US2592399A

US2592399A - Current-limiting fuse

Info

Publication number: US2592399A
Application number: US119476A
Authority: US
Inventors: William S Edsall; Kenneth W Swain; Frederick J Kozacka
Original assignee: Chase Shawmut Co
Current assignee: Chase Shawmut Co
Priority date: 1949-10-04
Filing date: 1949-10-04
Publication date: 1952-04-08
Anticipated expiration: 1969-04-08

Description

April 8, 1952 w. s. EDsALL x-:T A1. 2,592,399

CURRENT-LIMITING FUSE April 8, 1952 w. s. EDSALI. ET AL CURRENT-LMITING FUSE 5 Sheets-Sheet 2 Filed Oct. 4, 1949 m10- .Omw NEC' 025m; O

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UBLBNLLNBQ 83d SL'IOA am a a a I fw wf@ 572 eur/4Z H mm. d m w Mfr,

April 8, 1952 w. s. EDSALI. ET Ax. 2,592,399

CURRENT-LIMITING FUSE Filed Oct. 4, 1949 3 Sheets-Sheet 5 Patented Apr. 8, 1,952

William YsfEdsall, Boston, Mass., Kenneth W. Swain, Hampton VVFalls, N.-H., and Frederick-J. Kozacka, Amesbury, Mass., -assignors to The AChase-Shawmut Company, Newburyport, Mass., a corporation. of Massachusetts Application October 4, 1949,'Serial No. 119,476

42 Claims.

The' invention refers to fuses of the currentlimiting type which, lwhen subjected to an A.C. iaultcurrent of short-circuit current proportions, 'effect interruption of the circuit in less, for even substantiallyuless, than half a'cycle of thecurrent wave. Such fuses, when subjected to a D.C.'shortcircuit current having a similar rate `ofvrise as an A'.C.shortclrcuit current, effect :interruptionof the D.'C.current ina similarly `short period of time as in the case oi A.C'.1cur rent interruption. Thelprincipal object of the Sinventionis to providea low voltage vcurrent- 'limiting high current interrupting capacity "fuse Which is improved inveveryrrespect.compared to fuses of the same general type whichwere known heretofore.

Current-limiting'fusesare called to interrupt fault currents of 'short-'circuit current proportions before the currents reach the maximum `short-circuit current strength which the system or'circuit with which the fuses are associated is "capable of producing. lThat current vis generally referred to as the available short-circuit current. The `maximum current which isV allowedto'ilow ithrough a'circuitf which is protected' by a currentlimiting -fuse is 'called the let-through current. Obviously, the let-through current is `much 'smaller thanthe available short-circuit current. Anycurrent exceeding the let-through 'current is considered a short-circuitcurrent.

Current-limiting fuses are often alsorequired `to interrupt overload currents of `much smaller proportions than short-circuit currents if -the overload currents-are of 'relatively long duration. Small overload currents which a current-limiting fuse may beirequired to interrupt may beinthe order of eight to ten times the rated currentof the fuse.

Originally, -low voltage `current-linliting fuses were relatively compact on accountof the high heat dissipating'abilityof Vtheir pulverulent ller and on account-of the fact'that the varc energy H2-ndt which is dissipated in them is relatively Ain any other circuit interrupting device. In-

r'creasing concentration of loads made it necessary to provide current-limiting fuses of increased current carrying and interrupting'capacity rating'and the size and bulk of such currentlimiting fuses rhas tended lto increase in propor- .tion to their current carrying and interrupting capacity rating. Fuses for circuits capable of Vproducing'short-circuit currents in the order of several hundred thousand amperes are extremely lbulky and diilicult to accommodate where Yspace limitations are of a serious nature. Itis, therefore, an object of the invention'to provide a low voltage current-limiting-high current interrupting capacity fuse which is smaller and more com- `pactthan prior art current-limiting fuses *of a similar rating.

