GB2243941A

GB2243941A - Manufacture of mineral insulated electric cables

Info

Publication number: GB2243941A
Application number: GB9109688A
Authority: GB
Inventors: Edward Albert Filler; Alan Gidney
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1990-05-11
Filing date: 1991-05-03
Publication date: 1991-11-13
Anticipated expiration: 2011-05-03
Also published as: ITTO910349A1; IT1245538B; FR2662012B1; FR2662012A1; GB9010582D0; GB2243941B; GB9109688D0; ITTO910349A0

Abstract

In the manufacture of a mineral insulated electric cable by a continuous process in which a continuously fed strip 2 of metal is folded around one or more conductors 6 to form a sheath, the mineral insulating material is in the form of pellets 7 assembled on the conductor(s) and the conductor/pellet assembly is fed continuously into the sheath as it is being formed. The pellets may be heated prior to use to drive off any binder and treated with silicone fluid to render them hydrophobic. They may have grooves 11 to receive conductors and be coloured for identification. The cable may be reduced in diameter after formation of the sheath. <IMAGE>

Description

MANUFACTURE OF MINERAL INSULATED ELECTRIC CABLES This invention relates to the manufacture of mineral insulated electric cables, that is to say cables of the type consisting of one or more electrical conductor wires enclosed within a tubular metal sheath and insulated from the sheath by a mineral electrically insulating material in which the wires are embedded.

The term "mineral insulated electrical cables1: is to be understood to include, in addition to wiring cables for the conduction of electric current for general purposes, cables of the construction described above employed for other purposes, for example heating cables, thermocouple cables, thermocouple extension cables and thermocouple compensating cables. The electrically insulating material used in such cables is usually magnesium oxide, although other suitable materials such as alumina, could be employed.

Mineral insulated electrical cables are commonly made by either of two basic methods, namely a batch production or a continuous production process.

Batch processes predominate, in which a predetermined length of metal pipe is filled with granulate magnesium oxide and conductors, or with preformed pellets of magnesium oxide in which conductor wires are inserted. The relatively short length of tube, insulant and conductors is then homogeneously reduced in diameter to produce a smaller diameter but longer length finished cable.

These processes suffer from the disadvantage that the finished length of cable is restricted in length to that which can be obtained from a given limited length of starting tube.

The alternative is a continuous process in which tube is formed and filled simultaneously with granulate magnesium oxide and metal conductors in an "in-line" process, for example as described in GB 2041260 B. While this process does not suffer from the restricted length problems of the batch processes, the range of cable sizes tends to be restricted.

According to the present invention, in the manufacture of a mineral insulated electric cable, pre-formed pellets of mineral electrically insulating material are pre-assembled end to end on one or more electrical conductors, and the conductor or conductors carrying the pellets is/are fed continuously through a tube forming machine in which a continuously fed metal strip is folded into tubular farm around the pellets and its edges welded to form a surrounding sheath.

By this process the length of cable that can be formed is restricted only by the physical size of the take up mechanism, or by the problems of handling very long lengths of cable on site.

Following the formation of the tubular sheath, the resultant assembly may be passed through a series of reduction means to reduce the sheath to the desired overall diameter of the cable, followed by annealing and quenching where necessary in accordance with known mineral insulated cable forming techniques.

The pellets may be heat treated so as to have the robustness of fired porcelain, and their individual lengths may be chosen to enable the assembly of the conductor or conductors and the pellets to be bent round suitably sized drums containing an assembly of sufficient length for some hours continuous operation of the tube-forming and subsequent processes. Facilities may also be provided for joining such assemblies together end-to-end without interruption of the tube-forming, reduction and annealing processes.

Similarly, the take-up mechanisms may be provided with facilities to accumulate the formed cable while take-up spools, drums or formers are changed.

The pre-assembly of the pellets on the conductor or conductors may take place at a work station remote from the tube-forming machine, and the assembly stored on suitably dimensioned drums, as aforesaid, prior to feeding to the machine. However in other cases the pellets can be fitted on to the conductor or conductors automatically as the latter is/are being fed to the machine. For this purpose the pellets are conveniently of multi-section form. Where the cable comprises one or two conductors the pellets may be formed of two semi-cylindrical sections with one or a pair of longitudinal grooves in their planar faces which, on assembly of the sections, provide a channel or channels for accommodating the conductor or conductors.

Alternatively in the manufacture of a cable incorporating two or more conductors an inner section, having an appropriate number of longitudinal grooves in its outer surface, may be placed between the conductors, the conductors disposed within the grooves, and a sectional outer covering placed around the inner section to maintain the conductors within the grooves.

