WO1999021259A1

WO1999021259A1 - Improved medium voltage branch splice and method of making the same

Info

Publication number: WO1999021259A1
Application number: PCT/US1998/018907
Authority: WO
Inventors: Raymond Krabs; Carl J. Wentzel
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1997-10-22
Filing date: 1998-09-10
Publication date: 1999-04-29
Also published as: ZA988684B

Abstract

A branch splice for joining at least three shielded medium-voltage electric cables (11, 16, 21). Each of the cables has at least one conductor surrounded by a dielectric layer (13, 18, 23). To form the splice, a section of the cable dielectric layer is removed to expose a length of the conductor, and a section of an outer electrically conductive shield is removed to expose a length of the dielectric layer so that the exposed conductors of the cables can be joined using the connector. At least two of the cables are positioned in relatively close substantially parallel relationship with each other. The splice includes a stress controlling adapter (40) having an elastomeric body. The adapter body is provided with at least two hollow portions allowing the body to be placed onto the outer electrically conductive shield and the exposed section of the dielectric layer of the cables which are positioned in substantially parallel relationship. In one embodiment, the material of the stress controlling adapter (40) has a relative dielectric constant (epsilon) of greater than 3 and a dielectric strength of at least 10 kV/mm, and includes conductive microspheres uniformly distributed throughout an elastomeric matrix. In another embodiment, the adapter utilizes geometric stress control. The splice is completed by covering with an insulating layer (55) extensible across the splice so as to overlap a portion of the shield of each of the cables and a semi-conductive shielded layer positioned over the insulating layer which is connectable to each of the cable shields.

Description

IMPROVED MEDIUM VOLTAGE BRANCH SPLICE AND METHOD OF

MAKING THE SAME

The present invention relates to joints or splices, in particular to branch splices, sometimes known as bifurcating joints, breech joints, "Y" joints or tap joints particularly for use in medium voltage cable networks and a method of making the branch joints, as well as an adapter for use in the joints and covering means specially adapted to be applied to the geometry of a branch joint. As used herein, the terms "joint" and "splice" are used interchangeably.

Technical background In medium voltage power distribution networks, e.g. with system voltages between 5 and 36 kV, there is a requirement for a branch joint. A typical branch joint connects one cable on one side to two cables on the other side of the joint. This type of joint is known as a bifurcating, "Y" , breech, or branch joint.

There is a lesser requirement for a double branch joint in which two cables are joined to a further two cables. The latter type of joint may be described as an "H" joint, a double breech joint or a double branch joint. A particular difficulty with both single and double branch joints is that the geometry close to the connector joining the conductors of the medium voltage cables is complex and it is difficult to prevent formation of voids in areas of significant electrical stress. Branch joints may be made between cables of the same insulation type, e.g. between paper insulated cables or between polymeric insulated cables, or may be made between cables having differing insulation types, e.g. paper insulated cables and polymeric insulated cables, or between cables with differing types of dielectric cable insulation, e.g. between ethylene propylene rubber (EPR) insulated cables and cross- linked polyethylene insulated cables. Further, a branch joint may include cables having different conductive materials. Traditionally, copper conductors have been used for medium voltage cables. Recently, however, aluminum conductor cables have been used extensively. To maintain the same power rating, an aluminum conductor cable has to have a larger cross-sectional area of conductor than a copper conductor cable . This means that in a branch joint between copper conductor cables and aluminum conductor cables there may not only be a difference in insulation type but also in cable diameter and in particular in insulation diameter between the various cables . The considerable variations in cable types, cable designs, insulation types and conductor materials makes the design of a universally installable branch joint very difficult. In addition, any type of cable joint or termination which is made on site rather than under closely controlled factory conditions is a potential source of failure in the system. There is therefore a requirement for a branch joint which is relatively easy to assemble, accommodates the largest possible range of cable types and diameters and is reliable in operation. One type of branch joint utilizes either

"compound" or polymeric cross-linking resins in order to fill up the space between the connected cables and the outer earth shield of the joint. What is known in the industry as "compound" may be of bitumen basis or include oils and greases suitable for use in cables and cable joints. The material is normally poured hot and sets on cooling. As jointing in manholes and cable trenches is often done at remote sites, it is difficult to provide suitable and safe heating means to bring the compound into the liquid state suitable for pouring. Further, several accidents have occurred while pouring the compound and there have been considerable efforts to replace hot poured compound with cold-setting resins .

Branch joints using cold-setting resins are also known and have several disadvantages. The resins have a limited shelf life, and frequent exposure to some of the constituents in some resins before they have been cross-linked has been known to create allergies. Finally, the weight of resin necessary to fill up all the spaces in a large branch joint is considerable, resulting in heavy and bulky jointing kits. Accordingly, there have been considerable efforts to avoid the use of cross-linking resins in the manufacture of cable joints. Known solutions which avoid the use of either hot- poured compound or cross- linking resins include the use of either cold-shrink or hot-shrink pre-expanded tubular components. A further known solution is to use elastomeric tubular components which are not pre- expanded and which are installed on the cable by slipping or pushing the tubular components over the end of the cable. Such cable accessories are generally described as "push-on" . It is easier to design and install cable accessories such as terminations and joints with heat-shrinkable, cold-shrinkable or push-on tubular components when the accessory has a basic cylindrical geometry, for example, single-core in-line joints or single-core terminations. The branch joint, with its non-circular geometry, represents a significant technical challenge to those cable accessory systems which make use of tubular components. Still a further method of forming medium voltage joints is the use of self-amalgamating or vulcanizable rubber tapes . These tapes are used by wrapping the tape around the cable insulation, cable conductors and cable connectors in such a way as to build up a relatively void-free joint. The quality of the joint produced on site depends upon the skill of the jointer. Applying the tapes correctly and being able to fill up the unusual and irregular shapes which are present as voids within a branch joint is not easy. Such joints take a considerable amount of time to produce and are sensitive to the skill of the jointer as far as quality is concerned.

