Electric cable

Abstract: An electric cable 11 is manufactured by extruding a layer of electrically insulating material 13 around an electric conductor 12 to form an electrically insulated core. An intermediate layer of electrically insulating material 14 is then extruded around the core. Two such insulated cores with their intermediate layers 14 are laid up together a sheath 15 of electrically insulating material is extruded around the intermediate layers 14 and the insulated cores. The intermediate layer 14 is predominantly of a material which is incompatible with the material of the sheath so that the sheath does not form a bond with the intermediate layer and the sheath can easily be stripped away from the cores when preparing the ends of the cable for electrical connection to electrical components. In other embodiments the intermediate layer is incompatible with the core insulation so that the intermediate layer comes away with the sheath when the sheath is stripped. The intermediate layer may also be extruded over the insulated cores after they have been laid up together.

Description: The present invention relates to electric cables and is concerned with the problem of separating adjacent layers of cable insulation so as to facilitate stripping of the insulation when forming connections to electrical components. Conventional electric cable is formed by first extruding electrically-insulating plastics material, for example PVC, over an electric conductor either in the form of a single conductor wire or a bundle of twisted wire strands, to form an individually insulated conductor core. Two or more such insulated cores are laid up side by side, possibly with the inclusion of a bare earth conductor and a sheath, typically of PVC, is extruded over the bundle of insulated cores. If the sheath and the insulation of the cores is of the same or compatible plastics material there is a tendency for the sheath to bond to the insulation of the cores. Such bonding makes it more difficult to remove the desired amount of sheath when forming connections to electrical components. To prevent the PVC sheath adhering to the insulation of the cores it is known to coat the insulated cores with a powder material such as talcum powder. Other methods of preventing adhesions, such as wrapping the insulated cores with a layer of tape of a material such as polyester or polyethylene, or by coating the insulated cores with a liquid such as soap solution or wax, have been used. Examples of the forementioned can be found in DE-A-3144337. A problem with the use of powder is that it is difficult to control the layer thickness. A problem with the use of tapes is that it slows the production process and is difficult to apply to small cores. According to the present invention in a first aspect, an electric cable is constructed in accordance with claim 1. By "incompatible" it is meant that the material does not form a bond with the material of the sheath or the insulating material of the core, as the case may be. In some embodiments the material of the intermediate layer is incompatible with the material of the sheath. Then, the intermediate layer of material may or may not form a bond with the insulation material of the core. It is desirable however that it is strong enough and firmly enough secured around the insulated cores to withstand subsequent handling operations of winding or laying up the insulated cores and extruding the outer sheath over the cores without coming off. In other embodiments the material of the intermediate layer is incompatible with the core material only. Such an arrangement is preferable where it is desirable that the intermediate layer should come away with the sheath when the sheath is stripped to prepare a connection to an electrical component. Preferably the intermediate layer is of electrically-insulating material. The material of the intermediate layer may include additives, for example lubricants to make is more easily extrudable, and compounds to provide a limited degree of bonding to the sheath material or the core material. Where it is desired to provide a degree of bonding between the intermediate layer and the core, it is preferable to extrude the intermediate layer over the core before the core material has cooled after being itself extruded. On the other hand if it is desired that the intermediate layer should not be bonded to the core it is preferred that the core should have cooled before the intermediate layer is extruded. Preferably the insulation of the cores, the intermediate layer and the sheath are all of plastics material. In one particular example, the insulated cores are of PVC, the sheath is of PVC and the intermediate layer of material is of polythene. Other incompatible combinations of materials may be used. The intermediate layer may include additives to promote some bonding with the core, for example EVA. The intermediate layer prevents the sheath from bonding to the insulating material of the core and thus facilitates stripping. By extruding the intermediate layer the thickness of the layer can be controlled and the production process is not unduly slowed. The thickness of the intermediate layer of material is substantially less than the thickness of the insulation around the conductors and of the sheath.Preferably it is as thin as possible consistent with having the strength to resist