US2605371A - Fuse - Google Patents

Description

July 29, 1952 H H, FAHNOE 2,605,371

FUSE

Filed July l0, 1948 .1. QF'g n 25 38 Fig-2.

WITNESSES: INVENTOR Patented July 29, 1952 FUSE Harold H. Fahnoe, Wilkinsburg,/Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvaniav Application July 10, 1948, SerialNo. 38,031

(Cl. 200m-1,20)y

20Claims.

1 Y, This invention relates to electrical fuses which are adapted to interrupt an electrical" circuit on overloads, and particularly to increasing theability of such fusesA to withstand for short-times. moderate overloads, such asfmay be encountered in startingy motors or the like, without ,materially alter-ing the rated current. at which such fuses operateor thetime requiredfor their operation on heavier overloads such as those prompted vby. short-circuits.

lThis invention is of especial importance in fuses of the current limiting type,.such as those disclosed inthe copending application of Harold H. Fahnoe, Serial No. 582,867, filed March 15, 1945, and issued as Patent No. 2,496,704, dated February 7, 1950, on the-subject ofl Circuit Interrupters. As explained in the copending applica tion just referred to, most effective current and voltage limitation on vhigh overloads and shortcircuits is achieved whenthe overload is limited before `maximum current prevails in the first half cycle'of the overload. 1f limitation is not effected in this manner, the resulting surge on the 'rst half cycle of faultv current often is capable of doing considerable damage to the circuit and especially'to connected apparatus such as motors, transformers, or the like. It follows from this that the current limitingvmeans importantly becomes effectiveina small fraction of a half-'cycle of such overloads as are tobe limited, thisk amountingr in time to a fewhundred microseconds.

A variety of the fuses referred' to in the copending application gave rise to a problem incident upon the assurance of rapid overload limitation, namely that of preventing` interruption of current clue to moderate overloads, such as those of short duration which occur 'in'y starting an electric motor followed by the resumption of normal relatively low current conditions. By4

resorting to such measures vas adding more parallelfuse wires in the current limiting element of the fuse to compensate for motor inrush starting currents or the like, the peak current limit permitted by the fuseis correspondingly increased.

thus increasing the interrupting duty ofj the fuse,`

and the time-current characteristics on moderate short-circuits or overloads is correspondingly' slowed down.

, It is, accordingly, an object of this invention' to provide a current limiting fusewhichis capab1eofwithstanding light overload `currents for short periods of time, yet assures an extremely rapid blowing action where relatively large over.- load currents are encountered.

Another object of this invention is the provision'of a current limitingfuse in which fusion of the fusible portion or portions thereof is reliablyand effectively Iarrested in a relatively low current range by heat absorption. and without substantial impairment of desired rapid limitation of relatively large overloads.

A further object4 of thisv invention isthe provision of a fuse capable of permitting passage of inrush currents within lower. operating limits and of interrupting relatively large overload currents, and which upon being subjected to asuiicient current overload to give the blowing action, enables the rapid achievement of a high arc voltage contributing to the current limiting effect. y

Astill furthery object of this invention is to provide a fuse structure including fusible material, and heat absorbing means for cooperating with the fusible material, all supported in a novel andhighly useful manner interiorly of a tubular fuse holder.

These and other objects of this invention will..

Figure` 1 is asubstantially central longitudinal sectional view of a fuse constructed in accord- .ance withv the invention, with certain of the parts showninxelevation;

Fig. V2 is .an enlarged, partial, transverse crosssectional view ofy thefuse represented in Fig. 1, taken substantially on the line II--II thereof;

Eig. 3 is an enlarged, partial, transverse crosssectional view. of` the fuse represented inFig. 1, this being takenr substantially on the line III-III thereof; and

Fig; 4is anenlarged, partial, longitudinal, sectional view taken through one of the slotsor groovesv of the-fusesupporting member depicted in Fig. 1'.

Iny the particular embodiment of the invention disclosed' in the accompanying drawing, a limiting unit l0 kis provided for'limiting currentand voltage. This unit is encased `by a tubular fuse holder 2 made of insulating material such as fiber or synthetic resin, and by terminal caps 4` and' 6y overy the-'opposite endsof the tube. Terminal caps 4'- and 6=aremade of an electrically conductive material such, forfexample, as copper or` the like, and are preferably secured in position on the tubular holder, illustratively by the staking pins'8.

