CA1047135A - Medium and high voltage electric cable with readily peelable outer screen - Google Patents
Medium and high voltage electric cable with readily peelable outer screenInfo
- Publication number
- CA1047135A CA1047135A CA240,500A CA240500A CA1047135A CA 1047135 A CA1047135 A CA 1047135A CA 240500 A CA240500 A CA 240500A CA 1047135 A CA1047135 A CA 1047135A
- Authority
- CA
- Canada
- Prior art keywords
- insulation
- screen
- ethylene
- semi
- conductive
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B13/00—Apparatus or processes specially adapted for manufacturing conductors or cables
- H01B13/22—Sheathing; Armouring; Screening; Applying other protective layers
- H01B13/24—Sheathing; Armouring; Screening; Applying other protective layers by extrusion
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B7/00—Insulated conductors or cables characterised by their form
- H01B7/02—Disposition of insulation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B9/00—Power cables
- H01B9/02—Power cables with screens or conductive layers, e.g. for avoiding large potential gradients
- H01B9/027—Power cables with screens or conductive layers, e.g. for avoiding large potential gradients composed of semi-conducting layers
Landscapes
- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- Organic Insulating Materials (AREA)
- Extrusion Moulding Of Plastics Or The Like (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Conductive Materials (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE: A medium to high voltage electric cable having a conductor surrounded by an inner semi-conductive screen which is surrounded by an insulating layer of a synthetic polymeric material which is cross-linkable by peroxide agents. The insulating layer is surrounded by a semi-conductive layer of a synthetic poly-meric material which is degradable by said agents and which contains a conductive filler. The insulating layer and the outer screen are extruded simultaneously on the inner screen and the peroxide agent in the insulating layer de-grades the inner surface portion of the outer screen thereby enabling the peeling of the outer screen from the insulating layer with small forces and without damage to the insulating layer. The outer screen may be surrounded by a metal layer and a thermoplastic sheath.
Description
~LQ~7~3~i The present invention relates to a medium and high voltage electric cable, the core for which comprises a conductor covered with an inner semi-conductive screen which, in turn, is covered on its turn by an extruded in-sulation, the latter being surrounded by an outer semi-conductive screen which can be easily removed or peeled away from the insulation.
It is known that, in electric cables for medium and high voltages having a layer of insulation extruded about the conductor, the first screen, or inner screen, can be constituted by a semi-conductive tape wound around the conductor or by semi-conductive material extruded or applied in any other suitable manner. The extruded insulation hereinbefore used is natural rubber or a syn-thetic resin material, but, preferably, it is of the latter type, for example, ethylene-propylene rubber or cross-linked polyethylene.
The second semi-conductive screen, or outer semi-conductive screen, preferably is extruded on the in-sulation, because, in order to prevent trapping air between the screen and the insulation, it is necessary for the two layers to adhere throughout their facing surfaces.
Of course, the degree of adhesion of the layer of insulation and the layer forming the outer screen de-pends on the materials used, but the adhesion per se is ensured by a simultaneous cross-linking or curing of the insulation and of the extruded semi-conductive screen~
The latter can, but not necessarily, be made of the same material as the insulation and contains an additional con-ductive filler, for example, carbon black. The curing or cross-linking takes place by the use of appropriate
It is known that, in electric cables for medium and high voltages having a layer of insulation extruded about the conductor, the first screen, or inner screen, can be constituted by a semi-conductive tape wound around the conductor or by semi-conductive material extruded or applied in any other suitable manner. The extruded insulation hereinbefore used is natural rubber or a syn-thetic resin material, but, preferably, it is of the latter type, for example, ethylene-propylene rubber or cross-linked polyethylene.
The second semi-conductive screen, or outer semi-conductive screen, preferably is extruded on the in-sulation, because, in order to prevent trapping air between the screen and the insulation, it is necessary for the two layers to adhere throughout their facing surfaces.
Of course, the degree of adhesion of the layer of insulation and the layer forming the outer screen de-pends on the materials used, but the adhesion per se is ensured by a simultaneous cross-linking or curing of the insulation and of the extruded semi-conductive screen~
The latter can, but not necessarily, be made of the same material as the insulation and contains an additional con-ductive filler, for example, carbon black. The curing or cross-linking takes place by the use of appropriate
- 2 - ~
~7~35 agents selected according to the material chosen for the two layers.
