The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 166

January 15, 2024 12:23 pm

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 164, LWF looked at the recommendations and guidance for fire alarm sounders as per BS 5839-1. In part 165, we continue on that subject before discussing the requirements when NFPA 72 is the relevant standard.

When considering the number of sounders to install for a fire alarm system, as previously mentioned, it is preferable to have a larger number of sounders at a quieter sound level than a few sounders at a louder one, to ensure consistency of sound and sound level throughout the premises.

At least one sounder per fire compartment is necessary. Fire alarm sounders more than one separating wall away are likely to result in sound levels that are too low. Equally, the sound levels should not be set so high that they are too loud in any single area, as this can cause damage to the ears of the people in that area.

Even in a small building, there should be at least two alarm sounders.

Where a category P system is used, an external sounder or visual warning device may be required. The unit is red coloured and labelled ‘fire alarm’.

If mains-powered sounders are used to supplement 24 V DC sounders, then the electricity supply should be monitored.

NFPA 72

NFPA 72 has different requirements to BS 5839-1 and requires different levels or ‘modes’. They are as follows:

Public Mode – Fire alarm sounders must produce a level of 75 dBA at 10 feet, 15 dBA above the average ambient sounds, or 5 dBA above the maximum sound level, with a duration of at least 60 seconds. For example, in a shopping centre or mall, the sound level should be 15 dBA above the average sound level of any music, announcements of a non-safety nature etc. in each compartment.

Private Mode – Fire alarm sounders should produce a sound level of 45 dBA at 10 feet, with the sound level 10 dBA above the average ambient sound level or 5 dBA above the maximum sound level with a duration of at least 60 seconds. E.g. in a business environment which is only open to employees (who have received fire safety training).

Sleeping Mode – Fire alarm sounders that produce a sound level of 70 dBA at 10 feet, 15 dBA above average ambient sound or 5 dBA above the maximum sound level with a duration of at least 60 seconds. The sound level of the fire alarm sounders should be sufficient to wake all sleeping persons on the premises.

In part 167 of LWF’s series on fire engineering we will continue to discuss the requirements for fire alarm sounders as per NFPA 72, before looking at how fire alarm systems interface and interact with other safety measures. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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