PROTECTION FOR ELECTROMAGNETIC CONNECTION AND ELECTROSTATICAL DISCHARGE

Abstract: An arrangement for connecting electrically two pieces of electronic equipment (19, 20) that have good electromagnetic compatibility properties (EMC), e.g. are protected against electrostatic discharge (ESD), without impairing the properties of any of the pieces of equipment concerned. The arrangement enables work to be carried out on one of the equipments that impairs the EMC properties of this equipment without impairing the properties of the other euquipment connected electrically thereto. The arrangement comprises a tube-like element (4b; 5b; 12, 13) which connects a first cage (16) with a second cage (17). The tube-like element is connected electrically with the two cages and also with the shields (11) of the cables drawn between the cages, wherein the tube-like element is closed sealingly around the cable shields. The opening areas of the holes provided in the sides of the cages for passing the cables therebetween are adapted to the diameters of the cables with the aid of a displaceable plate.

Description: PROTECTION FOR ELECTROMAGNETIC CONNECTION AND ELECTROSTATICAL DISCHARGE FIELD OF INVENTION The present invention relates to an arrangement for mutually coupling electrically two pieces of electronic equipment, each having good EMC properties while retaining said properties. DESCRIPTION OF THE BACKGROUND ART It is known to enclose sensitive electronic equipment in an electrically conductive cage, a Faraday cage, to obtain good electromagnetic compatibility (EMC) with respect to said equipment, e.g. protection against electrostatic discharges (ESD) . In those situations when duplicated systems are used, two such pieces of equipment, or equipment, will normally stand side-by- side so that the first piece of equipment is able to take over the functions of the second piece of equipment should this second piece of equipment malfunction or requires inspection. It is, of course, possible to enclose both pieces of equipment in one and the same cage, thereby protecting both pieces of equipment. However, problems arise when the first piece of equipment needs to be repaired or serviced. In order to obtain access to this equipment, it is necessary to open the cage, which will impair the EMC properties also with respect to the second piece of equipment, therewith risking disablement of the second piece of equipment and therewith a total breakdown. It is therefore necessary to enclose each piece of equipment in its respective cage. In order for equipment that has good EMC properties in general to communicate with other equipment via cables for instance, it is necessary to provide holes in the cage that encloses the equipment and that constitutes the actual basis for said good EMC properties. Such holes enable radiation to leak into and out of the cage, resulting in poorer EMC properties. When two cages stand side-by-side, electrical discharges, ESD, from one piece of equipment in the first cage are able to pass out through the hole therein and wander through the air or along electric conductors and in through the hole in the second cage and there disable the equipment enclosed by this second cage. This is, of course, highly unfortunate and presents a specific problem to the application of duplicated systems. There are many different causes of electrostatic discharges, ESD. For instance, the equipment may be subjected to such discharge by a person coming into contact with the equipment or solely being present in the proximity thereof. In order to prevent such an occurrence, there are often provided special grounding or earthing points at which a person is able to connect up an electrically conductive arm band and therewith obtain the same ground, or earth, as the equipment, thereby preventing an ESD. Unfortunately, an ESD will sometimes occur before the person concerned has been able to ground his body at a grounding point, which may have catastrophic consequences in the case of sensitive equipment. A second person can cause an ESD, even though the first person is grounded or earthed. If the first person has had time to open the cage, the EMC properties of the equipment enclosed therein will be greatly impaired, and even if the actual discharge does not cause proDiems, tine electromagnetic radiation causeα oy cne αiscnarge may play havoc and create problems with respect to nearby equipment, by passing through holes that carry electric cables. Computers that are used for switching purposes in telephony are often duplicated, so that if the first computer is disabled, the other computer will take over. When repairing the first computer, it is extremely important that no electrostatic discharges caused by a careless technician will also disable the other computer, which would result in a total breakdown in operations. The problem is particularly serious in respect of new types of IC circuits that are extremely sensitive to static discharges . SUMMARY OF THE INVENTION The invention solves the aforesaid problem of enabling part of a duplicated system to be serviced without impairing EMC properties, e.g. without impairing ESD protection, of the other part of the system, by minimising the open hole area and sealingly enclosing the shield of the connection cables with a grounded electric conductor, and electrically connecting said conductor to the cages. More specifically, the solution is achieved with the aid of a tube-like element which connects the two cages. The inner diameter of the tube-like element is adapted to the diameter of the cables received in the hole so as to obtain a sealed joint. The holes provided in the sides of the cages are sufficiently large to accommodate an electric contact element of reasonable size, meaning that the holes may be substantially larger than the thickness of the cables received therein. These holes are covered witn an electrically conductive merruoer, ror instance a steel plate, such as to adapt the hole opening to the diameter of the cables. The object of the invention is to prevent an electrostatic discharge occurring in one cage or the equipment encased thereby from wandering through the air or through electric conductors to the other cage or the equipment encased thereby, so that the EMC properties of one part of a duplicated system will be retained even when the protection in the other part is partially non-existent, so as to be able to pass cables effectively from one piece of equipment to another while retaining EMC properties, e.g. ESD protection, and to be able to carry out repairs and servicing on one side of a duplicated system without impairing the EMC properties of the other side. The advantages thus afforded are good EMC properties between the two parts of a duplicated