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HI Fire Alarm study materials
HI Fire Alarm study resources

HI Fire Alarm: Control Units, Power Supplies, and Wiring

HI Fire Alarm · Work through your exam, one topic at a time.

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Control Units, Power Supplies, and Wiring

The Control Units, Power Supplies, and Wiring domain covers primary power, secondary power sizing, and the wiring methods that keep a system alive. Battery calculations appear on nearly every form of this exam.

Primary power

  • A dedicated branch circuit, supplied ahead of the main disconnect where required, with overcurrent protection per the manufacturer.
  • No GFCI protection on the fire alarm branch circuit.
  • A lock-on breaker or approved locking means, and a label at the disconnect and at the panel reading FIRE ALARM CIRCUIT CONTROL.

Secondary power

  • Batteries must carry the standby load for 24 hours plus 5 minutes of alarm, or 15 minutes of alarm where a voice evacuation system is provided.
  • Size batteries with an aging or derating factor (commonly 20 percent) and round up to the next standard size.
  • The charger must restore the batteries within the time stated by the manufacturer and code, and cannot be loaded beyond its rating.

Wiring methods

  • Power-limited fire alarm cable is marked FPL, FPLR, or FPLP and installed per Article 760.
  • Power-limited and non-power-limited fire alarm conductors may not share a raceway, cable, or enclosure without a barrier.
  • Ground-fault detection is required on the circuits so a fault annunciates as a trouble rather than silently disabling protection.
  • Interfaces to other systems (elevator recall, door release, suppression) must be supervised and listed for the purpose.

Worked example

Standby load 0.35 A for 24 hours; alarm load 2.5 A for 5 minutes. What battery? Reasoning: standby = 0.35 x 24 = 8.4 Ah. Alarm = 2.5 x (5/60) = 0.21 Ah. Total = 8.61 Ah. Apply a 20 percent aging factor: 8.61 x 1.2 = 10.3 Ah. Select a 12 Ah battery (next standard size), then verify the charger's recharge rating and the panel's maximum battery capacity. If the system had voice evacuation, the alarm period would be 15 minutes instead of 5.

Traps that cost marks

  • Putting the fire alarm circuit on a GFCI.
  • Using 5 minutes of alarm time for a voice evacuation system.
  • Sizing at the exact calculated value with no aging factor and no standard-size rounding.
  • Mixing power-limited and non-power-limited conductors in one raceway.
  • Ignoring the panel's maximum battery size and charger capacity.