This invention relates to enclosures for insulating electrical apparatus, to a method of insulati electrical apparatus, and to a kit of parts for use in making enclosures.

When a continuously shielded high voltage cable is spliced or connected to a component, it is frequently necessary that not only should the splice or termination be provided with a Layer of insulation having a thicknes appropriate to the voltage at which the cable is to be used, but that earth continuity be maintained across 'the splice or termination and that, where relatively high voltages are concerned, stress control be provided at the end of the cable shield to relieve the electrical stress at that point. There is thus a need, especially where relatively high voltage are concerned, for an enclosure for electrical apparatus, especially a cable termination or splice, that can be manufactured economically and applied with the minimum of time and skill.

According to the present invention there is provided an enclosure suitable for providing earth continuity in electrical apparatus, especially cable - terminations and splices, which enclosure comprises firs and-second hollow articles each- having at least one open end, the first hollow article being capable of being positioned about, and of being brought into close conformity with, the electrical apparatus and comprising a hollow electrically insulating component, and the seco

hollow article being capable of being positioned about, and of being brought into close conformity with, the first hollow article Cor a further layer installed on the electrical apparatus, the second hollow article comprising a hollow electrically insulating component within and axially substantially aligned with a hollow electrically conductive component. ϊn another aspect, the invention provides a method for insulating electrical apparatus, especially cable terminations and splices, which comprises positioning about the electrical apparatus a first hollow article having at lease one open end and comprising a hollow electrically insulating component and bringing the article into close conformity with the electrical apparatus, positioning about the installed first article, optionally after providing one or more further, preferably insulating, layers about the first article, a second hollow article having at lease one open end and comrpising a hollow electrically insulating component within and axially substantially aligned with a hollow electrically conductive component, and bringing the second article into close conformity with the first article or the said further layer or the outermost said layer.

Where relatively high voltages are concerned, for example voltages above about 15 kv (and in some cases above about 12 kv) the first article advantageously comprises a hollow stress grading component within the electrically insulating component. Alternatively, some other form of stress control could be used. One way of achieving this would be to taper the insulating layer at one or both ends of one or both of the articles.

It is to be understood that although in the first article the stress grading component (if present) is radially within the electrically insulating component and in. the second article the electrically insulating component is radially within the electrically conductive component, the components in each article are not necessarily longitudinally coextensive, and in each ^~$^^_~A ff

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article either component may if appropriate extend beyond the other component at one or both ends thereof.

The invention also provides electrical apparatus insulated" by an enclosure- or method- in accordance with the invention.

•5

The present invention makes it possible ..to provide an enclosure for, for example^ a cable termination or splice, in which earth continuity can be maintained and a relatively thick insulating layer, and if desired stress relief, can be provided by the use of two readily installed

10 components without the need to resort to, for example, complex tape winding procedures commonly used in the past for building up individual layers. The fact that a relatively thick insulating layer can be provided (by the use.-of a.f_i.rst--ar.ti.cle...comprising an outer electrically

15 ^* insulating layer and a second article comprising an inner electrically insulating layer, one or more further electrically insulating layers being installed, if desired, between the two said electrically insulating layers) is particularly advantageous when, for example,

20 terminating or splicing high voltage cable, for example . cables operating at 8 kv and above, particularly 12 kv and above. Thus, for example, there is no need for the prolonged heating,. ith the attendant risk of damage to the parts, that would be required in order to shrink

25 a heat-recoverable insulating layer of the thickness required for, for example, ;a 15 kv cable.

The invention also has the advantage that it avoids the need for the installer in the field to form interfaces between insulating layers and conductive layers or insulating layers and semi-conductive layers. When_such interfaces are formed, for example, when using tape winding procedures to build up individual layers, any voids which are formed between individual layers are adjacent to the conductive or semi-conductive layers, with

35 the risk of discharge activity in the voids leading to deterioration of the insulation. In accordance with the present invention, the interface formed by the installer

any voids formed are not subject to the same electrical stress as would be' the case if they were adjacent to s conductive or semi-conductive layer. Thus it will be noted that in, for example, splices protected in accordance with the invention, there can be interfaces in the dielectric above the connection between the inner conductors.

