In fire alarm systems, components are connected with circuits. Fire alarm circuit is those that get power from the fire alarm control unit.

a fire alarm circuit may be used to control a relay that switches a control circuit that is powered by the emergency control function equipment.

There are four types of fire alarm circuits:

• Initiating Device Circuits.
• Signaling Line Circuits
  • are used to provide two-way communications between an addressable fire alarm control unit and addressable field components.
• Notification Appliance Circuits
  • The fire alarm control panel can turn on that circuit and operate all of those appliances at the same time. There may be many notification appliance circuits in a building.
• Control Circuits.

A key requirement of NFPA 72 is that certain circuits be monitored for integrity. Some people refer to this as supervision. However. building. fire. and life safety codes typically use the word supervision to mean that a device or other system is being monitored or supervised by the fire alarm system.

This Initiating Device Circuit has a smoke detector. a rate- of-rise heat detector. and a manual fire alarm box. To monitor the integrity of the circuit. an end of line resistor is installed after the last initiating device. In their normal state. the initiating devices. which are wired in parallel on the circuit. Draw little or no electrical current.

For an Initiating Device Circuit. A a short is an alarm. In fact. Any low resistance fault or short across the circuit. such as a jumper wire. will result in an alarm for that Initiating Device Circuit. Unlike Notification Appliance Circuits and Signaling Line Circuits. a short does not produce a trouble condition.