The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 171

February 19, 2024 11:13 am

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 170, LWF discussed power supplies for fire alarm systems. In part 171, we continue discussing power supplies for category P fire alarm systems before beginning to look at hazardous areas.

The duration and power available to the standby power supply of a fire alarm system is important. The standards required depend upon the purpose of the system. In buildings with a category P system, where the building may be unattended and has no power supply monitoring link, the required duration is 24 hours longer than the building may be unoccupied (or 72 hours in total, whichever is less).

After the allowed time, there must be sufficient backup power to operate all fire alarm devices for 30 minutes.

Where the building could be unoccupied for longer periods, remote monitoring of the power supply is advisable.

BS 5839-1 contains methods for determining standby battery capacity.

The manufacturer of the fire alarm equipment will ensure their batteries supplied with the equipment are adequate for the standby period required by the design. It is necessary that realistic requirements and information are given at the design stage so that the resulting fire alarm system is suitable for purpose.

Hazardous areas

Extra precautions need to be taken when installing a fire alarm system in a hazardous area. It is important to ensure that the fire alarm equipment cannot cause a fire and that the fire hazard cannot damage the fire alarm equipment.

Flameproof equipment – In areas where it is necessary, detection equipment should be contained within a flameproof housing. In addition, if a fault occurs producing an electrical spark, the spark is contained within the housing and not into the potentially explosive environment.

Intrinsically safe equipment – Fire detection equipment installed in a potentially explosive environment should be fed through suitable barriers or isolators, limiting the electrical energy entering the hazardous area. If a fault occurs that causes a spark, the limited energy released would be insufficient to cause an explosion.

A Zener barrier is an electronic device designed to limit the current entering the hazardous area. The end-of-line resistor is used to monitor supply from the control panel to the detection devices.

In part 172 of LWF’s series on fire engineering we will look at fire alarm systems for construction sites. 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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