GB2150770A - Electrical connection of earth leads to cable sheath - Google Patents

Abstract

An elongate flexible strip of metal 8, having holes punched through it so as to leave jagged edges, is wound around insulated metal sheath 4 of cable 2 and is overlaid by an end 10 of a braided earthing strap 6. The punched strip and the earth braid are secured together by a worm drive clamp 14 such that the jagged edges pierce any insulation, for example an oxide coating, on the metal cable sheath. The region of the earth connection is subsequently protected by heat-shrinkable polymeric tubing. <IMAGE>

Description

SPECIFICATION Electrical connection to cables This invention relates to arrangements for and methods of making an electrical connection between the metal sheath of an electrical power cable and an electrical conductor. In particular, though not exclusively, the connection may be between an earthing conductor, which may be a braid, and a protective cable sheath of aluminium or lead, which may be corrugated, for carrying fault currents to earth.

Such a cable will be voltate rated at least at 110V, and typically may be a high voltage cable. By high voltage is meant voltages in excess of about 1 kV, and typically may be in excess of about 11 kV. The cable may have one or any number of currentcarrying cores.

It is to be noted that the metal sheath of a power cable, especially a high voltage cable, has to be capable of carrying a very large current, of the order of tens of thousands of amps for a time of the order of seconds, and of carrying a sustained current of the order of nundreds of amps, in the event of a failure in the cable or in associated equipment.

Accordingly, the connection from the sheath to a point at earth potential has to be able to withstand the high electrical and mechanical stresses involved, so as to ensure the safe earthing of any fault current.

Conventionally, earthing of the metal sheath of electrical power cables is effected by forming a plumb, and this requires, in addition to ensuring that the metal sheath is clean and free of any insulating coating, formed, for example, by oxidation, a great deal of skill in forming a good plumb connection. Furthermore, owing to the large amount of heat that has to be applied to melt the material, usually lead, to form the plumb, the danger exists that the cable, and especially the dielectric, may be damaged. For example, the oil of an oil-filled cable, which forms part of the insulation of the cable, may be displaced locally from the heated region. Furthermore, such damage may not be apparent at the time the plumb is formed, but could subsequently cause failure of the cable in service.

Alternatively, a bedding layer of copper braid is wrapped around a lead sheathed cable and an earthing braid is secured thereto by one or more roll springs. In some circumstances, however, the latter solution may not provide sufficiently good electrical contact. Particular care is required to obtain a good electrical connection to a corrugated aluminiumsheathed cable.

Although it is known from UK Patent Application Publication Nos 2105925 A and 2108780 A to arrange for projections or teeth to penetrate a cable shield so as to obtain earthing connection thereto, the arrangements disclosed therein effect rigid connection to the shield, and over a very limited region only. Furthermore, these arrangements consist of a large number of components, and also require acces to the inside of the shield to be earthed. UK Patent Application No 960291 provides a connector having lances for penetrating insulation to contact the braid shield of a cable, but the connector itself is a crimping ferrule and is accordingly limited in the strength of its mechanical connection, thus rendering it not very suitable for power cable connection, particularly with the very high currents that can be involved with a high voltage power cable.

It is an object of the present invention to achieve an electrical connection between the metal sheath of an electric power cable and an electrical conductor in a particularly convenient and effective manner.

In accordance with one aspect of the present invention,there is provided an arrangement for making an electrical connection between the metal sheath of an electrical power cable and an electrical conductor, the arrangement comprising an elongate flexible metal strip having a plurality of teeth extending therefrom that, in use, is arranged to be disposed around the sheath, and a clamping member, the clamping member being arranged, in use, to clamp the strip to the sheath and to the conductor such that the teeth pierce any insulating layer on the sheath and penetrate into the metal thereof, thereby to make said electrical connection.

The invention also provides in a further aspect, an electric power cable having a metal sheath, wherein an electrical connection is made between an electrical conductor and the sheath by an arrangement according to said one aspect of the invention.

In accordance with another aspect of the present invention, there is provided a method of electrically connecting an electrical conductor to a metal sheath of an electric power cable, wherein an elongate flexible metal strip having a plurality of teeth extending therefrom is disposed around the cable sheath and is clamped thereto by a clamping member so as to pierce any insulating layer thereon and to make electrical connection between said conductor and said sheath.

