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Electrical Diagrams

There are various types of diagrams that a technician may find useful in different aspects of their work. Some are like simple road maps in that they show where components are physically located, the wire paths between components, and connection points. The diagram shown in '''Figure 1''' is useful when replacing components since it shows component locations. It also identifies the wires by certain colors and identifies the specific component terminals that the wires are connected to.

'''Figure 1 - Wiring or Connection Diagram'''

The most common types of diagrams are '''connection'''('''Figure 1'''), '''ladder '''(or '''line'''), and '''wiring'''. Each type is best suited for certain purposes or aspects of maintenance of electrical equipment. Ladder and wiring diagrams are both referred to as ''schematic diagrams'' or just ''schematics''.

''Schematic'' diagrams are somewhat abstract and less pictorial than connection diagrams. They may or may not indicate exactly where the components and wires are physically located, but they will always show the current path between components and they will symbolically indicate the function of each component.

'''Figure 2''' is a combination ladder and wiring diagram of a ''reduced voltage motor starter. ''It does not resemble the actual equipment in any way. However, tracing our circuits on it is much easier than on a connection diagram, since components with related functions are usually grouped together.

'''Figure 2: Schematic Diagram'''

In '''Figure 2''', the ''input ''power lines (L1, L2, and L3) and the components that conduct electrical energy to the motor are shown on the right side of the diagram. This portion of the diagram also shows the ''series resistors'' (R1, R2, and R3) that reduce voltage to the motor during startup. The ''control ''section and its various components are shown on the left side of the diagram. Included in this section are the ''pushbutton switches'' (PB1 and PB2), ''relay coils'' (M1, TR1, and CR1) and associated ''contacts'', ''fuse'', and ''step-down transformer. ''M1 is a magnetic motor starter coil, TR1 is a timer relay, and CR1 is a contactor coil.

In this article, we begin with the building blocks and basic elements used to create diagrams, and in fact represent functional components, circuits, equipment, and systems. These elements are called '''''schematic symbols'''''.

Schematic Symbols

All schematics are composed of symbols to represent components. To make sense of the schematic diagrams and circuits, you must be able to recognize and distinguish the symbols.


''Transformers'' are used to step voltage up or down, isolate various types of panelboard meters from high voltage sources, and provide low voltage to control circuits. '''Figure 3''' shows the most common transformer symbol.

'''Figure 3 - Transformers'''


''Rectifiers'' or ''Diodes'' ('''Figure 4''') are primarily used to convert alternating current (AC) into direct current (DC). The most common applications are DC low voltage control circuits, DC power supplies, and battery chargers. The arrowhead represents the ''anode'' and the vertical bar represents the ''cathode''.

'''Figure 4 - Rectifiers or Diodes'''

Fuses and Circuit Breakers

''Fuses ''and'' circuit breakers'' are short-circuit and overload protection devices. Most often they will be found in series with the incoming power source or as the first device in a circuit it is designed to protect. Fuses ('''Figure 5''') incorporate a special low-melting point element that opens to interrupt circuit current in case of a short-circuit or overload. Circuit breakers ('''Figure 6''') may have both a ''thermal section'' to open the circuit when current is too high for too long and a ''magnetic section'' to open the circuit immediately to protect against short-circuit damage.

'''Figure 5 – Fuses'''

'''Figure 6 - Circuit Breaker with Thermal and Magnetic Trips'''

Thermal Overload Elements

''Thermal overload elements'' ('''Figure 7''') can be part of a circuit breaker or a separate device. They are frequently found in the input power lines to motors.

'''Figure 7 - Thermal Overloads'''


The basic purpose of any switch is to interrupt or close a circuit. However, there are numerous types of switches with various features. The features that distinguish one switch from another will usually determine its application. Some of the distinguishing features are:

  • Number of contacts
  • What force moves the contacts
  • Normal position (if any) of the contacts
  • Number of independent circuits the switch can control
  • Number of positions the moveable contact(s) can be positioned in

A switch must have at least two contacts. To close a circuit, the two contacts must come together; to open a circuit they must separate. On simple toggle switch symbols, as shown in Figure 8 and Figure 9, the contacts are not actually shown. However, you should consider one contact as being on the tip of the movable bar and the other contact on the stationary terminal.

