Control Units, Power Supplies, and Secondary Power
What this domain covers
Control unit functions, primary and secondary power, battery sizing with the derating factor, charger performance, and ground-fault and trouble supervision.
Core rules
- The control unit monitors initiating circuits, drives notification, annunciates alarm/supervisory/trouble, and may control releasing and fire-safety functions.
- Secondary power for a commercial protected-premises system is commonly 24 hours of standby followed by 5 minutes of alarm, with 15 minutes where a voice evacuation system is used. Confirm the exact figures in the edition your authority enforces.
- Battery calculation: standby current × 24 h, plus alarm current × (minutes ÷ 60), then multiply by 1.2 to cover aging and capacity tolerance.
- The charger must be able to restore the battery after a discharge while carrying the normal load, and battery terminals, voltage, and electrolyte (if wet) are inspected on a scheduled basis.
- Ground-fault detection is required on the system; a ground is a trouble, not an alarm.
- Notification appliance circuit expanders and power supplies must be supervised and synchronized with the main panel.
Worked example
Standby load 0.45 A, alarm load 2.6 A, non-voice system. Standby: 0.45 × 24 = 10.8 Ah. Alarm: 2.6 × (5 ÷ 60) = 0.22 Ah. Total 11.02 Ah; × 1.2 = 13.2 Ah required. Two 7 Ah sealed lead-acid batteries give 14 Ah and are acceptable, but only if the charger's recharge rating and the enclosure space also check out.
Traps that cost marks
Sizing to the nameplate amp-hour without the 1.2 factor; forgetting continuously powered accessories (annunciator, communicator, door holders) in standby; using 5 minutes on a voice system; mixing old and new batteries; assuming a charger is adequate because the batteries fit.