GB2227617A

GB2227617A - Termination of mineral insulated electric cable

Info

Publication number: GB2227617A
Application number: GB9001469A
Authority: GB
Inventors: Harry Baker
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1989-01-26
Filing date: 1990-01-23
Publication date: 1990-08-01
Anticipated expiration: 2010-01-23
Also published as: GB8901670D0; GB2227617B; GB9001469D0

Abstract

In a mineral insulated electric cable termination, a cut-back end of the cable 1 is sealed by a preformed insulating caps which comprises a tube 6 open at one end and closed at the other end by an end wall 7 integral with the tube and which is applied over the cut-back end of the cable sheath 4. The internal surface 8 of the tube 6 is roughened and effects a circumferentially continuous, fluid-tight resilient grip around the cable sheath 4. The end wall 7 of the tube 6 has at least one throughbore 9 and, integral with and extending lengthwise outwardly from the end wall, a flexible sleeve or flexible sleeves 10 in axial alignment with the throughbore or throughbores. The internal surface 11 of the or each sleeve 10 is roughened and effects a circumferentially continuous, fluid-tight resilient grip around the or a cable conductor 2 passing therethrough. <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.

As an additional precaution against water penetration along the compacted mineral insulated powder of a mineral insulated cable, it has been proposed to incorporate in the mineral insulating powder throughout the length of the cable a hydrophobic substance such as a silicone.

By rendering the compacted mineral insulating powder of a mineral insulated cable resistant to water penetration by the incorporation in the powder of a hydrophobic substance, the necessity to terminate such a cable by a sealing pot which requires skill and time to provide an effective waterproof termination may no longer apply and it is an object of the present invention to provide an improved mineral insulated cable termination which is simple and inexpensive and can be readily assembled by semi-skilled or unskilled personnel and which is especially, but not exclusively, applicable to a mineral insulated cable whose compacted mineral insulating powder incorporates a hydrophobic substance.

According to the invention, in the improved mineral insulated cable termination, a cut-back end of the cable is sealed by a preformed cap of electrically insulating material which comprises a tube open at one end and closed at the other end by an end wall integral with the tube and which is applied over the cut-back end of the cable sheath, the internal surface of the tube being serrated, knurled or otherwise roughened and effecting a circumferentially continuous, substantially fluid-tight resilient grip around the cable sheath and the end wall having at least one throughbore and, integral with and extending lengthwise outwardly from the end wall, a flexible sleeve or two or more flexible sleeves in substantially axial alignment with the throughbore or throughbores, the internal surface of the or each sleeve being serrated, knurled or otherwise roughened and effecting a circumferentially continuous, substantially fluid-tight resilient grip around the or a conductor passing therethrough.

By virtue of the fact that the internal surface of the tube and the internal surface of the or each flexible sleeve is serrated, knurled or otherwise roughened, each internal surface has a multiplicity of discrete areas of tight contact with the cable sheath or conductor, as the case may be, resiliently gripped by the tube or flexible sleeve with the result that risk of undesirable lengthwise movement of the preformed cap with respect to the cable and its conductor/s is substantially reduced.

The internal surface of the or each throughbore in the end wall of the cap may also be serrated, knurled or otherwise roughened and effect a circumferentially continuous, substantially fluid-tight resilient grip around the or a conductor passing therethrough.

Over at least a part of the length of the tube nearer its open end, the internal surface of the tube may have a plurality of axially spaced circumferentially continuous grooves which define between adjacent grooves axially spaced circumferentially continuous walls having roughened internal surfaces and effecting a plurality of substantially fluid-tight circumferentially continuous resilient seals axially spaced along the length of cable sheath surrounded by the tube.

The or each flexible sleeve extending lengthwise from the end wall of the cap may also have in its internal surface a plurality of axially spaced circumferentially continuous grooves which define between adjacent grooves axially spaced circumferentially continuous walls having roughened internal surfaces and effecting a plurality of substantially fluid-tight circumferentially continuous resilient seals axially spaced along the length of the conductor passing through the sleeve.

