Fire Alarm System Fundamentals and Design
Signals and circuits
A protected premises system produces three signal families: alarm, supervisory, and trouble. Alarm has priority, and supervisory and trouble signals must be distinct and annunciated. Design work turns on circuit class and style: Class A (redundant path, both paths supervised, no T-taps) versus Class B (single supervised path, T-taps permitted). Style designations (B, D, 4, 6, 7) are the style names for those classes on IDCs and SLCs; exam items swap them deliberately. Addressable is not a synonym for Class A.
Detection spacing
On smooth ceilings 10 ft or less, spot smoke detectors are spaced not more than 30 ft apart and not more than 21 ft from any wall. The 21 ft figure is the half-diagonal of the roughly 900 sq ft square each detector covers, which is why a nominally 30 ft grid still satisfies the wall rule. Heat detector spacing follows the listing, reduced for higher ceilings; beam pockets, joists, and slope change the layout.
Notification
Audible appliances must produce 15 dBA above average ambient or 5 dBA above the maximum sound level lasting 60 seconds or more, whichever is greater, and must not exceed 110 dBA at minimum hearing distance (above that, visible appliances are required). Sleeping areas call for 520 Hz low-frequency sound. Visible appliances are sized from the room dimensions, ceiling height, and mounting height (80 to 96 in. AFF) using the published candela table.
Worked example
A 90 ft by 60 ft smooth ceiling room under 10 ft high: three rows at 15, 45, and 75 ft satisfy the 30 ft spacing and 21 ft wall rule in the long direction; two rows at 15 and 45 ft do the same across 60 ft. Six detectors total.
Trap
Performance-based designs must still be documented and approved by the AHJ; the prescriptive spacing rules are defaults, not a ceiling on design responsibility.