The teeth of the metal strip need to be hard enough and sharp enough to pierce the insulating layer, for example an oxide coating, and to penetrate the metal of the sheath, but should not be so long as to pierce the metal of the sheath, since otherwise the underlying cable insulation and thus the electrical properties of the cable may be damaged. The "teeth" may accordingly be provided by any configuration or surface roughening treatment that achieves these results. The teeth may conveniently, but need not, be formed integrally with the metal strip, for example by piercing the strip to produce apertures therein having jagged edges. The metal strip thus provides a large number of points making electrical contact with the metal of the cable sheath, and ensures good, secure electrical connection thereto.

It will be appreciated that the electrical conductivity of the strip should be high, it should be flexible and not brittle so as easily to conform to the sheath, and this may require two-dimensional flexibility where the sheath is corrugated. Beryllium copper strip has been found to be particularly advantageous, but other materials, for example a hard brass, may be used where appropriate. It is to be noted that one size of strip, being elongate and flexible, may be supplied for use with a wide variety of cable sizes, i.e. diameters, thus making the arrangement very versatile.

The metal strip advantageously extends around the cable sheath for an appreciable portion of the circumference thereof, preferably for about threequarters of its circumference, thus providing electrical contact over a large area.

Advantageously the metal strip lies beneath the conductor which may be a braid, for example of copper, and a further metal strip, or a portion of the said metal strip, may be disposed on top of the conductor and beneath the clamping member of the arrangement.

The conductor, which typically will be for earthing any fault current, may be a braid, for example of copper, and thus is advantageously sealed, for example by solder, substantially to prevent the flow of water therealong. For the same reason, the conductor may advantageously be sealed to the cable sheath.

A protective sleeve, tubuiar or wraparound, may be disposed around the cable sheath in the region of the connection arrangement, and this may advantageously be of recoverable, for example heat-recoverable, polymeric material.

The clamping member of the arrangement preferably extends completely around the circumference of the cable sheath, and may be a worm drive clamp, or a roll spring for example. The clamping member will thus exert an inward pressure on the toothed metal strip to ensure secure mechanical connection between the conductor, the metal strip and the cable sheath. A roll spring has the advantage that whilst maintaining an inward pressure on the components, it is resilient and thus can accommodate movement of the cable. Such movement may be due to thermal cycling of the cable, or relative movement of the ground in which the cable is buried, for example.

The connection to a cable sheath provided by the arrangement and method of the present invention may be effected at or near a joint, which may be inline joint or a branch-off joint, or a termination of the cable, or at any point along its length.

An arrangement for and method of making an electrical connection between the metal sheath of a high voltage electrical power cable and an electrical conductor, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawing, which shows in partial section such a connection between a copper earthing braid and a corrugated aluminium sheath of the cable.

Referring to the drawing, a three-core 11 kV power cable has a PVC oversheath 2 that encloses a corrugated earth fault current carrying aluminium sheath 4. A section of the oversheath 2 about 70 mm long is removed to allow access for a copper braid 6 to make an earth connection to the metal sheath 4.

An elongate, flexible metal contact strip 8 is wrapped circumferentially around the sheath 4. The strip 8 is of beryllium copper, has a width of 3 cm, such as to extend across at least one valley of the sheath corrugations, is approximately 0.5 mm thick, and is of a length so as to extend about threequarters of the way around the circumference of the aluminium sheath 4. It is to be understood that the circumferential extent of the strip 8 around the sheath 4 may be greater that about threequarters, and may overlap itself, or alternatively may be a smaller amount, provided sufficiently good electrical and mechanical contact between these components is achieved. The strip 8 is pierced so as to have a large plurality of jagged edges or teeth extending from each of its major surfaces.One end 10 of the earthing braid 6 is splayed out so as to increase its surface area and is placed over the contact strip 8. A further length of the same material as the contact strip 8, forming a contact plate 12 is mounted on top of the braid end 10. It is to be understood that the contact plate 12 may be an integral part of the strip 8. A worm drive clamp 14 is tightened around the components 12,6, and 8. Thus, one end 10 of the earth braid 6 is securely clamped between two portions of contact strip 8, 12, and the teeth of the underlying contact strip 8 pierce any insulating layer, of aluminium oxide for example, on the cable sheath 4 and penetrate the metal of the sheath, forming a large plurality of electrical point contacts therewith extending over a major circumferential portion of the sheath.The pressure exerted by the clamp 14 also ensures continued good mechanical connection of the earth braid 6 to the sheath 4.

Since the interruption of the cable oversheath 2 could allow contamination, water for example, to enter between the oversheath 2 and the corrugated sheath 4, and in particular to flow inwards along the braid 6, the braid 6 is sealed with solder 16 so as to be water blocked a short distance from its end 10.