Figure 8 – OPEN Circuit

The circles represent what are called poles, contact pairs, or circuit paths. Since the switch in Figure 8 and Figure 9 only depicts one contact pair or circuit path, it is called a single-pole switch. The positioning or movement of the moveable bar is called throw. Since the moveable bar has only one back and forth movement, it's called single-throw. A full descriptive name, including all of the features, is single-pole single-throw switch. It is abbreviated as SPST.

A switch that can control two separate circuits with simultaneous On-Off (Open-Close) modes is called a double-pole single-throw (DPST) switch. Figure 10 shows the symbol for this type of switch.

Figure 10 - Double-Pole, Single-Throw Switch

A switch may be single, double, or multiple-pole. This switch with three sets of contacts is normally called a three-pole single-throw (3PST) switch (see Figure 11). Often, this version and the DPST are also called safety switches, and are used to open or close all of the power lines at the same time.

Figure 11 - Multiple-Pole Switch

The dashed line indicates that the contacts, or poles, in double or multiple pole switches are connected mechanically; all the contact pairs move together (at the same time) when an external force is applied.

There are switches that have more than one position or throw. In a two-position, double-throw switch, there are two possible places where a moveable contact can touch a fixed contact. There are three terminals. The terminal connected to the movable contact is called the "common" terminal. Figure 12 shows a single-pole double-throw switch. The switch controls two separate circuits, one at a time. Or, it can be said that this switch connects a common circuit to one circuit branch or another, one at a time. Figure 13 is a double-pole, double-throw switch.

Figure 12 - Single-Pole, Double-Throw Switch

Figure 13 - Double-Pole, Double-Throw Switch

Rotary and drum switches have numerous positions and connect one terminal to many others, one at a time. One or more of the positions may be open, with no connection, as seen in Figure 14.

Figure 14 - Rotary and Drum Switches

Some switches are momentary (spring-loaded). They are biased with a spring or a weight to return them to their normal position when there is no external operating force applied. The pushbutton switch in Figure 15 is an example of the type used to start a motor. It is the most common type of momentary switch.

Figure 15 - Momentary Contact Switch

Notice the "NO" designation just below the symbol. This is an abbreviation for normally open. This means an external force must be applied to close this switch. If the designation was "NC," as in Figure 16, the switch would be normally closed and force would have to exerted to open the switch.

Figure 16 - Normally Closed Switch

When some force stretches or compresses the spring, or lifts the weight, normally open contacts close and normally closed contacts open. When the force is removed, the contacts return to their normal condition.

In diagrams, spring-loaded switches are always shown in their'normal'position, without the external force. They may or may not display the NO or NC position designation.

The normal condition has nothing to do with whether the switch is more often open or closed. A freezer door light switch, for example, is a NC switch. Yet most of the time it is open, because the freezer door is shut and holding its spring compressed.


Special purpose switches relate to specific applications and the force that actuates them. That force can be:

  • A person
  • An electriccurrent
  • A moving part on amachine
  • A change in pressure, weight, fluid level, flow, or temperature

Manual Switches

Manual switches require some form of manual action by a person to turn power on or off. Three types of manual switches are shown below.

'''Figure 17 - Manual Switches'''

Limit Switches

These are usually spring-loaded, normally open, or normally closed switches equipped with a plunger, lever, or roller. A cam, ramp, or part on a machine pushes against the plunger, activating the switch. Normally open switches are drawn so they look as if they would fall open. Normally closed switches fall closed.

There are four types of limit switches as shown in '''Figure 18'''.

'''Figure 18 - Limit Switches'''

Flow, Pressure, and Level Switches

A paddle or sail in a flow stream will operate a switch in response to changes in flow (see '''Figure 19'''). Pressure switches use a diaphragm or piston; changing pressure behind it compresses a spring and activates the switch (see '''Figure 20'''). Fluid level switches use a float to sense a level change and activate the switch (see '''Figure 21''').

'''Figure 19 - Flow Switch'''

'''Figure 20 – Pressure Switch'''

'''Figure 21 – Level Switch'''

Temperature Switches

Thermostats, thermal overload breakers, and high limit and low limit temperature switches all operate when temperature rises or falls (see '''Figure 22''' and '''Figure 23''').

'''Figure 22 – Thermal Switch'''

'''Figure 23 – Bimetallic Strip'''

Many temperature switches have a bimetallic coil or strip that bends and operates contacts in response to temperature change (see '''Figure 23''' above).


Electrically operated switches are called ''relays''. Current in a relay coil produces magnetic force, which pulls in a moveable armature and switches the contacts from their normal condition (see '''Figure 24''').

