GB2297196A

GB2297196A - A cabinet for housing electrical apparatus

Info

Publication number: GB2297196A
Application number: GB9501132A
Authority: GB
Inventors: Ian James Mcleod
Original assignee: PAPST PLC
Current assignee: PAPST PLC
Priority date: 1995-01-20
Filing date: 1995-01-20
Publication date: 1996-07-24
Anticipated expiration: 2015-01-20
Also published as: GB9501132D0; HK1011827A1; GB2297196B

Abstract

Screening apparatus comprises a metal frame (2) having an airflow aperture (4) and fixing apertures (6). The fixing apertures (6) are spaced to correspond with those in a fan to be mounted in an electrical cabinet. The airflow aperture (4) is sized to correspond to an airflow aperture in the cabinet. The frame(2) is nickel plated mild steel and a web of welded mesh (8) is welded to an inner surface to extend over the aperture (4). A planar inner plate is received within the frame (2) on the opposite side of the welded mesh (8) to the aperture (4). Whilst reducing airflow by only approximately 1%, the screening apparatus has a screening effectiveness of the order of 25dB or better.

Description

A CABINET FOR HOUSING ELECTRICAL APPARATUS The present invention relates to a cabinet for housing electrical apparatus.

On 1st January 1995 EEC regulations in respect of radio frequency (rf) interference came into force. These require manufacturers to actively consider the amount of rf interference being caused by their products. This has led, in the electronics industry, to a number of new "screened" cabinets and enclosures being produced.

A totally screened cabinet has no access or exit for air and therefore cannot employ conventional methods of cooling, that is, bringing in cold air or exhausting hot air. There are proposals, for example, to build in heat exchangers, to deal with this problem but these tend to be expensive and space consuming.

In view of this, there is still a requirement to use airflow through the cabinet as the cooling mechanism, but of course, the provision of the necessary airflow aperture in the cabinet provides an aperture for both rf emissions and electro mechanical conduction (EMC).

There have been previous proposals for rf filters to be fitted into the airflow aperture of a cabinet, but none of these are particularly satisfactory. Thus, if the earlier proposed devices are able to provide a useful reduction in rf emissions, this is usually at the expense of a considerable reduction in the airflow.

It is an object of the present invention to provide a cabinet for housing electrical apparatus having an airflow aperture for conventional cooling but having screening apparatus in which the disadvantages of prior proposals are reduced.

According to a first aspect of the present invention, there is provided a cabinet for housing electrical apparatus and having a fan mounted therein to provide a flow of air for cooling the interior of the cabinet, and an airflow aperture through which air flows to or from the fan, the cabinet further comprising screening apparatus to reduce rf emissions through said airflow aperture, said screening apparatus comprising a frame of conductive material having a planar exterior surface through which an airflow aperture extends, and a mesh of conductive material extended within said frame across the airflow aperture therein and welded to said frame, wherein said frame is supported on said cabinet such that the airflow apertures through the frame and through the cabinet are substantially aligned, and such that said planar exterior surface of said frame is in contact with a conductive interior surface of said cabinet.

Because, in an embodiment of the invention, the frame has a planar exterior surface, good contact can be ensured between it and the cabinet. This ensures that the screening apparatus is effectively earthed and this, in its turn, ensures that the screening apparatus is an effective screen.

The conductive material mesh is welded to the frame, again ensuring good electrical contact, and thereby, good earthing and effective screening. However, the mesh can be arranged to provide very little interference with airflow, and, for example, to reduce the airflow by only about 1%.

Thus, the screening apparatus provided is able to provide effective screening without substantial reduction in the airflow.

In a preferred embodiment, the mesh is a welded mesh, for example, having four holes per inch (approximately ten holes per centimetre).

In a preferred embodiment, the frame is constructed of mild steel which has been nickel plated.

Although the intimate contact between the planar exterior surface of the frame and a surface of the cabinet can ensure good earthing of the screening apparatus, it is presently additionally preferred that the frame be fixed to the cabinet. This is preferably by means of conductive fixing screws extending through holes provided in both the cabinet and the frame.

Preferably, the screening apparatus additionally comprises an apertured conductive plate fixed within the frame to sandwich the welded mesh between the plate and the airflow aperture of the frame. The inner plate is fixed, for example, adhered to the frame and/or mesh, so that the screening apparatus is in a single piece.

