GB2275816A - Electric cables - Google Patents

Abstract

A "flat" electric cable having two or more insulated conductors and a bare earth continuity conductor laid parallel to one another and enclosed in a common sheath of polymeric material has a first side of the sheath flat or slightly convex to receive printed information. The second side of the sheath has a thickness over the earth conductor at least a third less than the first side, so as to facilitate the use of the earth conductor as a rip wire when stripping an end part of the cable for termination or jointing. Preferably all the conductors of the cable, including the earth conductor, are laid with their axes coplanar and the second side of the sheath is grooved to provide, a thickness within the required limit. Alternatively (Figure 2 ) the earth conductor may be out of plane with the other conductors. <IMAGE>

Description

Electric Cables This invention relates to electric cables and more particularly to cables of the kind, widely used in some countries for wiring domestic premises and other buildings for lighting and power supply, comprising two (or less frequently more) load-carrying conductors with individual coverings of polymeric insulating material and an earth continuity conductor without such an individual covering laid parallel to one another, that is without twisting, with the earth continuity conductor between the two (or two of the) insulated conductors and enclosed in a common sheath of polymeric material.

Such cables are normally made with all the conductors at least nominally coplanar. Current practice is to make the sheath with flat or slightly convex major faces on which information required to be displayed may be printed or embossed, and this implies that the sheath will be much thicker over the earth continuity conductor than over the insulated conductors.

Sometimes, to save material, grooves have been formed in both the major external faces of the sheath, but this deprives the cable of a flat area for marking.

For nearly 50 years, the only polymeric material used on a major scale as a base for the insulation and sheath of this type of cable has been polyvinyl chloride (PVC), which has relatively low tear strength particularly in the direction of flow during extrusion.

Many electricians have taken advantage of this and have used the bare earth continuity conductor as a rip wire to split the cable sheath longitudinally when required for temination or jointing.

PVC has an excellent balance of mechanical and electrical properties for low-voltage cables and is fully flame-retardant in the sense that it cannot be ignited in air by the application of a small flame, but if exposed to an external fire it may burn and in this case large amounts of dense smoke and acid fumes will be generated. Consequently there is an increasing tendency to use alternative "low-smoke-and-fume" or LSF materials in place of PVC-based ones where a fire hazard is perceived. Some of these materials, when formulated for adequate tensile strength, have a significantly higher tear strength then the common PVC-based sheathing compositions, and sheaths made from them are difficult to split by this technique.

The invention is of applicable to all polymeric shaeth material, including PVC, but has particular advantage when these LSF materials or other sheathing materials of relatively high tear strength are used.

In accordance with the invention an electric cable of the kind having two or more insulated conductors and a bare earth continuity conductor laid parallel to one another with the earth continuity conductor located between the two (or two of the) insulated conductors and enclosed in a common sheath of polymeric material is characterised by having a first side of the sheath flat or slightly convex to receive information and a second side of the sheath having a thickness over the earth continuity conductor at least a third less than the first side.

Preferably all the conductors of the cable, including the earth continuity conductor, are laid with their axes coplanar and the second side of the sheath is externally grooved to provide a thickness within the required limit.

Alternatively the earth continuity conductor may be out of plane with the other conductors by a sufficient distance to achieve the required dimensions.

The invention will be further described, by way of example, with reference to the accompanying drawings in which Figures 1 and 2 are cross-sections of two forms of cable in accordance with he invention.

In the preferred form of twin-and-earth cable shown in figure 1, there are two insulated conductors 1,2 laid parallel with an earth continuity conductor 3 placed centrally between them witnh their axes coplanar and enclosed by a sheath 4 of a conventional LSF sheathing composition. One major side 5 of the sheath is of conventional form, that is it is flat or slightly convex and is available for marking; the second major side 6 is formed with a groove 7 over the centre of the earth continuity conductor so that the thickness x of sheath between the base of the groove and the earth continuity conductor is about half its thickness y between the opposite side of the earth continuity conductor and the first side of the sheath.The groove 7 has the possible incidental benefits of enbling the polarity of the conductors to be deduced and of improving the security of fixing if used with clips modified to provide a complementary projection.

In the alternative design of Figure 2, a similar result is achieved (in terms of sheath thickness) by displacing the earth continuity conductor 3 from the plane of the insulated conductors 1,2 so that it is at least a third nearer the second major side 6 of the sheath then to the first. In this case both major faces can be flat or slightly convex so that the cable is of conventional appearance and can be marked on both sides if desired.

In a particular example made according to Figure 1, the insulated copper wires have a diameter (over insulation) of 2.81mum and the earth continuity conductor a diameter of 1.13mm. The minimum thickness of the sheath is to be l.lmm, and if the cable were of conventional design the thickness over the earth continuity conductor would be at least 1.94mm. By providing the groove on one side, this thickness is reduced to about 1.lem. When the sheath is made of an LSF sheath material with a tear strength of about 15-18 N/mm the result is that the force required to split the sheath by pulling back the end of the earth continuity conductor on the grooved side of the cable is about 30 N compared with about 70 N required if the earth-continuity conductor is pulled back on the other side of the cable (or on either side of a conventionally shaped cable made with the same sheathing material) and with about 47 N for a typical conventional PVC cable with the same size conductors and typical sheath thickness.

Claims (6)

1 An electric cable of the kind having two or more insulated conductors and a bare earth continuity conductor laid parallel to one another with the earth continuity conductor located between the two (or two of the) insulated conductors and enclosed in a common sheath of polymeric material characterised by having a first side of the sheath flat or slightly convex to receive information and a second side of the sheath having a thickness over the earth continuity conductor at least a third less than the first side.

2 A cable as claimed in claim 1 in which all the conductors of the cable, including the earth continuity conductor, are laid with their axes coplanar and the second side of the sheath is externally grooved to provide a thickness within the said limit.

3 A cable as claimed in claim 1 in which the earth continuity conductor is out of plane with the other conductors.

4 A twin-and-earth cable in accordance with any one of the preceding claims.

5 A cable as claimed in any one of the preceding claims having a sheath of an LSF material with a higher tear strength than that of PVC.

6 An electric cable substantially as described with reference to either Figure 1 or Figure 2 of the drawings.