The size and bulk lofla current-limiting fuse depend primarily upon the amount of arc energy fz`2r-dt which is Idissipated in the fuse. It was believed, heretofore, that the known designs of current-limiting fuses achieve the aim of minimizing the amount of arc energy which must necessarily be dissipated about aswell as that aim can possibly be achieved. It Was found, however, that it is possible to effect afdrastic decrease in the -amount of arc energy compared to the amount of arc energy Which is dissipated in current-limiting'fuses Aof known design. In accordance with this invention, a fuse is provided the size and bulk of which -arevreduced in accordance with the newly discovered possibility of further reduction of arc energy.

One of the major vdiiiiculties involved inthe design of current-limitingV fuses is their tendency to behave differently depending upon whether the'current under interruption is relatively'high or small. A current-limitingfuse is required'to achieve consistently sulciently 'rapid interruption irrespective of whether the current under in'- vvterruption is relatively high or small. 'It is, therefore, another object of this invention to'provide a current-limiting Tfuse which operates satis- Vfactorily both on the occurrence of fault currents of short-circuit'current proportions and of relatively small overload currents of inadmissible duration.

It has been found that the voltage gradient in a current-limiting fuse is infthe order of 300 volts per centimeter at the instant of arc initiation and that the voltage gradient at the point of arc initiation decreases rapidly with increasing arcing time. Because of this decrease of the voltage gradient at the point of arc initiation, the rate of rise of the total arc voltage-due to progressive Vgrowth of ther length of the arc gap on account or continued arcingwill be greatly reduced if the arcing time is not relatively short. It is, there- -iore, another object-ofV this invention to provide a current-limiting fuse in whichV the arc voltage Yrises so rapidly in case of interruption of relatively small overload currents that iinal arc eX- 'tinctionfis eifectedibefore the voltage` gradient at the point of arc initiation drops considerably.

'Current-limiting fuses which are made according to-this invention comprise a fuse link which is arranged in a casingand interconnects terminal lelements on opposite ends of the casing. The link has a plurality of portions of reduced 'cross-section' spaced' substantially equidistantly along substantially the entire length of the link Aand `a'plurality of portions of relatively larger cross-section intermediate the reduced cross- 'section portions. Thereduced cross-section portions have a sufficiently small cross-sectional area 'to initiate arcing at instantaneous currents far below the available short-circuit current of the vcircuit or the system with which the fuse is associated. 'The reduced cross-section portions of the link are veryshprtI and the length of theportions of relatively larger cross-section is a multiple of the length of the reduced cross-section portions. The fuse comprises further a pulverulent arc-extinguishing substance of predetermined arc-extinguishing capacity lilled into the casing and surrounding the link substantially throughout the total length thereof. The substance or iiller has a suiciently high arc-extinguishing capacity in relation to the arcs which are formed at said portions of reduced cross-section to effect complete interruption of any short-circuit current that the circuit with which the fuse is associated is capable of producing in less than 1/120 of a second after fault inception, i. e. less than half a cycle of a 60 cycle current wave. The total length of said reduced cross-section portions and of said relatively larger cross-section portions exceeds the length of the link vaporized under the most onerous circuit interrupting conditions to which the fuse may be subjected in the circuit with which it is associated.

Interruption of the short-circuit current by means of such a fuse is initiated by fusing of the link at the small cross-section portions. At the end of the interrupting process,-the small crosssection portions are entirely and the large crosssection portions partially, but not entirely, vaporized, not even under the most onerous circuit interrupting conditions to which the fuse may be subjected. Such a fuse consists, after blowing, of a series of spaced bodies of melted sand or fulgurites and intermediate metallic reinainders of the link. These remainders of the link are sometimes formed by a more or less solid piece of metal substantially in its original state with its length reduced by arcing and sometimes by a pattern of coarsely dispersed metal pieces.

