GB2230389A

GB2230389A - Electrical junction box

Info

Publication number: GB2230389A
Application number: GB8901686A
Authority: GB
Inventors: Christopher Moller
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-01-26
Filing date: 1989-01-26
Publication date: 1990-10-17
Also published as: GB8901686D0

Abstract

The familiar round 3- or 4-terminal plastics junction box traditionally used for joining permanent cables in a domestic environment has many limitations, including requiring individual conductors to be stripped to different lengths, and cracking because of the mechanical stresses on the box caused by the need to squeeze in the wires. The invention proposes a novel form of junction box (10) having a plurality of connectors (13) arranged in rows and columns, the connectors in each row or column respectively being in electrical contact and each column or row respectively being aligned opposite a cable clamp or slot (11). The rows and columns need not run perpendicularly. Each connecter includes springs and a bus bar engaged with a cable wire by resilient jaws (6) forced against a chamfered cam by a screw urging levers (6'), by a platen and rubber member or directly. The former equalise the connection forces. Alternatively, a screw-urged wedge is used. Insulation displacement can be used as an alternative. <IMAGE>

Description

JUNCTION BOX.

This invention relates to an electrical junction box.

The familiar round 3- or 4-terminal plastic junction box traditionally used for joining permanent cables in a domestic environment has many limitations. Among these are a tendency for the brass grub-screws to get cross-threaded or lost, and the box to fill up with wires (the latter because the point of entry of a cable to the box bears little relation to where the individual conductors need to go). These junction boxes also ideally require individual conductors to be stripped to different lengths, but in practice they seldom are. Further, the mechanical stresses on the box caused by the need to squeeze the wires in means that cracked boxes are extremely common. Although originally intended for up to four conductors per terminal, usage with up to six is common.

The end result is that although such junction boxes are very cheap to manufacture, when the cost of the installation time is included, they represent an expensive and unsatisfactory solution.

There is a need therefore for a junction box designed to minimise the amount of bending required for cables entering it, to provide a standardised length for stripping cables to, to reduce the number of actions required to install it, and to minimise the chance of its component parts being lost.

According to the present invention, there is provided a junction box having a plurality of connectors arranged in rows and columns, the connectors in each row or column respectively being in electrical contact and the connectors in each column or row respectively being aligned opposite a cable clamp or slot.

The columns and rows may be arranged perpendicular to one another or, alternatively, they may be at an angle to one another other than 90'.

There is effectively no limit on the number of cables and conductors that the box may be designed for.

Two specific embodiments of the invention will now be described (as examples of how the invention might be implemented for up to six cables, each containing no more than four conductors) with reference to the accompanying drawings in which: Fig. 1 provides a general overview of the first embodiment; Fig. 2 shows a section through the box of the first embodiment; Fig. 3 shows a selection of possible contact arrangements for the first embodiment; Fig. 4 shows a preferred cable clamp design.

Fig. 5 shows two methods of allowing a single screw to secure several wires.

Fig. 6 shows a way of increasing the reliability of poke-home connectors in a high-current environment.

Fig. 7 shows a second embodiment, using conventional screw-terminals, or insulation-displacement connectors.

Referring to Fig. 1, an electrical junction box comprises a moulded body of insulating (eg. plastics) material 10 incorporating several cable clamps 11 and 'poke-home' connectors 13. A cable 12 is stripped as shown, passed through a cable clamp, and the individual conductors poked into the connectors 13.

According to the design of the 'poke-home' connectors 13, it may be desirable then to secure the connection with a grip screw on the back (as shown in Fig. 3e and Fig. 5. As all the cables enter from the same side, the potential exists for mounting the box vertically with all the cables entering from the bottom. With a suitably designed lid/cover 10' (see Fig. 2), a degree of weatherproofing may thus be achieved.

Fig. 2 shows a section through the box.

Several possible sites for mounting holes 9 are shown.

In Fig. 3, several possible devices are shown for making the electrical connection. They have in common the following features: 1 an aperture of sufficient depth to prevent the wire 5 entering at an appreciable angle, 2 a common busbar (coming out of and/or going into the paper) to which the wires from each cable form a connection, 3 one or more springs to ensure adequate pressure with the contact surface.

The shape of the springs may Fig. 3a,b or may not Fig. 3c,d permit withdrawal of the wire from the same side as it was pushed in. If it does not, it may be desirable to provide an opening 4 to allow withdrawal (after cutting off) of a stub of wire from the opposite side to that at which it entered, in the event of a mistake being made, or a change in the wiring arrangement becoming necessary.

