter switch. During a tap-change operation, the flyback raises the potential between the two electrodes in the bottle, and some of the energy is dissipated in an arc discharge through the bottle instead of flashing across the diverter switch. Some arcing is unavoidable, and both the tap changer oil and the switch contacts will slowly deteriorate with use. In order to prevent contamination of the tank oil and facilitate maintenance operations, the diverter switch usually operates in a separate compartment from the main transformer tank, and often the tap selector switches will be located in the compartment as well. All of the winding taps will then be routed into the tap changer compartment through a terminal array. One possible design (flag type) of on-load mechanical tap changer is shown to the right. It commences operation at tap position 2, with load supplied directly via the right hand connection. Diverter resistor A is short-circuited; diverter B is unused.

In moving to tap 3, the following sequence occurs:

1. Switch 3 closes, an off-load operation. 2. Rotary switch turns, breaking one connection and supplying load current through

diverter resistor A. 3. Rotary switch continues to turn, connecting between contacts A and B. Load now

supplied via diverter resistors A and B, winding turns bridged via A and B. 4. Rotary switch continues to turn, breaking contact with diverter A. Load now supplied

via diverter B alone, winding turns no longer bridged. 5. Rotary switch continues to turn, shorting diverter B. Load now supplied directly via

left hand connection. Diverter A is unused. 6. Switch 2 opens, an off-load operation.

The sequence is then carried out in reverse to return to tap position 2.

Thyristor-assisted tap changers

Thyristor-assisted tap changers use thyristors to take the on-load current while the main contacts change over from one tap to the next. This prevents arcing on the main contacts and can lead to a longer service life between maintenance activities. The disadvantage is that these tap changers are more complex and require a low voltage power supply for the thyristor circuitry. They also can be more costly.

Solid state (thyristor) tap changers

These are a relatively recent development which uses thyristors both to switch the load current and to pass the load current in the steady state. Their disadvantage is that all of the non-conducting thyristors connected to the unselected taps still dissipate power due to their leakage current and they have smaller short circuit withstand capacity. This power can add up to a few kilowatts which has to be removed as heat and leads to a reduction in the overall efficiency of the transformer, in exchange for a compact design that reduces the size and weight of the tap changer device. Solid state tap changers are typically employed only on smaller power transformers.

References

Hindmarsh, J. (1984). Electrical Machines and their Applications, 4th ed.. Pergamon. ISBN 0-08-030572-5. Central Electricity Generating Board (1982). Modern Power Station Practice: Volume 4. Pergamon. ISBN 0-08-016436-6. Rensi, Randolph (June 1995), "Why transformer buyers must understand LTCs", Electrical World.

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Switchgears

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. Switchgear is already a plural, much like the software term code/codes, and is never used as switchgears.

The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

Substations

Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.

For industrial applications, a transformer and switchgear line-up may be combined in one housing, called a unit substation.

HousingSwitchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas- [or oil- or vacuum-] insulated switchgear used for transmission-level voltages saves space, although it has a higher equipment cost.

At small substations, switches may be manually operated, but at important switching stations on the transmission network all devices have motor operators to allow for remote control.

TypesA piece of switchgear may be a simple open air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6) sulfur hexafluoride gas. Other common types are oil [or vacuum] insulated switchgear.

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Circuit breakers are a special type of switchgear that are able to interrupt fault currents. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps. The quenching of the arc when the contacts open requires careful design, and falls into four types:

Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionise other than the contact material), so the arc quenches when it is stretched a very small amount (<2-3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device.

Several different classifications of switchgear can be made.

By the current rating. By interrupting rating (maximum short circuit current that the device can safely interrupt)

o Circuit breakers can open and close on fault currents o Load-break/Load-make switches can switch normal system load currents o Isolators may only be operated while the circuit is dead, or the load current is very

small. By voltage class:

o Low voltage (less than 1000 volts AC) o Medium voltage (1000-35,000 volts AC) o High voltage (more than 35,000 volts AC)

By insulating medium: o Air o Gas (SF6 or mixtures) o Oil o Vacuum

By construction type: o Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type) o Outdoor o Industrial o Utility o Marine o Draw-out elements (removable without many tools) o Fixed elements (bolted fasteners) o Live-front o Dead-front o Open o Metal-enclosed o Metal-clad

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o Metal enclose & Metal clad o Arc-resistant o By IEC degree of internal separation [2]

No Separation (Form 1) Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b) Terminals for external conductors separated from busbars (Form 2b, 3b, 4a,

4b) Terminals for external conductors separated from functional units but not

from each other (Form 3a, 3b) Functional units separated from each other (Form 3a, 3b, 4a, 4b) Terminals for external conductors separated from each other (Form 4a, 4b) Terminals for external conductors separate from their associated functional

unit (Form 4b) By interrupting device:

o Fuses o Air Blast Circuit Breaker o Minimum Oil Circuit Breaker o Oil Circuit Breaker o Vacuum Circuit Breaker o Gas (SF6) Circuit breaker

By operating method: o Manually-operated o Motor-operated o Solenoid/stored energy operated

By type of current: o Alternating current o Direct current

By application: o Transmission system o Distribution.

A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.

Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.

Functions One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear also is used to enhance system availability by allowing more than one source to feed a load.

SafetyTo help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. James Harry Castell invented this technique in 1922. For example, if only one of two sources of supply is permitted to be connected at a given time, the interlock scheme may

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require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.

Indoor switchgear can also be type tested for internal arc containment. This test is important for user safety as modern switchgear is capable of switching large currents.

References

1. ^ Robert W. Smeaton (ed) Switchgear and Control Handbook 3rd Ed., Mc Graw Hill, new York 1997 ISBN 0-07-058451-6 2. ^ IEC Standard EN 60439 part 1 Table 6A

Diode Bridge

A diode bridge or bridge rectifier is an arrangement of four diodes in a bridge configuration that provides the same polarity of output voltage for either polarity of input voltage. When used in its most common application, for conversion of alternating current (AC) input into direct current (DC) output, it is known as a bridge rectifier. A bridge rectifier provides full-wave rectification from a two-wire AC input, resulting in lower cost and weight as compared to a center-tapped transformer design.[1]

The essential feature of a diode bridge is that the polarity of the output is the same regardless of the polarity at the input. The diode bridge circuit is also known as the Graetz circuit after its inventor, physicist Leo Graetz.

Basic operationAccording to the conventional model of current flow originally established by Benjamin Franklin and still followed by most engineers today, current is assumed to flow through electrical conductors from the positive to the negative pole.[2] In actuality, free electrons in a conductor nearly always flow from the negative to the positive pole. In the vast majority of applications, however, the actual direction of current flow is irrelevant. Therefore, in the discussion below the conventional model is retained.

In the diagrams below, when the input connected to the left corner of the diamond is positive, and the input connected to the right corner is negative, current flows from the upper supply terminal to the right along the red (positive) path to the output, and returns to the lower supply terminal via the blue (negative) path.

When the input connected to the left corner is negative, and the input connected to the right corner is positive, current flows from the lower supply terminal to the right along the red path to the output, and returns to the upper supply terminal via the blue path.[3]

AC, half-wave and full wave rectified signals

In each case, the upper right output remains positive and lower right output negative. Since this is true whether the input is AC or DC, this circuit not only produces a DC output from an AC input, it can also provide what is sometimes called "reverse polarity protection". That is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards, or when the

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leads (wires) from a DC power source have been reversed, and protects the equipment from potential damage caused by reverse polarity.

Prior to the availability of integrated circuits, a bridge rectifier was constructed from "discrete components", i.e., separate diodes. Since about 1950, a single four-terminal component containing the four diodes connected in a bridge configuration became a standard commercial component and is now available with various voltage and current ratings.

Output smoothingFor many applications, especially with single phase AC where the full-wave bridge serves to convert an AC input into a DC output, the addition of a capacitor may be desired because the bridge alone supplies an output of fixed polarity but continuously varying or "pulsating" magnitude (see diagram above).

The function of this capacitor, known as a reservoir capacitor (or smoothing capacitor) is to lessen the variation in (or 'smooth') the rectified AC output voltage waveform from the bridge. One explanation of 'smoothing' is that the capacitor provides a low impedance path to the AC component of the output, reducing the AC voltage across, and AC current through, the resistive load. In less technical terms, any drop in the output voltage and current of the bridge tends to be canceled by loss of charge in the capacitor. This charge flows out as additional current through the load. Thus the change of load current and voltage is reduced relative to what would occur without the capacitor. Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage / current.

The simplified circuit shown has a well-deserved reputation for being dangerous, because, in some applications, the capacitor can retain a lethal charge after the AC power source is removed. If supplying a dangerous voltage, a practical circuit should include a reliable way to safely discharge the capacitor. If the normal load cannot be guaranteed to perform this function, perhaps because it can be disconnected, the circuit should include a bleeder resistor connected as close as practical across the capacitor. This resistor should consume a current large enough to discharge the capacitor in a reasonable time, but small enough to minimize unnecessary power waste.

Because a bleeder sets a minimum current drain, the regulation of the circuit, defined as percentage voltage change from minimum to maximum load, is improved. However in many cases the improvement is of insignificant magnitude.

The capacitor and the load resistance have a typical time constant τ = RC where C and R are the capacitance and load resistance respectively. As long as the load resistor is large enough so that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a smoothed DC voltage across the load.

In some designs, a series resistor at the load side of the capacitor is added. The smoothing can then be improved by adding additional stages of capacitor–resistor pairs, often done only for sub-supplies to critical high-gain circuits that tend to be sensitive to supply voltage noise.

The idealized waveforms shown above are seen for both voltage and current when the load on the bridge is resistive. When the load includes a smoothing capacitor, both the voltage and the current waveforms will be greatly changed. While the voltage is smoothed, as described above, current will

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flow through the bridge only during the time when the input voltage is greater than the capacitor voltage. For example, if the load draws an average current of n Amps, and the diodes conduct for 10% of the time, the average diode current during conduction must be 10n Amps. This non-sinusoidal current leads to harmonic distortion and a poor power factor in the AC supply.

In a practical circuit, when a capacitor is directly connected to the output of a bridge, the bridge diodes must be sized to withstand the current surge that occurs when the power is turned on at the peak of the AC voltage and the capacitor is fully discharged. Sometimes a small series resistor is included before the capacitor to limit this current, though in most applications the power supply transformer's resistance is already sufficient. Output can also be smoothed using a choke and second capacitor. The choke tends to keep the current (rather than the voltage) more constant. This design is not generally used in modern equipment due to the high cost of an effective choke compared to a resistor and capacitor.

Some early console radios created the speaker's constant field with the current from the high voltage ("B +") power supply, which was then routed to the consuming circuits, (permanent magnets were then too weak for good performance) to create the speaker's constant magnetic field. The speaker field coil thus performed 2 jobs in one: it acted as a choke, filtering the power supply, and it produced the magnetic field to operate the speaker.

Polyphase diode bridgesThe diode bridge can be generalized to rectify polyphase AC inputs. For example, for a three-phase AC input, a half-wave rectifier consists of three diodes, but a full-wave bridge rectifier consists of six diodes.

Three phase bridge rectifier.3-phase AC input waveform. Half-wave and full-wave rectified waveforms. Three-phase bridge rectifier for a wind turbine.

Lead-acid batteryLead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having the second lowest energy-to-weight ratio (next to the nickel-iron battery) and a correspondingly low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in cars to provide the high current required by automobile starter motors.

ElectrochemistryEach cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33.5% v/v (6 Molar) sulphuric acid (H2SO4). In the discharged state both electrodes turn into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulphuric acid and

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becomes primarily water. Due to the freezing-point depression of water, as the battery discharges and the concentration of sulphuric acid decreases, the electrolyte is more likely to freeze.

The chemical reactions are (charged to discharge):

Anode (oxidation):

Cathode (reduction):

Because of the open cells with liquid electrolyte in most lead-acid batteries, overcharging with excessive charging voltages will generate oxygen and hydrogen gas by electrolysis of water, forming an explosive mix. The acid electrolyte is also corrosive.

Practical cells are usually not made with pure lead but have small amounts of antimony, tin, calcium or selenium alloyed in the plate material.

Voltages for common usagesThese are general voltage ranges for six-cell lead-acid batteries:

Open-circuit (quiescent) at full charge: 12.6 V to 12.8 V (2.10-2.13V per cell) Open-circuit at full discharge: 11.8 V to 12.0 V Loaded at full discharge: 10.5 V. Continuous-preservation (float) charging: 13.4 V for gelled electrolyte; 13.5 V for AGM

(absorbed glass mat) and 13.8 V for flooded cells

1. All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes. 2. Float voltage recommendations vary, according to the manufacturer's recommendation. 3. Precise (±0.05 V) float voltage is critical to longevity; too low (sulfation) is almost as bad as

too high (corrosion and electrolyte loss)

Typical (daily) charging: 14.2 V to 14.5 V (depending on manufacturer's recommendation) Equalization charging (for flooded lead acids): 15 V for no more than 2 hours. Battery

temperature must be monitored. Gassing threshold: 14.4 V After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.

