GB2239981A - Mineral insulated cable - Google Patents

Abstract

A mineral insulated cable is formed by surrounding 19 one or more conductors 29 with an electrically insulating tape 20. The or each covered conductor is laid in a groove in preformed blocks (25 Fig 3) of mineral insulating material which are optionally covered with insulating tape 23. A metal tube 27 is formed 30, 32 around the covered blocks and the whole is then reduced to size by drawing or rolling 34, 35, alternately with annealing. Up to four blocks with different complementary shapes to give a circular cross-section may be used (Figs 13 to 18). A pair of insulating tapes (20a, 20b Figs 9 & 10) may be provided above and below a pair of conductors and secured in place by the blocks. The tapes may be of glass fibre, mica, silicone or PTFE, and may be coloured. Conductors of different polygonal cross-sections may be used to provide identification (figs 19 to 21). <IMAGE>

Description

METHOD OF AND APPARATUS FOR MAKING MINERAL INSULATED CABLE AND MINERAL INSULATED CABLE MADE BY SUCH METHOD OR APPARATUS.

The present invention relates to a method of and apparatus for making a mineral insulated cable and to a mineral insulated cable made by such a method or apparatus.

Mineral insulated cable comprises an outer metal tubular sheath, usually made of copper, containing one or more conductors embedded in an insulating mineral, usually magnesium oxide. Mineral insulated cable is used in applications where the cable has to withstand high temperatures or fires, for instance in emergency lighting systems and fire alarm systems. Such cables have conventionally been made by either a batch process or a continuous process.

In a known batch process, preformed blocks of mineral insulant having through-holes are inserted into a metal tube which will form the outer sheath in the finished cable. The holes in the blocks are aligned and conductor rods are inserted through the aligned holes. This arrangement forms a blank which is then further processed, for instance by repeated drawing or rolling and annealing to reduce the cross section and provide a finished cable. In alternative batch processes, the conductors are embedded in mineral insulant in powder form, the metal tube being arranged vertically and the powder being inserted from above. A ram may be used to compact the powder within the tube.

By their very nature, such batch processes are capable of producing cables of limited maximum length. Also, these processes have a relatively low rate of production, and the finished cable made by such processes is relatively expensive.

A known continuous process is illustrated in Figure 1 of the accompanying drawings, which illustrates manufacture of a mineral insulated cable having two conductor cores.

The conductors are made from a pair of copper rods 1 which are supplied continuously through bores in a spacer block 2. Copper strip 3 for forming the cable outer sheath is likewise continuously supplied to a tube forming mill illustrated diagrammatically by a pair of rollers 4 and 5. Powdered magnesium oxide 6 is fed under gravity from a hopper 7 through a tube 8 so as to fill the outer sheath. A welding station 9 welds the tube seam in the immediately vicinity of the rollers 4 and 5 so as to form a completed blank 10. The completed blank 10 is continuously fed to a plurality of rolling stages 11 and annealing stages 12, only one of each being shown in Figure 1.

In practice, the continuous process illustrated in Figure 1 has to be performed vertically, at least up to the first rolling stage 11. The speed of production is limited by the requirement that the tube be properly filled with magnesium oxide.

A recently developed continuous process is disclosed in British Patent Application No.8901911.filed on 28th January, 1989 in the name of the present applicant. This process will be described in more detail hereinafter but, briefly, the mineral insulant is formed into blocks having grooves into which the conductors are laid. An outer metal tube is formed around the blocks containing the conductors to complete a blank, which is then continuously fed to a plurality of rolling or drawing stages alternating with annealing stages. This process has many advantages compared with the known batch and continuous processes described hereinbefore, as explained in GB 8901911.1.

According to a first aspect of the invention, there is provided a method of making a mineral insulated cable, comprising at least partially surrounding a conductor with a layer of electrically insulating material and forming therearound a metal tube containing mineral insulant.

