Electrical Symbols and What They Mean to Controls Engineers
Using symbols instead of text provides a fast and effective way to describe an electrical system. Learn about the what, when, where, and why of commonly used electrical symbols from the perspective of a controls engineer.
Any controls engineer will have seen and dealt with electrical prints at some point in their career. The symbols used in electrical drawings are there to represent sensors, motors, and other devices. Controls engineers and technicians must understand those symbols to construct PLC programs that read sensors and control outputs. Below I outline commonly used symbols and where they might typically be found.
Figure 1. Controls engineers and technicians may use electrical symbols to construct PLC programs that read sensors and control outputs. Image used courtesy of Adobe Stock
Electrical Symbol Standards
Similar to electrical design codes having different standards per region, there are also different electrical symbols per standard. IEC 60617, IEC 61131, JIC, and ANSI Y32.2 are just some of the local and international standards used in electrical drawings. Some of the differences are slight, and some are very different.
Common Electrical Symbols
Below is a list of commonly used electrical symbols from the National Electrical Manufacturers Association (NEMA).
A coil symbol typically represents the coil of a relay that drives the contacts. This symbol can also represent a digital output from a PLC or an electrical load.
This symbol typically represents the contacts of a relay, with power on both coming into the contact and power leaving contact. When shown on an output page, references are commonly shown to what drives the contacts. It can also be used as a digital input to a PLC.
This above icon is also a contact symbol, shown in the normally-closed position. Sometimes these symbols are stacked on top of each other to represent the contacts of a relay. Terminal numbers are often provided on both sides of the contact to assist with wiring.
A capacitor charges and discharges in a cyclic manner. The symbol resembles a contact because power enters and leaves, similar to a contact. Capacitors are polarity dependent, so they should be represented accordingly.
There are two motor symbols depending on whether it is two-phase or three-phase. This symbol can be used to represent DC motors as well as AC motors. Pumps can also be represented with a motor symbol, as they typically have a motor attached to drive the pump.
The three-phase motor looks very similar to a two-phase motor; however, only three leads are depicted. It is important to differentiate between two-phase (split-phase) and three-phase motors, as the power requirements for a three-phase motor will be much higher, and the conductors should support this requirement.
Normally-closed Mushroom Button
Buttons and switches have a few different representations. This style is typically used for emergency stop buttons, even if they are not physically of the mushroom-style push button. A push button can be shown in a normally-closed or normally-open state. This symbol is of the normally-closed state.
Normally-open Mushroom Button
Normally open mushroom pushbutton. Same as the normally closed only shown in the open state.
Normally-closed Push Button
Normally-closed push buttons are a very common symbol used to represent user input. Like the mushroom push button, this symbol can be shown in the normally-open or -closed state.
Normally-open Push Button
Normally-open push buttons are similar to normally-closed ones; however, they are just shown in the normally-open state.
Illuminated Push Button
Some buttons have a light inside that can be controlled by an output from the PLC or by the electrical signal that goes through the contacts. Not all standards, such as the IEC standard, have a symbol for this device.
Fused Disconnect Switch
Main disconnect switches on electrical panels are represented with a vertical switch symbol—if fused, the fuse symbol is added to the bottom. These symbols can typically be found on the high-voltage page where the power comes into the system.
Switches are illustrated by two circles representing the terminals and a line branching between the circles, expressing the connecting contact.
There are many different switches, such as float switches, limit switches, and temperature switches. They all have a common representation of terminals with other symbols for their function.
Typically these symbols are shown on the PLC input page connecting to the designed inputs and can be normally closed or normally open.
Combining Symbols - Electrical Schematic
Sometimes these symbols are used individually, and sometimes they are combined. Below is an example of a motor control schematic.
Figure 2. Typical 3-wire latching control for motor start/stop logic.
Figure 2 shows a start push button that is normally open, a stop button that is normally closed, a motor contactor with auxiliary contact, and a motor overload.
Figure 3. Typical direct contactor/starter arrangement for driving a three-phase motor.
Figure 3 is a schematic of a three-phase motor showing three contacts, three overloads, and a motor.
The first schematic could be used to control the motor. If the stop button is not pressed and the start button is pressed, the “M” coil will energize, closing the contacts on L1, L2, and L3. An auxiliary contact is used to “seal in” the start button condition. If the stop button is pressed or the overload tripped, the coil will de-energize, opening the contacts.
By using symbols instead of written text, we break down language barriers and provide a fast and effective way to describe an electrical system.