Although 'poke-home' connectors are already in use, they are not widespread, because they are difficult to use with flexible cables, and in most environments, it is difficult to prevent lateral force on the contact. Furthermore, they do not cope well with widely differing wire diameters. However, in an environment where rigid single-conductor cables with similar current ratings are used exclusively, and a cable-clamp is provided, these obstacles can be eliminated and poke-home connectors may be the most appropriate tupe to use.

It is clearly essential to ensure that the connection has as low a contact resistance as possible, even when it is installed in difficult circumstances. Electricians are taught that after the traditional screw clamp has been screwed down, each of the wires should be pulled to ensure that a secure mechanical contact has been achieved. The implicit assumption is made that a secure mechanical contact implies a good electrical contact (this may or may not be true depending on circumstances!). For a poke-home connector to be acceptable to the trade, it is likely to be necessary that a secure mechanical contact is made, in addition to the electrical contact (though an effective cable clamp should actually make it unneccessary).

An alternative embodiment for the contact surface is therefore shown in Fig. 3e. In addition to the features described above, a flexible insulating carrier 6 (for example made of Nylon) is screwed towards the wire 5 by a screw 7. Due to the angled faces of the carrier, the spring contacts 3 will positively grip the conductor. It may be desirable to limit the gripping force on the screw thread to ensure that the spring contacts cannot actually sever the conductor. Note that in this example, the busbar 2 must have sufficient flexibility, and the carrier must be sufficiently distanced on the busbar from the next carrier, to allow the busbar to flex independently with the movement of each carrier. It may be desirable to provide an access hole 8 into which a small instrument may be inserted to facilitate release of the cable, in the event of changes in the wiring being necessary.

Note that the carrier 6 and screw 7 may be common to all four connectors specific to this cable, meaning that only a single action is needed to secure all the wires. A possible limitation of this scheme is that the force provided by the screw may be shared unequally between the several conductors (especially when one or more of them is absent, or the conductor sizes differ). Two further proposals are shown in Fig. 5a,b for overcoming this. Fig. 5a shows how a rigid plate 15 and a compressible material (such as rubber) 16 between the screw 7 and four independent carriers 6. This ensures that some force is applied, even to the smallest conductor.A better scheme involves sharing the force approximately equally between all four carriers, by means of levers as shown in Fig. 5b. (A similar scheme with levers of unequal length may be used for different numbers of terminals.) Note that although this has the effect of reducing the force on each carrier to a quarter, the amplifying effect of a shallow wedge angle of the leading edge of the carrier may well mean that the net force on the conductor is similar to that achieved with an individual screw for each conductor (though of course the number of screw turns required will increase).

A limitation of this scheme is that in the event of a good electrical contact not being made, a high current could cause heating of the connection.

This could cause any thermoplastic components to deform, further lessening the pressure on the contact surfaces, and further worsening the electrical contact. The traditional approach to this problem has been to use thermosetting plastics for the moulded components. However, they are brittle, and crack easily, as mentioned above. Fig. 6 shows an alternative approach that may be used to overcome this, in which the contact pressure is not dependent on the rigidity of the moulded parts.

The contact surface is replaced by a fixed metal cage 17, formed from a development of the busbar, or as a separate component spot-welded to it. The mating surface 18 may be contoured to ensure plural contact points with any conductor within the size range of interest. The movable thermoplastic carrier is replaced by a wedge of rigid non-thermoplastic material (eg metal or ceramic) 19, to force the conductor 6 against the contact surface. (A metal wedge would participate in making the connection, further increasing the number of contact points.) The angle of the wedge is chosen to be small enough to ensure a good translation of the screw pressure to the contact surface, but not so small as to require an unreasonably large number of turns on the screw, or to cause widely differing wedge positions for slight variations in conductor diameter.A larger angle will be easier to release, in the event of a wiring error, or cable changes becoming necessary. However, it is desirable that the various coefficients of friction are chosen such that simply releasing the screw will not be enough to allow the wedge to back out (as this could again cause contact pressure to lessen in the event of overheating). Releasing the wedge should only be achieved by a second action after releasing the screw, for example, pushing the conductor 6 further into the cage, or inserting some sharp object into a suitably-placed hole, to exert direct force on the wedge in the opposite direction to the screw.

Several methods are available to ensure that the conductor does not snag on the leading edge of the wedge during insertion, for example, an angled leading edge 20, and/or a leading or trailing spring 21,22. Arranging the force of the screw to act at or near the point marked 'a' on the wedge will ensure that the wedge tends to keep close to the top of the cage.