The method may be used to make cables having a single conductive core or cables having a plurality of conductive cores (multi-core cable). In the case of multi-core cables, each of the conductors may be at least partially surrounded with a layer of electrically insulating material, so that all cores are treated similarly. Alternatively, all of the cores except one may be so treated.

It is thus possible to provide a mineral insulated cable having improved insulation properties compared with known types of mineral insulated cables. Because of the improved insulation properties, the spacing between the conductor and the metal tube may be reduced and, in the case of multi-core cables, the spacings between the conductors can be reduced. Thus, a more compact cable can be provided for a given insulation rating and the quantity of mineral insulant, such as magnesium oxide, and metal in the surrounding metal tube can be reduced.

The cost of material per unit length is thus reduced compared with known types of mineral insulated cables.

In the case of a two-core cable, by providing only one of the core conductors with the layer of electrically insulating material, it is possible to identify the cores when installing the cable merely by inspection. For multi-core cables having more than two cores, the layers of electrically insulating material may be suitably coloured in order to permit visual identification of the cores. For instance, paint containing non-metallic pigment may be applied to one or more of the cores of a multi-core cable.

Preferably, the method further comprises supplying preformed blocks of mineral insulant having at least one groove, continuously laying the or each conductor into the or each groove, and continuously forming the metal tube around the blocks. The present invention may thus be used to improve the properties of a cable made by a method in accordance with GB 8901911.1. Preferably a layer of electrically insulating material is formed around the blocks before the metal tube is formed. The layer of insulating material around the blocks thus helps to hold the blocks in place for the subsequent tube forming step.

Preferably the or each layer of electrically insulating material comprises a tape provided on the or each conductor or on the blocks. The tape preferably forms a continuous layer and the or each layer may be formed by helically winding the tape, by lapping the tape around the conductor and/or blocks, by laying and forming the tape longitudinally around the conductors and/or blocks, or, in the case of preformed blocks, by sandwiching the or each conductor between layers of tape before laying the or each conductor into the groove or grooves in the preformed blocks. In general, the mineral insulated cables are finally formed by one or more cross sectional area reducing steps, such as drawing or rolling.The use of tapes of electrically insulating material around the conductors provides an abrasion barrier which reduces or prevents surface damage to the conductors during such drawing or rolling steps. The reduction in or elimination of surface irregularities on the conductors improves the insulation performance of the cable by reducing electrical stress points. Further, reductions in cross sectional area of the conductors caused by surface irregularities are reduced or eliminated so that conductors of smaller cross sectional areas can be used in order to provide a cable of a given current rating.

This permits smaller spacings and allows the final cable to be of reduced cross sectional area compared with known cables produced by known methods which, again, leads to a reduction in the cost of material per unit length in manufacturing the cables. In addition, because the conductor surface is smoother, the mineral insulant, such as magnesium oxide, adheres less well to the conductor surface and is easier to strip and clean when preparing cable ends. The tendency of moisture to track along the conductor or conductors and into the mineral insulant is therefore significantly reduced, thus improving the insulation properties of the ends of cables.

After drawing or rolling processes to finish the cable, the electrically insulating layer may not completely cover the conductor or conductors. However, during these cross section reducing steps, the layer tends to be deformed so as to fill the interstices between the particles of the mineral insulant. Thus, the insulation is improved in comparison with known methods.

Between each drawing or rolling step, the metal of the conductors and the tube, generally copper, is normally annealed in order to permit further drawing or rolling steps, and again after the final such step in order to prevent damage to the cable during installation resulting from bending of the cable. The electrically insulating material should therefore have properties which allow it to withstand the temperatures reached during annealing steps. It is possible to make use of an electrically insulating material which gives off gas during the annealing steps provided the gas pressure within the cable does not result in any damage. A possible benefit of this is that, following each annealing step, the gases condense onto the conductors to ensure a more complete coverage of the conductor surface.

