The LWF Blog

Fire Engineering Design and Risk Assessment – Fire Alarm Power Supplies – Part 27

November 24, 2016 12:27 pm

In the continuation of our blog series on Fire Engineering Design and Risk Assessment for architects and other parties involved in the design and build process, we have been looking at the different types of fire alarms and their use as part of a fire protection solution. Today, we’re going to discuss fire alarm power supplies.


Fire alarm control panels and associated devices operate at a low voltage – usually 24V DC and this is received via a built-in charger powered by the mains, or from a dedicated ELV DC power supply. In the event of a power failure, a standby supply is provided via batteries or generator. It is important that whatever back-up power source is in place for the fire alarm control panel and devices, it should be dedicated and not used for other purposes.


When the system is under normal operation and is powered by mains electricity, this too should be on a dedicated circuit, which is as close as possible to the origin of supply. This is so there is less chance of a power failure to the system, due to a circuit failure.


The protective device for the fire alarm system may be a miniature circuit breaker or a moulded case breaker, but whichever, it should be clearly marked in red and bear a clear message stating its purpose and that it should not be shut off.


The design of the power supply is important to ensure a consistent operation of the system. For instance, it is not acceptable for there to be any lapse or interruption if there is a transition from mains power to battery power. Once operating on battery or back-up power source, the potential duration is dependent upon the purpose of the system. Where detection and repair of a mains failure is anticipated within 12 hours of occurrence, the system should be able to run for 24 hours on back-up power source. If there is a possibility that the premises will be unoccupied for longer than 12 hours then the fire alarm system should be able to run for a minimum of 24 hours after discovery of the fault.


One reasonably common example is that if a building is to be left unattended from close of business on a Friday (usually 5pm) until 9am on a Monday morning, then the standby period on back-up power should be 88 hours.


When designing the power supply, the recommendations of BS 5839: 4 or NFPA 70, or a suitable local code should be adhered to. The British Fire Protection Systems Association has a formula which can be used to calculate the size of battery required, if retrospective checks need to be made. However, the fire alarm itself will have been fitted with appropriate batteries for the standby period required at the time of installation and subject to appropriate maintenance and checks, and sufficient consultation at the time of purchase, that should be adequate.


In our next blog we will look at how fire alarm equipment is placed in hazardous areas. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.


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