EP-0 136 154 describes an attempt to produce a branch joint making use of heat-shrinkable components. As the space between the two cables on the one side of a branch or bifurcating joint is not amenable to being filled using cylindrical components, this known proposal makes use of a malleable stress control material having an impedance between 5 x 10⁷ and 8 x 10⁹ Ohm cm. The stress control material between the two cables on the one side of the branch joint may include a void filling insulation composition which may be an extruded profile contoured to fit between the parallel cables. The stress control material proposed is an epihalohydrin-based composition containing a trihydrate as a filler. The composition described has an impedance of 3 x 10⁹ Ohm cm, is a malleable solid at room temperature, and has a viscosity at 70° C of about 10⁵ poise . Epihalohydrin polymer compositions which have stress grading properties and are suitable for use with hea -recoverable articles in cable accessories and in particular for encapsulation of high voltage electrical apparatus are described in US Patent No. 4 378 463. These compositions tend to absorb water in large quantities and then to become more conductive. Further, the volume resistivity of such materials tends to be low, i.e. a major contribution to the impedance at 50 Hz is provided by ohmic conductivity rather than by a pure loss-free capacitance. This means that such materials create considerable electrical power loss within the joints which manifests itself as heat energy generated within the stress control material. This may result in overheating of the joints at full load and voltage. A further problem with this known joint is that it is difficult to fill up the voids and irregular geometrical forms around the conductor connector in such a way as to achieve a completely void-free joint. As the stress control material is used as a void filler, it is under both radial and longitudinal electrical stress within the joint. Thus any voids within the stress control material may result in powerful discharges which may cause the joint to fail in the long term. Partial discharges may be observed even immediately after formation of the joint at the standard test voltages .

Branch joints of the product type DUP-TH and D3UP- TH have been supplied by the company Silec, Societe Industrielle de Liaison Electrique, Paris, France, which make use of pads and sheets of stress control mastic material which must be carefully placed and shaped to the irregular geometry of the branch joint. Completion of the joint requires skill on the part of the jointer in placing the various pads and sheets of mastic material in the correct positions. Both the joints known from EP-A-0 136 154 and from Silec use heat-shrinkable tubular components to complete the joint. These pre-expanded heat-shrinkable tubular components are normally shrunk on site by means of a propane gas burner. Several accidents have occurred with the use of open flames in the confined spaces typically experienced in jointing within manholes and cable trenches. It is therefore preferred to avoid the use of open flames when installing cable accessories and particularly when installing cable joints.

The push-on elastomeric technology has been adapted to the formation of tap joints by using a series of disconnectable, fully insulated and screened modular "T" shaped terminations joined by one or more bushings which are bolted together to form two-, three- or four-way splices. These joints are rated at 15kV, 25kV and 35kV, 600 amp dead break. Each interface is complex from both a molding and material standpoint, and as a result the complete joint is expensive.

The object of the present invention is to provide a branch joint which is relatively easy to install, accommodates the largest possible range of cable types and cable dimensions, does not require the use of open flame, provides a reliable means for reducing void formation in electrically stressed parts of the joint and which is reliable in operation. In addition, it is an object of the invention to provide a branch joint which is quick to remove and re-install, in the event of a cable failure or as a means for isolating a particular cable branch.

Summary of the invention

The present invention provides a branch joint between at least three shielded medium-voltage electric cables . Each of the cables has at least one conductor surrounded by a dielectric layer, a section of which has been removed to expose a length of the conductor, and an outer electrically conductive shield, a section of which has been removed to expose a length of the dielectric layer. The exposed conductors of the cables are joined together using a connector, with at least two of the cables being positioned in relatively close, substantially parallel relationship with each other. The joint comprises a stress controlling adapter having an elastomeric body provided with at least two hollow portions, thereby allowing the body to be placed onto the outer electrically conductive shield and the exposed section of the dielectric layer of the at least two cables being positioned in substantially parallel relationship with each other. In a high dielectric constant-type stress controlling adapter, the material of the stress controlling adapter has a relative dielectric constant of greater than 3 and a dielectric strength of at least 10 kV/mm. Covering means comprising an insulating layer extends across the joint so as to overlap a portion of the shield of each of the cables. A semi-conductive shielding layer is positioned over the insulating layer and is connectable to each of the cable shields. The stress controlling adapter could also utilize geometric stress relief in addition to or in place of the high dielectric constant material.

The present invention also provides a stress controlling adapter for a branch joint comprising an elastomeric body with a substantially elliptical cross- section and at least two hollow portions through said body having their centers being substantially on the major axis of the ellipse. The materials utilized for a geometric-type stress relief adapter would consist of a combination of conductive, insulating, and possibly high dielectric constant materials, while the material of the high dielectric constant type adapter comprises a matrix of dielectric plastic material and a content of microspheres conductive at least at the surface thereof and having a diameter of between 10 and 500 μm, said microspheres being evenly distributed in said matrix material.

The present invention also provides a kit of parts for establishing a branch joint between at least three shielded medium voltage cables, the kit including a stress control adapter having an elastomeric body provided with at least two hollow portion, thereby allowing the body to be placed onto the outer electrically conductive shield and the exposed section of the dielectric layer of at least two of the cables, wherein the material of the stress control adapter has a relative dielectric constant of greater than 3 and a dielectric strength of at least 10 kv/mm. The kit may also include a covering means comprising an essentially cylindrical single sleeve of elastomeric material which is pre-stretched and supported on at least one removable core and which can be caused to recover by removing the at least one core .

The present invention provides a method of forming a branch joint between a shielded, medium voltage electric cable and at least two other shielded, medium voltage electric cables, each of said cables having at least one conductor surrounded by a dielectric layer which in turn is surrounded by an electrically conductive shield, said joint being formed by removing a portion of the dielectric layer to expose a length of the conductor and a portion of the shield to expose a length of the dielectric layer, wherein the method comprises: positioning two of the cables in relatively close substantially parallel relationship to each other; applying a stress controlling adapter having an elastomeric body provided with at least two hollow portions allowing the body to be placed onto the outer electrically conductive shield and the exposed section of the dielectric layer of the at least two cables being positioned in substantially parallel relationship, wherein the material of the stress controlling adapter has a relative dielectric constant of greater than 3 and a dielectric strength of at least 10 kV/mm, to connect the conductors of the cables; applying a covering means comprising an insulating material and an outer semi-conductive shielding layer over the joint so as to extend across the joint and overlap a portion of the shield of each of the cables; and electrically connecting said shielding layer to the shield of each of the cable.

The dependent claims define further embodiments of the present invention. The invention, its advantages and embodiments will now be described with reference to the following drawings.

Brief description of the drawings

Fig. 1 is a schematic cross-sectional representation of a branch joint utilizing a high dielectric constant-type stress controlling adapter in accordance with the present invention.