It is well known in the art to provide fuseI links of low voltage fuses with portions of reduced crosssection alternating with portions of relatively larger cross-section. In a certain type of fuses comprising such links the large cross-section portions, because oi their large heat capacity, tend to delay the interruption of the circuit by the fuse'in case of protracted relatively small overload currents. The relatively large cross-section portions of the link are not involved in the interruption of large currents in the nature of short-circuit currents. The latter is effected at the portions of reduced cross-section only. Obviously, the present invention has no direct relation to the prior art time'lag fuses of the aforementioned t-ype. Y

It is also Well known in the art to provide the links or fusible elements of high voltagey fuses with a plurality of portions of reduced cross-section alternating with portions of relatively larger cross-section. These prior art fuses are not of the current-limiting type and the plurality of portions'of reducedcross-section'of their links is intended to provide a multibreak feature. As a general rule low voltage fuses do not call for a multibreak feature. The link of a current-limiting fuse is vaporized on the occurrence of a fault current of short-circuit current proportions suby stantially simultaneously at any point along the entire length of the link, resulting in insertion of arc resistance into the circuit at a more rapid rate than can be achieved in any prior art high voltage multibreak fuse. In a conventional currentlimiting low voltage fuse, on the occurrence of a short-circuit current a striation phenomenon occurs throughout the entire length of the link, resulting in the formation of multibreakathough the cross-section of the link is uniform throughout its entire length. This is an automatic multias constant as possible. This, in turn, tends to limit the inductive surge voltages which may occur in inductive high-voltage circuits when a current-limiting fuse is caused to blow and which may present a serious danger to circuit insulation. In low voltage circuits the danger resulting from inductive surge voltages is much less serious than in high voltage circuits and can effectively be eliminated by arranging a plurality of vfuse links in parallel in the circuit which fuse links melt and vaporize sequentially under the action of fault currents of short-circuit current portions. Hence inductive surge voltage control is no reason for providing the links of currentlimiting low voltage fuses with a plurality of portions of reduced cross-section. Though control of inductive surge voltages and the objects of this invention may both be achieved by means of links having a plurality of portions of reduced crosssection alternating with large cross-section portions, either case calls for different geometrical link coniiguration. A link having a plurality of reduced cross-section portions designed for inductive surge voltage control will not achieve the objects of the present invention. Vice versa, a link having a plurality of reduced cross-section portions designed to achieve the objects of the present invention will not achieve inductive surge voltage control. Inductive surge voltage control requires progressive insertion of arc resistance into the circuit. To achieve the objects of this invention, the length of the reduced cross-section portions ought to be Very short in comparison to the length of the large cross-section portions. Melting and Vaporization of the former under the action of a fault current of short-circuit current proportions results in insertion of but little arc resistance into the circuit. The subsequent melting and vaporization of the latter results in sudden insertion of a` large amount of arc resistance into the circuit, which is clearly the opposite of what is required for inductive surge voltage control.

Both the portions of relatively small crosssection and the intermediate portions of relatively large cross-section of links of prior art high-voltage current-limiting fuses are designed to be completely vaporized at all interruptions of a relatively onerous kind, whereas the large cross-section portions of the link of a fuse embodying the present invention are designed not to be completely vaporized under the most onerous circuit interrupting conditions to which the fuse may be subjected in the circuit with which it is associated. The significance of this relative excess of link length or link metal will now be explained.

In fuses embodying the present invention the function of the small cross-section portions and of the large cross-section portions of the link varies depending upon the magnitude of the current under interruption.

Currents-.in the'- nature oi` shortfcircuitcurrentsv lrise at'avery rapid ratefand` reach high magnitudes, unless previously interrupted. In fuses'embodying'the present. invention, on oc currence of` a short-circuitA or.y like fault, the reduced cross-section portions of the link reach their fusing temperature. virtually instantly which limits the let-throughicurrent to an unusually low value. The adjacent intermediate large cross-section portions ofthe link are still cool at the time the interrupting process is initiated at the small cross-section portions. Therefore, the large cross-section portions are capable of absorbing largeiamounts of heat during the subsequent phase of the interrupting process during whichthey are partially vaporized.

Currents in the nature of? relatively small protracted overloads result in a relatively slow rise of the temperature ofthe reduced cross-section portions of the link. The rate of rise of the temperature of the reduced cross-section portions is particularly -small Von account `of -the shortness oftheir. length `relative to the length of the large cross-section portions. The heat flow from the reduced cross-.section portions to the large'cross-section portions of the link extending over a considerable'period of time prior to initiation of the interrupting process results in a more even temperature distribution along the entire length of the link than in any prior art low voltage current-limiting fuse. Because of that even pre-heating of the entire link before the ltime .of` arc initiation, a smaller amount of arcenergyneeds tobe dissipated for producing the gap VYreunited for eilecting completeA interruption ofV the circuitthan is required withoutsuchV anI even pre-heating of the` entire link length.