Measuring the charge levelBecause the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries. It is relatively simple to determine the state of charge by merely measuring the specific gravity (S.G.) of the electrolyte, the S.G. falling as the battery discharges. Some battery designs have a simple hydrometer built in using colored floating balls of differing density. When used in diesel-electric submarines, the S.G. was regularly measured and written on a blackboard in the control room to apprise the commander as to how much underwater endurance the boat had remaining.

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Relative density

Relative density, sometimes called specific mass or specific gravity,[1][2] is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in modern scientific usage.

If a substance's relative density is less than one then it is less dense than the reference; if greater than one then it is denser than the reference. If the relative density is exactly one then the densities are equal; that is, equal volumes of the two substances have the same mass.

Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm equal to 101.325 kPa. Where it is not it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice the specific gravity as specified above is multiplied by 1000.[3] Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines, sugar solutions (syrups, juices, honeys, brewers wort, must etc.) and acids.

Basic formulasRelative density (RD) or specific gravity (SG) is a dimensionless quantity, as it is the ratio of either densities or weights

where RD is relative density, ρsubstance is the density of the substance being measured, and ρreference is the density of the reference. (By convention ρ, the Greek letter rho, denotes density.)

The reference material can be indicated using subscripts: RDsubstance/reference, which means "the relative density of substance with respect to reference". If the reference is not explicitly stated then it is normally assumed to be water at 4 °C (or, more precisely, 3.98 °C, which is the temperature at which water reaches its maximum density). In SI units, the density of water is (approximately) 1000 kg/m³ or 1 g/cm³, which makes relative density calculations particularly convenient: the density of the object only needs to be divided by 1000 or 1, depending on the units.

The relative density of gases is often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3. Relative density with respect to air can be obtained by

Where M is the molar mass and the approximately equal sign is used because equality pertains only if 1 mol of the gas and 1 mol of air occupy the same volume at a given temperature and pressure i.e. they are both Ideal gasses. Ideal behaviour is usually only seen at very low pressure. For example,

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one mol of an ideal gas occupies 22.414 L at 0 °C and 1 atmosphere whereas carbon dioxide has a molar volume of 22.259 L under those same conditions.

Temperature dependenceThe density of substances varies with temperature and pressure so that it is necessary to specify the temperatures and pressures at which the densities or weights were determined. It is nearly always the case that measurements are made at nominally 1 atmosphere (1013.25 mb ± the variations caused by changing weather patterns) but as specific gravity usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products) variations in density caused by pressure are usually neglected at least where apparent specific gravity is being measured. For true (in vacuo) specific gravity calculations air pressure must be considered (see below). Temperatures are specified by the notation Ts/Tr) with Ts representing the temperature at which the sample's density was determined and Tr the temperature at which the reference (water) density is specified. For example SG (20°C/4°C) would be understood to mean that the density of the sample was determined at 20 °C and of the water at 4°C. Taking into account different sample and reference temperatures we note that while SGH2O = 1.000000 (20°C/20°C) it is also the case that SGH2O = 0.998203/0.998840 = 0.998363 (20°C/4°C). Here temperature is being specified using the current ITS-90 scale and the densities[4] used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale the densities at 20 °C and 4 °C are, respectively, 0.9982071 and 0.9999720 resulting in an SG (20°C/4°C) value for water of 0.9982343.

The temperatures of the two materials may be explicitly stated in the density symbols; for example:

relative density: or specific gravity:

where the superscript indicates the temperature at which the density of the material is measured, and the subscript indicates the temperature of the reference substance to which it is compared.

UsesRelative density can also help quantify the buoyancy of a substance in a fluid, or determine the density of an unknown substance from the known density of another. Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones. Water is preferred as the reference because measurements are then easy to carry out in the field (see below for examples of measurement methods).

As the principal use of specific gravity measurements in industry is determination of the concentrations of substances in aqueous solutions and these are found in tables of SG vs concentration it is extremely important that the analyst enter the table with the correct form of specific gravity. For example, in the brewing industry, the Plato table, which lists sucrose concentration by weight against true SG, were originally (20°C/4°C)[5] i.e. based on measurements of the density of sucrose solutions made at laboratory temperature (20 °C) but referenced to the density of water at 4 °C which is very close to the temperature at which water has its maximum density of ρ(H2O) equal to 0.999972 g·cm−3 or SI units (or 62.43 lbm·ft−3 in United States customary units). The ASBC table[6] in use today in North America, while it is derived from the original Plato table is for apparent specific gravity measurements at (20°C/20°C) on the IPTS-68 scale where the density of

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water is 0.9982071 g·cm−3. In the sugar, soft drink, honey, fruit juice and related industries sucrose concentration by weight is taken from a table prepared by A. Brix which uses SG (17.5°C/17.5°CC)[1]. As a final example, the British SG units are based on reference and sample temperatures of 60F and are thus (15.56°C/15.56°C)[1].

MeasurementRelative density can be calculated directly by measuring the density of a sample and dividing it by the (known) density of the reference substance. The density of the sample is simply its mass divided by its volume. Although mass is easy to measure, the volume of an irregularly shaped sample can be more difficult to ascertain. One method is to put the sample in a water-filled graduated cylinder and read off how much water it displaces. Alternatively the container can be filled to the brim, the sample immersed, and the volume of overflow measured. The surface tension of the water may keep a significant amount of water from overflowing, which is especially problematic for small samples. For this reason it is desirable to use a water container with as small a mouth as possible.

For each substance, the density, ρ, is given by

When these densities are divided, references to the spring constant, gravity and cross-sectional area simply cancel, leaving

Relative density is more easily and perhaps more accurately measured without measuring volume. Using a spring scale, the sample is weighed first in air and then in water. Relative density (with respect to water) can then be calculated using the following formula:

where

Wair is the weight of the sample in air (measured in pounds-force, newtons, or some other unit of force) Wwater is the weight of the sample in water (measured in the same units).

This technique cannot easily be used to measure relative densities less than one, because the sample will then float. Wwater becomes a negative quantity, representing the force needed to keep the sample underwater.

Another practical method uses three measurements. The sample is weighed dry. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the

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displaced water has overflowed and been removed. Subtracting the last reading from the sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method works with scales that can't easily accommodate a suspended sample, and also allows for measurement of samples that are less dense than water.

Relative density and hydrometers

The relative density of a liquid can be measured using a hydrometer. This consists of a bulb attached to a stalk of constant cross-sectional area, as shown in the diagram to the right.

First the hydrometer is floated in the reference liquid (shown in light blue), and the displacement (the level of the liquid on the stalk) is marked (blue line). The reference could be any liquid, but in practice it is usually water.

The hydrometer is then floated in a liquid of unknown density (shown in green). The change in displacement, Δx, is noted. In the example depicted, the hydrometer has dropped slightly in the green liquid; hence its density is lower than that of the reference liquid. It is, of course, necessary that the hydrometer floats in both liquids.

The application of simple physical principles allows the relative density of the unknown liquid to be calculated from the change in displacement. (In practice the stalk of the hydrometer is pre-marked with graduations to facilitate this measurement.)

In the explanation that follows,

ρref is the known density (mass per unit volume) of the reference liquid (typically water). ρnew is the unknown density of the new (green) liquid. RDnew/ref is the relative density of the new liquid with respect to the reference. V is the volume of reference liquid displaced, i.e. the red volume in the diagram. m is the mass of the entire hydrometer. g is the local gravitational constant. Δx is the change in displacement. In accordance with the way in which hydrometers are usually graduated, Δx is here taken to be negative if the displacement line rises on the stalk of the hydrometer, and positive if it falls. In the example depicted, Δx is negative. A is the cross sectional area of the shaft.

Since the floating hydrometer is in static equilibrium, the downward gravitational force acting upon it must exactly balance the upward buoyancy force. The gravitational force acting on the hydrometer is simply its weight, mg. From the Archimedes buoyancy principle, the buoyancy force acting on the hydrometer is equal to the weight of liquid displaced. This weight is equal to the mass of liquid displaced multiplied by g, which in the case of the reference liquid is ρrefVg. Setting these equal, we have

or just

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Exactly the same equation applies when the hydrometer is floating in the liquid being measured, except that the new volume is V - AΔx (see note above about the sign of Δx). Thus,

Combining (1) and (2) yields

But from (1) we have V = m/ρref. Substituting into (3) gives

This equation allows the relative density to be calculated from the change in displacement, the known density of the reference liquid, and the known properties of the hydrometer. If Δx is small then, as a first-order approximation of the geometric series equation (4) can be written as:

This shows that, for small Δx, changes in displacement are approximately proportional to changes in relative density.

References

1. ^ Dana, Edward Salisbury (1922). A text-book of mineralogy: with an extended treatise on crystallography.... New York, London(Chapman Hall): John Wiley and Sons. pp. 195 - 200, 316. http://books.google.com/books?id=rCwaAAAAYAAJ&pg=PA156.

2. ^ Schetz, Joseph A.; Allen E. Fuhs (1999-02-05). Fundamentals of fluid mechanics. Wiley, John & Sons, Incorporated. pp. 111,142,144,147,109,155,157,160,175. ISBN 0471348562. http://books.google.com/books?id=YCSSolzuu9IC&pg=PP1.

3. ^ Hough, J.S., Briggs, D.E., Stevens, R and Young, T.W. Malting and Brewing Science, Vol. II Hopped Wort and Beer, Chapman and Hall, London, 1991, p. 881

4. ^ Bettin, H.; Spieweck, F.: (1990) (in German). Die Dichte des Wassers als Funktion der Temperatur nach Einführung des Internationalen Temperaturskala von 1990. PTB=Mitt. 100. pp. 195-196.

5. ^ ASBC Methods of Analysis Preface to Table 1: Extract in Wort and Beer, American Society of Brewing Chemists, St Paul, 2009

6. ^ ASBC Methods of Analysis op. cit. Table 1: Extract in Wort and Beer 7. ^ DIN51 757 (04.1994): Testing of mineral oils and related materials; determination of density

Construction of battery

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Plates

The principle of the lead acid cell can be demonstrated with simple sheet lead plates for the two electrodes. However such a construction would only produce around an amp for roughly postcard sized plates, and it would not produce such a current for more than a few minutes.

Gaston Planté realized that a plate construction was required that gave a much larger effective surface area. Planté's method of producing the plates has been largely unchanged and is still used in stationary applications.

The Faure pasted-plate construction is typical of automotive batteries. Each plate consists of a rectangular lead grid alloyed with antimony or calcium to improve the mechanical characteristics. The holes of the grid are filled with a mixture of red lead and 33% dilute sulphuric acid. (Different manufacturers have modified the mixture). The paste is pressed into the holes in the plates which are slightly tapered on both sides to assist in retention of the paste. This porous paste allows the acid to react with the lead inside the plate, increasing the surface area many fold. At this stage the positive and negative plates are similar; however expanders and additives vary their internal chemistry to assist in operation when in use. Once dry, the plates are then stacked together with suitable separators and inserted in the battery container. An odd number of plates is usually used, with one more negative plate than positive. Each alternate plate is connected together. After the acid has been added to the cell, the cell is given its first forming charge. The positive plates gradually turn the chocolate brown colour of lead dioxide, and the negative turn the slate gray of 'spongy' lead. Such a cell is ready to be used. Modern manufacturing methods invariably produce the positive and negative plates ready formed, so that it is only necessary to add the sulphuric acid and the battery is ready for use.

One of the problems with the plates is that the plates increase in size as the active material absorbs sulfate from the acid during discharge, and decrease as they give up the sulfate during charging. This causes the plates to gradually shed the paste during their life. It is important that there is plenty of room underneath the plates to catch this shed material. If this material reaches the plates a shorted cell will occur.

The paste material used to make battery plates also contains carbon black, blanc fixe (barium sulfate) and lignosulfonate. The blanc fixe acts as a seed crystal for the lead to lead sulfate reaction. The blanc fixe must be fully dispersed in the paste in order for it to be effective. The lignosulfonate prevents the negative plate from forming a solid mass of lead sulfate during the discharge cycle. It enables the formation of long needle like crystals. The long crystals have more surface area and are easily converted back to the original state on charging. Carbon black counteracts the effect of inhibiting formation caused by the lignosulfonates. It has been found that sulfonated naphthalene condensate dispersant is a more effective expander than lignosulfonate and can be used to speed up the formation of the battery plate. This dispersant is believed to function to improve dispersion of barium sulfate in the paste, reduce hydro set time, produce a stronger plate which is resistant to plate breakage, to reduce fine lead particles and thereby improve handling and pasting characteristics. It extends the life of the battery by increasing the end of charge voltage. The sulfonated naphthalene condensate polymer dispersant can be used in about one-half to one-third the amount of lignosulfonate and is stable to higher temperatures than lignosulfonate [1]

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About 60% of the weight of an automotive-type lead-acid battery rated around 60 Ah (8.7 kg of a 14.5 kg battery) is lead or internal parts made of lead; the balance is electrolyte, separators, and the case. [2]

Separators

Separators are used between the positive and negative plates of a lead acid battery to prevent short circuit through physical contact, mostly through dendrites (‘treeing’), but also through shedding of the active material.

Separators obstruct the flow of ions between the plates and increase the internal resistance of the cell.