If, on one hand, the achievement of a perfect adhesion between the extruded insulation and the outer semi-conductive layer provides the cable with very good electric characteristics, the same pe:rfect adhesion creates, on the other hand, serious problems of a mechani-cal nature at the time of manufacture of the cable ends and of the application of the accessories thereto, namely, when it is necessary to remove or to peel away the outer semi-conductive screen layer from the insulation layer.
Various methods are known at the present time for carrying out said peeling, and in general, they can be divided into two types. One method comprises merely the cutting of the semi-conductive screen with special knives, known in the art, and in peeling it away from the insulating layer. In this case, the action of removal or peeling enables one to remove the semi-conductive screen, but such action also involves the peeling of a small portion of the layer of insulation, or leaves on the layer of insulation a certain thickness of screen which must then be appro-priately scraped off the layer of insulation.
The second peeling method, which can be used, for example, when the insulation is made of ethylene-propylene rubber and the outer screen is also made of ethylene-propy-lene rubber loaded with carbon black or made of cross-linked polyethylene loaded with carbon black, comprises using a heat source for heating the outer screen, which can therefore be removed in a sufficiently easy mannex, due to the higher mechanical properties, in hot conditlon, of the semi~conductive screen with respect to the insulation.
~7~3~
However, in this method, the underlying insulation can be easily damaged. In summary, the cores constructed in accordance with known practices have the disadvantage of being provided with a screen so strongly adherent to the insulation underlying it that the peeling operation is quite difficult and the screen cannot be peeled away without employing special means for carrying out the peeling operation.
The principal object of the present invention is the provision of an improved electric cable having an e~-truded insulation, in which the outer semi-conductive screen can be easily peeled away in such a way as not to compromise the integrity of the extruded insulation and not to require additional operations, such as, for example, the use of heat sources, in addition to the usual cutting and tearing off operations.
More specifically, the principal object of the present invention is to provide an improved electric cable for medium and high voltage use, in which the core, or each core thereof, comp~ises a conductor covered with an inner semi-conductive screen, an extruded insulation of synthetic resin material and an outer semi-conduct:ive screen, also extruded and made of synthetic resin material and cured simultaneously with the insulation, characterized in that said insulation is constituted by a compound based on synthetic polymeric material cross-linkable with peroxide agents and said outer semi-conductive screen is constituted by a compound based on synthetic polymeric material which is degraded in the presence of peroxides and to which a conductive ~iller is added.
Other objects and advantages of the present in-vention will be apparent to those skilled in the art from 7~1L35i the following detailed description thereof, which should be considered in conjunction with the accompanying drawing, the single figure of which illus-trates, in perspective, and with portions of the layers removed, a cable con-structed in accordance with the invention.
In accordance with this invention there is provided an electric cable comprising a conductor having a layer of insulation therearound and a semi-conductive screen around and in contact with said layer of insulation, said insulation comprising a synthetic polymeric material which is cross-linkable by peroxide agents and said screen comprising a synthetic, polymeric material which is degradable by peroxide in said agents, said polymeric material of said screen being a rubber selected from the group consisting of butyl rubber, halogenated butyl rubbers and blends of either butyl rubbers or halogenated butyl rubbers with a polymer selected from the group consisting of ethylene-propylene-diene terpolymers, ethylene-propylene copolymers and ethylene-vinyl-acetate whereby the surface of said screen adjacent to said insulation is degraded by said peroxide and permits said screen to be detached readily from said insulation without substantial damage to said insulation.
The drawing illustrates a single-core cable 10 which comprises a conductor 11 which may comprise a plurality of stranded wires, as shown, or a single wire. The conductor 11 is covered by a first, or inner, semi-con-ductor screen 12, which preferably is made of extruded material, for exam~le, synthetic resin material, but which could also be constituted by a helically wound semi-conductive tape, such as a textile tape treated with semi-conduc-tive material.
An insulating layer 13, hereinafter simply called "insulation" and made of a synthetic resin material, is applied tigh~ly on the inner semi-conductive screen 12. A semi-conductive layer 14, extruded on the insulation 13, forms the second, or outer, semi-conductive screen and is also made of a synthetic resin material.
