US3012121A

US3012121A - Electric fuses

Info

Publication number: US3012121A
Application number: US758538A
Authority: US
Inventors: Hicks Bruce Clifford
Original assignee: ITE Circuit Breaker Co
Current assignee: ITE Circuit Breaker Co
Priority date: 1957-09-04
Filing date: 1958-09-02
Publication date: 1961-12-05
Anticipated expiration: 1978-12-05

Description

B. C. HICKS ELECTRIC FUSES Dec. 5, 1961 Filed Sept. 2. 1958 United States Patent 3,012,121 ELECTRIC FUSES Bruce Clifford Hicks, London, England, assigner, by mesne assignments, to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Sept. 2, 1958, Ser. No. 758,538 11 Claims. (Cl. 20d- 120) The present invention relates to high voltage electric fuses including current limiting fuses and it has for its object to provide a fuse construction capable of operating with current limiting action under short circuit conditions, whereas under overload it is possible to obtain any desired operating characteristic more particularly as regards its melting-time/current characteristic. A further object is to obtain a reliable method for the actuation of an indicator or striker pin or other like device on fault operation of the fuse.

In U.S. specification No. 2,800,554 and copending application Serial No. 629,886 a current limiting powder filled electric fuse has been described which permits the fusing factor, defined as the ratio of the minimum fusing current to the continuous current carrying capacity of the fuse, to be brought to a figure substantially less than 1.25 or even below that ligure. In these prior constructions the fuse included a plurality of load-sharing fusible elements disposed between conductive end caps and wholly embedded in the powder, and connected in parallel one with the other, one or more of which elements has an inherently diiferent fusing characteristic from another or others of said elements, but all of said elements have substantially similar current-carrying characteristics and are designed to fuse simultaneously under short-circuit.

In the arrangements referred to a iirst fusible element or a first group of elements is arranged to interrupt the current path on the occurrence of a small overload thereby resulting in the transfer of the whole of the current to another element or another group of elements which thereafter fuses.

In U.S. specification No. 2,800,554 all the elements are formed of the same material and of the same section, the rst element or each element of the first group being surrounded at one point by a mass of low melting point material, whereas in U.S. copending application Serial No. 629,886 elements formed of different materials and of corresponding cross-sections are employed for the same purpose.

More particularly the object of the present invention is to provide a fuse which may be a powder-lled fuse of the character deiined in the aforesaid specifications, or may be an open type fuse or an expulsion fuse, wherein any desired melting-time/current characteristic may be obtained by additional elements in heat transferring relation to the fusible element and is based on the recognition of the fact that in fuse structures of the type used in power distribution networks the temperature of the fusible element under operating conditions is different at different points of the length of the fusible element, and that various operating characteristics are obtainable by selective placing of the additional elements. In general the temperature is lowest near the end cap commonly terminating the fuse structure and to which the fusible element or elements is/ are attached.

A further object of the present invention is to provide a fuse incorporating a physically displ-aceable element, hereinafter termed a striker pin, which on fault operation of the fuse is adapted to perform some external function; it may for example be indicator means of some kind or it may actuate an isolator switch serving to disconnect the supply to the fuse. It will be realised that where such an isolator switch is employed a first essential is that thev isolator switch shall not open the circuit until the fault current flowing through the fuse has been interrupted by the proper operation of the fuse itself, since otherwise the current load would be transferred to the isolator switch and the latter would probably be damaged, as such switches are not designed to interrupt `any substantial How of current.

A second essential is that the operation of the striker pin must be completely reliable under all fault conditions from the minimum overcurrent fusing current to full short circuit in order to ensure that whenever the fuse operates on fault clearance the isolator switch shall operate in every case to isolate the circuit protected by the fuse to ensure proper safety precautions for the circuit protected by the fuse.

A more specific object of the present invention is therefore `to provide for correct and reliable operation of a striker pin of the character referred to, under every condition of fault operation; namely to give certainty of reliable operation of the striker pin and also to prevent premature operation thereof upon fault clearance to ensure that the current load is interrupted by the fuse and not by the isolator switch operated by the striker pin.

