A multicore electrical cable

Abstract

The multicore electrical cable 25 comprises at least two conducting cores 31 each coated with an insulating layer 32, and a circuit protective conductor core 14. The circuit protective conductor core

Description

Technical Field of the Invention The present invention relates generally to the field of electrical cables and more particularly to a multicore electrical cable including an internal circuit protective conductor. Background to the Invention The industry recognised norm is to have a bare (uninsulated) circuit protective conductor (CPC) 10 inside a cable 11 that carries the operating cores 12, 13 (stranded operating cores are illustrated) of the general building wire used for the majority of domestic wiring in the UK. An example of this configuration is shown in Figure 1. When a bare CPC is being installed, it is required that electricians use an over sleeving of Green / Yellow colour to identify the circuit protective conductor (commonly referred to as an ‘earth’). An electrician is required to cut a piece of sleeving and place this over every length of CPC that goes into a socket, switch, lighting accessory and the like. This is extremely time consuming if performed correctly. Further, the application of the sleeving in such a manner means that the sleeving can come free thereafter, which can cause a dangerous situation. Summary of the Invention According to one aspect of the invention, there is provided a multicore electrical cable comprising at least two conducting cores having each having a diameter and coated with an insulating layer, and a circuit protective conductor core, wherein the circuit protective conductor core is provided with an identifying cover layer provided to an outer portion of the circuit protective conductor core within the multicore electrical cable, the identifying cover layer thinner than the insulating layer of the at least two conducting cores. The electrical cable defined above overcomes the issues with the conventional manner of providing a circuit protective conductor (CPC) within the cable. The electrical cable includes an identifying layer provided to an outer portion of the CPC within the cable itself but which is thinner than the insulating layer of the at least two conducting cores. This means that even if the actual core of the cables is the same nominal diameter, the coated CPC will still be thinner than the at least two conducting cores overall. It also minimises material procurement, lowers waste, and saves time on electrical installations. It also ensures that the identifier of the CPC is present when the cable is opened in order to connect the cable electrically. Such a cable should also comply with the mechanical and electrical properties defined in the relevant standard. One example of a relevant standard is set out in British Standard BS6004 Table 4. The identifying layer may include at least one indicium to identify one or more properties of the CPC to an operator. For example, a green / yellow identifying layer may be applied to an outer portion thereof. Preferably the identifying layer is applied over the length of the coated circuit protective conductor core. The coated circuit protective conductor core may be reduced size when compared to the conducting cores. The identifying layer preferably has a reduced thickness when compared to the thickness of the insulating cover on the conducting cores. In this form, the identifying layer may be green / yellow in colour. The identifying layer may be protective, but not insulative. As the conducting cores are insulated, the coated circuit protective conductor core can be provided with an identifying layer which is not insulative, or only minimally so. The coated circuit protective conductor core is preferably located between the at least two conducting cores. The coated circuit protective conductor core is preferably located centrally in the cable, with a central axis thereof, coplanar with the central axes of the at least two conducting cores. As the coated circuit protective conductor core is preferably smaller in diameter as compared to the conducting cores even if the CPC is of the same core diameter, the identifying layer is preferably thinner and therefore, the overall size of the CPC will be smaller than the conducting cores, the coated circuit protective conductor core may be provided between the conducting cores but at any height between the conducting cores, provided that the upper limit of the coated circuit protective conductor core does not extend above the upper limits of the conducting cores (if two conducting cores are provided) and the lower limit of the coated circuit protective conductor core does not extend below the lower limits of the conducting cores (if two conducting cores are provided). The multicore electrical cable may comprise at least two conducting cores, but could include three or four conducting cores. If more than two conducting cores arc provided, the coated circuit protective conductor core is preferably provided in a void between the conducting cores. Each of the operating cores will preferably be insulated with a polyvinyl chloride material. Each of the cores will preferably have a high level of insulation resistance. The level of insulation resistance may vary with the nominal cross-section area of the conductor core. For example, at a nominal cross-section area of the conductor core of 1.0mm, the minimum insulation resistance at 70°C may preferably be O.OllMQ.km. At 1.5mm, the minimum insulation resistance at 70°C preferably be 0.01 IMQ.km. At 2.5mm, the minimum insulation resistance at 70°C preferably be O.OlOMQ.km. At 4mm, the minimum