Various materials have been used to make separators:

wood rubber glass fiber mat cellulose sintered PVC microporous PVC/polyethylene.

An effective separator must possess a number of mechanical properties; applicable considerations include permeability, porosity, pore size distribution, specific surface area, mechanical design and strength, electrical resistance, ionic conductivity, and chemical compatibility with the electrolyte. In service, the separator must have good resistance to acid and oxidation. The area of the separator must be a little larger than the area of the plates to prevent material shorting between the plates. The separators must remain stable over the operating temperature range of the battery.

Wooden separators were originally used, but deteriorated in the acid electrolyte. Rubber separators were stable in the battery acid.

Applications Wet cell stand-by (stationary) batteries designed for deep discharge are commonly used in

large backup power supplies for telephone and computer centers, grid energy storage, and off-grid household electric power systems.

Traction (propulsion) batteries are used for in golf carts and other battery electric vehicles. In vehicles such as forklifts, the battery can be used as a counterweight.

Motor vehicle starting, lighting and ignition (SLI) batteries (car batteries) provides current for starting internal combustion engines.

Gel batteries are used in back-up power supplies for alarm and smaller computer systems (particularly in uninterruptible power supplies) and for electric scooters, electrified bicycles and marine applications. Unlike wet cells, gel cells are sealed, with pressure relief valves in case of overcharging. In normal use they cannot spill liquid electrolyte.

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Absorbed glass mat (AGM) cells are also sealed and used in battery electric vehicles, as well as applications where there is a fairly high risk of the battery being laid on its side or over-turned, such as motorcycles.

Lead-acid batteries were used to supply the filament (heater) voltage (usually between 2 and 12 volts with 2 V being most common) in early vacuum tube (valve) radio receivers.

Lead-acid batteries are used in emergency lighting in case of power failure.

Large lead-acid batteries are also used to power the electric motors in diesel-electric (conventional) submarines and are used on nuclear submarines as well.

Cycles

Starting batteries

Lead acid batteries designed for starting automotive engines are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge. Repeated deep discharges will result in capacity loss and ultimately in premature failure, as the electrodes disintegrate due to mechanical stresses that arise from cycling. A common misconception is that starting batteries should always be kept on float charge. In reality, this practice will encourage corrosion in the electrodes and result in premature failure. Starting batteries should be kept open-circuit but charged regularly (at least once every two weeks) to prevent sulfation.

Deep cycle batteries

Specially designed deep-cycle cells are much less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged, such as photovoltaic systems, electric vehicles (forklift, golf cart, electric cars and other) and uninterruptible power supplies. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging.

Marine/Motor home batteries, sometimes called "leisure batteries", are something of a compromise between the two, able to be discharged to a greater degree than automotive batteries, but less so than deep cycle batteries.

Fast and slow charge and discharge

The capacity of a lead-acid battery is not a fixed quantity but varies according to how quickly it is discharged. An empirical relationship exists between discharge rate and capacity, known as Peukert's law.

When a battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, these chemicals at the interface, which we will call an "interface charge", spread by diffusion throughout the volume of the active material.

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If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. After a few hours this interface charge will spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start the car.

On the other hand, if the battery is given a slow charge, which takes longer, then the battery will become more fully charged, since then the interface charge has time to redistribute to the volume of the electrodes and electrolyte, and yet is replenished by the charger.

Similarly, if a battery is subject to a fast discharge (such as starting a car, which is a draw of some 200 amps) for a few minutes, it will appear to go dead. Most likely it has only lost its interface charge; after a wait of a few minutes it should appear to be operative. On the other hand, if a battery is subject to a slow discharge (such as leaving the car lights on, which is a draw of only 6 amps), then when the battery appears to be dead it likely has been completely discharged.

Valve regulated lead acid batteriesThe Valve Regulated Lead Acid (VRLA) battery is one of many types of lead-acid batteries. In a VRLA battery the hydrogen and oxygen produced in the cells largely recombine back into water. In this way there is minimal leakage, though some electrolyte still escapes if the recombination cannot keep up with gas evolution. Since VRLA batteries do not require (and make impossible) regular checking of the electrolyte level, they have been called Maintenance Free (MF) batteries. However, this is somewhat of a misnomer. VRLA cells do require maintenance. As electrolyte is lost, VRLA cells may experience "dry-out" and lose capacity. This can be detected by taking regular internal resistance, conductance or impedance measurements of cells. This type of testing should be conducted on a regular basis, as an indicator that more involved testing and maintenance may be required. Recent maintenance procedures have been developed allowing "dehydration" of cells that have experienced dry-out, often restoring significant amounts of the lost capacity.

VRLA types became popular on motorcycles because the acid electrolyte is absorbed into the medium which separates the plates, so it cannot spill. This medium also lends support to the plates which helps them better to withstand vibration. They are also popular in stationary applications such as telecommunications sites, due to their small footprint and flexibility of installation.

The electrical characteristics of VRLA batteries differ somewhat from wet-cell lead-acid batteries, and caution should be exercised in charging and discharging them.

Of the several types of VRLA batteries are the absorbent glass mat battery, gel battery, and sealed lead-acid battery.

Exploding batteriesExcessive charging of a lead-acid battery will cause emission of hydrogen and oxygen from each cell, as some of the water of the electrolyte is broken down by electrolysis. Wet cells have open vents to release any gas produced, and VRLA batteries rely on valves fitted to each cell. Wet cells

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may be equipped with catalytic caps to recombine any emitted hydrogen. A VRLA cell will normally recombine any hydrogen and oxygen produced into water inside the cell, but malfunction or overheating may cause gas to build up. If this happens (e.g. by overcharging the cell) the valve is designed to vent the gas and thereby normalize the pressure, resulting in a characteristic acid smell around the battery. Valves can sometimes fail however, if dirt and debris accumulate in the device, so pressure can build up inside the affected cell.

If the accumulated hydrogen and oxygen within either a VRLA or wet cell is ignited, an explosion is produced. The force is sufficient to burst the plastic casing or blow the top off the battery, and can injure anyone in the vicinity and spray acid and casing shrapnel to the immediate environment; an explosion in one cell may also set off the combustible gas mixture in remaining cells of the battery.

VRLA batteries usually show swelling in the cell walls when the internal pressure rises. The deformation of the walls varies from cell to cell, and is greater at the ends where the walls are unsupported by other cells. Such over-pressurized batteries should be isolated and discarded, taking great care using protective personal equipment (goggles, overalls, gloves, etc.) during the handling.

Environmental concernsCurrently attempts are being made to develop alternatives to the lead-acid battery (particularly for automotive use) because of concerns about the environmental consequences of improper disposal of old batteries and of lead smelting operations. Ni-Mn is already widely used in hybrid vehicles. Newer technologies are unlikely to displace lead-acid batteries owing to the much greater cost of potential alternatives. Nickel and Manganese are considerably more expensive than lead or antimony. As an example, at current (May 2008) prices quoted on the London Metal Exchange lead is about one tenth the price of nickel. Lead-acid battery recycling is one of the most successful recycling programs in the world. In the United States 97% of all battery lead was recycled between 1997 and 2001.[5] An effective pollution control system is a necessity to prevent lead emission. Continuous improvement in battery recycling plants and furnace designs is required to keep pace with emission standards for lead smelters.

AdditivesMany vendors sell chemical additives (solid compounds as well as liquid solutions) that supposedly reduce sulfate build up and improve battery condition when added to the electrolyte of a vented lead-acid battery. Such treatments are rarely, if ever, effective. Two compounds used for such purposes are Epsom salts and EDTA. Epsom salts reduces the internal resistance in a weak or damaged battery and may allow a small amount of extended life. EDTA can be used to dissolve the sulfate deposits of heavily discharged plates. However, the dissolved material is then no longer available to participate in the normal charge/discharge cycle, so a battery temporarily revived with EDTA should not be expected to have normal life expectancy. Residual EDTA in the lead-acid cell forms organic acids which will accelerate corrosion of the lead plates and internal connectors. Active material (the positive plate lead dioxide and negative plate spongy lead) changes physical form during discharge, resulting in plate growth, distortion of the active material, and shedding of active material. Once the active material has fallen out of the plates, it cannot be restored into position by any chemical

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treatment. Similarly, internal physical problems such as cracked plates, corroded connectors, or damaged separators cannot be restored chemically.

HydrometerNot to be confused with Hygrometer.

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity can be read directly. The scales may be Plato, Oechsle, or Brix, depending on the purpose.

Hydrometers may be calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits.

PrincipleThe operation of the hydrometer is based on the Archimedes principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced. Thus, the lower the density of the substance, the further the hydrometer will sink. (See also Relative density and hydrometers.)

HistoryAn early description of a hydrometer appears in a letter from Synesius of Cyrene to Hypatia of Alexandria. In Synesius' fifteen letter, he requests Hypatia to make a hydrometer for him:[1]

The instrument in question is a cylindrical tube, which has the shape of a flute and is about the same size. It has notches in a perpendicular line, by means of which we are able to test the weight of the waters. A cone forms a lid at one of the extremities, closely fitted to the tube. The cone and the tube have one base only. This is called the baryllium. Whenever you place the tube in water, it remains erect. You can then count the notches at your ease, and in this way ascertain the weight of the water.

RangesIn low density liquids such as kerosene, gasoline, and alcohol, the hydrometer will sink deeper, and in high density liquids such as brine, milk, and acids it will not sink so far. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top of the stem, and one for light liquids, on which the mark 1.000 is near the bottom. In many industries a set of hydrometers is used — covering specific gravity ranges of 1.0–0.95, 0.95–0.9 etc — to provide more precise measurements.

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ScalesModern hydrometers usually measure specific gravity but different scales were (and sometimes still are) used in certain industries. Examples include:

Baumé scale, formerly used in industrial chemistry and pharmacology Brix scale, primarily used in fruit juice, wine making and the sugar industry Oechsle scale, used for measuring the density of grape must Plato scale, primarily used in brewing Twaddell scale, formerly used in the bleaching and dyeing industries [2]

Commercial usesBecause the commercial value of many liquids, including sugar solutions, sulfuric acid, and alcohol beverages such as beer and wine, depends directly on the specific gravity, hydrometers are used extensively.

Maintenance precautionsOne precaution in workshops that handle large lead-acid batteries is a supply of ammonia solution to squirt on any spilled battery acid, to neutralize it. Surplus ammonia, and water, evaporate off, leaving a deposit of ammonium sulfate. Sodium bicarbonate (baking soda) is also commonly used for this purpose.

Primary (non-rechargeable) cells

Daniell cell | Lithium battery | Alkaline battery | Mercury battery | Zinc-carbon battery | Silver-oxide battery | Zinc-air battery | NiOx Battery

Rechargeable (secondary) cells

Lead-acid battery | Lithium-ion battery | Lithium-ion polymer battery | Lithium iron phosphate battery | Lithium sulfur battery | Lithium-titanate battery | Nickel-cadmium battery | Nickel hydrogen battery | Nickel-metal hydride battery | Nickel-iron battery | Sodium-sulfur battery | Vanadium redox battery | Rechargeable alkaline battery

Kinds of cells Battery | Concentration cell | Flow battery | Fuel cell | Voltaic pile

References collected from

1. United States Patent 5,948,567 2. David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition. McGraw-Hill, New York, 2002 ISBN 0-07-

135978-8 page 23.5 3. "Battery FAQ" at Northern Arizona Wind & Sun, visited 2006-07-23 4. #Saslow, Wayne M. (2002). Electricity, Magnetism, and Light. Toronto: Thomson Learning. ISBN 0-12-619455-6. pp.

302–304. Page 164 of 198

5. "Battery Council International". http://www.batterycouncil.org/LeadAcidBatteries/BatteryRecycling/tabid/71/Default.aspx. Retrieved on 2006-06-01.

6. U.S. Department of Energy, Primer On Lead-Acid Storage Batteries (pdf) 7. Environment Friendly Battery Recycling 8. Battery Council International (BCI), trade organization of lead-acid battery manufacturers. 9. BatteryUniversity.com 10. Car and Deep Cycle Battery Frequently Asked Questions 11. ToxFAQs: Lead 12. National Pollutant Inventory - Lead and Lead Compounds Fact Sheet 13. Case Studies in Environmental Medicine - Lead Toxicity 14. Battery Information 15. Dan's Data: Quick Guide to Memory Effect 16. Notebook Battery Guide By Chris Yano

Specific GravityOne of the key parameters of battery operation is the specific gravity of the electrolyte. Specific gravity is the ratio of the weight of a solution (sulfuric acid in this case) to the weight of an equal volume of water at a specified temperature. This measurement is usually measured using a Hydrometer. The specific gravity of a fully charged GB Industrial Battery is the industry standard of 1.285.

Specific gravity is used as an indicator of the state of charge of a cell or battery. However, specific gravity measurements cannot determine a battery's capacity. During discharge, the specific gravity decreases linearly with the ampere-hours discharged as indicated in the illustration below

Therefore, during fully charged steady-state operation and on discharge, measurement of the specific gravity of the electrolyte provides an approximate indication of the state of charge of the cell. The downward sloping line for the specific gravity during discharge is approximated by the equation below:

Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v – 0.845 = 1.285)

Or

Single-cell open circuit voltage = specific gravity + 0.845.