The limiter unit I0 includes a supporting rodl I 2"'of suitable insulating material suchi asffiber orv for example of brass, each threadedly engaging a corresponding one of the opposite inside Wall ends of the tubular fuse holder 2. Staking pins 8 advantageously reach into these end plugs for stability. An electrically conductive rod end cap I I, threadedly receiving one end of the rod I2, has a reduced diameter portion extending through aligned, substantially central openings in the end plug 25 and terminal cap 4. Similarly an electrically conductive rod end cap 56 threadedlv receives the other end of the supporting rod I2, and has a reduced diameter portion passing through aligned, substantially central openings in the end plug 26 and terminal cap 6. The reduced diameter portions of the rod end caps 25 and 26, conveniently are secured in electrically conductive relation to the respective terminal caps 4 and 6 as by soldering. A finely divided inert insulating material 38, such as sand, surrounds the current and voltage limiting unit II) and fills the space to the inside surface of tubular fuse holder 2 and of end plugs 25 and 26. The end plug 25 has a slightly tapered stopper 9 closing oif a suitable opening for insertion of the inert material 38. Terminal cap 4 is suiciently near the stopper to prevent the same from backing out of its related opening. The rod end cap 56 in the present embodiment is greater in length than rod end cap II, for reasons which appear more fully hereinafter.

The actual current and voltage limiting and are extinguishing structure embodied in the unit I comprises a plurality of tubes disposed on the supporting rod I2. Each of the tubes 20 has spaced grooves 22vrunning longitudinally along the outside surface thereof for receiving a fusible length or section of wire 24. The grooved tubes are preferably made of an electrically insulating material which is capable of evolving an arc extinguishing gas when in proximity to an electric arc, such for example as fiber or synthetic resin. Also, on the supporting rod I2, and preferably in alternate or spacing relation with the grooved tubes 20, are a plurality of heat wells 4U, each including a tube 2 I, such as copper or other suitable electrically conductive and heat absorptive material. The tubes 2i) and 2| as arranged in the present embodiment are easily tightened toward and clamped against each other in substantially coaxial relation on supporting rod I2 by advancing the rod end caps and 56 relative to one another. Grooves 22 are longitudinally aligned throughout the tubes 20 before tightening the end caps, or may be cut in alignment after thetightening operation. The bottoms of grooves 22 conveniently are at about the same level as the outside surfaces of heat well tubes 2i.

The fusible Wires or elements 24 extend in parallel electrical relation to each other from rod end cap 25 to the rod end cap 56, and each wire has fusible sections individually in the grooves of the tubes 20 which are connected in series with the heat wells 40. For this purpose wires 24 conveniently pass along in the grooves 22 and over the surface of the heat Well tubes 2|, there being connected in heat conductive and electrically conductive relation to the Well tubes as by an embedding layer of silver solder or the like 28. A plurality 0f Washers 34 are slipped onto each of the grooved tubes 20 and are frictionally held in spaced` relation to each other along the length of these tubes, fora purpose to be described. Washers 34 may be of any desired electrical insulating material, such, for example, as fiber or the like.

An additional mass of heat absorptive material for the heat wells 40, supplementing the absorptive effect of the tubes 2|, conveniently surrounds the solder bed 28 containing the fusible Wires 24. This mass, as represented in the accompanying drawing, includes a Wrapping of heat conductive wire over or in each of the solder beds 28 for reinforcing and strengthening purposes, and a superimposed segmented collar I5 of thermally absorptive material which may be the same as the material of sleeve 2|, is fastened by means of a layer of solder 39, for example of silver, to the Wire wrapping. Both the segmented collar I5 and the Wrapping wire 30 illustratively are made of copper. The segmented collar conveniently provides a circumferential groove for receiving a binding Wire I8 which assists in holding together the several soldered segments both before and after the soldering operation.