A metallic coverin~ 15 and a thermoplastic sheath 16 complete the cable 10, but it will be evident to those skilled in the art that the layers around the screen 1~ may be different from those illustrated.
~7~35 agents selected according to the material chosen for the two layers.
If, on one hand, the achievement of a perfect adhesion between the extruded insulation and the outer semi-conductive layer provides the cable with very good electric characteristics, the same pe:rfect adhesion creates, on the other hand, serious problems of a mechani-cal nature at the time of manufacture of the cable ends and of the application of the accessories thereto, namely, when it is necessary to remove or to peel away the outer semi-conductive screen layer from the insulation layer.
Various methods are known at the present time for carrying out said peeling, and in general, they can be divided into two types. One method comprises merely the cutting of the semi-conductive screen with special knives, known in the art, and in peeling it away from the insulating layer. In this case, the action of removal or peeling enables one to remove the semi-conductive screen, but such action also involves the peeling of a small portion of the layer of insulation, or leaves on the layer of insulation a certain thickness of screen which must then be appro-priately scraped off the layer of insulation.
The second peeling method, which can be used, for example, when the insulation is made of ethylene-propylene rubber and the outer screen is also made of ethylene-propy-lene rubber loaded with carbon black or made of cross-linked polyethylene loaded with carbon black, comprises using a heat source for heating the outer screen, which can therefore be removed in a sufficiently easy mannex, due to the higher mechanical properties, in hot conditlon, of the semi~conductive screen with respect to the insulation.
~7~3~
However, in this method, the underlying insulation can be easily damaged. In summary, the cores constructed in accordance with known practices have the disadvantage of being provided with a screen so strongly adherent to the insulation underlying it that the peeling operation is quite difficult and the screen cannot be peeled away without employing special means for carrying out the peeling operation.
The principal object of the present invention is the provision of an improved electric cable having an e~-truded insulation, in which the outer semi-conductive screen can be easily peeled away in such a way as not to compromise the integrity of the extruded insulation and not to require additional operations, such as, for example, the use of heat sources, in addition to the usual cutting and tearing off operations.
More specifically, the principal object of the present invention is to provide an improved electric cable for medium and high voltage use, in which the core, or each core thereof, comp~ises a conductor covered with an inner semi-conductive screen, an extruded insulation of synthetic resin material and an outer semi-conduct:ive screen, also extruded and made of synthetic resin material and cured simultaneously with the insulation, characterized in that said insulation is constituted by a compound based on synthetic polymeric material cross-linkable with peroxide agents and said outer semi-conductive screen is constituted by a compound based on synthetic polymeric material which is degraded in the presence of peroxides and to which a conductive ~iller is added.
Other objects and advantages of the present in-vention will be apparent to those skilled in the art from 7~1L35i the following detailed description thereof, which should be considered in conjunction with the accompanying drawing, the single figure of which illus-trates, in perspective, and with portions of the layers removed, a cable con-structed in accordance with the invention.
In accordance with this invention there is provided an electric cable comprising a conductor having a layer of insulation therearound and a semi-conductive screen around and in contact with said layer of insulation, said insulation comprising a synthetic polymeric material which is cross-linkable by peroxide agents and said screen comprising a synthetic, polymeric material which is degradable by peroxide in said agents, said polymeric material of said screen being a rubber selected from the group consisting of butyl rubber, halogenated butyl rubbers and blends of either butyl rubbers or halogenated butyl rubbers with a polymer selected from the group consisting of ethylene-propylene-diene terpolymers, ethylene-propylene copolymers and ethylene-vinyl-acetate whereby the surface of said screen adjacent to said insulation is degraded by said peroxide and permits said screen to be detached readily from said insulation without substantial damage to said insulation.
The drawing illustrates a single-core cable 10 which comprises a conductor 11 which may comprise a plurality of stranded wires, as shown, or a single wire. The conductor 11 is covered by a first, or inner, semi-con-ductor screen 12, which preferably is made of extruded material, for exam~le, synthetic resin material, but which could also be constituted by a helically wound semi-conductive tape, such as a textile tape treated with semi-conduc-tive material.
An insulating layer 13, hereinafter simply called "insulation" and made of a synthetic resin material, is applied tigh~ly on the inner semi-conductive screen 12. A semi-conductive layer 14, extruded on the insulation 13, forms the second, or outer, semi-conductive screen and is also made of a synthetic resin material.