The materials of the tapes used in the method must be flexible with reasonable tensile strength properties and good electrical properties. The tapes have to withstand the temperatures used during all of the manufacturing steps and must be substantially unaffected by, for instance, a welding operation performed when the metal tube is being formed. Additionally, the tapes preferably are made from an inert inorganic system which is flameproof. In general, it is preferable for no gases or fumes to be given off when the tape is heated and it is also preferable for the materials of the tape not to have any carbon content, which might otherwise adversely effect the insulating properties of the cable if the tape breaks down during use.

Various materials are suitable for the tapes, including glass fibre tape (which may be coated), mica (mica silicone bonded to glass tape), silicone tape, polytetrafluoroethylene, and various polyamide tapes.

Glass fibre tape has the advantage that it is cheap and can be coated, but has poor moisture blocking characteristics. Mica tapes have very good electrical insulating properties but have poor moisture blocking properties and present troubles with adhesives. Silicone tape has very good properties for this application, including the ability to seal to itself, but the temperature stability is not as good as other materials.

Polytetrafluoroethylene tapes also have very good properties for this application but temperature stability may be a problem for certain applications. Polyamide tapes have generally good properties and have the advantage that they do not melt. However, water permeability and temperature stability may not be adequate for some applications.

The above list is given merely by way of example, and any material having suitable properties may be used for the electrically insulating layer or layers.

The use of the electrically insulating layer, in addition to improving the insulating properties of the cable, can provide other advantages, such as helping to prevent moisture ingress into the final cables, keeping dust away from a welding region during forming of the metal tube, and preventing the ingress of moisture if the manufacturing process has to be stopped temporarily while cable is being made.

According to a second aspect of the invention, there is provided an apparatus for making a mineral insulated cable, comprising means for providing a layer of electrically insulating material which at least partially surrounds a conductor and means for forming therearound a metal tube containing mineral insulant.

According to a third aspect of the invention, there is provided a mineral insulated cable comprising a metal tube containing mineral insulant which contains a conductor provided with a layer of electrically insulating material.

According to a fourth aspect of the invention, there is provided a mineral insulated cable comprising a metal tube containing mineral insulant which contains a plurality of conductors, one of the conductors being visibly different from the or each other conductor.

It is thus possible to identify the conductors in order to facilitate making connections to the correct cores without requiring tests to be performed. This is particularly advantageous where the ends of a long cable are separated in such a way as to make communication difficult.

All of the core conductors may be made distinct from each other, for instance by colouring or by cross-sectional shape. However, it may be sufficient to make only one core conductor distinct since the relative positions of the conductors do not change along the length of the cable. Thus, if one core can be easily identified, the other cores can be identified with reference to it (allowing for lateral inversion or "minor-imaging" in the core patterns at opposite ends of a cable.

Whereas round section core conductors are normally used in mineral insulated cables, conductors having polygonal cross-sections may be used. Polygons with small numbers of sides are easier to identify for cores of small crosssectional size and triangular, square, pentagonal, and hexagonal sections may be used.

The present invention may be used with any of the known techniques for making mineral insulated cables. However, when used with the invention-bf GB 8901911.1, the advantages of both invention lead to a technique for the continuous production of mineral insulated cable of improved properties and reduced cost with respect to all other known techniques.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating a known continuous process for manufacturing mineral insulated cable, as hereinbefore described; Figure 2 is a diagram illustrating a method of and apparatus for making mineral insulated cable constituting an embodiment of the present invention; Figure 3 is a cross sectional view of a preformed block of mineral insulant for use in the method illustrated in Figure 2; Figure 4 is a cross-sectional view of conductors covered with an insulating layer; Figure 5 is a cross sectional view of parts of a mineral insulated cable before tube forming to form an outer sheath; Figure 6 is a cross-sectional view illustrating the start of tube forming;; Figure 7 is a cross sectional view of a finished mineral insulated cable constituting a preferred embodiment of the invention; Figure 8 is a diagram illustrating another method and apparatus constituting an embodiment of the invention; Figure 9 is a cross-sectional view illustrating an initial step of the method of Figure 8; Figure 10 is a cross-sectional view of parts of a mineral insulated cable before tube forming; Figures 11 to 18 are cross sectional views of different shapes of preformed blocks which may be used in preferred methods; and Figures 19 to 21 are cross-sectional views of mineral insulated cables constituting embodiments of the invention.

