Fire Detection and Alarm Systems
System anatomy
A fire alarm system comprises initiating devices, a fire alarm control unit, notification appliances, and the pathways that tie them together. NFPA 72 is a performance document: it describes what the system must do, not which product to buy. Pathway class matters - a Class A circuit survives a single open or ground fault, a Class B circuit does not. Addressable systems identify devices individually; conventional systems annunciate by zone, which is why zone size and labeling rules appear in questions.
Detection layout
Spot-type smoke detectors on smooth flat ceilings: 30 ft spacing and 900 ft² per detector, with no point more than 0.7 × spacing (21 ft) from a detector. Heat detectors: 50 ft and 2,500 ft², with 0.7 × spacing (35 ft) to walls. Reduce spacing for beams, joists, and ceiling heights outside the table basis. Duct detectors are required by NFPA 90A for air-handling units above 2,000 cfm. Waterflow switches must alarm within 90 seconds of sustained flow.
Notification and interfaces
Audible appliances must exceed ambient by 15 dBA and reach 75 dBA at the pillow in sleeping areas; visible appliances are spaced by candela rating. Elevator recall uses lobby, hoistway, and machine room detectors, with Phase I primary and alternate floors and Phase II in-car operation. Voice evacuation and mass notification fall under the emergency communication chapters. The supervising station type (central, proprietary, remote, auxiliary) sets the retransmission path. Chapter 14 inspection, testing, and maintenance includes annual testing, with smoke detector sensitivity checked after installation and at the interval the adopted edition specifies.
Worked example
A 60 ft × 40 ft room = 2,400 ft². At 900 ft² per detector, 2,400 / 900 = 2.7, so three detectors minimum; then verify that a 30 ft grid leaves no point more than 21 ft from a detector.
Traps
Applying 30 ft spacing blindly under a 20 ft ceiling; forgetting the 0.7 wall rule; confusing alarm, supervisory, and trouble signals; forgetting the 90-second waterflow criterion.