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NO versus NC related to alarms


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There is a lot of confusion regarding this topic as it relates to alarms.  I'd like to summarize what I've come to believe.  Please correct me where I am incorrect.  There are three devices that may be referred to when talking about NO and NC; switches, access points (doors/windows), and alarms.


For switches, specifically the magnetic reed switches but it also applies to other types and relays:

A switch is in its "Normal" state when de-energized.  This means when the magnet is away from the switch.  When a NO (normally open) switch is away from the magnet, the circuit is open.


An access point (door/window) is confusingly referred to as NO and NC as well.  A normally closed door, is a door that typically remains closed.  A normally open door is a door that you want to typically remain open.


Lastly, an alarm system can be set up for a normally open or normally closed loop.  From the alarms perspective, normal means un-violated.  NO would alarm when the circuit is closed.  NC would alarm when the circuit is opened.


This leads to 4 possible cases.  I made a table that will help getting my head around it.  Maybe it will help others as well.




Per the table, using a typical "normally open" switch on a typical door that is "normally closed", this would mean that you want the alarm to be "normal" or un-violated when the circuit is closed, meaning the alarm is set to NC.  Hope this is correct and helps.  Now I need to sort out my EOL series vs parallel confussion.

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I can see how confusion would arise from the difference between the idea of a switch or sensor that is NO or NC, and the alarm systems view of a circuit being NO or NC.


Generally alarm circuits are NC with an End of line resistor (EOLR) to enable supervision (one exception is fire sensors, which are generally NO with a resistor in parallel to the sensor for supervision).


The big advantage of the switch/circuit having continuity (via the switch being closed, or through a resistor in parallel) when secure is that you might have a door or window that is basically never opened, and without running power through the entire loop in the normal state, a wiring fault will go undetected.   By having a loop with constant current (at a known resistance), the controller can detect both shorts and open circuits.


And if you really want to get esoteric, we can talk about wetting current (aka "sealing current") and oxidation.

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