BACKGROUND OF THE INVENTION Electric fields of high magnitude adjacent to live parts in electric plants, and to insulators in electric plants, are a major problem, since they can cause damage to insulators, arma- tures, and other apparatuses. Overvoltage protections are used in many applications and include spark gaps and other means well known in the art. It has also been suggested to provide electrical insulators with conductors along the insulator body in order to provide a comparatively uniform voltage gradient along the insulator. It has also been suggested, SE-120373, issued 1947, to use a ceramic material with reduced electrical resistance in the insulator body to achieve a highly uniform voltage distribution. These early attempts, however, as far as is known to the inventors, have failed to solve the problems satisfactorily.

BRIEF DESCRIPTION OF THE INVENTION The invention is characterized in that the outer surface of the body and of the sheds mentioned in the preamble, is covered with a semiconductive layer having a surface resistivity of between 1 and 1000 MQ/square, that the semiconductive layer is in electrically conductive contact with the metal end fittings such that a certain electric current will flow over the surface of the body and of the sheds through the semiconduc- tive surface layer causing a voltage drop along the body between the end fittings when the insulator is in operation, that the voltage drop along the insulator between the end fittings is non uniform such that the voltage drop over at least a first length section of the insulator, including at least one entire shed, is greater than the voltage drop over other sections of equal length, said at least one first section being located where the voltage gradient along the insulator would be lower than in other sections of the insulator if the voltage drop would be uniform along the insulator between the end fittings, and that the non-uniformity of the voltage drop along the insulator is provided by at least one of the following features, namely

- that the number of squares of the surface of the body and of the at least one shed within said at least one first section in the direction of the current flow is significantly greater than in other sections of the insulator of equal length, and/or - that the surface resistivity in said at least one first section is significantly higher than in other sections of the insulator.

The greater number of squares per length unit of the surface of the body and of the at least one shed within the said at least one first section in the direction of the current flow can be provided in several ways, such as for example the following: - that the outer diameter of the body in the first section is significantly smaller than the outer diameter of the body in other sections, - that the shed or the sheds within the first section has/have outer diameter(s) which is/are significantly larger than the outer diameters of the sheds in other sections, - that the number of sheds per length unit within the first section is significantly larger than the number of sheds per length unit in other sections of equal length, - that the shed or sheds within the first section has/have underribs while the sheds in other sections have not, - and/or that the shed/sheds within in the first sections have underribs only adjacent to the body, while sheds in other sections have underribs only adjacent to the periphery ofthe sheds.

Also combinations of the above alternatives are conceivable.

The invention can be applied on hollow as well as on solid insulator bodies. The insulating material can in principle be any insulating material but is normally chosen among any material belonging to the group consisting of ceramic materials, including porcelain, glass and polymeric materials and combinations thereof. The semiconductive layer normally is a layer which contains a sufficient amount of tin oxide, preferably antimony and/or fluoride doped tin oxide, and has thickness such adapted to the conductivity of the surface material, that the semiconductive surface coating will attain a surface resistivity of 1-1000 MQ/square. Such surface coatings are per se well known in the art. How such coating materials can be produced, their exact chemical composition, and how they can be applied to the insulating member will therefor not be described in detail in this specification. It should be mentioned, however, that when the insulating member consists of porcelain, the semiconductive layer normally is a semiconductive glaze, which is 0.2-1.0 mm thick and contains totally 10-50 weight-% of tin oxide and antimony oxide or fluoride although also other semiconductive coatings containing tin

oxide with or without antimony or a fluoride or any other agent which makes the coating semiconductive, can be considered.

BRIEF DESCRIPTION OF DRAWINGS In the following, the above mentioned examples of how the non-uniformity of the voltage drop along the insulator can be provided, will be explained with reference to the accompanying drawings, in which Fig. 1A- Fig. 6A show hollow insulators where high-voltages, which normally exist at the ends of the insulator during normal use thereof in high-voltage systems, are at least partly balanced by the provision of an enhanced voltage drop in the central parts of the insulator, Fig. 1B- Fig. 6B show hollow insulators, e.g. breaker poles, where high over-voltages, e.g.

switching over-voltages inside the insulator at a distance from the ends thereof can be at least partly balanced by the provision of an enhanced voltage drop in the parts of the insulator adjacent to the end fittings thereof, Fig. 7 shows a conical bushing with sheds generally designed according to the same principles as the insulator showed in Fig. 2A, Fig. 8 shows a solid long rod suspension insulator designed in order to reduce the voltage gradient from the lower part of the insulator and increase it at the top part thereof, Fig. 9 shows a long rod suspension insulator designed for the same purpose as the insulator of Fig. 8, Fig. 10 shows at a larger scale the surface structure of any of the insulators, e.g. the encircled area of the insulator of Fig. 1A.

