GB2213328A - Earthing device - Google Patents

Abstract

To earth a metallic front panel (11) to the chassis (8) of a sub-rack having a forwardly open groove (9) with an inner portion (9b) containing a retaining strip (10) with screwthreaded apertures therein, an earthing device is provided comprising a U-section earthing clip (1) of spring steel with a base wall (2) and a pair of side walls (3, 4) in opposed spaced relation. The base wall (2), in operation of the device, bears against the retaining strip (10) and at a middle portion includes an aperture through which the shank (12b) of a clamping screw (12) can pass, the side walls (3, 4) have end portions formed with projections (6) which will penetrate into the front panel (11) upon tightening of the clamping screw (12) and the width of the base wall (2) is less than and the spacing between the side walls (3, 4) in a repose condition of the earthing clip (1) is greater than the width of an outer portion (9d) of the groove (9) whereby the earthing clip (1) can be inserted through the outer portion (9d) of the groove (9) by pressing the side walls (3, 4) towards one another and will be retained in position in the groove (9) when released. <IMAGE>

Description

EARTHING DEVICE The invention relates to an earthing device and particularly to an earthing device for earthing a metallic front panel of an electronic sub-rack to a chassis of the sub-rack.

One previously proposed way of earthing the front panels of subracks has been to replace the normal plastics retaining screw with a metal screw having a projection which, when the screw is pressed through the panel, cuts through the anodised surface of the panel and makes electrical contact with the base metal of the panel. The screw at its inner end engages in one of a plurality of screwthreaded apertures provided in a strip of metal received in a groove in the chassis of the sub-rack, which groove has a mouth which is narrower than the width of the inserted strip. While this method is effective in providing an earth path through the clamping screw via the apertured strip to the chassis, it is expensive and the screw looks odd compared with other plastics screws holding the panel.

According to the invention there is provided an earthing device for earthing a metallic front panel to the chassis of a sub-rack, which chassis of the sub-rack has a forwardly open groove with an inner portion containing a retaining strip with screwthreaded apertures therein, the screwthreaded apertures being accessible to a clamping screw having its head on the outer side of the panel and its shank extending through an aperture in the panel and through an outer part of the groove, wherein the earthing device comprises a generally U-section earthing clip of electrically conducting resilient material with a base wall and a pair of side walls in opposed spaced relation, the base wall in operation of the device bears against the retaining strip and at a middle portion includes an aperture through which the shank of a clamping screw passes, the side walls have end portions formed with projections which will penetrate into the front panel upon tightening of the clamping screw and the width of the base wall is less than and the spacing between the side walls in a repose condition of the earthing clip is greater than the width of an outer portion of the groove whereby the earthing clip can be inserted through the outer portion of the groove by pressing the side walls towards one another and will be retained in position in the groove when released.

The length of the earthing clip can be less than 15mm, for example 12nun, and can therefore be accommodated by front panels of the minimum width likely to be encountered.

Having the base wall bearing against the retaining strip prevents the device from seesawing end-to-end. The aperture in the base wall receiving the clamping screw can closely locate the base wall laterally within the groove and having the side walls sprung outwardly against side faces of the outer portion of the groove further locates the earthing clip such that any movement thereof upon tightening of the clamping screw is unlikely.

Preferably middle portions of the side walls are cut-away and the end portions of the side walls are formed as pointed tangs thereby readily to penetrate an anodising layer provided on an aluminium front panel.

Advantageously the earthing device is formed of spring steel and is formed by stamping from sheet spring steel and subsequent bending.

It can be seen that the earthing device can provide effective earthing without any special treatment being required to the front panel. This is significant since the front panel would be likely to be the component which required replacement in a plug-in unit in the event of failure and it is important therefore that it should not be specially adapted. Normally the clamping screws are captive screws retained on the front panels.

The invention is diagranniatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is an end view of an earthing device according to the invention Figures 2, 3 and ii are respectively a rear view, a side view and a perspective view of the device of Figure 1; and Figure 5 is a sectional plan view showing the ear thing device of Figures 1 to 4 in operation.

Referring to the drawings an earthing device comprises an earthing clip 1 of U-section having a base wall 2 and side walls 3 and 4. The base wall 2 has an aperture 5 in its middle portion and the middle portion of each side wall 3, 4 is cut-away and the outer corners are trimmed off to form each side wall 3, 4 with a pair of projecting tangs 6.

