US2658974A - High current carrying capacity current-limiting fuses - Google Patents

Description

Nov. 10, 1953 F. J. KOZACKA 2,658,974

HIGH CURRENT CARRYING CAPACITY CURRENT-LIMITING FUSES Filed Nov. 20, 1952 3 Sheets-Sheet 1 630:

I3 EO NOV. 10, 1953 KOZACKA 2,658,974

HIGH CURRENT CARRYING CAPACITY CURRENT-LIMITING FUSES Filed Nov. 20, 1952 5 Sheets-Sheet 2 Nov. 10, 1953 F. J. KOZACKA HIGH CURRENT CARRYING CAPACITY CURRENT-LIMITING FUSES 3 Sheet s-She et 3 Filed Nov. 20', 1952 by My;

Patented Nov. 10, 1953 HIGH CURRENT CARRYING CAPACITY CURRENT-LIMITING FUSES Frederick J. Kozacka, Amesbury, Mass., assignor to The Chase-Shawmut Company, a corpora.- tion of Massachusetts Application November 20, 195.2,Serial No. 321,599

14 Claims. (01. 200-120) This invention relates to high current carrying capacity current-limiting fuses, particularly fuses of this description having a relatively low voltage rating.

One object of the invention is to provide a current-limiting fuse capable of carrying currents in the order of several thousand amperes and being more compact than prior art current-limiting fuses having a comparable current rating.

Another object of the invention is to provide a current-limiting fuse comprising a great number of link elements forming parallel current paths, in which fuse the ratio of the total surface to the total cross-sectional area of said link elements is particularly large to correspondingly in crease the current carrying ability and the in terrupting efliciency of the fuse.

It is desirable to provide each link element of a fuse with a zone of minimum cross-sectional area to rapidly initiate arcing upon occurrence of currents of short-circuit current proportions, to limit short-circuit currents well below the peak of the available short-circuit current. Such zones of minimum cross-sectional area cause some 1 -1 losses during the normal operation of the fuse, i. e. as long as the fuse carries a current of load proportions. It is, therefore, a further object of the invention to provide a high current carrying capacity current-limiting fuse comprising a great number of link elements which form parallel current paths, in which each link element has a zone of minimum cross-sectional area, said zone of each link element being arranged in such a way as to maximize dispersal of heat and to equally distribute the heat dispersal duty among the two terminal elements of the fuse which are situated at opposite ends thereof.

Each zone of minimum cross-sectional area of each link element should be immersed in a fairly large mass of pulverulent arc-quenching filler arranged as far apart as possible from any portion of any adjacent link element which may be caused to fuse and be vaporized during the process of interrupting currents of short-circuit current proportions.

It is, therefore, still another object of the invention to provide a high current carrying capacity current-limiting fuse comprising a great number of link elements which form parallel current paths, each link element having a zone of mini mum cross-sectional area arranged in such a way as to associate said minimum cross-secticnal area zone with a relatively large quantity of relatively cool pulverulent arc quenching medium. This is achieved, according to one important feature of this invention, by arranging all points of minimum cross-sectional area of the link elements which points are often referred to as let-throughcurrent-limiting notches-away from the median plane of the casing of the fuse alternately immediately adjacent different terminal elements thereof.

Still another object of the invention is to pr0- vide a high current carrying capacity currentlimiting fuse comprising link elements designed to fuse sequentially on heavy fault currents wherein the link element to fuse last is associated with means imparting a high interrupting ability to the gap formed by the fusion thereof.

Further objects, advantages and features of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

For a better understanding of my invention reference may be had to the accompanying drawing in, which Fig. 1 is a view, partly in longitudinal section, of a high current carrying capacity current-limiting fuse embodying my invention;

Fig. 2 is a view, partly in transverse section, along line 2--2 of Fig. 1;

Fig. 3 is a side elevation of one of the terminal elements of the structure shown in Figs. 1 and 2;

Fig. 4 is a section along 5-4 of Fig. 3; and

Fig. 5 is a view, similar to Fig. l, of modified version of my invention.

The mode of operation of current-limiting power fuses, i. e. the way in which the short-circuit current is limited in and by such fuses is generally known in the fuse art. Reference may be had as regards this general aspect of my invention to United States Patent 2,592,399 to W. S. Edsall et al., April 8, 1952, Current-Limiting Fuses, assigned to the same assignee as the present invention.