Conventional fans for electrical apparatus cabinets are usually provided with screw holes for mounting the fan on fixing post or screws appropriately provided on the cabinet. In a particularly preferred embodiment, the screening apparatus is similarly provided with fixing holes so that it can be mounted on the same posts or screws as are provided for the fan. In this respect, it is preferred that the frame of the screening apparatus be designed to receive at least part of the fan therein.

The invention also extends to a screening apparatus for a cooled electrical cabinet, said screening apparatus comprising a frame of conductive material, said frame having a planar external surface through which an airflow aperture extends, a mesh of conductive material extended within said frame across said airflow aperture and welded to said frame, and means to support said frame on a cabinet between an airflow aperture of the cabinet and a cooling fan to provide screening for rf emissions without undue reduction in airflow.

In a preferred embodiment said screening apparatus is made to be in one piece to facilitate installation.

In a preferred embodiment the screening apparatus further comprises an inner plate fixed within the frame to sandwich said mesh between the plate and the airflow aperture.

In a preferred embodiment, the mesh is a welded mesh, for example, having four holes per inch (approximately sixteen holes per ten centimetres).

Preferably, the inner plate of the screening apparatus is an apertured conductive plate fixed within the frame to sandwich the welded mesh between the plate and the airflow aperture of the frame. The inner plate is fixed, for example, adhered to the frame and/or mesh, so that the screening apparatus is in one piece.

In a preferred embodiment, the screening apparatus is provided with fixing holes so that it can be mounted on the fixing posts or screws provided in a cabinet for a fan. It is preferred that the frame of the screening apparatus be designed to receive at least part of the fan therein.

Embodiments of the present invention will hereinafter be described, by way of an example, with reference to the accompanying drawings, in which: Figure 1 shows a front view of screening apparatus of the present invention; Figure 2 shows a section taken along the line A-A of Figure 1, Figure 3 is an exploded sectional view to illustrate the components of the screening apparatus, and Figure 4 shows the screening effectiveness of screening apparatus of an embodiment of the present invention.

Figures 1 to 3 show one embodiment of a screening apparatus of the present invention. This screening apparatus comprises a metal frame 2 having a circular aperture 4 therein and fixing apertures 6 extending therethrough. Preferably, the fixing apertures 6 are sized and spaced to correspond with those in a fan (not shown) which is intended to be mounted in an electrical cabinet.

Similarly, the aperture 4 is sized to substantially correspond to an airflow aperture (not shown) provided in the cabinet and through which air to or from the fan is to flow.

The frame 2 is preferably made of mild steel which has been nickel plated. To an inner surface thereof, surrounding the aperture 4, a web of welded mesh 8 is welded. Thus, the welded mesh 8 extends over the aperture 4. A substantially planar inner plate 10 also, preferably, of nickel plated mild steel is received within the frame 2 on the opposite side of the welded mesh 8 to the aperture 4. The inner plate 10 is fixed to the frame 2 and/or to the welded mesh 8, for example, by adhesive and causes the welded mesh 8 to be sandwiched between the inner plate 10 and the front apertured surface of the frame 2.

The resultant screening apparatus is in one piece and can be easily mounted within a cabinet for electrical apparatus. In this respect, the screening apparatus is preferably mounted on fixing screws or posts already provided for the fan. In this way the aperture 4 in the frame 2 is substantially aligned with the airflow aperture of the cabinet, and the welded mesh 8 extends over that airflow aperture. Care is taken to ensure that the front, substantially planar, surface of the frame 2 makes good contact with the inner surface of the cabinet. Preferably, this is a metal to metal contact. Similarly, the fixing screws for fixing the frame 2 to the cabinet are preferably metal, again to ensure that the screening apparatus is effectively earthed by the earthing for the cabinet, and hence that the screening apparatus has an effective screening capability.

The fan is preferably mounted so that a part thereof is received within the frame 2 and so that it is secured on the fixing posts or screws mounting the frame 2. Thus, the fan will be substantially coaxially arranged with the cabinet airflow aperture and with the aperture 4.

In one preferred embodiment, for use with a conventional fan for such purposes, the welded mesh 8 has four holes per inch (approximately sixteen holes per ten centimetres). It has been found that when the screening apparatus incorporates a mesh of such a size, the airflow reduction due to the screening apparatus is only of the order of 1%.

