GB2211366A

GB2211366A - Junction box

Info

Publication number: GB2211366A
Application number: GB8818781A
Authority: GB
Inventors: Howard Arthur Finch; Graham Stephen Ross
Original assignee: BARTON ENGINEERING Ltd
Current assignee: BARTON ENGINEERING Ltd
Priority date: 1987-10-16
Filing date: 1988-08-08
Publication date: 1989-06-28
Also published as: GB8818781D0

Abstract

A conduit box, in the form of a metal pressing, has a circular base (10), and a cylindrical peripheral wall (11), in which are formed, by pressing out, a plurality of flats (15), each provided with a knock-out disc (16), the flats having a circular, part-circular or polygonal profile, a metallic bush (19) being intended to be inserted through a circular hole in the box formed by removal of a knock-out disc, the bush having a head (20), a threaded shank (21) and an undercut (25), between the head (20) and the thread (22), the undercut having a frustoconical surface (29) which, if the head abuts the inner surface of a flat, causes deformation of a peripheral edge of said hole to create a tight seal between the box and the bush. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO ELECTRICAL ENCLOSURES This invention relates to improvements in or relating to electrical enclosures, particularly circular junction boxes for electrical conduit.

At present such conduit junction boxes are manufactured from steel or malleable iron and have at least one radially outwardly projecting, integral hollow spigot for connection of conduit. However an electrical installation may require the use of several different types of junction box, where each type includes a different number of conduit entries. This gives rise to problems for the manufacturer, wholesaler and installer of the junction boxes because each of said different types of junction box must be kept in stock.

It is well known to use an externally threaded, hexagon headed metallic bush for fixing conduit tube, rigid or flexible electrical wire or cable protection tube or fittings to an electrical enclosure, such as a conduit junction box. The bush is inserted through a hole provided in the electrical enclosure case so that an internally threaded coupling or protective tube can be connected to the electrical enclosure.

In some applications the dimensions of the bush and the hole in the enclosure case are precisely defined by relevant British Standards. Additionally in some cases the connecting tube is used as part of a path for electrically earthing the system, so that there must be a low resistance electrical contact between the threaded bush and the metal enclosure. In some applications it is also required that when tightened to secure a protective tube or fitting, the threaded bush must provide an adequate seal to prevent ingress of dust or water into the electrical enclosure. This seal has been provided by a washer, which may also have an attached earthing tag to provide the necessary electrical earth from the threaded coupling to the electrical enclosure.

Some British Standard Specifications require the threaded bush to have a thread undercut which is wider than the thickness of the metal electrical enclosure.

This is due to difficulty in threading accurately to the shoulder formed by the head of the bush. As a result considerable movement is possible between the bush and the hole in the enclosure case. For example if a threaded bush for an electrical conduit box has a 20mm nominal diameter and a 17.7mm to 18.0mum diameter undercut, and is to fit an enclosure hole of 20.25mm to 20.75mm diameter, the bush can be off-centre by between 1.125mm and 1.525mm, so that flats on the hexagonal head may no longer cover the hole.

One object of the invention is to provide a circular conduit junction box which may be easily adapted by an installer so as to give any number of conduit entries up to a maximum determined by the particular box.

A further object of the invention is to provide such a circular conduit junction box in combination with at least one bush which overcomes or reduces at least some of the above mentioned problems with known bushes.

According to a first aspect of the invention there is provided a conduit box comprising a cylindrical peripheral wall, a plurality of flats extending out of said wall, and each flat being formed with a knock-out portion.

According to a second aspect of the invention there is provided the combination of a conduit box comprising a cylindrical peripheral wall, a plurality of flats extending out of said wall, with each flat being formed with a knock-out portion, and a bush, the bush comprising a head, a hollow shank extending from the head and defining a central longitudinal axis, an external thread on the shank terminating at a position spaced from the head, and an undercut between said termination of the thread and said head, the undercut having a tapering surface which joins the head at a radial distance from said axis which is greater than the radius of a circular hole in said conduit box, formed on removal of one knock-out portion, through which hole the bush is intended to be received, so that, in use, at least part of the peripheral edge of said hole and/or of said surface at or adjacent said join is deformed if said head is received against a surround at one side of the hole.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary plan view showing a conduit junction box of the invention with a conduit affixed thereto, Figure 2 is a side view of the Figure 1 arrangement, Figure 3 is a sectional view on the line 3-3 of Figure 2, with only the conduit box itself being shown, Figure 4 is a part sectional view on the line 4-4 of Figure 1, and Figure 5 is an enlarged, scrap section of part of a bush used with the conduit box to affix conduit thereto.

Referring now to Figures 1 to 4, the conduit box comprises a flat circular base 10 formed integrally with an upstanding cylindrical peripheral wall 11. The box is preferably made from a steel pressing. The top of the wall 11 is formed with tabs 12 which may be provided with tapped holes 13, thereby allowing a lid (not shown) to be affixed to the box. A raised tapped hole 14 may be provided in the base to form a securing location for an electrical earth wire.

