Initiating Devices and Detection
Initiating devices include manual pull stations and automatic detectors: smoke (ionization, photoelectric), heat (fixed temperature, rate-of-rise), and specialty types (beam, air-sampling, flame). Know the operating principle of each and where it is appropriate.
Photoelectric smoke detectors respond to visible smoke particles (smoldering fires). Ionization detectors respond to small, invisible particles (fast-flaming fires) and are more prone to nuisance alarms from cooking and steam. Fixed-temperature heat detectors alarm at a set temperature; rate-of-rise detectors alarm when temperature climbs quickly. Heat detectors are slower than smoke detectors but more nuisance-resistant.
Spacing and placement are heavily tested. Detectors are generally spaced per listed spacing and manufacturer instructions; smoke detectors must be kept away from dead air spaces, HVAC supply diffusers, and the required clearance from walls and obstructions. Pull stations are typically mounted at a standard height and located along the path of egress near exits, so occupants encounter them on the way out.
Traps: assuming a smoke detector belongs in every space (kitchens and bathrooms are problematic); confusing rate-of-rise with fixed temperature; forgetting that the manufacturer's listed spacing governs over a generic rule.
Rule of thumb: smoldering fire → photoelectric; flaming fire → ionization; harsh environment → heat detector; path of egress → manual pull station.
Worked example: A detector in a commercial kitchen keeps nuisance-alarming. The correct fix is typically a heat detector or a properly rated detector, not a higher-sensitivity smoke detector.