3.2 Control Units & Power Supplies

The Fire Alarm Control Unit (FACP)

The fire alarm control unit (FACP), also called the fire alarm control panel, is the brain of the system. It is roughly the hub that every initiating, notification, and power topic on FAS-I connects back to. Its core jobs are:

• Receive inputs from initiating devices and circuits (IDCs and SLCs) - alarm, supervisory, and trouble conditions.
• Process and prioritize those inputs. The standard priority is alarm first, then supervisory, then trouble. An alarm always overrides a lower-priority condition on the display and signaling.
• Command outputs - activate NACs (horns/strobes), and drive auxiliary functions such as elevator recall, HVAC shutdown, door release, and relay modules.
• Supervise itself and its field wiring - monitor circuit integrity, battery condition, charger operation, and ground faults, annunciating any fault as a trouble signal.
• Annunciate the system state locally and at remote annunciators so responders can see status and zone information away from the main panel.

Knowing this input-process-output model lets you reason through most panel questions even without memorizing a specific manufacturer.

Primary (Main) Power

The primary power supply is the building's AC service. NFPA 72 requires the fire alarm system to be fed from a dedicated branch circuit - it serves the fire alarm system and nothing else. That circuit's over-current device (breaker) must be:

• Mechanically protected against accidental shutoff, commonly with a breaker lock.
• Identified in red and clearly marked, typically labeled "FIRE ALARM CIRCUIT."
• Accessible only to authorized personnel.

The point is simple: nobody should be able to kill the fire alarm system by flipping the wrong breaker, and if the breaker does open, it must be obvious which one it is.

Secondary (Standby) Power

If primary AC fails, the system must keep protecting the building. The secondary (standby) power supply is almost always a set of sealed lead-acid storage batteries kept charged by the panel. The battery charger continuously float-charges and supervises the batteries; a failed charger or a low/disconnected battery is annunciated as a trouble signal.

The governing requirement for most protected-premises systems is:

• 24 hours of standby (system quietly monitoring on battery), followed by
• 5 minutes of full alarm load (all required notification appliances energized).

Different occupancies have different numbers - emergency voice/alarm communication systems and certain supervising-station equipment use longer standby (for example 24 hours standby plus 15 minutes alarm for EVACS, or 60 hours for some central-station units) - but 24 hours + 5 minutes is the baseline number to know cold for FAS-I.

Sizing the Standby Battery

Battery sizing is a favorite FAS-I problem. You compute the total ampere-hours (Ah) the batteries must deliver, then add margin for aging. The procedure:

1. Standby load (Ah) = total standby current x standby hours.
2. Alarm load (Ah) = total alarm current x alarm time in hours (5 minutes = 5/60 = 0.0833 hour).
3. Add the standby and alarm amp-hours together.
4. Multiply the sum by the derating/safety factor of about 1.2 to account for battery capacity loss as the batteries age and to keep a margin. Then select the next-larger standard battery.

The 1.2 safety factor is the number to remember. It exists because a battery does not deliver its full nameplate capacity for its whole service life, so you intentionally oversize.

Worked Example

Suppose a panel draws 250 mA (0.25 A) standby and 1.5 A alarm:

• Standby: 0.25 A x 24 h = 6.0 Ah
• Alarm: 1.5 A x (5/60) h = 1.5 x 0.0833 = 0.125 Ah
• Subtotal: 6.0 + 0.125 = 6.125 Ah
• With 1.2 factor: 6.125 x 1.2 = about 7.35 Ah

So you would select the next standard battery at or above ~7.35 Ah (commonly a 7 Ah battery would be too small - you would move up to 8 Ah). Notice the standby term dominates because it runs for 24 hours while the alarm term runs only minutes; this is a common exam insight.

Trouble, AC-Loss, and Annunciation

The control unit must make every fault obvious. Trouble signals are distinct from alarm and supervisory and are typically a steady or pulsing audible plus a visible indicator at the FACP:

• AC (primary) power loss - annunciated as a trouble condition; the batteries automatically carry the load while the trouble shows the AC is gone.
• Low battery / battery disconnect / charger failure - trouble.
• Open circuit, ground fault, or loss of supervision on field wiring - trouble.

Annunciators are remote panels that repeat the system's status. A common placement is at the main entrance so arriving responders see which zone or device is in alarm without walking to the FACP. Annunciators can be lamp/LED-per-zone, alphanumeric LCD, or graphic.

Signal Priority Recap

Keeping this hierarchy straight - alarm over supervisory over trouble - resolves many panel and power questions, because the exam often asks what the panel does or displays when two conditions occur at once.