As can be seen best from Figures 1 and 3, a number of flats 15 are created by pressing out or otherwise shaping the peripheral wall 11. As shown in Figures 2 and 4, the profile of each flat is circular. However in one alternative embodiment, the profile of each flat could be only partly circular, having opposite, parallel straight sides normal to the base 10. In a further alternative embodiment the profile could be polygonal, for example hexagonal.

Each flat 15 is provided with a knock-out disc 16. One means of providing such a disc is to pierce a non-closed circular impression, leaving a small arc 17 of the impression unpierced in order to maintain connection of the disc 16 to the remainder of the flat 15 until the disc is knocked out. In the same way as for the flats 15, the base has knock-out discs 18 of different sizes.

In use, any number of the discs 16, 18 may be knocked out so as each to provide a circular hole which will accept a threaded bush 19, preferably made from electrically conducting material. The bush preferably has a hexagonal head 20 and a hollow cylindrical shank 21 which has an external screw thread 22.

The shank of the bush is passed through a hole provided by the removal of a knocked out disc and an internally threaded coupler 23 (Figures 1, 2 and 4) is threadedly engaged with the shank 21, the coupler being of a length sufficient to leave a portion of the coupler free to accept one end of an end-threaded conduit 24.

Alternatively the coupler 23 may be replaced by a coupling component having one end internally threaded, to engage on the shank 21, and its other end provided with any convenient means for clamping or fixing a non-threaded conduit.

Where the flats have a circular profile, the bush is rotated relative to the coupler 23 or the coupling component, this rotation preferably being effected by the use of a spanner-like tool engaged with the head 20 of the bush. This has the advantage that it is unnecessary to grip any part of the assembly which would thereafter require any scratched painted area to be touched up.

With the alternative profiles, the sides of the pressed out flats are preferably of such a dimension as to cause engagement between the flats on the head of the bush and the internal wall of the box, thereby preventing rotation of the bush relative to the conduit box while securing the bush to the box by means of the coupler 23 or the coupling component. Flats of polygonal profile provide better engagement with bush heads of similar polygonal profile. Alternatively the above-mentioned rotation may be prevented by locating the centre line of a knock-out disc sufficiently close to the base of the conduit box as to cause engagement between the head of the bush and the base of the conduit box.

The thread 22 of the bush 19 extends from the free end of the shank 21 towards the head 20, but terminates a little way short thereof, where, as shown best in Figure 5, there is a thread undercut 25 extending to the head 20. A central longitudinal axis A of the bush 19 is shown in Figure 5, which shows the undercut in detail.

At the end of the thread 22 is an inwardly sloping frustoconical surface 26 leading to a cylindrical surface 27 parallel to the axis A. From the surface 27 to a flat, inner axially facing surface 28 of the head 20 extends a frustoconical surface 29 which slopes outwardly, in this instance at 450, to the axis A. The width of the undercut, namely the distance between the end of the screw thread 22 and the plane of the surface 28, is greater than the thickness of a flat 15 (Figure 3). Most importantly, however, the junction of the surface 29 with the head 20 is at a radial distance from axis A which is greater than the radius of the hole provided by the knocking out of a disc 16.

Thus when the bush is initially engaged in a hole formed in a flat 15, the inner peripheral edge of the hole will engage against the surface 29 with the inner face 28 of the head slightly spaced from the inner surface of the flat 15. The coupling tube 23 (or equivalent component) is then engaged on the shank 21 and by appropriate relative rotation between the tube and the bush, the tube is screwed up towards the outer surface of the flat 15 until it engages against it, with the surface 28 of the head 20 abutting against the inner surface of the flat 15. This causes deformation of the peripheral edge of said hole at the inner surface of the flat 15 as it rides along the surface 29, to produce a seal between said edge and the surface 21.

This engagement between the surface 29 and the edge of the hole occurs whether the bush rotates and the tube is stationary or vice versa.

For a 20mm nominal diameter screwed fitting hole 16 (20.25 to 20.75mm), for example, it has been found that if the junction of surface 29 at surface 28 is between the limits 20.95mm diameter to 21.lmm diameter, the metal of a flat 15 of the conduit box will deform so as to form a seal and an electrical contact area of approximately lOsq.mm. Additionally the bush will of course self centre at the hole. The sizes given for this example have been found to work well and the tolerances can be held in mass production.

Thus the bush described will self centre on tightening, provide a dust and splashproof seal, provide good electrical earth contact, provide annular strengthening between the head and the undercut, and provide some measure of anti-vibration to prevent loosening of the bush.

The bush 10 and associated hole can of course be of sizes different to those specified and the coupling tube 23 and tube 24 can also be of different form.

