The aim of this article is to provide guidance on suitable measures to prevent and minimise the risk of spread of fire when installing wiring systems within domestic and similar types of premises.
Introduction
Sealing of wiring systems and openings is a fundamental requirement of the Building Regulations in England, Wales, Scotland and Northern Ireland.
In addition, Regulation Group 527.2 of BS 7671 highlights the need for sealing such systems where they penetrate an element of a building’s construction. The requirements of Regulation Group 527.2 and the relevant Building Regulations are intended to preserve:
– fire separation between areas of the building, and
– structural stability of the premises in the event of a fire. For example, in most domestic premises it is the loadbearing capacity of the floors that is threatened by early failure of the ceiling linings due to fire.
The requirements to seal openings apply not only for elements of the building construction that sub divide the building into fire compartments, in which many domestic dwellings consist of only a single fire compartment, but also apply wherever a wiring system penetrates any element of a buildings construction having a specific fire resistance.
Many modern forms of engineering construction such as timber framed buildings inherently have reduced levels of fire resistance in comparison to the traditional brick building. They rely heavily on such methods as fire compartmentation between separate areas, voids and/or cavity walls, as a means of maintaining the structural stability. Plasterboard and other similar types of lining are commonly used as a means of achieving adequate levels of fire resistance which must be maintained throughout the building.
Sealing requirements
Regulation Group 527.2 requires both
– external sealing, such as the filling of gaps around a cable where it passes through a plasterboard ceiling and into a luminaire for example, and
– internal sealing typically, within cable ducting, conduit and trunking systems of internal cross-sectional area not exceeding 710 mm2. However, wiring containment systems having a smaller cross-sectional area classifed by a relevant product standard as non- flame propagating in addition, offering ingress protection to IP331 need not be internally sealed (Regulation 527.2.3).
Note: A non- flame propagating wiring system is one that is liable to catch fire when exposed to a flame, but the flame will fail to propagate along the wiring system and will extinguish itself within a limited time. Such wiring systems must be in accordance with the relevant British Standards.
The method used for sealing openings must be such that the fire resistance of the element of the building construction is restored to its original level (if any) prior to penetration of the wiring system.
Regulation 527.2.4 requires the sealing arrangements to resist external influences to the same degree as the wiring systems with which they are used and to meet all of the following conditions:
i. It shall be resistant to the products of combustion to the same extent as the elements of the buildings construction,
ii. Provide the same degree of protection from water penetration as required for the element of the buildings construction,
iii. Be compatible with the material of the wiring system with which it is in contact,
iv. Allow thermal movement of the wiring system without reduction of sealing protection,
v. Have adequate mechanical stability to withstand any stresses that may arise through damage to the support of the wiring system due to fire.
Sealing Methods
Various types of fire-stopping products or solutions can be used for external sealing and may include intumescent mastics/gaskets, pillows, compounds and metal sleeves. Commonly used products such as intumescent materials have the ability to expand when exposed to heat therefore, sealing the opening around the cable system. Depending upon the type of product used a typical seal can last from 1 to 4 hours.
During the erection of a wiring system temporary sealing arrangements as required by Regulation 527.2.1.1 shall be put in place. This normally requires the use of such products as intumescent pillows or other similar types of removable fire-stopping. Typically, this method of fire stopping is often used in trunking or dividing walls within a roof space where cables are likely to pass through one area and into another.
Furthermore, where it has been necessary to disturb a sealing arrangement or fire barrier during alteration work Regulation 527.2.1.2 requires these prevention methods to be reinstated as soon as practicable. In addition, the applied reinstatement should be of the same type of materials/components as were originally used. The mixing and matching of systems and components is not supported by manufacturer’s fire test data and may compromise the fire integrity of the installation. Where it is not possible to clearly identify or obtain components for the original seal used, the whole seal should be replaced.
Accessories installed in timber framed cavity walls
Typically, many buildings incorporate cavity walls having a timber and plasterboard construction, not precluding other methods of construction. Appropriate measures shall be taken to maintain the fire integrity of such walls with the inclusion of installed equipment and/or accessories such as a consumer unit, cavity boxes and the like.
Cable entries must be provided with suitable sealing arrangements including, intumescent gaskets, grommets and/or fire sealants to maintain the degree of fire integrity of the equipment, as shown in Fig 1.
However, it must be noted that not all fire rated products qualify as a suitable fire-stopping product. For example, some expanding polyurethane (PU) foams which are suitable and tested for sealing linear gaps are not tested or suitable for cable or pipe penetrations, in all cases manufactures product data must be considered.
1 IP33 equates to resistance of small objects (2.5 mm) and water spray at an angle of up to 600 from the vertical.
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