The fuse wire 24, being of silver or similar material, has a high temperature coefficient of resistance, and is initially chosen to carry the rated current without undue heating, and yet melt on currents above the rated current. Such a wire is then provided with a plurality of reduced portions 36 (Fig. 4). By this arrangement the fusion time, at least for high values of current is speeded up and substantially corresponds to the fusion time required by a wire having about the same diameter as the reduced diameter portions 36. This speeding up effect has been found to be roughly proportional to the ratio between the area of the large portions of the fuse wire 24 and the reduction portions 36, the ratio being according to cross section. As a practical matter, this ratio may be made as high as possible, being limited of course by mechanical strength considerations. Such a fuse wire retains all of the normal time-current characteristics desired in a fuse for time values in excess of approximately two cycles without appreciable reduction in current carrying ability. Reduction in current carrying ability varies approximately in direct proportion to the length of portions 36, so that it is desirable to make these portions as short as possible. For short times of less than about one-fourth cycle, fuse wire 24 has an exceptionally fast melting time when the overload current is high. When the overload current is relatively low, however, the heat Wells 40 absorb enough of the heat to prevent the fuse wires from fusing. This feature givesy ample protection against blowing of the fuse wires, as for example when the related electrical circuit is carrying a motor starting or inrush current before normal operating currents are attained. The effect of increased or sustained overloads, such as short circuits is that of limiting the current and voltage by fusion of the Wires in an early part of the cycle of fault current. Under the latter conditions the heat wells 40 have substantially no arresting affect on fushion of the wires 24 and the melting time of less than about one-fourth half cycle.

As pointed out in the copending application hereinbefore referred to, the fuse wires 24 retain their current carrying ability Where the current is relatively low, and this is not substantially impaired by the introduction of reduced portions 36. Added to this in the present embodiment, is the effect of the heat wells, which extends the short-time current carrying ability of the fuse Wire to include appreciably increased overloads, such for example as currents ranging up to about eight times the rated current, all with retention of fast limiting characteristics for higher overloads.

Theactual limitation of current is achieved by.

ofthe fusible sections or lengths of thewire represented by thelength of the related grooved tube 2li. substantially instantaneously, thus exerting a limiting or choking effect on the current and preventing rise of the current beyond a predetermined value. The particular structure disclosed provides a'high arc voltage per unit of length of the fusible sections of wire 24, which interconnect' the wells 40 and lie intermediate the latter in the grooves 22 of tubes 20. rI'his conveniently is achieved by employinga large enough number of reduced portions 3G in the fusewire so that when these reduced portions melt, `a plurality of serially related arcs may be formed and the summation of the arc voltages of these series arcs will be greater than the arc voltage across asingle arc. Generally speaking, the arc voltage per unit of length is directly proportional to the number of series arcs orrestricticns per unitof length which may be effectively interrupted by fusion. This suggeststhat maximum arc voltage per unit of length may be attained by providing as many restrictions 36 perunit oflength as is:

physically possible. However, as the number of restrictions 36 per unit of length is increased, a point is eventually reached wherev a further increaser results in a decrease in current carrying ability. It has been found that at least for voltages above 600 volts, at least 4two restrictions 38 per effective inch of fuse wire 24, or a total of about ten shouldv be provided to obtain an effective rise in arc voltage with a fuse wire `which is not yof excess length. This corresponds approximately to a spacing ofy restrictions 36 not to exceed about fifteen times the large diameter of the fuse wire. t is desirable, however, Lto use the maximum number of restrictionsu possible Without. substantially impairing .the current carrying ability. The most desirable number of restrictions is dependent upon the size of the fuse wire, and appears to correspond roughly toa spacing of about three and` one-half times-the largest diameter of the wire. This is. an optimum` spacing, since spacings as low as about twice thelargest diameter of thefuse wiremay be employed with good results.

Another reason why the arcvoltage is rapidly built up to a value high enough to exert a current limiting effect, has to do with the grooves'22 in which the fuse Wiresare located. lGrooves 22 advantageously are madeas narrow. as possible, being limited only by the size of the fuse wires, and in practice illustratively are made nomore than 0.010 inch larger than the fuse wires. Preferably the grooves arey somewhat deeper than the diameter of the fuse Wires, so that the yWires will vclosely contact the groove on three sides and thus physically restrict the arc formed upon melting of the fuse wires, vthereby raising the arc voltage at the instant the arc is formedf Since fuse wires 24 are in close proximity .tothe Walls of groove 22, the arc formed upon inter--` ruption will cause the evolution of arc-extinguishing gas from the walls of the groove, and this gas will blast laterally through the arc to perform three functions, all of which act to furtherincrease the arc voltage and extinguish the arc. First, the blast of arc-extinguishing gas acts to. sweep metal vapor out of thearc stream and out of grooves 22 into the material'38 in which the metal vapor becomes dispersed and Upon fusion, a high'arc voltage Ais built up.:k

condensed into .separated particles insulated fromUr each other, so that'a `high resistance path `is maintained outside the grooves. Secondly, the

blast of varc.-extinguishing gas also acts to supply.;

url-ionized gas to :the arc ypath to further increase.

the resistance oftheaarc path. and toiextinguish. r` A third function of. the' transverse gas blast is to cause the series arcs-to.

