Fire Detection, Alarm, and Notification Systems
System anatomy
A protected-premises fire alarm system comprises initiating devices, a control unit, notification appliances, and the circuits linking them. Signals are alarm, supervisory, and trouble (plus security). Conventional systems use zones; addressable systems identify each device, enabling precise annunciation and drift compensation. NFPA 72 governs installation, performance, inspection, testing, and maintenance.
Detection technologies
Ionization detectors respond best to small, fast flaming particles; photoelectric (spot, beam, air-sampling) respond to smoldering smoke. Heat detectors are fixed-temperature, rate-of-rise, or combination, and are inherently slower. Duct detectors protect HVAC equipment and smoke spread, not the room. Gas and flame detectors serve special hazards.
Spacing rules
For smooth ceilings, spot-type smoke detectors are spaced at 30 ft (9.1 m), with the first detector no more than 0.7 × spacing (21 ft) from a sidewall and 4 in. minimum from the wall. Heat detectors use 50 ft spacing (35 ft to the wall). Spacing must be reduced for high ceilings, beams, and joists per NFPA 72 tables; solid beams deeper than the listing threshold create separate compartments.
Notification and interfaces
Audible signals must exceed ambient by 15 dBA and reach 75 dBA at the pillow in sleeping areas; visible appliances are rated in candela and must be synchronized when on the same circuit. Voice evacuation is required for many assembly, high-rise, and occupancies with staged evacuation. Interfaces include elevator recall, door release, HVAC shutdown, and suppression releasing service.
Worked example
A 90 ft × 60 ft smooth-ceiling room: along 90 ft, use three detectors at 21, 45, and 69 ft (gaps ≤ 30 ft, wall distances ≤ 21 ft). Along 60 ft, two detectors at 21 and 39 ft. Total 3 × 2 = 6 spot smoke detectors.
Traps
Questions blur detector spacing with coverage area, forget the 0.7 × S wall rule, or confuse supervisory and trouble signals. Duct detectors are not a room smoke detection substitute.