Electrical-Cable Shielding

This invention relates to methods of shielding electrical cables against electromagnetic interference, and electrical- cable bundles so shielded.

In order to provide effective shielding of a bundle of electrical cables against electromagnetic interference it is necessary to ensure that the shield, for example of braided wire, has electrical continuity and that connection of it to ground is of low resistance. It is normal to interrupt the shield where connection to ground is required and to re- establish electrical continuity with it using a solid, usually metal, fitting to which the broken or cut ends of the shield are attached. The required ground connection is then established by means of a ground-bonding lead from the solid fitting.

Interruption of the shield and using solid fittings as referred to above to maintain continuity of ground connection has disadvantages in that the solid fittings add weight and bulk to the cable-bundle and increase costs. It is an object of the present invention to provide methods of shielding a bundle of electrical cables against electromagnetic interference, and cable-bundles so shielded, for which such disadvantages are, at least to a significant extent, overcome.

According to one aspect of the invention there is provided a method of shielding a bundle of cables against electromagnetic interference, wherein the shield of the bundle is formed by an elongate electrically-conductive tape that extends lengthwise of the cable-bundle and is retained wrapped widthwise round the bundle with the longitudinal margins of the tape overlapping one another for electrical closed-circuit encirclement of the bundle, and wherein external electrical connection to the

shield is established via an electrically-conductive element which is secured to the cable-bundle within the shielding tape and which exits the shield between the overlapping margins of the tape.

According to another aspect of the invention there is provided a bundle of cables shielded against electromagnetic and electrical interference, wherein the shield of the bundle is formed by an elongate electrically-conductive tape that extends lengthwise of the cable-bundle and is retained wrapped widthwise round the bundle with the longitudinal margins of the tape overlapping one another for electrical closed-circuit encirclement of the bundle, and wherein an electrically- conductive element that is secured to the cable-bundle within the shielding tape exits the shield between the overlapping margins of the tape for providing an external electrical connection to the shield.

The electrically-conductive element of both the method and cable-bundle of the invention, may make electrical connection with the shielding tape inside or outside the shield. In the former case, the element may involve an electrically-conductive collar or sleeve through which the cable-bundle extends in securing the element to the bundle and which is in electrical contact with the electrically-conductive tape inside the shield. An electrically-conductive lug or tag may in this case extend from the collar or sleeve to exit the shield between the overlapping margins of the tape for use in establishing the external electrical connection to the shield. As an alternative, the collar or sleeve may be part of a cable transition device that is used in a cable harness where there is branching of one or more cables from (or into) the bundle, the branch cable or cables extending through a further collar or sleeve of the transition device which is inclined out of axial alignment with the first-mentioned collar or sleeve so as to project from the shield between the overlapping margins of the tape and which, where it projects from the shield, has a

terminal or lug for the external electrical connection to the shield.

In the circumstances in which the electrically-conductive element makes electrical connection with the shielding tape outside the shield, it may take the form of an elongate element that is secured at one end to the bundle within the shielding tape and after exiting the tape between the overlapping margins is retained wound round the outside of the shield in electrical contact with it; electrical contact between the shield and the element may be made also, or alternatively, where the element exits the shield between the overlapping margins of the shielding tape. The end of the element within the shield may be secured to a collar or sleeve through which the cable-bundle extends in securing the element to the bundle.

Where a collar or sleeve is used as referred to above to secure the element to the cable-bundle, it may be of a split-shell construction so that it can be assembled round the cable- bundle. It may however be threaded onto the bundle.

