GB2192498A

GB2192498A - Termination of mineral insulated electric cable

Info

Publication number: GB2192498A
Application number: GB08714165A
Authority: GB
Inventors: Alan Postill; Tobias Furman
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1986-06-17
Filing date: 1987-06-17
Publication date: 1988-01-13
Anticipated expiration: 2007-06-17
Also published as: GB2192498B; GB8714165D0; GB8614765D0

Abstract

A mineral insulated electric cable termination is prepared by threading on to the exposed lengths 2 of conductor a preformed electrically insulating button 6 having apertures through which the conductors pass and positioning the cut-back end of the cable 1 and the button threaded on the exposed conductors in a mould chamber 10 in such a way that the cut-back end of the cable and the button are spaced apart and effectively constitute end walls of the mould chamber. Hardenable electrically insulating material 7 in a semi-liquid state is injected into the mould chamber 10 and around the exposed lengths 2 of conductor until the chamber is filled with the hardenable material. The hardenable material 7 is permitted or caused to set and bond to the end face of the cut-back end of the cable 1 and to the button 6 to form a moisture-tight mineral insulated cable transmission. A sleeve 5 provided with an aperture for injection of the insulating material may extend between the cut back end and the button. Earthing continuity may be provided by clipping a metal collar around the cut back end of the cable, a conductor extending from the collar for passage through the button. <IMAGE>

Description

SPECIFICATION Termination of mineral insulated electric cable This invention relates to termination of mineral insulated electric cables, that is to say cables ofthe kind comprising at least one conductor insulated from a surrounding metal sheath, and where there is more than one conductor from the other conductor or conductors, by compacted mineral insulating powder, usually but not necessarily magnesium oxide.

It is an object of the present invention to provide an improved method ofterminating a mineral insulated electric cable.

According to the invention, the improved method comprises cutting back the cable sheath and removing the exposed mineral insulation to reveal a bare length ofthe or each conductor; threading on to the exposed length or lengths of conductor a preformed disc or button of electrically insulating material having an aperture or apertures through which the exposed length or lengths of conductor passes or pass; positioning the cut-back end ofthe cable and the disc threaded on the exposed length or lengths of conductor in a mould chamber in such a way thatthe cut-back end of the cable and the disc are spaced apart and effectively constitute at least parts of end walls ofthe mould chamber; injecting hardenable electrically insulating material in a semi-liquid state into the mould chamber and around the exposed length or lengths of conductor extending between the cut-back end of the cable and the disc until the mould chamber is substantiallyfilled with said hardenable material; and permitting or casing said hardenable material to set and bond to at least the end face of the cut-back end of the cable and to the discto form a substantially moisture-tight mineral insulated cable termination.

To ensurethatthe mould chamberwill besubstan tiallyfilled with said hardenable material, preferably the hardenable material is injected into the mould chamberfrom a fracturable sachet containing hardenable material in a semi-liquid state of a volume substantially equal to the volume of the mould chamber, so thatwhen the sachet is completely empty of hardenable material, the mould chamber should besubstantiallyfilled.

Preferably, injection ofthe hardenable material in a semi-liquid state into the mould chamber is effected through at least one port using a plungercapable of displacement in a cylinder connected to said port.

To compensate for any reduction in volume ofthe hardenable material on setting, afterthe hardenable material has been injected into the mould chamber preferably the volume of the mould chamber is redu ced to ensure that the mould chamber is substanti ally filled with hardenable material while it sets. Such reduction in volume of the mould chamber may be effected by u rg i ng the dise towa rds the cut-back end of the cable until hardenable material flows out ofthe space between the or each bare length of conductor and the boundary wall ofthe aperture in the disc through which it passes. Alternatively, the volume of the mould chamber may be reduced by means of a second plungerdisplacable in a cylinder connected to a second port opening into the mould chamber.

In some circumstances, a sleeve of metal or nonmetal may extend between the disc and the cut-back end of the cable where it may either abutthe end face of the cable sheath or overlap the cable sheath; in this case, the sleeve will have at least one aperture through its peripheral wall and will be so positioned in the mould chamber thatthe aperturewill be in alignment with the port through which said hardenable material in a semi-liquid state is injected into the mould chamber.

Where it is desired to provide for earth continuity, preferably prior to introduction of the cut-back end of the cable into the mould chamber, a collar of metal or metal alloy having an elongate electric earth conductor permanently connected thereto is clipped or otherwise electrically connected around the cut-back end of the cable sheath so that the earth conductor extends substantially parallel to the exposed length or lengths of conductor, the disc has an aperture through which the earth conductor passes, and the mould chamber is of such a shape and sizethatthe earth conductorwill be encapsulated within and el ectrically insulated from the cable conductor or conductors bythe hardenable material when it has set.

Preferred materials of which the disc or button may be made include aluminium oxide and other ceramic materials and bonded mica.

Preferred hardenable materials for injection into the mould chamber include silicone adhesive and polyethersulphone.

The invention also includes a mineral insulated cable termination made bythe improved method hereinbefore described.

The invention is further illustrated buy a description, by way of example, of the preferred method of terminating a mineral insulated electriccablewith reference to the accompanying drawing which shows a sectional side view of the mineral insulated electriccabletermination and of the mould employed in its manufacture.