The above equations permit electrical monitoring of approximate specific gravity on an occasional basis. As mentioned earlier, specific gravity measurements cannot be taken on sealed lead-acid batteries. Measurement of the cell open-circuit voltage has been used as an indicator of the state of charge of a sealed battery. More reliable methods for determining the state of charge of sealed batteries are under development. The specific gravity decreases during the discharging of a battery to a value near that of pure water and it increases during a recharge. The battery is considered fully charged when specific gravity reaches its highest possible value.

Specific gravity varies with temperature and the quantity of electrolyte in a cell. When the electrolyte is near the low-level mark, the specific gravity is higher than nominal and drops as water is added to the cell to bring the electrolyte to the full level. The volume of electrolyte expands as temperature rises and contracts as temperature drops, therefore affecting the density or specific gravity reading. As the volume of electrolyte expands, the readings are lowered and, conversely, specific gravity increases with colder temperatures.

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The specific gravity for a given battery is determined by the application it will be used in, taking into account operating temperature and battery life.

As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life. Typical specific gravities for certain applications are shown in Table 1.

Specific Gravity Applications

1.285 Heavily cycled batteries such as for forklifts (traction).

1.260 Automotive (SLI)

1.250UPS – Standby with high momentary discharge current requirement.

1.215 General applications such as power utility and telephone.

Represented in Table 2 (below), the electrolyte in a fully charged battery is still 62.48% water. Higher gravity acid, such 1.600 spgr, can be used to adjust the gravity of batteries that have been diluted due to repeated overflow caused over-filling. Note: Acid adjustments should only be performed by factory-trained technicians in a controlled environment

% Sulfuric Acid % Water Specific Gravity (68°F)

37.52 62.48 1.285

48 52 1.380

50 50 1.400

60 40 1.500

68.74 31.26 1.600

70 30 1.616

77.67 22.33 1.705

93 7 1.835In the selection of a battery for a given application, some of the effects of high or low specific gravity to be considered:

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Higher Gravity = Lower Gravity =More capacity Less capacityShorter life Longer lifeHigher momentary discharge rates Lower momentary discharge ratesLess adaptable to "floating: operation More adaptable to "floating" operationMore standing loss Less standing loss

A solution of higher specific gravity is heavier per unit volume than one of lower specific gravity. Therefore the more concentrated electrolyte created during charging sinks to the bottom of the battery jar creating a gradient in specific gravity. The gassing that occurs on overcharge serves as a "mixer" and makes the specific gravity uniform throughout the cell. To avoid erroneous readings, specific gravity measurements should only be taken after an equalizing charge and subsequent float charge for at least 72 hours.

AC/DC FormulasTo Find Direct

CurrentAC / 1phase 115v or 120v

AC / 1phase208,230, or

240v

AC 3 phaseAll Voltages

Amps whenHorsepower is Known

HP x 746 E x Eff

HP x 746 E x Eff X PF

HP x 746 E x Eff x PF

HP x 746 1.73 x E x Eff x PF

Amps whenKilowatts is known

kW x 1000 E

kW x 1000 E x PF

kW x 1000 E x PF

kW x 1000 1.73 x E x PF

Amps whenkVA is known

kVA x 1000 E

kVA x 1000 E

kVA x 1000 1.73 x E

Kilowatts I x E 1000

I x E x PF 1000

I x E x PF 1000

I x E x 1.73 x PF 1000

Kilovolt-Amps I x E 1000

I x E 1000

I x E x 1.73 1000

Horsepower(output)

I x E x Eff 746

I x E x Eff x PF 746

I x E x Eff x PF 746

I x E x Eff x 1.73 x PF 746

Three Phase ValuesFor 208 volts x 1.732, use 360For 230 volts x 1.732, use 398For 240 volts x 1.732, use 416For 440 volts x 1.732, use 762For 460 volts x 1.732, use 797For 480 Volts x 1.732, use 831

E = Voltage / I = Amps /W = Watts / PF = Power Factor / Eff = Efficiency / HP = Horsepower

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AC Efficiency and Power Factor Formulas

To Find Single Phase

Three Phase

Efficiency746 x HP E x I x PF

746 x HPE x I x PF x 1.732

Power Factor

Input Watts V x A

Input WattsE x I x 1.732

Power - DC CircuitsWatts = E xI

Amps = W / E

Ohm's Law / Power Formulas

P = watts

I = amps

R = ohms

E = Volts

Voltage Drop Formulas

Single Phase(2 or 3 wire)

VD = 2 x K x I x L CM

K = ohms per mil foot 

(Copper = 12.9 at 75°)

(Alum = 21.2 at 75°) Note: K value changes with temperature. See Code chapter 9, Table 8

CM= 2K x L x I VD

Three Phase VD= 1.73 x K x I x L CM

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L = Length of conductor in feet

I  = Current in conductor (amperes)

CM = Circular mil area of conductor

CM= 1.73 x K x L x I VD

To better understand the following formulas review the rule of transposition in equations.A multiplier may be removed from one side of an equation by making it a division on the other side or a division may be removed from one side of an equation by making it a multiplier on the other side.

1. Voltage and Current: Primary (p) secondary (s)Power(p) = power (s) or Ep x Ip = Es x Is

Ep = Es x Is Ip Ip = Es x Is

Ep

Is = Ep x Ip Es Es = Ep x Ip

Is

2. Voltage and Turns in Coil:Voltage (p) x Turns (s) = Voltage (s) x Turns (p)or Ep x Ts = Es x Ip

Ep = Es x Ip Ts Ts = Es x Tp

Ep

Tp = Ep x Ts Es Es = Ep x Ts

Tp

3. Amperes and Turns in Coil:Amperes (p) x Turns (p) = Amperes (s) x Turns (s)or Ip x Tp = Is x Ts

Ip = Is x Ts Tp Tp =Is x Ts

Ip

Ts =Ip x Tp Is Is = Ip x Tp

Ts

Calculating Motor Speed:

A squirrel cage induction motor is a constant speed device. It cannot operate for any length of time at speeds below those shown on the nameplate without danger of burning out.

To calculate the speed of an induction motor, apply this formula:

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Srpm = 120 x F            P

Srpm = synchronous revolutions per minute.120   = constantF       = supply frequency (in cycles/sec)P       = number of motor winding poles

Example: What is the synchronous of a motor having 4 poles connected to a 60 hz power supply?

Srpm = 120 x F            PSrpm = 120 x 60            4Srpm = 7200             4Srpm = 1800 rpm

Calculating Braking Torque:

Full-load motor torque is calculated to determine the required braking torque of a motor.To Determine braking torque of a motor, apply this formula:

T = 5252 x HP    rpm

T      = full-load motor torque (in lb-ft)5252 = constant (33,000 divided by 3.14 x 2 = 5252)HP    = motor horsepowerrpm = speed of motor shaft

Example: What is the braking torque of a 60 HP, 240V motor rotating at 1725 rpm?

T = 5252 x HP    rpmT = 5252 x 60     1725T = 315,120 / 1725

T = 182.7 lb-ft

Calculating Work:

Work is applying a force over a distance. Force is any cause that changes the position, motion, direction, or shape of an object. Work is done when a force overcomes a resistance. Resistance is any force that tends to hinder the movement of an object. If an applied force does not cause motion the no work is produced.

To calculate the amount of work produced, apply this formula:

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W = F x D

W = work (in lb-ft)F  = force (in lb)D  = distance (in ft)

Example: How much work is required to carry a 25 lb bag of groceries vertically from street level to the 4th floor of a building 30' above street level?

W = F x DW = 25 x 30W = 750 -lb

Calculating Torque:

Torque is the force that produces rotation. It causes an object to rotate. Torque consist of a force acting on distance. Torque, like work, is measured is pound-feet (lb-ft). However, torque, unlike work, may exist even though no movement occurs.

To calculate torque, apply this formula:

T = F x D

T = torque (in lb-ft)F = force (in lb)D = distance (in ft)

Example: What is the torque produced by a 60 lb force pushing on a 3' lever arm?

T = F x DT = 60 x 3T = 180 lb ft

Calculating Full-load Torque:

Full-load torque is the torque to produce the rated power at full speed of the motor. The amount of torque a motor produces at rated power and full speed can be found by using a horsepower-to-torque conversion chart. When using the conversion chart, place a straight edge along the two known quantities and read the unknown quantity on the third line.

To calculate motor full-load torque, apply this formula:

T = HP x 5252    rpm

T = torque (in lb-ft)HP = horsepower5252 = constantrpm = revolutions per minute

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Example: What is the FLT (Full-load torque) of a 30HP motor operating at 1725 rpm?

T = HP x 5252    rpmT = 30 x 5252     1725

T = 157,560     1725T = 91.34 lb-ft

Calculating Horsepower:

Electrical power is rated in horsepower or watts. A horsepower is a unit of power equal to 746 watts or 33,0000 lb-ft per minute (550 lb-ft per second). A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt. It is 1/746 of 1 horsepower. The watt is the base unit of electrical power. Motor power is rated in horsepower and watts.Horsepower is used to measure the energy produced by an electric motor while doing work.

To calculate the horsepower of a motor when current and efficiency, and voltage are known, apply this formula:

HP = V x I x Eff        746

HP = horsepowerV    = voltageI     = curent (amps)Eff. = efficiency

Example: What is the horsepower of a 230v motor pulling 4 amps and having 82% efficiency?

HP = V x I x Eff        746HP = 230 x 4 x .82        746HP = 754.4        746HP = 1 Hp

Eff = efficiency / HP = horsepower / V = volts / A = amps / PF = power factor

Horsepower Formulas

To Find Use FormulaExampleGiven Find Solution

HP HP = I X E X Eff.       746 240V, 20A, 85% Eff. HP

HP = 240V x 20A x 85%       746HP=5.5

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I I = HP x 746     E X Eff x PF

10HP, 240V, 90% Eff., 88% PF I

I = 10HP x 746      240V x 90% x 88%I = 39 A

To calculate the horsepower of a motor when the speed and torque are known, apply this formula:

HP = rpm x T(torque)         5252(constant)

Example: What is the horsepower of a 1725 rpm motor with a FLT 3.1 lb-ft?

HP = rpm x T         5252 HP = 1725 x 3.1         5252HP = 5347.5         5252 HP = 1 hp

Calculating Synchronous Speed:

AC motors are considered constant speed motors. This is because the synchronous speed of an induction motor is based on the supply frequency and the number of poles in the motor winding. Motor are designed for 60 hz use have synchronous speeds of 3600, 1800, 1200, 900, 720, 600, 514, and 450 rpm.

To calculate synchronous speed of an induction motor, apply this formula:

rpmsyn = 120 x f              Np

rpmsyn = synchronous speed (in rpm)f           = supply frequency in (cycles/sec)Np       =  number of motor poles

Example: What is the synchronous speed of a four pole motor operating at 50 hz.?

rpmsyn = 120 x f              Nprpmsyn = 120 x 50              4

rpmsyn = 6000 4rpmsyn = 1500 rpm

DEVICE FUNCTION NUMBERS (ANSI)

1. Master element 2. Time delay starting3. Interlocking relay4. Master contactor5. Stopping device

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6. Starting circuit breaker7. Anode circuit breaker 8. Disconnecting device 9. Reversing device 10. Sequence switch 11. _____________12. Over –speed device 13. Synchronous – speed device 14. Under – speed device 15. Speed matching device 16. _____________17. Discharge switch 18. Decelerating device 19. Starting – to –running contactor 20. Electrically operated value 21. Distance relay 22. Equalizer circuit breaker 23. Temperature control device 24. _____________25. Synchronizing device 26. Thermal device 27. under voltage relay 28. _____________29. Isolating contactor 30. Annunciated relay 31. Separate excitation device 32. Directional power relay 33. Position switch 34. Motorized sequence 35. Short – circuiting device 36. Polarity device 37. Undercurrent relay 38. Bearing protective device 39. ______________40. Field relay41. Field circuit breaker42. Running circuit breaker43. Manual selector device44. Sequence starting relay45. _______________46. Phase balance current relay47. Phase sequence voltage relay48. Incomplete sequence relay49. Thermal relay50. Instantaneous relay51. A.C time over current relay52. A.C circuit breaker53. D.C generator relay54. High speed D.C circuit breaker

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55. Power factor relay56. Field application relay57. Grounding device58. Power rectifier misfire relay59. Over voltage relay60. Voltage balance relay61. Current balance relay62. Time delay stopping relay63. Pressure level or flow relay64. Ground protective relay65. Governor66. Notching device67. A.C directional O/C relay68. Blocking relay69. Permissive control device70. Electrically operated rheostat71. _____________________72. D.C circuit breaker73. Load resistor contactor74. Alarm relay75. Position changing mechanism76. D.C over current relay77. Pulse transmitter78. Out of step protective relay79. A.C re-closing relay80. __________________81. Frequency relay82. D.C re-closing relay83. Auto selective control relay84. Operating mechanism85. Carrier receiver relay86. Locking out relay87. Differential relay88. Auxiliary motor89. Line switch90. Regulating device91. Voltage directional device92. Voltage and power directional relay93. Field changing contactor94. Tripping or trip free relay

Fire classes

Comparison of fire classesAmerican European/Australian Fuel/Heat sourceClass A Class A Ordinary combustiblesClass B Class B Flammable liquids

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Class C Flammable gasesClass C Class E Electrical equipmentClass D Class D Combustible metalsClass K Class F Cooking oil or fat

In firefighting, fires are identified according to one or more fire classes. Each class designates the fuel involved in the fire, and thus the most appropriate extinguishing agent. The classifications allow selection of extinguishing agents along lines of effectiveness at putting the type of fire out, as well as avoiding unwanted side-effects. For example, non-conductive extinguishing agents are rated for electrical fires, so to avoid electrocuting the firefighter.Multiple classification systems exist, with different designations for the various classes of fire. The United States uses the NFPA system. Europe and Australasia use another.