The method and apparatus illustrated diagrammatically in Figure 2 show all the steps required to make preformed blocks and finished mineral insulated cable. In the first step 15, a mineral insulating powder, such as magnesium oxide, is mixed and supplied to a powder granulating step 16. The granules of insulant are supplied to a tablet making step 17 which forms the mineral into the desired shape of the preformed blocks.

These blocks are then supplied to a heat treatment step 18 which ensures that the blocks have a sufficiently stable form for the subsequent steps.

The preformed blocks 25 have the shape shown in Figure 3 i.e. substantially hemi-cylindrical with a diameter of approximately 1" (2.5. centimetres) and a length of approximately 6" (approximately 15 centimetres). The flat surface of the block has two longitudinally extending grooves 26 which are also hemi-cylindrical in shape with a diameter of approximately 1/5" (approximately 5 millimetres).

Two copper conductors 29 are supplied in the form of continuous rods to a layer-forming station 19 which also receives tapes 20 of electrically insulating material from rolls 21. At the station 19, the tapes 20 are laid longitudinally of the conductors 29 and pressed around the conductors so as to form continuous layers as shown in Figure 4. Alternatively, the tapes may be lapped or wound around the conductors 29 so as to form the continuous layers. The covered conductors are supplied to another layer-forming station 22 which receives the blocks 25 from the heat treatment step 18 and another tape 23 of electrically insulating material from a roll 24.

The preformed blocks 25 are automatically supplied in facing pairs so as to entrain therebetween the two covered copper conductors 29. The opposing grooves 26 of the pairs of blocks 25 form continuous ducts containing the conductors 29.

The tape 23 is formed into a layer around the blocks 25 at the station 22 and holds the blocks 25 in place as shown in Figure 5.

The assembly of blocks 25 covered by the tape 23 and the conductors 29 covered by the tapes 20, together with a continuous strip 27 of copper from a roll 28, are supplied to a tube-forming mill 30 in which the strip 27 is formed into a tube around the blocks. The resulting seam is welded at a welding station 32 to form a continuous blank which is then supplied to a plurality of further processing steps. Figure 2 shows, purely by way of example, three rolling steps 33 to 35, each of which is followed by a respective annealing step 36 to 38, the final annealing step 38 being followed by a coiling step 39 for the finished mineral insulated cable.

Figure 6 illustrates the partly formed blank as supplied to the forming mill 30, whereas Figure 7 illustrates the finished blank after the welding. In fact, the rolling and annealing steps 33 to 38 do not alter the form, so that Figure 7 also illustrates the finished mineral insulated cable, having a weld seam at 40.

The method and apparatus illustrated in Figure 2 differ from those described in GB 8901911.1 by the inclusion of the layer-forming stations 19 and 22.

Figure 8 illustrates an alternative method of providing electrically insulating layers surrounding the conductors 29. Tapes 20a and 20b are continuously supplied from rolls 21a and 21b, respectively, so as to be disposed above and below the continuously supplied conductors 29, as shown in Figure 9. The blocks 25 are then applied from above and below so as to sandwich the conductors 29 and the tapes 20a and 20b as shown in Figure 10. An outer layer of tape may then be provided around the blocks 25 if desired, with further manufacturing steps being as shown in Figure 2.

Figure 11 illustrates the pairs of blocks 25, showing the cylindrical ducts 41 provided by the opposed grooves 26.

Figure 12 illustrates two blocks 42 which have grooves arranged to provide a single duct 43 for a single core cable. The step of laying the conductors in the grooves of the blocks may be performed in any suitable way. For instance, as described above, the blocks 25 are brought together around the continuously fed conductors 29.

However, in an alternative configuration, the lower blocks of the pairs are supplied so as to define two continuous grooves with the conductors being laid in the grooves from above. The upper blocks may then be placed on top so as to complete the laying in of the conductors.