the arc at current zero.

be looped-outwardly toward. filling material 38, thereby lengthening the arc path and, consequently, increasing the resistance'fthereof andthe voltage drop across it. A function of washers 34 on supporting rod` 20 will nowv be readily under-1 stood as preventing escape ofy the arcs from grooves 22 and'y consequently, keeping the aresfin prOXimityWiththe 'gas evolving material and the restricting action of the narrow. grooves, andl structure disclosed, because (1) the fusible wiresl themselves are capable of melting toestablish an.

arcvat least on such high currents which it is desired to limit, in a very short time, that is, in a very small fraction of` a half cycle, and (2) as soon as the arcis established. it is subjected to all ofthe factors enumerated above to create an arc voltage high enough to prevent any further rise of fault current. f

The; arc voltage, and the speed and efficiency of current limitation and interruption appear to be in some measure dependent upon ythe number and thickness of washers 34, vThese washers are `preferably 4made thick enough. as not to be materially eroded by the action of the arc. the number of washers v34 increases the arc volt.- age while increasing current limitation and interruptionspeed. A better understanding of this may be arrived at by considering that fusesv are known in the art which comprise one or more silver wires embedded in sand and that these fuses obtain currentlimitation by the formation of a semi-conductive core about the wire due to the action of the heat of the arc on the sand. However, such fuses must be excessivelyy long in order to obtain sufficient voltage drop to extinguish the arc, and increased length, of course, causes greater voltage surge at interruption. In the fuse constituting this invention the formation of any such semi-conductor is positively prevented by barrier plates 3.4, and this is one reason why unit I0 may be made relatively shortin length. The provision of barrier plates 34 (which themselves. are advantageously of a material capable of evolving an arcextinguishing gas when subjected to the heatxof an arc) thus appears to provide spaced deionizing zones along the length of unit il), which would explain why an increase in the number of barriers would provide an increase in arc voltage,-and also explains why a relatively high arc voltage per unit length is obtained with unit l2. Moreover, it is believed that this action of the barrier plates 34 undoubtedly contributes to rapid extinction of the arc. 1

Though there may be other reasons for the results obtained, tests show that a structure such as unit I0 of and by itself, is capable of limiting high overload currents in the first part of the rst half cycle thereof, without producing voltage surges substantially greater` than one and one-half times restored circuit voltage, and

It also appears that an increase :inr

is effective to interrupt the circuit at the following first or second current zero.

Further, the unit is thoroughly capable of absorbing heat resulting from relatively low overload currents of short duration, thus preventing interruption of the circuit on such overloads. The number and size of the heat wells 4D are correlated with the length of fuse wires 2:4 intermediate the wells so that the entire heat input due to momentary light overloads can be conducted to the wells before the centers of the fuse wires attain fusion temperature. Furthermore, a material such as copper is chosen for the heat wells because its specific heat is higher (about twice as high) than the material of which they fuse wires are formed. This means that for a given size of heat well a relatively large amount of heat can be stored therein. This heat lost from the wires to wells.

40, as well as some losses to the surrounding media, such as sand 38 and rod l2, thus limits the temperature rise of fuse wires 24 on light overloads of short duration. In addition, since wells 4D electrically and mechanically connect all of the parallel wires, increased uniformity in heating and in division of current flow through the several fuse wires is achieved. Moreover, since wells 40 effectively break up each fuse wire 24 into a plurality of serially arranged short fusible sections located between the wells, longitudinal expansion and contraction of the wires is not a factor in causing fuse wire bending or even breakage occurring in the fuses which employ longer lengths of fuse wire. One reason why peak currents on heavy overloads are not increased with the presen design is that fuse wires 24 intermediate wells 4G are not increased in number, and they act when the fuse blows on such heavy overloads in exactly thesame manner as if wells 40 were not present.