This object is attained in accordance with one form of the present invention by employing at least two fusible elements one of which includes or all but one of which` each includes an additional element having a different melting temperature from the fusible element` being positioned in heat transferring relation thereto and selected in dimensions and position so as to obtain a desirable operating characteristic for the fuse while ensuring that under overcurrent conditions that fusible clement not incorporating any additional element is the last to fuse, and said latter fusible element is shunted for part of its length by electrical connection means capable of operating the striker pin, for example by activating a charge of gasproducing substance or explosive material capable of actuating the striker pin.

Another object of the present invention is to provide a high voltage fuse having desirable operating characteristics and comprising at least one fusible element to which is attached mechanically or physically at least one relatively short additional element having a different melting temperature from the fusible element and positioned in heat-transferring relation thereto at a point which is selected in position as regards its temperatue so as to obtain a desired operating characteristic under overload. This additional element or each such element is preferably formed with an integral portion standing away from the fusible element and functioning in the manner of a cooling iin. Preferably this additional element consists of a different metal or alloy from the fusible element `and it has a lower melting point than said fusible element so that alloying of the additional element with the fusible element may occur. Such alloying may occur under overload conditions such as are designed to cause operation of the fuse and ensures the form-ation of a preferential melting zone which results in interruption of the circuit b y the fuse for a preferred overload ligure, giving a characteristic departing from the natural inverse time/ current basis of the fusible element.

The selection of t-he positioning of the additional element or elements may be determined experimentally or by calculation of the temperature gradient to give the required results, the important feature being that by varying the position of the additional element or elements and/or by providing one or more of them at different points any desired predetermined characteristic can be obtained.

The additional element may for example comprise a' sleeve of the second metal positioned around the main amener Amay be wrapped around the main fuse strip or wire to form a flattened tube around it. In an alternative a1- rangement the second metal may comprise a short piece of wire, several turns of which are coiled helically around the main fusible strip and flattened into contact therewith. The wire wrapping round the fusible element avoids intimate contact and alloying at normal temperature but by being physically very close tothe fusible element ensures speedy heat transfer and alloying at the selected point when there is a rise in temperature due to an overload current.

It will be realised therefore that in the design of a fuse according to the present invention for a specified operating characteristic, it is possible to vary the physical positioning of the additional element or elements on the one or more main fusible elements of the fuse structure and it is also possible to control the physical size or shape or manner of incorporation of the second metal to give the required result.

The invention will be further explained with reference to the accompanying drawings illustrating various features ofthe present invention and constructional embodiments of high voltage powder-filled fuses constructed according to the invention, andin which:

FIG. 1 is an explanatory diagram showing the features of a high voltage fuse incorporating four fusible elements and a striker pin device,

FIGS. 2 to 4 are diagrammatic viewsfshowing various forms of additional elements attached to a fusible element,

FIG. 5 is a view similar to FIGS. 2 to 4 illustrating anotherform of fuse which may be adopted according to the invention andalso illustrating a fusible element having an alternative form of the additional elements shown in FIGS. 2 to 4, and

FIG. 6 is a part-sectional view of one example of a high voltage fuse incorporating the features of the present invention. k

The features of the present invention will be appreciated by considering iirst the constructional example shown in FIG. 6; this comprises a shock resisting outer casing 7 of any suitable insulating material having conductive end caps 8 serving as terminal members for the fuse, the caps being rolled or swaged on to the ends of the casing 7 so as to form an intimate and close joint between the end caps and the casing 7. The end caps 8 are formed internally so as to support a ceramic former 8a carrying a plurality of fusible elements which may be strips or wires; in the embodiment shown there are four fuse wires or strips wound helically on the former 8a, these strips being indicated at 9, 10, 11 and 12. A filling of arc-quenching powder 13 is providedwithin the outer casing 7 and surrounds the fuse wires 9 to 12 wound on the former 8a, the latter being preferably longitudinally ribbed to support the fusible elements in such manner that they are enclosed substantially on all sides by the powder filling 13. i

