Read and Understand The Electrical Diagram

04/08/2023 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

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Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

Read & Understand The Electrical Diagram


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10. Read & understand the electrical diagramsElectrical ladder drawings are still one of the common and reliable tools used to troubleshoot equipment when it fails. As with any good troubleshooting tool, one must be familiar with its basic features to make the most of the diagram in the field. In other words, possessing a basic understanding of how the drawing is laid out — as well as the meaning of numbers and symbols found on the schematic — will make you that much more seasoned as a maintenance professional.

There are typically two distinct parts of a ladder drawing:

I. The power component:

The power portion consists of items such as the motor, motor starter

contacts and overloads, disconnect(s), and protective devices (fuses and

circuit breakers).

II. The control component:

The control part encompasses items that make the power components

do their work. For this discussion, we'll focus on the control portion of the

drawing. Let's take a look at the most common components.


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Fig: A sampling of common symbols you’ll find on the majority of ladder drawings. The normally open and normally closed symbols are added here to merely serve as an instructional aid.

Symbols. The symbols that go into making a ladder drawing are the ABCs of print reading (Fig). For instance, in an air compressor system there will be a symbol for a pressure switch. If the person doing the troubleshooting and repair does not recognize that symbol, it will be difficult to locate the switch to determine whether or not it is working properly.


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Normally Open (NO) and Normally Close (NC):Many times, the input devices are said to be either normally open (NO) or normally closed (NC). The normally open or closed status refers to the shelf state of the device. If a device is normally open, a resistance check of the device with a digital multimeter will give a reading of O.L. If the device is normally closed, a resistance check will give a reading of 0.0.The normally open and normally closed state of the devices is not labeled on the ladder drawing. Rather, you must recognize the symbol. A helpful hint in trying to determine whether the contacts are open or closed is to think of them in terms of gravity. If gravity is acting on the device, its normal state is as shown in the drawing. An exception to this concept is found in devices that contain springs. For instance, in the drawing of a normally open pushbutton, it appears that the pushbutton should be falling down and closing. However, there is a spring in the pushbutton that holds the contacts in the open (up) position.


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Rails: The drawing is called a ladder drawing because it resembles a ladder in the way it is constructed and presented on the paper. The two vertical lines (wires) that serve as a boundary for the control system and deliver the control voltage to the devices are called the rails. The rails may have overcurrent devices in them (fuses and/or circuit breakers) and may have contacts from control devices. These control lines may be thicker than the others to help better identify them. Like a real ladder, the rails are the supports for the rungs.

If the ladder drawing runs across several pages, the control voltage is carried from one page to the next along the rails. There are a number of ways that this may be represented on the drawing. One way this is done is with the use of continuation arrows at the bottom of the first page, which indicates that the rails continue on to another page. The page number on which the rails continue should be noted. On the page where the rails continue, there are also continuation arrows with the page number notation from which the control voltage lines originated.


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Fig. 2. In this circuit arrangement, the sequence of events could be described as such. When the pushbutton (PB) is pressed down, the circuit is completed and current will flow to energize the coil (CR1).

Fig. 2. In this circuit arrangement, the sequence of events could be described as such. When the pushbutton (PB) is pressed down, the circuit is completed and current will flow to energize the coil (CR1).

Rungs:

The rungs of the ladder are made up of wires and input devices that either allow current flow or interrupt current flow to the output devices. These lines may be thin lines when compared to the lines of the rails. From the placement of the input and output devices, you can determine the sequence of events that either energize or de-energize the outputs. The key to good troubleshooting is determining this sequence of events. Input devices are typically placed on the left side of the rungs, while the output devices are placed on the right (Fig. 2).


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Placement of input devices: The input devices are placed on the rungs in a way that indicates the current flow through the rung when there is a complete path to the outputs. There are several ways in which these input devices can be placed on the rungs, although as stated earlier, they are typically placed on the left side.

The STOP input devices are typically normally closed and are placed in series on the rung. This means that they are placed end to end in the drawing. In order for the current to flow through them, they must be in the closed position. Typical STOP input devices include components such as normally closed pushbuttons and mushroom head buttons, limit switches, and contacts for light curtains, photocells, and proximity switches.


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The START input devices are typically normally open and are usually placed in series with and after the STOP devices. If there is more than one START device, it is usually placed in parallel with the others.When the STOP and START devices are placed in order on the rung, the flow of current to the output devices can be seen. Understanding this flow is a great aide in troubleshooting. A key question to always ask yourself is: “What does it take to energize the output?”

Fig. 3. This circuit arrangement uses all three types of logic functions: AND, OR, and AND/OR.

Fig. 3. This circuit arrangement uses all three types of logic functions: AND, OR, and AND/OR.


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Here's a simple example to analyze. In Fig. 3, what does it take to energize the output CR1? When either start pushbutton is depressed, the path is complete, and the coil of CR1 is energized. In following the path for the current, you can see the logic of the placement of the input devices. This logic determines the decision making process of the input devices and the path for current as it travels to energize the outputs.Logic statements: There are several logic statements that can be used in placing the input devices in the rungs. The most common of these are the AND, the OR, and the combination AND/OR function. In Fig. 3, all three are present.1. The end-to-end placement of the stop and start pushbuttons makes an AND logic statement. In other words, both the stop AND the start buttons must be used to energize the coil.