Where the pellets are of multi-section form the sections may be held together in any convenient manner. Thus in some cases they may be assembled on the conductors automatically just prior to the tube-forming machine so that the formed sheath holds the composite pellets in place.

The advantages of the process described above are that the preformed pellets can be so dimensioned that the geometric disposition of the conductors in a cross section of the cable are predetermined to a greater extent than in alternative processes, and that the heat treatment of the pellets assists in the removal of various contaminants normally found in commercially available magnesium oxide. This enables improved electrical properties of the cable to be achieved, and maintained.

A further advantage of the process is that it can be carried out in a continuous manner without the problems associated with the introduction of magnesium oxide in powdered form into continuously formed tubes as commonly carried out hitherto, which inevitably gives rise to the presence of irritant, though non-toxic, dust in the atmosphere of the work place, as the production of pellets is such that the magnesium oxide is, in general, held in binders of various chemical formulations and is, therefore, not free to form dust. Where the binder is likely to be detrimental to the formed cable this can be driven off by heat treatment of the pellets or sections thereof prior to assembly on the conductor or conductors.

The pellets are conveniently such as to be readily fragmented into granular form during the reduction of the formed sheath.

The insulating material may be treated by the use of, for example, suitable silicone fluids, so as to render the insulating material hydrophobic, and thus maintain good electrical properties, unlike normal untreated magnesium oxide, for which special precautions against the absorption of atmospheric and other moisture are necessary.

The conductor or conductors employed in the manufacture of mineral insulated cable in accordance with the invention may be of stranded form, as an alternative to the solid conductors commonly employed in such cables at the present time.

A cable manufacturing process in accordance with the invention, and various modifications thereof, will now be described by way of example with reference to Figures 1 to 5 of the accompanying schematic drawings, in which Figure 1 represents part of apparatus for use in the manufacture of a mineral insulated electric cable utilising insulating pellets as illustrated in Figure 2, and Figures 3 to 5 illustrate alternative forms of insulating pellets suitable for use in a manufacturing process in accordance with the invention.

Referring first to Figure 1 this shows diagrammatically part of a tube forming machine, in which a copper strip 2 is folded by a series of rolls 3, only some of which are shown, into tubular form, and the edges welded together by an arc welding head 4 to form a closed sheath 5.

Into the sheath, as it is being formed, there is fed a pair of conductor wires 6, on which are supported cylindrical pellets 7 formed of magnesium oxide, the pellets having a pair of longitudinally-extending passages 8 (see Figure 2) through which the conductors pass. As the pellets enclose the wires before the welding position is reached, the wires are effectively protected against overheating during the welding process.

The pellets 7, which are heat treated, so as to have the robustness of fired porcelain, are threaded on to the conductor wires 6, either manually or automatically, at a work position remote from the tube forming machine, and the assembly is wound on to an appropriately sized drum 9, from which it is subsequently fed into the sheath 5 as the latter is being shaped.

The sheath 5, accommodating the pellets 7 and conductor wires 6, is then fed through a succession of reduction and annealing stages (not shown), which may be of a suitable known kind, and then on to a take-up spool. During the reduction stages the pellets 7 are crushed into granular form to completely fill the space between the conductor wires 6 and the formed sheath 5.

An alternative form of pellet 7.3, for use in the manufacture of mineral insulated electric cable incorporating two conductor wires, is illustrated in Figure 3. The pellet in this case is formed from three separate sections, namely an inner section 9 and two outer sections 10. The inner section 9 is in the form of a cylinder having in its cylindrical surface a pair of longitudinally-extending grooves 11 disposed diametrically opposite one another, and the two outer sections 10 are formed as half-sections of a hollow cylinder which fit closely around the inner section 9 as shown. In carrying out the manufacture, the inner section 9 is fitted between the conductor wires so that the latter lie within the grooves 11, and the two outer sections 10 are then fitted around the assembly to hold the wires in position.Any convenient means of securing the outer sections 10 around the inner section, which is not detrimental to the finished cable, may be employed.

Moreover instead of the conductor wires and pellets being assembled at a work station remote from the tube-forming machine 1, means may be provided for automatically fitting the sections of the pellets on to the conductor wires as a continuous process, immediately prior to the formation of the sheath tube 5, the latter then serving to hold the pellet assemblies in position on the wires.

A similar multi-section pellet may be used in the manufacture of cables employing more than two conductor wires, the inner section 9 of the pellet being provided with the appropriate number of longitudinal grooves. Thus the pellet 7.4 illustrated in Figure 4 is designed for use in the manufacture of cables having three conductor wires, the outer sections 10 of the pellet being formed as half-sections of a hollow cylinder, as in the case of the pellet shown in Figure 3, but the inner section 9, in this case, being formed with three uniformly spaced longitudinal grooves 11 for accommodating the three conductor wires.