Figs. 2a and 2b are schematic representations of a high dielectric constan -type stress controlling adapter in accordance with the present invention Fig. 3 is a further embodiment of the stress controlling adapter of Figs. 2a and 2b.

Fig. 4a and Fig. 4b and Fig. 4c are schematic cross-sectional representations of a covering means in accordance with the present invention.

Fig. 5 is a schematic representation of the covering means of Figs. 4a-4c in the expanded state.

Fig. 6 is a detailed schematic cross-sectional representation of a branch joint using the adapter of Figs. 2a and 2b, including the covering means of Fig. 4c.

Fig. 7 is a schematic cross-sectional representation of an alternative embodiment of a covering means .

Fig. 8 is an elevational view of a stress controlling adapter using geometric stress control.

Fig. 9 is a cross-sectional representation of the stress controlling adapter of Fig. 8.

Fig. 10 is a detailed schematic cross-sectional representation of an H-joint using the adapter of Fig. 8 including the covering means like that of Fig. 4a.

Fig. 11 is a detailed schematic cross-sectional representation of an H-joint using the adapter of Fig. 8 including the covering means like that of Figure 7.

Figs. 12a, 12b and 12c are schematic cross- sectional representations of the installations steps of one embodiment of the present invention.

Description of the illustrated embodiments of the invention In the following, the present invention will be described with respect to certain embodiments and with reference to certain drawings . The drawings are schematic only and certain dimensions may be exaggerated for clarity purposes. Further, the present invention will mainly be described with respect to forming a branch joint between single core screened cable. However, a skilled person would appreciate that the invention may be applied to three core screened cables by treating each individual core in the same way as will be described in the following for single core cables and then providing suitable outer mechanical protection for the complete joint. Further, the present invention will mainly be described with respect to a branch joint between polymeric insulated screened cables. The skilled person will appreciate that the present invention may also be applied to form branch joints between paper insulated cables or between paper insulated cables and polymeric insulated cables by making use of the universal cable adapter disclosed in US Patent Numbers 5,374,784 and 5,408,047, and EP-A-0 780 949 which are incorporated herein by reference. The incorporated references describe how the insulated paper cores of a medium voltage paper insulated cable may be transformed by means of a universal cable adapter into what is, in effect, a polymeric insulated cable. The cable may then be joined in accordance with the principles described below.

Fig. 1 is a schematic cross-sectional representation of a first embodiment of a partly completed branch joint 10 in accordance with the present application. Only the electrostatic shielding, insulating and stress control components of the branch joint 10 are shown in Fig. 1. The earth shield (e.g., ground) continuity and mechanical protection and sealing components of branch joint 10 are not shown. Cables 1, 2 and 3 are joined in branch joint 10. As shown in Fig. 1, cables 1, 2 and 3 are polymeric insulated single core wire shielded medium voltage screened cables but the invention is not limited thereto. In particular, in accordance with the present invention the insulation type, screening, earth shielding and number of cores in the cable may be different as has been explained above. In particular, the skilled person would appreciate that the branch joint 10 as shown in Fig. 1 may be the connection of a single core of a uhree-core cable or may be applied to a paper cable using an adapter as mentioned above.

Cables 1, 2 and 3 each include a conductor 15, 20, 25, insulation 14, 19, 24, an electrostatic screen 13, 18, 23, some form of earth shielding 12, 17, 22 which are wires as shown in Fig. 1 but may be any suitable material such as copper foil or aluminum foil or any other suitable form of earth shielding or drain wires . The outside of the cable may be protected by a suitable outer cable jacket 11, 16, 21, e.g. polyvinyl chloride (PVC) , polyethylene (PE) or high density polyethylene (HDPE) .

Conductors 15, 20, 25 may be of any suitable connector material such as stranded, solid or segmented aluminum or copper conductors. Insulation 14, 19, 24 may be any suitable insulation such as EPR, cross- linked polyethylene or paper insulation, in particular Mass Impregnated Non-Draining (MIND) paper insulation. The electrostatic core screen 13, 18, 23 may be any suitable core screen such as conductive or semi- conductive polymeric or rubber materials which may be strippable or removable by hand or by special tools, or a graphite layer with intercalated carbon paper. Each cable end of cables 1, 2 and 3 are prepared in a conventional way for jointing purposes, namely to expose a portion of the conductor 15, 20, 25, to expose and clean a length of insulation 14, 19, 24, to expose a length of electrostatic screen 13, 18, 23 and to expose and prepare the earth shielding 12, 17, 22 for subsequent connection across the joint.

In a preferred embodiment of the present invention, branch joint 10 includes a specially shaped void-filling and high dielectric constant-type stress- controlling adapter 40 which is located snugly around each of the cable ends of the cables 2 and 3. The joint is preferably insulated and further stress graded by a multi-layer sleeve 50 which includes an inner insulating layer, optional inner stress grading layers and an outer conductive layer to provide electrostatic screening. Sleeve 50 may be formed of one or more tubings and is preferably a single piece multi-layer cold-shrink pre-expanded elastomeric jointing component as described below. Sleeve 50 may also be formed from insulating, stress grading or conductive heat- shrinkable tubings or may be a combination of heat- recoverable and cold-shrink tubings or layers.

One preferred embodiment of the covering means will be described with reference to a single-piece cold shrink sleeve 50 but the invention is not limited thereto but only by the claims. Sleeve 50 preferably includes an optional centrally placed internal semi- conductive layer 53 which is sufficiently long to extend from the end of the insulation 14 of cable 1 onto the adapter 40 of cables 2 and 3. The internal semi-conductive layer 53 is directly in contact with connector 30 for connecting conductors 15, 20 and 25 and is therefore maintained at the conductor potential. Voids formed close to the connector 30, e.g. between conductors 15, 20 and 25 and other components such as the insulation 14, 19, 24, are surrounded by the semi- conductive layer 53 and are therefore stress-free in accordance with the well known principles of a Faraday cage. Integrally molded or formed with conductive layer 53 is an elastomeric insulation layer 55 sufficient to insulate the joint at the rated working voltage for the expected life of the joint. At each end of sleeve 50 an internal length of stress control material 51, 52 is provided which makes contact with the core screen 13 of cable 1 and with at least the adapter 40 for cables 2 and 3. Preferably, as shown in Fig. 1, the stress control layer 51, alternatively 52, does not extend longitudinally towards the center of the joint so far as to come close to or to contact the central semi- conductive layer 53. Preferably there is a gap left between these two components of at least 5 mm.