All the drawings show the insulators schematically and serve the only purpose to illustrate the principles of the invention. Especially the end fittings of all the insulators, and the central parts of the insulators of Fig. 1B-6B are shown only schematically. In reality, the insulators are also more elongated in relation to the cross-section and have more sheds than are shown in the drawings.

DESCRIPTION OF EMBODIMENTS With reference first to Fig. lA-Fig. 6A, Fig. 1Fig. 6B and Fig. 7, an elongated, hollow, insulating body is designated 1A, 2A, etc. .., 1B, 2B, etc. . 6B, and 7, respectively and in Fig. 8 and 9, an elongated, solid, long rod insulating body is designated 8 and 9,

respectively. On the hollow body there are a number of exterior sheds which in Fig. 1A are designated alA, blA, clA, dlA, and in Fig. 2A they are designated a2A, b2A, c2A, d2A, and in for example Fig. 6B they are consequently designated a6B, b6B, c6B and d6B. In Fig. 7, the four sheds are designated a7, b7, c7 and d7. Finally, in Fig. 8 the sheds are designated a8, b8, c8, d8, e8 and 18 and in Fig. 9 they are designated a9 . . e9.

The integrated insulating, elongated member consisting of the insualting body and the sheds thereon is exteriorly covered with a semiconductive layer 10, Fig. 10. The semiconductive surface layer 10 has a surface resistance of 1-1000 MQ/square and is deposited on the insulating member all the way from one end to the other. The semiconductive layer 10 may consist of a semiconductive glaze, when the insulating member is made of porcelain, in which case it is 0.2-1 mm thick. Also other semi- conductive coatings and other insulating materials, however, can be considered as is stated in the foregoing disclosure of the invention.

In the ends, the insulator has a metal fitting 11 and 12 which is in electrically conductive contact with the semiconductive layer 10 via a conductive cement 13, such that an electrical circuit is established when there is a difference voltage between the metal fittings. In the semiconductive layer 10 there will therefor flow a certain electric current between the end fittings 11 and 12 when there is a voltage difference between the end fittings causing a voltage gradient along the insulator body, the magnitude of which depends on the voltage drop which varies along the insulator body as will be explained in the following.

Strong electrical fields exist around the end portions of the insulator, causing high electrical stress on the insulator material because of steep voltage gradients adjacent to the end fittings 11 and 12. Such voltage gradients may cause problems like sparkings (corona) which may damage the glaze or corresponding surface. The high-voltages also may damage the apparatuses connected to the end fittings or details thereof.

With reference to Fig. lA-Fig. 6A, the high-voltages which normally exist at the ends of the insulators during normal use thereof in high-voltage systems, according to the invention are at least partly balanced by the provision of an enhanced voltage drop in the central parts of the insulator. "At least partly balanced" in this context shall not mean that there will be a uniform electrical stress distribution over the insulator but an improvement in that direction, so that the electrical stress where the voltage gradients are at most, are so much reduced that damages or other undesired phenomena are also

reduced. According to the embodiments shown in Fig. lA-Fig. 5A, this is achieved therein that the number of squares per length unit of the surface of the body 1 A and of the sheds blA and ciA within a central section IA in the direction of the current flow through the semiconductive layer 10 is significantly greater than in other sections of the insulator member. According to the embodiment of Fig. 1A, the increased number of squares per length unit within central section IA is achieved therein that the body 1A has a significantly smaller outer diameter than in other sections in combination with broader sheds blA and c1A (broader in the radial direction) in said section IA.

In the embodiment shown in Fig. 2A the greater number of squares per length unit in central section IA is achieved therein that the central sheds b2A and c2A have a significantly larger outer diameter than the sheds a2A and d2A in the end portions of the insulator.

In the embodiment shown in Fig. 3 A the greater number of squares per length unit of central section IA is achieved therein that the number of sheds b3 A, c3 A per length unit within central section IA is significantly larger than the number of sheds per length unit in other sections of equal length.

In the embodiment shown in Fig. 4A, the greater number of squares per length unit in central section IA is achieved therein that the sheds b4A and c4A have underribs 15 while the sheds a4A and d4A in other sections have not.

