Fire Alarm, Security, and Life Safety Systems
What this domain covers
Fire Alarm, Security, and Life Safety Systems tests circuit supervision, notification appliance circuits, battery calculations, detection devices, access control, and life-safety interfaces. NFPA 72 and NEC Article 760 are the controlling documents.
Core rules
- Fire alarm circuits are classified by performance. Class A circuits continue to operate with a single open or ground fault; Class B circuits do not. Style designations describe the same survivability concept for particular circuit types.
- Initiating device circuits and notification appliance circuits must be supervised. A conventional NAC typically uses an end-of-line resistor; an addressable system supervises devices individually and reports a trouble for a missing or extra device.
- Notification appliances are usually 24 VDC and are subject to voltage drop. Calculate the current of every horn, strobe, and speaker on the circuit, and verify that the last device receives at least its listed minimum operating voltage.
- Battery standby is usually 24 hours plus 5 minutes of alarm for commercial fire alarm systems, unless the occupancy code or the AHJ requires a longer time. Voice evacuation systems often require longer alarm time. Apply an aging or derating factor and use the battery manufacturer's data.
- Security systems use normally closed or normally open zones, end-of-line resistors, and supervised loops. Access control doors need fail-safe or fail-secure locks chosen by life-safety function, request-to-exit devices, and door-position switches.
- Life-safety interfaces include elevator recall, stairwell door release, HVAC shutdown, smoke dampers, and firefighter telephone. Coordinate these with the fire alarm control panel and the building code.
Worked example
A NAC has 12 smoke detectors at 0.05 A each and 4 horns at 0.5 A each. Alarm current is (12 × 0.05) + (4 × 0.5) = 0.6 + 2.0 = 2.6 A. If standby current is 0.2 A and the battery must supply 24 hours standby plus 5 minutes alarm, the ampere-hour demand is (0.2 × 24) + (2.6 × 5/60) = 4.8 + 0.217 = 5.02 Ah. Add 20 percent for aging and temperature: about 6.02 Ah, so a 7 Ah battery is the minimum practical size. The reasoning is to calculate both standby and alarm, not just one.
Traps that cost marks
Confusing Class A and Class B; sizing a battery on alarm current alone; ignoring NAC voltage drop; using unsupported cable; forgetting to supervise a tamper switch; and treating access control as a purely convenience system when it is part of egress.