GB2122370A

GB2122370A - Mineral insulated electric cable

Info

Publication number: GB2122370A
Application number: GB08314813A
Authority: GB
Inventors: Oliver Sanders Johnson
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1982-05-28
Filing date: 1983-05-27
Publication date: 1984-01-11
Also published as: GB8314813D0; GB2122370B

Abstract

A mineral-insulated electric cable has within the sheath (1) at least one tubular metal element (2) extending along it and accommodating one or more optical fibres (5). The tubular element may be constituted by one or more of the normal power carrying conductors or may be provided by a separate metal tube within the sheath. The introduction of the fibre or fibres is carried out after the reduction of the cable to its final diameter. <IMAGE>

Description

SPECIFICATION Mineral insulated electric cable This invention relates to mineral insulated electric cables, that is to say cables of the type comprising one or more electrical conductors, usually of copper, enclosed within a tubular metal sheath, and insulated from the sheath by a filling of compacted powdered insulating material.

Such cables are relatively expensive compared with conventional plastics-covered cable but nevertheless they are useful for a number of applications as they are less susceptible to mechanical damage and are also highly fire resistant.

In order to operate a number of slave devices, such as motors, circuit breakers, electrically operable valves and the like from a control centre, power cables to the various devices are frequently run radially, with switching taking place at the control centre, actuation of a device then following the energisation of the appropriate cores in a feeder cable. In a major installation such an arrangement may require a large number of individually insulated conductors, and it is accordingly not usually economix to utilise mineral insulated electric cables in such a system, despite their excellent fire resistant qualities.

The alternative is to supply every device requiring power from one or more ring mains, and to control the devices by local switching. This has the advantage that the number of conductors required to carry power to the devices is reduced, and renders the use of mineral insulated electric cables a more economic proposition.

Switching can, in such a case, be effected by photoresponsive devices actuated by light signals fed along optical fibres; however optical fibres, in view of their fragile nature, need to be protected from mechanical forces and also from fire. This is commonly done by surrounding the fibres by organic insulants so as to form a flexible cable, but although good properties can be achieved, the inclusion of such materials adds to the potential fire hazard.

An object of the present invention is to provide a form of mineral insulated electric cable which is suitable for use as a ring main in such a system, but which effectively avoids this problem.

According to the invention a mineral insulated electric cable has within the sheath at least one tubular metal element extending along it and accommodating one or more optical fibres.

In such a cable the fibre orfibres is/are well protected from mechanical and fire hazard, and when utilised in a control arrangement as above described the optical fibre or fibres can be used to carry control signals, thereby avoiding the need for separate control cables.

In a system incorporating a mineral insulated electric cable in accordance with the invention acting as a ring main for supplying power to a plurality of devices controlled from a central point, each device can be controlled by the passage of light signals along a respective optical fibre in the cable.

Alternatively two or more devices may incorporate a detector responsive to a unique control signai fed along a common optical fibre, An optical fibre can be incorporated in the cable, either by including a tubular metal element addition- alto the normal power carrying conductors, or by making one of the more of the power conductors itself of tubular form. In the manufacture of mineral insulated electric cables copper or aluminium rods, which are to form the conductors, and the powdered insulating material, commonly magnesium oxide, are first introduced into an oversize sheath tube and the assembly passed through a succession of reduction stages until the required cable diameter is obtained.

In the manufacture of a cable in accordance with the invention one would then start with one or more tubes, instead of solid rods, to form the ultimate conductors. In the case where the fibre or fibres is/are accommodated in a tube separate from the normal current carrying conductors, the tube need not be such a good electrical conductor, and may, in such a case be formed of some other metal, for example stainless steel.

The optical fibre or fibres are conveniently introduced into the tube or tubes after reduction of the cable to the required finished size. To effect the introduction the cable is preferably laid out in a straight length and the fibre threaded through it. As optical fibre is rigid and has a high compression strength, this can be done by pushing the fibre into the tube. For longer lengths the fibre can be carried in on a current of compressed gas. In either case it is desirable that the inner bore of the tube be smooth and the fibre finished with a low friction coating.

The formation of the sheath tube and the introduction of the conductors and powdered insulating material can be carried out in any convenient manner, for example by a continuous process as described in any of our co-pending patent applications Nos. 2041258,2041259 and 2041260.