Figure 4 shows the results of tests made on screening apparatus similar to that of Figures 1 to 3 and incorporating a welded mesh of four holes per inch. It will be seen that the shield effectiveness of the arrangement is generally of the order of 25dB or better and arrangement is generally of the order of 25dB or better and hence is considerably better than prior devices available.

Thus, a screening apparatus of the invention whilst only reducing airflow by approximately 1% does provide screening effectiveness of the order of 25dB or better. It is a one piece construction for easy installation and can be mounted on the standard fan fixing posts, again for easy installation. The screening apparatus does not increase audible noise and does not have any effect on the outside of the equipment. The screening apparatus is therefore a cost effective solution to the problems discussed.

It will be appreciated that variations and modifications to the illustrated embodiments may be made within the scope of the application.

Claims

1. A cabinet for housing electrical apparatus and having a fan mounted therein to provide a flow of air for cooling the interior of the cabinet, and an airflow aperture through which air flows to or from the fan, the cabinet further comprising screening apparatus to reduce rf emissions through said airflow aperture, said screening apparatus comprising a frame of conductive material having a planar exterior surface through which an airflow aperture extends, and a mesh of conductive material extended within said frame across the airflow aperture therein and welded to said frame, wherein said frame is supported on said cabinet such that the airflow apertures through the frame and through the cabinet are substantially aligned, and such that said planar exterior surface of said frame is in contact with a conductive interior surface of said cabinet.

2. A cabinet as claimed in Claim 1, wherein the conductive material mesh is welded to the frame, ensuring good electrical contact and good earthing and effective screening.

3. A cabinet in Claim 1 or Claim 2, wherein said conductive mesh is arranged to reduce the airflow through the airflow aperture by only about 1%.

4. A cabinet as claimed in any preceding claim, wherein said conductive material mesh is a welded mesh having about four holes per inch (approximately ten holes per centimetre).

5. A cabinet as claimed in any preceding claim, wherein said frame is constructed of mild steel which has been nickel plated.

6. A cabinet as claimed in any preceding claim, wherein the frame be fixed to the cabinet by means of conductive fixing screws extending through holes provided in both the cabinet and the frame.

7. A cabinet as claimed in any preceding claim, wherein the screening apparatus additionally comprises an apertured conductive plate fixed within the frame to sandwich the welded mesh between the plate and the airflow aperture of the frame.

8. A cabinet as claimed in Claim 7, wherein said inner apertured plate is fixed so that the screening apparatus is in a single piece.

9. A cabinet as claimed in any preceding claim, wherein the screening apparatus is provided with fixing holes so that it can be mounted within the cabinet on posts or screws provided for mounting the fan.

10. A cabinet as claimed in Claim 9, wherein the frame of the screening apparatus is arranged to receive at least part of the fan therein.

11. A screening apparatus for a cooled electrical cabinet, said screening apparatus comprising a frame of conductive material, said frame having a planar external surface through which an airflow aperture extends, a mesh of conductive material extended within said frame across said airflow aperture and welded to said frame, and means to support said frame on a cabinet between an airflow aperture of the cabinet and a cooling fan to provide screening for rf emissions without undue reduction in airflow.

12. A screening apparatus as claimed in Claim 11, wherein said screening apparatus is made to be in one piece to facilitate installation.

13. A screening apparatus as claimed in Claim 11 or Claim 12, further comprising an inner plate fixed within the frame to sandwich said mesh between the plate and the airflow aperture.

14. A screening apparatus as claimed in any of Claims 11 to 13, wherein the mesh is a welded mesh having about four holes per inch (approximately sixteen holes per ten centimetres).

15. A screening apparatus as claimed in any of Claims 11 to 14, wherein the frame is constructed of mild steel which has been nickel plated.

16. A screening apparatus as claimed in any of Claims 11 to 15, wherein the inner plate of the screening apparatus is an apertured conductive plate fixed within the frame to sandwich the welded mesh between the plate and the airflow aperture of the frame.

17. A screening apparatus as claimed in any of Claims 11 to 16, provided with fixing holes so that it can be mounted on fixing posts or screws provided in a cabinet for a fan.

18. A cabinet for housing electrical apparatus substantially as hereinbefore described with reference to the accompanying drawings.

19. A screening apparatus for a cooled electrical cabinet substantially as hereinbefore described with reference to the accompanying drawings.