A further advantage of the invention is that, as two separate insulating layers are used, it is possible to use layers of different dielectric constant, thus, for example, the insulating layer of the first article may have a higher dielectric constant than the insulating layer of the second article. The use of insulating layers of different dielectric constants may make it possible to make better electrical .use of the insulation; thus, for example, a more uniform stress across the total insulation may be obtained than would be possible with a single layer of uniform dielectric constant.

The invention also makes it possible to provide enclosures for electrical apparatus, for example cables, of a wide range of voltage classes using only a small set of relatively simple articles.

The hollow articles used in accordance with the invention are preferably open at both ends. The articles may have any desired configuration, and although the following discussion is primarily in terms of tubular articles, which are preferred, any other suitable configuration may be used unless this is clearly inappropriate in the circumstances. Although th is not essential, from a manu acturing point of view at least that portion of the first article comprising the stress grading layer is desirably of substantially uniform cross-section along its length in the stable or fully recovered state, that is to say the ratio of the thicknesses of the layers is advantageously substantiall constant along the length o'f the stress grading layer an the general configuration of the cross-section is -"$u f _ ^O

Although in the following discussion the invention • is described primarily in terms of a two-layer first . article having an inner stress-grading layer and an outer electrically insulating layer and a two-layer • second article having an inner electrically insulating layer and an outer electrically concuctive layer , it will be appreciated that where applicable to a particular

_* situation, one or Both, of the articles may comprise one or more further layers and/or, for example ^' , a separate

10 electrically insulating component -may be installed around the first article after installation thereof and before installation of the second article thereon. T -us, for example, a further lamina, for example an adhesive, may be present between the two essential laminae of

15 materials which will carry out a desired function. Furthermore one or both, articles may, for example, comprise one or more other laminae inside or outside the laminae essential to the invention; any additional laminae between the electrically insulating layer of the ^'

20 first article and the electrically insulating layer of th second article may, for example, be electrically insulati or could for example be. ^' conductive or semi-conductive.

Any suitable stress rel evin jiiaterial may be used as the stress grading inner ^' layer of the first a ticle _r :.

25 ^" and the material "used may- ha-ye electrical impedance characteristics wjaic . are r ist ve or capacitatiye , ^' r a. combination of both. Preferabl the stress grading layer is semi-conductive and comprises a polymeric matrix having dispersed therein a conductive filler, and

30 especiall rcarbon black .

• Polymeric materials - suitable for -use as the polymeric matrix include , for example , resins comprising , for example , polyolefins and olefin copolymers for

— 35 example- polyethylen , polypropylene , ethyiene/propylene-. copolymers , and polybutenes ; substituted polyolefins , particularly halogen-sxtbstituted polyolefins , for example polyvinyl chloride , polyvinylidene chloride , polyyinylide floried , Teflon 100 C _. a polytetraf luoroethylehe manufactur fsΪEA

by Du Pont) - Teflon ^* PFA (a copolymer of tetrafluoroethylene and' perf luoroalkoxy moieties manufactured by Du Pont) , Tefzel (a terpolymer of ^• ethylene , tetrafluoroethylene and perf luoroalkoxy . moieties manufactured by Du Pont) , Tefzel ( a

Ξ terpolymer of ethylene , tetrafluoroethylene and a fluorinated .monomer manufactured by- Du Pont) , Tefzel (a terpolymer of ethylene , tetrafluoroethylen and a fluorinated monomer manufactured by Du Pont) , and Halar ^• (a copolymer of ethylene and chlorotrif luoroethylene

Lo manufactured by Allied Chemicals) ; polyme s , for example

Hytrel ( a segmented polyether ester copolymer derived from terephthalic acid , polytetramethylene ether glycol ^' andl , 4-butanediol manufactured by Du Pont) ; and polyurethanes .