Ordinary combustibles

"Ordinary combustible" fires are the most common type of fire, and are designated Class A under both systems. These occur when a solid, organic material such as wood, cloth, rubber, or some plastics become heated to their flash point and ignite. At this point the material undergoes combustion and will continue burning as long as the four components of the fire tetrahedron (heat, fuel, oxygen, and the sustaining chemical reaction) are available.This class of fire is commonly used in controlled circumstances, such as a campfire, match or wood-burning stove. To use the campfire as an example, it has a fire tetrahedron - the heat is provided by another fire (such as a match or lighter), the fuel is the wood, the oxygen is naturally available in the open-air environment of a forest, and the chemical reaction links the three other facets. This fire is not dangerous, because the fire is contained to the wood alone and is usually isolated from other flammable materials, for example by bare ground and rocks. However, when a class-A fire burns in a less-restricted environment the fire can quickly grow out of control and become a wildfire. This is the case when firefighting and fire control techniques are required.This class of fire is fairly simple to fight and contain - by simply removing the heat, oxygen, or fuel, or by suppressing the underlying chemical reaction, the fire tetrahedron collapses and the fire dies out. The most common way to do this is by removing heat by spraying the burning material with water; oxygen can be removed by smothering the fire with foam from a fire extinguisher; forest fires are often fought by removing fuel by back burning; and an ammonium phosphate dry chemical powder fire extinguisher (but not sodium bicarbonate or potassium bicarbonate both of which are rated for B-class [clarification needed] fires) breaks the fire's underlying chemical reaction.As these fires are the most commonly encountered, most fire departments have equipment to handle them specifically. While this is acceptable for most ordinary conditions, most firefighters find themselves having to call for special equipment such as foam in the case of other fires.

Flammable liquid and gas

A CO2 fire extinguisher rated for flammable liquids and gasses

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Flammable or combustible liquid or gaseous fuels. The US system designates all such fires "Class B". In the European/Australian system, flammable liquids are designated "Class B", while burning gases are separately designated "Class C". These fires follow the same basic fire tetrahedron (heat, fuel, oxygen, chemical reaction) as ordinary combustible fires, except that the fuel in question is a flammable liquid such as gasoline, or gas such as natural gas. A solid stream of water should never be used to extinguish this type because it can cause the fuel to scatter, spreading the flames. The most effective way to extinguish a liquid or gas fueled fire is by inhibiting the chemical chain reaction of the fire, which is done by dry chemical and Halon extinguishing agents, although smothering with CO2 or, for liquids, foam is also effective. Some newer clean agents designed to replace halon work by cooling the liquid below its flash point, but these have limited class B[clarification needed] effectiveness.

Electrical

Electrical fires are fires involving potentially energized electrical equipment. The US system designates these "Class C"; the European/Australian system designates them "Class E". This sort of fire may be caused by, for example, short-circuiting machinery or overloaded electrical cables. These fires can be a severe hazard to firefighters using water or other conductive agents: Electricity may be conducted from the fire, through water, the firefighter's body, and then earth. Electrical shocks have caused many firefighter deaths.Electrical fire may be fought in the same way as an ordinary combustible fire, but water, foam, and other conductive agents are not to be used. While the fire is, or could possibly be electrically energized, it can be fought with any extinguishing agent rated for electrical fire. Carbon dioxide CO2, Halon and dry chemical powder extinguishers such as PKP and even baking soda are especially suited to extinguishing this sort of fire. Once electricity is shut off to the equipment involved, it will generally become an ordinary combustible fire.

Metal

Certain metals are flammable or combustible. Fires involving such are designated "Class D" in both systems. Examples of such metals include sodium, titanium, magnesium, potassium, steel, uranium, lithium, plutonium, and calcium. Magnesium and titanium fires are common, and 2006-7 saw the recall of laptop computer models containing lithium batteries susceptible to spontaneous ignition. When one of these combustible metals ignites, it can easily and rapidly spread to surrounding ordinary combustible materials. With the exception of the metals that burn in contact with air or water (for example, sodium), masses of combustible metals do not represent unusual fire risks because they have the ability to conduct heat away from hot spots so efficiently that the heat of combustion cannot be maintained - this means that it will require a lot of heat to ignite a mass of combustible metal. Generally, metal fire risks exist when sawdust, machine shavings and other metal 'fines' are present. Generally, these fires can be ignited by the same types of ignition sources that would start other common fires.Water and other common firefighting materials can excite metal fires and make them worse. The NFPA recommends that metal fires be fought with 'dry powder' extinguishing agents. Dry Powder agents work by smothering and heat absorption. The most common of these

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agents are sodium chloride granules and graphite powder. In recent years powdered copper has also come into use.Some extinguishers are labeled as containing dry chemical extinguishing agents. This may be confused with dry powder. The two are not the same. Using one of these extinguishers in error, in place of dry powder, can be ineffective or actually increase the intensity of a metal fire.Metal fires represent a unique hazard because people are often not aware of the characteristics of these fires and are not properly prepared to fight them. Therefore, even a small metal fire can spread and become a larger fire in the surrounding ordinary combustible materials.

Cooking oil

Laboratory simulation of a chip pan fire: a beaker containing wax is heated until it catches fire. A small amount of water is then poured into the beaker. The water sinks to the bottom and vaporizes instantly, ejecting a plume of burning liquid wax into the air.Fires that involve cooking oils or fats are designated "Class K" under the US system, and "Class F" under the European/Australasian systems. Though such fires are technically a subclass of the flammable liquid/gas class, the special characteristics of these types of fires are considered important enough to recognize separately. Specification can be used to extinguish such fires. Appropriate fire extinguishers may also have hoods over them that help extinguish the fire.

Fire extinguisher

A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user (i.e. no escape route, smoke, explosion hazard, etc.), or otherwise requires the expertise of a fire department. Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent which can be discharged to extinguish a fire.There are two main types of fire extinguishers: stored pressure and cartridge-operated. In stored pressure units, the expellant is stored in the same chamber as the firefighting agent itself. Depending on the agent used, different propellants are used. With dry chemical extinguishers, nitrogen is typically used; water and foam extinguishers typically use air. Stored pressure fire extinguishers are the most common type. Cartridge-operated extinguishers contain the expellant gas in a separate cartridge that is punctured prior to discharge, exposing the propellant to the extinguishing agent. This type is not as common, used primarily in areas such as industrial facilities, where they receive higher-than-average use. They have the advantage of simple and prompt recharge, allowing an operator to discharge the extinguisher, recharge it, and return to the fire in a reasonable amount of time. Unlike stored pressure types, these extinguishers utilize compressed carbon dioxide instead of nitrogen, although nitrogen cartridges are used on low temperature (-60 rated) models. Cartridge operated extinguishers are available in dry chemical and dry powder types in the US and in water, wetting agent, foam, and dry powder (ABC, BC, or D) types in the rest of the world.

Fire extinguishers are further divided into handheld and cart-mounted, also called wheeled extinguishers. Handheld extinguishers weigh from 0.5 to 14 kilograms (1 to 30 pounds), and are

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hence, easily portable by hand. Cart-mounted units typically weigh 23+ kilograms (50+ pounds). These wheeled models are most commonly found at construction sites, airport runways, heliports, as well as docks and marinas.History The first fire extinguisher of which there is any record was patented in England in 1723 by Ambrose Godfrey, a celebrated chemist. It consisted of a cask of fire-extinguishing liquid containing a pewter chamber of gunpowder. This was connected with a system of fuses which were ignited, exploding the gunpowder and scattering the solution. This device was probably used to a limited extent, as Bradley's Weekly Messenger for November 7, 1729, refers to its efficiency in stopping a fire in London. The modern fire extinguisher was invented by British Captain George William Manby in 1818; it consisted of a copper vessel of 3 gallons (13.6 litres) of pearl ash (potassium carbonate) solution contained within compressed air.

The soda-acid extinguisher was first patented in 1866 by Francois Carlier of France, which mixed a solution of water and sodium bicarbonate with tartaric acid, producing the propellant CO2 gas. A soda-acid extinguisher was patented in the U.S. in 1881 by Almon M. Granger. His extinguisher used the reaction between sodium bicarbonate solution and sulfuric acid to expel pressurized water onto a fire.[1] A vial was suspended in the cylinder containing concentrated sulfuric acid. Depending on the type of extinguisher, the vial of acid could be broken in one of two ways. One used a plunger to break the acid vial, whilst the second released a lead stopple that held the vial closed. Once the acid was mixed with the bicarbonate solution, carbon dioxide gas was expelled and thereby pressurize the water. The pressurized water was forced from the canister through a nozzle or short length of hose. The cartridge-operated extinguisher was invented by Read & Campbell of England in 1881, which used water or water-based solutions. They later invented a carbon tetrachloride model called the "Petrolex" which was marketed toward automotive use.The chemical foam extinguisher was invented around 1905 by Alexander Laurant of Russia, who first used it to extinguish a pan of burning naphtha. It works and looks similar to the soda-acid type, but the inner parts are different. The main tank contains a solution of water, foam compound (usually made from licorice root) and sodium bicarbonate. A cylindrical metal or plastic chamber holds about a quart and a half of 13% aluminium sulfate and is capped with a lead cap. When the unit is turned over, the chemicals mix, producing CO2 gas. The licorice causes some of the CO2 bubbles to become trapped in the liquid and is discharged on the fire as a thick whitish-brown foam Around 1912 Pyrene invented the carbon tetrachloride (CTC) extinguisher, which expelled the liquid from a brass or chrome container by a handpump; it was usually of 1 imperial quart (1.1 L) or 1 imperial pint (0.6 L) capacity but was also available in up to 2 imperial gallon (9 L) size. A further variety consisted of a glass bottle "bomb" filled with the liquid that was intended to be hurled at the base of a fire. The CTC vaporized and extinguished the flames by creating a dense, oxygen-excluding blanket of fumes, and to a lesser extent, inhibiting the chemical reaction. The extinguisher was suitable for liquid and electrical fires, and was popular in motor vehicles for the next 60 years. In the 1940s, Germany invented the liquid chloro bromo methane (CBM) for use in aircraft. It was more effective and slightly less toxic than carbon tetrachloride and was used until 1969. Methyl Bromide was discovered as an extinguishing agent in the 1920s and was used extensively in Europe. It is a low-pressure gas that works by inhibiting the chain reaction of the fire and is the most toxic of the vaporizing liquids, used until the 1960s. The vapor and combustion by-products of all vaporizing liquids were highly toxic, and could cause death in confined spaces. The carbon dioxide (CO2) extinguisher was invented (at least in the US) by the Walter Kidde Company in 1924 in response to Bell Telephone's request for an electrically non-conductive chemical for extinguishing the previously difficult to extinguish fires in telephone switchboards. It consisted of a tall metal cylinder containing 7.5 lbs. of CO2 with a wheel valve and a woven brass,

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cotton covered hose, with a composite funnel-like horn as a nozzle. CO2 is still popular today as it is an ozone-friendly clean agent and is useful for an extinguishing a person who is on fire, hence its widespread use in film and television.

In 1928, DuGas (later bought by ANSUL) came out with a cartridge-operated dry chemical extinguisher, which used sodium bicarbonate specially treated with chemicals to render it free-flowing and moisture-resistant. It consisted of a copper cylinder with an internal CO2cartridge. The operator turned a wheel valve on top to puncture the cartridge and squeezed a lever on the valve at the end of the hose to discharge the chemical. This was the first agent available for large scale three-dimensional liquid and pressurized gas fires, and was but remained largely a specialty type until the 1950s, when small dry chemical units were marketed for home use. ABC dry chemical came over from Europe in the 1950s, with Super-K being. Invented in the early 60s and Purple-K being developed by the US Navy in the late 1960s. Halon 1211 came over to the US from Europe in the 1970s, where it had been used since the late 40s or early 50s. Halon 1301 had been developed by DuPont and the US Army in 1954. Both work by inhibiting the chain reaction of the fire, and in the case of Halon 1211, cooling class A fuels as well. Halon is still in use today, but is falling out of favor for many uses due to its environmental impact. Europe and Australia have severely restricted its use, but it is still widely available in North America, the Middle East, and Asia.