Referring to the drawing, the metal sheath 4 ofthe mineral insulated electric cable 1 is cut-back and the exposed mineral insulation 3 is removed to reveal bare lengths 2 of conductor. A metal sleeve 5 is flitted tightly around the cable sheath 4 at the cut-back end of the cable 1 so thatthe sleeve protrudes beyond the cut-back end of the cable and the open end of the sleeve is closed by a preformed button 6 ofelectrically insulating material having apertures therethrough which is threaded on to the lengths 2 of conductor and which is fitted into the open end ofthe sleeve.The cut-back end of the cable 1, the metal sleeve 5 and the button 6 threaded on the exposed lengths 2 of conductor are then positioned in the chamber of a mould 10 in such awaythatthecut- back end of the cable and the button effectively constitute end walls of the chamber. Integral with the mould 10 is a transversely extending tubular limb 11 which has an internally screw-threaded bore which, at one end of the limb nearer the mould opens into a port 14with which an aperture 8 in the peripheral wall of the sleeve 5 is in alignment. In screwthreaded engagement in the internally screw threaded bore 12 ofthe limb 11 is a plunger 15.With the plunger 15 removed from the internally screwthreaded bore of the limb 11, a sachet (not shown) of hardenable electrically insulating material of a volume substantially equal to the volume ofthe chamber in the mould 10 is introduced into the internallyscrew-threaded bore. The plunger 15 is then screwed into the internally screw-threaded bore to cause the sachet to fracture and so inject hardenable electrically insulating material 7 in a semi-liquid state fromthesachetthroughtheportl4and aperture8 into the chamber of the mould 10 and around the exposed lengths 2 of conductor extending between the cut-back end ofthe cable 1 and the button 6 until the chamber is substantially filled with hardenable material 7. The hardenable material 7 is then permitted to set and bonds to the end face of the cut-backend ofthe cable 1 and to the disc 6 two form a substantially moisture-tight mineral insulated cable termination.

The mould 10 is then removed.

Claims

1. A method ofterminating a mineral insulated electric cable which comprises cutting back the cable sheath and removing the exposed mineral insulation to reveal a bare length ofthe or each conductor; threading on to the exposed length or lengths of conductor a preformed disc or button of electrically insulating material having an aperture or apertures through which the exposed length or lengths of conductor passes or pass; positioning the cut-back end ofthe cable and the disc threading on the exposed length or lengths of conductor in a mould chamber in such a way th at the cut-back end of the cable and the disc are spaced apart and effectively constitute at least parts of end walls ofthe mould chamber; injecting hardenable electrically insulating material in a semi-liquid state into the mould chamber and around the exposed length or lengths of conductor extending between the cut-back end of the cable and the disc until the mould chamber is substantially filled with said hardenable material; and permitting or causing said hardenable material to set and bond to at ieastthe end face of the cut back end of the cable and to the disc to form a substantially moisture-tight mineral insulated cable termination.

2. A method as claimed in Claim 1, wherein a sleeve of metal or non-metal extends between the disc and the cut-back end ofthe cable and has at least one aperture through its peripheral wall, the sleeve being so positioned in the mould chamberthatthe aperture is in alignment with the port through which said hardenable material in its semi-liquid state is injected into the mould chamber.

3. A method as claimed in Claim 1 or 2, wherein the hardenable material is injected into the mould chamberfrom a fracturable sachet containing hardenable material in a semi-liquid state of a volume substantially equal to the volume of the mould chamber.

4. A method as claimed in any one of the preceding Claims, wherein, to compensate for any reduction in volume ofthe hardenable material on setting, afterthe hardenable material has been injected into the mould chamber, the volume of the mould chamber is reduced to ensure that the mould chamber is substantially filled with hardenable material while it set.

5. A method as claimed in Claim 4, wherein said reduction in volume ofthe mould chamber is effected by urging the disctowardsthe cut-backend of the cable until hardenable material flows out ofthe space between the or each bare length of conductor and the boundary wall ofthe aperture in the disc through which it passes.

6. A method as claimed in any one of the preceding Claims, wherein injection ofthe hardenable material in a semi-liquid state into the mould chamber is effected through at least one port using a plunger capable of displacement in a cylinder connected to said port.

7. A method as claimed in Claim 6, wherein, to compensate for any reduction in volume ofthe hardenable material on setting, the volume of the mould chamber is reduced by means of a second plunger displaceable in a chamber connected to a second port opening into the mould chamber.

8. A method as claimed in any one of the preceding Claims, wherein, prior to introduction ofthecut- back end of the cable to the mould chamber, a collar of metal or metal alloy having an elongate electric earth conductor permanently connected thereto is clipped or otherwise electrically connected around the cut-back end ofthe cable sheath so that the earth conductor extends substantially parallel to the exposed length or lengths of conductor, the disc has an aperture through which the earth conductor passes, and the mould chamber is of such a shape and size that the earth conductor is encapsulated within and electrically insulated from the cable conductor or conductors bythe hardenable material when it has set

9. A method as claimed in any of the preceding Claims, wherein the disc or button is made ofalumi- nium oxide or other ceramic material or of bonded mica.

10. A method as claimed in any one of the pre- ceding Claims, wherein the hardenable material is silicone adhesive polyethersulphone.

11. A method ofterminating a mineral insulated electric cable substantially as herein before described with reference to the accompanying drawing.

12. A mineral insulated cable termination made by the method claimed in any one of the preceding Claims.