The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 170

February 12, 2024 12:05 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 169, LWF talked about radio-based fire alarm systems/wireless fire alarm systems. In part 170, we discuss power supplies for fire alarm systems.

Fire alarm control panels and associated devices usually operate at an extra low voltage (known as ELV). This is typically 23 V DC and is received from either a built-in charger/rectifier circuit, powered by the local mains AC supply, or from a dedicated ELV DC power supply.

If the mains power were to fail, standby ELV DC is supplied via batteries or occasionally, a generator. The power supply used for the fire alarm equipment should be for that purpose only.

The fire alarm system should connect to the mains supply via dedicated circuit with a point as close as possible to the origin of the supply within a building. Ideally, this is a spare fuse-way in a main switch panel, rather than a downstream distribution board. The reason for this is there is a reduction in the risk of loss of supply due to circuit failure. The protective device, be it a fuse, miniature circuit breaker (MCB) or moulded case circuit breaker (MCCB) should be clearly marked in red. It should also carry a means of preventing accidental operation and bear a notice stating ‘fire alarm – do not switch off’.

Care should be taken during the design to ensure there is no interruption to the power supply to fire alarm equipment during transition from mains to standby option. Operation of a single protective device should not interrupt supplies or cause the system to fail.

BS 5839-1 provides recommendations for construction of the power supply for fire alarm systems, as well as NFPA 70 or any local standards appropriate.

The duration and power of the back-up power supply for the fire alarm system depends on the purpose of the system. A category L system will require standby duration of 24 hours, with sufficient additional capacity to maintain the evacuate signal in all areas for 30 minutes.

If the premises are likely to be unoccupied and unsupervised for a period exceeding 24 hours, then it may be a remote monitoring service should be engaged to notify of any supply faults or increasing the standby duration.

In part 171 of LWF’s series on fire engineering we will continue discussing power supplies and backups for fire alarm systems for category P systems, before beginning to look at 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 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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