A metallic coverin~ 15 and a thermoplastic sheath 16 complete the cable 10, but it will be evident to those skilled in the art that the layers around the screen 1~ may be different from those illustrated.
3~
Moreover, the core, the expression "core" meaning herein the unit formed by the conductor ll, the first semi-conductive screen 12, the insula-tion 13 and the second semi-conductive screen 14, can form part of a single-core cable, as illustrated in the figure, or form part of a multi-core cable, such as a three-core cable.
The synthetic polymeric material constituting the -5a-1~
7~3S
insulation 13 is made of a compound, having appropriate electric characteristics, comprising polymeric materials which are cross-linkable with peroxide agents.
For the insulation 13, it is preferred to use an ethylene polymer, for example, cross-linked polyethylene, or a copolymer of ethylene with one or more comononers, desirably alkylene or vinyl ester comonomers, such as, ethylene-propylene, ethylene-vinyl-acetate, ethylene-ethyl-acrylate, ethylene-propylene-diene.
Any conventional agent for cross~linking ethylene polymers or copolymers which will degrade the inner surface of the semi-conductive layer extruded over the cross-linkable polymer may be used. Particularly suitable cross-linking agents are the organic peroxides, including the mono-functional or polyfunctional alkaryl and alkyl peroxides cross-linkin~ agents. Suitable monofunctional peroxides are those represented by the formula R - O - O - R' wherein R and R' may be the same or different and represents an alkyl or alkaryl radical such as a t-butyl or cumyl radical. Thus, useful monofunctional peroxides include:
dicumyl peroxide, t-butyl cumyl peroxide, di-tert hutyl peroxide. Suitable bifunctional peroxides may be repre-sented by the formula R" - O - O - X - O - O -~R"' wherein R" and R"' represent an alkyl radical preferably tert.-butyl and X is an alkyl, alkaryl or alkyne radical such as 1,4-diisopropyl benzene; 3,3,5-trimethylcyclo-hexane; 2,5-dimethylhexane: and 2,5-dimethylhexyne.
Useful bifunctional peroxides include: l,l-bis(tert.-butyl peroxy isopropyl) benzene; l,l-bis(tert.-butyl peroxy)-3,3, ~47~35 5-trimethyl cyclohexane; 2,5-dimethyl-2,5-bis~tert.-butyl peroxy) hexane and 2,5 dimethyl-2,5-bis~tert.-butyl peroxy) hexyne-3.
When the insulation 13 and the screen 14 are cured simultaneously, i.e., by extruding the material of the screen 14 over the insulation 13 before curing the latter and then curing, or polymerizing, both materials, the amount of the peroxide required to obtain the advantages of the invention is not critical and may, therefore, be the amount conventionally used in the cross-linking of the polymer of the insulation 13. For example, when mono-functional peroxides are used, two to five parts of such peroxides per hundred parts of rubber may be used, and when bifunctional peroxides are used, one-and-one-half to three parts per hundred parts of rubber may be used.
Conventionally, fillers selected as a function of the various requirements desired for the finished product, such as a greater dielectric strength, smaller dielectric losses, greater insulation constants, resistance to partial discharges, etc., are added at times to the compound con-stituting the insulation 13. Thus, to a compound ~ormed of an ethylene copolymer with one or more comonomers, calcium carbonate, calcined kaolin and/or other natural silicates, such as talc, etc., can be added in conventional amounts.
The outer semi-conductive screen 14 is made of a synthetic polymeric material which is degraded in the presence o~ peroxides. In practice, a copolymer of iso-butylene with isoprene, commonly known as butyl rubber, to which carbon black is added as a conductive filler, is the preferred material for the screen 14. However, examples 7~35 of other known materials which may be used are halogenated butyl rubbers or blends of such rubbers with other saturated or unsaturated rubbers, such as ethylene-propylene-diene terpolymers, ethylene-propylene copolymers, ethylene-vinyl-acetate, etc.
