Traditionally, electric power bushings have been made of an electric insulating part formed by a thermosetting resin such as epoxy moulded around an electric conductor. The task of the insulation has been to prevent electric discharges form upcoming between the conductor and the wait of a container through which the bushing protrudes. The thermosetting resin has been provided with a sufficiently high thickness to provide a satisfying functionality in this respect. However, moulding of thick, solid bodies of a thermosetting element is a costly process, and alternatives have been sought for.
Lately it has been suggested, for example in DE 102005059754, to let the bushing comprise a tubular body made of a thermoplastic resin that, in one thereof, is connected to the conductor and presents a thin-walled outer shell that is to be connected to the wall of a gas-tight container. There is an empty space between the conductor and the outer shell of the tubular body, and this empty space is in communication with the interior of the gas-tight container, which is suggested to be filled with an electrically insulating gas such as SF6. The use of thin-walled bushings of this type results in lower production costs in comparison to the use of thermosetting bushings of prior art.
However, for unusual situations, the use of an insulating gas alone might be insufficient in order to achieve a guaranteed prevention of electric discharges emanating from the conductor. Should there be any leakage of the insulating gas from the container, a device like the one suggested in DE 102005059754 is also very sensitive and likely to be subjected to electric discharges. Bushings of the kind lately suggested, using thin-walled insulation formed by a thermoplastic resin, will thus need to be re-designed in order to prevent the upcoming of electric discharges between the conductor of the bushing and the container wall. A disadvantage of thin walls of thermoplastic polymers might also be that they may be subjected to diffusion of moisture from the atmosphere into the container to which the bushing is connected.
The applicant has also realised that the same kind of design (thin-walled thermoplastic insulation instead of thick-walled thermosetting resin insulation) used for bushings may also be used for fuse canisters arranged so as protrude the walls of containers of electric power installations. Such canisters comprise an electric shield extending generally in the longitudinal direction of the canister and crosswise to the wall of the container, thereby intersecting the extension plane of the latter, wherein said shield is provided for the purpose of suppressing an electric field inside the canister and is carrying the same voltage as a conductor to which a fuse inserted into the canister is to be connected. Accordingly, the present invention is also applicable to this kind of devices.
It is an object of the invention to present a device as initially defined that remedies the above-mentioned drawbacks of prior art.