Heat-operated short-circuiting arrangements

Abstract

A heat-operated short-circuiting arrangement comprising an enclosure formed at least partly of a heat-shrinkable material; a pair of terminals for external connection of the arrangement which provide spaced surfaces within the enclosure; and at least one element of a fusible, electrically conductive material housed within the enclosure; the arrangement being such that the fusible element or elements melt on heating of the arrangement and the resultant molten material is caused to flow due to shrinking of the heat shrinkable material to a position where the molten material establishes electrical connection between said spaced surfaces of the terminals. The arrangement finds application in combination with an electrical device for short-circuiting a pair of terminals of the device in the event of overheating of the device.

Description

This invention relates to heat-operated short-circuiting arrangements.

It is an object of the present invention to provide such an arrangement which is of simple, inexpensive form.

According to the present invention a heat-operated short-circuiting arrangement comprises: an enclosure formed at least partly of a heat-shrinkable material; a pair of terminals for external connection of the arrangement which provide spaced surfaces within the enclosure; and at least one element of a fusible, electrically conductive material housed within the enclosure; the arrangement being such that the fusible element or elements melt on heating of the arrangement and the resultant molten material is caused to flow due to shrinking of the heat shrinkable material to a position where the molten material establishes electrical connection between said spaced surfaces of the terminals.

An arrangement in accordance with the invention finds application in combination with an electrical device for effecting short-circuiting of a pair of terminals of the electrical device, in the event of overheating of the device. In such an arrangement said enclosure is suitably formed partly by said device itself, and said pair of terminals of the short-circuiting arrangement are suitably constituted by said pair of terminals of the electrical device. In one such arrangement wherein the device is of generally cylindrical form and the terminals of the device provide annular surfaces spaced axially along the device, said enclosure is of annular form and encloses at least part of each of said annular surfaces. Such an arrangement suitably includes two said fusible elements which before melting are of annular form and are located by means of a spacer of electrically insulating material in positions respectively surrounding said enclosed parts of said annular surfaces.

One arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawing which is a sectional view of an electrical device in combination with two short-circuiting arrangements in accordance with the invention, the electrical device being an excess voltage arrester in this example.

Referring to the drawing, the arrester includes a hermetically sealed hollow, cylindrical, gas-filled enclosure comprising two cup-shaped metal end caps 1 and 2, a tubular metal central member 3 and two tubular ceramic members 4 and 5. The ceramic members 4 and 5 are sealed at one end into the end caps 1 and 2, and at the other end into opposite ends of the central member 3 of the enclosure.

Within the enclosure are housed a pair of axially spaced electrodes 6 and 7, the electrodes being brazed at one end to the end caps 1 and 2 respectively and extending towards each other through the ceramic members 4 and 5 respectively so that the gap between the electrodes lies within the central member 3.

Each electrode has an axial bore 8 or 9, the bore 8 in the electrode 6 communicating with a pumping stem 10 extending through the associated end cap 1. The pumping stem 10 is pinched-off in conventional manner during manufacture after the required gas-filling has been inserted in the enclosure. The bore 9 in the other electrode 7 is closed-off by the end cap 2.

In use of the arrester the end caps 1 and 2 are respectively connected to a pair of lines connected with the equipment which it is desired to protect against excess voltage, and the metal enclosure member 3 is grounded. On the occurrence of an excess voltage between either one of the lines and the other line, or ground, a discharge occurs between one or both electrodes 6 and 7 and the metal member 3 of the enclosure, thus effectively grounding the line or lines to which an excess voltage is applied.

If the discharge is maintained for any length of time or the discharge current is very high, the heat generated by the discharge may be sufficient to cause the arrester to fail, for example, by burning a hole in the metal member 3 and allowing gas leakage to occur. When this occurs the lines connected to the end caps 1 and 2 are left without protection against excess voltage, and it will not be apparent to a user that the arrester has failed.

