Source note. This article is based on the Fire Industry Association's published summary of the changes, Guide to the changes in BS 5839-1:2025 (Version 1, June 2025). It has not been checked line by line against the text of BS 5839-1:2025 itself, which is the authoritative document. Check clause wording in the standard before relying on it in a contract, certificate or dispute. The standard applies to the UK; other countries use different codes.
What changed in the 2025 revision
The FIA guide describes Section 3 (limitation of false alarms and unwanted fire alarm signals) as reviewed and reorganised into five clauses:
- Clause 29, responsibility for limitation of false alarms
- Clause 30, categories and causes of false alarms
- Clause 31, investigation of false alarms
- Clause 32, design process for limitation of false alarms
- Clause 33, measures to limit false alarms
Two practical changes stand out for anyone servicing systems.
- Every false alarm should be investigated. Subclause 29.6 recommends that the commissioning or handover organisation advises the user to arrange suitable investigation, and action where appropriate, on every occasion a false alarm occurs. The FIA's example of action is managerial change within the building.
- Categories of false alarm move into the commentary of clause 30. The commissioning or handover organisation should explain the categories to the user, so the user assigns the correct category to each incident.
The false alarm rate calculation
A new Annex F (informative) gives the calculation:
Afr = 100 × (Afn ÷ DAF)
- Afr is the false alarm rate
- Afn is the number of false alarms
- DAF is the number of automatic fire detectors
The FIA notes the trigger points are brought in line with the calculation that is recommended at every service visit, so the rate is something a servicing engineer should be working out routinely rather than only after a complaint. The annex also carries worked examples of the calculation and of which type of investigation follows.
Trigger points
| Situation | Recommended response | |---|---| | Rate over the previous 12 months exceeds 4 false alarms per 100 detectors per annum | Preliminary investigation | | System has more than 40 automatic fire detectors and the average rate exceeds 5 per 100 detectors per annum | In-depth investigation |
The FIA states these thresholds were not changed by the 2025 revision; they are now expressed in the same terms as the calculation.
Illustrative arithmetic
These figures are our own arithmetic to show the method, not examples taken from the standard.
| Detectors | False alarms in 12 months | Rate per 100 detectors | Outcome | |---|---|---|---| | 100 | 3 | 3.0 | Below both trigger points | | 100 | 5 | 5.0 | Above 4: preliminary investigation. Not above 5, so no in-depth investigation | | 250 | 14 | 5.6 | Above 4 and above 5: preliminary and in-depth investigation both indicated | | 30 | 2 | 6.7 | Above 4: preliminary investigation. In-depth threshold does not apply, as the system has 40 or fewer detectors |
A small system is very sensitive to a single event. Two false alarms on a 30-detector system is a rate above the preliminary trigger, which is why the count of detectors matters when reading the figure.
Detector choice
The FIA reports greater emphasis on multi-sensor detectors: where point smoke detectors may carry a higher risk of false alarms, the 2025 revision recommends selecting multi-sensor detectors instead, with Annex D giving guidance on detector selection.
Practical points for engineers
- Keep the false alarm log current and count only what belongs in it under the categories the user has been given.
- Record the rate at each service visit, not only when a threshold is crossed, so a trend is visible.
- When a threshold is crossed, record what was investigated and what was changed. That record is the evidence that the recommendation was followed.
- A rate is a prompt for investigation, not a diagnosis. Cause identification still comes from the event log and from fault-finding on the devices concerned.
For the broader picture of causes and prevention, see fire alarm false alarms and reducing unwanted alarms.
Test what you have learned with the free daily fire alarm fault challenge, which includes questions on this topic with the source shown for every answer.
Scope
This article covers BS 5839-1 as used in the UK. Ireland, the United States, Canada, Australia and New Zealand each apply their own standards and thresholds, so do not carry these figures into those markets.