To give an indication that fusion of the current limiting wires 24 has occurred in the unit I0, an indicator disc 68 is provided secured to a connecting rod 5B extending through a bore in the outer end of the rod end cap 56, to be normally held adjacent to end terminal cap 6 by a high resistance fusible strain wire 48 and a small tension spring 52. A coil compression spring 1U is provided between indicator disc 68 and cap 6 to bias the disc outwardly away from the end of the fuse. The large end coils 46 of spring 52 are adapted to be supported within fuse tube 2 and more particularly within the bore of rod end cap 56 by a shoulder in a small inner tube 44 of insulating material such as fiber or the like. The inner end of strain wire 48 is connected through spring 52 to the rod end cap Il by means of a high resistance fusible wire 66. High resistance wires 48 and 6U preferably are of such a high resistance as compared with fuse wire 24 that they normally do not carry any appreciable amount of current. However, as soon as wires 24 are melted, the arc voltage developed in unit I0 forces sufficient current through wires 60 and 48 to fuse these wires at substantially thel same time, thus releasing indicator disc 68 and permitting it to be moved by spring i6 outwardly to a clearly visible indicating position. Fusible wires 48 and 69 are of such high resistance relative Ato wires 24, on the order of several hundred times the resistance of wires 24, that they `do not affect the operation of unit (D in any appreciable way either before or during a circuit interrupting operation. l

The fuse construction described herein provides a simplified form of fuse which may be maderelatively small in size. It includes both current and voltage limiting means in a single unit, together with arc-extinguishing means.

, The limiting unit I0 has a definite, continuous current rating and the fusible sections of wires 24 will rapidly fuse on high overload currents which it is desired to limit, but will resist fusion of lower overload currents up to about 800 per cent of rated current and of short duration by aid of the heat Wells 4U, the latter also being compactly contained in the limiting unit I0.

Having described a preferred embodiment of the invention in accordance with the patent statutes, it is desired that the invention be not limited to this particular structure, inasmuch as it will be obvious to persons skilled in the art that many modifications and changes may be made in this particular structure Without departing from the broad spirit and scope of this invention. Accordingly, it is desired that the invention be interpreted as broadly as possible and that it be limited only as required by the prior art.

I claim as my invention:

1. A fuse having a definite, continuous current rating, comprising at least one fusible element having a maximum crosssection area not exceeding that which will fuse on currents in excess of rated current, a relatively large quantity of. good heat conducting material located in good heat conducting relation with said fusible element to prevent fusion thereof within a higher range of currents of short duration, and substantially all of said material having a relatively high specific heat.

2. A fuse having a definite, continuous current rating, comprising, serially arranged fusible elements, each of said elements having a maximum cross-section area not exceeding that which will fuse on currents in excess of rated current. a relatively large mass of good heat conducting material located at each pair of adjacent ends of said elements to prevent fusion thereof within a higher range of currents of short duration, and Substantially all of said material having a relatively high specific heat.

3. A fuse having a definite, continuous current rating, comprising a plurality of spaced parallel fusible elements each of which comprises serially arranged fusible sections, each of said sections having a maximum cross-section area not exceeding that which will fuse on currents in excess of rated current, and relatively large masses of good electrical conducting material each of which is common to and electrically connects corresponding sections of said elements, respectively.

4. A fuse having a definite, continuous current rating, comprising serially arranged fusible sections, each of said sections having a maximum cross-section area not exceeding that which will fuse on currents in excess of rated current and portions having a reduced cross-section area along the length thereof to substantially decrease melting time thereof on heavy overload; and at least one heat well interposed between said sections in heat conducting relation thereto to prevent fusion thereof within a range of currents of short duration.

5. A fuse having a denite, continuous current rating, comprising, a plurality of parallel fusible elements each of which comprises serially arranged fusible sections, each of said sections having a maximum cross-section area not exceeding that which will fuse on currents in `excess ofrated current and portions having a reduced cross-section area along the'length thereof to substantially decrease melting time thereof on heavy overload; and heat wells between said sections in heat conducting relation thereto to prevent fusion thereof within a range of currents of rvshort duration, with the heat wells between corresponding 'sections of said elements comprising single unitary commonl masses of heat conducting material, respectively. i

` of short duration, with said heat Well comprising a single unitary mass of heat conducting material common to all of said elements.

'1.A fuse having a definite, continuous current rating, comprising, at least one elongated fusible section having a maximum cross-section area not exceeding that which will fuse on currents in excess of rated current and portions having a reduced cross-section along the length thereof to substantially decrease melting time thereof on high overloads, and a sufficient large quantity of good heat conducting material located in heat conducting relation with said fusible section to prevent fusion thereof within a range of currents of short duration.

8. A fuse having a definite, continuous current rating, comprising, at least one elongated fusible section having a cross-section area not exceeding that which Will fuse on currents in excesss of rated current and portions having a reduced cross-section area along the length thereof to substantially decrease melting time thereof on high overloads, a suiiiciently large quantity of good heat conducting material located in heat conducting relation with said fusible section to prevent fusion thereof within a range of currents of short duration, and an elongated arc restrictive passage of insulating material receiving said fusible section for increasing the f arc voltage along the same under conditions of relatively high overload current.