Figures 13 and 14 illustrate two alternative forms of blocks 44 and 45. The blocks 44 shown in Figure 13 are continuously supplied so as to define two continuous diametrically opposite grooves 46. The blocks 45 in Figure 15 differ in that the grooves 47 are side-by-side and extend downwardly from the surface. The conductors are laid into the grooves 46 from the side whereas the conductors are laid into the grooves 47 from above. In order that the conductors be embedded within the mineral insulant, it may be sufficient merely to perform the rolling operations so that the mineral insulant closes around the conductors. However, it is also possible to fill the grooves 46 or 47, after the conductors have been laid therein, with mineral insulant. Suitably shaped preformed lengths of mineral insulant may be provided for this purpose.Alternatively, mineral powder or granules may be used, particularly with the blocks 46 shown in Figure 14. For the blocks shown in Figures 13 and 14, the method of providing insulating layers for the conductors shown in Figure 8 is inappropriate and the layer-forming step shown in Figure 2 is used.

Figure 15 shows a set of four identical blocks 48, each of which is generally quarter-cylindrical in shape with grooves extending longitudinally along the two flat surfaces of each block. When placed together as shown in Figure 15, the blocks 48 define four ducts 49 for receiving conductors in order to provide a four core cable. The blocks 50 shown in Figure 16 differ in that each is generally third-cylindrical in shape, these blocks being used to provide a three core cable. The methods of providing the conductors with insulating layers shown in both Figures 2 and 8 may be used with the blocks shown in Figures 15 and 16.

Figures 17 and 18 illustrate two possible forms of dissimilar pairs of blocks. The blocks 51 and 52 in Figure 17 differ from the blocks 25 in Figure 6 in that the block 51 has a longitudinal tongue 53 which extends between ducts 54 into a correspondingly shaped groove in the block 52. Figure 17 shows the block 51 disposed above the block 52, but the reverse arrangement is possible and may have advantages in that the tongue 53 assists in correctly locating the conductors during laying in.

The lower block 55 in Figure 18 is similar to the block 45 shown in Figure 14 but has a centre limb of reduced height for co-operating with a preformed upper block 46 to close the conductors within ducts 57. For the blocks shown in Figures 17 and 18, again the method of providing insulating layers shown in Figure 8 is inappropriate and that of Figure 2 should be used.

Preferred materials for the tapes 20, 20a, 20b, 23 are glass fibre, mica, silicone polytetrafluoroethylene, and polyamide, as described hereinbefore, but other materials may be used. For multi-core cables, the tapes may be differently coloured for the different conductors so as to aid identification, and one of the conductors may be left uncovered. In embodiments where the blocks 25 are surrounded by an insulating layer 23, the insulating characteristics of the cable are not substantially affected by leaving one conductor uncovered.

Identification of cores in a multi-core cable has previously caused difficulties because it is generally preferable to provide a symmetrical arrangement of cores and the cores themselves are identical. Thus, it was necessary to perform tests in order to establish which of the core ends at one end of a cable corresponded to a particular core end at the other cable end. For instance, a voltage could be applied between the particularly core end at the outer sheath at the other end and a test meter used at the one end to detect the presence of a voltage between the outer metal sheath and the core end. Such a procedure was necessary for even relatively short length of cable and represented a considerable inconvenience. For longer cables where the ends may be separated to such an extent that easy communication between the end was impossible, the problems were much more severe.

In order to identify cable cores in a convenient manner, it is necessary to make them visually distinct from each other, or at least to make one of the cores visually distinct. In general, the relative positions of the cores will not change along the length of a cable so that, for the more usual arrangement of cores, identification of one core will allow the other cores to be identified with respect to it (allowing for the lateral inversion of the core patterns at opposite ends of the cable).

One way of providing visible identification is to colour one or more of the cores or insulating layers surrounding the cores. Colours may be chosen as desired and, for increased convenience, all of the cores may be identified by different colours or colour patterns. However, for most applications, it may be sufficient to identify only one core by ensuring that it is coloured or is of a different colour or colour pattern compared with the other cores.

