Initiating Devices and Circuits
The Initiating Devices and Circuits domain covers how detection is wired and where each device is placed. Questions split cleanly into circuit-class items and placement items.
Circuit classes and styles
- Class A initiating device circuits (Style D) leave the panel and return. A single open does not disable any device, and there is no end-of-line resistor.
- Class B initiating device circuits (Style B) use an end-of-line resistor at the far end. A single open disables everything beyond the break, but the panel must annunciate a trouble.
- Addressable signaling line circuits follow the same logic: Class A (Style 6 or 7) versus Class B (Style 4).
- Power-limited fire alarm circuits must be separated from non-power-limited and power conductors, marked at intervals, and must not share a raceway without a barrier (NEC Article 760).
Placement rules that appear on every exam
- Spot smoke detectors on a smooth ceiling 10 ft or less: 30 ft spacing, 21 ft (0.7 x 30) maximum to any wall, 900 sq ft maximum per detector.
- Ceilings above 10 ft require reduced spacing using the code figures.
- Keep spot detectors at least 3 ft from supply diffusers and out of dead-air corners.
- Wall-mounted detectors must be listed for wall mounting and located 4 to 12 in below the ceiling.
- Manual stations: within 5 ft of the exit door, 42 to 48 in above finished floor.
- An elevator lobby detector is typically required within 21 ft of the elevator doors.
Worked example
A 60 ft by 40 ft room has a smooth 9 ft ceiling. Lay out spot smoke detectors. Reasoning: spacing is 30 ft and the wall limit is 21 ft. Use a 20 ft grid: rows at 10, 30, and 50 ft along the 60 ft dimension (three rows) and at 10 and 30 ft along the 40 ft dimension (two rows). That gives six detectors. Check: spacing 20 ft is under 30 ft, wall distance 10 ft is under 21 ft, and 400 sq ft per detector is under 900 sq ft. Six detectors works.
Traps that cost marks
- Forgetting the 0.7 wall rule and spacing only on the 30 ft dimension.
- Applying 30 ft spacing to heat detectors.
- Measuring wall distance diagonally instead of perpendicular.
- Placing a manual station at the wrong height or more than 5 ft from the door.
- Omitting the end-of-line resistor on a Class B circuit.