Control Units, Power Supplies, and Secondary Power
Domain 5 tests the fire alarm control unit, its power supply, and the secondary battery calculation. The arithmetic is simple, but the exam penalizes wrong standby times and missing derating.
Core rules
- Commercial protected-premises systems require 24 hours of standby followed by 5 minutes of alarm operation. Voice evacuation systems require a longer alarm period, typically 15 minutes.
- Batteries must be sized for standby load plus alarm load, then multiplied by a derating factor, commonly 1.2, to account for aging and inefficiency.
- The control unit must be on a dedicated circuit and marked as fire alarm power.
- Battery chargers must be capable of restoring the battery within the code time while carrying the normal load.
- Calculate with actual quiescent and alarm currents from the panel and device cut sheets, not guesses.
Worked example
Standby current is 0.35 A and alarm current is 2.5 A. Standby energy: 0.35 A times 24 h equals 8.4 Ah. Alarm energy: 2.5 A times 5 minutes, converted to 0.0833 h, equals 0.208 Ah. Total is 8.608 Ah. Multiply by 1.2 derating: 10.33 Ah. Select the next standard battery size, such as 12 Ah, and confirm charger capacity.
Traps
- Using 5 hours instead of 5 minutes for the alarm period.
- Omitting the 1.2 derating factor.
- Forgetting that voice systems require a longer alarm duration.
- Sizing batteries from nameplate current only, without device cut sheets.