GB2166912A - Improvement in electrical plugs - Google Patents

Abstract

The normal threaded nut or grub screw type of pillar terminal for connecting the wires of a flexible cable to the conducting pins of an electrical plug is replaced by a transversely pivoted cam-action clamping lever (14) which pinches the strands of a conductor (32) against an anvil (16) which may be the head of a pin or may be formed on a conducting tail connected to the relevant pin. Ideally, the clamping lever goes over-centre in the secure position. The plug top may also be releasably secured to the base either by a pair of similarly designed catches on the side of the base clamping appropriate pegs on the top; or by resilient fingers depending from the top which snap into notches in the external wall of the plug base. <IMAGE>

Description

SPECIFICATION Improvements in electrical plugs This invention relates to electrical plugs, and more particularly (although not exclusively) to solid-pin plugs, of the kind in which the cores of a flexible cable are secured to the pins by means of screws or nuts. Such securing means not only can cause severance of strands of a core due to overtightening but can also work loose, especially in inexpert hands. Furthermore, the need to fit, or refit, a plug on a cable involves the availability of a tool such as a screwdriver, and there is always the risk that the proper tool is not used, resulting in insecure connections being made. There is also the possibility that no tool is available, resulting either in the job not being done-with whatever consequences that may entail-or in some makeshift solution which may have dangerous consequences.

Conventional domestic plugs are of a twopart construction comprising a base or body carrying the pins and a top or cap which encloses the connections. The two parts are separable for the purpose of making the connections and are secured together by one or two screws. Again, it is not possible to fit a plug to a cable, or to repair a defective connection, without the use of a screwdriver, and sometimes the security of the finished job is dependent on ths ability of the person doing the job to tighten a screw to the proper degree. A person with weak fingers, or one who is unaware that tightness is necessary, can produce an unacceptable result.

It is an object of the present invention to mitigate or eliminate the foregoing hazards.

Another object is to produce a design of electrical plug which obviates the necessity for the use of tools of any kind in making connections, and preferably also in securing the top or cap to the base or body of a two-part separable plug.

The present invention consists essentially in the substitution for the conventional screw or nut system for clamping a core of a flexible cable to its appropriate plug pin of a friction clamping system including a pivoted cam-action lever which can be depressed to pinch a conductor tightly into contact with the head of the pin, or a lateral extension thereof-for example, in the form of a conducting "tail" suitably joined thereto with minimum practicable electrical resistance. The cam surface may be in the form of an arc of a circle pivoted about an axis offset from the centre of the circle, the chord of the arc being extended slightly in one direction to provide a tab by which the lever can be operated.

Preferably, the cam surface may be arranged that in the closed or operative position, in which the core is clamped, the cam is overcentre so as to be self-locking. Alternatively, the lever has a flat straight back edge or surface opposite the cam surface which lies in the plane of the top of the base so that, in the clamping position, the back edge or surface is in contact with the underface of the top or cap of the plug when it is secured in place so that the cam lever cannot move to release the core.

Ideally, the points of connection of a plurality of cores in a single cable to their respective pins would be equidistant from the point of entry of the cable to the plug. In the conventional design of popular domestic 3-pin plug, the achievement of equidistance is frustrated by the intrusion of a central screw securing the top or cap of the plug to its base or body. If this intrusive screw is eliminated, however, the equidistant criterion can be realised, and one aspect of the present invention is the substitution for a central securing screw of peripheral releasable locks for securing the top to the base until manually disengaged.

The locks may comprise relatively rigid interengageable components, or at least one component may be resilient and biased towards securing the top to the base. The former may include catches of similar design to the clamping levers, each operating in a recess in the base opening through the side wall thereof, one at each end of a diagonal of the nominally (or actually) rectangular base, the pivot of each catch being normal to the plane of the base and each catch when in the securing position having its outer or back face flush or substantially flush with the peripheral wall of the base. The recess is intersected by a socket opening through the top face of the base with its axis normal thereto, and a depending peg on the underside of the top is designed to enter the socket and penetrate below the level of the catch.The peg and the catch are designed to interengage when the top is properly fitted and the catches are pressed inwards to the flush position. Preferably, each peg is grooved where it registers with the catch so that interengagement of the two parts produces a mechanical interlock.