Furthermore, sealant material 18, for example a mastic, is wrapped around the sheath 4 so as to fill one of its valleys beneath the water-blocking solder 16, at the end of one section of the oversheath 2.

Finally, electrical insulation is restored in the region of the connection by covering the cutaway section and adjacent remaining portions of the oversheath 2 with a heat-recovered polymerictubular sleeve 20.

The earth braid 6 extends away from the cable between the oversheath 2 and the recoverable polymeric sleeve 20. It will be appreciated that a wraparound sleeve, with a suitable closure, would be used if access cannot be gained to an end of the cable. Since the screw portion of the clamp 14 is rather bulky, and may have sharp edges, the possibility of damage to the outer protective sleeve 20 may be avoided by insertion of a short length of padding 22, over the clamp 14. The padding 14 may conveniently be a piece of the removed cable oversheath 2.

It has been found that the above described arrangement will withstand a short circuit current of more than 14 kA for 1 second, and a continuous current of 300 A.

It will be appreciated that the connecting arrangement of the present invention may also be used for earthing, or other electrical connection, of other members, such as electrically conducting pipes.

Claims (21)

1. An arrangement for making an electrical connection between the metal sheath of an electrical power cable and an electrical conductor, the arrangement comprising an elongate flexible metal strip having a plurality of teeth extending therefrom that, in use, is arranged to be disposed around the sheath, and a clamping member, the clamping member being arranged, in use, to clamp the strip to the sheath and to the conductor such that the teeth pierce any insulating layer on the sheath and penetrate into the metal thereof, thereby to make said electrical connection.

2. An arrangement according to claim 1, wherein the metal strip extends around the cable sheath for an appreciable portion of the circumference thereof.

3. An arrangement according to Claim 1 or 2, wherein the metal strip extends in a circumferential direction around the cable sheath beneath the said conductor.

4. An arrangement according to Claim 3, wherein a further toothed metal strip, or a portion of the said metal strip is disposed between said conductor and the clamping member.

5. An arrangement according to any preceding claim, wherein the teeth are provided by the edges of apertures made in the metal strip.

6. An arrangement according to any preceding claim, wherein the clamping member comprises a worm drive clamp or a roll spring.

7. An arrangement according to any preceding claim, wherein said conductor comprises a braided strip that extends longitudinally of the cable sheath.

8. An arrangement according to any preceding claim, comprising a protective sleeve that in use is disposed around the cable sheath to enclose the electrical connection.

9. An arrangement according to Claim 8, wherein the protective sleeve comprises recoverable, preferably heat-recoverable, material, and is of tubularorwraparound configuration.

10. An electrical connection arrangement between an electrical power cable and an electrical conductor, substantially as hereinbefore described with reference to the accompanying drawing.

11. An electric power cable having a metal sheath, wherein an electrical connection is made between an electrical conductor and the sheath by an arrangement according to any preceding claim.

12. A cable according to claim 11, wherein the metal strip is sealed to the cable sheath so as substantially to prevent the flow of water along the strip.

13. A cable according to Claim 11 or 12, when dependent on Claim 8 or 9, wherein the cable metal sheath is enclosed within an outer insulating sheath of the cable that is removed to allow said electrical connection to be made, and wherein the protective sleeve is subsequently positioned over the connection and over adjacent portions of the outer cable sheath.

14. A cable according to any of Claims 11 to 13, wherein the metal cable sheath comprises aluminium, and said insulating layer comprises aluminium oxide.

15. A cable according to any of Claims 11 to 14, wherein the metal cable sheath is corrugated.

16. A method of electrically connecting an electrical conductor to a metal sheath of an electric power cable, wherein an elongate flexible metal strip having a plurality of teeth extending therefrom is disposed around the cable sheath and is clamped thereto by a clamping member so as to pierce any insulating layer thereon and to make electrical connection between said conductor and said sheath.

17. A method according to Claim 16, wherein the conductor is located between two portions of the metal strip, or between said metal strip and a further toothed metal strip.

18. A method according to Claim 16 or 17, wherein a protective sleeve of tubular or wraparound configuration, is positioned around the connection.

19. A method according to Claim 18, wherein the sleeve is recoverable, preferably by heat.

20. A method according to any of Claims 16 to 19, wherein the conductor is sealed to the cable sheath so as substantially to prevent the flow of water therealong.

21. A method of electrically connecting an electrical conductor to a metal sheath of an electric power cable, substantially as hereinbefore described with reference to the accompanying drawing.