Fire Alarm System Fundamentals and Design
System types and signals
Know the vocabulary: protected premises (local), auxiliary, remote station, central station, and proprietary systems describe how alarm signals leave the building (or whether they do). Internally, a system has initiating device circuits (IDCs), notification appliance circuits (NACs), and often signaling line circuits (SLCs) for addressable devices. Three signal classes must never be confused: alarm (fire condition), supervisory (off-normal condition such as a closed valve), and trouble (system integrity problem such as an open circuit or ground fault).
Design logic
Design starts with the sequence of operations matrix - a table naming every input, every output, and the logic between them. Conventional systems group devices into zones; addressable systems identify each device individually. Common features tested on exams: alarm verification (a brief delay confirming a smoke detector signal before general alarm), presignal (alarm signals at a constantly attended location before general evacuation), and positive alarm sequence (PAS), which allows roughly 15 seconds to acknowledge and about 180 seconds to investigate before the general alarm sounds - and if nobody acknowledges within that first window, the general alarm goes immediately.
Waterflow switches typically must alarm on sustained flow, with a retard (commonly in the range of 10 to 90 seconds) so that pressure surges do not create nuisance alarms. Valve tamper switches report as supervisory, not alarm.
Worked example
A designer proposes PAS for a dormitory. The corridor smoke detector activates; the panel starts its 15-second acknowledge timer with no audible evocation. The attendant acknowledges at 10 seconds, then has the investigation window (~180 s) to check the area. If the attendant had not acknowledged within 15 seconds, the evacuation signal would have sounded at once. Read the sequence matrix top-down and the timing questions answer themselves.
Traps
Treating supervisory as alarm; assuming instant waterflow alarm; forgetting that PAS requires a constantly attended location.