15

Examples of other polymeric materials suitable for use as the polymeric matrix include elastomers comprising , for example , copolymers of dienes with olefinically unsaturated monomers , for example e thy lene/p ropy lene/ non-conjugated diene terpolymers , styrene/budadienepolymers ,

20 butyl rubbers and copolymers of dienes with unsaturated polar monomers , for example acrylonitrile , methyl methacrylate , ethyle acrylate , vinyl pyridine and methyl vinyl ketone; halogen-containing elastomers , for example chloroprene polymers and copolymers , for example

25 neoprene , chlorinated polyethylene , chlorosulphonated polyethylene , and Viton Ca copolymer of vinylidene fluoride and hexaf luoropropylene manufactured by Du Pont). ; copolymers of olefins with olefinically unsaturated esters , for example elastomeric ethylene/vinyl acetate

30 polymers , ethylene/acrylic acid ester copolymers for example ethylene/ethyl acrylate and methacrylate copolymers and particularly ethylene/acrylic rubbers , for example Vamac C terpolymer of ethylene , methyl - acrylate and a cures ite monomer manufactured by Du Pont) _. ;

35 acrylic rubbers , for example polyethly acrylate , polybutyl acrylate , butyl aery late/ethyl acrylate . ' copolymers , and butyl acrylate/glycidyl methacrylate

copolymers; silicone elastomers, for example polydiorganos ^' iloxanes, copolymers, block copolymers, and terpolymers of mono ethylsiloxanes, ^• dimethylsiloxane methylvinylsiloxanes and methylphenylsiloxanes, fluorosilicones, for example those derived from 3,3,3- trifluoroprop l siloxane and carborane siloxanes; elastomeriσ polyurethanes-; and polyethers, for example epichlorohydrin rubbers.

Blends of the -above mentioned elastomers and resin may also be used. Particularly good results have been obtained using polyolefins, olefin copolymers and halogen-substituted olefin polymers. .

Any suitable carbon black may be used. Examples suitable carbon blacks that are currently commercially available are types HAF, SRF, EPC,-FEF, and. ECF. The amount of carbon black in the stress grading material w depend to some extent on the type of black used and the polymer matrix, but preferably the material comprises from 5 to 150 parts by weight of carbon black, per 100 parts by weight of polymer.

Alternatively there may be used as the stress grading inner layer a composition having electrical resistive characteristics as described in British Patent Nos. 1,470,01, 1,470,502 1,470,503 and 1,470,504, the disclosures of which are incorporated herein by reference. In place of the polymeric materials listed previously, the stress grading inner layer may comprise a fluid- coating, for example a mastic or a grease. Examples of suitable materials are given in British Pa Specification No. 1,526,397, the disclosures of which a incorporated herein by reference. The coating does not need to have mechanical strength, of itself; all that is required is that the coating stays in position during application and subsequently.

The stress grading inner layer preferably has a high permittivity, usually _t in excess of 20. This q corresponds to a specific ijpedance close to 10 ohm cm.

Preferably-the specific impedance lies in the range.107 to 10 ohm cm, measured at a frequency of 50 Hz.

The electrically insulating layer of the first article and the electrically insulating layer of the second article preferably comprise a substantially discharge resistant material and desirably, though not essentially, comprise a substantially track resistant, and preferably non-tracking, material. ^" By "non- tracking" there is meant a material which,is resistant to the formation of dendritic, carbonaceous, electrically conducting deposits on its surface under the influence of high electrical voltages. The insulating-layers may, for. example, comprise a polymeric material as listed, above, or a mixture of such polymeric materials, and may, for example, comprise a polymeric material as listed above, or a mixture of such polymeric materials, and may comprise an anti-tracking filler. Examples of suitable discharge and track resistant materials •comprising anti-tracking fillers are described in British Patent Nos. 1,041,503; 1,240,403; 1,303,432 and 1,337,951, the disclosures of which, are incorporated herin by reference. The electrically insulating layers may comprise the same or different materials.

