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If You Can't Stand the Heat...

Release Date: 31/07/2012

Fire kills - so what should electrical contractors be doing to make sure your customers’ installations are safe? Giuliano Digilio, the ECA’s Head of Technical Services gives his assessment.

As the certification body of the ECA can now offer contractors access to the industry leading BAFE SP203 Part 1 certification scheme, it is only fitting that we take this opportunity to discuss the best practice of installing fire alarms. 

Over two thirds of fire fatalities happen in the home which is why Part B Fire Safety (BS5839-6) specifies the level and location of fire alarms installed in domestic dwellings.

Where loft conversions or new rooms are added additional smoke alarms may be necessary to ensure that escape routes are covered.

It is important to select the correct type of fire detector for the area to reduce false alarms. There are many manufacturers but they all fall into one of three types: ionization smoke detectors, optical smoke detectors and heat detectors.

Ionization detectors recognise smoke by the reduction it causes in the current that flows between electrodes, in an ionization chamber within the detector. They detect fast burning, clean fires that produce small smoke particles.

Optical detectors detect smoke by means of the light scatter that results from the presence of a small light source within the detector. They identify slower, smouldering fires producing larger, more visible smoke particles. They are best suited to escape routes to prevent them becoming blocked with smoke.

Heat detectors reveal the temperature and at a set point - usually around 60°C - they trigger the alarm. They are immune from false alarms caused by cooking and excess dust. These are usually used in kitchens where smoke detectors would cause false alarms.

Within BS5839-6 there are six grades of fire alarm system ranging from A to F.

• Grade A - a full fire alarm system panel, detectors and manual call points which conforms to BS5839-1.

• Grade B - a fire detection and fire alarm system comprising fire detectors (other than smoke alarms and heat alarms), fire alarm sounders, and control and indicating equipment that either conforms to BS EN 54-2 (and power supply complying with BS EN 54-4) or to Annex C of BS 5839-6.

• Grade C - a system of fire detectors and alarm sounders (which may be combined in the form of smoke alarms) connected to a common power supply, comprising the normal mains and a standby supply, with central control equipment. These are usually warden call systems or detectors connected to intruder alarm panels.

• Grade D - mains powered interlinked smoke alarms with an integral battery backup.

• Grade E - mains powered interlinked smoke alarms with no battery backup.

• Grade F - battery powered smoke detectors.

We generally only concern ourselves with Grades A, C, D and E and of those, only Grades A and D are used regularly. The interlinks of Grades D and E can be via a radio link instead of hard wired, which can get round some difficult cabling routes.

The electrical connection for the fire alarm is as important as the selection and placement of the detector. For Grade A and B systems, the fire alarm needs a dedicated circuit, wired in fire resistant cable of the same fire rating as the fire alarm system. The circuit supplying the fire alarm system should not be protected by a residual current device (RCD) unless this is necessary to comply with the requirements of BS 7671 (regulation 522.6.101 & 522.6.102).

Where an RCD is necessary for electrical safety, a fault on any other circuit or equipment in the building should not be capable of resulting in isolation of the supply to the fire alarm system. This means that the fire alarm system would require its own RCBO (residual current operated circuit-breaker with integral overload protection).

Grade E systems (mains powered only) must be connected on their own independent circuit. You cannot connect these detectors to a circuit protected by an RCD. This precludes the use of Grade E detectors in domestic premises which conform to the 17th Edition of the Wiring Regulations.

Grade D (mains powered with battery backup) detectors can be connected via their own independent circuit or a regularly used lighting circuit. Either may be protected by an RCD. We recommend connection of Grade D detectors from a regularly used lighting circuit.

BS5839-6 has three categories which describe the level and area of coverage for the protection of life:

Category LD1:  a system installed throughout the dwelling, incorporating detectors in all circulation spaces that form part of the escape routes, and in all areas where fire might start, other than toilets, bathrooms and shower rooms.

Category LD2:  a system incorporating detectors in all circulation spaces that form part of the escape routes, and in all rooms that present a high fire risk to occupants (e.g. kitchen & living room).

Category LD3:  a system incorporating detectors in all circulation spaces that form part of the escape routes from the dwelling (e.g. hall, stairs & landings).

It is also important to correctly place the smoke detectors so that the sound pressure level is enough to warn occupants of the fire. BS5839-6 requires a level of 85db at the bedroom door with the door open. BS5839-6 requires detectors to be placed within 3 metres of all bedroom doors. Placement of detectors may impact maintenance - some have an alkaline battery backup which will need periodic changing. A balance of accessibility and optimal coverage needs to be achieved. Smoke detectors work within a 7.5 metre radius and heat detectors, a 5.3 metre radius. This provides some placement flexibility, but detectors need to be 300mm away from walls, light fittings or beams that might form dead air spaces preventing the smoke reaching the detector.

This combination of the right detector in the right location, that provides the best protection.