GB2094562A - High voltage electrical insulators for carrying tensile loads - Google Patents

Abstract

A high voltage electrical insulator for carrying tensile loads comprises one or more units 12, the or each unit having at least two spaced-apart metal elements 10, 11, preferably U- shaped units, which are interlinked and which are partially embedded in the insulating material of the unit 12 so that one metal element extends outwardly from one face of the insulating material and the other extends outwardly from the opposite face. The two metal elements overlap and preferably have substantially flat parallel faces in the overlapping region; insulating material lies between them however so that when a tensile load is applied to the outwardly extending portions of these elements they can transmit this tensile load via insulating material which is in compression between the elements. The insulating material covers the elements so that only one element is exposed where it extends out of one face and only the other element is exposed where it extends out of the opposite face of the insulating material. Elements in adjacent units may be bolted together. The insulating material of the or each unit 12 may be formed of a synthetic polymer such as a cycloaliphatic epoxy resin, preferably to form a plate-like body which has dependent flanges on its lower surface. These flanges may be in the form of a spiral partially embedded in the moulded plastics material.

Description

SPECIFICATION High voltage electrical insulators for carrying tensile loads This invention relates to high voltage electrical insulators for carrying tensile loads.

Porcelain or glass has been used as an insulating medium for high voltage insulators for many years. More recently however long rod insulators having a core of resin bonded glass fibre in an outer polymeric housing have been developed and are widely used for supporting overhead power lines. Insulators for such lines have two basic functions; they must support the conductor mechanically and they must insulate it electrically. The insluating medium necessarily has to carry a mechanical load. The insulating medium may be stressed therefore in tension or compression or shear or torsion or in a combination of these modes, according to the design of the insulator. In conventional porcelain and glass cap-and-pin insulators, the load is transmitted by a combination of shear and compression stresses. In a long rod insulator, the bulk of the material is in tension.Because of the high strength of resin bonded glass fibre in tension, the long rod design has been favoured for composite insulators and gives economic use of a relatively costly material. However such insulators have to be provided with long end fittings to transfer the load in shear and compression between the end fittings and the thin cylindrical core. This results in vulnerable interfaces. These interfaces are of three sorts. The first which is inescapable in the long rod design, is that between the resin bonded glass fibre core and the housing.

This runs the length of the insulation and is stressed electrically. If the bond between the housing and core is rigid and complete, the interface must stransfer mechanical load from the housing to the core by shear. If the housing is rigid and massive, the stress on the interface will be large. if the housing is thin and flexible, the stress will be small but a thin housing will transmit water vapour to the interface rapidiy. Many flexible materials such as might be used for a thin housing and which have good electrical surface characteristics such as for example PTFE, are difficult to bond to resin bonded glass fibre.

Various constructions have been proposed using a liquid interface to the resin bonded glass fibre but these rely on adequate seals. The second sort of interface, which is avoided if the insulator is formed using a one-shot moulding technique but which arises in the insulators having an assembled multi-part housing, is that which occurs in each of the joints between the sheds of the insulator.

These joints are vulnerable to the radial electric stresses which are produced by the combination of pollution layers on the insulator surface and any defective section of the longitudinal interface.

The third interface, which is unavoidable, is that between the metal fittings and the polymeric housing. These regions are subject to corrosion as well as to large electrical and mechanical stresses.

All the above-mentioned interfaces have given rise to problems when insulators are in service.

The first two interfaces commonly fail because of mechanical imperfections which result in moisture ingress and subsequent internal tracking and they are undoubtedly the most common cause of failure in this type of insulator.

Because of the vulnerability of the longitudinal interface and because the electrical (and possibly mechanical) integrity of the insulator is totally dependent on it, many other desirable design features have been sacrificed to achieve good interfaces. For example, to eliminate the intershed bonds by using a one-shot moulding technique, the shed profile has had to be greatly simplified, the thickness of the housing has had to be reduced and the housing material has to be chosen for good bond or for flexibility rather than for good surface characteristics. Consequently difficulty has been found in producing a reliable resin bonded glass fibre cored insulator which has a good performance in pollution conditions.