Instead of sloping at 450, the surface 29 can slope at a suitable different angle. Moreover the surface 29 could be curved, e.g. radiussed, rather than straight as shown in Figure 5. Instead of the metal of the flat of the conduit box deforming as the bush is tightened, the materials of the conduit box and the bush could be such that the bush surface 29 alone deforms or deforms in addition to the deformation of the peripheral edge of the hole in the flat.

A conduit junction box constructed in accordance with the present invention possesses the under-mentioned advantages: 1. On-site adaptation of the conduit box for connection of variable numbers of conduits is easily achieved by removing knock-out discs at whichever locations are required for conduit connection (connection being completed, as desribed above, by the fitting of a separate threaded bush and coupler at each connection point). The conduit box, being thus capable of adaptation to a variety of connection requirements, could be stocked as a single model in place of several different types of non-adaptable conduit boxes.

2. In fixing conduit to the conduit box, standard conduit couplers and standard threaded bushes can be used, such couplers and bushes being readily available for on-site conduit installation.

3. In the case where the conduit box, the threaded bush and the coupler are all made from electrically conducting components, the final connected conduit box assembly will provide electrical earth continuity between the conduit box and any affixed conduit.

4. The conduit box can be dimensioned so as to conform to the appropriate British Standard specification(s).

Claims

1. A- conduit box comprising a cylindrical peripheral wall, a plurality of flats extending out of said wall, and each flat being formed with a knock-out portion.

2. A conduit box as claimed in claim 1, wherein said knock-out portion is disc-shaped.

3. A conduit box as claimed in claim 2, wherein said knock-out portion is provided by piercing a non-closed circular impression, with a small arc of the impression unpierced to connect the disc-shaped portion to the remainder of the flat.

4. A conduit box as claimed in any one of claims 1 to 3, wherein each flat has a circular profile.

5. A conduit box as claimed in any one oE claims 1 to 3, wherein each flat has a partly circular profile with opposite parallel straight sides.

6. A conduit box as claimed in any one of claims 1 to 3, wherein each flat has a polygonal profile.

7. A conduit box as claimed in any one of the preceding claims, comprising a circular base in which at least one knock-out portion is formed.

8. A conduit box as claimed in any one of the preceding claims, formed as a steel pressing with each flat having been formed by pressing out a portion of said peripheral wall.

9. A conduit box and bush combination comprising a conduit box having a cylindrical peripheral wall, a plurality of flats extending out of said wall, each flat being formed with a knock-out portion, and a bush having a head, a hollow shank extending from the head and defining a central longitudinal axis, an external thread on the shank terminating at a position spaced from the head, and an undercut between said termination of the thread and said head, the undercut having a tapering surface which joins the head at a radial distance from said axis which is greater than the radius of a circular hole in said conduit box, formed on removal of one knockout portion, through which hole the bush is intended to be received, so that, in use, at least part of the peripheral edge of said hole and/or of said surface at or adjacent said join is deformed if said head is received against a surround at one side of the hole.

10. The combination as claimed in claim 9, wherein said knock-out portion is disc-shaped.

11. The combination as claimed in claim 10, wherein said knock-out portion is provided by piercing a non-closed circular impression, with a small arc of the impression unpierced to connect the disc-shaped portion to the remainder of the flat.

12. The combination as claimed in any one of claims 9 to 11, wherein each flat has a circular profile.

13. The combination as claimed in any one of claims 9 to 11, wherein each flat has a partly circular profile with opposite parallel straight sides.

14. The combination as claimed in any one of claims 9 to 11, wherein each flat has a polygonal profile.

15. The combination as claimed in any one of claims 9 to 14, wherein the conduit box has a circular base in which at least one knock-out is formed.

16. The combination as claimed in any one of claims 9 to 15, wherein the conduit box is formed as a steel pressing with each flat having been formed by pressing out a portion of said peripheral wall.

17. The combination as claimed in any one of claims 9 to 16, wherein said tapering surface is a frustoconical surface.

18. The combination as claimed in claim 17, wherein said surface slopes at 450 to said longitudinal axis.

19. The combination as claimed in any one of claims 9 to 16, wherein said tapering surface is radiussed.

20. The combination as claimed in any one of claims 9 to 19, wherein the width of said undercut is greater than the thickness of a flat of said conduit box.

21. The combination as claimed in any one of claims 9 to 20, wherein said bush is made of a material harder box than that of said conduit and said peripheral edge of the hole is thus deformed if said bush head is received, in use, against the interior surface of a flat with said shank passed through said hole.

22. The combination as claimed in claim 16, wherein the bush is made of electrically conductive material.

23. A conduit box substantially as hereinbefore described with reference to and as shown in Figures 1 to 4 of the accompanying drawings.

24. A conduit box and bush combination substantially as hereinbefore described with reference to and as shown in Figures 4 and 5 of the accompanying drawings.