YIn the accompanying drawings:

Fig. l is a longitudinal cross-sectionalview of a fuseembodying the present invention;

Fig. 2 is a cross-sectional view along line A-A of Fig. 1;

Fig. -3 is a diagram illustrating the` decrease with arcing time of the voltage gradient at the point of arc initiation;

Fig. 4- is a diagramillustrating the behavior of prior artcurrent-limiting fuses and of current-limiting fuses.` of the present invention under short-circuit conditions;

Fig. 5 isa longitudinal cross-sectional View of a' modification of fthestructure shown in Figs. 1=and.2;

Fig. 6 shows, in-cross-section, a structure simila-r` to that shownV in Fig. 5, however, having a smallerv interrupting, capacity. rating;

Fig. 7 showsin cross-section, a: modification of thestructureshown in Fig. 6 particularly de'- signed to effect rapid'interruption of a` circuit after along-time delay-in caseV ofi relatively small overloads;

Fig'. 8v. is a` cross-sectional view on a larger scaleA along B -B of Fig. 7;

9 is a. longitudinal. cross-sectional view of a current-limiting fuse having.' arnller designed. toevolvea, controlled, limited amount of gas;

Fig. l() shows in front elevation and partially Kinross-section a. composite fuse having a very high current-carrying capacityintended for use in.A circuits having. very highr available. short-circuitlcurrents; and

Fig. 11- is a cross-sectional' view. alongCe-'C of AFig-10.

In all'gixresathe. same rei'erenoefsigns-fare an plieditorlikezparts; l

VReferring now to Figs.. 1 and 2referenceV numerals; l.' and*- 2ivv are applied: tothe terminal elementsy or caps aarranged. on opposite. ends of casing,3. 'The fuse. link `4v interconnects the terminalel'ements Iandz2 and mayrbespotwelded to them. The terminal elements I and 2 and the fuseflinkrare inserted in an electric circuit having a: circuit voltage notexceeding 600 volts. Thisisthe voltage range for which the fuses according to the present invention are primarily intended; 'I'he circuit into which the fuse is insertedfhasfbeen indicated diagrammatically in Fig. 1 byfconductorsiaiand 5b. The fuse shown int Figs. 1` andl2 lends itself for use in A.C. circuits'as'well aszinzD.-C'; circuits, as will be more fully. explained.' below. The fuse link `4 has a plurality. of portions. 4a of reduced crossfsection spaced: substantially eduidistantly along substantially'the entirelength of link 4 and a plurality ofportions 4b of relatively larger crosssection intermediate said reduced cross-section portions. Each reduced cross-section portion has-a suiil'cientlyV small cross-sectional area to melt andito initiate arcing atr instantaneous current intensities far. below the available shortcircuit current of thecircuit into` which thefuse is inserted. The available short-circuit current oftheicircuit into 'which the `fuse is inserted may, for instanceybe inthe order of 10i peak amperes and arcing'may be initiatedat an instantaneous current intensity of' i; peak amperes, or 10% of tbefavailable` short-circuitcurrent. Each relativelylarger cross-section` portion 4b has a. length w-liiclrl isa multiple off the length'of'the reduced cross-sectionportions 4a. The casing 3 is lled with a pulverulent"arc-extinguishingller 6, e. g. cleaniquartz sand, surrounding fuse link 4. The filler 6- has a: suiciently high arc-extinguishing capacity in relationto they number o' points of are initiation-Which is obviously the same as the` number of portions 4a ofl reduced cross-section-to effect extinction of any short-circuit current arc that the circuit into which the fuse is insertedL is capable of producing in less than 1/120 of a second after'the time of fault inception. In other Words, if the fuse is inserted into an A.C. circuithaving a frequency of`60 cycles per second, the time during which a short-circuit currenty isepermitted to rise plus the arcing time will be less than half a cycle. If the fuse is designed to interrupt the circuit with a'minimum of aro energy, the time during which the short;- circuit current is permitted to rise plus the arce ing time will be much less than half a cycle of the current wave. A fuse according to the present invention, when interrupting a short-circuit currentina D.C. circuit, will have an arcing timeanddissipatean amount of arc venergy comparable to,th.e,A.-C. case provided that all other conditions are similar. The total length of the reduced cross-section portions 4a andof the relatively larger cross-section portions 4b exceeds the length of link 4 which is vaporized under the most onerous circuit interrupting conditions toY which the fuse may be subjected in the circuit into which it is inserted. Hence, the fuse link 4 will never` be entirely vaporizecl by arcing. The portions of vthe link which willnot be vaporized even under the most onerous circuit interrupting conditions, i. e. when the amount of arc energy is an optimum, has been indicated in all gures by cross-hatching. The cross-hatched portions of the link may either retain their complete integrity after the fuse has blown, or they may be broken up into a plurality of separated metal parts. However this'may be, the appearance of the cross-hatched portions of` the link after the fuse has blown is very different from the appearance of the other portions thereof since the cross-hatched portions retain substantially their metallic nature while the other portions are converted into fulgurites which are in the nature of glass rather than that of a metal.

