A BUSHING The invention relates to a bushing for sealingly leading a cable, pipe or the like through a wall, said bushing comprising a frame of a rigid material, which can be sealingly fitted in an opening formed in said wall, and furthermore one or more parallelopiped blocks of a resilient material, such as rubber, which are built up of two identical halves, which blocks, in their assembled condition, exhibit a bore which is adapted with small tolerances to the shape and the dimensions of a cable to be led through, which blocks are placed into said frame opening with a close fit in operative condition, wherein said halves are placed round a cable in enclosing relationship with respect thereto, whilst furthermore pressure means comprising a pressure plate are present, which press the blocks that have been placed in the frame opening together in the plane of the frame opening in their operative condition, such that a sealing compression is obtained of the block halves relative to each other, of the outside surfaces of the blocks relative to each other, between the walls of the bores and the outsides of the cables present therein, and also between the outside surfaces of the blocks and the inner walls of the frame, whilst furthermore a closing element is provided, which is fitted in the frame above the pressure plate.

Such a bushing is known from GB-A-2,186,440 as well as from EP-A-0 183 300.

One drawback of the known bushing is that it is not protected against electromagnetic overtension and/or interference.

The objective of the invention is to provide a bushing

which comprises a protection against electromagnetic overvoltage and/or interference, which is simple, inexpensive, chemically resistant, fire resistant, gastight and watertight.

In order to accomplish that objective, a bushing of the kind referred to in the introduction is according to the invention characterized in that said blocks are electrically conductive and that they are in contact with an electrically conductive cable shield of the cable in the operative condition of the bushing. The term electrically conductive sealing collar which is used in this framework is understood to mean that the material of the sealing collar is at least substantially electrically conductive, in particular in that additives have been added to the sealing material (preferably a resilient material such as rubber). Electrical conductivity has been realised by chemical means, therefore. Electrical installations and machines, wherein generally highly sensitive components are used for control and registration purposes, are highly susceptible to interference caused by various external electromagnetic fields and impulses. Said external electromagnetic fields and impulses may for example be generated by radar installations, thunderstorms, nuclear explosions and by electric/electronic equipment. In practice, steel constructions functioning as a Faraday cage are often placed round a space intended for electronic equipment in order to minimise such interference. Nevertheless, there is still a chance that electric cables, which are led into such a shielded space, will conduct undesirable electromagnetic fields and impulses into said electronic equipment. As a rule, said conduction will take place via the electrically conductive shielding ("braiding") of the bushed cables in question. According to the invention, said external, undesirable electromagnetic fields and impulses will

flow to earth along the electrically conductive shielding of a bushed cable, via the electrically conductive blocks. Thus, the occurrence of said interference in electronic equipment is prevented in a simple manner. Furthermore it has become apparent that the present bushing is also quite suitable for leading a cable through a steel deck or bulkhead of a ship.

It is noted that within the framework of the invention, the term"cable"as used herein is understood to include an electrically conductive conduit, tube, pipe or the like, for example a metallic tube or conduit, with which the present blocks are in electrically conductive contact in the operative condition.

In one embodiment of a bushing according to the invention, the blocks abut at least partially against the electrically conductive cable shield of the cable in the operative condition of the bushing. This makes it possible to fit the blocks easily and firmly round the electrically conductive cable shield.

In another embodiment of a bushing according to the invention, said blocks are made of (massive) rubber, to which carbon particles in the form of soot ("carbon black") have been added. Preferably, said rubber is EPDM rubber, with the carbon particles in particular having a concave, dish-like shape, preferably being of the type "Ketjenblack"EC 300J or 600JD. These types of soot, which are known under the trade name"Ketjenblack", have a unique morphology in comparison with conventional types of soot, which makes them uniquely suitable for the present electrically conductive application. The letters EPDM stand for Ethylene/Propylene/Diene/Monomer.

In another embodiment of a bushing according to the invention, blocks which are positioned one on top of the

other in the operative condition of the bushing are provided with a profile on their respective outsides. A slight sealing pressure is thereby required in order to obtain an accurate seal of the blocks between themselves, which blocks are positioned one on top of the other in this condition. More in particular, the blocks are provided, at least along part of their length, with circular inner ribs, wherein said blocks, whose outsides abut against an inner wall of the frame in the operative condition of the bushing, are at least partially provided with external ribs on said outsides.

Preferably, said external and/or inner ribs have a substantially serrated or triangular cross-section. This makes it possible to ensure an optimum sealing effect, whereby the serrated or triangular ribs are slightly compressed.

