Electrical Theory and Power Supplies
Scope
This domain tests the electrical calculations behind a reliable fire alarm system: Ohm’s law, series and parallel loads, notification appliance circuit voltage drop, primary and secondary power, and battery sizing. Questions often require a calculation, not just a definition.
Core rules
- Ohm’s law: E = I × R. Power: P = E × I. In series, current is common and resistances add; in parallel, voltage is common and branch currents add.
- Primary power is the commercial branch circuit; secondary power is usually sealed lead-acid batteries. The secondary supply must carry the standby load for the required time and then the alarm load.
- A common protected-premises requirement is 24 hours of standby followed by 5 minutes of alarm. Voice evacuation systems can require a longer alarm period; confirm the adopted edition.
- Battery capacity is computed from standby current × standby hours plus alarm current × alarm hours, then increased for aging and end-of-discharge voltage. A 1.25 factor is common.
- Voltage-drop calculations must use the total conductor length, both outgoing and return, and the appliance minimum operating voltage-not just the nominal 24 V.
Worked example
A panel draws 0.30 A standby and 2.40 A alarm. Required secondary capacity is 24 h standby plus 5 min alarm. Standby: 0.30 × 24 = 7.20 Ah. Alarm: 2.40 × (5 ÷ 60) = 0.20 Ah. Sum = 7.40 Ah. Apply a 1.25 aging/depletion factor: 7.40 × 1.25 = 9.25 Ah. Select a battery whose rated capacity at the end-of-discharge voltage is at least 9.25 Ah, commonly the next standard size, and verify the charger can recharge it within the code time. Do not simply pick a 7 Ah battery because the arithmetic sum was 7.40 Ah.
Traps that cost marks
- Forgetting device current in standby or alarm; every module, detector, and appliance matters.
- Using one-way cable length in voltage drop instead of the loop length.
- Checking only nominal voltage and ignoring the panel’s low-battery condition.
- Mixing Ah and amps without converting minutes to hours.
- Assuming all 24 V appliances operate down to the same minimum voltage.