Fire alarm battery and false alarm calculators (BS 5839-1:2025)

Calculator 01 · Clause 24.3 & Annex E

Standby battery calculator

Minimum battery capacity (C_min) for a valve-regulated lead acid standby battery, with the standby period set by the system category and how the building is used.

1 · System category

M or LLife protection (L1 to L5), or a manual system PProperty protection (P1 or P2) Combinede.g. L2/P2, L3/P2 or P1/M

2 · The building An automatically started standby generator serves the whole fire alarm system Including any remote power supplies. If any part of the system is not on the generator, leave this unticked. A power supply fault would be dealt with quickly Either the building is staffed or checked out of hours so a fault would be seen within 6 hours, or a power supply fault signals an alarm receiving centre that tells a keyholder straight away.

Longest time the building is likely to be empty For example, Friday 6pm to Monday 8am is 62 hours.

Longer standby period specified? Only if the specification, insurer or fire strategy asks for more than the standard. Leave blank otherwise.

3 · Currents

Currents per item from the manufacturers' data sheets, in milliamps. Alarm current is what each item draws with the system in alarm (3.40 counts every sounder and beacon running, plus alarm signals from the detectors and call points).

Item	Qty	Standby mA each	Alarm mA each
Totals		0 mA	0 mA

4 · Alarm period and derating

Alarm period (T₂)At least 30 minutes of evacuate signal after the standby period (24.3.5).

Battery maker's derating factorOnly used if the alarm load is heavy for the battery (see E.1.2). Leave blank to use 1.75.

Calculator 02 · Clause 31 & Annex F

False alarm rate calculator

The rate per 100 detectors per annum, and every trigger in clause 31 for a review, a preliminary investigation or an in-depth investigation. Work it out per system, not across a whole estate.

1 · The system

Automatic fire detectors Count each detector once: point smoke, heat and multi-sensor detectors, each beam, and each aspirating detector (not each sampling hole). Call points are not detectors.

2 · The previous 12 months

False alarms from automatic detectorsThese make up the rate.

False alarms from manual call pointsLeft out of the rate, as in the standard's own worked example, but they still count below.

Most from any one call point, detector or detector location

Unwanted fire alarm signalsFalse alarms passed on to the fire and rescue service, whether or not they attended.

3 · Since the last service visit

False alarms since the last service visitAll of them, whatever started them.

Most from any one call point, detector or detector locationLeave out false alarms with good intent (someone genuinely thought there was a fire).

A persistent cause of false alarms has been identified

FireCompliance Pro does both of these on every service visit. The engineer's form works out C_min from the currents they measure and puts it on the certificate, and the false alarm log is kept per system with the clause 31 rate in front of the engineer and your client.

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How these work, and their limits

Standby period (24.3.5). Category M and L systems need 24 hours, or 6 hours where an automatically started generator serves the whole system. Category P systems need 24 hours more than the longest time the building is likely to be empty, up to 72 hours in total, or 24 hours if a power supply fault would be noticed within 6 hours or is signalled to an alarm receiving centre that alerts a keyholder straight away. The generator relaxation never applies to P. A combined system (4.5) has to meet both, so the P period decides. After the standby period, 30 minutes of alarm is added.

Capacity (Annex E). C_min = 1.25 (T₁I₁ + D I₂T₂). D is 1 when C_min/20 is at least I₂; otherwise it is the battery maker's figure or 1.75. The check uses C_min worked out with D = 1. Where the result isn't a size you can buy, go up to the next size (E.1.3); the sizes shown are common ones, so check your supplier's range and what the panel's charger and battery space will take. On a 24 V panel the two 12 V batteries are in series, so each battery is the capacity shown, not half of it.

Annex E covers valve-regulated lead acid batteries only. For any other battery type the standard sends you to the manufacturer (E.2).
The charger has to bring the battery back to this capacity within 24 hours from its final voltage (24.3.4).
Extra indicating equipment that isn't the default means of control can have a shorter standby period of 4 hours in some cases (24.3.6); this calculator doesn't cover that.

False alarms (clause 31, Annex F). The rate is 100 times the false alarms in the previous 12 months divided by the number of automatic fire detectors. Following the standard's own worked examples, an aspirating detector or a beam counts as one detector, call points are not counted, and false alarms from call points are left out of the rate. Every false alarm should get a review. A preliminary investigation is needed if the rate is over 4, there have been more than 10 since the last visit, more than one (not counting good intent) has come from one device since the last visit, there is a persistent cause, or there have been more than 2 unwanted fire alarm signals in 12 months. With more than 40 detectors, an in-depth investigation is advised if the rate is over 5 or one device has caused more than 2 in 12 months; with 40 or fewer, if there have been more than 2 false alarms in 12 months. A Category M system has no detectors, so it has no rate.

These tools follow BS 5839-1:2025 clause 24.3, Annex E, clause 31 and Annex F, and are checked against the standard's worked examples. They are an aid for competent people, not a replacement for the standard, the manufacturer's data or your own judgement. Spotted something that doesn't match your reading of the standard? Tell us at hello@firecompliancepro.com and we'll fix it.