Preferably each of _. the insulating layers has a dielectric constant of from 2 to 4 and a volume resistivity of at least 10 , preferably at least 10 ohm cm. The two layers may have substantially the.-same or different properties provided that each is electrically insulating.

The conductive outer layer of the second article may comprise a woven or stranded metal braid or a conductive paint but preferably comprises a polymeric matrix having a conductive filler dispersed therin, or wires embeded therein which may replace the usual external braid. The polymeric matrix may, for example, comprise any of the polymeric materials listed previously,

may, for example, comprise metal particles or a conductive carbon black.- Particularly good results have been achieved using an electrically conductive polymeric composition as described in British Patent No. 1,294,665 the disclosure of which is incorporated herein by reference. The conductive outer layer preferably comprises from lO to 70 parts by weight of the conductive filler, based on the total weight of the polymeric matrix and . the filler.

The material of the conductive outer layer perferably has a volume resistivity of less that 5 x 10 ohm cm, and most preferably less than 100 ohm cm.

In general it can be said tat the mechanical, thermal and insulating requirements for each of the layers in the two articles should be appropriate to the cable or other electrical apparatus to be protected by the enclosure. Typically, for high voltage cables is is advantageous for the layers to be flexible and suitable for continuous use over a termperature range of at least

^• 40 to +7o°C. The preferred material for the insulating layers are ethyl polymers and copolymers, and elastomers, preferably containing additives to achieve good discharge resistance (see above) . The first and second hollow articles used in accordance with the present invention can- be positioned about and brought into close conformity with the electrical apparatns to be protected and the installed first article (or a layer thereon) respectively, and desirably, but ^' not essentially, the close conformity results in there being substantially no voids between contiguous layers. By close conformity is meant the property of an article to follow closely the contours of an underlying substrate. Such close conformity may be obtained, for example, by the use of articles that are elastomeric or heat recoverable, or both.

Where the first and/or second article is elastomeric it may be brought into close conformity with

. .

the underlying substrate by simply pushing it onto the substrate, the elasticity of the article enabling it ont the substrate, the elasticity -of the article enabling it to conform closely to the contrours thereof. In another embodiment an elastomeric first or second article may be "held-out" in a stretched state by an inner or outer hold-out member which can be removed or displaced, the elastic-stresses released thereby urging the article to recover into conformity with the substrate apparatus. Thus, for example, .a relatively rigid spirally wound core could be progressively unwound thus -allowing an elastomeric article positioned over the core to recover in conformity with the substrate. In a still further embodiment the article may be bonded to th hold-out member and the bond weakened, for example by solvent or mechanical treatment, to permit recovery.

Preferably, however, at least one of the hollow articles is heat-recoverable, and, advantageously,, both hollow articles are heat-recoverable. Usually, such articles recover, on heating, towards an original shape from which they have previously been deformed, but the term "heat-recoverable" as used herein also includes any article which on heating adopts a new configuration, even if it has not previously been deformed. In their most common form, heat-recoverable articles comprise a polymeric material exhibiting the property of elastic or plastic memory as described for example, in U.S. Patent 2,027,962 3,068,242 and 3,957,372. In- other articles, as described, for example in British Patents 1,434,719 and 1,440,524, an elastomeric member, which is held in a stretched state by a second member, which upon heating, weakens and thus allows the elastomeric member to recover. The disclosures of these Specifications are incorporated herin by regerence Where either or both of the articles is or are heat- recoverable, the stress grading layer of the first artic the insulating layer of the first article, the insulatin layer of the second article may each be independently

O

heat-recoverable, or one or more of the layers may be elastomeric, provided that the articles-(s) as a whole is or are heat-recoverable. It may in some cases be desirable for the second article to comprise an elastomeric electrically insulating layer "held out" by an electrically conductive layer that weakens on heating; in this case a relatively thick insulating, layer may be provided in the second article without the need for a proportionately long heating time for effecting recovery.