ClassificationInternationally there are several accepted classification methods for hand-held fire extinguishers. Each classification is useful in fighting fires with a particular group of fuel.

Type Pre-1997 Current Suitable for use on Fire Classes (brackets denote sometimes applicable)

Water Solid red Solid red A

Foam Solid blue Red with a blue band A B

Dry chemical (powder)

Red with a white band

Red with a white band A B C E

Carbon dioxide Red with a black band

Red with a black band (A) B C E F

Vaporising liquid (not halon)

Red with a yellow band

Red with a yellow band A B C E

Halon Solid yellow No longer produced A B E

Wet chemical Solid oatmeal Red with an oatmeal band A F

In Australia, yellow (Halon) fire extinguishers are illegal to own or use on a fire, unless an essential use exemption has been granted.

According to the standard BS EN 3, fire extinguishers in the United Kingdom as all throughout Europe are red RAL 3000, and a band or circle of a second color covering between 5-10% of the surface area of the extinguisher indicates the contents. Before 1997, the entire body of the fire extinguisher was color coded according to the type of extinguishing agent.

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EN3 does not recognize a separate electrical class - however there is an additional feature requiring special testing (35 kV dielectric test per EN 3-7:2004). A powder or CO2 extinguisher will bear an electrical pictogram as standard signifying that it can be used on live electrical fires (given the symbol E in the table). If a water-based extinguisher has passed the 35 kV test it will also bear the same electrical pictogram - however, any water-based extinguisher is only recommended for inadvertent use on electrical fires.

Type Old Code BS EN 3 Colour Code

Suitable for use on Fire Classes (brackets denote sometimes

applicable)

Water Signal Red Signal Red A

Foam Cream Red with a Cream panel above the operating instructions A B

Dry powder Blue Red with a Blue panel above the operating instructions (A) B C E

Carbon dioxide CO2

Black Red with a Black panel above the operating instructions B E

Wet chemical N/A Red with a Canary Yellow panel

above the operating instructions A (B) F

Class D powder Blue Red with a Blue panel above the

operating instructions D

Halon gas Green Now prohibited except under certain situations.[4]

In the UK the use of Halon gas is now prohibited except under certain situations.There is no official standard in the United States for the color of fire extinguishers, though they are typically red, except for Class D extinguishers, which are usually yellow, and water, which are usually silver, or white if water mist. Extinguishers are marked with pictograms depicting the types of fires that the extinguisher is approved to fight. In the past, extinguishers were marked with colored geometric symbols, and some extinguishers still use both symbols. The types of fires and additional standards are described in NFPA 10: Standard for Portable Fire Extinguishers, 2007 edition.

Fire Class Geometric Symbol Pictogram Intended Use

A Green Triangle Garbage can and wood pile burning Ordinary solid combustibles

B Red Square Fuel container and burning puddle Flammable liquids and gases

C Blue Circle Electric plug and burning outlet Energized electrical

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equipment

D Yellow Pentagram (Star) Burning Gear and Bearing Combustible metals

K Black Hexagon Pan burning Cooking oils and fats

The Underwriters Laboratories rate fire extinguishing capacity in accordance with UL/ANSI 711: Rating and Fire Testing of Fire Extinguishers. The ratings are described using numbers preceding the class letter, such as 1-A:10-B:C. The number preceding the A multiplied by 1.25 gives the equivalent extinguishing capability in gallons of water. The number preceding the B indicates the size of fire in square feet that an ordinary user should be able to extinguish. There is no additional rating for class C, as it only indicates that the extinguishing agent will not conduct electricity, and an extinguisher will never have a rating of just C.

Installation Fire extinguishers are typically fitted in buildings at an easily-accessible location, such as against a wall in a high-traffic area. They are also often fitted to motor vehicles, watercraft, and aircraft - this is required by law in many jurisdictions, for identified classes of vehicles. Under NFPA 10 all commercial vehicles must carry at least one fire extinguisher (size/UL rating depending on type of vehicle and cargo (i.e.. fuel tankers typically must have a 9.1 kg (20 lb). when most others can carry a 2.3 kg (5 lb).) The revised NFPA 10 created criteria on the placement of "Fast Flow Extinguishers" in locations such as those storing and transporting pressurized flammable liquids and pressurized flammable gas or areas with possibility of three dimensional class B hazards are required to have "fast flow" extinguishers as required by NFPA 5.5.1.1. Varying classes of competition vehicles require fire extinguishing systems, the simplest requirements being a 1A:10BC hand-held portable extinguisher mounted to the interior of the vehicle.

Types of extinguishing agents

Powder based agent that extinguishes by separating the four parts of the fire tetrahedron. It prevents the chemical reaction between heat, fuel and oxygen and halts the production of fire sustaining "free-radicals", thus extinguishing the fire.• Ammonium phosphate, also known as "tri-class", "multipurpose" or "ABC" dry chemical, used on class A, B, and C fires. It receives its class A rating from the agent's ability to melt and flow at 177 °C (350 °F) to smother the fire. More corrosive than other dry chemical agents. Pale yellow in color. • Sodium bicarbonate, "regular" or "ordinary" used on class B and C fires, was the first of the dry chemical agents developed. It interrupts the fire's chemical reaction, and was very common in commercial kitchens before the advent of wet chemical agents, but now is falling out of favor, as it is much less effective than wet chemical agents for class K fires, less effective than Purple-K for class B fires, and is ineffective on class A fires. White or blue in color. • Potassium bicarbonate (aka Purple-K), used on class B and C fires. About two times as effective on class B fires as sodium bicarbonate, it is the preferred dry chemical agent of the oil and gas industry. The only dry chemical agent certified for use in ARFF by the NFPA. Violet in color. • Potassium bicarbonate & Urea Complex (aka Monnex), used on Class B and C fires. More effective than all other powders due to its ability to decrepitated (where the powder breaks up into smaller particles) in the flame zone creating a larger surface area for free radical inhibition.

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• Potassium Chloride, or Super-K dry chemical was developed in an effort to create a high efficiency, protein-foam compatible dry chemical. Developed in the 60s, prior to Purple-K, it was never as popular as other agents since being a salt, it was quite corrosive. For B and C fires, white in color. • Foam-Compatible, which is a sodium bicarbonate (BC) based dry chemical, was developed for use with protein foams for fighting class B fires. Most dry chemicals contain metal stearates to waterproof them, but these will tend to destroy the foam blanket created by protein (animal) based foams. Foam compatible type uses silicone as a waterproofing agent, which does not harm foam. Effectiveness is identical to regular dry chemical, and it is light green in color (some ANSUL brand formulations are blue). This agent is generally no longer used since most modern dry chemicals are considered compatible with synthetic foams such as AFFF. • MET-L-KYL is a specialty variation of sodium bicarbonate for fighting pyrophoric liquid fires (ignite on contact with air). In addition to sodium bicarbonate, it also contains silica gel particles. The sodium bicarbonate interrupts the chain reaction of the fuel and the silica soaks up any unburned fuel, preventing contact with air. It is effective on other class B fuels as well. Blue/Red in color.

Foams

Applied to fuel fires as either an aspirated (mixed & expanded with air in a branch pipe) or non aspirated form to form a frothy blanket or seal over the fuel, preventing oxygen reaching it. Unlike powder, foam can be used to progressively extinguish fires without flashback.• AFFF (aqueous film forming foam), used on A and B fires and for vapor suppression. The most common type in portable foam extinguishers. It contains fluoro tensides which can be accumulated in human body. The long-term effects of this on the human body and environment are unclear at this time. • AR-AFFF (Alcohol-resistant aqueous film forming foams), used on fuel fires containing alcohol. Forms a membrane between the fuel and the foam preventing the alcohol from breaking down the foam blanket. • FFFP (film forming fluoroprotein) contains naturally occurring proteins from animal by-products and synthetic film-forming agents to create a foam blanket that is more heat resistant than the strictly synthetic AFFF foams. FFFP works well on alcohol-based liquids and is used widely in motor sports. • CAFS (compressed air foam system) Any APW style extinguisher that is charged with a foam solution and pressurized with compressed air. Generally used to extend a water supply in wild land operations. Used on class A fires and with very dry foam on class B for vapor suppression. • Arctic Fire is a liquid fire extinguishing agent that emulsifies and cools heated materials more quickly than water or ordinary foam. It is used extensively in the steel industry. Effective on classes A, B, and D. • Fire Ade, a foaming agent that emulsifies burning liquids and renders them non-flammable. It is able to cool heated material and surfaces similar to CAFS. Used on A and B (said to be effective on some class D hazards, although not recommended due to the fact that fireade still contains amounts of water which will react with some metal fires).

Cools burning material.

• APW (Air pressurized water) cools burning material by absorbing heat from burning material. Effective on Class A fires, it has the advantage of being inexpensive, harmless, and relatively easy to clean up. In the United States, APW units contain 2.5 gallons (9 liters) of water in

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a tall, stainless steel cylinder. In Europe, they are typically mild steel lined with polyethylene, painted red, containing 6-9 liters (1.75-2.5 gallons) of water. • Water Mist uses a fine misting nozzle to break up a stream of deionized water to the point of not conducting electricity back to the operator. Class A and C rated. It is used widely in hospitals for the reason that, unlike other clean-agent suppressants, it is harmless and non-contaminant. These extinguishers come in 1.75 and 2.5 gallon units, painted white in the United States and red in Europe. Wet chemical and water additives• Wet Chemical (potassium acetate, carbonate, or citrate) extinguishes the fire by forming a soapy foam blanket over the burning oil (saponification) and by cooling the oil below its ignition temperature. Generally class A and K (F in Europe) only, although newer models are outfitted with misting nozzles as those used on water mist units to give these extinguishers class B and C firefighting capability. • Wetting Agents Detergent based additives used to break the surface tension of water and improve penetration of Class A fires. • Antifreeze Chemicals added to water to lower its freezing point to about -40 degrees Fahrenheit. Has no appreciable effect on extinguishing performance.Clean agents and carbon dioxide

Agent displaces oxygen (CO2 or inert gases), removes heat from the combustion zone (Halotron, FE-36) or inhibits chemical chain reaction (Halons). They are labelled clean agents because they do not leave any residue after discharge which is ideal for sensitive electronics and documents.• Halon (including Halon 1211 and Halon 1301), a gaseous agent that inhibits the chemical reaction of the fire. Classes B:C for lower weight fire extinguishers (2.3 kg ; under 9 lbs) and A:B:C for heavier weights (4.1-7.7 kg ; 9-17 lbs). Banned from new production, except for military use, as of January 1, 1994 as its properties contribute to ozone depletion and long atmospheric lifetime, usually 400 years. Halon was completely banned in Europe resulting in stockpiles being sent to the United States for reuse. Although production has been banned, the reuse is still permitted. Halon 1301 and 1211 are being replaced with new halocarbon agents which have no ozone depletion properties and low atmospheric lifetimes, but are less effective. Currently Halotron I, Halotron II, FE-36 Clean guard and FM-200 are meant to be replacements with significantly reduced ozone depletion potential. • CO2, a clean gaseous agent which displaces oxygen. Highest rating for 7.7 kg (20 pound) portable CO2 extinguishers is 10B:C. Not intended for Class A fires, as the high-pressure cloud of gas can scatter burning materials. CO2 is not suitable for use on fires containing their own oxygen source, metals or cooking media. Although it can be rather successful on a person on fire, its use should be avoided where possible as it can cause chemical burns and is dangerous to use as it may displace the oxygen needed for breathing, causing suffocation. • Mixtures of inert gases, including Inergen and Argonite.

Class D

There are several Class D fire extinguisher agents available, some will handle multiple types of metals, others will not.• Sodium Chloride (Super-D, Met-L-X or METAL.FIRE.XTNGSHR) -contains sodium chloride salt and thermoplastic additive. Plastic melts to form an oxygen-excluding crust over the metal, and the salt dissipates heat. Useful on most alkali metals including sodium and potassium, and other metals including magnesium, titanium, aluminum, and zirconium.

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• Copper based (Copper Powder Navy125S) -developed by the U.S. Navy in the 70s for hard-to-control lithium and lithium-alloy fires. Powder smothers and acts as a heat sink to dissipate heat, but also forms a copper-lithium alloy on the surface which is non-combustible and cuts off the oxygen supply. Will cling to a vertical surface-lithium only. • Graphite based (G-Plus, G-1, Lith-X, Pyromet or METAL.FIRE.XTNGSHR) -contains dry graphite that smothers burning metals. First type developed, designed for magnesium, works on other metals as well. Unlike sodium chloride powder extinguishers, the graphite powder fire extinguishers can be used on very hot burning metal fires such as lithium, but unlike copper powder extinguishers will not stick to and extinguish flowing or vertical lithium fires. Like copper extinguishers, the graphite powder acts as a heat sink as well as smothering the metal fire. • Sodium carbonate based (Na-X)-used where stainless steel piping and equipment could be damaged by sodium chloride based agents to control sodium, potassium, and sodium-potassium alloy fires. Limited use on other metals. Smothers and forms a crust. Some metals, such as elemental Lithium, will react explosively with water, therefore water-based chemicals should never be used on any Class D fire due to the possibility of a violent reaction.Most Class D extinguishers will have a special low velocity nozzle or discharge wand to gently apply the agent in large volumes to avoid disrupting any finely divided burning materials. Agents are also available in bulk and can be applied with a scoop or shovel.