In one form of the invention one end cap 8 is provided with a striker pin device indicated generally at 14 which comprises an axially ejectable pin 15 'slidable in an outer casing of the device 14 and adapted to be-forced to the outside ofthe cap 8, as indicated in dotted lines, on fault operation of the fuse. FIhe actuation of the pin 15 may be effected in any convenient and well-known manner, preferably by means of a charge of gas-producing or explosive material in the closed inner end part of the casing of the device 14, said material or explosive being brought into operation by means of a heater wire connected at one end to the casing of the device 14, then passing through such material or explosive and continuing as an external connector wire 16 connected electrically to the fusible wire or strip 9 at a point located at some suitable distance from the connection of said wire or strip to the end cap 8; This arrangement ofthe striker pin mechanism and the connection 16 thereto has alreadyv been disclosed in British specification No. 713,663. It has however been found that the connecting wire 16, which also serves as a heating element for activating the charge of gas-producing or explosive material in the casing 14, is desirably formed of Ferry metal, which is an alloy mainly of nickel and copper containing approximately nickel. A wire formed of thisl alloy has the advantage that it maybe readily soldered at the required connection points, namely to the casing 14 and also to the fusible element 9 if desired. Furthermore, a wire consisting of Ferry metal alloy is not itself liable to deteriorate at the temperatures encountered in normal operation of the fuse.

Referring now to FIG. l ofthe drawings illustrating the connection and arrangement of the fusible elements 9, 1u, 11, 12 in one form of the invention, it will be seen that these elements are all connected in parallel to the end caps represented here in diagrammatic form at 81 and the striker pin mechanism is shown at 14 also connected electrically to one end cap 81and to an intermediate point of the fusible element 9.

All the fusible elements except the element 9 are provided in'accordance with one form of the present` invention with elements 17 attached thereto or otherwise in heat transferring relation therewith. These elements 17 consist of a metal having a different melting point from the fusible elements 9, 10,911, 12 so as to secure the operational characteristics explained below in greater detail. Further all the fusible elements 9 to 12 may be providedr with one, two or more ceramic beads or short refractory enclosing tubes 19, the purpose of which is to create predetermined arcing points in each fusible element during actual fault clearance of the fuse and they thus co-operate in securing correct operation of the fuse under fault clearance conditions.

In the preferred arrangement all the elements 9 to 12 are formed of silver strips and their resistance is substantially the same under normal load conditions but under moderate overload conditions the three fuse strips 10 to 12 provided with additional elements 17, preferably of tin,V

will melt due to the heating of said fuse strips and the alloying of the silver with the tin and consequently the whole load is transferred to the fourth element which now carries four times the current which it previously carried, and said fourth element 9 will thereafter melt and interrupt the circuit with a controlled time delay. Before it does so however there is an increase in the voltage drop across the element 9 comprising the shunt circuit actuating the device 14 and the said device is adjusted to operate consequent upon the additional Voltage drop across the fourth fuse element 9. This arrangement causes reliable operation of the trip device 14 which may be operatively connected to an isolating switch in the circuit feeding the fuse.

The :arrangement described has the effect of reducing the minimum fusing current of the fusible element, because when the elements begin to run hot, the aiiixed low melting point metal melts and alloys with the fuss strip and so lowers its melting point at predetermined positions so that the fuse operates. Under conditions of severe Y overloadhowever, there is insufficient time for the metals to diffuse together and give a lower melting point alloy so that the fuse has then a melting-time/ current characterisv tic similar to a fuse with the same fusible elements unmodified by the addition of the lower melting point metal.

It is thus possible to control separately the meltingtime/current characteristics for small and large melting currents.

This produces the desirable slow acting fuse elements which will melt on sustained small overloads; but which will allow large short duration currents to pass without the fuse operating.

Preferably all the fusible elements equipped with the additional elements (i.e. the elements lll to 12 in the illustrated examples) are identical, and where a striker pin` device, such as 14, is employed associated with another fusible element (ie. the element 9 in the examples), the latter element may be chosen or arranged so as to be the last to fuse under over-current fault conditions. This may involve a slightly greater cross-sectional area for the element concerned, but in many cases all the fusible elements (ie. all the elements 9 to 12 in the examples) may be identical. In other cases all the elements 9 to 12 may be suitably graded to ensure that they melt in predetermined order under overcurrent fault conditions.