For curing the butyl rubber, use can be made of one of the systems known in the art, for example, based on quinone compounds (p-quinone-dioxime; dibenzyl-p-quinone-dioxime) with or without sulfur and with or without an organic accelerator (benzo~hiazyl - disulfide) or simply based on sulfur and organic accelerators (telluriu~n-diethyl-dithiocarbamate, tetramethyl-thiouram-disulfide, mercapto-benzothiazole, etc.).
If desired, additives, such as l~w molecular weight polyethylene, paraffin wax, polyethylene oxide, silicone oils, metal stearates, stearic acid or cetyl : alcohol, can be added to the butyl rubber in order to facilitate its peeling.
The compound intended to form the insulation 13 and that intended to form the outer semi-conductive screen 14, the latter being extruded on the former, are cured at the same time in order to obtain between the two layers adhesion sufficient to exclude any trapping of air or moisture or of any type of impurity therebetween, thereby avoiding the occurrence of ionization phenomena (partial discharges).
The inner surace of the outer semi-conductive screen 14 is, therefore, adherent to the outer surface of the insulation 13. In other words, the butyl rubber layer, along its whole inner surface, is in the presence of peroxide, so that a slight thickness of the screen 14 of ~7~L3~;
butyl rubber, in proximity of the insulation 13, suffers a degradation. During the connection of the cable 10 to an accessory, the outer layers 15 and 16 are removed for the desired distance from the end of the cable 10 in a known manner, and the screen 14 is cut at selected portions thereof with suitable tools in a known manner. Thereafter, because of said degradation at the inner face of the screen 14 which provides a reduced resistance to stripping, the portion of the screen 14 which it is desired to remove may be readily stripped or peeled from the insulation 13 as indicated by the results of the tests described herein-after.
Tests were carried out on the removal of the outer screen 14 of medium voltage cables having an insu-lation 13 based on an eth~lene-propylene copolymer (EPM), by using three different compounds for the screen 14, one of a conventional type and the other two prepared according to the invention and the screen 1~ and the insulation 13 being cured simultaneously. The tests were effected in accordance with the U.S. AEIC N.6-73, Section D,1,3 Standards reported in the "Specification for Ethylene Propylene Insulated Shielded Power Cables Rated 5 to A6 KV".
According to said Standards, a force not less than 1.8 kg (4 lbs.) and not greater than 8.2 kg (18 lbs.) is required to remove from the insulation a strip of semi-conductive layer having a width of 1.27 cm (1/2 inch).
The composition of the semi-conductive compounds A, B and C used for the tests were the following:
_ g _ ~0~7~35 compound A (of conventional type) Parts b~ Weight Ethylene-propylene copolymer 100 (EPM rubber) semi-conductive carbon black 60 zinc oxide 6 paraffin oil 8 antioxidant (polymerized trimethyl-dihydroquinoline) triallyl-cyanurate di-cumyl-peroxide 2.5 Compound B (according to the invention) Isobutylene-isoprene copolymer 100 (butyl rubber) semi-conductive carbon black 60 zinc oxide 5 paraffin wax 10 p-quinone-dio~ime (GMF) 2 benzothiazyl-disulfide 4 Compound C (according to the invention) Isobutylene-isoprene copolymer 100 semi-conductive carbon black 60 zinc oxide 5 paraffin wax 7 poly-(ethylene-oxide) ~molecular weight 4000) 3 p-quinone-dioxime (GMF) 2 benzothiazyl-disulfide (BTS) 4 The test results showed the advantage which can be obtained with the present invention in the cables where the semi-conductive screen 14 and the insulation 13 are made in accordance with the invention, namely, the detach-ment of the screen 14 without damage to the underlying insulation 13. In fact, tests with the use of Compound A
of a conventional type showed that it was impossi.ble to remove the screen 14 from the insulation 13 under the ~L~47:~35 conditions provided by the hereinbeore-mentioned U.S.
Standard.
On the other hand, the use of Compounds B and C
according to the invention permitted achievement of results never obtained before, namely, by usi~g Compaund B a force of only 6.2 kg was necessary to detach a strip of the screen 14 having a width of 1.27 cm, and by using Compound C, an even smaller force of only 4.5 kg caused detachment of such a strip.
The advantages of the present invention were found even after a long accelerated thermal aging of the cable when Compounds B and C were used. In fact, even after such aging, which simulates the service conditions of the cable, ready peeling of the screen 14, without causing damage to the insulation 13, was observed.