Fire Extinguishing Ball

Several modern ball or "grenade" style extinguishers are on the market. They are manually operated by rolling or throwing into a fire. The modern versions of the ball will self destruct once in contact with flame, dispersing a cloud of ABC dry chemical powder over the fire which extinguishes the flame. The coverage area is about 5 square meters. One benefit of this type is that it may be used for passive suppression. The ball can be placed in a fire prone area and will deploy automatically if a fire develops, being triggered by heat. Most modern extinguishers of this type are designed to make a loud noise upon deployment.[6]This technology is not new, however. In the 1800s, glass fire grenades filled with suppressant liquids were popular. These glass fire grenade bottles are sought by collectors. [7] Some later brands, such as Red Comet, were designed for passive operation, and included a special holder with a spring loaded trigger that would break the glass ball when a fusible link melted. As was typical of this era, some glass extinguishers contained the toxic carbon tetrachloride.

Maintenance

Most countries in the world require regular fire extinguisher maintenance by a competent person to operate safely and effectively, as part of fire safety legislation. Lack of maintenance can lead to an extinguisher not discharging when required, or rupturing when pressurized. Deaths have occurred, even in recent times, from corroded extinguishers exploding.There is no all-encompassing fire code in the United States. Generally, most municipalities (by adoption of the International Fire Code) require inspections every 30 days to ensure the unit is pressurized and unobstructed (done by an employee of the facility) and an annual inspection by a qualified technician. Hydrostatic pressure testing for all types of extinguishers is also required, generally every five years for water and CO2 models up to every 12 years for dry chemical models.Recently the National Fire Protection Association and ICC voted to allow for the elimination of the 30 day inspection requirement so long as the fire extinguisher is monitored electronically. According to NFPA, the system must provide record keeping in the form of an electronic event log at the control panel. The system must also constantly monitor an extinguisher’s physical presence, internal

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pressure and whether an obstruction exists that could prevent ready access. In the event that any of the above conditions are found, the system must send an alert to officials so they can immediately rectify the situation. Electronic monitoring can be wired or wireless.In the UK, three types of maintenance are required:• Basic Service: All types of extinguisher require a basic inspection annually to check weight, correct pressure (using a special tool, not just looking at the gauge) and for signs of damage or corrosion, cartridge extinguishers are opened up for internal inspection & check weighing of the cartridge, labels are checked for legibility, where possible dip tubes, hoses and mechanisms checked for clear free operation. • Extended Service: Water, Wet Chemical, Foam & Powder extinguishers require every five years a more detailed examination including a test discharge of the extinguisher and recharging- on stored pressure extinguishers this is the only opportunity to internally inspect for damage/corrosion. By recharging fresh agent is used as they all have a shelf life, even water goes foul inside an extinguisher; Note: extinguishers should be percentage tested according to total number of units in any given area. Some extinguishers contain pressure in excess of 1.38 MPa (200psi) and this internal pressure over periods of time affects each brand & make differently depending on their placement & location. • Overhaul: CO2 extinguishers, due to their high operating pressure, are subject to pressure vessel safety legislation and must be hydraulic pressure tested, inspected internally & externally and date stamped every 10 years. As it cannot be pressure tested a new valve is also fitted. If replacing any part of the extinguisher (valve, horn, etc) with a part from another manufacturer then the extinguisher will lose its fire rating. This may invalidate insurance, as would incorrect or inadequate servicing if it were to be found. In the United States there are 3 types of service as well:• Maintenance Inspection: All types, annually (with the exception of water types which require a yearly recharge), consists of a physical maintenance and visual inspection. The extinguisher is checked to make sure it has good pressure (gauge in green or proper cartridge weight), has the correct volume of agent (tech weighs it), is within the required hydro test and internal maintenance intervals, is in good condition and all external parts are serviceable. Dry chemical and dry powder types are hit on the bottom with a rubber mallet to make sure the powder is free-flowing, which is called "fluffing" the powder. The tech will then attach a new tamper seal around the pin and a yearly service tag. • Internal Maintenance: Water-annually, foam-every 3 years, wet chemical and CO2, every 5 years, dry chemical, dry powder, halon and clean agents, 6 years. The extinguisher is emptied of its chemical and pressure to check for proper operation. All components are disassembled, inspected, cleaned, lubricated, or replaced if defective. Liquid agents are replaced at this time, dry agents may be re-used if in good condition, halon is recovered and re-used, but CO2 is discharged into the atmosphere. The extinguisher is then re-filled and recharged, after a "verification of service" collar is placed around the cylinder neck. It is impossible to properly install or remove a collar without depressurizing the extinguisher. Note: Cartridge-operated extinguishers should be visually examined, but do not require a verification of service collar.• Hydrostatic testing: Water, Foam, Wet chemical, and CO2, every 5 years. Dry chemical, dry powder, halon, and clean agents, every 12 years. Extinguishers installed on vehicles every 5 years regardless of type.Note: these are the required intervals for normal service conditions, if the extinguisher has been exposed to excessive heat, vibration, or mechanical damage it may need to be tested sooner.The agent is emptied and depressurized and the valve is removed. After a thorough internal and external visual inspection, the cylinder is filled with water, placed inside a safety cage, and pressurized to the specified test pressure (varies with the type, age, and cylinder material) for the

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specified time period. If no failure, bulges, or leaks are detected, the cylinder passes. The cylinder is then emptied of water and thoroughly dried. CO2 types have the test date, company's ID, etc. stamped on the cylinder, all other types get a sticker on the back of the cylinder. Once dry, the units are recharged. Unlike the UK, the US does not rebuild extinguishers and replace valves at specific intervals unless parts are found to be defective, with the exception of halon. Halon types are often given new o-rings and valve stems at every internal maintenance to minimize any leakage potential.OEM equipment must be used for replacement parts for the extinguisher to maintain its UL rating. If parts are unavailable, replacement is recommended, keep in mind extinguishers have a projected service life of about 25–35 years, although many are of such quality that they can outlast this, but realize that science is ever-changing, and something that was the best available 30 years ago may not be acceptable for modern fire protection needs.

Vandalism and extinguisher protection

Fire extinguishers can be a target of vandalism in schools and other open spaces. Extinguishers can be partially or fully discharged by a vandal, impairing the extinguisher's actual firefighting abilities.In open public spaces, extinguishers are ideally kept inside cabinets that have glass that must be broken to access the extinguisher, or which emit an alarm siren that cannot be shut off without a key, to alert people the extinguisher has been handled by an unauthorized person when a fire is not present.Fire extinguisher signsFire extinguisher identification signs are small signs designed to be mounted near a fire extinguisher, in order to draw attention to the extinguisher's location (Ex. If the Extinguisher is on a large pole the sign would generally be at the top of the pole so it can be seen from a distance) Such signs may be manufactured from a variety of materials, commonly self-adhesive vinyl, rigid PVC and aluminum.In addition to words and pictographs indicating the presence of a fire extinguisher , some modern extinguisher ID signs also describe the extinguishing agent in the unit, and summarize the types of fire on which it may safely be used.Some public and government buildings are often required, by local legal codes, to provide an ID sign for each extinguisher on the site.Similar signs are available for other fire equipment (including fire blankets and fire hose reels/racks), and for other emergency equipment (such as first aid kits).

Placement of fire extinguisher signsFire extinguisher signs are mounted above or to the side of the extinguisher they relate to.Most licensing authorities have regulations describing the standard appearance of these signs (e.g. text height, pictographs used and so on).

Photo-luminescent fire extinguisher signsPhoto-luminescent fire extinguisher signs are made with a photo luminescent phosphor that absorbs ambient light and releases it slowly in dark conditions - the sign "glows in the dark". Such signs are independent of an external power supply, and so offer a low-cost, reliable means of indicating the position of emergency equipment in dark or smoky conditions.Photo-luminescent signs are sometimes wrongfully described as being reflective. A reflective material will only return ambient light for as long as the light source is supplied, rather than storing energy and releasing it over a period of time. However, many fire extinguishers and extinguisher mounting posts have strips of retro reflective adhesive tape placed on them to facilitate their location in situations where only emergency lighting or flashlights are available now.

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References1. ^ The United States Patent and Trademark Office * Volume 192 - September 15, 1881 2. ^ "Staffordshire Past Track -"Petrolex" half gallon fire extinguisher". http://www.search.staffspasttrack.org.uk/engine/resource/default.asp?resource=14547. Retrieved 2009-05-25. 3. ^ "Halon Disposal". Ozone Protection. Australian Government Department of the Environment and Heritage (Australia). http://www.deh.gov.au/atmosphere/ozone/ods/halon/disposal.html. Retrieved 2006-12-12. 4. ^ a b c "The Fire Safety Advice Centre". http://www.firesafe.org.uk/html/fsequip/exting.htm. 5. ^ Wasserfilmbildendes Schaummittel - Extensid AFFFPDF 071027 intersales.info 6. ^ Chuck a ball to put out fire, Earth Times, 14 September 2007, http://www.earthtimes.org/articles/show/107481.html 7. ^ firegrenade.com 8. ^ CAIS16 - Safety signs in the catering industry 9. ^ Transport for London • Automatic Sprinkler Protection - Goram Dana, S.B.

DEFINITIONS & TERMINOLOGY

A Absolute Zero: -459.67°F or -273.15°C or 0 Kelvin. The temperature where thermal energy is at

minimum. Actuator: The mechanism of a switch which operates the contacts.

Alternating Current: (AC); Electrical current that changes (or alternates) in magnitude and direction of the current at regular intervals.

American National Standards Institute:ANSI;

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Amp: (ampere)The basic unit of current in an electrical circuit. One ampere is the rate of flow of electric current when one coulomb of charge flows past a point in the circuit in one second. Symbolically characterized by the letter "I" and sometimes "A" when used in formulas.

Ampere:A unit of electrical current named after French physicist André Marie Ampère (1775-1836), also see "amp".

Amplifier: An electrical circuit that increases the power, voltage or current of an applied signal.

Anode: A positive (+) electrode. The point where electrons exit from a device to the external electric circuit.

AWG: "American Wire Gauge" system used to determine wire size.

B Beta: The current that is gained by a transistor when it is connected in a common emitter circuit. Break: The act of the opening of an electrical circuit.

Bridge Rectifier: A full-wave rectifier where the diodes are connected in a bridge circuit. This allows the current to the load during both the positive and negative alternating of the supply voltage.

BTU: "British Thermal Unit", the amount of thermal energy required to raise one pound of water 1degree F. One BTU is equal to .293 watt hours. One kWh is equal to 3412 BTUs.

C Canadian Standards Administration: CSA Capacitor: A device used to store electrical energy in an electrostatic field until discharge.

Cathode: A negative (-) electrode. The point of entry of electrons into a device from an external circuit. The negative electrode of a semiconductor diode.

Celsiuss: A temperature scale. Also known as centigrade. Sea level water will freeze at 0°C and will boil at 100°C.

Charge: The measured amount of electrical energy that represents the electrostatic forces between atomic particles. The nucleus of an atom has a positive charge (+) and the electrons have a negative charge(-).

Chatter: The rapid on/off cycling of a relay caused by improper signal or adjustment, faulty contacts, or other malfunction.

Circuit: A full path of electrical current from a voltage source that passes completely from one terminal of the voltage source to another.

CMV: "Common Mode Voltage." The voltage which is tolerable between signal and ground.

Conductance: The measure of the ability of a material or substance to carry electrical current.

Conduction: The moving of electricity or heat through a conductor.

Conductor: A material used to conduct electricity or heat.

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Conduit: A tube, pipe or trough that carries and protects electric wiring.

Coulomb: A unit of electric charge. The amount of charge conveyed in one second by one ampere.

Current: The rate at which electricity flows, measured in amperes, 1 ampere = 1 coulomb per second.

Current Proportioning: In a temperature controller it is the output form that provides a current proportional to the amount of control that is required. Commonly it is the 4 to 20 milliamp current proportioning band that is used in the electronics industry.

Cycle: or Hertz; the measurement of the time period of one alternating electric current. In the United States this is commonly 60 cycles per second, or 60 Hertz.

Cycle Time: The time it takes for a controller to complete one on/off cycle.

D Delta: In a three phase connection all three phases are connected in series thus forming a closed circuit. Dielectric: Non-conducting material used to isolate and/or insulate energized electrical components.

Diode: A device having two terminals and has a low resistance to electrical current in one direction and a high resistance in the other direction.

Direct Current: (DC); Electrical current that flows consistently in one direction only.

E Efficiency: Output power divided by input power, (work performed in ratio to energy used to produce it). Electric circuit: An arrangement of any of various conductors through which electric current can flow

from a supply current.

Electricity: A form of energy produced by the flow of particles of matter and consists of commonly attractive positively (protons [+]) and negatively (electrons [-]) charged atomic particles. A stream of electrons, or an electric current.

Electrochemistry: Chemical changes and energy produced by electric currents.