Methods of shielding electrical cable-bundles, and shielded electrical cable-bundles, all in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a representative cross-section of tape used for shielding cables against electromagnetic interference in each of the examples to be described;

Figure 2 is illustrative of a plan view of the tape of Figure 1;

Figure 3 is illustrative schematically of the weave of a wire- mesh that forms part of the tape of Figures 1 and 2;

Figure 4 is a cross-sectional view of a bundle of electrical cables wrapped in the tape of Figures 1 to 3 for electromagnetic and electrical shielding of the cable-bundle;

Figures 5 and 6 are perspective views illustrative of a first device used for establishing a ground-bonding point of a bundle of cables shielded using the tape of Figures 1 to 3;

Figure 7 is a perspective view of part of a shielded cable- bundle incorporating the device of Figures 5 and 6;

Figures 8 and 9 are perspective views illustrative of a second device used for establishing a ground-bonding connection to a bundle of cables shielded using the tape of Figures 1 to 3;

Figures 10 and 11 are illustrative of alternative forms of attachment of a ground-bonding lead in the second device;

Figure 12 is a perspective view of part of a shielded cable- bundle incorporating the device of Figures 8 and 9;

Figure 13 is a perspective view illustrative of a third device used for establishing a ground-bonding point common to bundles of branched cables shielded using the tape of Figures 1 to 3; and

Figure 14 is a perspective view of part of a shielded cable- bundle incorporating the device of Figure 13.

Each of the three methods and devices to be described for electromagnetic-shielding of electrical-cable bundles, utilise shielding tape of the form illustrated in Figures 1 to 3.

Referring firstly to Figures 1 and 2, the shielding tape 1 used is of a three-layered, light-weight construction including a woven mesh 2 of bare metal-wire strands (for example, in a twill weave) . The mesh 2 is sandwiched between inner and outer

electrically-insulating layers 3 and 4 respectively. The layers 3 and 4 are formed by strips of plastics sheet (the thicknesses of the layers 3 and 4 and the mesh 2 are exaggerated in the drawings) . More particularly, the inner layer 3 is, for example, of polytetrafluoroethylene (PTFE) or polyvinyl chloride (PVC) , and the outer layer 4 is of a hard- wearing plastics material, for example, a ketone-based resin such as polyetheretherketone (PEEK) , for providing physical strength with resistance to abrasion.

As indicated schematically in Figure 3, the warp of the woven mesh 2 is formed by fine wire-strands 5 that run lengthwise of the tape 1, whereas the weft is formed by a fine wire- strand 6 running back and forth without break, in a narrow- fabric weave with the warp strands 5. The strands 5 and 6 are, for example, single wire-filaments or twisted pairs, of nickel-coated copper wire, and the strands 5 are fine enough that a group (in the case illustrated, of four) of them are drawn together through the dent-space of the loom in the weaving process; the gap G between the adjacent groups, resulting from the dents (and shown exaggerated in Figure 3) , is small enough that it does not materially affect the shielding provided.

The inner layer 3 is of a width to cover the inside face 7 of the woven mesh 2 apart from throughout a longitudinal marginal strip 8 of the face 7 at one longitudinal edge 9 of the bare mesh 2. The outer layer 4 correspondingly covers the outside face 10 of the mesh 2 apart from throughout a longitudinal marginal strip 11 of the face 10 at the other longitudinal edge 12 of the mesh 2. An edge 13 of the layer 4 lies beyond the edge 9 of the mesh 2 to carry a strip 14 of upstanding fibres that abuts the edge 9.

The strip 14 forms in conjunction with a second strip 15 of upstanding fibres, a pair of interengaging elements of a selectively-disengageable fabric-fastening of the kind, such

as that sold under the Registered Trade Mark VELCRO, in which hook-ended fibres of one element engage with upstanding loop- ended fibres of a second element; in this case, for example, the element 14 has the loop-ended fibres and the strip 15 has the hook-ended fibres. The strip 15, which is supported along the opposite edge 16 of the layer 4, has a central, red-coloured line 17 running throughout its length.

The mesh 2 together with the layers 3 and 4 and the strips 14 and 15 are retained in assembly with one another by- stitching.

The tape 1 is used for electromagnetic shielding of a bundle of electrical cables as will now be described with reference to Figure 4.

