Just as a point to keep in mind. Most systems are, as people have already said, wired normally closed. HOWEVER; they use a normally open switch to do it. It's all symantics but alarm people talk abut a "closed loop" system that actually uses NO switches that are held closed by the magnet or pressure when the system is armed. Most motion detector switches for a closed loop system are open until power is applied to the unit.
I only point this out so if you go to buy alarm switches you ask for "closed loop" or "open loop" switches. If you ask for NO or NC switches you may or may not get what you want; more than once I didn't.