The additional elements 1'7 may take various structural fonns, some of which are indicated by way of example on FIGS. 2 to 5.

In FIG. 2 each fusible element 1t) to 12 is shown as a thin strip consisting for example of silver, while the additional element 17 consists of a thin strip of another metal, tin for example, capable of alloying with the fusible element metal, which is wrapped more or less tightly around the strip 1@ to 12. There may be one or more turns of such strip to form the additional element or there may be rather less than one complete turn of such strip which may thus be of approximately C-shape. These additional elements may be loosely positioned on the str-ip to 12 or they may be more or less firmly pressed into position or if desired they may be soldered in position.

)FIGS illustrates by way of example a fusible element 10 to'12 in the form of a thin wire, of silverl for example, havinga1sleeve17 ofa metal-having a different melting point, for example. of tin, ranged as shown in FIG. 2 and as described above.

f FIG. 4 illustratesanother arrangement wherein the element 17 consists of several turns of wire wound helically on the strip or wire element 10 to 12. Such a length of wire could if desired be soldered or otherwise fixed to the support strip overall or at one or both ends as desired.

FIG. 5 illustrates another feature of the present invention and represents a fusible element (wire or strip) which may be the sole fusible element in a fuse or may be one of several identical elements. In this case a single fusible element 91 is shown adapted to be installed in a suitable fuse structure, for example somewhat similar to that shown in FIG. 6 but wherein a single fuse element only is employed. According to this arrangement the additional element 17 -is of relatively small dimensions at the point in contact with the fusible element 91, and comprises an outwardly directed tail or iin 18 which may be relatively massive; it provides an additional cooling surface to control the rate of heating of the element 17, considered as a whole, and thus the time taken for it to become effectively alloyed with the fusible element 91 under fault operating conditions. By using a relatively small area or mass of metal forming the element 17 in physical contact with the element 91 and a relatively massive tail part or fin 18, the element 17, considered as a whole, is held at a comparatively low temperature until the fusible element 91 itself has become substantially heated, and thus to some extent delays the alloying of the additional element 17 with the fusible element and the operation tends to be responsive to the overall temperaturc of the whole fusible element; this has been found to give improved operating properties.

The present invention thus includes a high voltage fuse incorporating a single fusible element as shown in FIG. 5 as well as a structure having a plurality of such identical elements connected in parallel, each such element being provided with an additional low melting point element 17 having an enlarged and preferably relatively massive tail portion or iin 18, and the complete structure oers the advantage that by appropriately selecting the positions of the additional elements having regard to the temperatures at different points of the fusible strip, any desired operating characteristics can be obtained and a fuse may be produced where therfusing factor as defined above has any desired value to meet any required specified operating characteristic.

Further as indicated with reference to FIGS. 1 and which otherwise may be ar- 6 the present invention also includes fuses as contemplated by FIG. 5, and comprising at least two fusible elements and a striker pin device 14 the operation of which is obtained by electrical connection to one of the fusible elements. In this case the latter fusible element is dcsigned so as to be the last of the elements to fuse under overcurrent fault conditions. This effect is obtained by arranging for the fusible element or elements other than that connected to the striker pin device to incorporate additional elements 17 of FIG. 5 which result in at least one of them fusing under overcurrent conditions and in transferring the current load to that element or those elements which are still conductive, which thereafter melts, the last element or one of the last elements to melt being associated with the device 14. The manner in which the fusible element or elements other than that to which the striker pin device 14 is connected operate under fault conditions need not be discussed in detail and forms no part of the essential operation of the present invention, the important consideration being that the last-to-fuse element shown at 9 in FIG. l, to which the striker pin device is connected, cannot operate until the other fusible element or elements have melted and the current load has therefore been transferred to the element 9. The device 14 cannot fail to act on fault operation of the fuse and if it is connected to an isolator switch it thus reliably ensures that the circuit enclosing the fuse is properly isolated from the supply. On the other hand however it cannot operate prematurely even under overcurrent fault conditions since it is associated with an element which is the last to fuse under such conditions and to which the fault current is transferred when the other elements fuse thus ensuring the maximum possible reliability and safety of operation.