Referring principally to Figure 4, the tape 1 is deployed running lengthwise of the bundle of electrical cables 20 with the inner layer 3 abutting the cables 20 of the bundle. The tape 1 is now wrapped widthwise round the bundle by folding its longitudinal margins round the bundle as indicated generally by arrows A in Figure 2. Wrapping of the tape 1 round the bundle is carried through to bring the margin 8 into overlap with the margin 11 and engagement of the element 14 onto the element 15 throughout the full length of the tape 1. Sufficiency of the area of engagement between the element 14 and the element 15 at each position throughout the length of the tape 1, so as to ensure that the tape 1 is correctly- wrapped and secured, is confirmed if the red line 17 cannot be observed anywhere along that length.

When the tape 1 is correctly wrapped, the margins 8 and 11 of the woven mesh 2 are in hard abutment with one another so as to ensure that there is good electrical contact between them and that optimum shielding is provided. With such contact, there is complete closed-circuit encirclement of the bundle, and low-resistance connection of that circuit with the weft

strand 6 and each warp strand 5. The strands 5 and 6 are clamped into couplings (not shown) at either end of the bundle to ensure good ground connection. Moreover, the shortest possible path for discharge from and lengthwise of the bundle is provided by the warp strands 5 running longitudinally of the shielding tape 1; this is of especial importance for lightning protection.

Electrical bonding to ground of the cable-shield is required at regular intervals along the length of the cable bundle, and this would normally be provided by cutting the tape 1 where connection to ground is required and re-establishing electrical continuity with it using a solid, usually metal, fitting inserted between the two cut ends of the shield. The required ground connection would then be established by attaching a ground-bonding lead to the solid fitting. However, in the present examples to be described, no cutting of the tape is required so there is no loss of continuity.

In the first example to be described with reference to

Figures 5 to 7, rather than cutting the tape, continuity is retained using a collar 30 with central bore 31 that is attached to the cable-bundle within the shield. More particularly, the collar 30 as illustrated in Figures 5 and 6, splits longitudinally into two shell-parts 32 and 33 which are assembled round the bundle of cables 20, and which are both electrically conductive; they may be of metal but for weight-reduction are preferably of metal-coated plastics.

The shell-part 32 has an elongate lug 34 extending from its outer surface 35. The lug 34, which is electrically conductive integrally with the shell-part 32, follows the curvature of the surface 35 with a small intervening gap 36 throughout about the first half its length.

In installing the collar 30, the two shell-parts 32 and 33 are assembled together encircling the cable-bundle 20, and

the shielding tape 1 is then closed over them both and the lengths of the bundle 20 either side of it, as illustrated in Figure 7. This is after a transverse strip of the plastics- sheet forming the insulating layer 3 of the tape 1 has been removed so that closing of the tape 1 round the collar 30 establishes good electrical contact between the collar 30 and the mesh 2 of the tape 1. The tape 1 is closed over the collar 30 with the bare margin 11 of the mesh 2 extending through the gap 36 under, and in electrical contact with the underside of, the lug 34, and the bare margin 8 of the mesh 2 extending over, and in electrical contact with the top along part of the length of the lug 34. The lug 34 accordingly exits the shield between the overlapping margins 8 and 11 of the tape 1, and the tape 1 is retained over the collar 30 with the two shell-parts 32 and 33 clamped together and the mesh 2 clamped in good electrical contact with them both, by a strap 37 (shown partly broken away) that is tightened round the shield onto the underlying collar 30; the strap 37 may have elasticity and may be replaced by a cable tie.

The lug 34 establishes a convenient terminal for ground- bonding of the shield without any interruption of the shield and without significant disturbance to the shielding tape 1 and its electrical continuity either circumferentially or lengthwise.

The second form of element for use in establishing a ground- bonding connection is illustrated by Figures 8 to 12 and will now be described.