damage during handling in the manufacturing process. in practice we have found that a thickness of between .01mm and .1mm is to be preferred. The present invention also provides a method of manufacturing an electric cable as set out in Claim 12. The present invention will further be described with reference to the accompanying drawings of which:- Figure 1 shows a cross-section of a first electric cable in accordance with the invention; Figure 2 shows a cross-section of a second electric cable in accordance with the invention; Figure 3 a cross-section of a third electric cable in accordance with the invention; Figure 4 shows in diagrammatic form apparatus for forming the cable in accordance with the invention; Figure 5 shows an alternative apparatus for manufacturing the cable in accordance with the invention; and Figure 6 shows a further apparatus for manufacturing the cable in accordance with the present invention. Referring to Figure 1, two-core electric cable 11 comprises two electric conductors 12, each individually insulated with a layer 13 of electrically insulating plastics material, for example PVC, to form an electrically-insulated core. Each individually-insulated core is individually covered with an intermediate layer or skin 14 of an electrically insulating plastics material, for example polyester or polythene, which is extruded over the core. The polythene material may also include additives in the form of a lubricant to facilitate extrusion and a polymer, for example BVA, to produce limited bonding to the core insulation. The two insulated cores with their overlying intermediate layers are then surrounded by a sheath 15, also of electrically insulating plastics material, for example PVC, which is extruded over the intermediate layers 14 on the two cores. The material of the intermediate layers 14 is a plastics material which is incompatible with the plastics material of the sheath 15 so that it does not form a bond. With this arrangement, when it is desired to strip the end of the cable to form a connection, the separating intermediate layer 14 allows the sheath 15 to be easily stripped away from the insulated cores. Figure 2 shows a cross section through a flat two-core-and-earth cable 16 in accordance with the invention in which the cable consists of two electrically insulated cores a bare earth conductor 17 surrounded by a sheath 15. In other respects the arrangement of Figure 2 is similar to the arrangement of Figure 1 and the corresponding components have the same reference numbers. Figure 3 shows an alternative flat two-core-and-earth cable 18 in accordance with the invention. In this embodiment the intermediate layer 14 is extruded over the insulated cores after they have been laid up together so that the layer 14 forms a common envelope around the cores over which the sheath 15 is extruded. In this embodiment it is preferred that the intermediate layer does not form a bond with the core insulation but is bonded weakly to the sheath so that the intermediate layer comes away with the sheath when the sheath is stripped away. To this end the intermediate layer is preferably extruded over the core insulation after the core insulation has cooled down from its own extrusion. The sheath material is preferably extruded over the intermediate layer whist the intermediate layer is still hot from its extrusion. The material of the intermediate layer may include additives which bond with the material of the sheath. In other respects the cable of Figure 3 is the same as that of Figure 2 and the same reference numerals have been used to indicate the corresponding components. Figures 4 to 6 show in somewhat schematic form alternative ways in which the cable of Figures 1, 2 and 3 might be manufactured. In the arrangement of Figure 4, electric conductor wire 12 is fed into the head 20 of a co-extruder 21. Two extruders 22 and 23 are attached to the extrusion head 20. The first extruder 22 contains PVC and extrudes the insulating layer 13 around the conductor 12. The second extruder 22 contains polythene and extrudes the intermediate layer 14 over the insulated core whilst the core insulation is still hot. Insulated cores with their separation layers formed in this way are then allowed to cool, laid up together, twisted and fed through another extruder (not shown) containing PVC which extrudes the layer of sheath material around the outside of the laid up twisted together insulated cores with their intermediate layer coverings. Figure 5 shows an alternative arrangement in which conductor 12 is fed into a first extruder 30 containing PVC where the layer 13 of insulation material is extruded around the conductor to form the insulated core. The insulated core then passes to a second extruder 31 which is placed in tandem with the first extruder 30 and which is filled with polythene. The second extruder extrudes the intermediate layer of material 14 over the insulated core whilst the core insulation is still hot. Insulated cores with the intermediate layers formed in this way are allowed to cool and then bundled together and passed through a further extruder (not shown) to apply the sheath 15 over the bundled cores. With these arrangements, a cable can be produced in which the intermediate layer is lightly bonded to the core insulation but the sheath material is not bonded to the intermediate layer. Figure 6 shows