GB2215921A

GB2215921A - Pre-assembled sealing pot for termination of mineral insulated electric cable

Info

Publication number: GB2215921A
Application number: GB8903374A
Authority: GB
Inventors: Alan Postill; Tobias Furman
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1988-02-15
Filing date: 1989-02-15
Publication date: 1989-09-27
Anticipated expiration: 2009-02-15
Also published as: GB2231735A; GB9004774D0; GB9008174D0; GB8903374D0; GB8803449D0; GB2215921B

Abstract

A pre-assembled sealing pot 1 which can be readily secured to a cut-back end of a mineral insulated electric cable 20 comprises a tube 2 of flexible electrically insulating material which at one end is closed by an end wall 3 and by a button 4 of ceramic or vitreous insulating material secured within the tube and having throughbores 5 for passage of the conductors 21 of the cable. At the other end 7, the tube 2 is open and fits over the cut-back sheath 22 of the cable 20. Integral sleeves 9 of the end wall 3 have bores 10 in axial alignment with respective throughbores 5 of the button 4. Between its closed and open ends, the tube 2 contains waterproof electrically insulating sealing compound 14 in a semi-liquid or jelly-like state. The sealing compound may be a hardenable adhesive or it may comprise a carrier liquid containing a filler, an anti-oxidant and a copper inhibitor. <IMAGE>

Description

TERMINATION OF MINERAL INSULATED ELECTRIC CABLE This invention relates to the termination of mineral insulated electric cables, that is to say cables of the 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 particularly vulnerable to water penetration and it is the general practice to protect jn end of a mineral insulated 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. Assembly of a sealing pot on an end of a mineral insulated electric cable 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 of the overall cost of the installation.

It is an object of the present invention to provide an improved pre-assembled sealing pot for sealing a cut-back end of a mineral insulated electric cable, which pre-assembled sealing pot is simple and inexpensive and can be readily secured to an end of a mineral insulated electric cable by semi-skilled or unskilled personnel.

According to the invention, the improved pre-assembled sealing pot comprises a tube of flexible electrically insulating material which at one end is closed by an end wall integral with the tube and by a button of ceramic or vitreous insulating material secured within said closed end of the tube and having a throughbore or two or more throughbores for passage of the conductor or conductors of the cable, which has integral with and extending lengthwise and outwardly.

from said end wall a flexible sleeve or two or more flexible sleeves in substantially axial alignment with the throughbore or throughbores of the button, which at the other end is open and of such an internal diameter that said end of the tube will fit over the cut-back sheath of the cable, and which, between its closed and open ends, contains waterproof electrically insulating sealing compound in a semi-liquid or jelly-like state.

When an end of a mineral insulated electric cable is to be terminated by the improved pre-assembled sealing pot, the cable sheath is cut back and the exposed mineral insulation is removed to reveal a bare length of the or each conductor. The bare length of the or each conductor is introduced into the open end of the tube of the pre-assembled sealing pot and is passed through the or a throughbore of the button so that the conductor protrudes from the aligned flexible sleeve and the open end of the tube is fitted over and permanently secured to the cut-back sheath of the cable.

Any suitable waterproof electrically insulating sealing compound may be employed in the improved pre-assembled sealing pot. In some circumstances, the sealing compound may be in a hardenable adhesive. When the sealing pot is required to pass the same tests with regard to its resistance to fire as a mineral insulated electric cable to which it is secured, preferably the sealing compound comprises a carrier liquid containing a filler, an anti-oxidant and a copper inhibitor.

The tube of the pre-assembled sealing pot may be permanently secured to the cut-back cable sheath using any convenient technique. In one suitable technique that may be employed, the part of the tube fitted over the cut-back sheath is secured to the sheath by a circumferentially continuous ring of metal or metal alloy which surrounds the tube and which is so swaged or otherwise radially inwardly distorted throughout its periphery that the tube makes a sound mechanical connection with the sheath or by an adjustable circumferentially continuous clip, e.g. a "Jubilee" clip. The tube may have in its outer surface, at or near its open end, a peripherally continuous groove for reception of the metal ring or clip.Where the sealing compound is a hardenable adhesive, the tube of the sealing pot may have such an internal diameter that, when the pot is applied two the cut-back end of a mineral insulated electric cable, it is radially spaced from the cable sheath so that hardenable adhesive will flow into the space between the tube and the cable sheath and, when it hardens, will permanently secure the tube to the cable.

Where the cable has two or more conductors, by virtue of the fact that the sleeves integral with the tube of the pre-assembled sealing pot are flexible, it is a simple matter to ensure that the conductors of the cable protruding from the sealing pot are transversely spaced apart by an adequate distance.