Referring initially to Figures 8 and 9, a plastics collar 40 having a bore 41 splits longitudinally into two shell-parts 42 and 43 which are assembled round the bundle of cables 20 within the shield. To this extent the collar 40 is similar to the collar 30, but in this case neither part 42 nor 43 need be electrically conductive or make electrical contact with the mesh 4 of the tape 1, and consequently no transverse

strip of the plastics-sheet forming the insulating layer 3 of the tape 1 need be removed. Also in this case, no lug corresponding to the lug 34 extends from either part 42 and 43, but instead a flexible ground-bonding lead 44 in the form, for example, of a woven strap of bare metal mesh, is attached to the shell-part 42. The lead 44 may be attached to the shell-part 42 as illustrated in Figure 10 by over- bonding it into the plastics material, or, as with the specific form of collar 40 shown in Figures 8 and 9, and as illustrated in Figure 11, by bonding it within a recess 45 of the external surface 46 of the shell-part 42.

The two shell-parts 42 and 43 are assembled together encircling the cable-bundle 20; in this regard, the bore 41 is not concentric with the external surface 46 so that the wall-thickness of the shell-part 42 is slightly increased at the location of the recess 45 so as to avoid constriction of the bore 41 there. The shielding tape 1 is then closed over the assembled collar 40 and the lengths of the bundle 20 either side of it. The lead 44 exits the shielded bundle 20 sandwiched between the overlapping margins 8 and 11 of the tape 1, so that electrical connection between it and the lead 44 is made where the bare margins 8 and 11 of mesh 2 bear on it. The electrical connection to the mesh 2 is enhanced by wrapping the lead 44 round the outside of the shielded bundle by more than one turn after appropriate removal of the underlying circumferential strip of the insulating layer 4 from the outer surface of the shielded bundle so as to bare the mesh 2 throughout that strip. The remaining, unwound portion 47 of the lead 44 is folded over onto itself at right angles, and is held at the fold together with the wound portion by means of a strap or clip 48 (shown partly broken away) that is tightened round the shield against the underlying collar 40. The wound portion of the lead 44 is thereby clamped firmly onto the strip of bared mesh 2 to give good electrical as well as physical connection to the shield,

and leave the unwound portion 47 available for direct connection to ground.

The use of collars such as the collars 30 and 40 for establishing grounding-point connections has the significant advantage that the location of any such grounding point can be readily adjusted or moved. This is achieved simply by- opening up the shielded bundle by parting the margin 8 of the tape 1 from its margin 11 and sliding the collar along the cable-bundle to the required location, making good whatever insulation has been removed in the former location and removing whatever is necessary in the new location, before closing the shielding up again. Furthermore, it is to be noted that because the lug 34 and the lead 44 exit the shielding between the margins 8 and 11 substantially tangentially to the cable-bundle, there is minimal disturbance or distortion of the shielding tape 1.

The general principles of using an electrically-conductive collar such as the collar 30 used in the example described above with reference to Figures 5 to 7, may be extended to the provision of a grounding-point connection where there is a transition within an electrical-cable harness . An example of such an extension of the principle, and of the provision according to a further aspect of the invention, of a branching- transition in a cable harness, will now be described with reference to Figures 13 and 14.

Referring to Figure 13, the transition device 50 has a cylindrical-bore 51 through its main sleeve-body 52 that flares off into a branch cylindrical-bore 53 within a sleeve side-arm 54 at right angle to the body 52. The device 50 is electrically conductive, being of metal-coated plastics construction, and splits longitudinally into two sections 55 and 56 so that it can be fitted to the cable harness with the main cable-run extending lengthwise of the bore 51 and the branched-off cables exiting through the bore 53. The two

sections 55 and 56 have cooperating lug-portions 57 and 58 that abut one another face-to-face to establish a common grounding lug of the device 50 when the sections 55 and 56 are assembled together.

