GB2313718A - Plug with bi-metal switch - Google Patents

Abstract

An electrical plug comprises a bistable member (22) which is movable between a first position connecting a terminal (26) to a pin (14) and a second position disconnecting the terminal (26) from the pin (14). The member (22) is movable from the first position to the second position in response to an increase in temperature caused by a current overload. The member (22) may carry a mass (30) and be reset by a lever (36) operated by a push-button (32).

Description

FUSE LINK This invention relates to a fuse link for an electric plug.

The connection system that is particularly well known in the United Kingdom calls for a fuse link in every plug. If there is an overload caused by equipment connected to an electric supply by the plug the fuse link in the plug will break. The known fuse links are in the form of cartridges that are clipped into the plug and which can be readily replaced when necessary.

In order to meet adequate safety standards the fuse cartridges must go open circuit after a predetermined time that an overload current is imposed.

Normally to meet such a standard the cartridges are made to comply with British Standard No.1362. This is now becoming an expensive standard to meet and the cost of the cartridge has become an unacceptably high proportion of the overall cost of the plug.

The present invention has been made in order to deal with this problem.

According to the invention there is provided a plug having at least one pin and a fuse link between the pin and a terminal for an electrical connector wherein said fuse link comprises a bi-stable member movable between a first position connecting the terminal to the pin and a second position disconnecting the terminal from the pin, said member being movable from the first position to the second position in response to an increase in temperature caused by a current overload.

In a preferred embodiment of the invention the bistable member comprises a so-called bimetal sheet actuator formed so that it will change its disposition, preferably with sudden or sharp action, when heated to a particular level. These bimetal actuators are well known and have been used for many years in electric kettles to provide automatic discontinuance of electric power to the kettle element when the kettle boils or if the kettle should overheat.

The bimetal actuator is in effect a switch which goes open circuit when heated to a predetermined level.

In order to return the bimetal actuator to closed circuit condition a mechanical force has, normally, to be imposed on the actuator. In the present invention this is preferably accomplished by providing a mass on the bimetal actuator. If the actuator is open circuit the plug can be tapped, firmly, onto a surface and the resultant force imparted to the mass on the actuator will cause the said actuator to return to the closed circuit condition. This arrangement is preferred to reset imparted by a positive mechanical link to the actuator such as a pin or the like. With a pin it would be possible to hold the actuator in the closed circuit position in circumstances where there is a current overload and the actuator is attempting to move into the open circuit position. However, with a mass on the actuator, as described above, it is impossible to hold the actuator in the closed position when there is a current overload.

A specific embodiment of the invention will now be described with reference to the accompanying drawing in which: Fig.l is a diagrammatic perspective view of a three pin plug; Fig.2 is a plan view of an actuator; and Fig.3 is a side elevation of a part of the actuator of Fig.2.

Fig.4 is a section through a plug with an alternative form of reset mechanism.

Referring to the drawing an electric plug 10 has a body 12 on which are mounted live pin 14, neutral pin 16 and earth pin 18. A bimetal actuator 20 in the form of an annulus having a tongue 22 is mounted, as by clips 24 in the body. The actuator 20 is positioned so that the tongue 22 can contact the end of the live pin 14 that extends through into the body. A terminal 26 is provided on the actuator 20 whereby a conductor 28 can be connected thereto.

The bimetal actuator 20 is configured so that the tongue 22 is normally in contact with the end of pin 14. A current overload will cause a rise in temperature which in turn causes the bimetal actuator to reconfigure such that the tongue 22 no longer contacts the pin 14 thus disabling electric supply between pin 14 and conductor 28.

Once the actuator has moved into the open circuit position as described above it can be reset to its normal position as follows. A mass 30 is provided on the tongue as shown in Fig.3. If the plug pins are tapped sharply on a horizontal surface the force applied to the tongue will cause the actuator to reset.

Another kind of reset mechanism is shown in Fig.4.

A push button 32 is provided in the body in the region between the live pin 14 and earth pin 34. The push button is arranged so that when depressed it will engage an end 38 of a beam 36. This causes the beam to pivot (anticlockwise as viewed in the drawing) so that the other end 40 of the beam urges the tongue 22 back to its closed circuit position in contact with the end of the live pin 14.

In the embodiment of Fig.4 it is also not possible to hold the actuator 20 in the closed circuit position when there is a current overload because the button 32 must be released in order to put the plug in a socket.

The invention is not restricted to the above described embodiment and many variations and modifications are possible.

Claims (7)

1. A plug having at least one pin and a fuse link between the pin and a terminal for an electrical connector wherein said fuse link comprises a bi-stable member movable between a first position connecting the terminal to the pin and a second position disconnecting the terminal from the pin, said member being movable from the first position to the second position in response to an increase in temperature caused by a current overload.

2. A plug as claimed in claim 1, wherein the bi-stable member comprises a bimetal sheet actuator.

3. A plug as claimed in claim 1 or claim 2, wherein a mass is provided on the bi-stable member.

4. A plug as claimed in any preceding claim, wherein the bi-stable member may be moved from the second position to the first position by way of a positive mechanical link.

5. A plug as claimed in claim 4, wherein the positive mechanical link comprises a pivotally mounted beam.

6. A plug as claimed in claim 5, wherein the beam is operated by way of a button located in the same face of the plug as the pins.

7. A plug substantially as defined herein withe reference to the accompanying figs. 1 to 3 or 4.