Control Units, Power Supplies, and Secondary Power
What the control unit does
The fire alarm control unit receives initiating signals, applies the site-specific sequence of operations, and drives notification and emergency control functions. It must be listed for the purpose, supervise its own initiating and notification circuits, and annunciate alarm, supervisory, and trouble conditions separately. The dedicated branch circuit supplying it must be identified as fire alarm and accessible only to authorized personnel.
Secondary power
The system needs a secondary source sized for standby plus alarm. The common commercial benchmark is 24 hours of standby followed by 5 minutes of alarm; voice evacuation systems often carry a longer alarm period (for example 15 minutes) per the table in the adopted edition. Batteries, UPS units, and generators all qualify, and the charger must be able to restore the battery after a discharge.
Battery sizing method
- Add standby current of all loads, multiply by standby hours.
- Add alarm current multiplied by alarm hours expressed as a fraction (5 min = 0.083 h).
- Apply a 1.25 multiplier for aging and derating, then select the next standard battery size.
Worked example
Standby current 0.15 A, alarm current 1.20 A, 24 hours standby and 5 minutes alarm. Standby: 0.15 x 24 = 3.6 Ah. Alarm: 1.20 x 0.083 = 0.10 Ah. Total 3.7 Ah, times 1.25 = 4.6 Ah. A 5 Ah battery meets the calculation, but a 7 Ah unit is often chosen for margin and availability.
Traps that cost marks
- Sizing on alarm current alone and ignoring 24-hour standby.
- Omitting auxiliary loads such as door holders or communicators.
- Forgetting the 1.25 derating factor.
- Specifying a battery the charger cannot recharge in the required time.