BS 5839-8:2023, in our own words

Commentary on Clause 18: naming the fire

The commentary records that the old practice of keeping the cause of an evacuation from occupants, for fear of panic, is now deprecated. People given accurate information are less likely to panic, more likely to do what is asked, and less likely to lose time working out why they are being moved. On that basis the standard now regards using the word "fire" in the evacuation broadcast as likely to be advantageous.

Commentary explains a clause. It is not itself a recommendation.

Cited in: Attention please: the two worst words to start an emergency announcement

Clause 18.2: clear, concise, and repeated where it helps

An emergency message should be clear and concise, delivered in a calm and authoritative way, and repeated where repetition reinforces its meaning.

Cited in: Attention please: the two worst words to start an emergency announcement

Clause 18.5: how an emergency message should sound

Emergency messages should be spoken slowly and clearly, without raising the voice, in plain language, and without jargon, abbreviations or colloquial expressions. Item c) adds that they should be assertive and convey the need for immediate action.

Cited in: Attention please: the two worst words to start an emergency announcement

Clause 18.8: the broadcast sequence, and the examples in its notes

Sets out how a pre-recorded evacuate broadcast is built: an attention-drawing signal of 2 s to 10 s, a brief silence, the evacuate message, then a further silence, with no more than 30 s between the start of one message and the start of the next, repeating until someone silences it. Alert broadcasts follow a similar pattern with their own timings.

The example wordings printed with this clause, including the familiar evacuate message that opens with "Attention, please", sit in its notes. In a British Standard a note is informative: it illustrates the recommendation and adds nothing to it.

Cited in: Attention please: the two worst words to start an emergency announcement

Clause 25.3.2: the battery type

The standby battery should be of a type expected to last at least four years in the conditions it will actually see in the system. Car starter batteries should not be used.

Cited in: Your next voice alarm battery quote should be smaller than your last

Clause 25.3.4: recharge within 24 hours

The charger should be able to bring a battery that has been discharged to its final voltage back up to the capacity Clause 25.3.5 calls for within a 24 h charging period. A note warns that fitting a larger battery than the design assumed, for example to cover an unforeseen extra load, can stretch the recharge time beyond that.

Clause 25.3.5: how long the standby battery must last

a) Normally, the battery should keep the system running for at least 24 h and then still have enough capacity to broadcast the evacuate message in every alarm zone for at least 30 min.

b) Where the total evacuation time of the building is likely to exceed 20 min, that 30 min is extended by a safety margin agreed by all the interested parties.

c) Where an automatically started standby generator serves the system, the standby period drops to at least 6 h, still followed by the 30 min evacuate broadcast.

Cited in: Peukert's law, and why the internet's version oversizes voice alarm batteries above 20 Ah, What battery manufacturers should publish, but do not, Your next voice alarm battery quote should be smaller than your last

Annex C, C.1: the battery capacity calculation

Annex C is normative: it is part of what the standard recommends, not an illustration of it. C.1 gives the formula for the minimum battery capacity. It adds the standby load over the standby period to the alarm load over the alarm period, each multiplied by a derating factor, and multiplies the total by 1.25.

The derating factors are to be derived from the battery manufacturer's data. They describe how much less than its 20 h rated capacity a battery actually delivers when it is discharged faster, which for the 30 min alarm period is a great deal.

Cited in: What battery manufacturers should publish, but do not, Your next voice alarm battery quote should be smaller than your last

Annex C, C.1.2 and its notes: the 1.25 factor, heat, and missing data

The 1.25 multiplier allows for some variation in ambient temperature and for battery ageing.

Note 1 adds that heat shortens battery life sharply: at a continuous 35 °C a valve-regulated lead acid battery can expect only about 60 % of the life it is specified for at 20 °C.

Note 2 accepts that derating factors are not always available from manufacturers' data, and offers an approximation read from a manufacturer's 20 h rate battery selection chart instead.

Cited in: Battery tools, What battery manufacturers should publish, but do not, Your next voice alarm battery quote should be smaller than your last

Annex C, C.2.1: speech at one eighth

Where the system broadcasts speech messages, the alarm load current is taken as one eighth of the full load current, because a real emergency message is a mix of silence, attention-drawing signal and speech. A note records that the one-eighth value derives from tests in BS EN IEC 62368-1 and has been verified by experiment.

Cited in: Your next voice alarm battery quote should be smaller than your last

Annex C, C.2.2: tone signals at a quarter

Where the system broadcasts signals imitating bells or sounders as its evacuate or alert signal, the alarm load is calculated as a continuous attention-drawing signal at 6 dB below full level, which is a quarter of full power.

Cited in: Your next voice alarm battery quote should be smaller than your last

Annex C, C.3: the worked example

An example calculation for a small system on a valve-regulated lead acid battery. It uses a derating factor of 1.9 for the 30 min alarm period, a figure that belongs to the battery data it was taken from rather than to batteries in general.

As printed, one step of the example adds 8.4 and 4.22 and writes 14.72; the true sum is 12.62. The worked example that doesn't work follows the consequences.

Cited in: What battery manufacturers should publish, but do not, Your next voice alarm battery quote should be smaller than your last