insulation resistance at 70°C preferably be 0.0077MQ.km. At 6mm, the minimum insulation resistance at 70°C preferably be 0.0065MQ.km. At 10mm, the minimum insulation resistance at 70°C preferably be 0.0065MQ.km. At 16mm, the minimum insulation resistance at 70°C preferably be 0.0052MQ.km. The multicore electrical cable is preferably substantially rectangular in cross-sectional shape. The outer sheath may be provided outside the electrical insulating layer surrounding the conducting cores and the identifying layer about the coated circuit protective conductor core. A bedding layer is preferably not provided about the cores. The coating composition may be applied directly about an outer side of the cores and will preferably define the outer shape of the multicore electrical cable. The conducting cores may be solid or stranded cores. The conducting cores may be single insulated or double insulated. To add a circuit protective conductor identifying coat layer to the cable whilst retaining all of the required insulation thickness requirements, and sheath thickness requirements, would significantly increase the diameter of the cable. This increase in diameter would make such a cable too large for use in the general wiring industry, as it would no longer fit within industry standardised sockets, light fittings, cable clips and the like. The multicore electrical cable of embodiments of the invention addresses this problem, ensuring the multicore electrical cable has reduced material thicknesses, whilst still complying with the mechanical and electrical performance characteristics of the relevant standards. This may be achieved by one or more of two key aspects, namely tooling design, and compound formulation. According to a second aspect of the invention, there is provided a tool for manufacturing a multicore electrical cable comprising at least two insulated operating cores, the tool comprising an extrusion tip and an extrusion die located in series, the extrusion tip comprising an opening for each of the at least two insulated operating cores and a secondary opening provided between the operating cores to position a coated circuit protective conductor core coextensively between the operating cores. In a preferred form, the extrusion die is located downstream of the extrusion tip. The secondary opening may be smaller in dimension than the dimension of the opening for each of the at least two insulated operating cores. The extrusion tip may comprise a keying structure or more preferably a keyway. The keying structure or keyway is preferably located on an external side of the extrusion tip. In one form, a keyway may be provided at or adjacent to an entry end of the extrusion tip on an outer side of the extrusion tip. The keyway preferably engages with a key to retain the extrusion tip in a fixed configuration as the cable is extruded. The exit end of the extrusion tip preferably has a shaped opening with a portion for each of the operating cores and the secondary opening therebetween. The secondary opening is preferably provided centrally between the openings for the respective operating cores. A central axis of the secondary opening is preferably coplanar with the central axes of the openings for each of the at least two conducting The extrusion die may comprise a keying structure. The keying structure is preferably located on an external side of the extrusion die. The keying structure on the extrusion die may have any shape and any configuration. As the purpose of the keying structure is to engage with a keyway, it is important that the shape and configuration of the keying structure correspond to that of the corresponding key way. The keying structure will preferably be received at least partially into a corresponding keyway to maintain the orientation of the extrusion die, preferably relative to the extrusion die. This alignment of the extrusion die relative to the extrusion tip will preferably maintain the orientation of the individual cores as they exit the extrusion tip, relative to the extrusion die to ensure that the multiple core electrical cable can be manufactured within the tight tolerances in terms of overall dimensions of an electrical cable required of an electrical cable for domestic use. According to a third aspect of the invention there is provided a method of forming a multicore electrical cable comprising at least two insulated operating cores, the method comprising the steps of feeding the at least two insulated operating cores into an extrusion tip comprising an opening for each of the at least two double insulated operating cores and a secondary opening provided between the openings to position a coated circuit protective conductor core coextensively between the operating cores, and feeding the coated circuit protective conductor core coextensively with the at least two insulated operating cores through the extrusion tip to form an internal core and coated circuit protective conductor core configuration within the cable. The method may be performed using a tool as provided in the second aspect and including the keying structure and at least one keyway discussed therein to ensure that the internal core and coated circuit protective conductor core configuration exiting the extrusion tip is maintained aligned with the extrusion die so that the coated circuit protective conductor core is positioned between the two insulated operating The conventional process that is used to twist cores together to form a multicore cable is unsuitable for use with the multicore electrical cable as it does not maintain the respective positions of the coated circuit protective conductor core between the insulated operating