GB2106724A

GB2106724A - Earth connection for metal sheath, of an electric cable

Info

Publication number: GB2106724A
Application number: GB08222803A
Authority: GB
Inventors: Dietmar Lorenz
Original assignee: Kabel & Lackdrahtfab GmbH
Current assignee: Kabel & Lackdrahtfab GmbH
Priority date: 1981-08-07
Filing date: 1982-08-06
Publication date: 1983-04-13
Also published as: DE3131274C2; DE3131274A1; GB2106724B; CH656981A5

Abstract

In an earth connection, the end part of the sheath, shield, screen or the like (20) is bent outwardly back in the form of strips 20(a) which are clamped between an inner spring ring and an outer contact ring which embrace circumferentially the cable end. The spring ring is biassed outwardly against the inner surface of the contact ring which is provided with at least one earthing lead or connection. The rings are preferably frustoconical in section and are arranged in opposition to one another. <IMAGE>

Description

SPECIFICATION Earth connection for sheath, shield, screen or the like of an electric cable This invention relates to an earth connection for the flexible metal component of a laminar sheath, shield, screen or the like of an electric cable, in particular of a power cable.

German Offenlegungsschrift No. 2445898 describes an earth connection for a cable in which the end part of an aluminium or aluminium alloytubular shield of the electric cable is axially notched to form strips, which are then clamped between the radially extending flanges of two contacts which are adapted to be screwed together. The above mentioned earth connection has good electrical and mechanical reliability, and the cost of installation is within required limits, but the necessary screwable contacts themselves are expensive and necessitate such a space requirement in the axial and, in particular, also in the radial direction, as is inconvenient or undesirable for many sealing end designs and in particularfor certain socket designs.

Finally, this latter earth connection appears to be little suited for cables which have both a copper wire shield and in addition a laminar sheath manufactured for example of aluminium foil with a synthetic material sheath extruded around it.

However, an increasing number of power cables it is not desired, because of operating safety, to dispense with the conventional cable sheild, which is manufactured from copper wire, but it is desired to interpose for example an insulating or semiconducting layer, e.g. of crepe paper, tissue or foil strips, under a laminar sheath having a metal, particularly aluminium, inner layer, which is intended to offer an additional safety against the penetration of water and which is preferably manufactured from longitudinally extending foil axially extending bonding joint.

Now in cables of this type, as a result of the largely insulating layer between the copper shield and the aluminium sheath, the contact protection otherwise existing is no longer ensured. It can therefore be assumed that operational switching processes and the current or voltage pulses associated with these, in turn induce voltages in the metallic sheath, which can represent a source of danger.

For these and other reasons it is desirable, in such cables to provide not only an earthing or through connection to the copper wire shield, but also to the laminar sheath. The same applies to laminar sheath cables, which have been known for a long time, for example from the field of telecommunications as external cables, and which do not have a copper shield, but where an earthing of the metal-layer or metal-foil is prescribed.

According to the present invention there is provided an earth connection for the flexible metal component of a laminar sheath, shield, screen or the like of an electric cable, wherein the end part of said sheath, shield, screen or the like is bent outwardly back in the form of individual strips which are clamped between two contact rings pushed onto the cable from the direction of the end thereof and embracing the end circumferentially and wherein an inner ring of said rings is a spring ring having a predetermined expansibility in a radially outward direction, and the outer ring of said rings is a contact ring which is of stable shape, fits over the spring ring and is provided with at least one earthing lead or connection, the arrangement being such that the spring ring presses with its outer surface the strips, which are bent back approximately parallel to the cable axis, against the inner surface of the contact ring.

The present connection is electrically safe and easy to install, but in particular is space-saving.

Such an arrangement makes it possible, in the case of synthetic material cables with conventional contact protection (copper shield) in combination with an additionally provided laminar sheath with a metallic component, to avoid undefined and possibly hazardous electrical conditions in this component. On the other hand, this earth connection, as an electrically reliable, mechanically sufficient stable and particularly also flat and space-saving design, is suitable for all types of cable, which have a metal foil, screen, shield or the like, whether it be for contact protection, humidity protection or whether it be a layer which satisfies both functions at the same time. It is a pre-requisite that this metal layer or shield be sufficiently flexible for normal purposes, which is the case for foil thicknesses of from 0.1 to 0.4 mm and in many cases thicker than this.

