Home security

[Roofman]

Could someone explain the how and why's of using normally open and or normally closed contact relays in a security system?

 

[WayneW]

The short answer is that normally closed circuits are generally used for security purposes to prohibit tampering and verify integrity. Bad guys or pranksters cannot simply snip a wire to defeat a zone/sensor.

Fire sensors usually use a normally open circuit for some reason, but most people aren't trying to tamper with fire circuits, other than arsonists.

 

[Sandpiper]

The short answer is that normally closed circuits are generally used for security purposes to prohibit tampering and verify integrity. Bad guys or pranksters cannot simply snip a wire to defeat a zone/sensor.

Fire sensors usually use a normally open circuit for some reason, but most people aren't trying to tamper with fire circuits, other than arsonists.

Correct, but I'd like to add some comments.

Fire sensors are required to have some type of supervision, though, such as and EOL resistor or EOL relay. It is this requirement that provides the tamper detection for fire circuits.

For burglar circuits, if you don't use an EOLR, the N/C circuit is definitely the most secure way to go. But if you DO use EOLRs then the difference in tamper detection becomes much less obvious. It would not be recommened to use N/O in burglary circuits unless an EOL is used on that circuit.

A N/O burglary circuit may be used in the case when the situation requires the zone contacts to be closed in the non-secure position, and open in the secure position. This is a very rare case, but it's nice to have the option.

 

[Neurorad]

Here's a decent read.

http://www.structuredhomewiring.com/TamperProofWiring.aspx

And welcome to CT.