Last edited 17 Dec 2020

Ionisation smoke alarm

The Technical Handbook – Domestic, published by Scottish Ministers to provide guidance on the building regulations suggests there are 4 main types of fire detector used in dwellings:

It is important that the right type of fire detector is used for different situations, as false alarms can result in occupants disabling fire detection and fire alarm systems.

The most common causes of false alarms are:

Ionisation smoke alarms operate on the principle that the electrical current flowing between electrodes in an ionisation chamber is reduced when smoke particles enter the chamber.

Ionisation smoke alarms are more sensitive to smoke containing small particles such as rapidly burning flaming fires but are less sensitive to steam. As a result of this, they are recommended for use in hallways and stairwells adjacent to bathrooms or shower rooms to reduce the number of unwanted false alarms.

Ionisation smoke alarms should conform to BS EN 14604: 2005 - Smoke Alarm Devices.

Characterising smoke from modern materials and evaluating smoke detectors was written by Raman Chagger and published by BRE in 2014 states that:

Ionisation detectors use a small radioactive source (americium-241) inside an ionisation chamber that contains charged electrodes. The chamber is arranged to allow a flow of air from outside. As the air enters it becomes ionized, generating an electric current between the charged electrodes. When smoke particles pass into the chamber the ions become attached to them and are carried away, leading to a reduction in the current. More ions are stripped away when there are many small particles, such as those generated during flaming fires. When a material is smouldering it tends to produce fewer but larger particles than it does when in flames. As these cause less current reduction, ionisation detectors are inherently less responsive to the large smoke particles generated during smouldering fires.


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