A relay is an electrically controlled switch. Electromechanical relays use an electromagnet to activate mechanical contacts, solid-state relays use semiconductor switches.

A latching relay stays in the current state when power is removed. A non-latching relay reverts to a known state when power is removed.

Relay contacts come in several varieties. The most common are:

A: normally open
B: normally closed
C: one normally-open and one normally-closed both actuated by a single coil
X: normally open, double make
Y: normally closed, double break
Z: one X and one Y, both actuated by a single coil
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