Preferably, the button of ceramic or vitreous electrically insulating material and/or the tube of the improved pre-assembled sealing pot has means for positively preventing movement of the button with respect to the tube in a direction towards the open end of the tube. In a preferred embodiment, the internal surface of the wall of the tube near the end wall of the tube has a peripherally continuous groove in which the button fits and within which it may be permanently secured by an appropriate cement.

To facilitate the introduction of the or each conductor of a mineral insulated cable into the or a throughbore of the button, over at least a part of the length of the or each throughbore nearer the open end of the tube, the internal diameter of the throughbore increases smoothly in a direction towards the open end of the tube to form, in effect, a throat into which a conductor can be readily guided.

Preferably, the tube and/or the button of ceramic or vitreous electrically insulating material has means for limiting the extent. to which the tube can be fitted over the cut-back sheath of a cable. Between the button and the open end of the tube, the wall of the tube may have an inwardly extending circumferentially continuous shoulder against which the end face of the cut-back sheath of a mineral insulated cable will abut but, preferably, the end face of the button nearer the open end of the tube has integral with and upstanding from the end face at least one projection against which the end face of the cable will abut when the cable is introduced into the tube to limit the extent to which the tube can be fitted over the end of the cable.Where the button has four throughbores uniformly circumferentially spaced around its longitudinal axis, the button preferably has a single projection upstanding from a central portion of the button.

Preferably, the button is made of a ceramic material such as alumina. The tube is preferably made of rubber or synthetic rubber, silicone rubber being preferred.

If desired, the open end of the tube of the improved pre-assembled sealing pot may be temporarily sealed by a thin disc of paper or plastics material which is so sealed to the open end of the tube that it can be readily peeled away when it is desired to use the pre-assembled sealing pot to terminate a mineral insulated electric cable.

The invention also includes a mineral insulated electric cable having an end terminated by the improved pre-assembled sealing pot hereinbefore described.

The invention is further illustrated by a description, by way of example, of a preferred pre-assembled sealing pot secured to an end of a mineral insulated electric cable with reference to the accompanying drawing which is a sectional side view of the preferred sealing pot secured to an end ofthe cable.

Referring to the drawing, the preferred pre-assembled sealing pot 1 comprises a flexible tube 2 of silicone rubber which at one end is closed by an end wall 3 integral with the tube and by a button 4 of alumina which abuts the end wall and which engages in a circumferentially continuous groove 13 in the internal surface of the wall in which it is permanently secured by means of an appropriate cement. The other end 7 of the tube 2 is open. The button 4 has four uniformly circumferentially spaced throughbores 5 for passage of the four conductors 21 of a mineral insulated electric cable 20. Over a part of the length of each throughbore 5 of the button 4 nearer the open end 7 of the tube 2, the internal diameter of the throughbore increases smoothly in a direction towards the open end of the tube to form, in effect, a throat 6.

Integral with and extending lengthwise from the external surface of the end wall 3 of the tube 2 are four flexible sleeves 9 whose bores 10 are in axial alignment with the throughbores 5 of the button 4. Each flexible sleeve 9 has, at its end remote from the button 4, a circumferential wall 11 which extends radially inwardly into the bore of the sleeve, the wall being of such an internal diameter that it will effect a wiping action on a conductor 21 of the cable 20 when the conductor is fed through the sleeve.

The end face of the button 4 nearer the open end 7 of the tube 2 has integral with and upstanding from a central portion of the end face a single projection 8 which, when the pre-assembled sealing pot 1 is applied over the cut-back end of a mineral insulated electric cable, will abut the end face of the cable 20 to limit the extent to which the tube 2 can be fitted over the cut-back sheath 22 of the cable.

The tube 2, between the end closed by the end wall 3 and button 4 and the open end 7, contains waterproof electrically insulating hardenable adhesive 14 in a semi-liquid state. The open end 7 of the tube 2 may be temporarily sealed by a thin disc of paper or plastics material (not shown) which is so sealed to the open end of the tube that it can be readily peeled away.

When it is desired to apply the pre-assembled sealing pot 1 to the cut-back end of a mineral insulated electric cable 20 having four conductors 21 electrically insulated from a surrounding copper sheath 22 by compacted mineral insulated powder 23, the thin disc of paper or plastics material (if present) temporarily closing the open end 7 of the tube 2 is removed and the protruding cable conductors 21 are inserted into the open end of the tube and into the throats 6 of the throughbores 5 of the button 4 and through the throughbores 10 of the sleeves 9, the radially inwardly extending circumferential wall 11 of each sleeve wiping any waterproof electrically insulating hardenable adhesive from the conductor as it passes therethrough.

