A building’s structural performance in the event of fire can be seriously compromised by electrical work that penetrates walls and ceiling linings. Mike Clark, Technical Director at the Electricity Safety Council, highlights some of the 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 modern domestic premises, it is the loadbearing 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’ loadbearing 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 inside the property.
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 loadbearing 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 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 domestic premises, which looks at 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
Direct impact
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.
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 and associated wiring systems; and 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 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.
The Electrical Safety Council recommends that, wherever possible, downlighters having 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 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, 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 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 loadbearing 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.
A weighty issue
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. It is increasingly popular, for example, 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 a fire to attack 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.
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 can head to the business and community section of the Electrical Safety Council’s website www.esc.org.uk where further practical advice and guidance is available.
FILLING THE GAP
The introduction of Approved Document P of the Building Regulations firmly put the onus on electrical contractors to avoid reducing the fire performance of a building as a result of the their installations. But with, for example, the growing use of downlighters which means many more ceiling penetrations, many electricians were unsure what measures they needed to take to satisfy these requirements.
The ESC guide addresses the impact that electrical installations will have on the fire performance of loadbearing and non-loadbearing fire separating elements, and also on the loadbearing capacity of structural walls and floors. Much of the guidance is related to the influence that the installed component will have on the performance of protective linings used to provide fire resistance to lightweight joisted or studded constructions. But in the case of the associated wiring, the need to prevent fire from passing through holes in all elements, whether solid or lightweight, is also addressed.
Modern forms of ‘engineered’ lightweight construction, using composite joists and studs, are much more vulnerable to inadequate installation than are the more traditional forms of joisted and studded constructions, and the guidance given clearly differentiates between the two. As the guide explains, many of these engineered elements have a low inherent fire resistance.
A building’s structural performance in the event of fire can be seriously compromised by electrical work that penetrates walls and ceiling linings. Mike Clark, Technical Director at the Electricity Safety Council, highlights some of the 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 modern domestic premises, it is the loadbearing 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’ loadbearing 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 inside the property.
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 loadbearing 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 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 domestic premises, which looks at 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
Direct impact
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.
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 and associated wiring systems; and 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 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.
The Electrical Safety Council recommends that, wherever possible, downlighters having 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 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, 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 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 loadbearing 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.
A weighty issue
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. It is increasingly popular, for example, 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 a fire to attack 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.
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 can head to the business and community section of the Electrical Safety Council’s website www.esc.org.uk where further practical advice and guidance is available.
FILLING THE GAP
The introduction of Approved Document P of the Building Regulations firmly put the onus on electrical contractors to avoid reducing the fire performance of a building as a result of the their installations. But with, for example, the growing use of downlighters which means many more ceiling penetrations, many electricians were unsure what measures they needed to take to satisfy these requirements.
The ESC guide addresses the impact that electrical installations will have on the fire performance of loadbearing and non-loadbearing fire separating elements, and also on the loadbearing capacity of structural walls and floors. Much of the guidance is related to the influence that the installed component will have on the performance of protective linings used to provide fire resistance to lightweight joisted or studded constructions. But in the case of the associated wiring, the need to prevent fire from passing through holes in all elements, whether solid or lightweight, is also addressed.
Modern forms of ‘engineered’ lightweight construction, using composite joists and studs, are much more vulnerable to inadequate installation than are the more traditional forms of joisted and studded constructions, and the guidance given clearly differentiates between the two. As the guide explains, many of these engineered elements have a low inherent fire resistance.