Referring now to the drawing, reference numeral I has been applied to a tubular insulating casing. Casing I consists of an insulating material having high heat resistance and heat shock resistant quality. Various ceramic compounds might be used for making casing I, but I prefer to use glass cloth laminates which are impregnated with a synthetic resin. Such laminates are preferable on account of the great mechanical strength inherent in this type of material. Spaced conductive terminal elements 2a, 2b are arranged on opposite ends of casing I. Each of these terminal elements 2a, 21) comprises a cylindrical link receiving portion to having a relatively large diameter and a coaxial stud-shaped connector portion 2?) having a relatively small diameter. The stud-shaped connector portions 2b are intended to be inserted into a fuse holder or cut-out (not shown) having tubular or tulip type contacts adapted to engagethe stud-shaped connector portions 2b. A fuse holder of this type has been disclosed in my copending patent application for Fuses and Fuse Holders filed Octoher 9, 1952, Ser. No. 313,862, assigned to the same assignee as the present invention. A plurality of transversely arranged steel pins 3 of which only one is shown in the drawing secure casino l to the link-receiving terminal element portions 2a. Thus casing l operates as a spacer between the pair of terminal elements 2a, 217. Since terminal elements 2a, 2b--which consist preferably of p per-involve a relatively large mass, it may be desirable to provide some additional structure for securely spacing terminal elements 211. 2b. Said additional spacing structure is preferably in the shape of an insulatin column 451, 41) arranged coaxially within insulating casing l. Insulating casing l and insulating column 4a, 4b define a toroidal space 5. The above referred-to insulating column id, 41) comprises a radially outer tubular member 4a and a radially inner tubular member 4b. The former is arranged in abutting relation to the juxtaposed surfaces 6 of the linkreceiving portions 2a of terminal elements 2a, 2b. Said juxtaposed surfaces are each provided with a recess 1 adapted to receive the axially outer ends of the tubular member ib. The latter is provided with metal caps 8 on the axially outer ends thereof and metal caps 8 project into the aforementioned recesses I and are firmly held by friction therein. If desired a filler rod to of insulating material may be arranged within tubular member 4b. Transverse steel pins 9 of which but one is shown tie caps B to the tubular member 4?). A system of grooves I0 is arranged on the juxtaposed surfaces 6 of terminal elements 2a, 2b. Grooves it! are arranged radially with respect to casing and terminal elements 2a, 2b and angularly displaced with respect to each other. The angle of displacement should preferably be less than degrees, e. 18 degrees, and should be an integer fraction of 360 degrees. A composite link structure generally designated by reference numeral ll connects conductively the spaced pair of terminal elements 2a, 22). Link structure I! consists of a pluralit of flat stampings l2 arranged radially with respect to casing I and terminal elements 2a, 2b. Each of said plurality of stampings I2 is arranged with the axially outer edges thereof in one pair of juxtaposed grooves H] and conductively connected to terminal elements 2a, 2b by appropriate solder means. Each stamping or link unit [2 is subdivided into a plurality of narrower stri -shaped link elements lila. Link elements lZa form elemental current paths in parallel. All link elements I2a which are integrated into one link unit or stamping I: are tied together by the axially outer cross-strips 12b of which each is inserted in one of the grooves l 0. Link elements 12a have each a plurality of zones of restricted crosssectional area and each link element has a zone of minimum cross-sectional area I20. It is the size of this zone 20 of minimum cross-sectional area which determines the peak value of the letthrough current allowed to flow in each link element 12a and consequently the peak value of the let-through current allowed to flow through the Cal entire fuse structure. It will readily be seen from Fig. 1 that the zones [20 of minimum cross-sectional area of alternate link elements [2a are arranged immediately adjacent to the same terminal element 2a, 2?). There is a zone l2d of reduced cross-section and a zone I20 of still smaller,

'1. e. of minimum cross-section located immediately adjacent to each other on immediately adjacent link elements i2.

