Note: Descriptions are shown in the official language in which they were submitted.
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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
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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.
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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
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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.
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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
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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
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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:
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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
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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.