cores with sufficient precision to deliver the tight tolerances in terms of overall dimensions of an electrical cable required of an electrical cable for domestic use. The secondary opening may be smaller in dimension than the dimension of the opening for each of the at least two insulated operating cores. The running parameters of the manufacturing machine differ for each multicore cable to account for different composite cable characteristics. The tool of the second aspect and the method of the third aspect incorporates ‘high shoulders’, which enhances the mechanical protection at the outer corners of the multicore electrical cable, as this is where performance characteristics were most difficult to achieve. The enhanced shoulders give a thicker minimum point at this area of the cable, whilst still allowing the multicore electrical cable to be compliant with industry standardised accessories. According to a fourth aspect, there is provided a coating composition for a multicore electrical cable comprising a resin, a plasticiser, a stabilizer and at least one filler. The composition may preferably comprise polyvinylchloride with a K-value of between 35 and 80, di(2-propylheptyl)phthalate, at least one filler and a stabiliser. The coating composition may be provided outside the electrical insulating layer surrounding the conducting cores and the identifying layer about the coated circuit protective conductor core. Use of the coating composition may obviate the need to provide a bedding layer about the cores. The coating composition may be applied directly about an outer side of the cores and will preferably define the outer shape of the multicore electrical cable. A preferred ratio of the components may be 450-460 rcsin:210-225 plasticiser: 15-25 stabilizer:300-315 filler. In one form, the resin may be a polyvinylchloride resin. A PVC resin such as k70 PVC is particularly preferred. A suitable resin for use may have one or more of the following characteristics: K Value 70+15 Bulk Density 460 - 520 (g / 1) Sieve Analysis <5% retention on 250 micron Black spec < 10 The K-value is a characteristic of the polyvinylchloride (PVC) resin which describes the length of the polymer molecules. It is usually a measure of the molecular weight of PVC based on measurements of viscosity of a PVC solution. It ranges usually between 35 and 80. In an embodiment, a K-value of approximately 70 has been found to be optimal. In one form, the plasticiser may be or include stabilised Di(2-propylheptyl) phthalate. A suitable plasticiser for use may have one or more of the following characteristics: Ester content > 99.0% Viscosity 115-140 mPa*s Density 0.960-0.966 g / cm3 Refraction index 1.42-1.485 nD / 20 Water content < 0.05% The stabilizer may be or include a Ca / Zn Stabiliser. A suitable stabiliser for use may have one or more of the following characteristics: Metal content 47.8 - 50.8 % Heat Test MATHIS ±10% PV4 Dehydrochlorination +10% PV7 Although any type of filler may be used, a calcium carbonate filler is preferred. A suitable filler for use may have one or more of the following characteristics: Calcium Carbonate >97.0 % Acid insoluble matter <2.0 % Sieve Analysis <1% retention on 45 micron Dry brightness vs absolute >87.5% A particularly preferred composition may be: Resin k70 PVC 445,000-458,000 p.p.m Plasticiser Stabilised Di(2-propylheptyl) phthalate 212,000-222,000 p.p.m Stabiliser Ca / Zn Stabiliser 15,000-25,000 p.p.m Filler Calcium Carbonate 300,000-315,000 p.p.m Use of this composition as the coating layer for the circuit protective conductor within a multicore electrical cable gives enhanced mechanical and electrical performance. This allows us to meet the necessary performance characteristics whilst applying a lesser amount of material. The composition has a high abrasion resistance that protects the composite cable from installation practices expected (such as difficult cable routes, and the cable being pulled along concrete floors and building work). The composition has a high impact resistance that protects the composite cable from impacts, such as impacts from vehicle doors in tight parking situations. The composition has good thermal stability to achieve operating temperatures of 90°C that allow for higher current carrying capacities. The composition has good UV stability which allows long term operation in sunlight for exterior use. This preferred internal configuration of the cores within the cable may create a multiple core electrical cable with ‘high’ or flared shoulders to ensure that multiple core electrical cable can be manufactured within the tight tolerances in terms of overall dimensions of an electrical cable required of an electrical cable for domestic use. Detailed Description of the Invention In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a plan view of a prior art electrical cable with a bare circuit protective conductor inside a cable that carries two, stranded operating cores. Figure 2 is a plan view of an electrical cable according to an embodiment. Figure 3 is a schematic sectional view of an electrical cable according to an embodiment in which the cores are solid cores. Figure 4 is a schematic sectional view of an electrical cable according to an embodiment in which the cores are stranded cores. Figure 5 is a sectional side view along line A-A of an extrusion tip for the operating cores and circuit protective conductor for a 3-core tool embodiment of the present invention. Figure 6 is a front view of the extrusion tip illustrated in Figure 5. Figure 7 is a sectional side view along line B-B of an extrusion die for a 3-core tool embodiment of the present invention showing the alignment