Fire Alarm Systems
Fire Alarm Systems
This domain tests how well you apply NFPA 72 and NEC Article 760 to the life cycle of a fire alarm: initiation, notification, control, and power. Expect questions on device spacing, circuit class, addressable versus conventional operation, monitoring, and battery calculations.
Core rules
- A fire alarm system has initiating device circuits (IDC), notification appliance circuits (NAC), and a control unit. Conventional IDCs report a zone; addressable systems identify the individual device.
- Class A circuits continue to operate after a single open; Class B circuits do not. Know which pathway the design requires and how a fault is annunciated.
- Spot-type smoke detectors on a smooth ceiling use 30 ft (9.1 m) spacing and a 900 sq ft (83.6 m²) maximum area per detector, with detectors no more than half the spacing from walls. Beams, high ceilings, and air changes require adjustment.
- Notification must be audible and visible per the adopted code; sleeping areas, ADA visible signals, and synchronized NACs are common exam items.
- Power supplies must be listed for fire alarm, supervised, and sized for standby plus alarm. Battery calculations use the current draw of all devices.
Worked example
A 90 ft × 60 ft smooth-ceiling room needs spot smoke detectors. Area = 5,400 sq ft; 5,400/900 = 6 detectors. A 30 ft grid gives positions at 15, 45, and 75 ft across the 90 ft dimension and 15 and 45 ft across the 60 ft dimension - a 3 × 2 layout, six detectors. Check that no point is more than 15 ft from a detector and that the layout is not blocked by beams.
Traps
Do not mix IDC and NAC wiring; do not assume 30 ft spacing survives a beam or high ceiling; do not skip battery standby; do not treat addressable devices as automatically power-limited without checking Article 760.