1 2446271

FIRE RESISTANT BACK BOX

This invention relates to a novel fire resistant back box for use with electrical fittings, particularly sockets and switches.

In buildings it is commonplace for most if not all internal walls to be constructed from a self supporting frame made for example of wood or metal sandwiched between plasterboard, with the gaps between the wood filled with sound-proofing/heat insulating material. When installing electrical fittings into such walls, there may be problems in securing the fitting without damaging the wall and/or the fithng.

An object of the present invention is to provide a novel fire resistant back box which can be used with a wide range of electrical fittings in a wide range of installations and which in use is efficient at minimising the risk of fire spread, should that occur.

According to one aspect of the present invention, a fire resistant box suitable for fitting into a cavity and adapted to receive an electrical fitting comprises a substantially rigid box open at its front face for receiving an electrical fitting, said box having a flange extending around the periphery of the edge defining said front face, said box further being provided with at least one means for clamping the box in position within a cavity and said box having on its internal back surface a layer of solid intumescent material.

According to a further aspect of the present invention, a fire resistant box suitable for fitting into a cavity and adapted to receive an electrical fitting comprises a substantially rigid box open at its front face for receiving an electrical fitting, said box having a flange extending around the periphery of the edge defining said front face, said box further being provided with at least one means for clamping the box in position within a cavity and said box having on its internal back surface a layer of solid intumescent material arranged such that in the event of a fire and/or excessive heat said layer of solid intumescent material will expand to substantially seal off any openings in the box. Examples of these openings are where cables pass through the back or side wall(s) of the box. Preferably in the event of a fire andlor excessive heat, the solid intumescent material will also fill the interior of the box.

Potential advantages of the box according to the present invention, which in the electrical trade is usually referred to as a uback box, are that: 1. The construction inhibits the spread of fire because the solid intumescent material expands under excessive heat to substantially seal off any openings in the box and probably also to fill the interior of the box.

2. The box is relatively easy to clamp into place.

3. It may be possible to for a fitter to clamp the box into place by means of a one-hand operation.

Although the main application is in internal, load bearing and non-load bearing walls it is also possible to use the box in the ceiling or floor of a building. Metal is the preferred material for the box, but it would be possible to use any rigid fire resistant material available. The plastics materials commonly used for switches etc would not be suitable as they are not fire resistant.

In a preferred embodiment, the clamping means comprises one or more spring-loaded lugs provided with teeth and being slidably mounted within slots at each side of the box. In use the box can be positioned in a cavity and then the lugs released so that the spring forces the teeth against the back face of the board or other laminar member surrounding the cavity. Preferably two clamping means are provided, one in each side of two opposite side faces of the box.

Preferably the solid intumescent material comprises a woven or non-woven fibrous pad, e.g. of short-clipped fibres.

The intumescent material may be based on graphite or other intumesecent materials which upon exposure to extreme heat expand in volume but remain substantially coherent i.e. have char stability. Preferably the intumescent material comprises graphite stabilised with mineral fibre. An example of a graphite based material is that supplied by Technical Fibre Products Limited as Intumescent Material' and consists of a high temperature Rockwool fibre mat containing easily dispersed foliating graphite bonded in acrylic. A specific example is TFP 64854 having a pad thickness of 2.3mm and an expansion ratio of about 18:1 available from Technical Fibre Products Limited. By expansion ratio we mean the expansion resulting upon exposure of the material to excessive heat, which in the case of an 18:1 would be 18 times the original volume.

The box preferably has a substantially rectangular (e.g. square) cross-section with four side walls and a back. Weakened knock outs' can be provided to allow passage of cables in and out of the box. The box preferably conforms to British Standard BS4662: 1970-Specification for boxes for the enclosure of electrical accessories'. This sets out standard sizes for such back boxes so that it is possible to fit products from a number of manufacturers without changing the back box. For example, the box is may be in the following sizes: 132mm x 72mm x 37mm (shown in Figure 1 to 5), 132mm x 72mm x 45mm, 72mm x 72mm x 37mm or 72mm x 72mm x 45mm. However, the size of the box is a matter of convenience for the user and the invention is not limited to just these sizes.

It will be appreciated that certain features of the invention which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are for brevity described in the context of a single embodiment, may also be provided separately or in any suitable combination.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a perspective front view from one side of a box; Figure 2 shows a front view of the box; Figure 3 shows a side view of a the box; Figure 4 shows a perspective front view from the opposite side of the box as shown in Figure 1.

Figure 5 shows an end view of the box in Figures 1 to 4 with the fibrous sheet of intumescent material in place.

Referring to the Figures 1 to 5, a rectangular metal box 1, open at the front, is provided with two clamping assemblies 2(a) and 2(b) on two of its opposing sides. Each clamping means consists of a spring- loaded lug 6(a), 6(b) slidably mounted in T' shaped slots 3(a), 3(b) and each lug having a set of two teeth 4(a),4(b) -one tooth each side of each lug. The spring loading is achieved by means of coil springs 10(a), 10(b). The teeth are slidably mounted within slots 5(a), 5(b). Box 1 has weakened areas 7, 8, 9, any of which may be knocked out to provide passage for cables. A flange 11 extends around substantially the entire periphery of the front edge of the box. A solid sheet of fire resistant intumescent material 13 is fitted against the internal surface of the back (12) of the box; this is not shown in Figures 1 to 4 for reasons of clarity but is hown in Figure 5. To install the box 1, e.g. in a partition wall, lugs 6a, 6b are pushed against the force of the respective springs lOa, lOb and locked in place by locating lugs 6a, 6b against the surfaces at the neck of each T slot 3a,3b. This action retracts the four teeth 4a, 4b and to allow the box to be located within a cavity. Once the box is in place, then releasing lugs 6a, 6b cause the four teeth 4a, 4b to protrude through their respective slots 5a. 5b and hence cause the teeth 4a and 4b to hold the box in situ via the pressure of the coil springs lOa and lOb. The final clamping is achieved by the assembly of the respective electrical fitting's mounting screws into the tapped holes in 6a and 6b against the flange 11.

It should be noted that in Figures 1 to 4 one clamping assembly 2(a) is shown in spring open' mode i.e. with coil spring 10(a) open, whereas clamping assembly 2(b) is shown in spring compressed' mode i.e. with coil spring 10(b) closed (compressed). This is merely for illustrative purposes; in practice both would be either open' or compressed' (closed)' In the event of a fire the intumescent pad 13, which is located against the internal back surface of the box, expands and fills the box cavity with a fire retardant compound. This intumescent compound also seals all the apertures that may be in the back box, for example where the cable entries are, thus preventing the spread of fire from one side of the dividing wall/partition to the other.