To overcome this difficulty the arrester is provided with two heat-operated short-circuiting arrangements 11 and 12. On overheating of the arrester, before the arrester fails, the arrangements 11 and 12 respectively short-circuit the end caps 1 and 2 to the metal enclosure member 3, thus grounding the lines to which the arrester is connected.

The short circuiting arrangement 11 comprises two identical annular members 13 of an electrically conductive fusible material having a low melting point, for example, an alloy of tin and lead in the proportion 3:2 by weight. The members 13 are of rectangular cross-section with a portion of reduced external diameter at one end to provide shoulders 14. The members 13 fit loosely around the enclosure of the arrester with the shoulders 14 facing one another, one of the members 13 surrounding the end of the end cap 1 sealed to ceramic member 4, and the other member 13 surrounding the end of the metal member 3 sealed to ceramic member 4. The two members 13 are located with respect to one another by an insulating tubular spacer 15 whose ends engage the shoulders 14. The short-circuiting arrangement 11 is completed by a sleeve 16 consisting of a heat shrinkable material, for example, Kynar (Registered Trade Mark) supplied by Raychem Limited. The sleeve 16 is first formed to a diameter slightly greater than the members 13 and spacer 15, and placed around the members 13 and the spacer 15. The sleeve is then shrunk by heating to a temperature below the melting point of the members 13 until the ends of the sleeve 16 grip the body of the arrester tightly. Hence, the sleeve 16 and the adjacent part of the arrester form an enclosure housing the members 13, the spacer 15, and the adjacent ends of the end cap 1 and the metal enclosure member 3.

The short-circuiting arrangement 12 is identical with the arrangement 11, except that it surrounds the adjacent ends of end cap 2 and metal member 3.

When the arrester overheats in operation the members 13 melt. Due to further shrinking of the sleeve 16 the molten material is forced along the space 17 between the spacer 15 and the ceramic member 4, thereby establishing electrical connection between the end cap 1 and the grounded metal member 3. The short-circuiting arrangement 12 similarly connects the end cap 2 to the metal member 3.

Claims

We claim:

1. A heat-operated short circuiting arrangement comprising: an enclosure formed at least partly of a heat-shrinkable material; a pair of terminals for external connection of the arrangement which provide spaced surfaces within the enclosure; at least one element of a fusible, electrically conductive material housed within the enclosure; and means providing a space within said enclosure between said spaced surfaces through which, on heating of the arrangement, molten material, produced by melting of said fusible material is caused to flow due to shrinking of said heat shrinkable material to establish electrical connection between said spaced surfaces.

2. An arrangement according to claim 1 wherein said fusible material is an alloy of tin and lead.

3. An arrangement according to claim 1 in combination with an electrical device having a pair of terminals; electrical connections between said pair of terminals of the device and said pair of terminals of the short circuiting arrangement; and a thermal connection between said enclosure and said electrical device so that said pair of terminals of the device are short circuited in the event of overheating of the device.

4. An arrangement according to claim 3 wherein said device is an excess voltage arrester.

5. An arrangement according to claim 3 wherein said enclosure is formed partly by said device.

6. An arrangement according to claim 5 wherein the device is of generally cylindrical form, said pair of terminals of the device provide annular surfaces spaced axially along the device, and said enclosure is of annular form and encloses at least part of each of said annular surfaces.

7. An arrangement according to claim 6 including two said fusible elements which before melting are of annular form and are located by means of a spacer of electrically insulating material in positions respectively surrounding said enclosed parts of said annular surfaces.

8. An arrangement according to claim 7 wherein said spacer fits loosely around the device to provide, between the spacer and the device, said space through which the molten material is caused to flow to establish electrical connection between said spaced surfaces.

9. An arrangement according to claim 8 wherein the ends of the spacer locate on shoulders provided on said fusible elements before melting.

10. An arrangement according to claim 6 wherein said heat-shrinkable material is in the form of a sleeve coaxially surrounding the device.