key. Figure 8 is a front view of the extrusion die illustrated in Figure 7. Figure 9 is a schematic sectional view showing the relative positioning of an extrusion tip and an extrusion die in normal use. An example of a multicore electrical cable 25 is illustrated in Figures 2 to 4. The multicore electrical cable 25 comprising two conducting cores 26, 27 having each having a diameter, and a coated circuit protective conductor core 14. As shown, the coated circuit protective conductor core 14 is of a smaller diameter as compared to the diameters of the two conducting cores 26, 27. This electrical cable includes the preferred green / yellow identifier of the circuit protective conductor core (CPC) within the cable itself. This ensures that the identifier of the CPC is present when the cable is opened in order to connect the cable electrically. Such a cable should also comply with the mechanical and electrical properties defined in BS6004 Table 4. As shown, the coated circuit protective conductor core 14 has a layer of green / yellow identifier 29 applied to an outer side thereof. The layer of green / yellow identifier 29 is preferably applied over the length of the coated circuit protective conductor core 14. As shown, the coated circuit protective conductor core 14 is preferably not only reduced size when compared to the conducting cores 26, 27 but is covered with a reduced thickness of the layer of identifier 29. In this form, the identifying layer 29 is green / yellow in colour. The identifying layer 29 may be protective, but not insulative. As shown, the coated circuit protective conductor core 14 is located between the two conducting cores. The coated circuit protective conductor core 14 is located centrally in the cable 25, with a central axis thereof, coplanar with the central axes of the two conducting cores 26, 27. The multicore electrical cable 25 is substantially rectangular in cross-sectional shape. The outer sheath 28 may be provided outside the electrical insulating layer surrounding the conducting cores 26, 27 and the identifying layer 29 about the coated circuit protective conductor core 14. A bedding layer is not provided about the cores. The coating composition may be applied directly about an outer side of the cores 26, 27 and defines the outer shape of the multicore electrical cable 25. The conducting cores 26, 27 may be solid core as shown in Figure 3 or stranded core as shown in Figures 2 and 4. The wire(s) 31 of the conducting cores 26, 27 is covered with an outer layer of insulating cover 32 within the outer sheath 28. To achieve the preferred positioning of the coated circuit protective conductor core within the cable as described above, a tool for manufacturing a multicore cable is shown in Figures 5 to 8. The tool comprises an extrusion tip 16, an example is illustrated in Figures 5 and 6 and an extrusion die 17, an example is illustrated in Figures 7 and 8 located in series, as illustrated schematically in Figure 9. This example is for a dual conducting core and coated circuit protective conductor core cable tool. As shown best in Figure 6, the extrusion tip 16 comprises an opening 18 for each of the two insulated operating cores and a secondary opening 19 provided adjacent to one of the openings 18 to position a coated circuit protective conductor core 14 coextensively beside the one of the operating cores. In the illustrated configuration, the dimension of the secondary opening 19 is larger than the dimension of the coated circuit protective conductor core itself to allow the ripcord to be fed without snagging or friction into the secondary opening 19. As shown, the central axis of the secondary opening 19 is coplanar with the central axes of the openings 18 for each of the at least two conducting cores. This forms a cable as illustrated in Figures 3 and 4. In use, the extrusion die 17 is located downstream of the extrusion tip 16 as shown schematically in Figure 9. As shown, the exit end of the extrusion die has a shaped opening 22 to define the outer shape of the cable by coextruding the outer or cover sheath about the internal configuration exiting the extrusion tip 16. In the illustrated embodiment, the extrusion die 16 comprises a keyway 20. The key way 20 is preferably located on an external side of the extrusion tip 16 and engages with a key (not shown) on the extruder to maintain the orientation of the extrusion tip 16 to ensure that position and orientation of the two insulated operating cores and the coated circuit protective conductor core is maintained. A keying structure 21 is located on an external side of the extrusion die 17 as shown in Figures 7 and 8. The keying structure 21 engages with a key way (not shown) on the extruder to maintain the orientation of the extrusion die 17 relative to the extrusion tip 16 to ensure that position and orientation of the two insulated operating cores and the coated circuit protective conductor core is maintained as the cover sheath 28 is coextruded about the two insulated operating cores and the coated circuit protective conductor core. In use of this embodiment, the two insulated operating cores are fed into the extrusion tip 16 through the opening 18 for each and the coated circuit protective 5 conductor core 14 is fed at the same time into the secondary opening 19 provided adjacent to one of the openings 18, to form an internal configuration. The internal configuration exiting the extrusion tip 16 is then fed through the extrusion die 17 with the cover sheath composition to form the multicore cable. The one or more embodiments are described above by way of example only. 10 Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