system, minimisation of the risk of disabling both sides of duplicated equipment as a result of an electrostatic discharge. Other advantages are that the solution is simple and inexpensive, that the solution enables cables having large electric contact devices in relation to cable thickness to be passed from one piece of equipment to other pieces of equipment, that ESD spikes will not wander from one piece of equipment to the other, and that the one cage will still provide good EMC properties and be protected against radiation caused by ESD, for instance, even when the other cage is open and has lost its ESD protection. BRIEF DESCRIPTION OF THE DRAWINGS rne invention win now be described in more αecaii witn reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which Fig. 1 illustrates two cages in perspective; Fig. 2 is a perspective view of one embodiment; Fig. 3 is a perspective view of one part of one embodiment; Figs. 4-6 illustrate alternative embodiments schematically and in cross-section; and Figs. 7-8 illustrate schematically parts of different embodiments. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates schematically first electronic equipment 19 enclosed in a first cage 16, second electronic equipment 20 enclosed in a second cage 17, and cables 10 which function to connect the two pieces of equipment 19, 20 electrically. The sides of the cages are comprised of an electrically conductive material and may include small holes for ventilation purposes, for instance. The diameter of the holes determine how much electromagnetic radiation is able to leak from the cage and consequently the requirements set by the authorities or determined by the sensitivity of peripheral equipment to radiation will determine the size of the holes. Figure ^ 2 is a perspective view of one embodiment of an inventive arrangement. Reference numeral 1 identifies one side of the first cage and reference numeral 2 identities one side of the second cage. Reference 5a identifies a first plate having a first U-shaped bar 5b located on its lower edge. Reference 4a identifies a second plate which has a second U- shaped bar 4b located on its upper edge. Reference 3 identifies a third plate. Reference 6 identifies screws and reference 7 identifies a first hole in the first cage, and reference 8 identifies a second hole in the second cage. Figure 3 illustrates the cables 10 and the cable shields 11. Figure 5 illustrates an electrically conductive stocking 12. Figure 6 illustrates an electrically conductive tube 13 and two nuts 14 and 15. Figure 8 illustrates shutter means 18. Mutually corresponding elements in the various Figures have been identified with the same reference signs. The second plate 4a is attached to the bottom edge of the second U-shaped bar 4b flush with the short side, as shown in Figure 2. The second U-bar 4b is inserted into the first hole 7, such that the plate 4a will lie in abutment with the cage side beneath the hole 7. The length of the U-bar 4b is adapted to the distance between the cages, so that the opposite short side of the U-bar will lie in abutment with the side 2 of the second cage. The size of the holes 7, 8 may be adapted to the cable contact devices, which can be substantially larger than the cables 10 themselves, so as to enable cables and contact devices to be drawn readily between the two cages 16, 17. The first U-bar 5b is attached to the plate 5a in the same way as that in which the plate 4a and the bar 4b are attached to each another. The first U-bar 5b is inserted into the hole 8, such that the plate 5a will be in abutment with the side 2 of tne second cage over the hole 8, and such that tne opposing short side of the U-bar will be in abutment with side 1 of. the first cage. The first bar 5b will therewith face downwardly. The bar 5b is slightly smaller than the bar 4b therewith enabling it to fit into the bar 4b. Because the plate 5a is slotted, the plate can be attached to the cage side so that the bar 5b will press against the cables with the plate covering the upper part of the hole 8 at the same time. When the first plate 5a is in an upwardly displaced position, cables fitted with contact devices can be led from the first cage 16 to the second cage 17. The cables 10 are peeled so as to expose the cable shields 11 between the cages 16 and 17. Subsequent to having drawn the cables between the cages, the plate 5a is pressed downwards so as to obtain good contact between the cable shields 11, the U-bar 4b and the U-bar 5b. This will result in the formation of a tight tube between the cages 16 and 17 and also in electrical contact between the formed tube, the cable shields, and the first and the second cages. The plate 3 is fastened in the side 1 and moved down so as to seal the hole 7. The plate 5a functions to seal the hole 8 on the side 2 of the second cage in a similar manner. Figure 3 shows the second plate 4a and the third plate 3 in position with the cables drawn through the hole. A different method of fastening the plate 3 has been used in this case. In this embodiment, the plate 3 moves behind two smaller plates 9 which are removably attached to the first cage side 1. Subsequent to having moved the plate 3 down to its active position, the plates 9 are secured more firmly with the aid of spring elements for instance, or with the aid of screws as in the illustrated case. Figure 4 illustrates this embodiment in cross-section prior to the plate 3 being secured but subsequent to having moved down the plate 5a. Figure 5 shows an alternative embodiment in which there is used an electrically conductive stocking 12 of the kind typically used to shield cables. The stocking is attached to side 1 of the first cage with the aid of an annulus, which is secured with screws. The stocking is thereafter kept loose and the cables and contact devices drawn through the stocking. Subsequent to having drawn all cables through the stocking, the stocking is tightened to reduce its diameter and thereby sealingly embrace the cables, and is then secured to side 2 of the second cage. Figure 6 illustrates a solution that utilizes a tube 13 which is secured with the aid of nuts 14, 15. In this case, the internal diameter of the tube itself need not be adapted to the actual diameter taken-up by the cables 10. On the other hand, the active diameter can be adapted by filling the cavity defined between the cable shields 11 and the inner wall surface of the tube with an electrically conductive material, for instance steel wool or buckled or crunched aluminium foil. Figure 7 shows the plate 5a attached to the side 2 of the second cage. Figure 8 illustrates a shutter means 18 that can be used as an alternative to the plate for adapting the hole opening to • the diameter presented by the cables.