The stress-grading component (if present) of the first article may if desired be a continuous layer extending from one end to the other of the article and a first article having such a stress-grading layer has the advantage (see below) that it can be manufactured by 5 coextrusion of the layers. In other embodiments, however, the stress grading layer may extend over only part of the length of the first article provided that in use it can provide the necessary stress relief. _ The invention also comprehends the use of a tapered portion of the insulating layer(s) to produce a stress con adjacent to the or each cable shield end. This tapered portion preferably does not extend beyond the end of the cable shield, and it has been found to be _c surprisingly advantageous to space the tapered portion longitudinally"from the end of the shield, thus producing smoother stress grading than in the case where the tapered portion ends at the shield end.

Thus, for example, in the case of shielded high 0 voltage cable, the stress grading must extend for a sufficient distance over the dielectric from the end of the shield to provide the necessary stress control at the end of the shield.

In one embodiment wherein the stress grading layer 5 extends over only part of the length of the first article., the stress grading layer is substantially centrally disposed along the length of the first article, and in this

case, the stress grading- layer preferably extends for at least 60% of the length of the a'^ticle and ^' most preferably for at least 75% of its length.

In a second embodiment wherin the stress grading layer extends over only part of the length of the first article is electrically conductive. In .use such an electrically conductive portion may be positioned around, for example, a splice between the central conductors of two cables or the connection of a cable conductor to an electrical component to provide improved electrical characteristics in the splics or connection. In a particularly preferred arrangement, the inner surface of the first article comprises an •electrically conductive layer centrally disposed along the length of the article and a stress grading layer o. each side of the electrically conductive layer, which stress grading layers may or may not extend to the ends of the article. For some purposes, it is- advantage to provide electrically conductive layers at the ends of the article as a means of establishing earth contact, in which case the inner surface of the preferred arrangement just mentioned comprises an electrically - conductive layer disposed between the two stress grading portions, all three being between the end conductive layers.

In some cases, although not for example when the stress grading layer has non-linear characteristics, it is advantageous for the stress grading layers in this arrangement to contact the conductive layer. When the article is to be used for protecting, for example, the. termination of a cable to an electrical component one of the stress grading layers itiay- be omitted. The invention also provides a first article in accordance with the invention when the inner wall of the article is sub-_ . stantially without steps and comprises at least one stre grading portion and at least one electrically conductive portion.

Where the use of an electrically conductive layej

round, for example, a cahle splice is desirable thi.s may, of course, be a conductive layer applied, for example painted, on a continuous stress-grading layer. Alternatively, for example a conductive layer could, be installed round the splice Before the installation of the first article thereon.,.-or the/articl -comprising _^ the stress grading layer and the ^' insulating laye .may e partially recovered round a, conductive insert to form ^' a recoverable article which can substantially be •recovered around, for example, a splice.

10 Whether or not an electrically conductive inner layer is used it may in some circumstance ^" Be found to be advantageous to provide the ^' space adjacent to the exposed electrical conductor, for exiample the region surrounding the crimped centra.1 conductors.of a cable,

15 splice, with a void-filling material. Such a meteria.1 may be a grease, for example a silicone grease/ a mastic, or a hot melt adhesive. ^" A particularly- suitable void-filling materia.1 is described in German Offenlegungsschrift 2,748,371, the ^" disclosure of which

20 is incorporated herin by reference. ^' A substance- R cB- acts, for example as- an adhesive sealant, or void filling material ma_y also.Be ^" used, for example/ Between the first and second articles, Tim ^' s, for example, such, a substance may be provided on t least part of the interior

25 of the second article and/or at least of the exterior, of the first article.

