GB2231735A

GB2231735A - Termination of mineral insulated electric cable

Info

Publication number: GB2231735A
Application number: GB9008174A
Authority: GB
Inventors: Alan Postill; Tobias Furman
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1988-02-15
Filing date: 1990-04-11
Publication date: 1990-11-21
Also published as: GB8803449D0; GB2215921B; GB2215921A; GB9004774D0; GB8903374D0; GB9008174D0

Abstract

A mineral insulated electric cable 20 has an end terminated by a sealing pot 1 comprising a tube 2 of flexible electrically insulating material which at one end of the tube fits over the cut-back sheath 22 of the cable and which at the other end of the tube is closed by an end wall 3 integral with the tube and spaced from the end face of the cable and by a button 4 of ceramic or vitreous insulating material secured within the closed end of the tube. Each cable conductor 21 passes through axially aligned throughbores in the button 4 and end wall 3. Integral with and extending lengthwise and outwardly from the end wall 3 are flexible sleeves 9, the bore 10 of each sleeve being in axial alignment with the axially aligned throughbores of the end wall 3 and button 4. The space within the pot 1 between the end face of the cable 20 and the end wall 3 is filled with waterproof electrically insulating sealing compound. The sealing compound may be a hardened 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 an 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 a mineral insulated electric cable having an end terminated by an improved sealing pot which is simple and inexpensive and readily secured to the end of the cable by semi-skilled or unskilled personnel.

According to the invention, a mineral insulated electric cable has an end terminated by a sealing pot comprising a tube of flexible electrically insulating material which at one end of the tube fits over the cut-back sheath of the cable, which at the other end of the tube is closed by an end wall integral with the tube and spaced from the end face of the cable and by a button of ceramic or vitreous insulating material secured within said closed end of the tube, the or each conductor of the cable passing through axially aligned throughbores in the button and end wall, and 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 said axially aligned throughbores and surrounding the conductor or conductors protruding from said end wall, the space within the pot between the end face of the cable and said end wall being filled with waterproof electrically insulating sealing compound.

Preferably, the button of ceramic or vitreous electrically insulating material and/or the tube of the sealing pot has means positively preventing movement of the button with respect to the tube in a direction away from said closed 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.

Over at least a part of the length of the or each throughbore of the button remote from the closed end of the tube, preferably the internal diameter of the throughbore increases smoothly in a direction towards the end face of the cable 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 limiting the extent to which the tube is fitted over the cut-back sheath of the cable. Between the button and the end of the tube remote from its closed end, the wall of the tube may have an inwardly extending circumferential shoulder which constitues said limiting means and abuts the end face of the cut-back sheath of the cable but, preferably, the end face of the button remote from the closed end of the tube has integral with and upstanding from the end face at least one projection which constitutes said limiting means and which abuts the end face 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.

The space within the sealing pot between the end face of the cable and the end wall of the pot is filled with any suitable waterproof electrically insulating sealing compound. In some circumstances, the sealing compound may be a hardened adhesive. When the terminated mineral insulated electric cable is required to pass tests with regard to its resistance to fire, preferably the sealing compound comprises a carrier liquid containing a filler, an anti-oxidant and a copper inhibitor.

The tube of the sealing pot is permanently secured to the cut-back cable sheath by any convenient technique. In one suitable technique, 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 of the pot may have in its outer surface, at or near its end remote from said end wall, a peripherally continuous groove in which the metal ring or clip is disposed.Where the sealing compound is a hardened adhesive, the tube of the sealing pot may have such an internal diameter that it is radially spaced from the cable sheath so that hardened adhesive fills the space between the tube and the cable sheath and permanently secures the tube to the cable.

