GB2269946A

GB2269946A - Earthing of enclosures

Info

Publication number: GB2269946A
Application number: GB9317080A
Authority: GB
Inventors: Brian David Hamblin; Rheinhold Barlian
Original assignee: BARTEC (UK) Ltd; Bartec
Current assignee: BARTEC (UK) Ltd; Bartec
Priority date: 1992-08-20
Filing date: 1993-08-17
Publication date: 1994-02-23
Anticipated expiration: 2013-08-17
Also published as: GB9217671D0; GB2269946B; GB9317080D0; GB2269946A8

Abstract

An electrical enclosure of non-conducting or substantially non-conducting material which contains an earth plate (12) preferably moulded into a wall (10) of the enclosure. A bush (16) of conducting material is connected to the earth plate and extends through the wall of the enclosure so that an earth connection can be made from the exterior of the enclosure, but without imposing any load on the earth plate. <IMAGE>

Description

EARTHING OF ENCLOSURES This invention relates to the earthing of an electrical enclosure.

An electrical enclosure is a term which is intended to describe housings, boxes or the like containing terminals or other electrical equipment and having apertures in the walls thereof through which electrical conductors are led to the terminals and/or equipment.

When the enclosure is made of essentially non-conducting material it is now the practice to provide an earth plate within the enclosure which usually, but not essentially, has tapped apertures to receive the entry portion of a cable gland or other termination that enters the enclosure. Many different arrangements for earth plates within electrical enclosures have been proposed. A particularly preferred construction comprises an enclosure of plastics material with the earth plate or plates moulded into the enclosure so that at least a part of the earth plate is exposed inside the enclosure.

Because of the importance of ensuring that equipment is properly earthed it is often necessary to provide a separate external earth connection to the earth plate inside the enclosure. One proposal to meet that requirement comprises providing for the earth plate in the bottom of the enclosure to extend through a side wall and project from the exterior of the enclosure across the top of one of the mounting feet of the enclosure. An external earth conductor can then be secured to the projecting part of the earth plate, for example by a bolt which extends through the earth plate and into the adjacent mounting foot. There are risks associated with this type of construction.

Although the earth plate is moulded into the plastics enclosure, there is no proper bond between the plastics and the metal earth plate. Uncontrolled tightening of the bolt onto the mounting foot may distort the earth plate sufficiently to separate it from the surrounding plastic so that a capillary track for water is created through the side wall of the enclosure between the said earth plate and surrounding plastic.

Penetration of water into an electrical enclosure can have highly damaging consequences. A similar consequence can occur even when there is no external earth connection, but where the enclosure is not properly seated on its feet, for example due to uneven mounting surface, which again can distort the enclosure and separate the earth plate from the surrounding plastic material where the earth plate projects through the side wall of the enclosure. Excessive torque applied to the earthing foot can crush or damage the sandwiched plastics under the earth point leading to an inadequate earth. Where sensitive electronic connections, for small currents, for example of the order of a few milliamps, are made it is often required to use a clean earth which is independent of the main earth which can have continuous signals on it.

The present invention has been made in order to deal with this problem.

According to the invention there is provided an enclosure having an earth connection, wherein said enclosure is of non-conducting or substantially non-conducting material and an earth plate is provided inside the enclosure, characterised in that means for receiving an earth connection is secured to the earth plate and extends at least partially through a wall of the enclosure so that an earth connection can be made from the exterior of the enclosure and without imposing any load on the earth plate.

As already stated the enclosure of the invention may be of non-conducting or substantially non-conducting material. Substantially non-conducting materials include plastics materials loaded with conducting materials to produce anti-static properties or radio frequency interference shielding.

The earth plate may be provided on one or more interior faces of the enclosure, it may be captured in one or more walls of the enclosure so as to provide a sandwich type construction or may be in part on one or more interior face of the enclosure and in part contained in one or more enclosure walls. The earth plate may be provided in any part of the enclosure.

Where the enclosure has a removable lid or cover an earth plate can be in or on the lid or cover. The earth plate can be held in place by any suitable means.

A preferred way of securing the earth plate to the enclosure is to mould it in place during manufacture of the enclosure.

In one embodiment of the invention receiving means for an earth connection extends preferably at right angles to the earth plate through the wall of the enclosure to the exterior of the enclosure. An earth connection can be fitted to the receiving means without imposing any load upon the earth plate.