Although preferred embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that var~us modifi-cations may be made without departing from the principles of the invention.
Moreover, the core, the expression "core" meaning herein the unit formed by the conductor ll, the first semi-conductive screen 12, the insula-tion 13 and the second semi-conductive screen 14, can form part of a single-core cable, as illustrated in the figure, or form part of a multi-core cable, such as a three-core cable.
The synthetic polymeric material constituting the -5a-1~
7~3S
insulation 13 is made of a compound, having appropriate electric characteristics, comprising polymeric materials which are cross-linkable with peroxide agents.
For the insulation 13, it is preferred to use an ethylene polymer, for example, cross-linked polyethylene, or a copolymer of ethylene with one or more comononers, desirably alkylene or vinyl ester comonomers, such as, ethylene-propylene, ethylene-vinyl-acetate, ethylene-ethyl-acrylate, ethylene-propylene-diene.
Any conventional agent for cross~linking ethylene polymers or copolymers which will degrade the inner surface of the semi-conductive layer extruded over the cross-linkable polymer may be used. Particularly suitable cross-linking agents are the organic peroxides, including the mono-functional or polyfunctional alkaryl and alkyl peroxides cross-linkin~ agents. Suitable monofunctional peroxides are those represented by the formula R - O - O - R' wherein R and R' may be the same or different and represents an alkyl or alkaryl radical such as a t-butyl or cumyl radical. Thus, useful monofunctional peroxides include:
dicumyl peroxide, t-butyl cumyl peroxide, di-tert hutyl peroxide. Suitable bifunctional peroxides may be repre-sented by the formula R" - O - O - X - O - O -~R"' wherein R" and R"' represent an alkyl radical preferably tert.-butyl and X is an alkyl, alkaryl or alkyne radical such as 1,4-diisopropyl benzene; 3,3,5-trimethylcyclo-hexane; 2,5-dimethylhexane: and 2,5-dimethylhexyne.
Useful bifunctional peroxides include: l,l-bis(tert.-butyl peroxy isopropyl) benzene; l,l-bis(tert.-butyl peroxy)-3,3, ~47~35 5-trimethyl cyclohexane; 2,5-dimethyl-2,5-bis~tert.-butyl peroxy) hexane and 2,5 dimethyl-2,5-bis~tert.-butyl peroxy) hexyne-3.
When the insulation 13 and the screen 14 are cured simultaneously, i.e., by extruding the material of the screen 14 over the insulation 13 before curing the latter and then curing, or polymerizing, both materials, the amount of the peroxide required to obtain the advantages of the invention is not critical and may, therefore, be the amount conventionally used in the cross-linking of the polymer of the insulation 13. For example, when mono-functional peroxides are used, two to five parts of such peroxides per hundred parts of rubber may be used, and when bifunctional peroxides are used, one-and-one-half to three parts per hundred parts of rubber may be used.
Conventionally, fillers selected as a function of the various requirements desired for the finished product, such as a greater dielectric strength, smaller dielectric losses, greater insulation constants, resistance to partial discharges, etc., are added at times to the compound con-stituting the insulation 13. Thus, to a compound ~ormed of an ethylene copolymer with one or more comonomers, calcium carbonate, calcined kaolin and/or other natural silicates, such as talc, etc., can be added in conventional amounts.
The outer semi-conductive screen 14 is made of a synthetic polymeric material which is degraded in the presence o~ peroxides. In practice, a copolymer of iso-butylene with isoprene, commonly known as butyl rubber, to which carbon black is added as a conductive filler, is the preferred material for the screen 14. However, examples 7~35 of other known materials which may be used are halogenated butyl rubbers or blends of such rubbers with other saturated or unsaturated rubbers, such as ethylene-propylene-diene terpolymers, ethylene-propylene copolymers, ethylene-vinyl-acetate, etc.
For curing the butyl rubber, use can be made of one of the systems known in the art, for example, based on quinone compounds (p-quinone-dioxime; dibenzyl-p-quinone-dioxime) with or without sulfur and with or without an organic accelerator (benzo~hiazyl - disulfide) or simply based on sulfur and organic accelerators (telluriu~n-diethyl-dithiocarbamate, tetramethyl-thiouram-disulfide, mercapto-benzothiazole, etc.).