To provide electrostatic shielding, i.e. a continuation of the core screens 13, 18, 23, sleeve 50 is provided with an outer semi-conductive layer 54 which is preferably integrally formed or molded with the insulation layer 55. An outer protective jacket (not shown) enclosing the entire joint can be applied to provide mechanical protection and sealing against moisture ingress. Preferably the protective jacket includes an outer polymeric tubular article which may be a heat-recoverable tubing or more preferably a pre- expanded tubing supported on a removable core . Removal of the core causes collapse of the protective sleeve. To provide sealing at the end of the joint with more than one cable, a breakout can be used, e.g. the HDBB heat shrinkable breakouts supplied by Minnesota Mining and manufacturing Co. St. Paul, Minnesota, USA which include an internal coating of sealing adhesive. To provide additional mechanical protection to the joint, a metal joint case (not shown) can be provided intermediate the jointed cables and the outer protective tubular protection. The joint case may be connected to the screen wires of the cables to provide grounding and may comprise a woven metal sock over the jointing area. . The adapter 40 according to one embodiment of the invention consists of a homogeneous material having a relative dielectric constant which is larger than that of the insulation of the cable, for example larger than 4 and preferably larger than 5 but smaller than 10. Normally, materials capable of electric field control or stress grading have a low dielectric strength. On the other hand, insulation material has no stress controlling properties. The invention contemplates that the material for adapter 40 is a compromise between both types of materials. If the material of the adapter 40 is provided with a dielectric constant which is larger than that of the insulation of the cable, it is possible to obtain a limited electric field controlling effect although the material of the adapter 40 is also a good electrical insulator.

One embodiment of adapter 40 is shown schematically in Figs. 2a and 2b. It consists of an elastomeric body of circular or elliptical cross- section with at least two hollow tubular portions 43, 44 through the body. The inner diameters of hollow portions 43, 44 are chosen so that the adapter 40 may be pushed over the insulation of cables 2 and 3 to provide a snug, void-free fit. Silicone grease or other similar lubricant may be applied to insulation 19, 24 of cables 2, 3 before fitting the adapter 40. Adapter 40 extends from the end of the insulation 19, 24 across the dielectric and overlaps the cable shields 18, 23 (see Fig. 1) . The ratio of the larger to smaller axis of the elliptical cross-section of adapter 40 shown in Fig. 2b is preferably in the range 1.0 to 1.5, more preferably 1.15 to 1.25.

Optionally an additional stress grading layer 49 may be provided on the outside of adapter 40. The material of stress grading layer 49 preferably has a considerably lower 50 Hz impedance than that of the material for the bulk of adapter 40. For instance, the layer 49 may be Vinyl-Methyl-Silicone rubber vulcanized at high temperature including carbon black. Preferably, the dissipation factor is below 0.1. The dielectric constant of the material may be between 15 and 25, preferably about 15. The stress controlling layer 49 consists preferably of a stress controlling material which is known from the German patent specification DE 30 08 264. It is a permanent flexible dielectric material having an electrical volume resistivity at room temperature of at least 10⁶ Ohm cm. The base material is silicone rubber or polyethylene or ethylene propylene diene copolymerisate (EPDM) with a content of a finely divided conductive material to increase the relative dielectric constant. The conductive material may comprise strongly structured dust-fine particles of a weakly conductive, electrically polarizable material in a mass content of up to about 350g per Kg base material. Preferably, the conductive material is carbon black.

Fig. 3 shows an embodiment of the adapter 40 in which each of the hollow portions 43, 44 have been expanded and are supported by a removable cores 45, 46, respectively. Suitable removable support cores are known from DE 37 15 915, DE 39 43 296, EP 0 101 472, EP 0 117 092, EP 0 702 444, US 5,589,667 or US 3,515,798. With reference to Fig. 2b, when an elastomeric body such as adapter 40 is expanded, the thinner material in the region 41 at the ends tends to absorb all the strain whereas the region 42 hardly elongates due to its considerable thickness. This means that the material for adapter 40 in accordance with the embodiment shown in Fig. 3 needs to have a high value of elongation before break and a low cold-set, i.e. a low value of permanent deformation when the adapter 40 is expanded, stored for a considerable length of time and then released. Incorporating considerable amounts of carbon black into the material of the adapter 40 tends to make it mechanically stiff, resulting in a reduced elongation before break. In accordance with one preferred embodiment of the present invention, the material 40 preferably includes an elastomeric matrix and substantially evenly dispersed microspheres. Such a material is sufficiently stress grading to control the stress around and between the cores of cables 2 and 3 in the joint while maintaining a high dielectric strength, low lost factor and high value of elongation. The adapter 40 according to the invention can be easily manufactured, e.g. by extrusion, injection or transfer molding. The adapter 40 according to the invention can be reliably used for a relatively broad medium voltage range and for different sizes and types. The installation of the adapter 40 may take place without tools.

According to the invention, the relative dielectric constant of the material of the adapter 40 is 3 or larger. Preferably, it is between 3 and 10, more preferably between 4 and 10 and most preferably between 5 and 10. Preferably the ohmic resistivity is 10¹² ohm cm or higher, typically 10¹³ ohm cm or higher.