Referring to Figure 14, the shielding of the main cable-run is provided by wrapping with the tape 1 described above, and is continued through the transition at the device 50 without break and on throughout the bundle 59 of cables remaining after the transition (the latter bundle is represented in the drawing by just three cables) . In this regard, the tape 1 is wrapped widthwise round the main cable-run to and from the device 50 with its margin 8 overlapping the margin 11 and retained by engagement of the fabric fastening strips 14 and 15. The overlapping with consequent fastening of the tape 1, extends onto the body 52 of the device 50 with the lug formed by the abutting lug-portions 57 and 58 projecting between the overlapping margins 8 and 11. The tape 1 is nonetheless pulled taut round the base of the side-arm 54 by the tightness of the adjoining overlapping portions of the tape 1 pulled up onto the body 52. In this regard, the device 50 flares inwardly to form a neck 60 (Figure 13) at the junction of the side-arm 54 with the body 52, and the margin 11 is folded under the strip 15 in this region to facilitate the tight, close fitting of the tape 1 there; the flaring inwardly at the neck 60 also has the advantage of maintaining a constant cross-sectional area in transition from the bore 51 to the bore 53.

Where the tape 1 is pulled taut round the neck 60, the bare margin 8 and folded-over margin 11 of the mesh 2 are in direct contact with the electrically-conductive surface of the device 50, but the electrical connection between the device 50 and the shielding mesh 2 may be enhanced by removal of a portion of the insulating layer 3 where it comes into contact with the body 52. An external strap or tie (not shown) may be tightened round the tape 1 where it covers the body 52 to ensure that contact is maintained.

It is of note that the shielding of the main cable-run is provided by the tape 1 in one unbroken, continuous run in spite of the transition. Shielding of the branched bundle 61 (represented in the drawing by just three cables) is provided along its length by a further tape (not shown) corresponding to the tape 1 wrapped widthwise round the bundle 61 and the side-arm 54. The end of this tape located on the side-arm 54 is secured there by an encircling clip or band, with its shielding mesh 2 in surface electrical contact with the side- arm 54

As with the ground-connecting devices described above with reference to Figures 5 to 12, the transition device 50, can be readily introduced with minimum disturbance into a cable-bundle that is already fitted with the overlapping wrap-around tape. It is simply necessary to open up the shielding tape temporarily where the transition and bond to ground is required, by parting the overlapping margin of the tape from its other, underlying margin in that location, and after fitting the two sections 55 and 56 round the bundle and ensuring adequate electrical integrity, closing up the shielding tape.

Additional insulation and resistance to abrasion and other mechanical effects may be provided simply by adding additional layers to the shielding tape 1. Mechanical protection may be modified by the choice or mixture of fibres, and/or their diameters, within a textile layer. More especially, fibres or other non-conductive fibres may be included between some or all of the wire strands 5 in the mesh 2 of the shielding tape 1 to improve the stability of the mesh 2.

As a further possible modification of the shielding tape 1, the layer 3 may be extended round the edge 12 of the mesh 2 to cover the strip 11. When the modified tape 1 is wrapped round the cable-bundle, the extended portion 21 of the layer 3

enhances the integrity of the electrical insulation of the mesh 2 from the cable-bundle, but separates the strip 11 of the mesh 2 from its direct electrical contact with the strip 8. It has been found that this separation does not materially detract from the operation of the shielding tape, especially in relation to high-frequency interference. The closed-circuit encirclement of the bundle by the mesh 2 is now of low impedance with a small capacitative component instead of being solely resistive.

In some circumstances it may not be necessary or desired to provide insulation between the bare mesh 2 and the cables, so then a saving in cost and weight can be achieved simply by omitting the inner insulating layer 3 from the tape 1. Similarly, the outer insulating layer 4 may be omitted to the same end, where appropriate. In each case the omission of insulation may usefully extend the area of electrical contact with the mesh 2 required for ground-bonding contact in the devices described above.