At least the second article of the present invention can be formed so as- to have ^" a substantially uniform 0 cross-section along its length, at least in the staBle or freely- recovered state, if the articles are recoverable, thus enabling the article to be produced by relatively inexpensive extrusion methods. This is a considerable advantage over prior art designs which requently requi e

-... sophisticated moulding operations. Where the stress grading layer extends along the entire length of the inner article, this article ^, m also be extruded. Other methods, for example moulding or casting, could of coursjarξ

be used for either of the articles. However, the preferred method production, where this is possible, is by extrusion of each of the articles, followed if necessary by treatment to render the products recoverable. This, treatment may involve, for example, crosslinking by

5 ionising radiation or by chemical crosslinking agents, followed by expansion, for example, using differential gas pressure or a mandrel. Each of the articles could also, of course, be built up from the individual layers, the layers being bonded together if".necessary. Examples ^'

10 of suitable adhesives are given in British Specification ' Nos. 1,434,719 and 1,440,524.

The dimensions of the articles used according to the invention-will of course vary ^" depending on the application, and the relative thicknesses of the various layers in the

15 two articles will be dictated to some extent by the required electrical properties of the enclosure, but examples of suitable dimensions are from o.5 to 4.0 mm, preferably 1.0 to 3.0 mm, for the stress grading layer, from 0.5 to 6 .0 mm, preferably 1.0 to 3.0 mm, for the

20 insulating layer of the first article, from 1.0 to 20.0 mm, preferably 3.0 to 10.0 mm, for the insulating layer of the second article, and from 0.5 to 4.0 mm, preferably 1.0 to 3.0 mm, for the conductive layer. Typical dimensions for an enclosure for a 12 kv splice are 2 mm for the stress

25 grading layer and for the inner insulating layer, 4 mm for the outer insulating layer and 1 mm for the conductive layer, while typical dimensions for a 24 kv splice are 2 mm for the stress grading layer and the inner layer, 8 mm for the outer insulating layer and

30 1 mm for the outer conductive layer.

Where, for example, the enclosure of the invention is installed on a shielded component, for example, a shielded high voltage cable, the stress grading layer ₃ 5 (if presnet) of the first article and the conductive layer of the-second article are required to be in electrical contact with the shield (and thus with each other), at least after the enclosure is installed on the cable, and

this may be -achieved by an appropriate configuration of the ends of the enclosure or by the provision of means for making electrical contact between the layers, a common connection being made to the shield. Eleftrical contact between the stress grading layer and the shield and the conductive layer and the shield may be direct or indirect.

In one simple form, for example, the ends of the enclosure may be profiled so that, on engagement, both the conductive layer and the stress grading layer contact the cable shield, and indirect, electrical contact between these two layers is thereby made through the shield. Alternatively the ends of the enclosure may be so arranged that the- insulating intermediate ^' layers terminate just before the stres rading layer and electrically conductive layer thus allowing them to come into- direct electrical contact, a common connection then being made to the shield; if the stress grading layer is interposed between the shield and the conducting layer, the voltage drop across this layer should in general be negligible and preferably a spearate low resistance connection (for example By means of a conductive strap or braid as described below should be madel. Another possibility is that indirect electrical contact may be provided by conductive members which fit on the ends of the enclosure. Such members can, for example, be metal straps or braids or moulded parts formed from conductive polymeric materials, which, may, if desired, be heat- recoverable. The moulded parts may, for example, be annular members which have grooved faces--and can fit over the ends of the enclosure, and may advantageously be provided with an internal coating of a sealant, for example -mastic or-hot -melt adhesive, ^" to-give environmental protection to the ends of the enclosure. The internal coating of sealant should either be conductive, or arranged in such a way that the conductive moulded part can.provide the required electrical continuity.

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The enclosures of the invention are particularly suitable for protecting terminations and splices in high voltage power cables, for example those operating a voltages in excess of 8 kv and especially in excess of . 12 kv.