1. A multicore electrical cable comprising at least two conducting cores having each having a diameter and coated with an insulating layer, and a circuit protective conductor core, wherein the circuit protective conductor core is provided with an identifying cover layer provided to an outer portion of the circuit protective conductor core within the multicore electrical cable, the identifying cover layer thinner than the insulating layer of the at least two conducting cores.

2. A multicore electrical cable as claimed in claim I wherein the coated circuit protective conductor core has a layer of green / yellow identifier applied to an outer portion thereof.

3. A multicore electrical cable as claimed in claim 2 wherein the layer of green / yellow identifier is applied over an entire length of the coated circuit protective conductor core.

4. A multicore electrical cable as claimed in any one of the preceding claims wherein the coated circuit protective conductor core is located between the at least two conducting cores.

5. A multicore electrical cable as claimed in any one of the preceding claims wherein the coated circuit protective conductor core is located centrally in the cable, with a central axis thereof, coplanar with a central axis of the at least two conducting cores.

6. A multicore electrical cable as claimed in any one of the preceding claims wherein the multicore electrical cable is substantially rectangular in cross-sectional shape defined by an outer sheath provided outside the at least two conducting cores and the coated circuit protective conductor core.

7. A tool for manufacturing a multicore electrical cable comprising at least two insulated operating cores, the tool comprising an extrusion tip and an extrusion die located in series, the extrusion tip comprising an opening for each of the at least two insulated operating cores and a secondary opening provided betweenthe operating cores to position a coated circuit protective conductor core coextensively between the operating cores.

8. A tool as claimed in claim 8 wherein the secondary opening is smaller in diameter than the opening for each of the at least two insulated operating cores.

9. A tool as claimed in claim 7 or claim 8 wherein the extrusion tip may comprise a keying structure or a keyway located on an external side of the extrusion tip.

10. A tool as claimed in any one of claims 7 to 9 wherein an exit end of the extrusion tip has a shaped opening with the openings for each of the at least two insulated operating cores and the secondary opening therebetween.

11. A tool as claimed in any one of claims 7 to 10 wherein the secondary opening is provided centrally between the openings for the openings for each of the at least two insulated operating cores.

12. A tool as claimed in claim 11 wherein a central axis of the secondary opening is preferably coplanar with a central axis of the openings for each of the at least two insulated operating cores.

13. A tool as claimed in any one of claims 7 to 12 wherein the extrusion die comprises a keying structure.

14. A tool as claimed in claim 13 wherein the keying structure is located on an external side of the extrusion die.

15. A method of forming a multicore electrical cable comprising at least two insulated operating cores, the method comprising the steps of feeding the at least two insulated operating cores into an extrusion tip comprising an opening for each of the at least two double insulated operating cores and a secondary opening provided between the openings to position a coated circuit protective conductor core coextensively between the operating cores, and feeding the coated circuit protective conductor core coextensively with the at least twoinsulated operating cores through the extrusion tip to form an internal core and coated circuit protective conductor core configuration within the cable.

16. A method as claimed in claim 15 further including the step of maintaining the alignment of the at least two insulated operating cores and coated circuit protective conductor core configuration exiting the extrusion tip with the extrusion die so that the coated circuit protective conductor core is positioned between the two insulated operating cores.

17. A method as claimed in claim 15 or 16 further including the step of coextruding an outer sheath about the at least two insulated operating cores and coated circuit protective conductor core to form an electrical cable.

18. A method as claimed in any one of claims 15 to 17 wherein the secondary opening is smaller in diameter than the opening for each of the at least two insulated operating cores19. A coating composition for a multicore electrical cable comprising a resin, a plasticiser, a stabilizer and at least one filler.

20. A coating composition as claimed in claim 19 wherein the resin comprises polyvinylchloride with a K-value of between 35 and 80, and preferably approximately 70.

21. A coating composition as claimed in claim 19 or claim 20 wherein the plasticiser comprises di(2-propylheptyl)phthalate.

22. A coating composition as claimed in any one of claims 19 to 21 wherein a ratio of the components may be 450-460 resin:210-225 plasticiser: 15-25 stabilizer:300-315 filler.

23. A coating composition as claimed in any one of claims 19 to 22 wherein the stabilizer is or includes a Ca / Zn Stabiliser.

24. A coating composition as claimed in any one of claims 19 to 23 wherein a calcium carbonate filler is used.

25. A multicore electrical cable as claimed in claim 6 wherein the coating composition of any one of claims 19 to 24 is used as the outer sheath.

Images