Various methods are conceivable to provide an insulating material for the adapter 40 with an increased relative dielectric constant without reducing the dielectric strength too much. According to an embodiment of the invention, a matrix of a dielectric plastic material and a content of microspheres is provided with the microspheres conductive totally or only at the surface thereof, having a diameter distribution of between 10 and 500 μm and being uniformly distributed in the matrix material, the compound thus achieved having a relative dielectric constant equal to or greater than 3 , more preferably greater than 4 and less than 10 and most preferably greater than 5 and less than 10 and a dielectric strength of at least 5 kv/mm. The ohmic resistivity is preferably 10¹² ohm cm or higher. Silicone rubber may be used as the matrix material, in particular liquid silicone rubber. However, other matrix materials can be used, for example, acrylester rubber (ANM), cellulose acetate (CA) , epoxide ( (EP) , nitrile rubber (NBR/NCR) , polyamide (PA) , polyacrylate (PAR) , polycarbonate (PC) , polyimide (PI) , styrenebutadiene rubber (SBR) , silicone (SI) or vinylacetate (VAC) . It is preferable that the matrix material have reasonable molding or extrusion properties and the ability to be molded by known molding processes such as extrusion or injection molding. It is further preferable that the structure or constitution of the matrix material be such that the microspheres are not crushed or deteriorated by such molding process. To avoid high pressures in extrusion or molding presses it is preferred that the material has a relatively low viscosity during extrusion or injection into the mold. The diameter range of the microspheres may be between 10 and 500 μm. Preferably, according to an embodiment of the invention, the diameter distribution is between 10 and 250 μm and more preferably between 30 and 90 μm. These small spheres can be simply processed with conventional plastic molding methods and effect a satisfactory homogeneous distribution in the molded article. The relative dielectric constant obtained with this mass is equal to or greater than 3 , with the dielectric strength being at least 5 kV/mm, more preferably at least 10 kV/mm. The resistivity is greater than 10¹² ohm cm. The microspheres can be made of metal. Preferably, glass spheres are used, particularly hollow glass spheres or bubbles as, for example, known from "Scotchlite™ Glass Bubbles Hollow Micro Glass Spheres" product information and specification of 3M Company, St. Paul, Minnesota of January 1, 1993. They are made of low alkali borosilicate glass and are chemically inactive. The bubbles have a size distribution of 96% in the range of 20 to 120 μm and of 60% in the range of 40 to 80 μm. If metal spheres are used, electrical conductivity is automatically available. If, however, glass spheres are used, a surface coating with a metal is mandatory. The coating may consist of, for example, aluminum, nickel, silver, or the like. The metallic coating can be as thin as practical since significant current does not flow. Therefore, the coating may have, for example, a thickness of 0.001 μm.

According to a further embodiment of the invention, it may be appropriate to provide the microspheres with an insulating coating. This applies to metal microspheres and to metal coated glass microspheres as well. In this case, mutual touching of some microspheres is not substantially injurious compared to the desired insulating effect. Such a coating may be very thin, for example 0.0004 μm. The preferred material for this coating is to be selected such that it is compatible with the metal or the metal coating in order to avoid a chemical reaction and to provide a sufficient adhesive capacity. For example, aluminum can be used for the metal coating and aluminum sub oxide as insulating coating. The coating of the non-conductive glass spheres with a metal can be carried out by conventional technologies, e.g. the sputter deposition process.

It has been found through experimentation that the system works both with microspheres having the additional insulating oxide layer as well as with microspheres which have the bare metal coating. The reason for this is that the microspheres under normal circumstances are insulated from each other by the matrix material. Only very occasionally does contact between microspheres occur which changes the electrical properties and then by only an insignificant amount. The additional coating may, however, be advantageous under certain circumstances. For example, it has been observed that during the molding process a higher concentration of microspheres has been observed at corners of the mold cavity as a consequence of material flow behavior.

In particular with the use of glass microspheres, it is preferable that they are not crushed by the molding process. Therefore, liquid silicone rubber may be advantageously used because its viscosity is relatively low before curing. A high viscosity could lead to a high pressures during injection or extrusion molding resulting in crushing of the spheres and, in addition, may prevent the spheres from being uniformly distributed in the plastic matrix during the mixing process .

To form adapter 40 in accordance with one embodiment, liquid silicone rubber as matrix material is mixed with metallic coated glass bubbles. The glass bubbles may be included in the range 2 to 12% volume percent .

The specific gravity of the glass bubbles may be 0.6. Their hydrostatic compression strength may be 7000 N/cm². The size distribution of the spheres may be as follows :

Sieve Size (μm) Percent

Passing Through

88 100 62 93.7

44 73.7

31 50.5

22 30.5

16 15.8 11 7.4

7.8 2.1

5.5 0.0

The glass bubbles may be coated with aluminum, with the coating having a thickness of 10 nm while the insulating layer of aluminum oxide coated on the metallic coating has a thickness of 4 nm. The specific resistivity of the material produced is constant above 2% by volume of bubbles and has a value in the order of 6 x 10¹³ Ohm cm. The dielectric strength is in the range of 18 kV/mm which is particularly satisfactory for the medium voltage range. The dissipation factor is about 0.0001. At the volume content of 2% of bubbles, the relative dielectric constant is somewhat above 3 while it is 4 at a volume content of 10%. In addition to the above mentioned material for the adapter 40, the following compositions can be provided :

1) 100 parts methylvinyl silicone rubber (HDU)

(e.g. silicone rubber R 420/40 U of the German company Wacker) .

10 parts carbon black N 765 0.9 parts dicumyl peroxide. The following electrophysical properties are achieved: Relative dielectric constant ε_r = 4.6 Resistivity: = 3 x 10¹⁴ Ohm cm.

Dielectric strength: = 10 kV/mm

2) Substances and parts as mentioned above, however carbon black type N 683 is used instead of N765. The following electrophysical properties are achieved: Relative dielectric constant ε_r = 3.5 Resistivity: = 5 x 10¹⁴ Ohm cm.

Dielectric strength = 12 kV/mm

It has been found that it is possible to optimize the electrical properties of the material of the adapter 40 according to the requirements of the specific intended use. The main properties, namely the relative dielectric constant and the dielectric strength, can be varied within certain limits by selecting the diameter range of the glass bubbles, the thickness of the aluminum coating and the concentration of the glass bubbles in the matrix material as described in PCT/US95/06125, which is incorporated herein by reference. For example, using glass bubbles of the sieving range of 32-50 microns and an aluminum coating thickness of 44.3 Angstroms and a percentage weight of 5%, a relative dielectric constant of 3.85 and a dielectric strength of 16.5 kV/mm is obtained. Alternatively, when choosing the same sieve size of the glass bubbles, namely 32-50 microns, but an aluminum coating of 23.6 Angstroms and a percentage weight of 3%, the dielectric constant would be reduced to 3.45 and the dielectric strength increased to 21 kV/mm.

The adapter 40 described above provides void filling and stress grading functions between and around the cores of the two cables 2,3. The sleeve 50 provides insulation and electrostatic screening for the completed joint and is shown schematically in cross- section in Fig. 4a, Fig. 4b and Fig. 5. Sleeve 50 may optionally include additional stress grading layers 51, 52. Sleeve 50 is annularly cylindrical and includes earlier described outer layer 54, middle layer 55, and an inner layer which includes median portion 53. Optionally, two end portions 51, 52 may be provided which are spaced from the median portion 53. The outer layer 54 has a uniform wall thickness and consists of semi-conductive material, e.g. semi-conductive Vinyl- Methyl-Silicone rubber. The middle layer 55consists of an insulating cured liquid-silicone rubber.