'''Figure 24 - Relay Switches'''

When the coil ('''Figure 25''') is energized, the contacts move to the active position. A relay is drawn simply as a circle with a label, and represents the connections to energize the coil of the relay. Relay contacts will be shown on the drawing as normally open or normally closed contacts with the coil name designation, in this example CR1.

'''Figure 25 - Relay Coil'''

Relay contacts ('''Figure 26''') may be shown either normally open or normally closed.

'''Figure 26 - Relay Contacts'''

Relays often have many poles, or independent contact sets, including both normally open and normally closed contacts. Some small relays are so sensitive that the coil will operate on the current produced by a photocell. Huge circuit breakers and switchgear in utility lines have coils that need many amperes to activate, and the contacts are designed to withstand high currents and voltages. There are many sizes and types in between:

  1. '''''Contactors''''' switch currents to power-using equipment. The main contacts are normally open, but there are sometimes auxiliary contacts operated by the same coil that may be either NO or NC. The relay in a magnetic motor starter is a contactor. The contacts are often replaceable.
  2. '''''Control Relays''''' provide the logical decision making in most control circuits. Their coils operate on either AC or DC voltages, and their multiple sets of contacts switch relatively small currents.
  3. '''''Solid state relays''''' do the same things as other relays, but work completely differently. There are no coils, contacts, or moving parts. Solid state devices--often transistors or silicon controlled rectifiers--switch a circuit on or off in response to an input current or voltage signal.

Special Relays

'''''Latching relays''''' have a latch that holds the contacts in their active condition even after coil current has been shut off. Some latches are tripped manually, but more often another electromagnet pulls the latch out, allowing contacts to return to their inactive state. Both coils will appear on the schematic separately (see '''Figure 27''').


'''''Timed relays''''' include a pneumatic timing device or dashpot like the air cylinder on a storm door. This gives the relay a pre-set, often adjustable time delay. The coil is shown on a diagram labeled with a ''T'' or ''TR ''(see '''Figure 28''').

'''Figure 28 - Timed Relay'''

Timed relays can provide four possible types of delay:

'''Figure 29 – Timed Relay'''

1. Delayed contact closing when the coil is energized. This turns something ON some time after a switch sends a control signal.

'''Figure 30 – Timed Relay'''

2. Delayed contact opening when the coil is energized. This turns something OFF some time after a switch sends a control signal.

'''Figure 31 – Timed Relay'''

3. Delayed contact closing when the coil is de-energized. This turns something ON some time after a switch stops sending a control signal.

'''Figure 32 – Timed Relay'''

4. Delayed contact opening when the coil is de-energized. This turns something OFF some time after a switch stops sending a control signal.


A timer switches a circuit on or off at a certain time or after a certain period of time. Timers can provide more accuracy than pneumatic delay relays. Electro-mechanical timers have a motor and gear set that operates contact sets at a predetermined time. Solid state timers have no moving parts but are shown on drawings the same way. Often, the time can be reset with an independent signal, or by turning off the timer power. The symbol may show the contacts directly controlled (see '''Figure 33''').



Counters switch a circuit on or off after a certain number of events have occurred. Each time an input switch is tripped, a voltage is sent to the input, and the counter advances until its pre-set number is reached. The symbol is similar to a relay or timer (see '''Figure 34''').

'''Figure 34 - Counter Switch'''


Resistors are used to limit current flow and create voltage drops. You will recall that earlier in this module we mentioned resistors being used in a ''reduced voltage motor starter. ''Resistors are generally classified as either ''fixed ''or ''variable ''(adjustable). '''Figure 35''' depicts the symbol for a fixed resistor.

'''Figure 35 - Fixed Resistor'''

''Variable'' (adjustable) resistors are used in applications where minor adjustments to current or voltage levels may be required. The two symbols used to represent adjustable resistors are shown in '''Figure 36'''.

'''Figure 36 - Variable (Adjustable) Resistors'''


Capacitors are devices that can store energy. There are numerous categories and types of capacitors. The symbol used to represent a capacitor is depicted in '''Figure 37'''.

'''Figure 37 - Capacitor'''


Ground connections are used as reference points when troubleshooting electrical equipment and circuits. Schematic diagrams always include ground symbols to indicate these reference points. The three most common symbols are shown in '''Figure 38'''.

'''Figure 38 - Ground Symbols'''