GB2293497A

GB2293497A - Repairing cable installations

Info

Publication number: GB2293497A
Application number: GB9518337A
Authority: GB
Inventors: Alan Friday; Matthew Holt; John Owen
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1994-09-10
Filing date: 1995-09-08
Publication date: 1996-03-27
Anticipated expiration: 2015-09-08
Also published as: GB9518337D0; GB2293497B; GB9418263D0

Abstract

A power cable installation is repaired using a precut length 6 (about 3m for an 11kV distribution cable) of replacement cable each end of which has already been prepared by cutting back to expose end portions of its individual cores and by fitting stress control means and crutch sealing means thereto. Preferably at least the end portions are enclosed for protection in storage and transit. In the Figure, showing only one end of the damaged cable 1, a corresponding length of the cable is cut out and discarded and the two remaining cut ends of the damaged cable are prepared for jointing and jointed to the already prepared ends of the precut short length brought to site, the electrical connections being covered stress control sleeves 12. Since two cable ends, instead of four, are prepared for jointing at the site of the damage, around two hours' site work by a two-man jointing team is saved. <IMAGE>

Description

Repairing Cable Installations This invention relates to a method of repairing an electric multicore power cable installation that has been damaged by accident or misadventure, for example by mechanical excavation equipment in the course of the provision, maintenance or modification of other underground services. It relates primarily (but not exclusively) to the repair of distribution cable installations in the range from about 1 to 35kV, with the main application in the United Kingdom likely to be at the llkV distribution level.

Except where damage is minor and superficial, a length of cable containing the damaged portion needs to be cut out and replaced, requiring the making of two joints at the respective ends of the short replacement cable length. This is a precision operation requiring the use of skilled labour, and at llkV will typically require a full day's work by a team of two jointers, or if access allows half a day's work by two teams working on respective joints. This is not only expensive, but may entail undesirably long consumer "outages" (supply failures) if alternative supply routes have insufficient spare capacity. There is thus a need for repair techniques in which the requirement for skilled site labour is reduced and the completion of the repair speeded up.

This invention is based in part on the recognition that the length of cable to be removed is in a considerable proportion (possibly a majority) of cases largely determined not by the extent of the damage but by the spacing required to accomodate the two joints at the ends of the replacement cable length.

In accordance with the invention, a method of repairing a power cable installation comprises bringing to the damage site a precut length of replacement cable each end of which has already been prepared by cutting back to expose end portions of its individual cores and by fitting stress control means and crutch sealing means thereto; excavating (in the usual case of an underground cable) a corresponding length of the damaged cable; cutting out and discarding a predetermined length of the damaged cable substantially equal to the said precut length and including the damaged portion thereof; preparing the two remaining cut ends of the damaged cable for jointing; forming electrical connections between the so-prepared cut ends and respective ends of the said precut length of replacement cable;; and applying insulation, stress-control and protective means to those connections to complete two cable joints.

For an llkV installation, a single precut length of about two to four metres will serve for a large proportion of damage incidents, but if desired two or three different precut lengths could be stocked (and if desired taken to site for selection after the extent of damage has been assessed).

The cost of cable in the short precut length is a minor part of the cost of the repair, and it is economically satisfactory to stock precut lengths only of the largest current-carrying capacity required anywhere in the installation, so that the number of prepared precut lengths of replacement cable needed to be stocked can be very small even as small as one if the need to use conventional techniques in cases of unusually extensive damage is accepted.

Preferably the precut length is of a polymeric-insulated power cable, such as a crosslinked polyethylene insulated power cable. If the damaged cable is of another type (for example a traditional paper-insulated power cable), then the joints to be completed will be transition joints of appropriate and generally conventional design.

Preferably the stress-control and crutch sealing means pre-applied to the precut length of cable are resilient or heat-shrunk ones and may be identical to those that would conventionally be used to make an "indoor type" termination between the same cable and a transformer or other terminal equipment not exposed to the weather. The expression "resilient means" as used herein includes conventional premoulded and cold-shrink components.

The length of damaged cable to be excavated will need to be greater than the length of the precut replacement cable by the length of one of the cable joints to be formed, plus a reasonable clearance for access; since this is a predetermined length, excavation can be completed in a single operation and the excavating equipment and labour released from site (even if the jointing team has yet to arrive).

The cutting out of the damaged cable end may be entirely conventional, except that the predetermined length to be removed needs to be adhered to; and the preparation of the remaining cable ends may be entirely conventional.

The electrical connections may be made by conventional mechanical or crimped connectors, as desired; in some cases it may be practicable to pre-fit appropriate connectors to the prepared conductor ends of the replacement cable length, but where different sizes and types of conductor are liable to be encountered in the damaged cable, it may be preferable to select an appropriate connector on site from an available range.

Preferably stress-control around each of the said electrical connections in the eventual cable joints is provided by using a sleeve of high-permittivity stress-control material, in accordance with our prior EP0578349A. Alternatively, preformed sleeves with conductive layers forming stress-control electrodes may be used; other conventional stress-control techniques are applicable in principle but likely to require extra jointer time.