Another pellet 7.5 for use in the manufacture of cables incorporating three conductor wires is illustrated in Figure 5. The inner section 9 of the pellet is the same as that shown in Figure 4, but the outer section 10 is formed as three identical sections of a hollow cylinder, each extending over a respective one of the grooves 11 of the inner section. These sections may be differently coloured for ease of identifying the adjacent conductor wires of the completed cable. This is particularly beneficial.According, therefore, to another aspect of the invention, in the manufacture of a mineral insulated electrical cable in which preformed pellets of mineral electrically insulating material are pre-assembled end to end on a plurality of conductor wires, and surrounded by a metal sheath which is subsequently passed through a series of reduction means, the pellets are formed in sections and the sections adjacent different ones of the conductor wires are at least partly differently coloured.

Such manufacture may be carried out as continuous production process in accordance with the first aspect of the invention as described above, although it may also be applicable to cables made by a batch production process.

Moreover it will be appreciated that the invention is not restricted to the use of multi-section pellets of the constructions illustrated.

Although, in the example described, the pellets are formed of magnesium oxide, other high temperature mineral or synthetic mineral material, suitable for the manufacture of mineral insulated cables and capable of being fabricated into pellet form, may alternatively be used. For example the pellets may be formed of alumina or suitable compounds thereof.

The pellets may also be impregnated with a suitable hydrophobic material such as a silicone fluid to reduce the likelihood of moisture absorption from the atmosphere.

Although a vertical tube-forming machine has been represented in Figure 1, it will be appreciated that the invention is equally applicable for the manufacture of mineral insulated electric cables involving tube-forming processes other than vertical, for example horizontal or at any intermediate angle. Moreover single or multi-head arc welding equipment may be utilised for seam closure.

Although in the processes described above, the conductors, and the metallic strip which forms the sheath are of copper, it will be appreciated that they can be of any other metal or alloy having sufficient ductility to allow for the reduction in diameter and increase in length produced in the last stages of the processes.

Claims

1. The manufacture of a mineral insulated electric cable wherein pre-formed pellets of mineral electrically insulating material are pre-assembled end to end on one or more electrical conductors, and the conductor or conductors carrying the pellets is/are fed continuously through a tube forming machine in which a continuously fed metal strip is folded into tubular form around the pellets and its edges welded to form a surrounding sheath.

2. The manufacture of a mineral insulated electric cable according to Claim 1 wherein following the formation of the tubular sheath, the resultant assembly is passed through a series of reduction means to reduce the sheath to the desired overall diameter of the cable.

3. The manufacture according to Claim 1 or 2 wherein the pre-assembly of the pellets on the conductor or conductors takes place at a work station remote from the tube-forming machine, and the assembly is stored on drums prior to feeding to the machine.

4. The manufacture according to Claim 1 or 2 wherein the pellets are of multi-section form and are fitted on to the conductor or conductors as the latter is/are being fed to the machine.

5. The manufacture according to Claim 4 in which the cable contains one or two conductors wherein the pellets are formed of two semi-cylindrical sections with one or a pair of longitudinal grooves in their planar faces which, on assembly of the sections, with the faces in contact, provide a channel or channels for accommodating the conductor or conductors.

6. The manufacture according to Claim 4 in which the cable contains two or more conductors wherein an inner pellet section, having an appropriate number of longitudinal grooves in its outer surface, is placed between the conductors, the conductors disposed within the grooves, and a sectional outer covering assembled around the inner section to maintain the conductors within the grooves.

7. The manufacture according to Claim 6 wherein the sectional outer covering of each pellet, is formed with a number of sections corresponding to the number of grooves, and each section is disposed over a respective groove.

8. The manufacture according to Claim 7 wherein the sections forming the outer covering are at least partly differently coloured from one another.

9. The manufacture according to any preceding claim wherein the pellets are heat treated during manufacture.

10. The manufacture according to Claim 5 or 6 wherein the formed pellet sections are heat treated prior to assembly.

11. The manufacture according to any preceding claim wherein the insulating material forming the pellets is subjected to a treatment which renders it hydrophobic.

12. The manufacture according to Claim 11 wherein the pellets are impregnated with a silicone fluid.

13. The manufacture of a mineral insulated electric cable carried out substantially as shown in and as hereinbefore described with reference to Figure 1 and any one of Figures 2 to 5 of the accompanying drawings.

14. A mineral insulated electric cable manufactured in accordance with any preceding claim.

15. An insulating pellet for use in the manufacture of a mineral insulated electric cable according to Claim 1, and substantially as shown in and as hereinbefore described with reference to any one of Figures 2 to 5 of the accompanying drawings.