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings-, in which:

Figures 1 and 2 are axial sections through two different first articles suitable for use in accordance with the invention;

Figure 3 is a longitudinal section through a second article suitable for use in accordance with the invention;

Figure 4 is a longitudinal section through one half of a splice protected in accordanc with, the invention;

Figure 5 is a longitudinal section through a sleeve of electrically insulating material 20 having an inner lining of stress grading material 21, and innermost layers of electrically conducting material 22 and 23 at its central and end portions. Such a sleev is suitable for use as the first articl in some embodiments of the present invention. The end conductive layers 2 are optional;

Figures 6 to 8 show schematically the tapering of electrically insulating layer to form a stress cone suitable for use in the sec article according to the present invent

The figures are not drawn to scale.

Referring now to the drawings. Figure 1 shows a tube indicated generally by the reference numeral 1 _/

the invention. The tube 1 comprises an inner layer 2 of stress grading material and an outer layer 3 of electrically insulating material. In this embodiment, the stress grading layer 2 extends for the whole length of the tube 1.. .

In the embodiment shown...in.JEigure-2, however, a tube generally indicated by the reference numeral 4 comprises an electrically insulating outer layer 5 and, on the inner surface of the layer 5,. an electrically conductive layer 6 which is centrally situated along the length of the tube 4 and dows not extend to the ope ends 7 and-8 of the tube 4. Each of two stress grading layers, 9 and 10 respectively, extends from a respective open end of the tube 4 towards the electrically conductive layer 6. Advantageously, unless the stress grading layer has non-linear characteristics, each of the stress grading layers 9 and 10 touches the electrically conductive layer 6 (i.e. the distances x and y in Figure 2 are advantageously zero) . It will be noted that the inner wall of the tube 4 is smooth and without steps which could lead to ^' undesired voids in the final structure.

Figure 3 shows a tube generally indicated by the reference numeral 11 suitable for use as a second article in accordance with the invention. The tube 11 comprises an inner electrically insulating layer 12 and an outer electrically conductive layer 13.

Figure 4 shows one half of a cable splice having an enclosure made using the first article (outer tube) of Figure 3; the right hand side of the splice (not shown) may be substantially identical to the left hand side.

When making the protected splice of Figure 4, an end of a high voltage cable generally -indicated-by the numeral 14 is stripped to expose a length of the shield 15, a length of the dielectric 16 and a length of the centre conductor 17. A crimp sleeve 18 may then be installed over the exposed centre conductor 17 and the exposed centre

_.

conductor of the cable (not shown) to which a splice is to be made, and crimped to provide the desired electric connection. A heat-shrinkable inner tube 4, which has • previously been slipped over one of the ^' cables is then positioned around the splice and heated so that it recovers into contact with the splice. As can be seen

Figure 4, the inner tube.4 is so designed and positione .that the electrically conductive layer 6 recovers round' the crimp connection and the adjacent end of the expose dielectric 16 of each of the cables, while the stress grading layers 9 and 10 (of which only layer 9 is shown in Figure 5) , .recovers round the exposed portion of th cable shield 15 and the adjacent end ^" of the cable dielectric 16.

To complete the enclosure around the splice, a heat-shrinkable outer tube 11 which has previously been slipped over one of the cables is then positioned around the shrunk inner tube 4 and is heated to cause i to recover into contact with the inner tube 4, the arrangement being such that there is electrical contact between the outer electrically conductive layer 12 of t tube 11 and the exposed cable shield 15. If desired an electrically insulating void filling substance, for example a grease, (not shown) may be present on the exterior of the tube 4 and/or the interior of the tube to ensure that there are no voids between the tubes aft recovery thereof. A further electrically insulating tube (not shown), may also, if desired, be positioned between inner tube 4 and outer tube 11, for example by shrinking of a suitable heat-shrinkable tube around the inner tube 4 after installation thereof and an electrical connection between the outer conductive laye and the cable shield 15 is made by means of a metal str or braid (not shown in Figure 4) . As can be seen from Figure 4, the invention makes it possible to provide a relatively thick insulating layer using heat-shrinkable. parts which, because they themselves are not of extreme thickness, can be shrunk

in an acceptable time and without the danger of charring that accompanies the -lengthy heating required to shrink a relatively thick electrically insulating heat _" shrinkable layer.