9. A fuse having a definite, continuous current rating, comprising, serially arranged fusible sections, each of said sections having a crosssectionarea not exceeding that which will fuse on currents in excess of rated current and portions having a reduced cross-section area along the length thereof to substantially decrease melting time thereof on high overload, heat wells between said sections to prevent fusion thereof within a range of currents of short duration, and insulating material adjacent said fusible sections capable of evolving an arc extinguishing gas for increasing the arc voltage under conditions of relatively high overload current.

l0. A fuse having a definite, continuous current rating, comprising, a plurality of parallel fusible elements each of which comprises serially arranged fusible sections, each of said sections having a cross-section area not exceeding that which will fuse on currents in excess of rated current and portions having a reduced cross-section area along the length thereof to sustantially decrease melting time thereof on high overload, heat wells between said sections f to prevent fusion thereof within a range of currents vof lshort duration, and elongated arc restrictive passages of insulating material receiving vsaid fusible lsections for increasing the arc voltage along the same under conditions of relatively high overload current.

l1. A fuse comprising, at least two spaced parallel elongated fusible sections, a single mass of heat conducting material located in heat `conducting relation with both of said fusible sections to prevent fusionthereof within a range of currents of short duration.

12. A fuse comprising, at least two spaced parallel elongated fusible sections, a mass of good electrical and heat conducting material electrically and mechanically connecting said fusible sections to prevent fusion thereof within a range of currents. of'short durations 13. A fuse comprising, elongated fusible sections, spaced longitudinally grooved elements receiving said lfusible sections in the grooves thereof, for increasing the arc voltage along the same under conditions of relatively high overload current, arc escape prevention means longitudinally spaced along said grooved elements and the groove paths thereof, and heat absorptive means between said grooved elements in heat conducting relation with said fusible sections to prevent fusion thereof within a relatively low range of currents of short duration.

14. A fuse comprising, elongated fusible sections, spaced longitudinally grooved elements receiving said fusible sections in the grooves thereof, for increasing the arc voltage along the same under conditions of relatively high overload current, arc escape prevention means longitudinally spaced along said grooved elements and the groove paths thereof, and good electrical and heat conducting means located between said grooved elements and being in electrical and heat conducting relation with said fusible sections to prevent fusion thereof Within a relatively low range of currents of short duration.

15. A fuse comprising, a plurality of parallel fusible elements each of which comprises serially arranged fusible sections, spaced substantially tubular heat wells in heat conducting relation with corresponding fusible sections of all of the fusible elements to prevent fusion thereof within a relatively low range of currents of short duration, substantially tubular elements between said heat wells and having longitudinal external grooves for receiving said fusible sections and increasing the arc voltagealong the same under conditions of relatively high overload current, arc escape prevention washers longitudinally spaced on said grooved tubular elements and over the groove paths thereof, and a rod extending longitudinally through said heat wells and grooved tubular elements for supporting the same as a unit.

16. Afuse comprising, a plurality of spaced parallel fusible elements, means at corresponding ends of said elements for electrically connecting them in parallel circuit relation, and a plurality of heat wells spaced along the length of said elements, each of said wells comprising a unitary mass of good heat conducting material extending into heat conducting relation with corresponding points of all of said elements.

17. A fuse comprising, a plurality of spaced parallel fusible elements, means at corresponding ends of said elements for electrically connecting them in parallel circuit relation, and a plurality of means of electrical conducting material spaced along the length of said elements, each electrically connecting corresponding points of all of said elements.

18. A fuse comprising, a plurality 'of spaced parallel fusible elements, means at corresponding ends of said elements for electrically connecting them in parallel circuit relation, and a plurality of heat Wells spaced along the length of said elements, each of said Wells comprising a unitary mass of good electrical and heat conducting material extending into electrical and heat conducting relation withl corresponding points of all of said elements.

19. A fuse comprising, an elongated fusible element, supporting means for said element comprising spaced sections of insulating material and interposed sections of good electrical and heat conducting material arranged longitudinally of said element and mechanically engaging the same.

20. A fuse comprising, a plurality of spaced,

REFERENCES CITED The following references are of record in the nie of this patent:

UNITED STATES PATENTS Number s Name Date 2,004,191 Bussmann June 11, 1935 2,055,866 Jung et al Sept. 29, 1936 2,209,823 Lohausen July 30, 1940

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