What is the difference between NO (normally open) and NC (normally closed) alarm contacts, and why do I care?
Normally closed contacts complete a circuit when they are in their “normal” state — i.e. when a door or window being monitored is closed. When the protected door or window opens, the circuit is broken, triggering an alarm. Normally open contacts complete the circuit when the door or window being protected is opened. Normally closed contacts are more prevalent in DIY and household installations, probably because they have a built-in failsafe; if the circuit is cut, a normally closed alarm zone will sound an alarm.