The optional end portions 51, 52 may consist of stress controlling dielectric material, e.g. Vinyl- Methyl-Silicone rubber vulcanized at high temperature. The dissipation factor is below 0.1 The dielectric constant is between 15 and 25, preferably about 15. The optional stress controlling portions 51, 52 consist preferably of a material which is known from the German patent specification DE 30 08 264. It is a permanent flexible dielectric material having an electrical volume resistivity at room temperature of at least 10⁶ Ohm cm. The base material is silicone rubber or polyethylene or ethylene propylene diene copolymerisate (EPDM) with a content of a finely divided conductive material to increase the relative dielectric constant. The conductive material may comprise strongly structured dust-fine particles of a weakly conductive, electrically polarizable material in a mass content of up to about 350g per Kg base material. Preferably, the conductive material is carbon black. The median portion 54 acts as an electrode and consists of semi-conductive Vinyl-Methyl-Silicone rubber-Mixture vulcanized at high temperature . In the sleeve 50 shown schematically in cross- section in Fig. 4a, the portions 51, 52 and 53 are embedded in the insulation layer 55 but the outer and the inner surface of the sleeve 50 are smooth. The sleeve 50 is formed by injection molding wherein preferably first the portions 51, 52 and 53 are formed and thereafter the layers 54 and 55 are formed. When molding the sleeve, a method is applied preferably which is described in the German patent specification DE 36 33 884, incorporated herein by reference. The layerwise injection molding takes place by means of a plurality of cores having a shape corresponding to the cavity of the article 50, the cores being cyclically advanced through a series of mold cavities. In each of the mold cavities the cores are provided with a layer of material by injection molding, resulting in a further partial article until the last molding operation, with a free core being present at the beginning of the series, and a finished sleeve 50 being removable at the end of the series, sleeve 50 being separable from the core . All layers of sleeve 50 shown are of elastic material so that it can be radially stretched to a sufficient pre-expanded diameter to slip of the cables during jointing. The portions 51, 52 and 53 have preferably the same wall thickness. In the expanded state, one or more support coils 60, 62 are introduced (Fig. 5) for example as known from DE 37 15 915, DE 39 43 296, EP 0 101 472, EP 0 117 092, EP 0 702 444, US 5,589,667 or US 3,515,798. Adjacent convolutions of each support coil 60, 62 may be interconnected in circumferential areas so that the coil may withstand the inherent radial forces of sleeve 50. In the embodiment shown in Fig. 5, the left-hand end portion of the coil 60 is led back through the coil and can be manually gripped at 61. The right-hand end portion of the coil 62 is led back through the coil and can be manually gripped at 63. By withdrawing the coils 60, 62 from the sleeve, the sleeve 50 shrinks radially. No special tools are required.

In accordance with one embodiment of the present invention it is preferred if jointing element 50 is an essentially cylindrical sleeve having two different portions: a first portion 51, 53, 54, 55 with a first recovered inner diameter and a second portion 52, 53, 54, 55 with a second larger recovered diameter as shown schematically in cross-section in Fig. 4a. The second portion may be circular, oval or elliptical in cross- section in order to adapt better to the shape of the adapter 40. Between the two portions, a transition zone 56 with a gradually increasing diameter may be provided. Such a sleeve 50 is adapted to accommodate a single cable on the first portion side and the two cables 2 , 3 on the second portion side. Alternatively, sleeve 50 may find independent application in single- core straight joints in which there is a considerable difference in cable sizes. Different support coils 60 and 62 with differing diameters may be used to hold sleeve 50 in the expanded condition as shown schematically in Fig. 5. Alternatively, sleeve 50 may be held in the expanded state by a single removable support core (not shown) having varying cross-sections according to the first portion, the transition region and the second portion of sleeve 50. The conical end portions 57, 58 are optional and may improve release from a molding press.

As will readily be recognized by those skilled in the art, sleeve 50 is not required to have differing diameters from one end to another. This is especially true, for example, in the case of an H-joint, which joins 4 cables. Such a sleeve 50 and H-joint are illustrated in Figs. 7, 10 and 11. Fig. 4b shows a further embodiment of the sleeve 50 in accordance with the present invention which may also find independent application outside the field of branch joints. Items with the same reference numbers in Figs . 4a and 4b generally refer to the same elements of the sleeve 50 and may be made from the same materials except when indicated to the contrary below. Sleeve 50 as shown in Fig. 4b is a generally cylindrical sleeve having an outer conductive layer 54 and an inner median conductive layer 53 for forming a Faraday cage around the connector of a medium voltage joint. Sleeve 50 as shown in Fig. 4b also has a layer 55 which may be insulating as has been described in detail for the sleeve shown in Fig. 4a. However, in accordance with the present invention, the layer 55 may be made from similar stress grading and insulating materials as described with respect to the adapter 40. In particular, the layer 55 may be made from an elastomeric material as a matrix in which microspheres are substantially uniformly embedded. In particular, the material of layer 55 may comprise a matrix of dielectric plastic material and a content of microspheres conductive at least at the surface thereof and having a diameter of between 10 and 500 μm, preferably between, 10 and 250 μm and more preferably between 30 and 90 μm, the microspheres being evenly distributed in the matrix material . The matrix material may be silicone rubber. The microspheres may be glass spheres coated with a metal, preferably aluminum, nickel or silver. The microspheres maybe coated with a thin insulating layer preferably aluminum suboxide. Sleeve 50 as shown in Fig. 4b may be expanded and held in the expanded state using at least one removable core as has been described with respect to the sleeve 50 shown in Fig. 4a. The conical end portions 57, 58 are optional. Fig. 4c shows a further embodiment of the sleeve 50 in accordance with the present invention which may also find independent application outside the field of branch joints. Items with the same reference numbers in Figs. 4a, b and c generally refer to the same elements of the sleeve 50 and may be made from the same materials except when indicated to the contrary below. Sleeve 50 as shown in Fig. 4c is a generally cylindrical sleeve having an outer conductive layer 54 and an inner median conductive layer 53 for forming a Faraday cage around the connector of a medium voltage joint. Sleeve 50 as shown in Fig. 4c also has a layer 55 which may be insulating as has been described in detail for the sleeves shown in Figs. 4a and b. However, in accordance with the present invention, sleeve 50 according to Fig. 4c has more pronounced conically shaped end pieces 57, 58 as defined by the outer conductive layer 54. The outer conductive layer 54 of these conically shaped end pieces 57, 58 extends to the inner diameter of the insulating layer 55 and may extend beyond this to form semiconductive end tubular portions 59. Semiconductive end portions 59 preferably have an inner diameter identical to that of the abutting portions of the insulating layer 55 and the transition between end portions 59 and the inner surface of layer 55 is preferably smooth without any step or discontinuity. The conical portions 57, 58 provide geometrical stress grading at the ends of sleeve 50. Geometric stress cones for stress grading as provided by end portions 57, 58 are known to the skilled person in the art. As shown in the Fig. 6 which shows a detail of a branch joint in accordance with the present invention, the conically shaped end portions 57, 58 and the semiconductive portions 59 may be installed on the cables 1,2 and 3 so that the end portions 59 partially overlap the ends of the stress control adapter 40. The end portions 59 should be aligned with the core screens 13, 18, 23, so that the ends of the core screens 13, 18, 23 are located near the middle of end portions 59. The smallest diameter region of the conical portion "S" of the insulating layer 55 which forms the stress cone is aligned with the end of the core screens 13, 18, 23 so that the start of the cone portions 57, 58 is at the prepared end of the core screens 13, 18, 23 or displaced therefrom slightly towards the middle of the joint.