In another embodiment of the invention the receiving means for an earth connection is secured to an earth plate in the base of the enclosure and projects upwardly therefrom. An earth connection can be made to the receiving means by a fastening member which is used to fix the enclosure to a support surface. This arrangement can be used where a clean earth is not important.

There are many ways by which the earth connection receiving means can be secured to an earth plate in the enclosure, for example it can be riveted, welded or threadedly connected. In the latter case the connection can be improved if the thickness of the earth plate is increased, for example by folding a part of the earth plate on itself to double the thickness thereof.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig.1 is a vertical section through a part of the side wall of an enclosure; Fig.2 is a section on the line II-II of Fig.3; Fig.3 is a plan view of the corner of an enclosure including a different embodiment of the invention; and Fig.4 is a transverse section through the corner of an enclosure showing a further embodiment.

Referring to Fig.1 an enclosure side wall 10 of plastics material has an earth plate 12 moulded in so as to be flush with the inner surface of the side wall.

The upper edge 14 of the earth plate is preferably in-turned as illustrated. This avoids the upper edge of the earth plate being engaged when equipment is fitted into the enclosure. Alternatively edge 14 can be chamfered or flat.

A bush 16 of conductive material is also moulded into the side wall 10. The base 18 of the bush is riveted or connected in some other suitable manner to the earth plate and extends at substantially right angles to the plane of the earth plate across the width of the side wall to, or just adjacent to, the outer surface of the side wall. The bush is internally profiled, for example it may be tapped so as to receive an earth connection.

It will be appreciated that the earth connection made using the above described assembly does not distort the assembly in such a way as to create a capillary path through the side wall. There may be some torque applied to the bush when the earth connection is made.

The outer surface of the bush can be knurled or otherwise shaped or profiled in order to maintain the bush in position.

Referring to Figs.2 and 3 instead of mounting the bush to extend through the side wall it can be mounted on a part of an earth plate 20 moulded into the base 22 of an enclosure. A bush 24 extends upwardly from the earth plate into the side wall 26 of an enclosure. In this embodiment the bush has a profiled throngh bore 28 which is aligned with a bore 30 which extends through the side wall. The bore 30 is used for fixing the enclosure to a support surface, for example by means of a bolt which passes through the bore 30. The provision of bush 24 enables an earth connection to be made by the said bolt.

Again the bush can be externally knurled profiled to maintain it in position when the earth connection is made. It will also be appreciated that even if there should be some movement of the bush relative to the surrounding plastics material of the side wall that would not create any possibility of leak into the enclosure.

Fig.4 shows another embodiment of the invention, in which an earth plate 12 moulded into the side wall 10 of an enclosure is folded back on itself at, or along, part of one vertical edge as at 32 so as to provide a greater thickness of metal. A tapped bore 34 can be made at the location at 32 where the earth plate 12 is folded on itself to receive a stud, bush or the like 36, for making an earth connection.

In this embodiment also there is no risk of any leak being created when the earth connection is made.

The invention is not limited to the above described embodiments and many modifications and variations can be made. The invention can be used to provide a so-called "clean earth" within an enclosure.

Claims

1. An enclosure having an earth connection, wherein said enclosure is of non-conducting or substantially non-conducting material and an earth plate is provided inside the enclosure, characterised in that means for receiving an earth connection is secured to the earth plate and extends at least partially through a wall of the enclosure so that an earth connection can be made from the exterior of the enclosure and without imposing any load on the earth plate.

2. An enclosure as claimed in Claim 1, wherein at least a part of the earth plate is provided at or adjacent an inner surface of the enclosure.

3. An enclosure as claimed in Claim 1 or Claim 2, wherein at least a part of the earth plate is captured in a wall of the enclosure.

4. An enclosure as claimed in any preceding claim, wherein the earth receiving means extends through at least part of the wall at an angle to the earth plate to which said earth receiving means is secured.

5. An enclosure as claimed in any preceding claim, wherein the earth receiving means is secured to an earth plate in or adjacent to the base of the enclosure and projects upwardly therefrom in a side wall of the enclosure.

6. An enclosure as claimed in any preceding claim, wherein the thickness of the earth plate at the location where the earth receiving means is secured thereto is increased.

7. An enclosure as claimed in any preceding claim, wherein the earth receiving means comprises a bush.

8. An enclosure as claimed in any preceding claim, wherein means are provided on the outer surface of the earth receiving means for retaining the said earth receiving means in position in the enclosure wall.