The LWF Blog

Fire Engineering Design and Risk Assessment – Fire Alarm Control Equipment – Part 23

November 3, 2016 4:07 pm

In this blog series for Architects and those involved in building design, we have been looking at Fire Alarms in some detail to assist with providing suitable fire protection. In part 22, we talked about the ways of alerting building occupants to fire and how this differs when the system is installed for property protection, all as per the recommendations of BS 5839: 1. Today, we’ll cover recommendations as per NFPA 72. The NFPA is based in the U.S. but the standards are in use in many places across the world.


NFPA 72 approaches Fire Alarms differently to BS 5839:1, because it breaks down areas of a building and categorises them as requiring different levels or modes. They are as follows:


Public – Where the alarm sounders are used in an area categorised as public, they should produce a sound level of 75 dBA within 10 feet, and it should also achieve 15 dBA above average ambient sound level and 5 dBA above the maximum sound level, for a duration of at least 60 seconds. This means that in areas such as workshops, where drilling or other machinery may intermittently cause loud noise, the fire alarm sounder must be at least 5 dBA above that sound level.


Private – Alarm sounders should produce 45 dBA at 10ft, 10 dBA above average ambient sound or 5 dBA above the maximum sound level, with a duration of at least 60 seconds.


Sleeping – Where occupants of a building may sleep on the premises, sounders should produce a sound level of 70 dBA at 10ft, 15 dBA above the average sound level, 5 dBA above the maximum sound level for a duration of at least 60 seconds.


Maximum sound levels must not exceed 120 dBA at any point. NFPA 72 also mentions the use of ‘temporal audible signals’ by which they mean an alarm whose pattern of timing of the siren is recognisable and attributable to that building or premises. An additional benefit of having a recognisable timing to the alarm is that this timing can also be applied to any other devices, such as visual warning flashing lights.


The type and level of visual alarm and other options should be determined locally as NFPA 72 does not look at this aspect in any detail. The country or area of application should have guidelines from which to work when providing alarm protection to people with hearing difficulties or other disabilities which may affect an individual’s ability to hear and respond to a fire alarm.


In our next blog, we will look at other fire safety measures which may be controlled by the Control and Indicating Equipment panel of a fire alarm system. 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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