In the above description of the invention, stress control adapter 40 has been described as a high dielectric constant-type stress control adapter. However, a stress control adapter 40' may also relieve electrical stress by using geometric stress control. Figs. 8 and 9 illustrate a stress control adapter 40' which uses the geometry of the adapter materials to control electrical stress. Stress control adapter 40' includes insulative portion 80 and conductive portion 82. Insulative portion 80 and conductive portion 82 are preferably made of silicone elastomers which have sufficient elasticity to be radially expanded and relaxed to be placed onto a cable connection or termination. Preferred silicone elastomers for use in insulative portion 80 include, but are not limited to, liquid silicones available as Baysilone ^® LSR series numbered 2030-2040, available from Bayer Corp.,

Elastosil® LR3013/40 to 3003/50, available from Wacker Silicones Corp., Silastic® 9280-30 to-40 series from Dow Corning, "KE 1950-30 to 1950-40", available from Shincor Silicones Inc., and "LIM 6030-D1, and 6040-D1", available from General Electric Corp. ; as well as gum silicones available as Silastic® M2809 from Dow

Corning, Elastosil® 4000/40 through 4000/70 from Wacker Silicones Corporation, Tufel® I SE 846, and Tufel® II 94405, available from General Electric, "SVX-14007B" , available from Shincor Silicones Inc. and "HWP AC3537", available from Bayer Corp. For use as the conductive portion, electrically conductive silicones such as Elastosil® R573/50, available from Wacker Silicones and "KE-3611U", available from Shincor Silicones. Other suitable materials will be known to those skilled in the art. As best seen in Figs. 10 and 11, the interface between insulative portion 80 and conductive portion 82 is shaped such that when adapter 40' is placed on cables prepared for splicing, conductive portion 82 overlaps the end of the electrostatic screen 18, 23 of the cable and smoothly transitions away from the cable insulation 19, 24, thereby moving the concentration of electrical stress away from cable insulation 19, 24 and into adapter insulative portion 80. Adapter 40' thereby provides greater insulation to accommodate the electrical stress and reduces the electrical stress in the joint.

Fig. 10 illustrates an H splice using geometric stress control adapter 40' in combination with sleeve 50 like that illustrated in Fig. 4a. As described above, sleeve 50 of Fig. 10 uses high dielectric constant materials to provide additional electrical stress control in the joint. In contrast, Fig. 11 illustrates an H splice using geometric stress control adapter 40' with sleeve 50 as shown in Figs. 4c and 6. Sleeve 50 of Fig. 11 uses geometric stress control methods to provide additional electrical stress relief in the joint. As can be clearly seen, geometric-type stress control adapter 40' may be used with a high dielectric constant-type stress control sleeve or with a geometric-type stress control sleeve. Similarly, a high dielectric constant-type stress control adapter 40 may be used with a high dielectric constant-type stress control sleeve or with a geometric-type stress control sleeve .

The sequence of jointing the branch joint 10 in accordance with the present invention is shown schematically in cross-section in Figs. 12a to 12c. As shown in Fig. 12a, the cables 1 - 3 are first prepared for jointing in the conventional way by exposing conductors 15, 20, 25, insulation 14, 19, 24, core screens 13, 18, 23, folding back drain wires 12, 17, 22 or securing shielding tapes if provided, e.g. by tack soldering and cutting back cable jackets 11, 16, 21. Expanded jointing element 50 is then parked on cable 1 with the larger end towards the joint middle. Stress control adapter 40 is then pushed over the exposed dielectric 19, 24 of cables 2, 3 respectively until it overlaps core screens 18, 23. Alternatively, if adapter 40 is pre-expanded as shown in Fig. 3, the adapter 40 is placed over the cable dielectrics and support cores 45, 46 are removed by pulling ends 47, 48 to cause adapter 40 to collapse onto the cores. The conductors 15, 20 25 are then connected with a suitable connector 30 by soldering, crimping or by a screw connector. The joint now appears as in Fig. 12b.

Then, jointing element 50 is centered over the joint and caused to collapse by removing cores 60, 62 (Fig. 5) by pulling on ends 61, 63 respectively. The joint now appears as shown in Fig. 12c.

Finally, the shielding wires or tapes 12, 17, 22 are joined to provide electrical continuity across the joint and suitable joint mechanical protection and sealing across the joint are provided by conventional techniques .

In the above, a kit for forming a branch joint has been described consisting basically of two components, a sleeve 50 to provide insulation and screening and an adapter 40 to provide insulation and stress grading in the area of the complex geometry between the plurality of cables on one side of the joint. While the invention has been shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention. For instance, the installation sequence shown in Figs. 12a to 12c has been described with reference to a sleeve 50 shown in Fig. 4a, however, the present invention includes any of the covering means described above, e.g. the sleeve 50 of Fig. 4b or of 4c, or a sequence of cold-shrink or heat-shrink tubing fulfilling the same functions. Further, the installation of the adapter 40 has been shown in Figs. 6 a-c without an outer stress controlling layer 49, however, the present invention includes the use of an adapter with an additional stress controlling layer 49 in combination with any of the sleeve 50 described in accordance with the present invention, e.g. as described with reference to Figs. 4a, b, c or d.