Upon occurrence of a current of short-circuit current proportions the zones [20 of minimum cross-sectional area fuse first and arcing is initiated at the points of link elements l2a where the zones are located. The are gaps thus initially formed tend to grow in a direction longitudinally of link elements l2a. The rate of gap growth is relatively slow in the direction toward the immediately adjacent terminal element 2a 2b but very rapid in the direction toward the terminal element 2a, 2b remote from the respective point I20 of arc initiation. The rate of gap growth is slow in one direction because the link elements (2a are relatively cool in the direction toward the immediately adjacent terminal element 2a, 2b. The rate of gap growth is rapid in the other direction because the link elements [2a are considerably preheated and have a relatively high temperature in the direction toward the terminal element 2a, 2b remote from the respective point 420 of arc initiation. At the time of initiation of an are at any point He the temperature at the zone of reduced cross-section 12d situated immediately adjacent to the respective point Mo on the immediately adjacent link ele ment I2 is relatively low. Hence each point 5% of arc initiation is surrounded by a region of relatively low temperature tending to limit at any point of arc initiation the peak of the temperature ever reached during the interrupting process.

Since the number of points i227 of minimum cross-sectional area situated immediately adjacent each terminal elernent 2a, 2b is equal, the cooling duty of both terminal elements 2a, 2b is equal during the normal operation of the fuse and the temperature of both terminal elements tends also to be equal.

The toroidal space 5 which is defined by casing l and by the additional spacing structure 4a, 4b is filled with a suitable pulverulent arc-quenching filler I3 as, for instance, quartz sand. Filler i3 fills all the interstices formed between the link units 12 as well as the spaces formed between the individual link elements In.

It is desirable in certain instances to provide the link structure of a current-limiting fuse with means which cause disintegration of the link structure not only upon occurrence of currents of short-circuit current proportions but also upon occurrence of relatively small but protracted overloads. A means for achieving this end frequently used is a metallurgical effect which occurs between the metals of which the link structure of current-limiting fuses is preferably made-i. e. silver or copper-and metals which have a lower fusing point or fusing temperature than the metal of which the link structure proper is made. Such a low fusing temperature metal is tin. When a fuse link of silver or copper is provided with an appropriate overlay of tin, or with another tin element in initirnate contact with the link metal. the latter dissolves into the tin upon fusion of the tin. The resulting alloy has a higher specific resistance than silver or copper, respectively, and causes increased heating of the link at the point Where the metallurgical reaction between the low fusing temperature metal and the high fusing temperature metal is taking place. This reaction causes in the ultimate stage thereof a disruption of the fuse link accompanied by the initiation of an arc. Upon interruption of the are thus formed by the action of a pulverulent arcquenching filler the break formed at the point of disruption of the link structure constitutes an effective insulator between the separated portions of the link structure.

Each of the link elements 12a shown in Fig. l is made of silver and is provided with a tin rivet Hi. Tin rivets E i fuse upon occurrence of an overload of inadmissible duration and initiate by their fusion by virtue of the above described metallurgical process the ultimate formation of an insulating break at each point where a rivet M is originally located. It is apparent from Fig. 1 that the rivets l4 arranged in immediately adjacent link elements !2a are out of registry. In other words, the distances of rivets Hi from each of the pair of terminal elements 2a, 2b varies from each link element [2a to the link element [2a immediately adjacent thereto. This arrangement of rivets i4 is eifective in precluding any substantial heating of the break zone formed at the point of each rivet [4 in each link element lZa b the heat generated at the break zone in the link element immediately adjacent to the link element under consideration.

The portions I20 of link elements IZa are very short and this minimizes I 1 losses; on the other hand, the portions [2c are highly effective, on account of their small cross-sectional area, in limiting short-circuit currents tending to rise to high peak values. Another factor tending to limit the magnitude of the short-circuit current is the large ratio of the total surface of the link units or stampings If. to the total cross-sectional area of the link elements l2a at the points [20 thereof where their cross-sectional area is smallest. Still another factor tending to limit the peak values of the let-through current is the use, for making the stampings l 2, of metals having a high conductivity since this is conducive to relatively small cross-sectional areas for given current ratings. While stampings [2 may be made either of copper or of silver, silver is preferable since its specific heat is much smaller than that of copper which in turn tends to reduce the peak of the let-through current. The sub-division of the entire link structure into an unusually great number of link elements in parallel results in a higher current density during the normal operation of the fuse, i. e. while currents of load proportions are circulating in the circuit. In other words, the unusually great number of link elements 52a in parallel enables to reduce the cross-section thereof for a given current rating. The length of the link elements i2a is relatively short. If the circuit voltage is in the order of 250 volts the lengthof the link elements 12a is in the order of 2 to 3 inches. This permits a most effective heat exchange between the link elements I2a and the cool terminal element 2a, 2b. The highly effective cooling of link elements 12a permits in turn a further increase of the current density. As a result of the geometry of the various parts of the fuse shown in the drawing and as a result of the interaction of the above factors the current-carrying ability of that fuse is in the order of 2000 to 4.000 amperes, or higher, depending upon the dimensions of the various parts.

