Compromising fire resistance performance in the home

Electrical work in UK homes can critically limit their fire defences

The structural ability of domestic buildings to resist and withstand fire can be critically reduced by electrical work within walls and ceiling linings. In this article Mike Clark, Technical Director of the Electrical Safety Council, the UK-based not-for-profit organisation, explains some of the related issues.

Fire safety in domestic buildings generally requires that certain walls, floors and ceilings provide fire separation. The construction of fire compartments helps to isolate fire, heat and smoke to limit its spread through the property and also helps to keep escape routes safe. Equally, it is important that the structure resists collapse and provides occupants with valuable time to get out of the building in the event of a fire.

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

In fact, in most UK 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 separating function. 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 to provide protection for firefighters engaged in search and rescue in the property.

The capacity of homes to withstand fire is crucial - for victims and firefighters

Crucially, 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, and can also compromise the load-bearing capacity. Electrical work in this context might include the installation of recessed downlighters, flush-mounted sockets, wall-mounted flat-screen TVs or elaborate pendant lighting, to name but a few examples. In fact, much of the installed electrical equipment and electrical accessories commonly found in contemporary homes can impact on the fire resistance performance of the building. 

With this in mind the Electrical Safety Council has produced a new Best Practice Guide for designers, installers, verifiers and inspectors of electrical installation work in new and existing UK domestic premises, looking at the fire safety aspects. 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.

Fire hazards from electrical installations and the need to enhance fire protection

Many modern forms of engineered construction, such as lightweight joists, have an inherently lower level of fire resistance when compared to more traditional forms of construction, and are heavily reliant on the 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 and ‘making good' after any penetration to accommodate electrical equipment and associated wiring.

Many common electrical items have a direct and significant influence on fire resistance

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 the replacement with glass, metal or plastic that does not provide the same level of fire protection to the structural members, causing a reduction in the fire resistance performance of the element.

Take, for example, downlighters - generally installed as standard in modern homes and often retro-fitted in older homes. When exposed to a 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.

Downlighters with integral fire protection should be used

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 when incorporated in a ceiling of the type into which it is to be installed. Generally, the tests should have been carried out in accordance with British Standards 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 or 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.

Electrical accessories such as sockets directly affect a building's fire performance

There are similar issues for flush-mounted accessories (such as switches, socket-outlets, data and telephone points) which have recessed back boxes. 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 face plate 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 back box that incorporates integral fire protection, or be fitted with a proprietary fire protection pad. 

Additional hazards influencing fire performance

In addition to items that have a direct influence on the fire performance of floors or walls, other hazards are posed by installations that have an indirect influence on the performance of these elements if the lining provides some or all of their support.

Heavy electrical equipment may cause premature structural failure

It is increasingly popular to mount heavy equipment such electrical appliances/electronic gadgets such as TVs, speakers, flat screen installations etc 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 simply 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 if the board is weakened by fire.

The resulting early failure of these protective linings will allow fire attack on the studs and joists which, if of poorly-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.

This article touches on just a few of the 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. Those interested in finding out more should visit the business and community section of the Electrical Safety Council's website, www.esc.org.uk, where further practical advice and guidance is available.

Mike Clark - Technical Director, Electrical Safety Council (UK)