Other designs of insulators are possible which eliminate the interfaces carrying the total working voltage. These include the well-known cap-andpin design, the insulated bucket type described in U.K. Patent Specification No. 1148502 and insulators in which metal elements, between successive insulating elements, are swaged to engage between complementary ripples on the surfaces of the insulating elements.

The present invention is directed to an improved form of insulated construction which avoids electrically stressed internal interfaces and which very substantially will retain its mechanical integrity and strength even if the insulating material fails.

According to the present invention, an electrical insulator for carrying tensile loads comprises one or more units, the or each unit having at least two spaced-apart metal elements which are interlinked and which are partially embedded in insulating material so that one metal element extends outwardly from one face of the insulating material and another extends outwardly from an opposite face of the insulating material, individual metal elements overlapping but having the insulating material between them so than when a tensile load is applied to the outwardly-extending metal elements they remain electrically insulated but can transmit loads via insulating material which is in compression between the elements, the insulating material covering the elements so that only one element is exposed where it extends out of one face of the insulating material and only another element is exposed where it extends out of the opposite face of the insulating material.

With this construction the insulating material may be in the form of a plate but can readily be of relatively complex shape, e.g similar to sheds of known types of high voltage insulators. The insulating material conveniently is moulded around the metal elements. The insulating material on each unit may extend outwardly from a longitudinal axis to form a multi-shed insulator unit. Individual units can be joined by connecting a metal portion of one unit to a metal portion of an adjacent unit, e.g. using a pin or a bolt The load bearing area for the insulating material between the metal elements can be large without necessitating any significant increase in the overall size of the insulator.This material between the metal members is under compression when the insulator carries a tensile load and the metal members will still be interlinked even if the insulating material between them is fractured.

The metal members, when the insulator assembly is in use, are at differing electric potentials and they are commonly referred to as electrodes.

The metal members, in one convenient form of construction each comprise a U-shaped link.

Preferably each link is symmetrical about a plane through the longitudinal axis of the insulator. Such U-shaped members may be interlinked by arranging them in orthogonal planes about the axis of the unit and the insulating material can be moulded around and between them with the arms of one member protruding from one face of the insulating material and the arms of the other member protruding from the opposite face.

Provided the region where the members overlap is completely embedded in the insulating material, the two members are electrically insulated by the insulating material between them and the external leakage path from the exposed part of one member to the exposed part of the other member would be outwardly over one surface of the insulating material and then back along the opposite surface. As previously explained this insulating material is made into a generally platelike form and hence the path can be made quite long. The material may be shaped in the form of a conventional shed or sheds on an insulator. For an insulator to be used to support a load vertically, the upper surface may be made of generally smooth form whilst the lower surface may have dependent concentric flanges or the like.

Although U-shaped links are convenient, they may be of other form. Closed loop links may be employed and such an arrangement may be used if one or more links are completely embedded in the insulating material. It is convenient in such a case that the metal elements protruding from the surface of the insulating material have two arms for joining to an adjacent unit even if there are closed loop links embedded within the insulating material. Although it is possible to embed a number of links in insulating material, for convenience of moulding, bearing in mind that the links must be kept out of contact with one another whilst the material is moulded around them, in general only two or possibly three links would usually be employed for convenience of manufacture.

The insulating material in a unit may be a single material which can be moulded around the links.

In some cases however it may be preferrred to use a composite construction, for example having a mechanically stronger material around the links or at least those parts of the links which are to be embedded in insulating material and then having another material, chosen for example for its surface characteristics, moulded around the overlapping portions of the links. Preferably in such an arrangement the first material is completely embedded within the second material.