The same reference signs have been used in Fig. 5 as in Fig. 1 to indicate like parts, and Fig. 5 calls merely for a description of the details thereof which differ from those shown in Fig. 1. According to Fig. 5 the inner member 4b of insulating column 4a, 4b is used as a fuse casing. Casing 4b is used as a fuse casing. Casing 4b accommodates link 30 provided with a plurality of circular equidistant perforations. Casing 30 contains a highly effective pulverulent arc-quenching filler 3| such as quartz sand. The perforations of link 30 determine zones of restricted cross-sectional area and the cross-sectional area of these zones is so correlated to the cross-sectional area of the zones of minimum cross-sectional area [20 of link elements lZa that fusion of link 30 is initiated only subsequent to initiation of fusion of link elements i201. The link element 30 to fuse last is of increased interrupting ability because it is of increased length and has a larger number of points of break than link elements l2a and because it is surrounded by a separate strong casing 41) permitting a very high rise of the pressure prevailing therein. In the embodiment of the invention shown in Figs. 5 caps 8 are conductively connected to link 39 and to terminal elements 2a, 21).

It will be understood that, although but one embodiment of this invention has been shown and described in detail, the invention is not limited thereto. It will also be understood that the illustrated embodiment may be modified or other embodiments made without departing from the spirit and scope of the invention as set forth in the accompanying claims.

It is claimed:

1. A high current carrying capacity currentlimiting fuse comprising a tubular insulating casing, spaced conductive terminal elements arranged on opposite ends of said casing, a pulverulent arc-quenching filler within said casing, a link structure conductively interconnecting said terminal elements and immersed in said filler, said link structure including a plurality of wide ribbon-type link units arranged radially with respect to said casing and radially with respect to said terminal elements and angularly displaced with respect to each other, each of said plurality of link units being subdivided into a plurality of narrower link elements forming elemental cur rent paths in parallel, said link elements having zones of restricted cross-sectional area including zones of minimum cross-sectional area, said zones of minimum cross-sectional area of alternate said link elements being arranged immediately adjacent to the same of said terminal elements.

2. A high current carrying capacit current limiting fuse comprising a tubular insulating casing, a pair of spaced conductive terminal elements each arranged on an opposite end of said casing, a pulverulent arc-quenching filler within said casing, a link structure conductively interconnecting said terminal elements and immersed in said filler, said link structure including a plurality of flat stampings arranged radially with respect to said casing and angularly displaced less than 30 degrees with respect to each other, each of said plurality of stampings being subdivided into a plurality of narrower spaced strips forming elemental current paths in parallel, said strips having zones of restricted cross-sectional area including zones of minimum cross-sectional area, said minimum cross-sectional area zones being arranged alternately immediately adjacent juxtaposed surfaces of said pair of terminal elements.

3. A high current carrying capacity currentlimiting fuse comprising a tubular insulating casing, a pair of conductive terminal elements each arranged on an opposite end of said insulating casing and secured by said insulating casing in spaced position, an additional structure for spacing said pair of terminal elements, said additional structure being in the shape of an insulating column arranged within said insulating casing and extending along the longitudinal axis thereof, said insulating casing and said insulating column defining a toroidal space, a pulverulent arcquenching filler within said toroidal space, a link structure conductively interconnecting said terminal elements and immersed in said filler, said link structure including a plurality of flat metal stampings arranged radially with respect to said casing and angularly displaced less than 30 degrees with respect to each other, the width of each of said plurality of metal stampings being equal to preponderant portion of the radial width of said toroidal space, each of said plurality of stampings being subdivided into a plurality of narrower strips forming elemental current-paths in parallel, said strips having zones of restricted cross-sectional area including zones of minimum cross-sectional area, said minimum cross-sectional area zones being arranged alternately immediately adjacent juxtaposed surfaces of said pair of terminal elements.