In the embodiment shown in Fig. SA, the greater number of squares per length unit in central section IA is achieved therein that the sheds b5A and c5A have underribs 15 only adjacent to the body 5A, while the sheds a5A and d5A have underribs 15 only adjacent to the periphery of the sheds.

In the embodiment shown in Fig. 6A, the non-uniformity of the voltage drop along the insulator is provided in a different mode than in the embodiments of Fig. lA-Fig. 5A, namely therein that the surface resistivity in the central section IA is significantly higher than in the sections adjacent to the end fittings 11 and 12. The higher resistivity in section IA - or the lower resistivity in the regions between section IA and the end fittings - can be achieved therein that the semiconductive layer is thicker in the end sections, e.g.

50-200 % thicker than in section IA. It is also possible to achieve the lower resistivity in the end sections therein that the surface layer 10 is brought to contain a higher amount of tin oxide or corresponding agents than in midsection IA.

The insulators of Fig. 1Fig. 6B are designed to counteract over voltages inside the hollow insulator body at a distance from the end fittings 11, 12. For example, the insulating member, including the hollow insulator body, can be a breaking chamber of a circuit breaker, wherein reference numeral 16 schematically indicates a fixed arcing contact and reference numeral 17 indicates a moving arcing contact. The high electrical fields that are generated around the contacts 16, 17 at the breaking occasion can cause damage to neibouring parts of the insulator. The purpose of the designs illustrated in Fig.

1B-Fig. 6B is to, at least to some degree, level out the high voltage peaks. This is carried out according to the same principles as have been described with reference to Fig. 1 A- Fig. 6A but "the other way round". This is achieved by providing a greater number of squares ofthe surface ofthe body 1B, 2B etc. and ofthe sheds blB, c1B; b2B, c2B etc.

within the central section IB in the direction of the current flow than in the sections adjacent to the end fittings 11 and 12. Therefor, - in the embodiment of Fig. 1B, the insulator body 1B has a larger diameter, and the sheds blB and clB are shorter in the radial direction than in the outer sections, - in the embodiment of Fig. 2B, the sheds b2B and c2B in central section IB are much shorter than the sheds a2B and d2B in the outer sections, - in the embodiment of Fig. 3B, there are a greater number of sheds a3B, b3B, c3B, d3B in the outer sections than in central section IB, - in the embodiment of Fig. 4B only the sheds b4B and c4B in central section 113 have no underribs, while the sheds a4B and d4B between central section IA and end fittings have, and - in the embodiment of Fig. 5B the sheds a5B and c5B in midsection I have underribs 15 only adjacent to the periphery of the sheds, while sheds a5B and d5B in other sections have underribs only adjacent to the body 5B.

In the embodiment of Fig. 6B corresponding result is achieved therein that the central section IB has a lower surface resistance than the sections adjacent to the end fittings.

It should be understood that the drawings and the foregoing description only are intended to illustrate a number of means which the artisian can utilise for isolators according to the principles of the invention. These means can be combined with each other in a very great number of different combinations. For example, any or all of the features of the insulators of Fig. 1A, Fig. 2A, Fig. 3A, Fig. 4A or 5A, and Fig. 6A very well can be combined with each other in a single insulator. Correspondingly, also any or all of the features of the insulators of Fig. 1B, Fig. 2B, Fig. 3B, Fig. 4B or 5B, and Fig.

6B can be combined with each other in a single insulator.

Fig. 7 illustrates a bushing of the type used for transformers and has typically a conical shape. The sheds a7-d7 are designed according to the same principles as in the insulator illustrated in Fig. 2A, which means that the sheds b7 and c7 in a midsection I7 are longer in the radial direction than the end sheds a7 and d7 in order at least partly to compensate for the high voltages in the region ofthe end fittings 11 and 12.

In the embodiment of Fig. 8, the sheds a8, b8 and c8 in a section II remote from the suspended high-voltage line 19 are broader in the radial direction than the sheds d8, e8 and 18 nearer said line 19.

Fig. 9 illustrates a suspension insulator where the number of squares per length unit of the insulator successively increases from the lower end, where the high-voltage line 19 is suspended, towards the upper end fitting 11, which is connected to the earthed power- line pylon. As is shown in the drawing, this is achieved therein that the length of the sheds in the radial direction successively increase towards the upper shed a9 adjacent to the upper, earthed end fitting 12.