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False Alarm Rate Calculator
Check a system's false alarm rate against the BS 5839‑1 Annex E benchmark rates, from detector count and activations over the past 12 months.
Indicative check only — a single elevated reading doesn't automatically mean a fault; look at the trend and the source of each activation.
Methodology
How this is assessed.
BS 5839‑1's informative Annex E sets out benchmark false alarm rates, expressed per 100 detectors per year, as a way of judging whether a system's performance is drifting outside what's normal for its type and occupancy.
This calculator applies the two triggers most commonly used by installers and maintainers day to day: for systems with 40 or more detectors, a rate above 1 false alarm per 20 detectors per year (5 per 100) warrants investigation; for smaller systems, 3 or more false alarms in a rolling 12 months is the trigger, since the detector count is too small for a rate to be meaningful.
| Rate (per 100 detectors/year) | Reading |
|---|---|
| ≤ 1 | Within the general benchmark for a well‑performing system. |
| ≤ 1.33 (1 per 75) | Benchmark for 24‑hour/shift‑occupied premises such as hospitals and care homes. |
| ≤ 2 (1 per 50) | Within general guidance for a normal system. |
| > 5 (1 per 20), 40+ detectors | Investigation trigger — review detector selection, siting and maintenance history. |
| ≥ 3 alarms/year, <40 detectors | Investigation trigger for smaller systems, applied as a count rather than a rate. |
Three or more false alarms from a single detector or call point, regardless of the overall system rate, is also worth investigating on its own — it usually points to a specific siting or contamination issue with that device rather than a system‑wide problem.
System running a high false alarm rate?
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