Fire Alarm System Design

It is now required by the Building Regulations to fit new or refurbished dwellings with a fire detection and alarm system that complies with BS 5839-6:2013. The minimum for a dwelling of up to three storeys not in multiple occupation or for rent is for smoke alarms in the hallway of each level with the alarm installation being grade D. Which means the alarms have a 'mains' electricity supply with an integral battery back up, the alarms must also be interlinked either by hard wiring or wireless.

As with any standard it is tempting to make the minimum the norm, this is a mistake every room of the dwelling should be separately assessed as to the risk of fire and where considered necessary an alarm fitted.

There are four different types of detector available at the present time, ionisation, optical (also described as photo electronic), heat and combined alarms (ionisation and optical). There are also combined smoke and CO₂ detectors.

The ionisation type are very sensitive to small particles of smoke produced by fast flaming fires, such as paper and wood, and will detect this type of fire before the smoke gets too thick. They are marginally less sensitive to slow burning and smouldering fires which give off larger quantities of smoke before flaming occurs.

The optical type are more effective at detecting larger particles of smoke produced by slow-burning fires, such as smouldering foam-filled upholstery and overheated PVC wiring. They are marginally less sensitive to free burning flaming fires.

The heat type detect the rise in temperature from a fire and are insensitive to smoke, they can therefore be installed in kitchens.

The combined type (Ionisation and Optical) are effective at detecting slow-burning as well as flaming fires – which are both common types of fire.

If in any doubt as to what type to use and where to install alarms your local fire authority should be consulted alternatively since it will be the local building control officer that signs off the system they to may be approached.