Circuits, Wiring, and Power Supplies
Fire alarm circuits fall into initiating device circuits (IDC), notification appliance circuits (NAC), and signaling line circuits (SLC). Know Class A vs. Class B wiring. Class B is a two-wire circuit that can be compromised by a single open; Class A provides a redundant return path so a single open does not disable the circuit. This is a favorite exam trap: Class A survives a single open; Class B does not.
Power supplies: primary power (usually 120 VAC branch circuit) plus secondary (standby) power (batteries) sized to carry the system for a required duration. Know that battery calculations must account for standby current plus alarm current. Supervision of circuits and power is required so opens, shorts, and grounds are detected.
Wiring must be per NEC Article 760, with proper cable types, overcurrent protection, and separation from other circuits. Ground faults must be detected and reported as trouble.
Rule of thumb: if a question describes a single break in a circuit and asks whether it still works, Class A = yes, Class B = no. For batteries, standby current × standby hours plus alarm current × alarm hours = required capacity.
Worked example: A Class B NAC with one open conductor loses notification beyond the break. Reconfiguring as Class A provides a return path so the circuit remains functional.