Fire Alarm Systems: Components, Circuits, and Operation
Core concepts
A fire alarm system is a chain: initiation (smoke, heat, duct, waterflow, manual pull), notification (horns, strobes, speakers), control (FACP), and supporting circuits (power, supervisory, signaling line). Know the difference between an initiation device circuit (conventional zone wiring) and an addressable signaling line circuit (SLC), and between notification appliance circuits (NAC) and relay-controlled outputs. Understand Class A vs. Class B wiring: Class B is a two-wire circuit that loses the entire circuit on an open; Class A returns the circuit to the panel so a single open leaves all devices operational. Know supervision - open, short, ground, and power-supply faults must annunciate - and trouble vs. alarm vs. supervisory signals.
Traps in questions
- Confusing a supervisory signal (valve closed, low air) with a trouble signal.
- Assuming addressable means wireless or that SLC carries speaker audio.
- Mixing alarm verification and presignal with simple delay timers.
- Forgetting that duct detectors typically supervise airflow and often shut down units rather than trigger general evacuation.
Rule of thumb
For a Class B circuit, one open = loss of everything beyond the break; for Class A, one open = no loss of function. If the question asks 'how many devices are lost,' answer for the circuit class named.
Worked example
A Class B NAC feeds 12 appliances in series wiring; an open occurs after the fourth device. Devices 5–12 drop out and the panel shows a NAC trouble. Same layout in Class A: all 12 continue to operate and a trouble annunciates.