Pilot Devices

7 Switches and Pushbuttons

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Pilot Devices Overview

These are some of the most basic control devices that we use to start and stop our motors. They can be provided with fuses for overcurrent protection, and if a switch is horsepower rated, it is designed to make and break line current to a motor.


Pushbuttons are momentary contacts that come available as either normally open (NO) or normally closed (NC) contacts, although many pushbuttons come with both NO and NC sets of contacts.

When you depress the button, you change the state of the contacts, and when you release the button, a spring returns the contacts to their original state.

These are ideal for use in low-voltage control circuits.



Switches are identified by the number of conductors (poles) they connect to, and the number of positions (throws) they can switch to. Switches are also rated for voltage and power and must be operated within their limits.

Single-pole, single-throw (SPST)

This switch is used where we only need to break a single line in a 120V single-phase supply, providing that it is connected to an ungrounded-circuit conductor.

Single-pole, double-throw (SPDT)

This switch connects a single-line conductor to either of two possible switch legs. A common household example of this is in a three-way switch loop, which allows control of a single load (usually a light) from two different locations.

In industrial settings, it is commonly used as a Hand – Off – Auto selector switch.

Single-pole, double-throw

Double-pole, double-throw (DPDT)

The most common application of these devices is as a 4-way switch, which is used in conjecture with two three-way switches to allow for control from three or more locations. As a general rule, a light can be controlled from any number of locations, providing you start and end your switching with three-way switches and use as many 4-way switches in between as necessary.

Double-pole, double-throw

Double-pole, single-throw (DPST)

This switch looks like a standard SPST switch from the outside but inside, instead of interrupting only one current-carrying conductor, it interrupts two. This makes it suitable for controlling and isolating loads that operate at 240V single-phase.

Triple-pole, single-throw (3PST)

These are used to interrupt current to three-phase circuits and motors. With a single external handle, three current-carrying conductors can be opened or closed at once. The switches often come with housing for the mounting of fuses to provide overcurrent protection. The switch must be horsepower rated if it is used to interrupt current to a motor.

Isolating switch

An isolating switch is connected upstream of a power circuit and is NOT horsepower rated and so it is not meant to interrupt current flow. Isolating switches are not meant to control motor loads. Rather once a motor has been properly shut off, an isolating switch can be used for lockout purposes. An isolating switch can have any number of pole contacts, but to be used for lockout purposes it must provide only a single throw option.



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Basic Motor Control Copyright © 2020 by Aaron Lee and Chad Flinn is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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