GA Fire Alarm: Fire Alarm Systems study guide

Fire Alarm Systems is the heaviest technical domain on the Georgia exam. It tests whether you can lay out initiating devices and notification appliances, pick the right circuit class, prove the power supply and voltage drop, and then inspect and test the finished system to NFPA 72.

Initiating device layout

Spot smoke detectors on a smooth ceiling: 30 ft on center, no more than 21 ft (0.7 x 30) from a sidewall, and 900 sq ft maximum per detector.
Spot heat detectors on a 10 ft smooth ceiling: 50 ft on center, 25 ft from walls, 625 sq ft per detector for units rated 135-174 F. Raise the ceiling and spacing shrinks; use NFPA 72's height multiplier instead of the flat 10 ft numbers.
Keep spot detectors at least 3 ft from supply diffusers so air changes do not delay response.
Duct detectors belong in the return or supply duct where other codes require them (typically air handling over 2,000 cfm). They are never a substitute for area detection.

Circuit classes

Class B: a single open disables every device beyond the break. Cheap, common, and a favorite trap answer.
Class A: the circuit leaves and returns to the panel, so one open does not disable the circuit. Often required in high-rise and assembly designs.
Class X (addressable): short-circuit isolation keeps the rest of the loop operating after a fault.

Notification and power

Public mode: 65 dBA, or 10 dBA above average ambient, measured 5 ft above the floor.
Sleeping areas: 75 dBA, or 15 dBA above ambient, measured at pillow level.
Hold at least 20.4 V at the last appliance on a 24 VDC NAC, which is 85 percent of rated voltage and the listed minimum for most appliances.
Battery sizing: 24 hours of standby followed by 5 minutes of alarm, based on calculated loads.

Worked example

A 24 VDC NAC serves four horn/strobes at 0.15 A each; the run is 250 ft of #14 AWG (3.07 ohms per 1,000 ft), so 500 ft of conductor. Load = 0.60 A. Resistance = 500 x 3.07 / 1,000 = 1.54 ohms. Voltage drop = 0.60 x 1.54 = 0.92 V. End voltage = 24 - 0.92 = 23.1 V, comfortably above 20.4 V, so the circuit passes. Appliance loads are actually distributed, so this lumped series model is conservative, which is why it is accepted for a quick field check.

Traps that cost marks

Confusing spacing (30 ft) with coverage (900 sq ft) on beamed or high ceilings.
Sizing the battery for standby only and ignoring the alarm leg of the 24 h + 5 min calculation.
Claiming a Class B circuit supervises both opens and grounds.
Forgetting that duct detector activation must shut down the air handler when the code requires it.