Reverting now to FIG. 6 of the drawings, it will be seen that the various features explained with reference to FIGS. l and 5 of the drawings have been incorporated therein. The ceramic beads 19 shown in FIG. 1 are suitably spaced upon the fusible elements 9 to 12 as indicated, while the additional elements 17 of low melting point metal are attached to the

fusible elements

10, 11 and 12 as indicated, and in this case the elements are Shown as having the form indicated on FIG. 5. It should` be understood however that the additional elements 17 may be of the form shown on any of FIGS. 2 to 4.

Various structural forms of the additional elements 17 have been referred to above. In each case they preferably consist of a relatively low melting point material, such as tin, while the main fusible elements 9-12 consists of a highly conductive metal such as silver. These two metals, namely silver and tin, have (like many other pai-rs of metals) the property of becoming alloyed at certain temperatures; the so-alloyed points (of lower melting temperature than the unalloyed fusible element) initiate the required circuit interrupting action under overload conditions. Other metals and metal alloys can however be used such as silver-gold alloys, copper, bismuth, cadmium alloys or the like. In some cases silver may be used as the Second metal where the main fusible strip or wire consists of a metal or alloy of higher melting point.

The additional element 17 may as indicated above be applied in relatively loose physical contact with the main fusible strip or wire, or it may be iirmly attached, adhered or bonded to the main fusible strip or wire, and

it could for example comprise a bead permanently fixedV to the main fusible strip, or it may comprise a deposit fused to the main fusible strip or wire, or for example soldered or welded thereto. Similarly where the second metal comprises several turns of wire, said wire can be` iixedly connected to the fuse strip or wire at one point at least by fusion or by soldering or welding.

In the design according to the invention, a powderiilled high rupturing capacity fuse as shown in FIG. 6 may be taken as a typical example. The silver fuse 7 strips or wires 9 to 12 are of a size sufficient to carry the largest short-duration current required and to carry the continuous operating or full (rated) load current without its temperature going above say 150 C. A bead 17 or the like (see FIGS. 2 to 5) of low melting point metal for example tin, is xed at a suitable position on each of the elements 9-12 to be so equipped; when the fuse strips or Wires concerned (which would normally melt at 960 C.) reach a temperature near 230 C. (the melting point of tin) fusion of the tin into the silver occurs giving a low melting point alloy in the element, so that it melts readily and interrupts the current. Under very short duration heavy currents however, the heating is almost adiabatic and no appreciable alloying occurs. The low melting point element 17 may be arranged or atiixed so that it cools the fuse strip at the point of attachment by virtue of its shape and mass; the effect is enhanced when the arrangement of FIG.l is adopted.

There may be different numbers `of additional elements 17 on some or all of the different fusible strips, the arrangement being however identical for all the fusible strips so provided. Y

In all cases the position of said additional metal element or elements 17 is chosen to secure the required operating characteristics. In general this implies that the additional elements shall be placed comparatively close torone end cap 8 and where ceramic elements 19 are provided surrounding sections of the fusible strips in known manner, said additional elements are positioned between an end cap 8 and such a ceramic element 19.

It will of course be realised .that the foregoing discussion in connection with the operating properties of the fusible device according to the present invention is concerned more particularly with the operation of the fuse under overcurrent or overload conditions rather than severe faultconditions in the nature of a short circuit, where, in the case of those `forms of fuse embodying a plurality of fusible elements, all such elements melt substantially simultaneously to interrupt the fault current with a current limiting action in a'manner wellknown in this type of fuse, and that the features described herein are designed particularly to obtain specialised operating properties under overcurrent or overload fault conditions other than those which would be regarded as short circuit conditions.

