Control Equipment, Power Supplies, and Standby Power
Domain 5 - Control Equipment, Power Supplies, and Standby Power
This domain tests the fire alarm control unit's power architecture and the battery calculation, which is the most common calculation question on the exam.
Control unit and primary power
The control unit must be listed for fire alarm service. Primary power comes from a dedicated branch circuit provided with a means to prevent accidental disconnection, such as a breaker lock. The circuit supplies no other loads.
Secondary (standby) power
Standby power is typically sealed lead-acid batteries, or a listed UPS or generator. Transfer to standby must be automatic and the charging means must restore the battery after a discharge. Battery capacity is sized for the end of useful life, not the day of installation.
Standby capacity calculation
- Typical commercial requirement: 24 hours of standby followed by 5 minutes of alarm; voice evacuation systems require a longer alarm duration (15 minutes).
- Formula: Ah = (I_standby × 24) + (I_alarm × alarm hours), then add roughly 20% for aging and derating.
Worked example
Standby current 0.150 A; alarm current 2.40 A; 24 h standby plus 5 min alarm.
- Standby: 0.150 × 24 = 3.60 Ah
- Alarm: 2.40 × (5 ÷ 60) = 0.20 Ah
- Total: 3.80 Ah; × 1.2 = 4.56 Ah Select a 7 Ah battery, then confirm the charger is rated for that battery and that recharge time is met.
Auxiliary power
Addressable loops have their own current budget. When notification load exceeds the panel's capability, use listed auxiliary power supplies with synchronization, and keep the 24 VDC distribution within voltage-drop limits.
Traps
Skipping the 20% derate; using quiescent current for the alarm leg; ignoring charger rating; forgetting that batteries age and must be dated and replaced.