In an advantageous embodiment of the present earth connection, the inner spring ring consists of a sleeve manufactured with pre-stressing from spring brass, bronze or the like, with axially extending slots alternately directed from opposite ends of the ring.

The gauge and width of the rings varies according to the type of cable or to the outer diameter of the metal layer. In general a length of approximately 15 to 50 mm and a gauge of approximately 4 to a maximum 10 mm, are sufficient for both rings, but the outer contact ring is constructed thicker than the spring ring, in order to guarantee stability of shape.

The contact ring must have at least one connection or earth lead, via which a current which may be induced or brought about through earth leakage, can be carried off. In this it is preferred that the electrically effective cross-section of the connection or earthing lead corresponds at least to the electrically effective cross-section of the laminar sheath or screen, which is to be earthed. Basically an arrangement is to be recommended, in which several connections or earthing leads are arranged approximately evenly distributed around the circumference of the contact ring, since the use of a single lead, of correspondingly large diameter, can lead to difficulties, at least with cables provided for large transmission capacities.

These connections or earthing leads can advantageously be attached to the contact ring in production, and are preferably fastened on the front face of the contact ring to point to the input side of the cable, thereby to achieve a flat construction, by soldering them into bores formed in one end face of the contact ring. If, particularly for lower power cables, the use of a smaller number or of only one single lead is indicated, this can also be carried out prior to positioning using corresponding supporting or clamping elements provided on the contact ring.

Where the metallic component is made of aluminium, it may be desirable to employ measures to improve contact on the outer surface of the spring ring and/or on the inner surface of the contact ring, such measures to improve contact being basically known.

(cf e.g. DE-OS 24 45 898).

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings, which illustrate diagrammatically and by way of example an embodiment thereof, and in which: Figure lisa longitudinal section through a cable with a completely installed earth connection; Figure 2 is a perspective view of a contact ring with earthing leads soldered into bores; and Figure 3 is a perspective view of a spring ring.

Referring now to Figure 1,there is shown a completely installed earth connection for the laminar sheath of a cable, such as may be found at a cable connection socket or at a sealing end for a cable. The input end of the cable lies to the right in the drawing.

The inner conductor of the cable is not visible, neither is an insulating layer of cross-linked polyethylene, which has become largely usual for power cables. A semi-conducting layer 10 situated over it, is visible to the left in the drawing, where it appears beneath an additional semi-conducting wound strip 12 at the finishing edge 14 thereof. A copper wire shield 16 lies over the semi-conducting wound strip 12, and a relatively thin intermediate layer 18, for example, of insulating or semiconducting crepe paper or strip of textile material lies over the shield. Over the intermediate layer 18 lies a sheath comprising a inner layer, consisting preferably of aluminium and shown in the drawing as a black line, and an outer layer 22, which because of the desired water diffusion resistance preferably consists of polyethylene ratherthan polyvinylchloride.Approximately in the centre of the drawing, the oblique finishing edge 24 of this outer layer can be seen, while to the left of this can be seen a spring ring 26 and a contact ring 28 externally surrounding the spring ring. The two rings are frustoconical in section and are arranged in opposition to one another, and the layer 20 is clamped between the end of the layer 20 being, if required, cut to form strips 20a. As can be seen, the thinner spring ring 26, which is under pre-stressing, presses from the inside against the thicker contact ring 28 of more rigid construction, without for instance pressing in an undesired manner onto the cable.To the left of the drawing, in front of the spring ring 26, wire toroid 30 can be seen, which can be assembled radially in front of the spring ring 26, in order to facilitate the folding-back of the metal foil strips 20a and in particular to avoid their being damaged. Close in front of this toroid 30 the intermediate layer 18, situated between the copper shield 16 and the aluminium layer 20 finishes; the individual copper leads 32 of the shield 16 are bent around and run directly along the outer surface of the contact ring 28 to the rear in the direction of the cable input. In the region of the edge 24 of the outer sheath 22 the individual shield leads 32 meet the earthing leads 34 which extend parallel and which, as shown in Figure 2, are soldered into bores formed in the front face 36 of the contact ring 28. As can be seen, a particularly space-saving solution can thus be realized.Accordingly, for example, a shrinkage tube 37 can be applied over the entire earth connection arrangement and can be bonded to the right in the drawing if required through the application of a layer of a hot melt adhesive and continues to the left in the drawing and covers a stress core 38 which is normally situated there and is only indicated quite diagrammatically, in the case of a connection socket normally to beyond a similar earth connection, arranged in mirror symmetry, of a connection cable on the other side of the stress core. Between the shrinkage tube 37 and the semi-conducting layer 10 present on the cable, a semi-conducting wound layer 40 is indicated, which is only applied on assembly, and which is in connection with the construction of the stress core 38.Its design is basically known, as is the further (also not shown) encasing and completing of the arrangement shown, for example by means of PVC-protective pipe and casting resin introduced in between, so that this will not be dealt with in more detail. Finally, Figure 1 shows a supporting element 42 in the form of a screw bolt arranged so that it can be screwed in the contact ring 28.

