Optical smoke alarm

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

• Optical smoke alarms.
• Ionisation smoke alarms.
• Multi sensor alarms.
• Heat alarms.

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

The most common causes of a false alarm are:

• Fumes from cooking.
• Steam from bathrooms, shower rooms and kitchens.
• Tobacco smoke.
• Dust.

Optical smoke alarms detect the scattering or absorption of light within the detector chamber. They are more sensitive to slow smouldering fires such as fires involving soft furnishings and bedding.

A likely source of this type of fire is in a principal habitable room (a frequently used room by the occupants of a dwelling for general daytime living purposes) from the careless disposal of smoking materials. Polyurethane foam found in some furnishings may ignite and begin to smoulder producing large particles of smoke. Because of this, optical smoke alarms are recommended in principal habitable rooms. However if the room is used by a heavy smoker, this could give rise to some false alarms from tobacco smoke.

As optical smoke alarms are less sensitive from fumes caused by toasting bread or frying or grilling food, they are also recommended where a principal habitable room is open plan with a kitchen, and in hallways and stairwells adjacent to kitchens, to reduce the amount of unwanted alarms from cooking fumes. Most unwanted alarms occur during cooking.

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

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

• Carbon monoxide detector.
• Domestic smoke alarms DG525.
• Fire detection and alarm system.
• Fire detector.
• Heat alarm.
• Ionisation smoke alarm.
• Multi-sensor alarm.
• New requirements for fire detection and alarm network systems IP 12 13.
• Smoke detector.
• The causes of false fire alarms in buildings.
• The role of codes, standards and approvals in delivering fire safety.