Power Supplies: Primary and Secondary Power
Primary power
The control unit is fed by a dedicated branch circuit serving only the fire alarm system, taken from the building supply so it is not switched off with lighting or other equipment. The disconnecting means must be clearly marked (for example, Fire Alarm Circuit, Do Not Disconnect) and arranged so it can be locked. Overcurrent protection follows the manufacturer's instructions, and ground-fault protection of the fire alarm branch circuit is generally not permitted because a tripped GFCI would silently remove power.
Secondary power
Battery capacity is calculated for 24 hours of standby followed by 5 minutes of alarm signaling, or 15 minutes where a voice evacuation system is installed. The load must include the panel, all loop-powered devices, notification appliance circuits, and the communicator or IP device. Apply a 20 percent margin for battery aging. The charger must be able to restore the battery within 24 hours, and the panel must annunciate low battery and charger failure before the system is lost. Float voltage on a 24 V lead-acid string is roughly 27.6 V (13.8 V per 12 V battery).
Traps
Leaving the communicator out of the calculation, using 5 minutes for a voice system, omitting the aging margin, and forgetting that a generator-backed circuit can still drop during transfer.
Worked example
Standby 0.20 A x 24 h = 4.8 Ah. Alarm 1.5 A x 5/60 h = 0.125 Ah. Total 4.93 Ah x 1.2 = 5.9 Ah, so specify a 7 Ah battery.