The LWF Blog

Fire Engineering Design and Risk Assessment – Siting of fire Alarm Detectors – Part 19

October 6, 2016 2:54 pm

In this blog series for Architects and others within the building industry who have an interest in fire engineering design and fire risk assessment, we have been looking recently at the different types of detectors you might find in use with a fire alarm system. In part 18, we discussed the spacing and placement of detectors and in this blog, we look at beam and flame detectors.


Beam detectors are most commonly found in large areas without much footfall. An example would be a storage facility or warehouse. BS5839:1 states that the length of the area of such a space should not exceed 100m. When considering positioning of beam detectors, they should not be positioned closer than 500mm to a wall or partition, however, the length of the beam may overlap by up to 3m.


In cases where people may walk in the area of the beam, the proposed installation should be adjusted to between 2m and 7m above the floor, so as to fall above head height. While BS5839:1 offers some guidance to follow regarding the use and installation of beam detectors, the NFPA guidance suggests reference to the manufacturer’s instructions.


In previous blogs, we established that heat and smoke detectors are each triggered when either sufficient heat or smoke rises towards the ceiling where the detectors are placed. Flame detectors work in a different way.


A flame detector monitors the frequency of light in the protected area, with types being available to monitor infrared and ultraviolet light. The detector monitors an area for signs of the particular frequencies of light which correspond to a fire and then are triggered into an alarm signal.


As they do not require direct input from the fire – as in the case of smoke and heat detectors – there is not a need to place the detectors at ceiling height and they should be installed as per the manufacturer’s instructions. Where the situation warrants it, more than one flame detector can be installed to cover the same area from different angles to ensure the earliest possible alert to a fire.


As we have now covered the siting and use of different types of detector, as a part of the fire alarm system, it seems we should turn our attention towards the control and indicating equipment, sometimes abbreviated to CIE. In the next blog of this series, we will look at where the control panel should be sited and contributing factors, along with relevant legislation.


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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