Thus the links where they protrude from the insulating structure would be protruding solely from the second material. If there is a perfect interface between the links and the second material no moisture can penetrate and thus the conditions of the interfaces between the links and the first material and between the two materials are not critical. If the interface between the first and second materials is perfect, then the interface between the links and the second material is not critical. It is therefore desirable that the second material should achieve a good bond preferably with both the metal links and with the first material. As an example a cycloaliphatic epoxy resin might be used for the second material and a fibre glass mat/epoxy or polyester resin for the first material.In such a case the insulation around the links utilising the first material could be built up by individually machine wound and tensioned resinimpregnated fibres or cloths, covered by resinimpregnated glass tapes to give a profile providing adequate length of the interface between the two insulating materials in the region between the links.

Using U-shaped links, the two arms of the link may be arranged to form tongues of a clevis. To reduce the surface electric stress in the air or over the insulating surface between these tongues, it may be desirable to provide conductive material, e.g. a metal foil, around each of the links within the insulating structure. Such a foil can readily be incorporated in the first insulating material, particularly conveniently if that material is formed as decribed above using tensioned fibres covered by impregnated glass tapes. Alternatively a metal or conducting resin electrode could be incorporated on the surface of the unit between the tongues of the clevis or these tongues could be shaped so that they are close together if this is found to be necessary to reduce the stress in the air between the tongues.

A U-shaped metal link may be of plate material formed into a U. As explained later a closed loop of metal may be shaped into a U. This latter arrangement enables the metal to be used of substantially cylindrical cross-section. The cylindrical cross-section of the metal is preferable electrically. Use of a loop shaped into a U also permits of a lighter weight construction than a plate type metal link.

In a construction, such a loop formed into a U, from each of the two opposite faces of the insulating material, the protruding portions of the metal elements may comprise two pairs of arms.

The two arms in each pair may be joined to form a U-shape adapted to receive a pin or bolt passing through the two U-shapes for securing to another insulator and in a chain of such units.

The invention includes within its scope an electrical insulator comprising one or more insulator units, the or each unit having two Ushaped metal elements with their bases parallel and spaced apart and with the arms of one extending in the opposite direction to the arms of the other and with the base of each link passing between the arms of the other and electrical insulating material surrounding and lying between the bases of the U-shaped elements. As previously explained, the insulating material is of generally circular form extending outwardly from an axis normal to the bases of the two U's.

More specifically the invention includes within its scope an electrical insulator comprising one or more insulator units, the or each unit having a chain of three or more metal links, at least the intermediate links in the chain being closed loop links and the links being arranged in a straight line along an axis for taking a tensile load along that axis, electrical insulating material being provided around each intermediate link or all the intermediate links, the insulating material extending between the links to prevent contact therebetween, the two end links extending outwardly from opposite faces of the insulating material.

With the constructions described above the insulating material will be subject to relatively modest tensile and shear stresses at the place where the metal elements enter into the insulating material. It might be desirable to absorb the strain of the tongues by incorporating a sheath of resilient material around them for a short distance into the body of the insulator.

The following is a description of a number of embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of an insulator unit; Figure 2 shows two half cross-sections, respectively in orthogonal planes about the axis, of one embodiment of insulating unit; Figure 3 is a part-section through another construction of insulating unit having a composite construction; Figures 4 and 5 are part-sections through further embodiments of insulating units; Figures 6, 7 and 8 are diagrams illustrating yet further constructions of insulating units; Figures 9 and 10 are longitudinal part-sections in orthogonal planes through the axis of yet another construction of insulating unit; Figure 11 is a diagram illustrating a detail of a fixing in the unit of Figures 9 and 10; and Figure 12 is a diagram illustrating a modification of the elements in the unit of Figures 9and 10.

Referring to Figure 1 there is shown diagrammatically an insulator unit comprising two U-shaped links 10, 11 formed of metal which are linked without touching one another, that is to say the base of the U of each link crosses between the two arms of the U of the other link but these base portions are held apart by insulating material moulded around and in between the bases of the links as shown at 12. This insulating material completely covers the base portions of the U's of both links and holds these links separate from one another in rigid relationship. The moulded insulating material is, in this embodiment, formed as a substantially circular plate with a smooth external surface. For a suspension insulator, the plate is horizontal in use and its upper surface is convex.The edges of the plate are smooth and rounded but, on the underside, dependent flanges may be provided as is weli-known for sheds for high voltage suspension insulators. Each of the Ushaped members has two arms which protrude outwardly from the insulating material and, in this particular embodiment, these arms have apertures 13 for accepting a fixing bolt or pin (not shown) whereby adjacent units may be joined one to another to form an insulator string.