Alternatively, the top may be provided with resilient depending fingers integral therewith or anchored thereto and adapted to be sprung outwards to pass over the side wall of the base. The latter is provided, at the bottom of the side wall, with undercut notches which register with the fingers when the top is correctly assembled to the base. Each finger has an inturned nib or lug which can snap into its respective notch when the top is in place to hold it there. The "roof" or top wall of each notch, assuming that the plug is standing on its pins on a horizontal surface, is profiled so that the nib or lug on the respective resilient finger snaps home in the securing poeition.

The top ie released by prising the lugs out of their notches and pulling the top away from the base.

With either type of lock, the plug top can be hinged to the base--preferably along the edge adjacent the earth pin in a conventional 3-pin domestic plug.

Alternative practical embodiments of the present invention will now be described, by way of illustration thereof only, with reference to the accompanying drawings in which: Figure 1 is a plan view of the base or body of a domestic plug according to the present invention; Figure 2 is a scrap sectional elevation of a pin with its attendant clamping lever, the section being taken on the line A-A of Figure 3; Figure 3 is a plan view, partly in section, of a clamping lever; Figure 4 is a scrap section through the base and top of a plug taken on the line B-B of Figure 1; Figure 5 is a scrap view of the cable entry of the plug of Figure 1; Figure 6 is a view similar to Figure 1 of a modified design; Figure 7 is a side elevation of a top or cap of the plug shown in Figure 6; Figure 8 is a side elevation of an alternative design;; Figure 9 is a plan of the base only of Figure 8 and Figure 10 is a scrap section through a corner of the top and the base showing a spring clip in position.

Referring first to Figures 1-3, the plug has a base or body 10 which carries the usual 3 flat pins (not shown) of a standard 13-amp plug and the usual line fuse 12 in circuit with the live pin. The earth and neutral pins are overlaid by respective clamping levers 14. The head 16 of each pin 18 (Figure 2) is recessed into the material of the base or body 10 of the plug and is flanked on three sides by walls 20 of the same material as the base 10.

Each clamping lever 14 is somewhat boatshaped in side elevation, and has a flat "deck" or top face 22 and a partly arcuate "keel" or cam face 24. The lever is of the same width as the head 16, and is pivoted at 26 between a pair of opposed flanking walls 20. As shown in Figure 3, the pivot is preferably in the form of a pair of short studs 28 which slide in a common bore through the lever and are urged apart by an intermediate compression spring 30. The studs 28 can be pressed mutually inwards against the spring 30 to allow the lever 14 initially to be introduced between the opposed walls 20, and on reaching a position of coincidence with the axis 26 in the base 10, the studs are forced out into collinear holes in the walls.

The location of the axis 26 and the contour of the cam face 24 are interrelated in such a way that as the clamping lever 14 is pressed down onto the core 32 lying on the head 16 the pressure rises until the lever is in the horizontal position shown in full lines in Figure 2.

Although it is ideal that in this position the lever 14 is "over-centre", and hence. self-locking, for a given set of dimensions of the lever and its components there is a relatively narrow range of wire thicknesses of the core conductor 32 which will permit this. As a precaution, therefore, the top face 22 of the lever 14 is located so as to be in contact with the underside of the top or cap, indicated in Figure 2 by the broken lines 34, when the latter is secured in its closed position. Irrespective, therefore, of the thickness of the wire 32, the clamping lever 14 will always hold the core securely in tight contact with the head of the pin 18.

In order to enable the clamping mechanism to accommodate a wider range of wire thicknesses at 32, the lever 14 is formed with an extended tab 36 which is resiliently flexible so that when the cam surface 24 is prevented by the thickness of the wire from moving to its over-centre seif-locking position, the tab part 36 can yield under pressure of the top or cap 34, thus maintaining tight contact with the core.