In orderto assemble the earth connection just described the polyethylene outer sheath 22 is stepped with a peeler down to the aluminium layer 20 to form the edge 24. In order to obtain a clean edge of the metal layer 20, it may be formed eith circumferential indentations so that the foil material lying in between can be easily removed. Notches approximately 30 mm. long are then made axially in the direction of the input side of the cable in the metal layer 20 or the screen of aluminium, copper or the like.After the spring ring 26 has been slipped over the bared end of the layer 20 and possibly after the assembly of the wire toroid 30, the free ends of the strips 20a are bent back over this wire toroid 30 and placed onto the outer surface of the spring ring 26 which tapers slightly towards the input side of the cable. Preferabiy, the spring ring 26 has been previously pre-stressed by means of a circumferentially extending wire which has been twisted. The contact ring 28 is pushed onto the spring ring 26 from the left in Figure 1 so that at least a portion of the latter is covered by the contact ring, whereupon the bracing wire which lies to the right of the contact ring and which is not shown, is removed so that the spring ring 26 expands and the contact ring is slipped over completely.

The spring ring 26 can be made longer than the contact ring 28, so that compression by means of the bracing wire is facilitated and this wire can be removed after the contact ring 28 has been completely slipped over the spring ring 26. The contact ring 28 has earthing leads 34 which are soldered into bores on the front face 36 of the ring and which together with the folded round individual wires 32 of the copper shield, after the assembly is completed are carried out from the shrinkage tube 37 and at a position provided on the sealing end are earthed jointly or in the case of the manufacture of a connection socket are connected, for example by means of a pressure connector, with the copper strand coming from the other earth connection.

Figure 2 is a perspective view of the contact ring 28 with the soldered in earthing leads 34, which are distributed approximately equiangularly around the front fact 36. It will be appreciated that other modifications may be used and it is possible for connection to take place only within the framework of assembly, for instance by means of locking, screwing or the like. The solution shown, however, has proved to be particularly reliable.

Figure 3 is a perspective view of the spring ring 26 and clearly shows the notches 44. The spring 26 preferably consists of spring brass or spring bronze, and can be tinned at least on its outer surface to improve the electric contact.

A corresponding surface improvement can also be applied to the contact ring 28, which is normally manufactured from brass, copper or the like. Due to the oppositely directed conicity of the two rings 26 and 28, the shape of the notches 44 and the relatively large contact surface, lying substantially parallel to the cable outer sheath, for the strips 20a, which are of aluminium, no particular contact problems occur.

However, surface measures to improve contact can be used such as, for example, a contact paste, known per se, which can be introduced priorto the engagement of the rings 26 and 28.

