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Limiting false fire alarms

by Ian Chapman
NBS Technical Author

Here, we report on the new recommendations on limiting false alarms, constituting an entire section in BS 5839-1:2002 Fire detection and alarm systems for buildings. Code of practice for system design, installation, commissioning and maintenance.

The number of false alarms recorded by the Office of the Deputy Prime Minister has increased over recent years, largely in line with the increased use of automatic fire detection systems. Between October 2001 and September 2002 UK local authority fire brigades responded to just over 1 million fire alarms, of which almost half were false.

This high proportion of false alarms has a significant impact on UK local authority fire brigades. False alarms reduce their availability for real fires and are a financial liability. They cost industry in lost production time and affect the safety of building occupants by increasing evacuation time in real fire situations (crying wolf).

As a result, the new code gives a great deal of attention to minimizing false alarms. When a false alarm occurs it now has to be recorded, and categorized as one the following:

• Unwanted alarms – alarms in which the system has operated as designed, or has responded as expected to its environment, accidental damage or action by people.
• Equipment false alarms – alarms generated from a fault within the system equipment.
• Malicious false alarms – alarms in which someone intentionally triggers a fire alarm knowing that there is no fire.
• False alarms with good intent – these are alarms in which someone intentionally triggers a fire alarm believing that there is a fire.

If it is not possible to establish the cause of the false alarm it should be recorded as ‘unknown’.

Whilst all false alarms are undesirable the code accepts that the complete elimination of false alarms is not possible. It recommends that at each annual service the number of false alarms during the previous year be recorded and expressed as the number per 100 detectors. The code suggests that in clean, well managed environments a rate of 1 false alarm per 100 detectors should be achievable, whereas in more industrial applications a rate of 1 per 75 detectors is more realistic.

Where the annual rate exceeds 1 per 20 detectors, or more than two false alarms are initiated by any single manual call point or automatic fire detection device, an in-depth investigation should take place, followed by appropriate remedial action. In systems with 40 or fewer automatic detectors, an in-depth investigation should take place if more than 3 false alarms occur in any rolling 12 month period.

In many cases equipment faults are blamed for the false alarms, but the cause is usually incorrect selection or siting of detectors. Common causes of false alarms are fumes from cooking processes, steam from bathrooms and showers, tobacco smoke, dust, insects, and aerosol sprays.

False alarms are also caused by building work, so the code recommends that:

‘Tender documents for contract work to be carried out in a protected area should contain a clause making the contractors responsible for informing their staff of the presence of any automatic fire detectors, and of the precautions to be adopted when working’.

Specifications should also require contractors to submit statements identifying methods of working to avoid false alarms. Temporary measures to prevent false alarms, such as replacing smoke sensors with heat sensors or covering detector heads, should be agreed with the fire authority beforehand.

The code outlines a selection of design measures that can be used to limit false alarms. Manual call points should not be positioned where malicious damage can occur. For example, in leisure centres – with the agreement of all interested parties – manual call points could be installed in staff locations but not within public spaces. Automatic smoke detectors generate more false alarms than any other type of detector – careful consideration should be given to their selection and siting. The code recommends that smoke detectors should not be installed in kitchens, toilets or where steam occurs during normal operation.

With advances in electronic technology the fire alarm industry can provide systems which are more resilient and produce fewer false alarms, but their success depends upon designers knowing enough about the building detail and its eventual use to make informed choices. Sophisticated single- and multi-sensor detectors are available which use algorithms to filter out temporary abnormal readings which, in the past, would have generated false alarms.

A comprehensive solution to minimizing false alarms will not be found quickly. By adopting the recommendations of the new code, the incidence of false alarms should reduce – but only time and statistics will tell.

Written November 2003, updated July 2007

 

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