Power Supplies & Batteries
Primary and secondary power sources, battery standby calculations, voltage drop, power-limited transformers, and capacity margins.
- Identify primary and secondary power source requirements for fire alarm systems
- Perform battery standby capacity calculations
- Explain the 24-hour standby plus 5-minute alarm requirement
- Describe battery types and their maintenance requirements
- Calculate total NAC current draw for battery sizing
Lesson 1
Primary & Secondary Power Sources
Primary Power
The primary power source for a fire alarm system is the building's AC electrical supply. NFPA 72 requires:
- Dedicated branch circuit for the fire alarm panel
- Circuit breaker must be locked in the ON position
- Circuit must be clearly identified at the electrical panel
- No other loads on the fire alarm branch circuit
- Connection ahead of any ground fault interrupter
Dedicated Circuit Required
The fire alarm panel must have its own dedicated branch circuit that serves no other loads. The circuit breaker must be mechanically locked ON and marked "FIRE ALARM". Sharing a circuit with other equipment is a code violation.
Secondary Power (Batteries)
The secondary power source provides backup when primary AC power fails. For most fire alarm systems, this means sealed lead-acid (SLA) batteries located inside or adjacent to the fire alarm control panel.
Switchover Requirements
The system must automatically transfer to secondary power when primary power fails, and transfer back when primary power is restored. The switchover must occur without loss of any alarm signals or system functions.
Fire alarm panels require a dedicated, locked-on branch circuit for primary power. Secondary batteries must provide 24 hours standby plus 5 minutes of alarm operation. Switchover must be automatic with no loss of function.