Two different mineral insulated electric cables in accordance with the invention will now be described by way of example with reference to Figures 1 and 2 of the accompanying schematic drawing, which represent transverse sections through the two cables.

Referring first to Figure 1, the cable comprises a tubular copper sheath 1 containing a plurality (in this case two) copper conditions 2 separated from each other and from the sheath tube 1 by a filling 3 of powdered magnesium oxide. The sheath tube 1 also contains an inner metal tube 4, which may also be of copper, but may alternatively be of another metal such as stainless steel, this inner tube extending along the sheath tube parallel to the conductors 2, and being also insulated from them, and from the sheath tube 1, by the powderfilling 3.In the manufacture of the cable the powder filling 3 is compacted around the conductors 2 and tube 4, after they and the powder have been introduced into the sheath tube 1, by passing the latter through a series of reduction stages until the required cable diameter is obtained, the assembly being passed through an annealing furnace after each reduction step in the conventional manner.

In accordance with the invention the inner tube 4 contains at least one optical fibre 5 (only one being shown) extending along the length of the cable.

The optical fibre 5 is introduced into the cable after the diameter of the latter has been reduced to the required finished size. This is conveniently achieved by laying the cable out in a straight length and either mechanically pushing the fibre into the inner tube 4, or by carrying it in on a current of gas fed through the tube. In either process it is desirable that the bore of the tube 4 be smooth and the fibre finished with a low friction coating, such as polyurethane enamel.

In the second embodiment the inner tube 4 is omitted and one of the current carrying copper conductors 2 is made hollow as shown at 6, the optical fibre or fibres 5 being accommodated within this conductor.

The optical fibre or fibres can be introduced into the hollow conductor, following the final reduction of the sheath tube, in a similar manner to that described with reference to Figure 1.

if desired both conductors may be hollow and contain a respective optical fibre, which could be desirable in some cases for passing signals to different control elements.

Although in both embodiments only two currentcarrying conductors have been shown, it will be appreciated that a cable in accordance with the invention may have more than two such conductors.

In addition it could have two or more hollow conductors each accommodating one or more optical fibres.

In a system incorporating a mineral insulated electric cable in accordance with the invention to supply current to one or more slave devices to be controlled remotely, any convenient means can be provided for introducing control signals into the optical fibre or fibres, suitable detectors and converters being provided for converting the light signals into electrical signals for actuating switches controlling the operation of the devices.

It will be noted that as optical fibres are electrical insulators they present no problems when encapsulated in live conductors. Moreover signals transmitted along optical fibres are immune to electromagnetic interference.

Claims

1. A mineral-insulated electric cable provided with at least one tubular metal element which accommodates one or more optical fibres and is located within and extends along the cable sheath.

2. A cable according to Claim 1 wherein said element is a power conductor.

3. A cable according to Claim 1 wherein said element is of stainless steel.

4. A cable according to any preceding Claim wherein a said fibre is provided with a low-friction coating.

5. A cable according to Claim 4 wherein said coating is of polyurethane enamel.

6. A control system comprising a plurality of electrically operable slave devices connected to a ring main wherein said ring main incorporates at least one mineral-insulated cable as claimed in any preceding Claim and at least one said device is associated with local photoresponsive switching means which is remotely operable via a said optical fibre in said cable.

7. A control system as claimed in Claim 6 wherein said optical fibre is common to two or more said devices.

8. A method of manufacturing a mineralinsulated electric cable having at least one tubular metal element located within and extending along the cable sheath, in which powdered insulating material is compacted around the element by passing the sheath through a series of reducing stages, and in which at least one optical fibre is introduced into the tubular element after the sheath diameter has been reduced by said reducing stages.

9. A method according to Claim 8 in which the fibre is provided with a low friction coating.

10. A method according to Claim 9 in which the fibre coating consists of a polyurethane enamel.

11. A method of manufacturing a cable as claimed in any one of Claims 8 to 10 wherein said fibre is pushed into the tubular metal element.

12. A method of manufacturing a cable as claimed in any one of Claims 8 to 10 wherein said fibre is carried through the tubular metal element by a current of compressed gas.

13. A mineral-insulated cable substantially as described hereinabove with reference to Figure 1 or Figure 2 of the accompanying drawing.