GB2192499A

GB2192499A - Termination of mineral insulated electric cable

Info

Publication number: GB2192499A
Application number: GB08714166A
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: GB8614766D0; GB2192499B; GB8714166D0

Abstract

An open ended sleeve 5 of rubber or plastics material is applied to the cut-back end of a cable sheath 4 and forms a mould into which a semi-liquid hardenable insulating bonding material 6 is injected. A preformed electrically insulating button 7 having a diameter equal to or greater than the internal diameter of the sleeves is slid over the conductors into the open end of the mould and towards the end face of the cable 1 until bonding material 6 flows into and fills the space between the conductors 2 and the boundary walls of the apertures in the button through which the conductors pass. The sleeve may be removed after the material 6 has set. Preferably, prior to the application of the sleeve, a collar of metal having an elongate earth conductor is clipped around the cut back end of the cable sheath, the earth conductor extending longitudinally for passage through the button 7. <IMAGE>

Description

SPECIFICATION Termination of mineral insulated electric cable This invention relates to the termination of mineral insulated electric cables, that isto 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.

Compacted mineral insulating powder is part icularlyvulnerable to water penetration and it is the general practice to protect an end of a mineral insulated electric cable against the ingress of moisture by a metal sealing pot which is screwed or otherwise secured to the cable sheath and which is filled with waterproof sealing compound. Fitting of the sealing pottothe cable sheath in such a way as to provide an effective waterproof termination requires a degree of skill and the time taken to terminate mineral insulated electric cables of a mineral insulated electric cable installation contributes to a substantial proportion ofthe overall costofthe installation.

It is an object ofthe present invention to provide an improved method ofterminating a mineral insulated electric cable which is simple and inexpensive and which can be effected bysemi-skilled or unskilled personnel.

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; applying to the cutback end ofthe cable sheath an open-ended sleeve of rubber or plastics material in such a way thatthe sleeve tightly fits the cable sheath and protrudes beyond the cut-back end ofthe cable to form a mould from which a bare length of the or each conductor protrudes; injecting a hardenable electrically insulating bonding material in a semi-liquid stateintothe open end of the mould so-formed; threading over the bare length ofthe or each conductor a preformed button or disc of electrically insulating material which has an aperture or aperturestherethrough for the bare length or lengths of conductor, which button or disc has a diameter substantially equal to or slightly greaterthan the internal diameter of the sleeve; sliding the preformed button or disc into the open end of the mould and towards the end face of the cable until bonding material flows into and sub stantiallyfills the space between the or each bare length ofthe conductor and the boundary wall ofthe aperture in the button ordiscthrough which it passes; and permitting or casing the bonding material to set.

Preferably, the sleeve of rubber or plastics material is retained in place to form a partofthe mineral insulated electric cable termination but, in some circu mstances,the sleeve may be removed after the bonding material has set.

Where it is desired to provide for earth continuity, preferably prior to application ofthe sleeve of rubber or plastics material on to the cut-back end ofthe cable sheath,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 thatthe earth conductor extends lengthwise with respectto the bare length or lenghts of conductor and will be radially inwardly spaced from the wall of the sleeve, and the preformed button or disc has an aperture through which the earth conductorcan pass.

In all cases, before hardenable bonding material in a semi-liquid state is injected into the mould, preferably a moisture-resistant sealing material in a liquid orsemi-liquid state is applied to the exposed face of the compacted mineral insulation atthe cut-back end of the cable.

Preferred materials of which the sleeve may be made include silicone rubber and filled polytetrafluoroethylene; the preformed button or disc preferably is made of a ceramic material, filled polytetrafluoroethylene or bonded mica.

Preferred hardenable bonding materials include a modified ceramiccementand a preferred moistureresistant sealing material is the sealing material sold underthetrade nameWR 1107 resin.

The invention also includes a mineral insulated el ectric cable termination made by the improved method hereinbefore described.

