Fault Finding — Common Fire Alarm Faults
Purpose
This document covers how to diagnose and resolve the most common fire alarm faults — false alarms, zone faults, sounder faults, communication faults, and power faults.
Fire alarm faults are more serious than intruder alarm faults. A fire alarm that fails to activate in a real fire can have fatal consequences. Do not sign off a system as operational until the fault is properly resolved.
False Alarm — Detector Activated Without a Fire
False alarms are the most common reason for a fire alarm service call. Repeated false alarms cause occupants to ignore the alarm — a serious life-safety risk.
Step 1 — Read the panel event log
Identify the exact device and time of activation. On an addressable panel this is a specific detector address and location. On a conventional panel it is the zone number.
Step 2 — Identify the cause
Environmental causes (most common):
| Detector location | Likely false alarm cause |
|---|---|
| Near kitchen | Cooking fumes, steam, toast smoke |
| Near bathroom | Steam from shower |
| Near boiler room | Dust, steam, condensation |
| Near air conditioning outlet | Turbulence affecting detection chamber |
| Storage area | Dust from disturbed goods |
| Near loading bay | Diesel fumes from vehicles |
Detector condition:
- Dirty/dusty — clean or replace
- Insect in detection chamber — replace
- Detector has been painted — replace immediately; paint blocks sensing
Wiring causes:
- Intermittent cable joint — flex the zone cable to see if zone state changes
- Moisture in a junction box — inspect all junction boxes on the zone
Step 3 — Resolve
Options in order of preference:
- Relocate the detector away from the source of interference
- Replace with a multi-sensor detector — significantly more resistant to cooking, steam, and dust
- Change detector type — e.g. replace optical with heat detector in kitchen environments
Do not simply isolate a detector to stop false alarms unless absolutely necessary as a temporary measure — only with the customer's documented consent and a plan to reinstate.
Zone Fault (Conventional) — Open Circuit
The panel cannot detect the EOL resistor — the zone circuit is broken.
Causes: Cable break; loose connection at panel terminal, detector base, or call point; failed detector base; EOL resistor missing or failed.
Diagnosis:
- Disconnect the zone at the panel terminals
- Measure resistance across the zone wiring — should read approximately the EOL resistor value (e.g. 4.7kΩ)
- If open circuit: trace the cable from the panel outward, measuring at each junction point until the break is located
Zone Fault (Conventional) — Short Circuit
The zone wiring is shorted, causing a zone alarm or specific short fault.
Causes: Cable damage (two cores touching); moisture in junction box; detector base with shorted contact.
Diagnosis:
- Disconnect the zone at the panel
- Measure resistance across the zone wires — should not be zero or near-zero
- If shorted: isolate sections by disconnecting at junction boxes and measuring each section until the short is located
Loop Fault (Addressable) — Device Missing or Offline
The panel reports a device at a specific address is missing or not communicating.
Check:
- Is the device powered? Check cable connections at the base.
- Is the device correctly seated in its base? Remove and reseat.
- Is the address set correctly? Compare with the device schedule.
- Is there a loop wiring fault between the previous device and this one?
Temporary isolation: If a device must be taken out of service, isolate it at the panel (Engineer Menu → Isolate → Device [address]). Never leave devices isolated without recording it and arranging reinstatement.
Sounder Fault — Sounder Not Activating
Check:
- Measure voltage at the sounder terminals during an alarm — should be the panel's sounder output voltage (typically 24V)
- Check polarity — sounders are polarity-sensitive (reverse polarity = no output)
- Check the EOL device at the end of the sounder circuit is present and correct
- Substitute with a known-working sounder to confirm whether the sounder head is faulty
Power Fault — Mains Failure
Check:
- Is the panel's mains fuse/MCB intact and switched on?
- Is there a wider mains fault at the premises?
- Is the mains cable undamaged and connected at both ends?
During mains failure: The panel operates from battery. BS 5839-1 requires minimum 24-hour standby on battery. Monitor battery voltage if mains fault is sustained.
After mains restores: Battery recharges automatically. A low battery fault may show for several hours — normal.
Power Fault — Battery Fault
Check:
- Is there also a mains fault? (Battery may have discharged)
- Test battery voltage — each 12V battery should read 12.7V+ at rest
- If below 11V, swollen, leaking, or won't hold charge — replace
- Check battery connections are secure and not corroded
Replacement interval: 4 years. Replace during annual service if approaching or exceeding this age.
Communication Fault — ARC Signalling Failure
Check:
- Is the communication module powered and connected?
- Is the network/broadband connection at the premises active?
- Confirm ARC receiver IP address and port are correct in panel programming
- Send a manual test signal and confirm receipt with the ARC
If the communication module has failed (no LEDs, no response): replace the module.
Panel Won't Reset After Alarm
Check:
- Is there still a detector in alarm? The panel will not reset while any device is active.
- Is there still a call point activated? Reset all MCPs using the test key.
- Is engineer-level reset required? Some systems require an engineer code to reset after an alarm. Use the engineer code.
- Are there zone or loop faults? Clear all faults before attempting a reset.