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

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

Referring to the drawing, the mineral insulated electric cable 20 has four conductors 21 electrically insulated from a surrounding copper sheath 22 by compacted mineral insulating powder 23 and the sheath is cut-back so that the cable conductors protrude from the end face of the cable. The cut-back end of the cable 20 is terminated by a sealing 1 pot comprising a flexible tube 2 of silicone rubber which at one end of the tube fits over the cut-back cable sheath 22 and which at the other end of the tube is closed by an end wall 3 integral with the tube and spaced from the end face of the cable 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 tube in which it is permanently secured by means of an appropriate cement.The protruding cable conductors 21 pass through four uniformly circumferentially spaced throughbores 5 in the button 4, through axially aligned bores in the end wall 3 and through axially aligned bores 10 of four flexible sleeves 9 integral with and extending lengthwise from the external surface of the end wall. Over a part of the length of each throughbore 5 of the button 4 nearer the end face of the cable 20, the internal diameter of the throughbore increases smoothly in a direction towards the end face of the cable to form, in effect, a throat 6 which facilitates initial introduction of the associated cable conductor 21.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 is a tight fit on the conductor 21 passing therethrough and has effected a wiping action on the conductor as the conductor was fed through the sleeve. The end face of the button 4 nearer the end face of the cable 20 has integral with and upstanding from a central proportion of the end face a single projection 8 which abuts the end face of the cable and limits the extent to which the tube 2 is fitted over the cut-back cable sheath 22. As will be seen on referring to the drawing, the tube 2 is of such an internal diameter that it is radially spaced from the outer surface of the cable sheath 22, the end of the tube remote from the end wall 3 having a circumferential wall 12 which extends radially inwardly into the bore of the tube and effects a tight fit with the cable sheath. The space within the pot between the end face of the cable and the end wall 3 of the tube 2, and the space between the cable sheath 22 and the tube 2, are filled with a hardened waterproof electrically insulating adhesive which permanently secures the sealing pot to the cut-back end of the cable.

Claims

CLAIMS:

1. A mineral insulated electric cable having an end terminated by a sealing pot comprising a tube of flexible electrically insulating material which at one end of the tube fits over the cut back sheath of the cable, which at the other end of the tube is closed by an end wall integral with the tube and spaced from the end face of the cable and by a button of ceramic or vitreous insulating material secured within said closed end of the tube, the or each conductor of the cable passing through axially aligned throughbores in the button and end wall, and 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 said axially aligned throughbores and surrounding the conductor or conductors protruding from said end wall, the space within the pot between the end face of the cable and said end wall being filled with waterproof electrically insulating sealing compound.

2. A terminated mineral insulated electric cable 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 positively preventing movement of the button with respect to the tube in a direction away from said closed end of the tube.

3. A terminated mineral insulated electric cable 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 terminated mineral insulated electric cable as claimed in Claim 3, wherein the button is permanently secured in the peripherally continuous groove by an appropriate cement.

5. A terminated mineral insulated electric cable 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 remote from the closed end of the tube, the internal diameter of the throughbore increases smoothly in a direction towards the end face of the cable to form, in effect, a throat.

6. A terminated mineral insulated electric cable as claimed in any one of the preceding Claims, wherein the tube and/or button of ceramic or vitreous electrically insulating material has means limiting the extent to which the tube is fitted over the cut-back sheath of the cable.

7. A terminated mineral insulated electric cable as claimed in Claim 6, wherein, between the button and the end of the tube remote from its closed end, the wall of the tube has an inwardly extending circumferential shoulder which constitutes said limiting means and abuts the end face of the cut back sheath of the cable.

8. A terminated mineral insulated electric cable as claimed in Claim 6, wherein the end face of the button remote from the closed end of the tube has integral with and upstanding from the end face at least one projection which constitutes said limiting means and which abuts the end face of the cable.

9. A terminated mineral insulated electric cable 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 terminated mineral insulated electric cable 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 is a tight fit on the or a conductor of the cable passing therethrough.

11. A terminated mineral insulated electric cable as claimed in any one of the preceding Claims, wherein the waterproof electrically insulating sealing compound comprises a hardened adhesive.

12. A terminated mineral insulated electric cable as claimed in Claim 11, wherein the tube of the sealing pot has an internal diameter such that the tube is radially spaced from the cable sheath so that hardened adhesive fills the space between the tube and the cable sheath and permanently secures the tube to the cable.

13. A terminated mineral insulated electric cable as claimed in any one of Claims 1 to 11, wherein 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.

14. A terminated mineral insulated electric cable as claimed in any one of Claims 1 to 11, wherein the part of the tube fitted over the cut-back sheath is secured to the sheath by an adjustable circumferentially continuous clip.

15. A terminated mineral insulated electric cable as claimed in Claim 13 or 14, wherein the metal ring or clip is disposed in a peripherally continuous groove in the outer surface of the tube at or near its end remote from said end wall.

16. A terminated mineral insulated electric cable 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.

17. A terminated mineral insulated electric cable as claimed in any one of the preceding Claims, wherein the button is made of alumina,

18. A terminated mineral insulated electric cable substantially as hereinbefore described with reference to and as shown in the accompanying drawing.