Fire Detection Devices and Circuits
This domain tests device selection and placement: what a detector responds to, where it belongs, and how the circuit behaves when something fails.
Detector types
Ionization smoke detectors respond to sub-micron particles and react quickly to flaming fires, but they nuisance-alarm on cooking and steam. Photoelectric detectors respond to larger particles and suit smoldering materials and most commercial spaces. Beam and air-aspirating detectors cover atria and other high spaces where point detectors are impractical.
Spacing rules to quote
- Smooth flat ceiling, smoke detectors: 30 ft maximum spacing and no point more than 15 ft from a detector, which yields a 900 sq ft maximum area per detector.
- Keep detectors at least 3 ft horizontally from supply air diffusers and at least 4 in. from sidewalls; wall-mounted units sit 4 to 12 in. below the ceiling.
- Heat detectors follow their listed spacing plus NFPA 72 reduction for ceiling height; never reuse the smoke detector grid.
- Duct detectors are installed per NFPA 90A for air-handling systems above 2,000 cfm, mainly for equipment shutdown and smoke spread control.
Worked example
A 60 ft by 40 ft room with a smooth ceiling is 2,400 sq ft. Area alone suggests three detectors (2,400 divided by 900). Spacing controls instead: along 60 ft, detectors at 15 ft and 45 ft give 30 ft spacing with 15 ft to each wall. Along 40 ft, a single row 20 ft from the walls fails the 15 ft limit, so two rows are required. Result: four detectors at the intersections of 15/45 ft and 10/30 ft.
Traps
Sizing by area only; forgetting the sidewall distance limit; counting duct detectors toward ceiling coverage; specifying ionization detectors next to kitchen or shower nuisance sources.