NO, NC, COM SPST Definitions

The simple items that make or break your circuit.

This is part 2 of the basic electricity class that we teach titled “the control”

NO: Normally Open. The circuit will be open until the switch actuator is operated.
NC: Normally closed. The circuit will be closed until the switch actuator is operated.
Momentary: As soon as the button is released it returns to its previous state. Holding the button down will either make or break the circuit, depending on if you wired the NC or NO contacts.
Maintained: Best described as a push/pull action, similar to a light switch. Pushing the button in and it will “maintain” this position until it is pushed again (like a light switch).
SPST: single pole, single throw. The switch has only one moving contact and one stationary contact; it is either normally open (NO) or normally closed (NC).
SPDT: single pole, double throw. The switch has one moving contact and two stationary contacts (NO & NC); when operated, the contacts reverse states.
DP: double pole. It is available in single throw and double throw configurations; it allows for two separate circuits to be opened simultaneously from one switch.
 
Common Contact Forms:
SPST-NO: Single pole, single throw, normally open switch keeps circuit A-B open. This could be used in a system with a fail secure device. When the switch is operated, contact closes, completing circuit A-B. Electricity would flow to the device causing it to release.
SPST-NC: Single pole, single throw, normally closed switch keeps circuit A-B closed. This could be used in a system with a fail safe device. When the switch is operated, contact opens, breaking the circuit A-B. Electricity would cease to flow to lock, causing it to release.
SPDT: Single pole, double throw switch provides a closed circuit, A-B and an open circuit, A-C. This could be used in systems with a fail safe device in circuit A-B and a light in circuit A-C. When the switch is operated, circuit A-B opens, releasing the device and circuit A-C closed, turning the light on. The device and light would have to be rated at the same voltage.
DPDT: Double pole, double throw switch provides a set of NO and NC contact for controlling two separate circuits. The swingers of each contact set are mechanically attached together, as indicated by the dotted line (Not to be confused with a Double SPDT, which are independent). Whenever the switch is operated, both contact sets change position simultaneously. When shown on a drawing, the contact set may be drawn in separate locations for convenience. They will be identified as belonging to one switch; that is; the switch may be labeled S1 and the contact sets labeled S1-A and S1-B.

For more information here is the section from the class: Section 2-The Control