If desired, additives, such as l~w molecular weight polyethylene, paraffin wax, polyethylene oxide, silicone oils, metal stearates, stearic acid or cetyl : alcohol, can be added to the butyl rubber in order to facilitate its peeling.
The compound intended to form the insulation 13 and that intended to form the outer semi-conductive screen 14, the latter being extruded on the former, are cured at the same time in order to obtain between the two layers adhesion sufficient to exclude any trapping of air or moisture or of any type of impurity therebetween, thereby avoiding the occurrence of ionization phenomena (partial discharges).
The inner surace of the outer semi-conductive screen 14 is, therefore, adherent to the outer surface of the insulation 13. In other words, the butyl rubber layer, along its whole inner surface, is in the presence of peroxide, so that a slight thickness of the screen 14 of ~7~L3~;
butyl rubber, in proximity of the insulation 13, suffers a degradation. During the connection of the cable 10 to an accessory, the outer layers 15 and 16 are removed for the desired distance from the end of the cable 10 in a known manner, and the screen 14 is cut at selected portions thereof with suitable tools in a known manner. Thereafter, because of said degradation at the inner face of the screen 14 which provides a reduced resistance to stripping, the portion of the screen 14 which it is desired to remove may be readily stripped or peeled from the insulation 13 as indicated by the results of the tests described herein-after.
Tests were carried out on the removal of the outer screen 14 of medium voltage cables having an insu-lation 13 based on an eth~lene-propylene copolymer (EPM), by using three different compounds for the screen 14, one of a conventional type and the other two prepared according to the invention and the screen 1~ and the insulation 13 being cured simultaneously. The tests were effected in accordance with the U.S. AEIC N.6-73, Section D,1,3 Standards reported in the "Specification for Ethylene Propylene Insulated Shielded Power Cables Rated 5 to A6 KV".
According to said Standards, a force not less than 1.8 kg (4 lbs.) and not greater than 8.2 kg (18 lbs.) is required to remove from the insulation a strip of semi-conductive layer having a width of 1.27 cm (1/2 inch).
The composition of the semi-conductive compounds A, B and C used for the tests were the following:
_ g _ ~0~7~35 compound A (of conventional type) Parts b~ Weight Ethylene-propylene copolymer 100 (EPM rubber) semi-conductive carbon black 60 zinc oxide 6 paraffin oil 8 antioxidant (polymerized trimethyl-dihydroquinoline) triallyl-cyanurate di-cumyl-peroxide 2.5 Compound B (according to the invention) Isobutylene-isoprene copolymer 100 (butyl rubber) semi-conductive carbon black 60 zinc oxide 5 paraffin wax 10 p-quinone-dio~ime (GMF) 2 benzothiazyl-disulfide 4 Compound C (according to the invention) Isobutylene-isoprene copolymer 100 semi-conductive carbon black 60 zinc oxide 5 paraffin wax 7 poly-(ethylene-oxide) ~molecular weight 4000) 3 p-quinone-dioxime (GMF) 2 benzothiazyl-disulfide (BTS) 4 The test results showed the advantage which can be obtained with the present invention in the cables where the semi-conductive screen 14 and the insulation 13 are made in accordance with the invention, namely, the detach-ment of the screen 14 without damage to the underlying insulation 13. In fact, tests with the use of Compound A
of a conventional type showed that it was impossi.ble to remove the screen 14 from the insulation 13 under the ~L~47:~35 conditions provided by the hereinbeore-mentioned U.S.
Standard.
On the other hand, the use of Compounds B and C
according to the invention permitted achievement of results never obtained before, namely, by usi~g Compaund B a force of only 6.2 kg was necessary to detach a strip of the screen 14 having a width of 1.27 cm, and by using Compound C, an even smaller force of only 4.5 kg caused detachment of such a strip.
The advantages of the present invention were found even after a long accelerated thermal aging of the cable when Compounds B and C were used. In fact, even after such aging, which simulates the service conditions of the cable, ready peeling of the screen 14, without causing damage to the insulation 13, was observed.
Although preferred embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that var~us modifi-cations may be made without departing from the principles of the invention.