For applications where relatively sharp edges on the core conductors can be tolerated, the cores may be identified by providing different cross-sectional shapes. For instance, whereas the cores are normally circular, one of the cores may be formed with a polygonal cross-sectional shape. This is illustrated in Figures 19 and 20, where a square section core is shown. In Figure 19, a mineral insulated cable comprises two cores and, apart from the corr-sectional shape of the core 129, is identical to the cable illustrated in Figure 7.

Figure 20 illustrates a four core cable making use of insulated blocks 48 of the type shown in Figure 15.

Three of the cores 29 are circular whereas the fourth core 129 is square. Otherwise, this cable differs from that of Figure 19 in that it does not have insulating layers 20 surrounding the core conductors.

Figure 21 illustrates a four core cable in which the insulating layers 20 and 23 are not provided. Otherwise, the cable of Figure 21 differs from that of Figure 20 in that all four cores have different cross-sectional shapes. Thus, a first core 29 is round, a second core 129 is square, a third core 229 is triangular, and a fourth core 329 is hexagonal. It is thus possible to identify each of the cores of the cable shown in Figure 21 without reference to the others and, for some applications, this may be preferable.

The presence of relatively sharp edges along the core conductors greatly increases the local electrical field and the field is greater the sharper the edge. Thus, the lowest field is provided by a circular section core, the greatest field is provided by a triangular core, and other regular polygonal shapes create fields whose strength fall as the number of sides, and therefore edges, increases.

However, because the core conductors are subjected to rolling or drawing to reduce the cross-sectional size of the cable, and the cores may therefore end up being of very small cross-sectional size, as the number of sides of the polygonal cross-sectional shape increases, it becomes less easy to identify the number of sides. Thus, in general, polygons having relatively low numbers of sides are preferable for the cross-sectional shapes of the core conductors. If necessary, the spacings between cores and between the outer sheath and cores can be increased so as to provide adequate insulation in the presence of the increased electrical fields in the vicinities of the edges. Thus, different arrangements of cores may be used to provide cables particularly suited to different applications.

Further, cross-sectional shapes having one or more curved sides may be used, such as sector-shaped cross-sections.

Claims (41)

1. A method of making a mineral insulated cable, comprising at least partially surrounding a conductor with a layer of electrically insulating material and forming therearound a metal tube containing mineral insulant.

2. A method as claimed in Claim 1, in which the cable is a multi-core cable and each core conductor is at least partially surrounded with a layer of electrically insulating material.

3. A method as claimed in Claim 1, in which the cable is a multi-core cable and the or each core conductor except one is at least partially surrounded with a layer of electrically insulating material.

4. A method as claimed in Claim 2 or 3, in which at least two core conductors are at least partially surrounded with layers of electrically insulating material of different colours.

5. A method as claimed in any one of Claims 2 to 3, in which the cross-sectional shape of one of the core conductors differs from the cross-sectional shape of the or each other core conductor.

6. A method as claimed in Claim 5,- in which the cross-sectional shapes of the core conductors differ from each other.

7. A method as claimed in any one of the preceding claims, supplying preformed blocks of mineral insulant having at least one groove, continuously laying the or each conductor into the or each groove, and continuously forming the metal tubes around the blocks.

8. A method as claimed in Claim 7, in which a further layer of electrically insulating material is provided so as to surround at least partially the blocks before forming the tube.

9. A method as claimed in any one of the preceding claims, in which the or each layer or further layer of electrically insulating material comprises a tape.

10. A method as claimed in Claim 9, in which the or each layer or further layer is formed by helically winding the tape.

11. A method as claimed in Claim 9, in which the or each layer or further layer is formed by lapping the tape.

12. A method as claimed in Claim 9, when dependent on Claim 7 or 8, in which the or each layer is formed by sandwiching the or each conductor between layers of tape before laying the or each conductor into the groove or grooves in the blocks.

