Maintaining fire performance

The design and construction of the fabric of a building is fundamental to its performance in the event of a fire, with fire safety in buildings generally requiring that certain walls, floors and ceilings provide fire separation.

Fire compartments help to isolate fire, heat and smoke to limit the spread of a fire through the property, and they also help to keep escape routes safe so that occupants have the valuable time they need to get out, and firefighters have time to get in.

However, any electrical work that penetrates the linings that form the walls and ceilings of a home can significantly reduce the fire-separation capabilities of those elements, with potentially dangerous consequences.

Such work can include the installation of flush-mounted consumer units, concealed and recessed luminaires including downlighters, flush-mounted socket outlets and electrical switches.

Equally important is the preservation of the structural stability of the building. In fact, in most modern domestic premises, it is the load-bearing capacity of the floors that is threatened by the early failure of ceiling linings, not the fire separation.

Protection for occupants

In a typical two-storey house, the floor between ground and first floor has never been fitted with anything other than a minor separating function in the event of a fire, because none of the doors to the rooms off the staircase have to be fire-resisting. Indeed, open-plan stairs are common.

The upper floor, however, is required to provide 30 minutes’ load-bearing capacity to prevent complete structural collapse, giving some protection to occupants should they be trapped upstairs, and also providing protection for firefighters engaged in search and rescue in the property.

Again, this essential load-bearing capacity can be compromised by incorrectly designed and installed electrical work.

Of course, in addition to the risk of the electrical installation reducing the fire separation capabilities of those elements that need to resist fire spread or to remain structurally sound in a fire, a poorly constructed installation can itself potentially be the cause of a fire in a dwelling, for example, due to heat generated by loose connections.

However, this is an issue separate from the impact of an electrical installation on the fire performance of the building structure.

What is clear is that, when all of these factors are considered, contractors carrying out electrical work in homes have a critical role in ensuring that the fire safety of the premises is preserved.

With this in mind, the latest in the Electrical Safety Council’s series of Best Practice Guides concentrates on providing practical advice and guidance for designers, installers, verifiers and inspectors of electrical installation work in new and existing domestic premises from the fire safety aspect (taking into account the publication of BS 7671: 2008).

As with all the guides in the series, Electrical Installations and their impact on the fire performance of buildings: Part 1 – domestic premises: Single family units is available to download free of charge from the council’s website, www.esc.org.uk.

Recessed and flush-mounted accessories

Many modern forms of engineered construction, such as lightweight joists, have an inherently lower level of fire resistance when compared with more traditional forms of construction, and

are heavily reliant on plasterboard or similar linings for achieving the requisite level of fire separation.

The fire resistance of these elements can be easily compromised by inadequate fire sealing or poor making good after any penetration to accommodate electrical equipment and associated wiring.

Electrical equipment that has a direct and significant influence on the fire performance of buildings includes:

flush-mounted consumer units, concealed and recessed luminaires including downlighters, flush-mounted socket outlets, flex outlet plates and data points, flush-mounted switches, detection and control devices, recessed wall luminaires, concealed speakers, associated wiring systems, ventilation fans and related ductwork.

Installation of these items requires the removal of a part of the ceiling or wall lining and replacing it with glass, metal or plastic, which does not provide the same level of fire protection to the structural members. This lessens the fire performance of the element.

Take, for example, downlighters, commonly found in modern homes. When exposed to fire from below, downlighters may provide far less protection to a cavity and the structural elements within it than the plasterboard they are replacing, unless suitable precautions are taken by contractors.

The Electrical Safety Council recommends that, wherever possible, downlighters with integral fire protection are selected for use in all ceilings where the lining that is to be penetrated is the sole means of keeping fire and heat out of the cavity.

There are a number of types of downlighter available, and it is important that the type selected for a particular application has test evidence to support its fire performance.

Generally, the tests should have been carried out in accordance with BS 476: Part 21: 1987 or BS EN 1365-2. More details can be found in the Electrical Safety Council’s guide.

However, not all designs and styles may be available with integral fire protection, especially where higher lighting levels and/or larger coverage is required. In these situations, additional fire protection may be fitted at the time of installation in the form of a fire hood, an insulated fire-protective box or similar. Such separate forms of protection must be fit for purpose and not be easily dislodged or compromised after installation by subsequent work.

There are similar issues for flush-mounted accessories such as switches, socket outlets, and data and telephone points, which have recessed backboxes. The large knock-out sections, many times greater in diameter than the cables passing through them, make them very permeable in a fire after the faceplate has been destroyed by the heat. This permeability will allow hot gases into the cavity of a wall much more rapidly than the plasterboard.

Where flush-mounted accessories penetrate each face of a 30-minute fire separating or load-bearing plasterboard-lined wall within the same cavity space, each accessory should be fitted with a backbox that incorporates integral fire protection, or be fitted with a proprietary fire protection pad. Again, more details are given in the Best Practice Guide.

Other hazards are posed by elements that have an indirect influence on the performance of floor and wall linings if the lining provides some or all of their support.

An increasing trend is to mount heavy equipment such as TVs, speakers and flat-screen installations on wall brackets and to hang heavy luminaires, lighting track and equipment such as projectors from the ceiling. In the event of a fire, the weight of such items may lead to the premature failure of the lining material.

Unfortunately, plasterboard linings are just not designed to carry such weights under fire conditions and, unless these items are fixed back to the structural members in the wall or floor, they will pull down the linings, once the board is weakened by the fire.

The resulting early failure of these protective linings will allow fire attack on the studs and joists which, if of engineered construction, is likely to lead to premature structural failure. It is therefore crucial for the fire performance of a building that all heavy equipment mounted on the face of walls or hung from the ceiling is supported completely independently from the fire-protective plasterboard linings.

Get acqainted

Clearly, there are many issues to be considered in order to maintain the fire resistance of walls and ceilings in domestic premises that have been penetrated or partially penetrated in the process of installing electrical equipment and wiring, and here I have touched on just a few.

Contractors responsible for carrying out electrical work in domestic premises are urged to acquaint themselves with the issues covered in the Best Practice Guide.