In another embodiment of a bushing according to the invention, anchoring strips are provided, which can be fitted on at least one side of the frame, in a direction parallel to the plane of the frame. More in particular, the blocks are externally provided with slots thereby, so that the anchoring strips can be inserted into said slots in the blocks from outside. The anchoring strips function to support the blocks in a direction perpendicularly to the longitudinal direction of the bushed cable, tube or the like. This prevents the blocks from moving out of the frame, whilst furthermore an improved internal transmission of pressure is achieved, so that again a smaller pressure is required for sealingly pressing the blocks together in the plane of the frame opening. It is noted that within this framework the term"anchoring"trips as used herein is understood to include other anchoring elements such as plates and the like as well.

The invention furthermore relates to a method for

manufacturing a bushing according to the invention, wherein said blocks are made of an electrically conductive rubber by incorporating concave, dish-like carbon particles into the rubber by means of a vulcanizing process. The rubber is in particular EPDM rubber, whilst the carbon particles are preferably of the type"Ketjenblack"EC 300J or 600JD.

The invention will be explained in more detail hereafter with reference to figures illustrated in a drawing of a preferred embodiment of the present invention, wherein: Figure 1 is a schematic front view of the present bushing; Figure 2 is a schematic, perspective view of the frame of Figure 1; Figure 3 shows parallelopiped blocks of the bushing as shown in Figure 1, which blocks are provided with outer ribs having a serrated cross-section and with slots for receiving anchoring strips; and Figure 4 shows a bushed cable of Figure 1 separately in perspective view.

In Figure 1, a bushing 1 according to the invention can be distinguished, which consists of a frame 2 of a rigid material, which must be sealingly fitted in an opening in a wall. Two parallelopiped rubber blocks 3,4 each consisting of two identical halves are to be provided in said frame opening, which blocks 3,4 contain bores 5 for leading a cable, a tube, a pipe, a conduit or the like therethrough. A"blind"block 6 is inserted into frame 2 in those places where no cable is to be led through. Pressure means in the form of a pressure plate 8 press the blocks 3,4,6 which are present in the frame opening together in the operative condition, in such a manner as to obtain a sealing compression of the respective halves of the blocks 3,4 relative to each

other, of the respective outside surfaces of blocks 3, 4,6 relative to each other, between the walls of bores 5 and the outsides of the cables present therein, and also between the outside surfaces of blocks 3,4,6 and the inner walls of frame 2. After a desired compression has been effected, a closing element in the form of a block 8 of a compression-resistant material is placed into the free space 9, between pressure plate 7 and frame 2, after which the pressure plate 7 can be released.

Figure 3 shows one half 10 of a parallelopiped block 3 and several complete parallelopiped blocks 3, which are dimensioned to extend outwards on either side of frame 2 in the operative condition of the bushing, in a direction transversely to the plane of the frame, over a distance 11 (for example 60 cm in the case of a frame width of likewise 60 cm, so that the width of the whole is 180 cm). The advantage of having the blocks extend outwards on at least one side of frame 2, in a direction transversely to the plane of the frame, in the operative condition of the bushing is that the bushing possesses a very good resistance to fire in that direction, due to the increased amount of fire resistant rubber, for example. Blocks 3 are provided with circular inner ribs 12 along at least part of their length, whereby blocks 3, whose outsides abut against an inner wall of the frame in the operative condition of the bushing 1 according to the invention, are at least partially provided with outer ribs 13 on said outsides. Blocks 3 are provided with slots 14, so that anchoring strips (not shown) can be inserted into slots 14 from outside.

As already explained above, blocks 3 abut against the electrically conductive cable shield 15 ('braiding") of a bushed cable 16 (Figure 4). To this end, cable 16 has been stripped in situ of its metal shield 17 and also of

its electrically insulating shield 18. Blocks 3 are electrically conductive, due to the incorporation of concave, dish-like carbon particles of the type "Ketjenblack"EC 300J or 600JD therein by means of a vulcanizing process, as a result of which external, undesirable electromagnetic fields and impulses flow to earth along the electrically conductive shield of the bushed cable 16, via the electrically conductive blocks 3. It is noted that the"braiding"15 of the bushed cable 16 is only"exposed"along the depth of the frame.

This means that blocks 3 enclose said"braiding" entirely, whereby cable 16 still contains its metal and electrically insulating shields before as well as after said blocks 3.