Any space between the end wall of the cap and the end face of the cable may be substantially filled with a moisture resistant sealing compound which may either be in the form of a grease or in the form of a hardened material, such as a thermoset resin.

Additionally, or alternatively, between the end wall of the cap and the end face of the cable may be housed a button of ceramic or vitreous material which has a throughbore or throughbores in axial alignment with the throughbore or throughbores in the end wall for maintaining conductor separation when there is more than one conductor and for assisting in threading of the conductor or conductors through the throughbore or throughbores in the end wall of the cap. The button may have integral with and upstanding lengthwise and outwardly from the button a rigid sleeve or two or more rigid sleeves which is or each of which is a tight fit in the or a throughbore in the end wall of the cap and which or each of which effects a circumferentially continuous tight fit with the or a conductor of the cable.The or each rigid sleeve upstanding lengthwise and outwardly from the button may protrude from the or a throughbore in the end wall of the cap and into the bore of the flexible sleeve associated with said throughbore, which flexible sleeve has a stepped bore and, in addition to effecting a circumferentially continuous, substantially fluid-tight resilient grip around the or a conductor passing therethrough, also effects a circumferentially continuous, substantially fluid-tight resilient grip around said protruding part of the rigid sleeve of the button.

The cap preferably is made of moulded rubber or rubber-like material or of moulded resilient plastics material.

The invention is further illustrated by a description, by way of an example, of two preferred forms of mineral insulated cable termination with reference to the accompanying drawings in which: Figure 1 is a side view, partly in section and partly in elevation, of a first preferred form of mineral insulated cable termination, and Figure 2 is a similar view of a second preferred form of mineral insulated cable termination.

Referring to Figure 1, the first preferred form of mineral insulated cable termination comprises a mineral insulated electric cable 1 which comprises two conductors 2 insulated from each other and from a surrounding metal sheath 4 by compacted magnesium oxide (not shown) impregnated throughout the length of the cable with liquid dimethylpolysiloxane. One end of the cable 1, cut-back to expose lengths of the conductors 2, is sealed by a preformed cap 5 of silicone rubber which comprises a tube 6 open at one end and closed at the other end by an end wall 7 integral with the tube. The open end of the tube 6 is applied over the cut-back end of the cable sheath 4 so that the end wall 7 is spaced from the end face of the cable, the internal surface 8 of the tube being roughened and effecting a circumferentially continuous, fluid-tight resilient grip around the cable sheath. The end wall 7 has two throughbores 9 and, integral with and extending lengthwise outwardly from the end wall, two flexible sleeves 10 in axial alignment with the throughbores, the conductors 2 passing through the throughbores and associated flexible sleeves. The internal surface 11 of each flexible sleeve 2 is roughened and effects a circumferentially continuous, fluid-tight resilient grip around the conductor 2 passing therethrough. The space between the end wall 7 of the preformed cap 5 and the end face of the cable is filled with a moisture resistant sealing compound which may either be in the form of a grease or in the form of a thermoset resin.

The second preferred form of mineral insulated insulated cable termination shown in Figure 2 has the same components as the first preferred form of mineral insulated cable termination shown in Figure 1 and corresponding components have been given the same numerical reference. However, in the second preferred form of mineral insulated cable termination shown in Figure 2, the internal surface 8 of the tube 6 of the preformed cap 5 has over that part of its length surrounding the cable sheath 4 a plurality of axially spaced circumferentially continuous grooves 16 which define between adjacent grooves axially spaced circumferentially continuous walls having roughened internal surfaces and effecting a plurality of fluid-tight circumferentially continuous resilient seals axially spaced along the length of cable sheath surrounded by the tube. The roughened internal surface 11 of each flexible sleeve 10 has a plurality of axially spaced circumferentially continuous grooves 18 which define between adjacent grooves axially spaced circumferentially continuous walls having roughened internal surfaces and effecting a plurality of fluid-tight circumferentially continuous resilient seals axially spaced along the length of the conductor 2 passing through the sleeve.