Referring now to Figure 4, the top or cap is secured to the base 10 by two catches 38 which are similar in appearance to the clamp levers 14, and are located on the base 10 at opposite ends of a diagonal. Each catch 38 is located in a recess 40 in the base in which it is pivoted on a vertical axis (i.e. normal to the plane of the base 10). The latter is formed with a socket 42 which extends down from the top face of the base to a point below the level of the catch 38, which partially projects thereinto. The top 34 has two pegs 44 formed integrally therewith to register with and project into the respective sockets 42 for substantially the full depth thereof. Each peg 44 has a shallow notch or groove 46 formed in the side facing the catch 38 so as to register therewith when the top is mounted firmly on the base. Preferably, the parts are dimensioned so that, when a catch 38 is pressed home flush with the outer surface of the plug, the "keel" or cam surface corresponding to the part 24 on the clamp levers 14 tightens on the bottom of the notch 46 and goes over-centre. The top 34 is thus securely locked to the base 10 and can only be released by the deliberate release of both catches 38.

Figure 5 shows a method of clamping the cable 50 in the cable entry to the plug. A shallow depression 52 ie formed in the base 10 to receive the cable, and a latch 54 is pivoted at 56 on the base 10 to one side of the depression. The other end of the latch 54 has a lug 58 which, whsn the latch is clamping a cable, fits into a slot in the base. A pad 34a on the underside of the top 34 presses on the latch to force it down onto the cable.

Figures 6 and 7 illustrate a modification of the plug shown in Figure 1. Since the central area of the base 10 is no longer occupied by a screw holding the top 34 on the base 10, it is possible to locate a clamping lever 14 there. The clamping lever for the line pin L can be reorientated parallel to the fuse 12, while the lever 14 for the neutral pin N can be placed on the opposite side of the middle lever 14 and connected back to its pin by a "tail" 60. A similar but shorter tail 62 connects the middle lever to the earth pin E.Each of the tails 60, 62, and that which conventionally carries one of the spring clips for the fuse 12 terminates at its outer end in an anvil (not shown) similar to the head 16 (Figure 2) and which is anchored to the base 10 so that the lever 14 which overlies it functions in the same way as in Figures 1-3 to clamp a respective one of cores 32 (brown, blue and yellow/green in the current international colour coding for domestic appliances). Thus it is possible to achieve a' layout of the terminal connections to the pins of a 3-pin plug which calls for equal-length cores 32.

To cooperate with the base 10 of Figure 6, the plug top 34 of Figure 7 has a plurality of pads 64 on its underside each adapted to register with and press on a respective pin 18 or lever 14 to maintain the entire assembly firm and secure when the top 34 is locked into place. It may be that the parts will be dimensioned so that the underside of the plug top 34 will be flush except for the two locking pegs 44 and the cable clamp pad 34ai.e. the pads 64 will disappear, or perhaps become resilient inserts in the material of the plug top 34.

Referring now to Figures 8-10, the modified plug has its top 34 hinged at 70 to its base 10. The top has a pair of depending resilient fingers 72 each having at its free end an inturned nib or lug 74. The fingers 72 are of sufficient length to allow their lugs to engage respective undercut notches 76 at opposite ends of the plug wall opposite the hinge 70. The undercuts 76 are profiled so as to cause the lugs to engage in them with a snap action. When the top 34 is swung down onto the base 10, the spring fingers 72 slide over the external wall of the base 10 until they register, in the closed position of the top 34.

with their respective undercut notches 76. The lugs 74 are then pressed into place in the notches 76 to lock the top 34 in its closed position. In order to release the top, the lugs 74 are prised out of their notches 76 and the top is hinged upwards to reveal the pin connections.

Various modifications of shape may be made in respect of detail components, as will be understood. For example, the boat-shaped or fish-bellied contour of the clamps 14 may be replaced by a simple transverse rib, the rib and the top surface 22 forming an angle bracket whose arms are of unequal length (the rib bein the shorter) and the pivot 26 being in the form of two short trunnions projecting laterally at the angle. This is a well-known form of over-centre clip, and if the rib itself is curved, so that the clip resembles the letter "J",it may be possible to build in the necessary resilience in the rib to accommodate a range of wire thicknesses of the core 32 to ensure that the clamp always goes over-centre in the clamping position.

Although the foregoing description has concentrated on the standard 13-amp, 3-pin domestic plug, it is to be understood that the invention is not limited to the number of pins nor to the domestic field. Any number of pins may be provided, and the plug may be designed for industrial or commercial use.