Claims (3)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electric cable comprising a conductor having a layer of insulation therearound and a semi-conduc-tive screen around and in contact with said layer of insulation, said insulation comprising a synthetic polymeric material which is cross-linkable by peroxide agents and said screen comprising a synthetic, polymeric material which is degradable by peroxide in said agents, said polymeric material of said screen being a rubber selected from the group consisting of butyl rubber, halogenated butyl rubbers and blends of either butyl rubbers or halogenated butyl rubbers with a polymer selected from the group consisting of ethylene-propylene-diene terpolymers, ethylene-propylene copolymers and ethylene-vinyl-acetate whereby the surface of said screen adjacent to said insulation is degraded by said peroxide and permits said screen to be detached readily from said insulation without substantial damage to said insulation.
2. An electric cable as set forth in claim 1, wherein said cable further comprises a semi-conductive screen intermediate said conductor and said layer of insulation.
3. An electric cable as set forth in claim 2, wherein said polymeric material of said insulation is polymer selected from the group consisting of ethylene-propylene copolymer, ethylene-vinyl-acetate copolymer, ethylene-ethyl acrylate copolymer and ethylene-propylene-diene terpolymer.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT29862/74A IT1026543B (en) | 1974-11-27 | 1974-11-27 | ELECTRIC CABLE PERFECTED FOR MEDIUM AND HIGH VOLTAGES |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1047135A true CA1047135A (en) | 1979-01-23 |
Family
ID=11228572
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA240,500A Expired CA1047135A (en) | 1974-11-27 | 1975-11-26 | Medium and high voltage electric cable with readily peelable outer screen |
Country Status (11)
| Country | Link |
|---|---|
| JP (1) | JPS5162372A (en) |
| AR (1) | AR208932A1 (en) |
| AU (1) | AU497407B2 (en) |
| BR (1) | BR7507452A (en) |
| CA (1) | CA1047135A (en) |
| DE (1) | DE2552895A1 (en) |
| FR (1) | FR2293039A1 (en) |
| GB (1) | GB1516708A (en) |
| IT (1) | IT1026543B (en) |
| SE (1) | SE422381B (en) |
| ZA (1) | ZA756712B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2908457C2 (en) * | 1979-03-05 | 1986-09-18 | kabelmetal electro GmbH, 3000 Hannover | Process for the continuous production of casings, in particular insulation, for electrical cables |
| EP2444460B1 (en) | 2010-10-25 | 2019-03-06 | Imerys Industrial Minerals Greece S.A. | Milled expanded volvanic glass as lamellar filler |
| CN105825914A (en) * | 2016-01-27 | 2016-08-03 | 苏州翠南电子科技有限公司 | Novel composite silicone rubber solid insulation bus |
-
1974
- 1974-11-27 IT IT29862/74A patent/IT1026543B/en active
-
1975
- 1975-08-19 JP JP50100555A patent/JPS5162372A/en active Pending
- 1975-10-16 AR AR260840A patent/AR208932A1/en active
- 1975-10-23 GB GB43626/75A patent/GB1516708A/en not_active Expired
- 1975-10-24 ZA ZA00756712A patent/ZA756712B/en unknown
- 1975-10-30 AU AU86157/75A patent/AU497407B2/en not_active Expired
- 1975-11-03 FR FR7533516A patent/FR2293039A1/en active Granted
- 1975-11-12 BR BR7507452*A patent/BR7507452A/en unknown
- 1975-11-24 SE SE7513181A patent/SE422381B/en not_active IP Right Cessation
- 1975-11-25 DE DE19752552895 patent/DE2552895A1/en not_active Withdrawn
- 1975-11-26 CA CA240,500A patent/CA1047135A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| FR2293039B1 (en) | 1978-05-12 |
| SE7513181L (en) | 1976-05-28 |
| FR2293039A1 (en) | 1976-06-25 |
| AR208932A1 (en) | 1977-03-15 |
| GB1516708A (en) | 1978-07-05 |
| IT1026543B (en) | 1978-10-20 |
| DE2552895A1 (en) | 1976-08-12 |
| SE422381B (en) | 1982-03-01 |
| AU8615775A (en) | 1977-05-05 |
| BR7507452A (en) | 1976-08-10 |
| JPS5162372A (en) | 1976-05-29 |
| ZA756712B (en) | 1976-09-29 |
| AU497407B2 (en) | 1978-12-14 |
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