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
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
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.
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, 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.
An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.
A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.
A device for making or breaking the connection in an electric circuit.
A momentary contact device that has a built in spring to return the button to its normal position once release. Available with either normally-open, normally-closed or both sets of contacts.
The conducting part of a switch that makes or breaks a circuit.
A contact that under normal conditions does not have continuity through it. When the contact changes its state it permits the flow of current by closing its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
A contact that under normal conditions has continuity through it. When the contact changes its state it interrupts the flow of current by opening its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
The rate at which work is done. It is measured in watts (W), or joules per second (J/s).
An electrical circuit that uses three current carrying conductors, called Line 1, Line 2, and Line 3, which have a 120° phase shift in the voltage and current waveforms between them. The Power Circuit of three-phase motors is an example of a three-phase circuit.
A switch that is used to lockout an electrical circuit once a motor has been turned off to ensure it is completely de-energized. Not designed to interrupt the flow of current.
In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.