The invention is further illustrated by a description, by way of example, ofthe preferred method of terminating a mineral insulated electriccablewith reference to the accompanying drawing which shows a sectional side view of a mineral insulated electric cable termination made by the method.

Referring to the drawing, the metal cable sheath 4 of a mineral insulated electric cable 1 is cut-back and the exposed mineral insulation 3 is removed tore- veal bare lengths2 of conductor. An open-ended sleeve 5 of plastics material having at one end an internal diameter slightly less than the external diame ter ofthe cable sheath 4 is applied to the cut-back end ofthe sheath in such a way that the sleeve tightlyfits the sheath and protrudes beyond the cut-back end of the cable 1 to form a mould into which the cut-back end of the cable extends and from which the bare lengths 2 of the conductor protrudes.Hardenable electrically insulating bonding material 6 in a semiliquid state is injected into the open end of the mould so formed to substantially fill the mould and a preformed button 7 of electrically insulating material having apertures therethrough for the bare lengths 2 of conductor is threaded over the bare lengths of conductor. The button 7 has a diameter slightly greaterthan the internal diameter of the sleeve at the end of the sleeve remote from the cut-back end ofthe cable 1 and has, on its outer face, tubular extensions 8 in axial alig nment with the apertu res throug h which the bare lengths 2 of conductor pass. The button 7 is slid into the open end of the mould and towards the end face of the cable 1 until bonding material 6 flows into and substantially fills the space between each bare length 2 of conductor and the boundary wall of the aperture in the button and of tubular extension 8 through which it passes. The bonding material 6 is then permitted to set.

Claims

1. A method ofterminating a mineral insulated electric cable which comprises cutting backthe cable sheath and removing the exposed mineral insulation to reveal a bare length of the or each conductor; applying to the cut-back end of the cable sheath an open ended sleeve of rubber or plastics material in such a way that the sleeve tightly fits the cable sheath and protrudes beyond the cut-back end ofthe cable to form a mould from which a bare length of the or each conductor protrudes; injecting a hardenable electrically insulating bonding material in a semiliquid state into the open end ofthe mould so formed; threading over the bare length ofthe or each conductor a preformed button ordiscofelectrically insulating material which has an aperture of aper turestherethrough for the bare length or lengths of conductor, which button or disc has a diametersubstantially equal to orslightly greaterthan the internal diameter of the sleeve; sliding the preformed button or disc into the open end of the mould and towards the end face of the cable until bonding material flows into and substantially fills the space between the or each bare length of conductor and the boundary wall of the aperture in the button or disc through which it passes; and permitting or causing the bonding material to set.

2. A method as claimed in Claim 1, wherein the sleeve is removed afterthe bonding material has set.

3. A method as claimed in Claim 1 or 2, wherein, priorto application of the sleeve of rubber or plastics material on to the cut-back end ofthe cable sheath, a collar of metal or metal alloy having an elongate electric earth conductor permanentiy connected thereto is clipped or otherwise electrically connected around the cut-back end of the cable sheath so that the earth conductor extends lengthwise with respect to the bare length or lengths of conductor and will be radially inwardly spaced from the wall ofthe sleeve, and the preformed button ordisc has an aperture through which the earth conductor can pass.

4. A method as claimed in any one of the preceding Claims, wherein, before hardenable bonding material in a semi-liquid state is injected into the mould, a moisture resistant sealing material in a liquid or semi-liquid state is applied to the exposed face of the compacted mineral insulation at the cutback end of the cable.

5. A method as claimed in any one of the preceding Claims, wherein the sleeve is made of silicone rubber or filled polytetrafluoroethylene.

6. A method as claimed in any one of the preceding Claims, wherein the preformed button or disc is made of a ceramic material, filled polytetrafluoroethylene or bounded mica.

7. A method as claimed in any one of the preceding Claims, wherein the hardenable bonding material is a modified ceramic cement.

8. A method of terminating a mineral insulated electric cable substantially as hereinbefore described with reference to the accompanying drawing.

9. A mineral insulated electric cable termination made by the method claimed in any one ofthe preceding Claims.