Control Panels, Power Supplies, and Circuit Design
The panel as the system's brain and battery
A fire alarm control unit monitors initiating circuits, drives notification circuits, and manages power. Candidates must understand primary (AC) power, secondary (battery) standby, and how calculations prove the battery can carry the load through the required standby period plus alarm time. Battery sizing depends on quiescent current, alarm current, and any auxiliary loads - and on derating for aging and temperature.
Circuit design fundamentals
- Notification appliance circuits (NACs): regulated vs. unregulated, and how voltage drop limits appliance count and wire gauge.
- Voltage drop calculations: the classic exam item. Longer runs and smaller wire reduce end-of-line voltage; appliances have a minimum operating voltage.
- Class A vs. Class B wiring and how each tolerates faults.
- Addressable signaling line circuits (SLCs) and device loading limits.
- Separation of power-limited from non-power-limited wiring.
Traps
Many questions hinge on where the battery calculation starts: standby load excludes notification appliances, which are added only during the alarm period. Another trap is treating nameplate current as the design value - always use the manufacturer's listed alarm current. Also expect items on ground faults presenting as troubles, and on shared neutrals or improperly tapped 24V aux power causing intermittent faults.
Rule of thumb
Size batteries by summing standby current × standby hours plus alarm current × alarm duration, then apply the derating factor. If the computed capacity is close to the next standard size, go up - never down.
Worked example: Panel quiescent 0.10 A for 24 hours = 2.4 Ah; alarm load 1.5 A for 5 minutes ≈ 0.125 Ah; total ≈ 2.53 Ah before derating. With a 1.25 safety factor, specify at least ~3.2 Ah, so a 4 Ah or 5 Ah battery is appropriate.