A more conventional embodiment of the junction box is shown in Fig. 7. Traditional screw terminals Fig. 7a do not suffer Yrom the heating problems mentioned above. A junction box may be implemented using these, and still benefit from many of the advantages of the arrangement described above, provided that the terminals 24 are arranged on the arc of a circle (as shown), to keep the individual conductors the same length.

Provided that the range of conductor sizes supported is small, the size of each individual screw terminal can be kept to a minimum. (It is suggested that it is better to add additional screw terminals, than to attempt to cater for more than one conductor in any one terminal.) It may be desirable to have one or more sets of terminals slightly larger, to accomodate older wiring, which had a larger cross-sectional area for a given current-carrying capacity. The tip 23 of the screw may be burred, to reduce the probability of the screw being lost.

If the range of conductor sizes supported is very small (and use with older multi-strand conductors is not permitted), the possibility exists with this arrangement of using an insulation displacement connector, being an enlarged version of that commonly found in telephone sockets (Fig. 7b).

This would further reduce installation time, by eliminating the need to strip the end of each individual conductor. However, a special tool will be required to ensure reliable insertion of the conductor in the terminal. The connector comprises a two-prong metal cantilever 25 (which could be duplicated to increase current-carrying capacity), housed (for safety, not strength) in an insulating carrier 26. The dimensison of the aperture between the prongs, and the elasticity of the metal, are such that, when the conductor is in place, sufficient contact pressure is maintained on the conductor by the metal cantilever alone. Therefore, contact heating, though it may result in plastic deformation of the carrier 26, will not result in breaking of the contact.

The cable clamp may be the familiar screw-down type found in most domestic 13 Amp plugs, or preferably, a twin-jaw nylon clamp as shown in Fig. 4. Besides obviating the need to tighten screws, this has the additional advantage of securing the cable with the correct orientation for the proposed first embodiment. The clamp comprises two nylon jaws 14 between which the cable 5 is inserted by pushing simultaneously to the left and away from the observer.

Claims

CLAIM

1. A junction box having a plurality of connectors arranged in rows and columns, the connectors in each row and column respectively being in electrical contact and the connectors in each column and row respectively being aligned opposite a cable clamp or slot.

2. A junction box as claimed in Claim 1, wherein the connectors are within recesses the apertures into which are arranged in rows and columns in one major face of a cuboidal block, the aligned cable clamps or slots being adjacent that face and pointing theretowards.

3. A junction box as claimed in Claim 1, wherein the connectors are disposed generally in a plane, the aligned cable clamps or slots being generally co-planar therewith and pointing theretowards.

4. A junction box as claimed in either of Claims 1 and 2, wherein the columns and rows of connectors are arranged perpendicular to one another.

5. A junction box as claimed in either of Claims 1 and 4, wherein the columns or rows of connectors relating to each cable clamp or slot are disposed generally equidistant from that clamp or slot.

6. A junction box as claimed in any of the preceding Claims, wherein the connectors are "poke-home" connectors.

7. A junction box as claimed in Claim 6, wherein each connector has a recess of sufficient depth to prevent the wire entering at an appreciable angle, and one or more spring to ensure adequate pressure with the contact surface.

8. A junction box as claimed in Claim 7, wherein the shape of the spring(s) does not permit withdrawal of the wire from the same side as it was pushed in.

9. A junction box as claimed in Claim 8, wherein there is provided at the rear of the recess an opening to allow withdrawal (after cutting off) of a stub of wire from the opposite side to that at which it entered.

10. A junction box as claimed in any of Claims 6 to 9, wherein each connector has a rear-positioned grip screw mechanism, to assist in providing a secure mechanical contact.

11. A junction box as claimed in Claim 10, wherein the grip screw mechanism involves a flexible insulating carrier within the recess, the carrier having angled front faces that co-operate with corresponding angled abutment surfaces near the mouth of the connector aperture, the deformation of the carrier as it is moved forwards by the screw causing the spring contacts therewithin positively to grip the conductor.

12. A junction box as claimed in Claim 11, wherein a single carrier and screw is common to all the connectors in this column/row thereof.

13. A junction box as claimed in Claim 12, wherein to assist the force provided by the screw being shared equally, there is a rigid plate and a pad of compressible material between the screw and independent carriers, or there is a plurality of floating levers transmitting the force to the carriers.

14. A junction box as claimed in any of the preceding Claims, wherein each cable clamp is a twin-jaw flexible plastics clamp of the type shown in Figure 4.

15. A junction box as claimed in any of the preceding Claims, wherein the connectors and cable clamps/slots are formed within a moulded insulating body.

16. A junction box as claimed in any of the preceding Claims and substantially as hereinbefore described.