The U-shaped metal elements are each symmetrical with respect to the axis of the assembly and are aligned along this axis.

It will be seen that, in such an insulator string, when a tensile load is supported, the insulating material between the bases of the U-shaped links in each unit is under compression. The insulator therefore does not rely for its mechanical integrity on the insulating material; if the material between the links should crock, it still remains in position.

There is no electrically stressed internal interface in the units. The basic design is simple and a simple disc-type insulator unit has a good electrical performance. However the insulating material can be made of much more complex profile and yet still be easily moulded around the metal elements. It is readily possible to make the spacing between the arms of one of the links slightly less than the spacing between the others so that, in forming an assembly, the arms of the links of one unit may be located between the arms of a link of an adjacent unit with the apertures 13 aligned. A simple pin or bolt through these apertures secures the units together.For the same corona inception voltage, the tongue cross-section of an insulator assembly as described above can be small relative to the pin size of a cap-and-pin type of insulator assembly having similar mechanical and electrical performance because there are two tongues on the face of the insulator.

The insulating material, where it is subjected to a compressive load, can have a large load-bearing area without necessitating any significant increase in the overall size of the insulator since the size of the plate of moulded insulating material is dictated primarily by electrical considerations. It will be noted that the metal members are of simple shape. Unlike a cap-and-pin insulator assembly, the absence of the cap will result in an improved electrical resistance co-efficient and dry bands should form on both the top and bottom surfaces of the insulating material thereby giving better pollution performance. By appropriate shaping of the metal elements, the electrical and mechanical stresses can easily be made fairly uniform.It is readily possible to effect routine testing of an assembly of units for defects and the presence of a defective unit in an assembly of units need not cause rejection of a complete assembly.

Figure 2 illustrates in more detail the construction of one form of insulator unit in accordance with the present invention. This figure comprises two half cross-sections through the axis of the insulator assembly but at right angles to one another. The unit comprises two U-shaped links 10, 11 with moulded insulating material 1 2 between the bases of the U-shaped links and completely surrounding the base portions of the metal links. The upper surface 14 of the moulded insulating material 1 2 is smooth and slightly convex and the edge is rounded. Dependent flanges are provided on the underside, as in known types of insulator shed profile. In the particular embodiment illustrated the ratio of the leakage path to the effective length of a unit may readily be three or more.The leakage path is the distance around the insulator surface from a point on the metal element 10 where it emerges from the upper surface of the insulating body portion 12 to a point on the link 11 where it emerges from the lower surface of the insulating material. Thus the leakage path to effective length ratio can readily be made comparable with known types of insulator untis such as cap-and-pin porcelain units. The compressive load-bearing area, that is to say the region betweeen the base portions of the two links within the moulded insulating material, is very large and the compressive stress on the material for a given tensile load may typically be less than one tenth of the tensile stress which would arise in a resin-bonded glass fibre core type insulator of similar electrical and mechanical performance.

The links 10, 11 conveniently are made of mild steel. Various types of insulating material may be employed, for example epoxy resin glass fibre or polypropylene. It will be readily apparent however that the small mechanical stresses make possible the use of a wide range of materials. With the large load-bearing area, the compressive stress in the insulating material is very small and it is readily possible with many materials to obtain the required mechanical strength.

In Figure 2 the arms of the upwardly-extending U-shaped link are shown at 20, 21; because the half cross-sections are orthogonal, one arm is shown in side elevation and the other in section.

These arms have the aforementioned apertures 13 and the apertures are aligned with corresponding apertures 22 in downwardly-extending arms 23 of an adjacent insulator unit. The two arms can then be secured together by a pin or other connector (not shown).

