The LWF Blog

Fire Engineering Design and Risk Assessment – Types of Fire Alarm Detector – Part 13

August 25, 2016 9:29 am

In our recent blogs for Architects and building planners, we’ve been looking at active fire protection through fire alarms. In the last blog we said that there are various different detectors – smoke, heat and flame being the most common – and that there are choices to be made again under those category headers. Today, we will begin to go through each type of detector to give an overview of their potential for use.


A Point-type Smoke Detector is one of the most commonly used in general environments where you would not ordinarily expect to find smoke. This category can be further broken down into two types – ionisation chamber detectors and optical scatter chamber detectors (which can also be known as photoelectric smoke detectors).


An Ionisation chamber detector works by the current between two electrodes being uninterrupted, unless by smoke, when it is triggered. While the concept is sound, it is true that these detectors are more effective in cases of small particle fire, which is a rapidly burning fire and would possibly not be effective if a large smoke particle fire was started. A large particle fire might be a non-flaming, smouldering fire such as the kind found in some furniture.


An Optical chamber detector works on a completely different system to Ionisation chamber detectors. In these detectors, a light beam is scattered or absorbed by smoke particles. Interruptions of the beam cause the alarm to trigger. In this case, the detector is more sensitive to large smoke particle fires and less sensitive to small particle fires. An optical detector is the most commonly used smoke detector due to the lower frequency of false alarms experienced.


A combination detector is an often popular choice due to it combining the benefits of smoke and heat detection in one unit. It is usually the case that it comprises an optical chamber detector and a flat-response heat detector. A modern combination unit therefore, monitors a number of states in the surrounding area to determine whether the alarm should be triggered.


A Point-type Heat Detector is one which monitors for changes in the surrounding temperature. This unit will contain a fixed temperature element which will trigger at a pre-set level. A more advanced unit might also include an element which monitors the rate of temperature increase, the idea being that a rapid increase in temperature will trigger the alarm. A heat detector is commonly less sensitive than a smoke detector and you can find them in use in areas where non-fire smoke or particulate matter might cause false alarms if a smoke detector were used.


Next week, we will continue looking at the different types of detector. 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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