4. A high current carrying capacity current limiting fuse comprising a tubular casing, a pair of spaced conductive terminal elements each ar ranged on an opposite end of said casing, a pair of systems of grooves each on a juxtaposed surface of one of said pair of terminal elements, said grooves of said systems being arranged radially with respect to said casing and angularly displaced less than 30 degrees with respect to each other, a pulverulent arc-quenching filler within said casing, a link structure conductively interconnecting said terminal elements and im-- mersed in said filler, said link structure including a plurality of fiat stainpings each inserted into a pair of juxtaposed grooves of said pair of systems of grooves, each of said plurality of stampings comprising a plurality of relatively narrow strips f rming elemental current paths in parallel, said strips having zones of restricted cross-sectional area including zones of minimum cross-sectional area, immediately adjacent said strips having minimum cross-sectional area zones arranged alternately immediately adjacent a different of said pair of terminal elements.

5. A high current carrying capacity currentlimiting fuse according to claim 4 wherein said pair of terminal elements is spaced by an insulating column arranged within said casing and extending along the longitudinal axis thereof, said insulating column and, said casing defining a toroidal space, and wherein the width of each of said plurality of metal stampings is equal to a preponderant portion of the radial width of said toroidal space.

6. A high current carrying capacity current limiting fuse comprising a tubular insulating casing, a pair of spaced conductive terminal elements each arranged on an opposite end of said casing, each of said pair of terminal elements consisting of a cylindrical link-receiving portion having a relatively large diameter and a coaxial stud-shaped connector portion having a relatively small diameter, a pulverulent arc-quenching filler within said casing, a link structure immersed in said filler and conductively interconnecting said link-receiving portion of each of said pair of terminal elements, said link struc-- ture including a plurality of flat ribbon-type units arranged radially with respect to said linkreceiving' portion of each said pair of terminal elements, each of said plurality of link units being angularly displaced with respect to the other and each of said plurality of link units including a plurality of relatively narrow spaced strips forming elemental current paths in parallel, said strips having zones of minimum cross-sectional area for initiating arcing upon occurrence of a current of short-circuit current proportions, i1nmediately adjacent said strips having minimum cross-sectional area zones arranged alternately immediately adjacent different juxtaposed surfaces of said link-receiving portion of said pair of terminal elements.

'7. A high current carrying capacity currentlimiting fuse comprising an insulating casing, a pair of spaced conductive terminal elements each arranged on an opposite end of said casing, a pulve'rulent arc-quenching filler within said casing, a link structure conductively interconnecting said pair of terminal elements and immersed in said filler, said link structure including a plurality of link elements arranged to form current paths in parallel, each of said plurality of link elements having a minimum cross-sectional area for limiting the let-through current allowed to flow therein, said minimum cross-sectional area of some of said plurality of link elements being displaced in longitudinal directional direction thereof with regard to said minimum cross-sectional area of the other of said plurality of link elements arranged immediately adjacent there-to, said minimum cross-sectional area of each of plurality of link elements being arranged away from the median plane of said casing alternately immediately adjacent different surfaces of said pair of spaced terminal elements.

8. A high current carrying capacity currentlimiting fuse comprising an insulating casing, a pair of spaced conductive terminal elements each arranged on an opposite end of said casing, a pulverulent arc-quenching filler within said casing', a composite link structure conductively interconnecting said pair of terminal elements and immersed in said filler, said link structure includ ing a plurality of link elements arranged to form a plurality of current-paths in parallel, each of said plurality of link elements having a point adapted to initiate interruption of the respective link element at a temperature below the fusing temperature of the metal of which link element is made by the action of a metal situated at said point and having a fusing temperature below said fusing temperature of said metal of which said link element is made, link. elements situated immediately adjacent to each other having said point arranged out of registry.

9. A high current carrying capacity currentlimiting fuse comprising an insulating easing, a pair of spaced conductive terminal elements on opposite ends of said casing, a pulverulent arcquenching filler within said casing, a plurality of link elements conductively interconnectinc, said terminal elements and immersed in said filler, a low fusing temperature element of a metal having a lower fusing temperature than the metal of which said plurality of link elements is made associated with each of said plurality of link elements to cause disintegration thereof on the occurrence of protracted overloads by an alloying action between said metal of which said plurality of link elements is made and said metal having a lower fusing temperature, the distance of said low fusing temperature element from each of said pair of terminal elements varying from each of said plurality of link elements to the other of said plurality of link elements situated immediately adjacent thereto.