Figure 3 is a diagram illustrating a unit using two different insulating materials. Figure 3 is a half section along the axis of an insulator generally similar to that illustrated in Figure 1 and has two U-shaped links 31, 32 arranged as in Figure 1. A fibre glass mallepoxy or polyester resin 33 is provided around the base portion of each of the links. This insulating coating around this part of each of the links may be applied by moulding or by building up with individually machine wound and tensioned resin-impregnated fibres or cloth. These fibres may be covered by resin-impregnated glass tapes to achieve a profile around the bases of the two links such as to ensure that when a second material 34 is moulded around the first material 33, there is an adequate length of interface between the two insulating materials.The material 33 is selected for its mechanical strength.

The outer material 34 is chosen for its surface characteristics and electrical properties and might typically be a cycloaliphatic epoxy resin. In this construction it will be seen that if the interface between each metal element and the material 34 is perfect, no moisture can penetrate. Thus the further interfaces are not critical. If the interface between the metal and the material 34 is not perfect, then there must be a good interface between the two insulating materials 33, 34. The interface between the material 33 and the metal again is not critical. It is preferable therefore that the material 34 should consistently achieve a good bond, pereferably with both the metal and with the other insulating material.

In the particular embodiment illustrated in Figure 3, a metal foil 35 is provided around each of the metal elements within the material 33. This is to reduce the surface electric stress in the air between the two arms of each metal element where they extend outwardly from the insulating material and also to reduce the maximum electric stress in the resin. Alternatively a metal or conducting resin electrode could be incorporated on the surface of the unit between the two arms of a metal element or each metal element or these arms could be shaped so that they lie close together In this construction, the insulating material will be subject to relatively modest tensile and shear stresses in the regions where the metal elements extend out from the insulating material.A sheath of resilient material, for example silicone elastomer, may be provided around each of these arms of the metal element and extending for a short distance into the body of the insulator to absorb such strains.

It has been mentioned that the external part 34 of the unit might be formed of a cycloaliphatic epoxy resin. This material has been found to have an excellent record in bonding to resinimpregnated fibre glass but it is known that degradation may occur in the cycloaliphatic epoxy resin over prolonged periods due to exposure and electric discharges. Recent work has shown that the effective leakage path may be reduced by as much as 50% of its original value. The construction described above readily enables an adequate leakage path to be obtained. There are however a number of techniques available for increasing the leakage path without significantly changing the overall size of the insulator unit. One possibility is to incorporate a fiiler such as aluminatrihydrate in the resin. Another way of increasing the leakage path is illustrated in Figure 4.Figure 4 is a side elevation, partly in section, of part of an insulator unit having two metal elements 41, 42 with insulating material 43. This unit may be constructed in any of the ways described above. As shown in Figure 4, a spiral strip of insulating material 44 is moulded into the insulating material 43 so as to extend downwardly therefrom on the underside of the insulator unit so thereby forming a structure with a series of narrow passages of relatively large depth-to-spacing ratio.

Such a device will trap pollution and it has been found that very little dirt penetrates into the far end of such passages. As shown in Figure 4, the downwardly-extending depth of the insulating strip varies along its length so that some of the entrances to the passages are in effect recessed with their entrances lying at different levels, thereby reducing the risk of the dirt which has accumulated on the edges of the strip from bridging the gaps and short-circuiting part of the leakage path.

If an insulator is polluted, discharges may occur over polluted regions. The more separate discharges there are in series, the better the pollution performance of an insulator should be even if dry bands are spanned by several discharges at once. This is because of the high arc resistance at low currents and the increase in effective resistance of a wet surface caused by current concentration at the arc roots. By moulding in a spiral or helico-spiral or a series of complete or even partial rings of water-repellant material, such as polytetrafluoroethylene, which has improved water-repellant properties compared with the main body of the insulating material, then it is possible to produce artificial dry bands which will give the desired effect.Such a construction is shown in Figure 5 which shows part of an insulator of the form described with reference to Figure 1 but which has rings 45 of PTFE moulded into the insulating material 46 so as to extend downwardly out of that material on the undersurface of the shed. In such an arrangement, surface treatment of the buried portion of the PTFE may be desirable to improve the bonding. However, even if puncture of this PTFE sheet or tracking of the buried portion occurs, it should have no major effect on the performance of the insulator.