Electrode: An anode (+) or cathode (-) conductor on a device through which an electric current passes.

Electroduct: An interconnected arrangement of parts for carrying high-voltage electricity.

Electrodynamic: The interaction of magnetism and electrical current.

Electrokinetics: The behavior of charged particles and the steady motion of charge in magnetic and electric fields.

Electrolysis: Electric current passing through an electrolyte which produces chemical changes in it.

Electrolyte: An electrically conductive fused salt or a solution where the charge is carried by ionic movement.

Electromagnet: A coil of wire wound about a magnetic material, such as iron, that produces a magnetic field when current flows through the wire.

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Electromagnetic field: Electric and magnetic force field that surrounds a moving electric charge.

Electron: A fundamental negatively (-) charged atomic particle that rotates around a positively (+) charged nucleus of the atom.

Environmental Protection Agency: EPA;

F Factory Mutual:FM; Farad: The unit for capacitance. A capacitor that stored one coulomb of charge with one volt across it

will have a value of one farad.

Field cell: Commonly used in generators and motors, it is an electromagnet formed from a coil of insulated wire that is wound around a soft iron core.

Field-Effect Transistor (FET): A three terminal semiconductor device. In a "FET" the current is from source to drain because a conducting channel is formed by a voltage field between the gate and the source.

Filament: The element inside a vacuum tube, incandescent lamp or other similar device.

Filter: A circuit element or components that allows signals of certain frequencies to pass and blocks signals of other frequencies.

Fluorescent: The quality of having the ability to emit light when struck by electrons or another form of radiation.

Flux: The rate of transfer of energy.

Forward resistance: When there is current through a semiconductor p-n junction it is the resistance of a forward-biased junction.

Forward Voltage: The voltage that is applied across a semiconductor junction to permit forward current through that junction and the device. Forward voltage is also known as "bias."

Frequency: Also known as Hertz, it is the number of complete cycles of periodic waveform that occur during a time period of one second.

G Gain: The increase of the power level, current or voltage of a signal. In an amplifier it is the ratio of the

output to the input signal levels. Ground: A reference point at zero potential with respect to the earth. In an electronic circuit it is the

common return path for electric current. A conducting connection between the earth and an electrical circuit or electrical equipment. Also, the negative side of DC power supply.

Grounded: A connected path to earth or to a conductive body that has a reference potential to earth.

Grounded Conductor: A circuit conductor that is grounded to become part of the electric circuit by design and intent.

Grounding Conductor: The conductor that is used by intent to connect the grounded circuit of an electrical wiring system or equipment to a grounding electrode with reference to earth.

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H Hard Wired: That part of a circuit which is physically interconnected. Hazardous Location: An area in which combustible or flammable mixtures are or could be present.

I I: Intensity. The commonly used symbol used to represent Amperes when used in formulas. I = Intensity

= Current = Amps = Amperes. Impedance: The opposition to electrical flow.

Infrared: The form of radiation used to make non-contact temperature measurements. In the electromagnetic spectrum it is the area beyond red light from 760 nanometers to 1000 microns.

Ingress Protection Ratings: European environmental ratings. Similar to NEMA ratings in the USA. IP;*site has NEMA comparisons

Institute of Electrical and Electronics Engineers: IEEE;

Interface:The method by which two devices or systems are connected and interact with each other.

International Brotherhood of Electrical Workers:IBEW;

International Electrotechnical Commission:IEC;

International Organization for Standardization:ISO;

Intrinsically safe: A device, instrument or component that will not produce any spark or thermal effects under any conditions that are normal or abnormal that will ignite a specified gas mixture. Electrical and thermal energy limits are at levels incapable of causing ignition. It is common practice to use external barriers with intrinsically safe installations.

Instrument Society of America:ISA;

Isothermal: A process that is kept at a constant temperature.

J Joule: The basic of thermal energy. The work done by the force of one Newton acting through a distance

of one meter.

K Kilovolt: kV; One thousand volts. Kilovolt amperes: Kva; One thousand volt amps.

Kilowatt: Kw; One thousand watts.

Kilowatt Hour: Kwh; One thousand watt-hours.

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Lag: The time delay between the output signal and the response time of the receiver of the signal. Latching logic: The modification of a signal that causes the output to remain energized until it is released

by intent.

Latent heat: The amount of heat needed to convert one pound of water to one pound of steam. Latent heat is expressed in BTU per pound.

Leakage current: A small current leaking from an output device in the off state caused by semiconductor characteristics.

Lesco: London Electric Supply Corporation (LESCo) established in 1887.

Light Emitting Diode: LED; A solid state light source component that emits light or invisible infrared radiation.

Load: The electrical demand of a process. Load can be expressed or calculated as amps (current), ohms (resistance) or watts (power).

M M: Symbol for Mega, one million. Magnetic Blow-out Switch: A switching device used in switching high DC loads. It contains a small

permanent magnet which deflects arc in order to quench it.

Magnetic Field: A region of space that surrounds a moving electrical charge or a magnetic pole, in which the electrical charge or magnetic pole experiences a force that is above the electrostatic ones associated with particles at rest.

Magnetic Flux: Expressed in webers, it is the product of the average normal component of the magnetic intensity over a surface and the area of that surface.

Make: To close an electrical circuit. To establish an electrical circuit through the closing of a contact, switch or other related device.

Manual Reset Switch: A switch in a controller that manually resets after exceeding the controllers limit.

Maximum Load Current: see; "Maximum Power Rating".

Maximum Operating Temperature: The maximum temperature at which a device can be safely operated.

Maximum Power Rating: The maximum watts that a device can safely handle.

Mean Temperature: The average temperature of a process.

Microamp: One millionth of an amp.

Micron: One millionth of a meter.

Microvolt: One millionth of a volt.

Mil: One thousandth of an inch.

Milliamp: mA; One thousandth of an amp.

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Millimeter: mm; One thousandth of a meter.

Milli volt: mV; One thousandth of a volt. The difference in potential needed to cause a current of one milli ampere flow through a resistance of one ohm.

Modulated Light Source Control: MLS; A photoelectric control that operates on pulsed infrared radiation at a specific frequency, and responds only to that frequency of pulse. MLS is used frequently in areas where ambient light may cause problems with other types of sensors.

Momentary switch: A switch with contacts that are made with actuating force and released when that force is removed.

Mueller Bridge: A highly accurate bridge configuration that is used to measure three-wire RTD thermometers.

N National Electrical Code: NEC: A set of regulations pertaining to electrical installation and design in the

interest of the protection of life and property. The NEC is adopted by NFPA and approved by ANSI. It is the preferred standard of guidelines used by most electrical regulatory boards in the USA.

National Electrical Manufacturers Association:NEMA;

National Fire Protection Association:NFPA;

N.C.: Normally Closed.

N.O.: Normally Open.

O Occupational Safety & Health Administration:OSHA; Ohm: The unit by which electrical resistance is measured. One ohm is equal to the current of one ampere

which will flow when a voltage of one volt is applied

Ohmmeter: A meter used to measure electrical resistance in units of ohms.

On/Off Controller: A controller whose action is either fully on or off.

Open Circuit: An electrical circuit that is not "made". Contacts, switches or similar devices are open and preventing the floe of current.

Operating Temperature: The range of temperature over which a device may be safely used. The temperature range which the device has been designed to operate.

OR Logic: The output that is produced when one or more inputs are present.

Output: The energy delivered by a circuit or device. The electrical signal produced by the input to the transducer.

P Phase: The time based relationship between a reference and a periodic function.

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Phase Proportioning: A form of control where the power supplied to a process is controlled by limiting the phase angle of the line voltage.

PID: A three mode control consisting of time Proportioning, Integral and Derivative rate action.

Plasma: An ionized gas containing about equal numbers of positive and negative charges, which is a good conductor of electricity, and is affected by a magnetic field.

Polarity: Magnetically, opposite poles, north and south. In electricity, oppositely charged poles, positive and negative.

Potentiometer: A variable resistor.

Power Dissipation: The amount of power that is consumed and converted to heat.

Power Supply: The part of a circuit that supplies power to the entire circuit or part of the circuit. Usually a separate unit that supplies power to a specific part of the circuit in a system.

Process Meter: A panel meter with zero and span adjustments commonly scaled for signals such as 1-5 volts, 4-20mA, etc.

Proximity Sensor: A sensor or switch with the ability to detect it’s relationship to a metal target without making physical contact.

Proximity Switch: see; "Proximity Sensor".

PSIA: Pounds per square inch absolute. Pressure commonly in reference to vacuum.

PSID: Pounds per square inch differential. The difference in pressure between two points.

PSIG: Pounds per square inch gage. Pressure in relationship to the ambient air pressure>

Pulse: A rise and fall of voltage, current, or other faction that would be constant under normal conditions. A pulse that is intentionally induced will have a finite duration time.

Q Quality Control: Inspection, analysis and action required to ensure quality of output. Quantum: One of the very small discrete packets into which many forms of energy are subdivided.

Quantum Electronics: Applying molecular physics to electronics.

Quap: A hypothetical nuclear of a quark plus an antiproton.

Quark: A hypothetical basic subatomic nuclear particle believed to be the basic component of protons, neutrons, etc.

Quartz: A form of silicone dioxide. Commonly used in the making of radio transmitters and heat resistant products.

Quartziodine Lamp: A high-intensity incandescent lamp with a quartz bulb containing an inert gas of iodine or bromine vapor.

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Quasiparticle: A unit of energy in solid-state physics with mass and momentum but that does not exist as a free particle.

Q-value: The amount of energy released in a nuclear reaction. It is expressed in atomic mass units, or in million electron volts (MEV).

R Rectifier: A device that converts AC voltage to pulsating DC voltage. Relay: A Solid State relay is a switching device that completes or interrupts a circuit electrically and has

no moving parts. A Mechanical relay is an electromechanical device that closes contacts to complete a circuit or opens contacts to interrupt a circuit.

Resistance: The resistance to electrical current. Resistance is measured in ohms.

Response Time: The amount of time it takes for a device to react to an input signal.

RFI: Radio Frequency Interference.

Rheostat: A variable resistor.

Ripple: A fluctuation in the intensity of a steady current.

Root Mean Square: RMS; AC voltage that equals DC voltage that will do the same amount of work. For an AC sine wave it is 0.707 x peak voltage.

RTD: Resistance Temperature Detector.

S SCR: Silicone Controlled Rectifier. Series Circuit: A circuit which may have one or many resistors and/or other various devices connected in

a series so that the current has only one path to follow.

Supply Current: Current Consumption. The amount of amps or milliamps needed to maintain operation of a control or device.

Supply Voltage: The range of voltage needed to maintain operation of a control or device.

System International: SI; The standard metric system of units.

T Thermistor: An electrical resistor composed of semiconductor material, whose resistance is a known

rapidly varying function of temperature. Thermocouple: Two dissimilar metals connected at a point, which produces an electrical current whose

magnitude is dependent upon the temperature at the junction point.

Thermoelectricity: Electrical energy produced by the action of heat.

Threshold Response: Response to the change in the level of the input signal.

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Thyristor: A solid-state switching device for semiconductors to convert AC current in one of two directions controlled by an electrode.

Time Delay Before Availability: The delayed period of time when outputs are turned off when power is initially applied.

Transducer: A device that transfers power or energy from one system to another, such as taking a physical quality and changing it to an electrical signal.

Transient: A sudden and unwanted increase or decrease of supply voltage or current.

Transient Protection: Protective circuitry to guard against spikes that might be induced on the supply line.

Transistor: A device incorporating semiconductor material and suitable contacts capable of performing electrical functions (such as voltage, current or power amplification) with low power requirements.

Triac: A solid-state switching device used in switching AC wave forms.

U UHF: Ultra High Frequency Underwriters Laboratories:UL;

V Vacuum: Pressure that is less than atmospheric pressure. Vector: The magnitude and time phase of a quantity, represented by a plotted line.

Velocity: The speed or time rate of change of displacement.

VF: Variable Frequency.

VHF: Very High Frequency.

Volt: Voltage; The unit of electromotive force (EMF) that causes current to flow. One volt causes a current of one amp through a resistance of one ohm.

Voltage Drop: The difference in potential measured between two points caused by resistance or impedance.

Voltmeter: A meter used to measure units of volts.

VOM: Volt-ohm Meter.

W Watt: The unit of power. One watt equals one joule per second, 1/746th horsepower. Watt-hour: The power of one watt operating for one hour, and equal to 3,600 joules.

Weber: The standard unit of magnetic flux.

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Working Standard: The standard that is used to make comparison measurements or calibrations.

X XFMR: Symbol used to denote transformer. XMTR: Symbol used to denote transmitter.

X ray: An electromagnetic radiation produced when the inner satellite electrons of heavy atoms have been excited by collision with a stream of fast electrons return to their ground state, giving up the energy previously imparted to them.

Y Y: Symbol used for wye configuration for three phase electrical connections.

Z Zener Diode: A silicone semiconductor that maintains a fixed voltage in a circuit. Zener Effect: The pronounced curvature in reverse voltage current that is characteristic of a diode.

Zero Adjustment: The adjustment of a display that results are zero on the display corresponding to a non-zero signal.

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