GB2317750A

GB2317750A - Screening of enclosures

Info

Publication number: GB2317750A
Application number: GB9619402A
Authority: GB
Inventors: Brian David Hamblin; Rheinhold Barlian
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-09-17
Filing date: 1996-09-17
Publication date: 1998-04-01
Also published as: GB9619402D0

Abstract

A system for screening an electric or electronic enclosure of the type made of plastic panels, and particularly for screening radio frequency energy. The system includes an earth plate (20) provided in the enclosure and one or more pieces (26, Fig.4) adapted to cover any gap in the earth plate to form a metal housing with an open top, and a cover (32) for closing the open top of the housing. The system may be effected in enclosures already provided with an earth plate, to which additional pieces are added to fill any gaps in the coverage of the earth plate and therefore provide full screening.

Description

SCREENING OF ENCLOSURES This invention relates to the screening of enclosures of electrical equipment.

It is now common practice for enclosures of electrical equipment such as junction boxes and the like to be made of plastics rather than metal. The advantage of that is that non-metallic enclosures are not corroded like metal enclosures and can often stand up to the ambient conditions, for example in gas hazardous areas, better than metal enclosures. With non-metallic enclosures it is necessary, in order to meet various directives and regulations, to provide screening so as to prevent radio frequency energy generated outside or inside the enclosure from passing into or out of the enclosure as the case may be. Loading of the nonmetallic material of the enclosure with an anti-static does not provide the necessary screening effect.

It is known that earthing of enclosures can be provided by means of internal metal plates. These plates can be included during moulding of the non-metallic enclosure or a separate piece fitted during manufacture or afterwards. The provision of such an internal metal plate enables all internal components to be earthed via the plate to a special earth conductor or to the armour of cable conductors entering the enclosure. These earth plates will provide some screening, but they are not designed to cover the entire surface of the enclosure and, therefore, their screening properties are incomplete.

Screening of the whole outer and/or inner surface of an enclosure has been provided by covering the surface with a conductive metal based paint deposited for example by electro deposition or by simple painting or spraying methods. However, these methods are expensive and they are not readily applicable to existing equipment particularly equipment that already forms part of an electrical or electronic installation.

The object of the present invention is to provide a screen for electrical or electronic enclosure that is simple and inexpensive and which can be fitted to existing enclosures.

According to the invention there is provided a screen system for an electrical or electronic enclosure, said enclosure comprising a base, walls on the base and a removable lid, said system comprising an earth plate in the enclosure one or more pieces adapted to cover any gap in the earth plate on the base and/or walls of the enclosure to define a metal housing having base walls and an open top and a cover for closing the open top of the metal housing, said cover still permitting the lid to be sealingly secured to the enclosure.

A particular advantage of the invention is that enclosures already provided with earth plates can be fully screened by the provision of as many pieces as are required to fill any gaps in the coverage of the earth plate over the base and walls of the enclosure and the provision of the cover. However, the invention can be applied to original equipment during fitting of an earth plate thereto.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Fig.l is a plan view of an enclosure with the lid and cover removed; Fig.2 is a section on the line II-II of Fig.l, but including the lid and cover shown separated from the rest of the enclosure; Fig.3 is a plan view of an earth plate prior to shaping for reception in the enclosure; and Fig.4 is a perspective view of a corner piece.

Referring to the drawing a non-metallic enclosure, generally 10, includes a base 12 and walls generally 14.

In the embodiment shown in the drawing the enclosure is a substantially cruciform plan with side walls 16 at the ends of each arm of the cross being linked by inturned corners 18.

A cruciform sheet metal earth plate 20 as shown in Fig.2 has its arms 22 folded upwardly along the dotted lines 24 so that it can be fitted into the enclosure.

It will be apparent that the earth plate once located in the enclosure covers the base and the side walls 16.

The inturned corners 18 not covered by the earth plate 20 are covered by corner pieces 26 shaped and sized to match the corners 18. One corner piece is illustrated in Fig.4. Each corner piece is an L-shaped metal sheet with out-turned flanges 29 along each edge. A slot 28 is formed between the flanges 29b at each side and the adjacent flanges 29a and 29c at the ends. When the corner pieces are placed in position around the corners 18 the slots 28 receive the edge of arms 22 of the earth plate, the flanges 29b along the sides of the corner pieces overlie the adjacent edges of arm 22 of the earth plate and in addition the flanges 29c at the bottom of the corner pieces overlie the base portion of the earth plate. Preferably the corner pieces are fitted onto the earth plate prior to location in the enclosure. The earth plate together with the corner pieces form an open topped metal housing.

The enclosure is closed by a lid 30 which can be secured to the side walls by screws or the like (not shown). A simple metal cover 32 is provided for closing the open top of the housing formed by the earth plate and corner pieces. The cover 32 has downturned flanges 34 rather like the lid of a biscuit tin for engaging the outer sides of the free ends of arms 22 to hold the cover 32 in place. Provided the cover 32 extends to the tops of the corner pieces 26 and there is no gap between the cover 32 and corner pieces 26, it is not necessary to provide any interconnection between the cover 32 and the corner pieces 26.

The invention is not restricted to the above described embodiment. For example the corner pieces can be made integral with the arms 22 of earth plate.

Moreover the invention is not only applicable to enclosures of the shape described in the specific embodiment. Other shaped enclosures can equally benefit from the invention with appropriate modification to the shape of the earth plate, corner pieces and cover as necessary to provide a complete metal housing within the enclosure.

The cover can be linked to the earth plate or the enclosure by a simple strap, hinge or the like to prevent the cover being lost and also to ensure that the cover is replaced before the lid is fitted onto the enclosure.

The strap can be conductive and provide an earth connection between the cover and the earth plate.

Claims

1. A screen system for an electrical or electronic enclosure, said enclosure comprising a base, walls on the base and a removable lid, said system comprising an earth plate in the enclosure and one or more pieces adapted to cover any gap in the earth plate on the base and/or walls of the enclosure to define a metal housing having base walls and an open top and a cover for closing the open top of the metal housing, said cover still permitting the lid to be sealingly secured to the enclosure.

2. A screen system according to claim 1, wherein the earth plate comprises a folded sheet metal plate.

3. A screen system according to either claim 1 or 2, wherein said one or more pieces adapted to cover any gap in the earth plate comprises folded sheet metal plates.

4. A screen system according to any preceding claim, wherein said one or more pieces are integral with the earth plate.

5. A screen system according to any preceding claim, wherein any of said pieces adapted to cover any gap in the earth plate are provided with slots to receive the earth plate.

6. A screen system according to any preceding claim, wherein the cover is provided with down-turned flanges for engaging the earth plate.

7. A screen system according to any preceding claim, wherein the cover is linked to the earth plate to prevent loss of the cover and to ensure that the cover is replaced before the lid is fitted onto the enclosure.

8. A screen system according to claim 7, wherein the cover is linked to the earth plate by means of a conductive strap which provides an earth connection between the cover and the earth plate.

9. A screen system substantially as hereinbefore described and illustrated.