This invention refers to a fixation system for medium and high- voltage switching poles, insulators, circuit breakers (CB) and general devices, with an insulating housingthat is provided with holes, in which screws are screwed into, in order to fix the device on a support, or a further housing, according to the preamble of claiml .

State of the art Insulating parts in CB or switchgear need to be fixed to other parts. Screws are often used for this purpose. To provide a reliable thread for said screws in insulating parts, dowels are often used. The dowels are permanently fixed to the insulating parts, e.g. by gluing or by integral casting.

It is also possible to save the dowels and to use screws that form or cut the thread directly in the insulation material.

Insulating parts in Medium Voltage or High Voltage CB or switchgear are exposed to electrical fields. Depending on the geometries of live parts, grounded parts and insulating parts, the electrical field is usually not homogeneous. In contrary, areas of high electrical field density can occur especially in the border areas of insulation media and conductors, e.g. where solid insulation parts end and air begins at the surface of a conductive part, also referred to as triple points. Said high electrical field densities can result in partial discharges, limiting the lifetime of the equipment.

Dowels can have an outer surface with relative large radii and without sharp edges. When made of conductive material (e.g. brass), the electrical field density at the transition from the dowel to the insulating part can be relatively low. As no air is involved at this transition, the field strength of the material of the insulating part (e.g. duroplast, BMC or thermoplastic material) can be exploited for a compact design of the switchgear.

If the dowels shall be saved, the screws are directly inserted into the insulating material. The screws, that are usually made of electrically conductive material like steel, have relatively sharp edges at their thread. These edges will increase the density of the electrical filed compared to the smooth surface of the dowels. Further, for the insertion of the screws, the insulating part will provide a hole that will not fully be filled by the inserted screw. Consequently, gas will be present in the region of the high electrical field density. Partial discharges may occur to an extend that is not acceptable for a reliable operation of the CB or switchgear.

Object of the invention

So the object of the invention is, to realize a mechanically optimized fixation of the pole part and/or the circuit breaker, which is optimized also in its performance of dielectric withstand. Invention

In order to fulfill that, the present invention proposes to make the inner surface of the hole conductive, by deposing a conductive layer for example. The inserted screw will electrically connect to this conductive layer and then to earth potential, or said conductive layer extends to another part, that is directly connected to earth potential, so that the effective surface for the electrical field will be the inner surface of the hole, and no longer the surface of the screw. Further, there will no longer be an electrical field in gas. Triple points are then avoided.

For making the inner surface of the hole conductive, it is proposed to apply a conductive varnish to said surface. This conductive varnish can e.g. use silver, coper or graphite particles as conductive component.

Optionally, it is proposed to activate the surface prior to applying the varnish to improve the adhesive strength and the long-term reliability of the varnishing. Said activation can be obtained e.g. with a plasma or flame treatment or with a chemical treatment.

Further advantageous embodiments are mentioned in the depending claims.

Embodiment of the invention is shown in the drawing

Figure 1 : state of the art, using a dowel

Figure 2: state of the art, not using a dowel

Figure 3: first embodiment of the invention Figure 1 shows the commonly used dowel 2 that is permanently fixed into the insulator 1. A screw 3 fixes the insulator to the conductive ground plate 4 that represents earth potential. The electrical potential of plate 4 is shown with the thick line 5. The other electrical potential shall be represented by the thick line 6. The exposition of line 5 towards line 6 is showing a relatively large radius at the top side of the dowel 2. There are triple points 7 at line 5, at the lateral ends of insulator 1 , but as these points are relatively far away from the line 6 and as this situation is the same for all figures shown here, these points 7 are not subject of the present invention.

Figure 2 shows the state of the art when a screw 3 is directly inserted or screwed into a hole 8 of the insulating material 1. Line 5 now shows the outline of the screw 3 pointing towards the other electrical potential of line 6. Beside the sharp edges 9 of the end of the thread of the screw, there are additional triple points 10 exposed to the other electrical potential. Here, partial discharges are likely to occur first when the overall dimensions are small compared to the applied voltage. Figure 3 shows the solution that is proposed in this invention.

Due to the added conductivity of the hole 8, the line 5 shows now the relatively large radius of the hole 8. Any triple points in the region around the screw 3 are avoided. There are still sharp edges of the screw, but these are not critical from the electrical point of view, as they are shielded by the ground plate 4 and by the more exposed area 11 of the hole 8.

The conductive layer at the inside of the hole 8 needs to be electrically connected to the potential of the plate 4, which can be e.g. the ground potential or any other potential. This connection can be realised by the conductive screw and / or by pressing the plate 4 to a conductive region at the collar of the hole 8. In the latter case, the proposed principle also works with electrically non-conductive screws.

Beside insulators, the proposed method can be applied to all kinds of casted parts in MV CBs or switchgears, like e.g. bushings or poles.

The conductive material can be deposed metals layers, or conductive varnishes, or conductive foils.

Reference list:

1 Insulator

2 Dowel

3 Screw

4 Plate

5 Thick line

6 Line

7 Triple points

8 Hole

9 Sharp edges

10 Triple points

11 Exposed area