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2. The parallel placements of the start buttons makes an OR logic statement. Either start pushbutton will complete the path and energize the coil.3. Placing the stop pushbutton in series with the parallel start pushbuttons supports the AND/OR logic statement.Placement of the output devices. As noted earlier, the output devices are placed on the right side of the ladder drawing.

Unlike input devices, it's important that the output devices only be placed in parallel. If they are placed in series, electrical theory says that the voltage will drop across the resistance of each output. If this happens, they will not operate properly.Outputs include items such as lights, coils, solenoids, and heating elements. In addition to the commonly accepted symbols shown in Fig. 1, letters and numbers also help to identify the output devices.


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Fig. 4. As noted by the numbers on the right rail, this circuit arrangement indicates there is a normally open contact in Rung 2 and a normally closed contact in Rung 3.

Fig. 4. As noted by the numbers on the right rail, this circuit arrangement indicates there is a normally open contact in Rung 2 and a normally closed contact in Rung 3.

Coils will typically have contacts associated with them. These contacts will change state when the coil is energized. The changing contacts will either complete or open the path for current.

As noted in Fig. 4, when the pushbutton is pressed down, the path is completed, and current will flow to energize the coil. When the coil is energized, the contacts associated with the coil will change state. The red light will go on, and the green light will go off.


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Locating the contacts: In the ladder drawing, the contacts associated with the coil can be located using a cross-reference system. The rungs are usually numbered on the left side of the rail. A number on the right side of the rail references the contacts associated with the coil (Fig. 4). These numbers are the rungs in which the contacts can be found. A number without a line under it references an open contact. Drawing a line under the number on the right rail references a closed contact.

Let us see a teaching video for “Reading Electrical Diagram”


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“How to read an Electrical Diagram”


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Reference:1. www.realfixerrealfast.com

http://www.realfixerrealfast.com/


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Wires and connections

Component Circuit Symbol Function of Component

Wire

To pass current very easily from one part of a circuit to another.

Wires joined

A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.

Wires not joined

In complex diagrams it is often necessary to draw wires crossing even though they are not connected. The simple crossing on the left is correct but may be misread as a join where the 'blob' has been forgotten. The bridge symbol on the right leaves no doubt!

Power Supplies


Cell

Supplies electrical energy. The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.

Battery

Supplies electrical energy. A battery is more than one cell. The larger terminal (on the left) is positive (+).


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Solar Cell

Converts light to electrical energy. The larger terminal (on the left) is positive (+).

DC supply

Supplies electrical energy. DC = Direct Current, always flowing in one direction.

AC supply

Supplies electrical energy. AC = Alternating Current, continually changing direction.

Fuse

A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.

Transformer

Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.

Earth (Ground)

A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.


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Output Devices: Lamps, Heater, Motor, etc.


Lamp (lighting)

A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.

Lamp (indicator)

A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.

Heater

A transducer which converts electrical energy to heat.

Motor

A transducer which converts electrical energy to kinetic energy (motion).

Bell

A transducer which converts electrical energy to sound.

Buzzer


Inductor (Coil, Solenoid)

A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.


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Switches


Push Switch (push-to-make)

A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.

Push-to-Break Switch

This type of push switch is normally closed (on), it is open (off) only when the button is pressed.

On-Off Switch (SPST)

SPST = Single Pole, Single Throw. An on-off switch allows current to flow only when it is in the closed (on) position.

2-way Switch (SPDT)

SPDT = Single Pole, Double Throw. A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.

Dual On-Off Switch (DPST)

DPST = Double Pole, Single Throw. A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.

Reversing Switch

(DPDT)

DPDT = Double Pole, Double Throw. This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.

Relay

An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit. NO = Normally Open, COM = Common, NC = Normally Closed.


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Resistors


Resistor

A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit. Some publications use the old resistor symbol:

Variable Resistor (Rheostat)

This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

Variable Resistor (Potentiometer)

This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.

Variable Resistor (Preset)

This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.


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Capacitors


Capacitor

A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

Capacitor, polarised

A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

Variable Capacitor

A variable capacitor is used in a radio tuner.

Trimmer Capacitor

This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.


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Diodes


Diode

A device which only allows current to flow in one direction.

LED Light Emitting Diode

A transducer which converts electrical energy to light.

Zener Diode

A special diode which is used to maintain a fixed voltage across its terminals.

Photodiode

A light-sensitive diode.

Transistors


Transistor NPN

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

Transistor PNP

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

Phototransistor

A light-sensitive transistor.


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Audio and Radio Devices


Microphone

A transducer which converts sound to electrical energy.

Earphone


Loudspeaker


Piezo Transducer


Amplifier (general symbol)

An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.

Aerial (Antenna)

A device which is designed to receive or transmit radio signals. It is also known as an antenna.


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Meters and Oscilloscope


Voltmeter

A voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!

Ammeter

An ammeter is used to measure current.

Galvanometer

A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.

Ohmmeter

An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.

Oscilloscope

An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Sensors (input devices)


LDR

A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor

Thermistor

A transducer which converts temperature (heat) to resistance (an electrical property).

Logic Gates

Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the Logic Gates page. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).


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LET’S READ A DIAGRAM


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ANY QUESTION?

THANK YOU!Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

Education

Read and Understand The Electrical Diagram