Conducting segments as well as insulating material would also have the effect of inhibiting flashover. Such segments would have to be substantial electrodes to dissipate heat but they could easily be incorporated on the underside of a disc insulator in a similar way to the insulating material described above.

Figures 6, 7 and 8 illustrate further developments of a basic concept similar to that described with reference to Figure 1. In Figure 6 there is shown an insulator unit with two Ushaped metal links 51, 52 and disc-type insulating material 53 moulded around the two links in the region where they are interlinked. In Figure 6 there is shown a relatively simple design of insulator unit having insulating material 53 with a generally convex upper surface but of concave form around the outer part of the lower surface. Such a unit may be extended as shown in Figure 7 in which there is a closed metal link 54 extending around two U-shaped metal links 55, 56 and with insulating material around the closed loop and part of the U links 55, 56. The insulating material may be formed with two sheds 57, 58. The closed link 54 must be insulated from the U links 55, 56.

This insulation could be achieved by wrapping insulating material around the links before assembly in a mould or by moulding the sheds in two parts as shown; the moulding may be effected for the part above the line AA to form the upper half of the insulator and then the remainder of the insulator below the line AA may be moulded. In each case the relevant links can be held in position while the moulding is effected. The interface AA could be made as good an insulating join as possible or an electrode could be incorporated between the two sheds and connected to the closed link 54. Such an arrangement reduces the number of components for a given string length in an insulator assembly and gives an increase in the ratio of the creepage path length for a given overall diameter.However, with such an arrangement, it is much more difficult to form complex shed profiles. The introduction of further closed links is possible as shown in Figure 8 where the insulator has two U-shaped metal links 60, 61 with two closed metal links 62, 63 between them.

It is thus possible to make a long rod insulator in this way although it would be necessary to join and position the closed links in the manner described with reference to Figure 7.

It will be appreciated that insulators such as have been described above may be used with a wide variety of designs. Relatively cheap massproduced low-strength insulators can readily be obtained using a simple moulding with waterrepellant insulating materials. On the other hand more complex composite constructions having a very high strength and good pollution characteristics can also be produced as has been more specifically described.

Figures 9 and 10 illustrate a design of insulator which is generally similar to the plate-type design as previously described but the metal elments are designed to reduce the amount of metal used, for a given conductor radius, in the high stress regions. For this purpose the metal parts are formed from double links so that four metal elements extend outwardly from each end of the disc of insulating material. Referring to the drawings, there are shown two metal links 70, 71 each an endless closed loop bent into a U and arranged so that two parts of one of the links extend through the other link. The links are separated by moulded insulating material 73.

Where the parts of the link 70 lie outside the insulating material, on the upper surface in this case, they lie close together so that they can lie between two parts of the loop of a link 76 (similar to link 71) on the next insulator so that two insulating units can be assembled together by means of a pin 77. This also reduces the electric stress in the air and across the surface of insulating material in the region between the portions of a link where they protrude from the surface. In this particular construction, the pin 77 is secured in position by a phosphor bronze strip 78 sprung into position in a bore 79 through the pin.

As shown in Figure 12 a metal plate 80 may be lapped around the links where they will be within the insulating material to give increased mechanical strength over the high stress region.

The arrangement illustrated in Figures 9 to 12 finds particular application in lower strength units.

The metal may be steel, which may be galvanised, or aluminium alloy. The insulating material might typically be polypropylene or a silica-filled cycloaliphatic resin. The arrangement of for example Figure 2 finds more particular application with higher strength units, particularly with an epoxy resin glass fibre or with medium strength units when using a weaker material such as polypropylene.

