The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 151

September 25, 2023 10:30 am

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 150, LWF began to consider the different types of fire detection device and their purpose. In part 151, we will continue to talk about the available types of fire alarm detector and what their best uses are.

The detectors chosen for use with a fire alarm system should be suitable for the environment in which they are placed and optimal for the types of fire that are most likely to occur. A fire alarm operating with unsuitable fire detectors can be disruptive due to false alarms, or may fail to provide an alert to a fire at an early enough stage.

Ionisation Chamber Smoke Detectors

Ionisation chamber smoke detectors are designed to trigger when the electrical current between two electrodes is interrupted by smoke particles. When smoke from a fire reaches the detector, the current is reduced and a notification is sent to the control and indicating equipment, which raises the alarm.

An ionisation chamber smoke detector is most efficient when detecting rapidly burning fires which emit small-particle smoke. The detectors are much less effective at detecting large smoke particles which are produced by dense smoke, smouldering fires and a fire involving PCV or polyurethane foam.

Ionisation chamber smoke detectors should be used only in locations where they are most suitable, as improper siting and use can lead to a high incidences of false alarms.

It should be noted that an ionisation chamber smoke detector has an element of radioactive content and appropriate disposal means should be sought.

Optical Chamber Smoke Detectors

An optical smoke alarm (also called photo-electric smoke alarm) works when the light produced is scattered or absorbed by smoke particles. The alarm contains a pulsed infrared LED which emits a beam of light into the sensor chamber to check for smoke particles.

Optical chamber smoke detectors are most efficient at detecting large particles found in dense smoke but are less sensitive to smaller smoke particles.

The use of optical chamber smoke detectors is commonplace, partly due to the lower frequency of false alarms and their ability to detect smouldering fires.


In part 152 of LWF’s series on fire engineering, we will continue exploring the types of smoke and heat detectors for fire alarms. 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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