AU606045B2 - Drop-out expulsion fuse - Google Patents

Description

7(y4 COMMONWEALTH OF AUSTRALIA FORM PATENTS ACT 1952 COMPLETE SPECIFICATION FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: and ;s r, ;;;Priority: Related Art: see S Name of Applicant: BROWN BOVERI (AUSTRALIA) PTY. LIMITED S*.**Address of Applicant: 25 Garling Road, Marayong, New South Wales 2148, Australia l ***..Actual Inventor: WOLFGANG JOSEPH REBHOLZ Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney ,i *...:Complete Specification for the Invention entitled: "DROP-OUT EXPULSION FUSE" The following statement is a full description of this invention, including the best method of performing it known to me/us:- Complete of PH4231 dated 16th January, 1986 1

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liCY"dOLLARS I i This invention relates to drop-out expulsion fuses as used in high voltage, overhead, electricity distribution systems, and more particularly to a fusible element for use in such fuses.

Simple expulsion fuses conventionally comprise a tubular insulating body with a metallic head fitting at one end and a metallic tail fitting at the other. Those fittings are adapted to electrically and mechdnically engage opposite ends of a fuse holder, in turn adapted for connection to incoming and outgoing portions of an overhead line conductor. A fuse element is positioned within the fuse body and is connected at one end to a terminal adapted to make contact with the fuse head fitting. The other end of the fuse element is connected to a flexible, conducting pigtail which extends through the body to the tail fitting, to which it is connected.

When the current in the line conductor exceeds a o o• *predetermined value for a predetermined time the fuse element melts and an arc is created between the said 0 terminal and the adjacent end of the pigtail. Tho heat of the arc causes the material of the body, or a lining thereon, to produce copious amounts of steam and/or de-ionising gas which, if all goes well, violently expels the pigtail from the fuse body to lengthen and extinguish the arc and so open circuit the line conductor; hence, the name expulsion fuse.

In the case of a drop-out expulsion fuse one of the fittings, usually the tail fitting, is in two parts I pivotally or otherwise linked together. The two parts of 2-

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SHELSTON WATERS PATENT ATTORNEYS CLARENCE STREET. SYDNEY

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Cables: 'Valid' Sydney Telex: 24422 that fitting are normally held fixedly each to each by tension in the pigtail; which tension is maintained without stressing the fuse element by means of a strain cord in parallel with the fuse element which, like the fuse element, is mechanically connected at its ends to said terminal and one end of the pigtail respectively.

When the fuse element melts, and if all goes well, the strain cord is destroyed and that tension disappears.

This allows the two part fitting to collapse causing the 10 other fitting to retract from its engagement with the fuse holder which, in turn, allows the fuse body to swing downwardly under the influence of gravity to a fully isolating position. The fact that a fuse has dropped out may readily be observed from ground level enabling the blown fuse to be identified and replaced more quickly than otherwise would be the case.

ee*e In some instances the strain cord is a high S' resistance conductor. In those instances rupturing of the fuse element transfers all the cui.rent to the strain cord which then fuses to effect its mechanical destruction.

Such systems tend to suppress the desired formation of an arc as they provide a continual current path until the strain chord has been destroyed which tends to reduce the buildup of electric potential between the terminal and the end of the pigtail. They also produce considerable more molten material as a result of destruction of the strain i cord.

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@9 11 0 eSSO 1115 110 S 11 11 1151 2.0 9*0901 In other instances the strain chord is a high-tensile plastics thong and the heat of the arc is relied upon to destroy it. This latter is preferred from the point of view of reducing the risk of emission of hot metal particles when the fuse operates; but occasionally, especially with a light duty fuse and a small current to be ruptured, the heat of the arc is insufficient to destroy the integrity of the strain cord and as a consequence there is a fuse malfunction at least to the extent that there is no visible drop-out indication that the fuse has blown.

Further, it has been found that merely surrounding the fusible element and strain cord with a plastics sleeve does not ensure destruction of the strain cord, as once the fusible element has been destroyed and if insufficient heat has been generated to destroy the strain cord, the conductive path is opened and further fusion prevented. Prior art devices have overcome this problem by making the strain cord conductive with the disadvantages described above.

An object of the present invention is to provide a drop-out expulsion fuse of the kind having a plastics or other non-conductive strain cord which is more reliable insofar as the destruction of non-conducting cords is concerned, is more retentive of molten material by comparison with prior known fuses and retains the molten 4

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material in such a manner to continue to provide a current path through this material once the fuse element has been destroyed.

The invention achieves that object by the provision of a drop-out expulsion fuse having a fuse body with a head terminal at one end and a tail terminal at its other end, a fuse element and non-conductive strain cord in said fuse body and each interconnected between said head and tail terminals, a shrink-fitted sheath of insulating material encasing central portions of the fuse element and strain cord, such that, in use, molten material produced by destruction of the fuse element by an overload current is retained by the sheath to ensure destruction of the strain cord by maintaining a conductive path through said fuse until the strain cord is destroyed.