Claims (22)

1. An electrical insulator for carrying tensile loads comprising one or more units, the or each unit having at least two spaced-apart metal elements which are interlinked and which are partially embedded in insulating material so that one metal element extends outwardly from one face of the insulating material and another extends outwardly from the opposite face of that material, the individual metal elements overlapping but having the insulating material between them so that when a tensile load is applied to the outwardly extending portions of said metal elements, they remain electrically insulated but can transmit the tensile load via the insulating material which is in compression between the elements, the insulating material covering the elements so that only one element is exposed where it extends out of one face of the material and only another element is exposed where it extends out of the opposite face of the insulating material.

2. An electrical insulator as claimed in claim 1 wherein the insulating material for each unit is in the form of a plate.

3. An electrical insulator as claimed in claim 1 wherein the insulating material extends outwardly from a longitudinal axis to form a multi-skirt insulator unit.

4. An electrical insulator as claimed in any of the preceding claims wherein units are joined together by a pin or bolt connecting a metal portion of one unit to a metal portion of an adjacent unit.

5. An electrical insulator as claimed in any of the preceding claims wherein each metal element comprises an U-shaped link.

6. An electrical insulator as claimed in any of the preceding claims wherein each metal element is symmetrical about a plane through the longitudinal axis of the insulator.

7. An electrical insulator as claimed in claim 5 wherein each metal element is of generally platelike form with the bases of the U-shaped elements in parallel planes normal to the axis of the assembly.

8. An electrical insulator as claimed in claim 7 wherein the arms of the U of each metal element form tongues of a clevis.

9. An electrical insulator as claimed in claim 5 wherein each metal ement is a closed loop formed into a U-shaped element.

1 0. An electrical insulator as claimed in any of claims 1 to 5 or claim 9 wherein the parts of each metal element protruding from the insulating material are of substantially cylindrical crosssection.

11. An electrical insulator as claimed in claim 10 wherein, from each of the two opposite faces of the insulating material, the protruding portions of the metal element comprise two pairs of arms.

12. An electrical insulator as claimed in claim 11 wherein the two arms in each pair are joined to form a U-shape adapted to receive a pin or bolt passing through the two U-shapes, for securing to another insulator and in a chain of such units.

13. An electrical insulator as claimed in any of the preceding claims wherein the insulating material is moulded around the links.

14. An electrical insulator as claimed in any of claims 1 to 11 wherein the links are at least partially coated with one insulating material and wherein a different insulating material is moulded around the coated links.

1 5. An electrical insulator as claimed in either claim 13 or claim 14 and incorporating one or more further conducting electrodes in the insulating material.

1 6. An electrical insulator as claimed in any of the preceding claims and having resilient material around the metal elements in the regions where they enter the insulating material.

1 7. An electrical insulator comprising one or more insulator units, the or each unit having two U-shaped metal elements with their bases parallel and spaced apart and with the arms of one extending in the opposite direction to the arms of the other and with the base of each link passing between the arms of the other and electrical insulating material surrounding and lying between the bases of the U-shaped elements.

1 8. An electrical insulator as claimed in claim 1 7 wherein the insulating material is of generally circular form extending outwardly from an axis normal to the bases of the two U's.

19. An electrical insulator as claimed in claim 18 and for supporting a vertical tensile load wherein the insulating material has a smooth convex upper surface and has one or more dependent ribs or flanges on its underside.

20. An electrical insulator comprising one or more insulator units, the or each unit having a chain of three or more metal links, at least the intermediate links in the chain being closed loop links and the links being arranged in a straight line along an axis for taking a tensile load along that axis, electrical insulating material being provided around each intermediate link or all the intermediate links, the insulating material extending between the links to prevent contact therebetween, the two end links extending outwardly from opposite faces of the insulating material.

21. An electrical insulator as claimed in claim 20 wherein the insulating material has substantially circular symmetry about said axis and is shaped to form one or more radially extending sheds.

22. An electrical insulator substantially as hereinbefore described with reference to Figure 1 or Figure 2 or Figure 3 of Figure 4 of Figure 5 or Figure 6 or Figure 7 or Figure 8 or Figure 9, 10 and 11 orFigure9,10,11 and 1 2 of the accompanying drawings.