DESCRIPTION OF THE BACKGROUND ART Electrical high-voltage insulators of porcelain have conventionally a glaze layer, which has several functions. It is to increase the mechanical strength of the insulator, make the surface smooth and thereby prevent attachment of impurities onto the surface of the insulator, and give the surface a good wear resistance. However, the attachment of impurities onto the surface of the insulator cannot be completely avoided. Eventually the surface becomes worn, and in certain areas such as desert regions, this wear can be significant due to sandstorms etc. If as well the surface becomes moist, then the impurities deposited on the surface can become conductive, give rise to short-circuits and in this way cause significant power losses.

To prevent significant power losses due to moist insulators, special insulators with a glaze layer made of an electrically semiconductive material are used to a certain extent.

Such semiconductive glazes contain typically about 30% tin oxide, doped with a certain amount of antimony oxide to increase the electrical conductivity. The small electric currents passing over the surface of the insulator through the glaze layer cause heating of the surface and thereby counteract the deposition of moisture on the surface by condensation and/or assist in drying the surface. It is realized that the current leaks created by the semiconducting glaze layer are not desirable from a power-loss point of view; however the alternative--larger power losses due to moist, dirty surfaces--is considered worse. The technique has however another serious disadvantage and limitation, which is that the semiconductive glaze layer cannot be fired in a reducing furnace atmosphere, which is normally required in the production of electric porcelain insulators due to the chemical compositions of the clays used by most manufacturers.

While the application of a semiconductive glaze on the porcelain body can be used when the porcelain is of a type that can be fired in oxidizing furnace atmosphere, this is a technique which cannot be applied for glazed porcelain insulators fired in reducing atmosphere, non-glazed porcelain insulators and nor for insulators made of glass or of polymeric materials. For non-glazed porcelain insulators and for glass and polymeric

insulators there is today no efficient solution of the problems related to power losses due to moist insulators.

DESCRIPTION OF THE INVENTION The present invention aims to attack the complex problem described above. In particular, the invention aims to offer an electric insulator of porcelain, glass or a polymeric material with a semiconductive surface coating.

It is also an object of the invention to use a composition of the surface coating which readily can be applied either in connection with the manufacture of the insulator or on existing insulators.

In accordance with the invention, it is also possible to provide a certain hydrophobic surface character of the surface coating.

These and other objects can be obtained therein that the surface of the insulator body is covered with at least a semiconductive layer having a surface resistivity of 1-1000 MQ/square, and that the outermost layer consists of mainly a polymeric material, preferably a hydrophobic polymeric material.

Thesemiconductive layer can be the outermost layer, wherein it contains conductive and/or semiconductive particles consisting of one or several materials belonging to the group of materials consisting of conductive or semiconductive metal oxides, carbides, borides and nitrides, e.g. tin oxide, which may be doped with antimony oxide or a fluroide; graphite and metals, distributed in the polymeric matrix.

It is however, also possible that the semiconductive layer essentially consists of an inorganic substance between the insulator body and an outermost layer of a hydrophobic, polymeric material.

The polymeric material can be any of the materials belonging to the group of materials consisting of silicone rubber, polyethylene, polyurethane, polyacrylate, epoxy resins, polytetrafluoroethylene (PTFE, Teflon), nylon, polymerized ethylene, propyl adiene monomer (EPDM), alkyd resin, polystyrene, butyl methacrylate, methyl methacrylate, and other co-polymers.

Suitably, the polymeric, hydrophobic material substantially consists of any of the silicone rubbers which are referred to as RTV- 1 silicone rubbers (RTV = room temperature vulcanizing, 1 = l-component). Typically, RTV-1 silicone rubbers consist of siloxanes, such as polydimethyl siloxanes, together with cross-linkers and solvents. The conductive or semiconductive powder particles, such as antimony doped tin oxide, metal powder and/or graphite, is dispersed in the RTV-1 composition in a sufficient amount to make the finished, cured coating semiconductive with a surface resistivity of 1-1000 MQ/square and is vigorously stirred in the composition such that the powder will be evenly distributed in the mixture.

The RTV-1 silicone rubber and conductive and/or semiconductive powder mixture contains some solvent, which e.g. can be selected among conventional organic solvents, such as ketones, ethers, including ligroin and other petroleum fractions; esters, aromatic, alifatic and chlorinated hydrocarbons. The mixture also can contain other fillers than the said conductive and/or semiconductive powder particles in order to provide a proper viscosity suitable for the mode of application of the mixture on the substrate. In principle, any deposition technique can be applied, such as mechanic or by means of sprayguns.

Suitably, the composition is applied on the surface of the insulator in a layer which has a thickness from about 0.2 to 1.0 mm. The RTV-1 silicone rubber cures in a humid atmosphere therein that OH-groups of the polysiloxane react with the cross-linker to form curable products. This cross-linking reaction or vulcanization takes less than 24 h in humid atmosphere at room temperature when the deposited layer has a thickness of max.

1.0 mm, but the vulcanization rate can be increased by raising the temperature.

Further features and aspects of the invention, as well as modifications thereof, are apparent from the appending claims and from the following description of an example.

EXAMPLE A fine (1-1000 nm diameter) graphite powder was dispersed in ethanol to form a slurry containing 29.2 weight-% graphite. The slurry was mixed with a room temperature vulcanisable silicone rubber of type Powersil 566 (trade name), manufacture Wacker Chemie GmbH, which was diluted with about 50 % petroleum ether (ligroin).The weight ratio of (the graphite+ethanol slurry)/(silicon rubber+solvent) was 1/4. The mixture was vigorously stirred, so that the graphite powder was evenly distributed in the silicone rubber matrix. Then the mixture was deposited on a glazed porcelain test piece to form an approximately 0.5 mm thick layer on the glaze. The sample was cured (vulcanized) at room temperature in a normally humid atmosphere for about 15 h, wherein the ethanol and the ligroin were at least to a substantial degree evaporated, while at the same time water was absorbed in the deposited material during the vulcanization process. The cured surface layer therefor contained graphite in an amount of about 6 % of the weight of the silicone rubber material prior to vulcanization. The surface layer had a surface resistance of about 45 MQ/square.