10. A high current carrying capacity currentlimiting fuse comprising an insulating casing, a pair of spaced conductive terminal elements each arranged on an opposite end of said casing, a pulverulent arc-quenching filler within said casing, a link structure conductively interconnecting said pair of terminal elements and immersed in said filler, said link structure including a plurality of fiat stampings arranged radially with respect to said casing and angularly displaced with respect to each other, each of said plurality of stampings comprising a plurality of relatively narrow spaced strips forming current paths in parallel, a low fusing temperature element made of a metal having a lower fusing temperature than the metal of which said plurality of strips is made associated with each of said plurality of strips to cause disintegration thereof on the occurrence of protracted overloads by an alloying action between said metal of which said plurality of strips is made and said metal having a lower fusing temperature, the distances of said lower fusing temperature element from each said pair of terminal elements varying from each of said plurality of strips to the next adjacent of said plurality of strips, each of said plurality of strips having a zone of restricted cross-sectional area determining the peak of the let-through current flowing therein, said zone of restricted crosssectional area of each of said plurality of strips being situated away from the median plane thereof, and the distances of said zone of minimum cross-sectional area of each of said plurality of strips from each of said pair of terminal elements varying from each of said plurality of strips to the next adjacent of said plurality of strips.

11. A high current carrying capacity currentlimiting fuse comprising an insulating casing, a pair of spaced conductive terminal elements each arranged on opposite ends of said casing, a pulverulent arc-quenching filler within said casing, a plurality of ribbon-type fuse elements conductively interconnecting said pair of terminal elements and immersed in said filler, each of said plurality of fuse elements having zones of restricted cross-sectional area including a letthrough current determining zone of minimum cross-sectional area, each said zones of minimum cross-sectional area being situated away from the center portion of each of said plurality of fuse elements and the distances of each said zones of minimum cross-sectional area from each of said pair of terminal elements varying from each of said plurality of fuse elements to the next adjacent of said plurality of fuse elements.

12. A current-limiting fuse comprising a first tubular insulating casing, spaced conductive terminals arranged on opposite ends of said first casing, a first pulverulent arc-quenching filler ,within said first casing, a link structure including a plurality of link elements conductively interconnecting said terminal elements and immersed in said first filler, said plurality of link elements forming elemental current paths in parallel, a second tubular insulating casing arranged within said first casing, a second pulverulent arc-quenching filler within said second casing, a fuse link within said second casing immersed in said second filler, said fuse link forming a current path parallel to the current paths formed by said plurality of link elements, and said fuse link being designed to fuse subsequent to fusion of each of said plurality of link elements.

13. A current-limiting fuse comprising a first tubular insulating casing, spaced conductive terminals arranged on opposite ends of said first casing, a first pulverulent arc quenching filler within said first casing, a second tubular insulating casing within said first casing, a second pulverulent arc quenching filler within said second casing, a plurality of fusible elements arranged inside of said first casing and outside of said second casing, a single fusible element of greater length than said plurality of fusible elements arranged in said second casing, the current-carrying capacity of each of said plurality of fusible elements being less than the current carrying capacity of said fusible element to cause said fusible element to fuse only upon fusion of each of said plurality of fusible elements.

14. A high current carrying capacity currentlimiting fuse comprising a first tubular insulating casing, spaced conductive terminal elements arranged on opposite ends of said first casing, a first pulverulent arc quenching filler within said first casing, a link structure conductively interconnecting said terminal elements and immersed in said first filler, said link structure including a plurality of wide ribbon-type link units arranged radially with respect to said first casing and radially with respect to said termial elements and angularly displaced with respect to each other, each of said plurality of link units being subdivided into a plurality of narrower link elements forming elemental current paths in parallel, each of said link elements having a plurality of zones of restricted cross-sectional area, a second tubular insulating casing coextensive with the geometrical axis of said first casing, a second pulverulent arc quenching filler within said second casing, a link element within said second casing immersed in said second filler, said link element having a plurality of zones of restricted cross-sectional area and forming a current path parallel to the current paths formed by said plurality of link units, said link element having a larger current carrying ability than each of said plurality of link elements to fuse subsequent to fusion of each of said plurality of link elements.

FREDERICK J. KOZACKA.

No references cited

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