Figure 6 illustrates the tapering of the insulating - laye -of an .article suitable-for -use as the second article of the present invention,' in the form where the conductive layer 36 makes electrical contact with the cable shield 31 at a distance from the end 32 of the shield substantially equal to the thickness of the

10 insulating layer overlying the shield ends. ^• This arrangement in itself is described and claimed in our copending British Patent Application No.

Figure 7 illustrates the ending of.the tapered • _j c stress cone at the end 32 of the cable shield, without direct contact between the conductive layer 36 and the shield 31.

Figure 8 illustrates the preferred arrangement wherin the stress cone ends at a position 34 which is 20 longitudinally spaced from the cable shield, this arrangement producing improved stress grading as already mentioned.

The following examples illustrate the invention:

25 EXAMPLES ^' 1 ^' to 4

In order to provide splices between 24 kv cables with enclosures in accordance with the invention, inner and outer tubes (whic had the structures shown in Figures 2 and 3 respectively and the dimensions 30 specified below) were made, the dimensions, which for the inner and outer tubes, are fully recovered dimension, being mm unless other wise specified: -

35

OMP

Example Cable sizes for _^ which suitable Inner tube outer ( given as cross- tube sectional area of 1 a b c d e d' D' cable conductor in m.2 ) .

^• 1 16-70 460 150 160 150 13 28 24 39

2 95-185 500 150 200 150 19 34 30 45

3 240-300 530 150 220 150 24 39 ^' 35 50

4 400-800 750 150 440 150 29 44 40 55

The inner tube in each case was moulded, the insulating ^* layer 5 comprising modified polyolefin, the electrically conductive layer 6 (thickness approximately 2 mm) comprising carbon black filled polyolefin and the stress grading layers 9 and 10 (layer thickness approximately 2 mm) comprising semi-conductive modified polyolefin. The outer tubes were coextruded, with the inner electrically insulating layer 12 comprising modified polyolefin and the outer electrically conductive layer le (layer thickness approximately 2 mm) comprising carbon black filled polyolefin. The tubes were then heated and expanded radially to approximately twice their original dimateer and cooled in the expanded state. The heat- recoverable tubes so produced were then installed by the method described in connection with Figure 4 and average i life test were carried out on the protected splices. The results obtained demonstrated that excellent electrical results could be obtained using an enclosure in accordance with the invention.

EXAMPLE 5

An enclosure according to Example 2 was converted to a heat-shrinkable form and installed on a splice between two 24 kv cables . The splice was subjected to a 40 kv/AC for 1 ,000 hours and voltage was afterwards increased by 5 kv/500 hour steps . The splice passed 1 , 000 hours at 40 kv, 500 riours -at .-45 kv. 'and after 300

hours at 50 kv no breakdown had occurred. The splice withstood 10 shots of 150 -kv impulse and also withstood 30 minutes DC at a 100 kv.

Combinations of elements according to the present invention which may prove useful at various voltages include the"hollow second article -according to the- invention (laminate of electrically insulating material inside electrically conducting material) surrounding a hollow first article comprising: -

For 12-15 kv £al a single separate layer of 0 electrically insulating material, optionally with void filler:

(b) same as (a) with additional

^• - separate layer of"stress grading 5 ^" material inside the separate layer of insulating material;

(c) a laminate of a layer of stress grading material within and substantially axially aligned 0 with an outer layer of electrically insulating material;

For 24-25 kv (d) same as (c) with addition of void ^' filler; 5 (el same as (c) with addition of electrically conducting layer at least on a central portion of the innermost surface of the stress grading material;

^J For: 36.kv .. Cfl .same as (e) ;

Cgl same as (e) with insulation in the said second article shaped to provide stress cone(s);

35 (h) same as (e) with addition of further article (c) between (e) and the said second article;

(i) same as (e) with additio of further article (e) between first (e) and th said second article.