Initiating Devices and Circuits
What this domain covers
Spot detection spacing, manual station placement, duct and waterflow devices, circuit styles, and the difference between alarm and supervisory inputs.
Core rules
- Smoke detectors, smooth ceiling: space no more than the listed spacing (commonly 30 ft), keep detectors within half the listed spacing (15 ft) of walls measured at right angles, at least 4 in from side walls, and no ceiling point more than 0.7 × listed spacing from a detector.
- Wall-mounted smoke detectors sit 4 to 12 in below the ceiling.
- Keep detectors at least 36 in from supply diffusers; provide a detector within 3 ft of the peak of a peaked ceiling.
- Heat detectors use their own listed spacing (often 15 to 25 ft) and the same 0.7 diagonal check.
- Manual pull stations: 42 to 48 in above finished floor, within 5 ft of the exit door, unobstructed.
- Duct detectors are required for larger air-handling units and must be installed with sampling tubes per the manufacturer and tested at the remote port.
- Waterflow switches must alarm on sustained flow (commonly within 90 s); tamper switches are supervisory.
Worked example
A 60 ft × 40 ft smooth-ceiling room with 30-ft listed smoke detectors. Place columns at 15 and 45 ft (30-ft spacing, 15 ft to walls) and rows at 10 and 30 ft (20-ft spacing, 10 ft to walls). Four detectors. Worst-case diagonal to a corner is the square root of (15² + 10²) ≈ 18 ft, under the 21-ft limit (0.7 × 30). The layout passes.
Traps that cost marks
Putting detectors in dead-air pockets above doorways or next to diffusers; T-tapping an addressable signaling line circuit; mixing conventional zone wiring with addressable devices on the same terminals; forgetting end-of-line supervision; setting alarm verification without accounting for the added delay.