None of the features according to the present linvention modify significantly the operation of the circuit under short circuit conditions where in accordance with the well-known current limiting features the fuse should operate sufficiently rapidly to terminate the fault current during the rst half cycle. The provision of the additional elements 17 does not aiect the property of the fusible elements `9-12 to'terminate a short circuit current in the manner of a current limiting fuse, since under short circuit conditions the whole length of the fusible strip or of Veach of the strips melts substantially simultaneously.

It will of course be realised that the additional elements 17 in any ofthe various forms herein specified may be applied to open-type fuses and to expulsion fuses in a manner which can be readily `appreciated by those skilled in the art.

What I claim is:

l. A high voltage electric fuse of thecurrent limiting type comprising an outer casing, conductive end caps sealed into intimate contact with said casing, a striker pin device associated with one end cap, said striker pin device comprising a casing member, a charge of explosive material, an axially displaceable pin adapted to be projected externally of said end cap on activation of said explosive material and a heating wire for activating said explo-sive material, a ceramic former extending within the casing between said end caps, a number of fusible .elements wound on said ceramic former and extending between said end caps, said number being more than two,

a plurality of additional elements consisting of a different metal capable of alloying with the fusible elements and in heat transferring relation thereto, said elements being applied to all but one of said fusible elements and the positions thereof in regard to the lengths of the respective fusible elements being selected to obtain predetermined melting-time/ current characteristics under v over-current fault operation, a powder lling in said casing surrounding said ceramic former and enclosing the fusible elements and the additional elementsk thereon and a connection from the heating Wire of said striker pin device to an intermediate point in the length of that fusible element not provided with additional elements, said element being the last to melt under over-current fault conditions.

2. A. high voltage electric fuse according to claim l Y 'whereink said additional element conslsts of a portion of comparatively small dimension in physical contact with Vthe fusible element and a massive ltail portion standing Youtwardly therefrom.

3. A high voltage electric fuse according to claim l wherein the yadditional element consists of a thin strip of a second metal pressed into physical contact with the fusible element.

4. A high voltage electric fuse according to claim 1 wherein the additional element consists of a length of Wire of the second metal, several turns of which :are wound onto the fusible element, ysaid turns being pressed into intimate contact with the fusible elements.

5. Ak high voltage electric fuse comprising holder means, a single fusible element carried by said holder and an additional element of a different metal capable o-f alloying with the fusible element at a temperature corresponding to an over-current fault current to produce a local area of reduced melting temperature and initiate operation of lthe fuse, said additional element comprising a body portion and an outwardly directed tail portion of said different metal, said body portion being a small mass in physical contact with the fusible element, said outwardly directed tail formed integrally with said small mass serving as a cooling element, and its position on the fusible element being selected to obtain predetermined melting-tirne/current characteristics.

6. A. fuse comprising conductive connector blocks disposed at opposite ends of the fuse, conductive connectors respectively connected to said conductive blocks for connecting said fuse in a circuit, a plurality of highly conductive fusible elongated elements extending betweenk on'short circuit faults, an outer casing, a filling of arc quenching powder within said casing surrounding vall of said strips in heat transferring relation thereto, all of said strips and said powder filling being selected to operate with current limiting action upon operation of said fuse, a mass of low melting -point metal individual to `all but a iirst of said strips in conductive ycontact with a rst short portion of the length of its associated strip, said low melting point metal alloying with the material of the fusible elements under predetermined overload conditions thereby assuring that said first st-rip will be the last of said strips to rupture under said overload conditions, meansrincluding an igniter wire constructed to indicate when said fuse has blown, said igniter wirebeing connectedv in electrical parallel with only a portion o-f said first strip, Isaid igniter wire being constructed and positoned to rupture under overload conditions before said iirst strip ruptures but after the remaining strips have ruptured, said masses of low melting point metal being constructed in a manner such that said iirst short portions of said strips are cooler than the adjacent portions of `said strips upon the occurrence of over current faults, said masses each having an eye portion surrounding its associated strip and a tail portion eX- tending from said eye portion in a radial direction from the axis of its associated fusible element.