It is assumed that the clamping levers 14 would not normally be of an electrically conducting metal, but there is no restriction on the material. Provided that the contact surfaces of the pins 18 or their tails 60 or 62 and of the conductors 32 are clean, the levers 14 could be of an insulating material such as a ceramic or a dimensionally stable synthetic resin plastic. This could be of advantage where the conductors 32 are in the form of standard "spade" terminals welded, soldered or crimped onto the wire of each core.

In all forms of construction where a tail connects the clamp lever 14 to a pin 18-whether directly (as at 60, 62 in Figure 6) or via a fuse 12-the tail has an anvil which is deemed to be the mechanical equivalent of the head 16 in Figure 2. Consequently.

wherever in the claims appended hereto the head of a pin is referred to as the anvil which cooperates with a clamp lever. the terminology is deemed to include an anvil formed or fixed on a tail connected to that pin.

A safety feature made possible by the invention and not normally found in conventional plugs is that each clamping lever 14 can be coloured to match the international colour code for domestic wiring. either by applying colour to the levers during their manufacture or by affixing coloured tabs during assembly of the plug. The relatively large flat surface of the top 22 of a clamping lever lends itself ideally to such identification, whereas the head of a screw is small and vulnerable to surface damage by a sorewdriver. In contexts other than domestic, of course, other colour codes may apply, or other forms of identification may be adopted-e.g. numerals or letters, or a combination thereof.

Claims (13)

1. An electrical plug for connecting a flexible cable to a source of electric power and having pins for insertion into respective coacting sockets in a power supply circuit wherein the head of each pin constitutes an anvil on which the bared end of the appropriate conductor is laid, and a clamp lever is pivoted about a fixed axis at right angles to that of the pin and is formed with a cam surface to bear on the said end with increasing pressure as the clamp lever is depressed.

2. A plug according to claim 1 wherein the clamp lever has a flat top adapted to lie, when in its operative position, substantially in the plane of the top of the plug base so as to be engaged by the underside of the plug cap or top when the latter is in its position of normal use.

3. A plug according to claim 1 or 2 wherein the cam surface is profiled so that the clamp lever goes over-centre when in its operative position.

4. A plug according to claim 1, 2 or 3 wherein the clamp lever is pivoted on an extension of the head of the pin with which it is associated.

5. A plug according to claim 4 wherein the plug has at least 3 pins and the clamp levers associated with all of them are aligned parallel to the plane containing two of them.

6. A standard domestic 3-pin plug according to claim 5 wherein the clamp levers are aligned parallel to the plane containing the live and neutral pins.

7. A plug according to any preceding claim wherein the top or cap is secured to the base by means of a similar clamp lever pivoted on the base adjacent an edge thereof about an axis normal to the plane of the base and engageable in a groove on a peg or skirt formed on the cap.

8. A plug according to claim 7 of the generally rectangular box-like pattern of a conventional 13-amp 3-pin domestic plug wherein two clamps are located at the respective ends of a diagonal of the base and the top or cap has integral depending peg or skirt formations adapted, in the closed position of the top or cap. to project into respective recesses in the base adjacent the clamps for positive engagement thereby when they are moved to the operative position.

9. A plug according to any of claims 1-6 wherein the plug top is secured to the base by means of spaced resilient fingers depending from the top, each having an inturned lug at its free end, and the base has an equal number of undercut notches each located so as to register with the lug on a respective finger when the top is fully closed, the resilience of each finger urging its lug fully into its notch.

10. A plug according to claim 9 wherein the top wall of a notch, viewed when the base rests by its pins on a horizontal surface, is contoured so that the lug on the respective finger engages therein with a snap action.

11. A plug according to any preceding claim wherein the top is hinged to the base.

12. A plug according to any preceding claim wherein the base has a formation on one edge to receive the cable whose cores form the conductors to be clamped by the clamp levers, and a depending block or wall on the top or cover registers with the formation and is adapted to compress the cable when the top or cap is secured in its normal closed position.

13. An electrical plug for connecting a flexible cable to a source of electric power substantially as hereinbefore described with reference to Figures 1-3; Figure 4; Figure 5; Figures 6 and 7 or Figures 8-10 of the accompanying drawings.