Initiating Device Circuits, Signaling Line Circuits, and Wiring
Circuit types and wiring
Initiating device circuits (IDCs) connect conventional devices and are supervised for opens, grounds, and shorts; end-of-line (EOL) resistors provide the supervision reference. Signaling line circuits (SLCs) connect addressable devices and carry data; they are supervised and may be Class A, B, or X. Notification appliance circuits (NACs) power audible/visible appliances and are also supervised.
Wiring requirements
NFPA 72 and the NEC require fire alarm cable to be listed and installed per its listing. Power-limited circuits may use lower-rated cable; non-power-limited circuits require different separation and wiring methods. Circuits must be separated from other wiring, protected from physical damage, and installed so that a single fault does not disable both the primary and redundant path. Survivability requirements apply where circuits must remain operational during a fire (e.g., high-rise, assembly).
Grounding and isolation
Ground faults must be detected and annunciated. Addressable SLCs use isolators so a short in one segment does not disable the entire loop. Shielded cable, where used, must be grounded at one end only.
Traps
- Confusing IDC (conventional, zone) with SLC (addressable, data).
- Forgetting EOL resistors and supervision reference.
- Assuming all fire alarm cable is interchangeable - listing and circuit class matter.
- Missing that a single ground fault must annunciate but not necessarily disable the system.
- Overlooking survivability requirements in high-rise or assembly occupancies.
Rule of thumb
Supervision is the theme: every initiating, signaling, and notification circuit must detect open, ground, and short conditions. If the question describes a circuit that fails silently, it is wrong.