7. A high voltage electric fuse comprising holder means, a yfusible element carried by said holder means and an additional element in heat transferring relation to said fusible element, said additional element having a different melting point from the Afusible element and its position on the fusible element -between the two terminal means being selected to obtain predetermined meltingtime current characteristics; said additional element being comprised of a metal having a lower melting point than the ffusible element and capable of alloying therewith on heating of the fusible element to an extent corresponding to over-current conditions, to control the melting-time/ current characteristic of the fusible element; said additional element comprising body portion and a tail portion of said metal; Isaid body portion having relatively small dimensions and being in physical contact with the tail portion serving as a cooling iin .formed integrally with said body portion.

8. A high voltage electric fuse comprising an outer casing, conductive end caps on said casing, a fusible element extending between said end caps and an additional element of a metal having a different melting point in heat transferring contact with `said fusible element and capable of alloying with said fusible element on heating thereof -by the current carried thereby under Ifault conditions to initiate fuse operation, the position of said additional element being selected to obtain desired operating characteristics; said additional element having a small area in physical contact with the fusible element and an integrally formed massive tail portion standing away therefrom, said tail portion and said additional element being formed of the same material.

9. A fuse comprising conductive connector blocks disposed at opposite ends of the fuse, conductive connectors respectively connected to said conductive blocks for connecting said fuse in a circuit, a plurality of highly conductive fusible elongated elements extending between and connected at the respective ends thereof separately to said conductive connector blocks, said elements being in the form of thin strips of the same length and of the same cross-section to insure even sharing of the load current between said elements and simultaneous operation on short circuit faults, an outer casing, a filling of are quenching powder within said casing surrounding all of said strips in heat transferring relation thereto, all of said strips and said powder filling being selected to operate with current limiting action upon operation of said fuse, a mass of low melting point metal individual to all but a first of said strips in conductive contact with a lirst short portion of the length of its associated strip, said low melting point metal alloying with the material of the fusible elements under predetermined overload conditions ,thereby assuring that said rst strip will be the last of said strips to rupture under said overload conditions, means including an igniter wire constructed to indicate when said fuse has blo-wn, said igniter wire being connected in electrical parallel with only a portion of said rst strip, said ignite-r wire being constructed and positioned to rupture under overload conditions before said first strip ruptures but after the remaining strips have ruptured, said masses of low melting point metal being constructed in a manner such that said first short portions of said strips are cooler than the adjacent portions of said strips upon the occurrence of over current faults, said masses each comprising a first portion and a massive tail portion of said low melting point metal, said first portion of comparatively small dimension in physical contact with its associated strip; said massive tail portion extending outwardly therefrom.

l0. A high voltage electric fuse comprising holder means, a single fusible element carried by said holder and an additional element of a different metal capable of alloying with the fusible element at a temperature corresponding to an over-current fault current to produce a local area of reduced melting temperature and initiate operation of the fuse, said additional element comprising a small mass of metal in physical contact with the fusible element and an outwardly directed tail formed integrally with said small mass serving as a cooling element, and its position on the fusible element being selected to obtain predetermined melting-time/current characteristics, a ceramic `bead enclosing a different part of the fusible element.

11. A high voltage electric fuse comprising holder means, a plurality of fusible elements carried by said holder each having an additional element of a different metal capable of alloying with its associated fusible element at a temperature corresponding to an over-current fault current to produce a local area of reduced melting temperature and initiate operation of the fuse, said additional element comprising a small mass of metal in pysical con-tact with the fusible element and an outwardly directed tail formed integrally with said small mass serving as a cooling element, and its position on the fusible element being selected to obtain predetermined meltingtime/current characteristics, all said fusible elements being supported by a ceramic former comprising the said holder means and the whole structure being enclosed by an outer casing carrying terminal and caps.

References Cited in the tile of this patent UNITED STATES PATENTS 2,639,350 Cox May 19, 1953 2,688,061 Kozacka Aug. 31, 1954 2,794,095 Kozacka May 28, 1957 2,800,554 Dannenberg et al `uly 23, 1957 2,866,037 Stewart Dec. 23, 1958 FOREIGN PATENTS 40,074 France lan. l2, 1932 (Addition to No. 684,241) 570,441 Germany Feb. 16, 1933 713,663 Great Britain Aug. 18, 1954