The insulation and protective means to complete the joint preferably comprises a preformed non-metallic joint shell that has an internal earthed metallic screen and is filled with casting resin (which need not be of high-voltage electrical grade if the preferred stress-grading means are used).

At least the end parts of the precut and prepared length of replacement cable are preferably enclosed for protection in storage and transit, suitably by wrappings of a metal/plastics foil laminate or other appropriate sheet material, and the invention includes a precut and prepared cable length with at least its end parts so enclosed.

The invention will be further described, by way of example, with reference to the accompanying drawing, which is a diagrammatic longitudinal section of one end of a repair to a damaged cable being made by the method of this invention.

The invention is illustrated in relation to the repair of a damaged cable 1 having insulated and screened conductors 2 (normally there will be three or exceptionally four conductors, but for clarity only two have been drawn and one of them only partially), bedding 3, corrugated metal sheath 4 and oversheath 5.

On the reporting or detection of damage, there is taken from stock a three-metre length of crosslinked polyethylene cable 6 having insulated and screened conductors 7 equal in number and equal or larger in current-carrying capacity to those of the damaged cable 1, a wire earth screen or armour 8 and plastics sheath 9, already prepared for use by cutting back each of its ends and fitting a "cold-shrink" sealing and stress-control fitting of the kind sold by BICC Components Limited (a subsidiary of the applicant) under the trademark BICC as BICC Stretch Rubber Terminations comprising a glove 10 and a sleeve 11,11 for each core. For a typical three-core cable with cross-section in the range 120-400mm2, BICC Stretch Rubber Termination no. EPDX-32 will be appropriate.The prepared cable ends ends are protected in storage and transit by wrappings of metal foil/plastics laminate to ensure they arrive at site in a clean and dry condition.

Meantime, a five-metre length of the damaged cable, with the damaged portion roughly in the middle, is exposed by excavation, and a central three-metre length A cut out and disposed of as scrap. The remaining ends of the damaged cable 1 are cut back and prepared for jointing in the usual way. Electrical connections (not visible) are made between corresponding conductor ends 2,7 with suitable mechanical connectors after stress control sleeves 12 (in accordance with the teaching of EP0578349A) have been threaded over the ends of one conductor of each pair. These are then positioned, as shown, over the electrical connections, and the sheath 4 of the damaged cable 1 and the wire screen or armour 8 of the cable length 6 connected to each other and to a perforated metal earth screen 13 for the joint.

A plastics joint shell 14 and a filled polyurethane resin (not shown) occupying the void spaces in the shell, primarily for waterproofing, complete the joint.

The other end of the repair is substantially identical.

Because only two cable ends, not four, have to be prepared on site, we estimate that approximately two hours site work (four man hours) can be saved, and the cable returned to service about two hours earlier than using conventional techniques.

Claims

CLAIMS:

1 A method of repairing a power cable installation comprising bringing to the damage site a precut length of replacement cable each end of which has already been prepared by cutting back to expose end portions of its individual cores and by fitting stress control means and crutch sealing means thereto; excavating (in the usual case of an underground cable) a corresponding length of the damaged cable; cutting out and discarding a predetermined length of the damaged cable substantially equal to the said precut length and including the damaged portion thereof; preparing the two remaining cut ends of the damaged cable for jointing; forming electrical connections between the so-prepared cut ends and respective ends of the said precut length of replacement cable; and applying insulation, stress-control and protective means to those connections to complete two cable joints.

2 A method in accordance with claim 1 of repairing an llkV power distribution cable installation, in which the precut length of replacement cable is about two to four metres.

3 A method as claimed in claim 1 or claim 2 in which the precut length is of a polymeric-insulated power cable.

4 A method as claimed in claim 1 or claim 2 in which the precut length is of a crosslinked-polyethylene insulated power cable.

5 A method as claimed in any one of the preceding claims in which the stress-control and crutch sealing means pre-applied to the precut length of cable are resilient or heat-shrunk ones.

6 A method as claimed in any one of the preceding claims in which connectors are pre-fitted to the prepared conductor ends of the replacement cable length.

7 A method as claimed in any one of the preceding claims in which stress-control around each of the said electrical connections in the eventual cable joints is provided by using a sleeve of high-permittivity stress-control material in accordance with EP0578349A.

8 A method as claimed in any one of the preceding claims in which the insulation and protective means to complete the joint comprises a preformed non-metallic joint shell which has an internal earthed metallic screen and is filled with casting resin.

9 A method of repairing a damaged power cable installation substantially as described with reference to the drawing.

10 For use in the method claimed in Claim 1, a precut length of replacement cable each end of which has already been prepared by cutting back to expose end portions of its individual cores and by fitting stress control means and crutch sealing means thereto and which has at least its said end portions enclosed for protection in storage and transit.

11 A precut length of replacement cable as claimed in claim 10 in which the said end portions are enclosed in wrappings of sheet material.

12 A precut length of replacement cable as claimed in claim 10 in which the said end portions are enclosed in wrappings of metal/plastics foil laminate.

13 A precut length of replacement cable substantially as described with reference to the drawing.