The LWF Blog

Fire Engineering Design and Risk assessment – Types of Fire Alarm Detector – Part 14

September 1, 2016 11:42 am

In these Fire Engineering Design and Risk Assessment blogs written for those working in Architecture and Building Planning, we have recently been looking at types of fire alarm detector. In last week’s blog we established that the nature of the fire risk, the type of building and business and other factors affect the type of alarm detector which should be chosen. Today’s blog will continue looking at different types of detector and their potential for use.


In many types of building, especially those with relatively low ceiling heights, point type heat and smoke detectors are suitable, but where there are very high ceiling heights, those point type detectors could cease to be effective, as well as being difficult to service and maintain.


Optical Beam Detectors


Where high ceiling heights are a consideration, dedicated optical beam detectors are much more suitable. They work by sending an infrared beam between a transmitter sited on one side of the area to a receiver on the other. If a fire occurs, the amount of infrared light will be reduced as smoke builds up and interferes with the beam. Additionally, the excess heat produced by a fire will affect the signal received also.


Beam detectors should be installed at just under ceiling height in the vast majority of situations, except where the ceiling heights are extremely high. In the case of extremely high ceiling heights, a smoke layer can develop below the ceiling height and so, in this case, below the detector line. Where this is the situation, the potential height of the smoke layer should be calculated and the beam detector installed below that height.


Aspirating Systems


An Aspirating System works through specifically designed pipework which has been drilled at regular intervals and installed in the area to be protected. A pump then draws samples of air through the pipes to a very sensitive detector – one which is usually between 10 and 200 times more sensitive than a point detector – to analyse for smoke content in the air samples. Additionally, a filter can be installed to remove particles of dust which may have been ‘inhaled’. The system also includes an electronic control panel which indicates the presence of smoke and allows the user to control the output relays, for example.


An Aspirating System is very useful in those buildings where sensitive and expensive equipment is housed, such as in server rooms. This is because it’s important to detect smoke before a fire combusts into flames. Another area in which Aspirating Systems are becoming more well-used is in those buildings or parts of buildings where accessing a fire alarm system for maintenance would be difficult. Additionally, because of the nature of the system, i.e. using pipework, it is a much more sympathetic system to use in heritage properties as the pipework can be concealed quite easily and the appearance of the rooms is largely unchanged.


More information on the design and installation of Aspirating Systems can be found in BS 5839:1.


In next week’s blog, we will continue looking at the different types of detector starting with Flame Detectors. 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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