As can best be seen in Figures 1 and 4, each side wall 3, 4 diverges outwardly from its junction with the base 2 to a swan-neck portion 7 and thereafter extends substantially parallel to the other side wall and at a spacing slightly greater than the width of the base wall 2.

The length of the base wall 2 is advantageously 12mm, the width of the base wall 2 is advantageously 3mm, the aperture 5 has a diameter of 2.6mm and the distance between the tangs 6 and the far face of the base wall 2 is preferably 4.8mm.

Referring to Figure 5, a chassis member 8 of a sub-rack has a groove 9 therein having four defined portions that is to say a narrow inner portion 9a, a wide next outer portion 9b, a narrower intermediate portion 9c and a narrow outer portion 9d. A retaining strip 10 with a plurality of screwthreaded apertures therein is received in the next outer portion 9b and is of a width such that it cannot pass outwardly into the intermediate portion 9c. A front panel 11 of anodised aluminium is retained by a screw 12 having a head contained within a plastics member 12a and a shank 12b engaged in one of the screwthread apertures in the retaining strip 10, the plastics member 12a having a portion 12c which retains the screw 12 captive in the front panel 11.

The earthing clip 1 of Figures 1 to 4 is received in the intermediate portion 9c and the outer portion 9d of the groove 9. The base wall 2 of the clip 1 is pressed against the retaining strip 10, the shank 12b of the screw 12 passes through the aperture 5 in the base wall 2 and the tangs 6 penetrate the rear face of the front panel 11 to ensure electrical connection between the front panel 11 and the chassis 8 by way of the earthing clip 1 and the retaining strip 10.

The base wall 2 of the clip 1 is narrow enough to permit the clip 1 to be inserted into the groove 9 from the front i.e. by movement from left to right as viewed in Figure 5 by squeezing towards one another the side walls 3, 4 of the clip 1. After the clip 1 is in position in the portions 9c and 9d of the groove 9, the screw 12 mounting the panel 11 can be engaged in the retaining strip 10, passing firstly through the aperture 5 in the base wall 2 of the clip 1. The base wall 2 of the clip 1 should lie squarely against the retaining strip 10 such that approximately equal force will be applied by the front panel 11 to each of the four tangs 6 upon tightening of the screw 12 thereby to prevent rocking and tilting of the clip which if permitted could permit full tightening of the clamping screw without penetration of the panel.

Claims (6)

1. An earthing device for earthing a metallic front panel to the chassis of a sub-rack, which chassis of the sub-rack has a forwardly open groove with an inner portion containing a retaining strip with screwthreaded apertures therein, the screwthreaded apertures being accessible to a clamping screw having its head on the outer side of the panel and its shank extending through an aperture in the panel and through an outer part of the groove, wherein the earthing device comprises a generally U-section earthing clip of electrically conducting resilient material with a base wall and a pair of side walls in opposed spaced relation, the base wall, in operation of the device, bears against the retaining strip and at a middle portion includes an aperture through which the shank of a clamping screw passes, the side walls have end portions formed with projections which will penetrate into the front panel upon tightening of the clamping screw and the width of the base wall is less than and the spacing between the side walls in a repose condition of the earthing clip is greater than the width of an outer portion of the groove whereby the earthing clip can be inserted through the outer portion of the groove by pressing the side walls towards one another and will be retained in position in the groove when released.

2. An earthing device according to claim 1, wherein the length of the earthing clip is less than 15mm and can therefore be accommodated by front panels of the minimum width likely to be encountered.

3. An earthing device according to claim 2, wherein the length of the earthing clip is 12mm.

4. An earthing device according to any one of claims 1 to 3, wherein middle portions of the side walls are cut-away and the end portions of the side walls are formed as pointed tangs thereby readily to penetrate an anodising layer provided on an aluminium front panel.

5. An earthing device according to any one of claims 1 to 4, formed of spring steel by stamping from sheet spring steel and subsequent bending.

6. An earthing device for earthing a metallic front panel to the chassis of a sub-rack substantially as hereinbefore described and illustrated with reference to the accompanying drawings.