Claims: CLAIMS 1. An arrangement for electrically connecting with the aid of shielded cables (10) a first piece of equipment (19) that has good electromagnetic compatibility properties (EMC) , e.g. shielded against electrostatic discharges (ESD) , by virtue of being enclosed in a first electrically grounded cage (16) , to second piece of equipment (20) having good EMC properties, e.g. shielded against ESD, by virtue of being enclosed in a second electrically grounded cage (17) , by drawing the cables through holes (7, 8) in the two cages (16, 17) without impairing the EMC properties of said two pieces of equipment, characterized in that the first cage (16) is connected to the second cage (17) through the medium of a tube-like element (4b, 5b; 12; 13) ; in that the tube-like element (4b, 5b; 12; 13) is disposed in the proximity of the holes (7, 8) ; in that the active internal diameter of the tube-like element (4b, 5b; 12; 13) is adapted to the combined diameter of the cables (10) ; in that the tube-like element (4b; 5b; 12; 13) is in electrical contact with the shielding (11) on respective cables (10) ; in that the active area of the holes (7, 8) in said cages is adapted to the diameter taken-up by the cables (10) that are passed therethrough; and in that the tube-like element (4b; 5b; 12; 13) is in electrical contact with the two cages (16, 17) . 2. An arrangement according to Claim 1, characterized in that the tube-like element is comprised of two elongated U-shaped bars (4b, 5b) ; in that the first U-shaped bar (5b) has a first plate (5a) attached in the short side and extending away from the rounded part of the bar (5b) with the normal parallel with the axis of said U-bar; in that the first plate (5a) is fastened in the side (2) of the second cage; and in that the second U-bar (4b) is fastened in the side (1) of the first cage; in that the first plate (5a) can be displaced perpendicularly to the axis. of the U-bar such as to push the first U-bar (5b) into the second U-bar (4b) and therewith form a tube of adjustable internal diameter; in that the opening area of the hole (7) in the first cage can be shielded with a plate (3) having a semi-circular recess in the bottom of said plate and being displaceable in a radial direction. 3. An arrangement according to Claim 1, characterized in that the tube-like element is an electrically conductive stocking (12) whose internal diameter can be reduced by stretching. 4. An arrangement according to Claim 1, characterized in that the tube-like element is comprised of a typical electrically conductive tube (13) ; and in that the active diameter of the tube can be adapted by filling the cavity between cable shields (11) and the inner surface of the tube with an electrically conductive material. 5. An arrangement according to Claim 4, characterized in that the electrically conductive material is steel wool. 6. An arrangement according to Claim 4 , characterized in that the electrically conductive material is buckled aluminium foil. 7. An arrangement according to any one of Claims 3 to 6, characterized in that the diameter of the holes (7, 8) can be adapted with the aid of shutter means (18) . 8. An arrangement according to any one of Claims 1 to 8, characterized in that the cages (16, 17) are constructed as cabinets in which the equipment (19, 20) are arranged. 9. An arrangement for controlling switched telephone traffic comprising at least two cages (16, 17) that enclose electronic equipment (19, 20) having good EMC properties, characterized in that the equipment in said cabinet are mutually connected electrically by cables (10) through the medium of an arrangement according to any one of Claims 1 to 8.

Inventors: ENSTROEM KENT, ESSLAND HANS-OLOV, KASSMAN BJOERN, SVENSSON ANDERS, TORHAGE ERIK, ENSTROEM, KENT, ESSLAND, HANS-OLOV, KASSMAN, BJOERN, SVENSSON, ANDERS, TORHAGE, ERIK
Applicants: ENSTROEM KENT, ERICSSON TELEFON AB L M, ESSLAND HANS OLOV, KASSMAN BJOERN, SVENSSON ANDERS, TORHAGE ERIK, ENSTROEM, KENT, ESSLAND, HANS-OLOV, KASSMAN, BJOERN, SVENSSON, ANDERS, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL), TORHAGE, ERIK
Application year: 1997
Main classification: H05K9
IPC codes: H01R13, H02G3, H05K9