Initiating Devices and Detection
Know the device families
Smoke: ionization (fast-flaming fires), photoelectric (smoldering), spot, projected-beam, air-sampling, and duct-mounted. Heat: fixed-temperature, rate-of-rise, and combination units, plus linear heat detection. Flame: ultraviolet and infrared, used where fast, fuel-fed fires are the hazard. Waterflow switches on sprinkler/standpipe systems. Supervisory devices such as valve tamper switches and low-air switches. Manual pull stations as the occupant's own initiator.
Placement rules candidates must own
- Duct detectors are required by the mechanical code for air-handling units above 2,000 cfm; they protect the duct and air handler, and never substitute for area detection.
- Waterflow switches must operate on flow equal to or greater than that from the smallest sprinkler and may incorporate a retard not exceeding 90 seconds to ride out pressure surges.
- Manual stations mount with the operating handle 42 in to 48 in above the finished floor, within about 5 ft of the exit doorway, unobstructed, and on the egress path.
- Ceiling-mounted spot detectors stay at least 4 in from a sidewall; wall-mounted detectors are placed roughly 4 in to 12 in below the ceiling; detectors are kept about 3 ft away from supply diffusers and out of the dead-air pocket at the wall/ceiling junction.
Traps: classifying a tamper switch or duct detector trouble as an alarm; assuming a smoke detector's 30 ft spacing applies unchanged to a 30 ft ceiling; forgetting that heat detectors have their own listed spacing and must be derated for high ceilings; and placing a pull station more than 48 in high because the door frame interfered.
Rule of thumb: if people must escape through it, there is a manual station within 5 ft of it, reachable at 42–48 in.