RC-33 NC Recessed Wired Security Window Door Contact Sensor Alarm Magnetic Reed Switch White 10 Pair

The switches designed for a "closed loop" alarm system, meaning when the window is closed the switch is shorted / ON / closed. That way if someone snips you cable the alarm will go off or you will know there is something wrong. I think that is why they call this NC switch. In reality, I might agree with some of the other reviews, the switch is open when not next to the magnet so I can see the confusion it is normally open. While they don't explain this in the description, I believe it is standard in the alarm industry to call these switches NC, other similar products posts describe NC switches this way for closed loop alarm systems.

Perfect for replacing old sensors

Per every reference document I could read, and there were many - a normally closed switch is one, that when away from an influence (in this case a magnet), would allow current to pass and then, when a magnet is nearby, it opens. The definition is based on no influence of the magnet unless it gets placed there. Think of the starter on a car - it is normally not always spinning until you turn the key - that is normally open until something is done to close it.In this product's case and maybe your application - when the switch is in the door jamb and the magnet is in the door and the door is closed, no current will go through the circuit. Open the door and current will start flowingI had to order another set from another seller and return these.Please - if I am wrong, let me know. But otherwise I would be careful to consider what you want these switches to do.

THESES ARE NOT N/C THEY ARE N/O CONTACTSThat means they do not make contact when the other ½ is pulled away

World great to replace an old sensor that my new ring retrofit kit did not like. Tip that worked for me is I noticed the old sensors intermittently opening during the day, so they seemed to be failing when they heated up with the sun. I had about 10 wired together, so I took a hair dryer around to see if I could force one to fail. Found the right one out of ten, so so far only had to replace the one. Retrofit alarme seems to be a little picky with old sensors, but these are working good with it so far.