Insertion of the cut-back end of the cable 20 into the tube 2 is continued until the projection 8 of the button 4 abuts the end face of the cable, hardenable adhesive in a semi-liquid state having been forced to flow into the space between the cable sheath 22 and the surrounding tube. The adhesive 14 is then permitted or caused to harden so that the sealing pot 1 is permanently secured to the cut-back end of the cable by, and is filled with, hardened waterproof electrically insulating adhesive.

The improved pre-assembled sealing pot of the present invention has several important advantages. It can be easily mass produced and can be readily secured to an end of a mineral insulated electric cable by semi-skilled or unskilled personnel. It is of slim, attractive appearance and can be pre-packed in a strip or bag of, say, cellophane, The flexible sleeves of the tube of the sealing pot provide for further sleeving and/or bending of the cable conductors, as required.

Claims

CLAIMS:

1. A pre-assembled sealing pot for sealing a cut-back end of a mineral insulated electric cable, which sealing pot comprises a tube of flexible electrically insulating material which at one end is closed by an end wall integral with the tube and by a button of ceramic or vitreous insulating material secured within said closed end of the tube and having a throughbore or two or more throughbores for passage of the conductor or conductors of the cable, which has integral with and extending lengthwise and outwardly from said end wall a flexible sleeve or two or more flexible sleeves in substantially axial alignment with the throughbore or throughbores of the button, which at the other end is open and of such an internal diameter that said end of the tube will fit over the cut-back sheath of the cable, and which, between its closed and open ends, contains waterproof electrically insulating sealing compound in a semi-liquid or jelly-like state.

2. A pre-assembled sealing pot as claimed in Claim 1, wherein the button of ceramic or vitreous electrically insulating material and/or the tube of the sealing pot has means for positively preventing movement of the button with respect to the tube in a direction towards the open end of the tube.

3. A pre-assembled sealing pot as claimed in Claim 2, wherein the internal surface of the wall of the tube near the end wall of the tube has a peripherally continuous groove in which the button fits.

4. A pre-assembled sealing pot as claimed in Claim 3, wherein the button is permanently secured in the peripherally continuous groove by an appropriate cement.

5. A pre-assembled sealing pot as claimed in any one of the preceding Claims, wherein, over at least a part of the length of the or each throughbore of the button nearer the open end of the tube, the internal diameter of the throughbore increases smoothly in a direction towards the open end of the tube to form, in effect, a throat.

6. A pre-assembled sealing pot as claimed in any one of the preceding Claims, wherein the tube and/or button of ceramic or vitreous electrically insulating material has means for limiting the extent to which the tube can be fitted over the cut-back sheath of a cable.

7. A pre-assembled sealing pot as claimed in Claim 6, wherein, between the button and the open end of the tube, the wall of the tube has an inwardly extending circumferentially shoulder which constitutes said limiting means and against which the end face of the cut-back sheath of a mineral insulated cable will abut.

8. A pre-assembled sealing pot as claimed in Claim 6, wherein the end face of the button nearer the open end of the tube has integral with and upstanding from the end face at least one projection which constitutes said limiting means and against which the end face of the cable will abut when the cable is introduced into the tube.

9. A pre-assembled sealing pot as claimed in Claim 8 in which the button has four throughbores uniformly circumferentially spaced around its longitudinal axis, wherein the button has a single projection upstanding from a central portion of the button.

10. A pre-assembled sealing pot as claimed in any one of the preceding Claims, wherein the or each flexible sleeve of the tube has, at or near its end remote from the button, a circumferential wall which extends radially into the bore of the sleeve, the wall being of such an internal diameter that it will effect a wiping action on the or a conductor of the cable when the conductor is fed therethrough.

11. A pre-assembled sealing pot as claimed in any one of the preceding Claims, wherein the waterproof electrically insulating sealing compound comprises a hardenable adhesive.

12. A pre-assembled sealing pot as claimed in any one of Claims 1 to 10, wherein the waterproof electrically insulating sealing compound comprises a carrier liquid containing a filler, an anti-oxidant and a copper inhibitor.

13. A pre-assembled sealing pot as claimed in any one of the preceding Claims, wherein the button is made of alumina.

14. A pre-assembled sealing pot as claimed in any one of the preceding Claims, wherein the open end of the tube of the sealing pot is temporarily sealed by a thin disc of paper or plastics material which is so sealed to the open end of the tube that it can be readily peeled away.

15. A pre-assembled sealing pot for sealing a cut-back end of a mineral insulated electric cable substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

16. A mineral insulated electric cable having an end terminated by the pre-assembled sealing pot as claimed in any one of the preceding Claims.