an arrangement in which a single extruder 40 with a single extrusion head 41 is used. Conductor 12 is fed into the extrusion head in a first pass with the extruder loaded with PVC and the insulated layer 13 is extruded around the conductor. The extruder is then changed for one loaded with polythene and the insulated core is passed through the extrusion head 41 a second time. The intermediate layer 14 is thus extruded over the insulated core. After the extrusion of the intermediate layer 14, insulated cores with intermediate layers are bundled together and a sheath layer is then extruded around the bundle in an extruder (not shown) to for the cable. Instead of applying the intermediate layers over the individual insulated cores as described above with reference to Figures 4, 5 and 6, it is possible to apply the intermediate layer over a bundle of insulated cores to construct a cable as shown in Figure 3. The apparatus of Figures 4, 5 and 6 can be used for this process. Instead of feeding bare conductor into the extrusion head 20,30 or 41, one or more insulated cores are fed together into the extrusion head and the intermediate layer 14 and the sheath 15 are applied in turn over the insulated cores. If the apparatus of Figure 4 or 5 is used, the intermediate layer will still be hot from its extrusion when the sheath is extruded over it. This process can be used to produce a cable in which there is light bonding between the intermediate layer and the sheath. If the apparatus of Figure 6 is used, the intermediate layer will have cooled when the sheath is extruded. This latter method can be used if it is desired to produce no bonding between the intermediate layer and the sheath so that the intermediate layer is left behind on the core insulation when the sheath is stripped away. Although polyethylene has been described as material for the separation layer, any appropriate separation material could be used. The requirements are that it should be an extrudable material which is incompatible with at least the material of the sheath material. The choice of incompatible material will be dependent upon the material of the sheath and the layer of insulation around the cores. The layer of material should preferably be itself electrically insulating. The layer of material should be thinner than the insulation around the conductors and the layer of the sheath material. A thickness of between .01mm and .1mm for the intermediate layer is preferred.

Claims: An electric cable comprising one or more electric conductors (12) individually coated with a layer of electrically insulating material (13) to form one or more insulated cores, a sheath (15) of electrically insulating material arouad the said one or more insulated cores and an intermediate layer located immediately between and adjacent the sheath and the insulated core or cores, characterised in that the intermediate layer (14) is predominantly of a plastics material which is incompatible with the material of the sheath (15), or with the material of the core or cores, or with both materials, and is extruded over the insulated core or cores An electric cable according to claim 1 in which the intermediate layer (14) of material is applied individually over the individual insulated cores. An electric cable according to claim 1 in which a common intermediate layer (14) of material is applied over a bundle of the insulated cores. An electric cable according to any of the preceding claims in which the intermediate layer of material (14) is incompatible with the insulating material of the cores. An electric cable according to any of the preceding claims in which the insulation (13) of the cores, the intermediate layer (14) and the sheath (15) are all of plastics material. An electric cable according to any of the preceding claims in which the insulating material (13) of the cores is PVC. An electric cable according to any of the preceding claims in which the insulating material of the sheath (15) is PVC. An electric cable according to any of the preceding claims in which the intermediate layer of material (14) is of electrically insulating material. An electric cable according to any of the preceding claims in which the intermediate layer of material (14) is of polythene. An electric cable according to any of the preceding claims in which the thickness of the intermediate layer of material (14) is substantially less than the thickness of the insulation around the conductors and of the sheath. An electric cable according to any of the preceding claims in which the thickness of the intermediate layer (14) is between .01mm and .1mm. A method of manufacturing an electric cable comprising extruding a layer of electrically insulating material around an electric conductor to form an electrically insulated core, extruding an intermediate layer of material around the said core or a bundle of such cores, and extruding a sheath of electrically insulating material around the intermediate layer and the insulated core or cores, the intermediate layer being predominantly of a plastics material which is incompatible with the material of the sheath, with the material of the core or cores, or with both materials.

Inventors: RODGERS THOMAS, VALE JOHN P, RODGERS, THOMAS, VALE, JOHN P.
Applicants: DRAKA UK LTD, DRAKA UK LIMITED
Application year: 1996
Main classification: H01B7
IPC codes: H01B13, H01B7