13. A method as claimed in any one of Claims 9 to 12, in which the tape comprises glass fibre, mica, silicone, polytetrafluoroethylene, or polyamide or a combination of two or more thereof.

14. A method as claimed in any one of the preceding claims, in which the mineral insulant is magnesium oxide.

15. A mineral insulated cable made by a method as claimed in any one of the preceding claims.

16. An apparatus for making a mineral insulated cable, comprising means for providing a layer of electrically insulating material which at least partially surrounds a conductor and means for forming there around a metal tube containing mineral insulant.

17. An apparatus as claimed in Claim 16, for making a multi-core cable, in which the layer providing means is arranged to provide a layer of electrically insulating material at least partially surrounding each core conductor.

18. An apparatus as claimed in Claim 16, for making a multi-core cable, in which the layer providing means is arranged to provide a layer of electrically insulating material at least partially surrounding the or each core conductor except one.

19. An apparatus as claimed in any one of Claims 16 to 18, further comprising means for supplying preformed blocks of mineral insulant having at least one groove and means for laying the or each conductor into the or each groove.

20. An apparatus as claimed in Claim 19, further comprising further means for providing a further layer of electrically insulating material which at least partially surrounds the blocks.

21. A mineral insulated cable made by an apparatus as claimed in any one of Claims 16 to 20.

22. A mineral insulated cable comprising a metal tube containing mineral insulant which contains a conductor at least partially surrounded by a layer of electrically insulating material.

23. A cable as claimed in Claim 22, comprising a plurality of core conductors, each of which is at least partially surrounded by a layer of electrically insulating material.

24. A cable as claimed in Claim 22, comprising a plurality of core conductors, each except one of which is at least partially surrounded by a layer of electrically insulating material.

25. A cable as claimed in Claim 23 or 24, in which at least two core conductors are at least partially surrounded by layers of electrically insulating material of different colours.

26. A cable as claimed in any one of Claims 23 to 25, in which the cross-sectional shape of one of the core conductors differs from the cross-sectional shape of each other core conductor.

27. A cable as claimed in Claim 26, in which the cross-sectional shapes of the core conductors differ from each other.

28. A cable as claimed in any one of Claims 22 to 27, in which a further layer of electrically insulating material is provided between the mineral insulant and the metal tube.

29. A cable as claimed in any one of Claims 22 to 28, in which the electrically insulating material comprises glass fibre, mica, silicone, polytetrafluoroethylene, or polyamide or a combination of two or more thereof.

30. A cable as claimed in any one of Claims 22 to 29, in which the mineral insulant is magnesium oxide.

31. A method of making a mineral insulated cable substantially as hereinbefore described with reference to Figures 2 to 21 of the accompanying drawings.

32. An apparatus for making a mineral insulated cable substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 8 of the accompanying drawings.

33. A mineral insulated cable substantially as hereinbefore described with reference to and as illustrated in Figures 7 and 19 to 21 of the accompanying drawings.

34. A mineral insulated cable comprising a metal tube containing mineral insulant which contains a plurality of conductors, one of the conductors being visibly different from the or each other conductor.

35. A cable as claimed in Claim 34, in which each of the conductors is visibly different from each other conductor.

36. A cable as claimed in Claim 34 or 35, in which the visible difference is provided by colour difference.

37. A cable as claimed in Claim 34 or 35, in which the visible difference is provided by difference in crosssectional shape.

38. A cable as claimed in Claim 37, in which a first of the conductors is of circular cross-sectional shape and a second of the conductors is of polygonal crosssectional shape.

39. A cable as claimed in any one of Claims 34 to 38, in which a layer of electrically insulating material is provided between the mineral insulant and the metal tube.

40. A cable as claimed in Claim 39, in which the electrically insulating material comprises glass fibre, mica, silicone, polytetrafluoroethylene, or polyamide or a combination of two or more thereof.

41. A cable as claimed in any one of Claims 34 to 40, in which the mineral insulant is magnesium oxide.