Single pole (SP) switches can be either single throw or double throw, which refers to the number of conducting positions; that is, the number of terminals that will complete the circuit. They are abbreviated as SPST or SPDT accordingly. SP switches consist of two brass terminals, connecting wires, the actuator or toggle switch and an enclosure that is most frequently plastic.
One characteristic of single pole, single throw switches are that they have “On” and “Off” markings on them because those are the only two options. SPDT switches are slightly different because they are changeover switches that toggle between two circuit options. The maximum voltage and current of switches is a necessary specification to keep in mind in addition to knowing the difference between AC and DC currents and what the situation requires.
Single pole switches are very common and are used in virtually every building that uses electricity: light switches are SP. The large category of power switches or buttons are SPST, but other simple function devices use these switches as well, such as doorbells.
Single pole switches have few parts because of the basic function they provide. As the switch is flipped (or pushed in the case of push button SPST switches), a strip of conducting material is brought into contact with both terminals, closing the circuit and allowing electricity to flow through the actuator. The operator is protected from the current because the components are safely contained inside the enclosure. When the switch is toggled back, the conductor breaks contact with both terminals and the circuit is open.
Single pole, double throw switches are similar in operation. One actuator is flipped from one circuit line to another. There are two terminals that the switch toggles between, removing the “Off” setting. SP double throw switches are used to change circuits from a higher voltage current to lower current, for example, if the lights need to remain on but at a lower intensity. SPST and SPDT can be used in combination to create more options. When two SPST switches are used in combination, they essentially form a double pole, single throw switch.
Though other switches can do the same thing in one enclosure, single pole switches are very common, comparatively inexpensive and easy to install. Some switches may be illuminated through use of a small bulb inside the plastic shell that covers the actuator. Timers, faceplates and additional enclosures are also available.