The supporting element 42 for the reciprocal locking of the rings may take various forms. However, a screw bolt 42, proves to be particularly simple, and after the contact ring 28 has been slipped over the spring ring 26, it is turned to prevent lateral slippage of the spring ring 26, which could only take place to the right because of the frustoconical construction ofthe rings.

Finally, it is to be understood that the present earth connection is applicable to all types of metallic foil or the like shields or screens orthe like which are to be earthed, irrespective of whether or not the cable in addition is equipped with a wire shield or the like, and irrespective of whether a metallic layer is applied running longitudinally or lapped around the cable or wound and whether it may, if required, be coated with paper or synthetic material, which can be recommended for instance in the case of a PE outer sheath on the sheath side. The main field of use of the present earth connection is for power cables. However, it may be used also for cables with a communications function, in particular laminar sheath cables, where an earthing of the metallic layer is desired or is prescribed.

Claims

1. An earth connection for the flexible metal component of a laminar sheath, shield, screen or the like of an electric cable, wherein the end part of said sheath, shield, screen or the like is bent outwardly back in the form of individual strips which are clamped between two contact rings pushed onto the cable from the direction of the end thereof and embracing the end circumferentially, and wherein an inner ring of said rings is a spring ring having a prefdetermined expansibility in a radially outward direction, and the outer ring of said rings is a contact ring which is of stable shape, fits over the spring ring and is provided with at least one earthing lead or connection, the arrangement being such that the spring ring presses with its outer surface the strips, which are bent back approximately parallel to the cable axis, against the inner surface of the contact ring.

2. An earth connection as claimed in Claim 1, wherein the spring ring is a sleeve, manufactured with pre-stressing from spring brass, bronze or the like, and formed with alternately arranged notches running substantially in the axial direction of the ring.

3. An earth connection as claimed in Claim 2, wherein the spring ring has approximately a thickness of 2 to 8 mm, and an axial length of approximately 15 to 50 mm, and wherein the notches have a length of 50 to 90% of the total axial length of the ring and a thickness of approximately 0.5 to 2.5 mm, the angular distance between two notches directed from the same side being approximately 10 to 50 degrees.

4. An earth connection as claimed in any one of Claims 1 to 3, wherein the contact ring has a gauge of 4 to 15 mm and is thicker than the associated spring ring.

5. An earth connection as claimed in any one of Claims 1 to 4, wherein the electrically effective cross-section of the earthing lead(s) or connection(s) at least corresponds to the electrically effective cross-section of the laminar sheath, shield, screen or the like.

6. An earth connection as claimed in any one of Claims 1 to 5, wherein several earthing leads or connections are distributed around the circumference of the contact ring.

7. An earth connection as claimed in any one of Claims 1 to 6, wherein the earthing lead(s) or connection(s) is(are) soldered into bores formed in the contact ring.

8. An earth connection as claimed in Claim 7, wherein the earthing lead(s) or connection(s) are fitted in the front face of the contact ring pointing to the input side of the cable.

9. An earth connection as claimed in any one of Claims 6 to 8, wherein the earthing lead(s) or connection(s) is(are) solid or stranded wires of copper.

10. An earth connection as claimed in any one of Claims 1 to 9, wherein at least one clamping or supporting element is provided for securing the spring ring and contact ring against undesired slippage in the axial direction.

11. An earth connection as claimed in any one of Claims 1 to 10, wherein the spring ring and/orthe contact ring has or have a frusto conical crosssection.

12. An earth connection as claimed in any one of Claims 1 to 11 for a cable with an aluminium sheath, sheild or screen, wherein the parts of the spring ring and/or contact ring which come into contact with these rings are at least partially shaped so as to be capable of penetrating the aluminium oxide layer.

13. An earth connection as claimed in Claim 12, wherein said parts are corrugated or roughened.

14. An earth connection as claimed in Claim 12, wherein a contact paste is applied between the spring ring or contact ring and the strips.

15. An earth connection for the flexible metal component of a laminar sheath, shield, screen or the like of an electric cable substantially as hereinbefore described with reference to the accompanying drawings.