The improved mineral insulated cable termination of the present invention is simple and inexpensive and can be readily assembled by semi-skilled or unskilled personnel. It is especially suitable for use with a mineral insulated cable whose compacted mineral insulating powder incorporates a hydrophobic substance.

Claims

1. A mineral insulated electric cable termination wherein a cut-back end of the cable is sealed by a preformed cap of electrically insulating material which comprises a tube open at one end and closed at the other end by an end wall integral with the tube and which is applied over the cut-back end of the cable sheath, the internal surface of the tube being roughened and effecting a circumferentially continuous, substantially fluid-tight resilient grip around the cable sheath and the end wall having at least one throughbore and, integral with and extending lengthwise outwardly from the end wall, a flexible sleeve or two or more flexible sleeves in substantially axial alignment with the throughbore or throughbores, the internal surface of the or each sleeve being roughened and effecting a circumferentially continuous, substantially fluid-tight resilient grip around the or a conductor passing therethrough.

2. A mineral insulated electric cable termination as claimed in Claim 1, wherein, over at least a part of the length of the tube nearer its open end, the internal surface of the tube has a plurality of axially spaced circumferentially continuous grooves which define between adjacent grooves axially spaced circumferentially continuous walls having roughened internal surfaces and effecting a plurality of substantially fluid-tight circumferentially continuous resilient seals axially spaced along the length of cable sheath surrounded by the tube.

3. A mineral insulated electric cable termination as claimed in Claim 1 or 2, wherein the or each flexible sleeve extending lengthwise from the end wall of the cap has in its internal surface a plurality of axially spaced circumferentially continuous grooves which define between adjacent grooves axially spaced circumferentially continuous walls having roughened internal surfaces and effecting a plurality of substantially fluid-tight circumferentially continuous resilient seals axially spaced along the length of the conductor passing through the sleeve.

4. A mineral insulated electric cable termination as claimed in any one of the preceding Claims, wherein any space between the end wall of the cap and the end face of the cable is substantially filled with a moistureresistant sealing compound.

5. A mineral insulated electric cable termination as claimed in Claim 4, wherein the moisture resistant sealing compound is in the form of a grease.

6. A mineral insulated electric cable termination as claimed in Claim 4, wherein the moisture resistant sealing compound is in the form of a hardened material.

7. A mineral insulated electric cable termination as claimed in any one of the preceding Claims, wherein, between the end wall of the cap and the end face of the cable, is housed a button of ceramic or vitreous material which has a throughbore or throughbores in axial alignment with the throughbore or throughbores in the end wall.

8. A mineral insulated electric cable termination as claimed in Claim 7, wherein the button has integral with and upstanding lengthwise and outwardly from the button a rigid sleeve or two or more rigid sleeves which is or each of which is a tight fit in the or a throughbore in the end wall of the cap and which or each of which effects a circumferentially continuous tight fit with the or a conductor of the cable passing therethrough.

9. A mineral insulated electric cable termination as claimed in Claim 8, wherein the or each rigid sleeve upstanding lengthwise and outwardly from the button protrudes from the or a throughbore in the end wall of the cap and into the bore of the flexible sleeve associated with said throughbore, which flexible sleeve has a stepped bore and, in addition to effecting a circumferentially continuous, substantially fluid-tight resilient grip around the or a conductor passing therethrough, also effects a circumferentially continuous, substantially fluid-tight resilient grip around said protruding part of the rigid sleeve of the button.

10. A mineral insulated electric cable termination as claimed in any one of Claims 1 to 7, wherein the internal surface of the or each throughbore in the end wall of the cap is roughened and effects a circumferentially continuous, substantially fluid-tight resilient grip around the or a conductor passing therethrough.

11. A mineral insulated electric cable termination as claimed in any one of the preceding Claims1 wherein the cap is made of moulded rubber or rubber-like material or of moulded resilient plastics material.

12. A mineral insulated electric cable termination substantially as hereinbefore described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawings.