I've just removed the alarm reed relay sensor from the frame of my front door because it's become unreliable, often 'sticking' in its passive state instead of triggering the alarm. But I now face some ambiguity about ordering a replacement, because none of those I've found, like these
actually specify whether the contacts are NO or NC. And the phrase itself appears to mean different things to different people!
The sensor I removed is what I call NC, because (with no magnets in sight) my continuity tester gives me a signal on the two active wires. But of course when the sensor is in position and the door or window holding the magnet is closed, so that the magnet is close to the reed, then the state is reversed, so the circuit is open. If an intrusion occurs, the circuit is closed again, triggering the alarm.
When I asked the tech support guy at CPC he described the above Elmdene sensors as NC, which was what I wanted to hear. But on further checking it turned out that by 'normally' he meant the state when the alarm is set, awaiting possible intrusion. So, in MY terms, these are NO!
And the guy at Elmdene confirmed this, adding that the 'normal' state required for an alarm sensor is that it OPENS the circuit when intrusion occurs. (Makes sense, because cutting the wires would then trigger it.)
So the only conclusion I can reach is that my 15 year old alarm is different in that respect. Or that I installed it wrongly. Or that there is some setting on its circuit board for switching between the two 'trigger states'.
Any insight or advice would be appreciated please.