Claims

What is claimed is:

1. A branch joint between at least three shielded medium-voltage electric cables, each of the cables having at least one conductor surrounded by a dielectric layer, a section of which has been removed to expose a length of the conductor, and an outer electrically conductive shield a section of which has been removed to expose a length of the dielectric layer so that the exposed conductors of the cables are joinable using a connector with at least two of the cables being positioned in relatively close substantially parallel relationship with each other, the joint comprising a stress controlling adapter having an elastomeric body provided with at least two hollow portions allowing the body to be placed onto the outer electrically conductive shield and the exposed section of the dielectric layer of the at least two cables being positioned in substantially parallel relationship, wherein the material of the stress controlling adapter has a relative dielectric constant of greater than 3 and a dielectric strength of at least 10 kV/mm, and covering means comprising an insulating layer extensible across the joint so as to overlap a portion of the shield of each of the cables and a semi-conductive shielding layer positioned over the insulating layer which is connectable to each of the cable shields .

2. A branch joint in accordance with claim 1 wherein the relative dielectric constant of the material of the stress controlling adapter is greater than 4.

3. A branch joint in accordance with claim 1 wherein the material of the stress controlling adapter comprises a matrix of dielectric plastic material and a content of microspheres conductive at least at the surface thereof and having a diameter of between 10 and 500 ╬╝m, the microspheres being evenly distributed in the matrix material .

4. A branch joint in accordance with claim 3 wherein the matrix material of the stress controlling adapter is silicone rubber.

5. A branch joint in accordance with claims 3 wherein the microspheres have a diameter between 30 and 90 ╬╝m.

6. A branch joint in accordance with claims 3, wherein the microspheres are glass spheres coated with a metal selected from the group consisting of aluminum, nickel and silver.

7. A branch joint in accordance with claim 3, wherein the microspheres are coated with a thin insulating layer.

8. A branch joint in accordance with claim 7, wherein the insulating layer is aluminum suboxide.

9. A branch joint in accordance with claim 1, wherein the stress controlling adapter is provided with an additional outer stress control layer the relative dielectric constant of which being substantially larger than that of the material of the stress controlling adapter.

10. A branch joint in accordance with claim 1, wherein the stress controlling adapter has the form of an elastomeric push-on article which is positionable to overlap the shield of the parallel cables and a length of the exposed adjacent dielectric layers.

11. A branch joint in accordance with claim 1, wherein the stress controlling adapter has the form of a dimensionally recoverable tubular article which is positionable to overlap the shield of the parallel cable and to extend over a length of the exposed adjacent dielectric layers.

12. A branch joint in accordance with claim 11 wherein the dimensionally recoverable tubular article is supported on removable cores and is caused to recover by removing the cores .

13. A branch joint in accordance with claim 1 wherein the covering means comprising an insulating layer and a semi-conductive shielding layer further include at least one stress control means which is positionable to overlap the shield and a length of the exposed adjacent dielectric layer of at least one cable.

14. A branch joint in accordance with claim 13 wherein the relative dielectric constant of the material of the stress controlling adapter is greater than 4.

15. A branch joint in accordance with claim 13 wherein the material of the stress controlling adapter comprises a matrix of dielectric plastic material and a content of microspheres conductive at least at the surface thereof and having a diameter of between 10 and 500 ╬╝m, the microspheres being evenly distributed in the matrix material.

16. A branch joint in accordance with claims 15 , wherein the microspheres have a diameter between 30 and 90 ╬╝m.

17. A branch joint in accordance with claim 1, wherein the covering means further include a stress control layer which is positionable to overlap the stress controlling adapter whereby the relative dielectric constant of the stress control layer is substantially larger than that of the material of the stress controlling adapter.

18. A branch joint in accordance with claim 1, wherein the covering means further include at each end thereof a stress cone which is positionable to overlap the stress controlling adapter.

19. A branch joint in accordance with claim 1, wherein the insulating layer of the covering means has at least the same relative dielectric constant and dielectric strength as the material of the stress controlling adapter .

20. A branch joint in accordance with claim 19 wherein the insulating layer of the covering means consists of essentially the same material as the stress controlling adapter.

21. A branch joint in accordance with claim 20 wherein the relative dielectric constant of the material of the stress controlling adapter is greater than 4.

22. A branch joint in accordance with claim 20 wherein the material of the stress controlling adapter comprises a matrix of dielectric plastic material and a content of microspheres conductive at least at the surface thereof and having a diameter of between 10 and 500 ╬╝m, the microspheres being evenly distributed in the matrix material .

23. A branch joint in accordance with claims 22, wherein the microspheres have a diameter between 10 and 250 ╬╝m.

24. A branch joint in accordance with claim 1, wherein the covering means form an essentially cylindrical single sleeve of elastomeric material which is pre- stretched and supported on at least one removable core and which is recoverable by removing the at least one core.

25. A branch joint in accordance with claim 24 wherein the sleeve in a first longitudinal portion at one end of the sleeve has an inner diameter adaptable to a single cable and in a second longitudinal portion at the other end of the sleeve has a larger circular or elliptical inner cross section adaptable to at least two cables, the first and second longitudinal portions being connected through a third longitudinal portion which provides a smooth transition between the first and second longitudinal portion.

26. A branch joint in accordance with claim 25 wherein the sleeve is supported by a single removable core having varying cross sections according to the first, second and third longitudinal portions of the sleeve respectively.

27. A branch joint in accordance with claim 1, wherein the covering means further include an inner layer of semi-conductive or conductive material which is positionable to cover the connector and/or the conductors of the cables .

28. A branch joint in accordance with claim 27 wherein the inner layer of semi-conductive or conductive material is capable of overlapping an exposed portion of the stress controlling adapter.

29. A kit for establishing a branch joint between at least three shielded medium-voltage electrical cables comprising a stress controlling adapter having an elastomeric body provided with at least two hollow portions allowing the body to be placed onto the outer electrically conductive shield and the exposed section of the dielectric layer of at least two cables, wherein the material of the stress controlling adapter has a relative dielectric constant of greater than 3 and a dielectric strength of at least 10 kV/mm, and covering means comprising a substantially cylindrical insulating layer and a semi-conductive shielding layer positioned over the insulating layer.