In order to understand switches, it is necessary to understand the terminology associated with switches.  First, a switch is a device that controls the flow of electricity by either stopping the flow (the switch is “off” or more correctly it is “open”), or by diverting the flow, as with a changeover switch. When the switch is “on” it is more correctly referred to as “closed”; this basic distinction leads to the concept of opening or closing the circuit via a switch. Switches come configured as normally open (n.o.) or normally closed (n.c.) depending on the nominal state.

A switch’s function is designated by the number of circuits controlled by the switch (poles); the number of contacts, or electrical pathways that the switch controls (throws).  In the simplest configuration, an on/off switch would be a normally open, single pole, single throw switch (n.o. SPST); one line in and one line out, with actuation closing the circuit.

A single pole double throw (SPDT) is also sometimes referred to as a “change over” switch (SPCO). In this switch, the actuation will divert the flow of electricity from one pathway to another.  A double pole single throw is like two SPST switches controlled by one actuator, while a double pole double throw is equivalent to a two SPDT switches, controlled by a single actuator.

– Lyle, PYS Electrical Technician