The invention thus provides the advantages of using a non-conductive strain cord while also ensuring correct operation of the expulsion fuse by ensuring the strain cord is destroyed by the molten material produced by melting of the fuse and subsequent arc.

By way of example, a preferred embodiment of the invention is described hereinafter with reference to the accompanying drawings.

Fig. 1 is a partly sectioned front elevation of an end portion of a fuse core suitable for incorporation in a drop-out fuse assembly.

5 Fig. 2 is a sectional elevation taken on line 2-2 of Fig. i.

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5* 0 *9 *5 In use the illustrated core would be encased within a tubular fibre or other insulatory fuse body and connected at each end to metallic head and tail fittings on the respective ends of the fuse body. That body and those fittings may be quite conventional and thus need not be illustrated or described in detail.

The core itself comprises an auxiliary fibre or other insulatory pressure tube 3 closed at one end by a metal cap 4 and plug 5. The plug 5 terminates at one end in a screwed stem 6 whereby it may be mechanically secured and electrically connected to the fuse head fitting. At its other end the plug 5 terminates in a generally tubular socket 7.

A flexible, braided conductor pig-tail 8 extends through the open end of tube 3 into one end of a double-ended, metallic socket element 9 therein. The pig-tail 8 is held in the socket 12 constituting one end of socket element 9 by a swaged indention 10 formed in the socket wall after the pig-tail was inserted.

The other end of the socket element 9 is a socket 11 formed in the same way as is the socket 7.

In so far as it has been described above the illustrated fuse core is conventional.

6 L -e i 1- c 9;u r ii A fuse element 17 and a strain cord 13 span from socket 11 to socket 7. They are held in place by swaged indentations 14 in the respective sockets.

In accordance with the invention the fuse element 17 and strain cord 13 are encased in a shrink fitted sheath of insulatory plastics material.

The fuse element 17 may be a length of wire, for example, of tin-silver alloy, say 96.5% tin and silver.

The strain cord 13 may be a length of polyester *.ee cord. In the manufacture of the fuse, the strain cord 13 oaf. and the fuse element 17 are laid side by side and a length of heat shrinkable plastics tube, for example a teflon tube, is sleeved over both the cord and the element. The cord and element protrude beyond the ends of the plastics tube, and the latter is warmed so as to shrink and become the sheath 15. That operation may be carried out as a preliminary operation producing, a fusible element, namely the fuse element, cord and sheath oeso combination, which may be incorporated in the normal too**: manner in an otherwise conventional fuse.

*A further feature of the preferred embodiments of .i the invention is the provision of a pliable, plastics tubular sheath 16 over the pig-tail 8. In this instance, the tubular sheath may be either heat shrunk on to the pig-tail or may be a neatly fitting tube secured in i Li .i w~ i L place, for example by being gripped by the socket 12 along with the pigtail.

This tubular sheath 16 on the pig-tail provides some lateral support for it and tends to prevent the pig-tail buckling or kinking and therefore becoming jammed in the fuse body during fuse operation. This, in turn, enables a mechanically weaker fuse body to be used with consequent saving in cost.

The term "strain cord" as used herein, is intended to cover any elongate tensile non-conductive element of a size and composition suitable for withstanding the tensile forces in the pig-tail of a drop-out fuse. it embraces braided or twisted cords, multi- and mono-filaments.

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Claims (4)

1. A drop-out expulsion fuse having a fuse body with a head terminal at one end and a tail terminal at its other end, a fuse element and a non-conductive strain cord in said fuse body and each interconnected between said head and tail terminals, a shrink-fitted sheath of insulating material encasing central portions of the fuse element and strain cord, such that, in use, molten material produced by destruction of the fuse element by an overload current is retained by the sheath to ensure destruction of the strain cord by maintaining a conductive path through said fuse until the strain cord is destroyed.

2. A drop-out expulsion fuse accortding to claim 1 wherein said sheath is a length of plastics tube heat shrunk on to the fuse element and strain cord.

3. A drop-out expulsion fuse according to claim 1 or claim 2, having a flexible conducting pigtail connected to said tail terminal and at its other end to a fuse tail fitting wherein said pigtail is encased in a tubular plastics sheath.

4. A drop-out expulsion fuse according to anyone of the preceding claims wherein said shrink-fitted sheath extends between said head and tail terminals and is attached thereto. A drop-out expulsion fuse substantially as described herein with reference to the accompanying drawings. DATED this 22nd Day of October, 1990 BROWN BOVERI (AUSTRALIA) PTY. LIMITED Attorney: PETER R. HEATHCOTE Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS 1 9