GB2204996A

GB2204996A - Pressure-sensitive cut-out for capacitor

Info

Publication number: GB2204996A
Application number: GB08809215A
Authority: GB
Inventors: David Ronald Snowdon
Original assignee: PED CAPACITORS Ltd
Current assignee: PED CAPACITORS Ltd
Priority date: 1987-05-22
Filing date: 1988-04-19
Publication date: 1988-11-23
Anticipated expiration: 2008-04-19
Also published as: GB2204996B; FR2616007A1; GB8712165D0; GB8809215D0

Abstract

A pressure-sensitive cut-out device is described for use in capacitors which in the event of an internal fault condition cause an internal build up of pressure. The device comprises at least one frangible conductor (20) mounted on an end wall (16) of the capacitor casing, and separated from the interior of the capacitor casing by a deformable wall or diaphragm (23), which moves outwardly towards the frangible conductor (20) due to internal over pressure, to cause the conductor (20) to fracture. Two or more such frangible conductors may be incorporated to break two or more separate electrical circuits between external terminals and the capacitor plates within the casing. The frangible conductor comprises metal-plated carbon or metal-loaded carbon, a brittle ceramic substrate carrying one or more conductive tracks, or a weakened metal wire. Broken pieces are retained by the cap-like diaphragm (23), which may also move overcentre to hold broken conductors apart. <IMAGE>

Description

Title: Components relating to electrical capacitors Field of invention This invention relates to electrical capacitors and particularly to a capacitor incorporating a pressuresensitive cut-out device which responds to an increase of pressure within the capacitor caused by overheating or by gas produced by electrical breakdown.

Background to the invention The principle of using a deformable element, the movement of which is linked to and causes one or more conductors to break, is well known to those versed in the art of capacitor manufacture. For example, UK Patent Application No. 2174544A discloses a capacitor including such a device, in which a breakable conductor is mounted either directly upon the capacitor casing or within a separate cartridge inserted as a discrete component into the capacitor casing, and is broken, in the event of overpressure in the casing, by deformation of the casing itself or by deformation of the body of the separate cartridge, respectively.

It is an object of the present invention to provide a pressure-sensitive cut-out device for use in a capacitor in which the disadvantages of the constructions described in UK No. 2174544A, namely dependence upon the deformation of the casing to break the conductor in the first instance, and an increase in the length of the casing in the second, are mitigated.

Summary of the invention According to the present invention a pressure sensitive cut-out device for use in a capacitor to safeguard the latter in the event of a build up of pressure therein comprises a frangible conductor mounted on an end wall of the capacitor casing and separated from the interior of the casing by a deformable wall, movement of which due to over-pressure within the casing will cause fracture of the conductor.

Preferably the deformable wall forms a cover for the conductor so that if and when broken, the pieces are contained within the cover.

The frangible conductor is preferably carried upon a separate lid forming one end of the capacitor casing, which lid may also carry the deformable wall or cover.

According to another aspect the invention consists in a capacitor including a pressure-sensitive cut-out device of the type recited in the preceding paragraphs.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a section through a capacitor embodying the invention, Figure 2 is a similar view of the capacitor of Figure 1 in which the frangible conductor has been broken by internal pressure build up, Figure 3 is an enlarged section through the cover of the capacitor in which the frangible conductor is mounted, and Figure 4 is an end view from underneath of the conductor of Figure 3 with the cover omitted.

As shown in Figure 1, a capacitor element 12 is housed in a cylindrical casing 14 made of aluminium or plastics to which a separate lid or upper end wall 16 is fitted. The capacitor element 12 is encapusaluted with epoxy or polyurethane resin 18 by means of the so-called "resin fill and invert" method.

Referring now also to Figures 3 and 4, a frangible conductor 20, in the form of a rod or bar, is mounted directly under the separate lid 16 and is supported therefrom by pairs of pillars 22. Formed inside the lid 16 is an annular collar 17 on to which a cap-like cover 21 is secured as a push fit to enclose the conductor 20. The cover 21 comprises an integrally moulded flexible diaphragm 23 inside which is formed a lug 27, acting as a pressure point which is directed towards the conductor 20.

