arc supression

7/15/2019 arc supression

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ARC SUPPRESSION

Arc suppression is the reduction of sparks formed when current-carrying contacts are separated. The

spark is a luminous discharge of highly energized electrons and ions, and is an electric arc

There are several possible areas of use of arc suppression methods, among them metal film depositionand sputtering, arc flash protection, electrostatic processes where electrical arcs are not desired (such

as powder painting, air purification, PVDF film poling) and contact current arc suppression. In industrial,

military and consumer electronic design, the latter method generally applies to devices such as

electromechanical power switches, relays and contactors. In this context, arc suppression refers to the

concept of contact protection

Arc suppression as contact protection

Every time an electrical power device (for example: heaters, lamps, motors, transformers or similar

power loads) turns on or off its switch, relay or contactor transitions either from a closed to an openstate (break arc) or from an open to a closed state (make arc & bounce arc), under load, an electrical arc

occurs between the two contact points (electrodes) of the electromechanical power switch, relay or

contactor. The break arc is typically more energetic and thus more destructive.

The energy contained in the resulting electrical arc is very high (tens of thousands of degrees

Fahrenheit), causing the metal on the contact surfaces to melt, pool and migrate with the current. The

extremely high temperature of the arc cracks the surrounding gas molecules creating ozone, carbon

monoxide, and other compounds. The arc energy slowly destroys the contact metal, causing some

material to escape into the air as fine particulate matter. This very activity causes the material in the

contacts to degrade quickly, resulting in device failure.

Arc suppression is an area of interest in engineering because of the destructive effects of the electrical

arc to electromechanical power switches, relays and contactors’ points of contact

(left) Pristine contacts from a relay

(right) The nearly destroyed contacts from a relay operated under power for nearly 100,000 cycles



Mechanical relays are a very common and versatile method for switching a variety of electrical signals

and power. The mechanical contacts in relays provide good isolation between the circuits activating

them and the signals being switched, and can often handle large amounts of current. However, there

are some effects related to mechanical contacts that must be considered when designing a system using

relays, particularly when they are switching power.

(The contacts on manually operated mechanical switches experience these effects too, but since these

switches are usually operated less frequently for uses such as switching equipment power on and off,

these effects are usually less of a concern.)

Contact Wear, Arcing, and Noise

Every time the contacts of a mechanical relay or switch are closed or open, there is a certain amount of

wear. For a very brief time, only a small section of the contact is touching, and all of the current must go

through this part of the contact. If the current being switched is large, part of the contact is degraded ordestroyed. (The manufacturer of the component will include life expectancy information in their product

specifications.)

Also, because there is a very small gap in the contacts for a brief time when the contacts are broken, an

electrical arc may be generated across the gap if the voltage is high enough. This arc will produce a

certain amount of radio frequency interference (RFI) through the air, and noise in the system power

supply, and may affect nearby electronic circuits in a number of ways.

How much wear on the contacts and how much RFI and noise generated depends on:

The voltage and current being switched

Whether the voltage being switched is AC or DC

The type of load (resistive versus inductive)

How quickly the relay operates (how long the contact area and gap are small)

What type of contact protection or arc and noise suppression circuitry is used

(Many other factors, such as system wiring, grounding, etc.)

Protection and Suppression Circuitry

Because of these effects it is sometimes desirable to include contact protection and arc suppression

circuitry to relays, however, most companies that manufacture products that can use their relays in a

variety of applications do not include this circuitry. Why? Well, unless it is known that the relays will be

switching particular types of power signals, adding this circuitry would reduce the versatility of the

product by preventing it from being used to switch other signals properly.



For this reason it is sometimes up to the system designer to add protection and suppression circuitry

appropriate to the power being switched if the system experiences any problems due to the effects of

this switching.

Example Circuitry

The figure below shows one of the most common ways of protecting relay contacts and suppressing

noise and arcing caused by the contacts. Note that there are many ways of doing this, and that the

circuit shown below is only one of many methods. In particular, RFI and power supply noise problems

may be difficult to eliminate and may require some experimentation or the use of more advanced

suppression circuitry.

The component values specified in the figure come from several sources and are included as a

comparison of these sources.



ARC CHUTES

A set of insulating barriers on a circuit breaker arranged to confine the arc and prevent it from causing

damage

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings have metal

plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils or permanent

magnets deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through

the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and

then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize 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 very quickly: typically the arc is extinguished

between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and

construction of the device.

Arc chutes are an integral part of a modern industrial or power circuit breaker.

As the circuit breaker contacts open, the arc chute segments, cools, and moves the arc away from the

contacts. Moving the arc away preserves the integrity of the contacts for a longer service life. Standard

arc chute designs absorb about 80% of the energy released during an interruption.

These drawings show a typical arc chute in cross-section:

In addition to the standard arc chute plates, the arc chutes include a series of metallic grids and meshes

that significantly reduce the ionized gas released during an interruption. The grids and meshes absorb

up to 90 –95% of the arcing energy, considerably more than the 80% seen in standard arc chutes. The

emissions are deionized and cooled, resulting in pressure and arcing reductions. The new arc chute



assembly also allows for smaller electrical clearances that provide smaller installed volumes and smaller

equipment footprints

.

