NO vs NC Basics

Normally Open (NO) and Normally Closed (NC) are simple concepts and yet cause confusion even among experienced technicians. The problem is what is normal?

Door Contacts

A standard wired alarm door contact is, in its uninstalled state, open. However, when installed in a door jam with a magnet on the door, the circuit becomes a normally closed circuit. It is important for an installer to be aware of the differences between a component's NO/NC state and how it will be used in an alarm circuit. 

If a door is wired with a supervised circuit, then an installer will need to install an NC contact so that when exposed to a magnet field (as in the door is no closed) the contact opens. Supervised door circuits are rare, but do exist.

Relays In Components

Relays pose their own problems with NO and NC. In most cases, the relay terminals are labeled with the relay in a non-energized state. However, if the relay can only be used in an energized state, the installer must always factor this into account when wiring the device. This is very common on smoke duct detectors. The image below is from an analog duct detector. Notice the warning in the box. The manufacturer is warning the installer that the label is for a non-energized device. 

duct detector connections label
Smoke Duct Detector Wiring Label 

All circuits in a fire system are wired as a supervised circuit. In this installation, you would use terminals 4 (NC)  and 15 (C) with a resistor in parallel. The trouble relay is held open for normal operation and on power failure, the relay returns to a closed or shorted state, triggering the trouble notification.  For the alarm circuit, which is also wired as a supervised circuit, you would use terminals 6 (C) and 17 (NO). This is due to the fact that the relay for the alarm circuit is not held open by the device. It takes a positive action by the device to trigger an alarm. If it the alarm relay functioned like the trouble relay, then a power failure would cause an alarm. 

Standalone Relays

Unlike relays built into components where the manufacturer knows how you are going to use the components, standalone relays have an untold number of uses.

 You should never trust the NO/NC labeling on a relay. Use your ohm meter to meter out the state of the relay in the manner in which you are going to use the relay. Some manufacturers will print the NO / NC terminals for the relay component, then they power up the relay on their board, which reverses the terminals labeling.  

Metering out a relays state (open or closed) is always the best way to avoid issues.