One end of the conductor 20 is connected by a flexible wire lead or connection 24 to an external terminal 26 mounted on the lid 16, and the other end is connected by a similar wire connection 28 to one end of capacitor element 12. The other end of capacitor element is connected by flexible wire lead or connection 30 to the other external terminal 32 on the lid 16. The lead 30 passes through a sealed hole (not shown) in the cover 21, and it is surrounded by an insulating sleeve 34 within the body of casing 14.

The cover 21 is sealed against the collar 17, and the wire holes are also sealed by inverting the capacitor case whilst the resin 18 is liquid. Such sealing prevents the ingress of gas into the chamber formed by the cover 21.

A discharge resistor 36 may be mounted between the terminals 26 and 32.

In the event of a fault condition within the capacitor element 12, a build-up of gas pressure causes the diaphragm 23 to bow inwardly of the cover 21, as shown in Figure 2, breaking or shattering the frangible conductor 20 and interrupting the flow of current to the capacitor element 12.

Where the conductor 20 is of a brittle nature, it will shatter and the pieces will be retained with the cover 2i.

Alternatively, if the conductor 20 is of a different construction, e.g. a metal wire with a weakened portion, which can be broken by the pressure point 27, the diaphragm 23 may be of a bistable (or over-centre) form, retaining the position shown in Figure 2 in the event of a reduction of pressure in casing 13, thus keeping the two ends of conductor 20 apart and prevent reconnection or arcing.

This construction may be used with capacitors which are housed in casings 14 made either of plastics or aluminium.

The lid 16 may be moulded of a rigid plastic material, as shown, or may be machine fabricated from a composite material such as Tufnol (R.T.M.).

The frangible conductor 20 may be fabricated of carbon, or of metal-loaded carbon, all or part of which may be metal plated to enable electrical connection to be made to each end.

Alternatively the conductor 20 may comprise a brittle substrate such as ceramic, with a conductive coating or track bonded, printed or deposited on all or part of its surface in a known manner.

The conductor 20 is shown as a rod or bar in the embodiment described, but with appropriately modified supporting pillars 22 it could alternatively be in the form of a plate or disc.

The capacitor element 12, which is typically wound of metallised polymer film, need not necessarily be encapsulated in the resin 18.

It is to be noted that although the described embodiment only shows the lead being broken the invention is not limited to such an arrangement and a second frangible connector (not shown) may be provided between the appropriate external terminal and the lead 30, which is also acted on and broken by upward displacement of the diaphragm 23. Where the frangible connector is a conductive coating or track on a brittle substrate such as ceramic, two such tracks/coatings may be applied in parallel to the same substrate.

Claims

1. A pressure sensitive cut-out device for use in a capacitor to safeguard the latter in the event of a build up of pressure therein, characterised by at least one frangible conductor mounted on an end wall of the capacitor casing and separated from the interior of the casing by a deformable wall, movement of which due to over-pressure within the casing will cause fracture of the conductor.

2. A cut-out device according to claim 1, wherein the frangible conductor is fabricated of carbon, or of metalloaded carbon, all or part of which is metal plated to enable electrical connection to be made to each end.

3. A cut-out device according to claim 1, wherein the conductor comprises a brittle substrate such as ceramic, with one or more conductive coatings or tracks bonded, printed or deposited on all or part of its surface.

4. A cut-out device according to claim 1, wherein the frangible conductor is in the form of a plate or disc.

5. A cut-out device according to claim 1, 2, 3 or 4, in which the deformable wall forms a cover for the conductor so that if and when broken, the pieces are contained within the cover.-

6. A cut-out device according to any of claims 1 to 5, in which the frangible conductor is carried upon a separate lid forming one end of the capacitor casing.

7. A cut-out device according to claim 6, wherein the lid also carries the deformable wall or cover.

8. A cut-out device according to any of claims 1 to 4, which includes a second frangible conductor also fracturable by outward movement of the end wall of the casing to break a second conductive path within the capacitor.

9. A cut-out device according to any of claims 1 to 8 when incorporated in a capacitor.

10. A capacitor when fitted with a pressure sensitive cutout device comprising at least one frangible conductor mounted on an end wall of the capacitor casing and separated! from the interior of the casing by a deformable wall, movement of which due to overpressure within the casing will cause fracture of the conductor.

11. A capacitor according to claim 10, wherein a discharge resistor is mounted between the external terminals.

12. A capacitor with a pressure sensitive cut-out device constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.