The circuit breaker of the present invention includes an electrical arc extinguishing apparatus. The

electric arc extinguishing apparatus includes a first sidewall in a spaced relationship with the second

sidewall with a top arc plate mounted between the first and second sidewalls. A plurality of

intermediate arc plates are mounted between the first and second sidewalls below the top arc plate

with each in a spaced apart relationship. A bottom arc plate is mounted between the first and second

sidewalls below and apart from the intermediate plates forming an arc chute. One embodiment of the

electric arc extinguishing apparatus includes a top arc plate having an arc runner extending into the arc

chute. Another embodiment includes a bottom arc plate that has an arc runner extending into the arc

chute. A further embodiment includes an arc runner extending into the arc chute from both the top arc

plate and the bottom arc plate. The electric arc extinguishing apparatus can also be provided with two

end caps with each end cap having an interior cavity with one leg above each arc plate mounted in the

cavity of one end cap and the other leg of each arc plate mounted in the cavity of the other end cap.During a short circuit condition operation of the circuit breaker, the end caps expel a gas which assists in

extinguishing the arc generated between the contacts of the circuit breaker. The circuit breaker also

includes a first terminal and a second terminal mounted in the molded case of the circuit breaker. The

first contact is electrically coupled to the first terminal and the second contact is electrically coupled to

the second terminal. An operating mechanism having an ON position, an OFF position and a TRIPPED

position is coupled to the second contact. An intermediate latching mechanism is mounted in the

housing and is coupled to the operating mechanism. The trip unit coupled to the second contact and the

second terminal is selectively operative with the intermediate latching mechanism.



MAGNETIC BLOW OUT COILS

Blowout coils overcome this condition by providing a magnetic field, which pushes the arc away

from the contact area.The magnetic blowout operation is shown in fig-ure 1-18. It is important

that the fluxes remain in theproper relationship.

Otherwise, if the direction of thecurrent is changed, the direction of the blowout flux willbe

reversed and the arc will actually be pulled into thespace between the contacts.When the

direction of electron flow and flux areasshown in figure 1-18, the blowout force is upward.

Theblowout effect varies with the magnitude of the currentand with the blowout flux. The

blowout coil should bechosen to match the current so the correct amount offlux may be

obtained. The blowout flux across the arcgap is concentrated by the magnetic path provided

bythe steel core in the blowout coil and by the steel polepieces extending from the core to

either side of the gap.

A method of arc suppression that uses magnetic coils to create a magnetic field that pushes arc

upward until it breaks



Blow out coil is connected in series with contacts and hence carrying current always when contact is

closed. While opening of the contacts with load, heavy arcing takesplace between contacts and this

arcing current through blowout coil produces magnetic flux and motor torque, which expells the arc to

the arc chamber for extinguishing.

Magnetic blowouts use blowout coils to lengthen and move the electric arc. These are especially useful

in DC power circuits. AC arcs have periods of low current, during which the arc can be extinguished with

relative ease, but DC arcs have continuous high current, so blowing them out requires the arc to be

stretched further than an AC arc of the same current. The magnetic blowouts in the pictured Albright

contactor (which is designed for DC currents) more than double the current it can break, increasing it

from 600 A to 1,500 A.

Sometimes an economizer circuit is also installed to reduce the power required to keep a contactor

closed; an auxiliary contact reduces coil current after the contactor closes. A somewhat greater amount

of power is required to initially close a contactor than is required to keep it closed. Such a circuit can

save a substantial amount of power and allow the energized coil to stay cooler. Economizer circuits are

nearly always applied on direct-current contactor coils and on large alternating current contactor coils.

A basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on

the contactor design). The coil may be energized at the same voltage as the motor, or may be separately

controlled with a lower coil voltage better suited to control by programmable controllers and lower-

voltage pilot devices. Certain contactors have series coils connected in the motor circuit; these are used,

for example, for automatic acceleration control, where the next stage of resistance is not cut out until

the motor current has dropped

Unlike general-purpose relays, contactors are designed to be directly connected to high-current load

devices. Relays tend to be of lower capacity and are usually designed for both normally closed and

normally open applications. Devices switching more than 15 amperes or in circuits rated more than a

few kilowatts are usually called contactors. Apart from optional auxiliary low current contacts,

contactors are almost exclusively fitted with normally open ("form A") contacts. Unlike relays,contactors are designed with features to control and suppress the arc produced when interrupting

heavy motor currents.

When current passes through the electromagnet, a magnetic field is produced, which attracts the

moving core of the contactor. The electromagnet coil draws more current initially, until its inductance

increases when the metal core enters the coil. The moving contact is propelled by the moving core; the

force developed by the electromagnet holds the moving and fixed contacts together. When the

contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position

and opens the contacts.

For contactors energized with alternating current, a small part of the core is surrounded with a shading

coil, which slightly delays the magnetic flux in the core. The effect is to average out the alternating pullof the magnetic field and so prevent the core from buzzing at twice line frequency.

Because arcing and consequent damage occurs just as the contacts are opening or closing, contactors

are designed to open and close very rapidly; there is often an internal tipping point mechanism to

ensure rapid action.

Rapid closing can, however, lead to increase contact bounce which causes additional unwanted open-

close cycles. One solution is to have bifurcated contacts to minimize contact bounce; two contacts



designed to close simultaneously, but bounce at different times so the circuit will not be briefly

disconnected and cause an arc.

A slight variant has multiple contacts designed to engage in rapid succession. The first to make contact

and last to break will experience the greatest contact wear and will form a high-resistance connection

that would cause excessive heating inside the contactor. However, in doing so, it will protect the

primary contact from arcing, so a low contact resistance will be established a millisecond later.

Another technique for improving the life of contactors is contact wipe; the contacts move past each

other after initial contact on order to wipe off any contamination.

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arc supression