The required number of portions 4a of reduced cross-section and the total length of the reduced cross-section portions 4a and of the relatively larger cross-section portions 4b depends upon such factors as the circuit voltage, the available short-circuit current and its rate of rise, the ratio between the cross-sectional area of the reduced cross-section portions 4a and the relatively large cross-section portions 4b, the metal of which link 4 is made, the arc-extinguishing capacity of ller B, the pressure prevailing within casing 3 during interruption, etc. Correct data for the required number of reduced cross-section portions lia, and for the total length of the reduced cross-section portions 4a and of the relatively larger cross-section portions `4b can readily be determined by power laboratory tests. Such tests can be greatly simplied by subjecting sets of experimental fuse structures of the type shown in Figs. 1 and 2 to various interrupting conditions and observing in a radioscope the pattern resulting from blowing of the fuses. If that pattern approaches that indicated in Fig. l, i. e. comprises a number of fulgurites resulting from vaporization of portions of the link and melting of the surrounding pulverulent ller alternating with more or less metallic portions, this is an indication that the fuse is proportioned substantially in accordance with the teachings of the present invention. After having found a relativelyough approximation to the required proportions, proportions assuring optimum effectiveness of the fuse structure may be determined by continued testing of varying structures along the established lines.

A point to consider in proportioning a fuse according to the present invention may be the inductive voltage surge that is permissible in a particular circuit. On the basis of a given limitation of the permissible inductive voltage. surge, the number of portions 4a of restricted crosssection may be determined as follows:

Let r be the total arc resistance of a given fuse and rx the arc resistance developed at each point of reduced cross-section of the fuse link. Then v mi; (l) Assuming the fuse is placed in a D.C. circuit whose circuit voltage is E volts and whose inductance is L henries. The voltage equation for such a circuit is A =Er.,i (2) wherein Us is the instantaneous arc voltage and 1a the instantaneous arc resistance.

Because of the limited insulation level of any electric system, it may be necessary to impose certain limitations as to the instantaneous voltage di Ln along 'theinductance In low'voltage systems the permissible inductive voltage may be in the order of several hundred percent of the circuit voltage. It may, however, be assumed that arc resistance rs -be introduced into the circuit at such a rate as to induce exactly the voltage 2E in the inductance L. Then In the above equations z' is the current which nows in the circuit at the time tz() of arc initiation. The value rx must be determined experimentally. t may be necessary to admit higher induced voltages than 2E, and the above calculation can be repeated in a similar fashion for any permissible upper limit of the inductive voltage A fuse structure ofthe type shown in Figs. l and 2 may require special consideration of the problem of limiting the inductive voltage incident upon blowing of the fuse, particularly in case of a fuse used in circuits involving higher circuit voltages than the 600 volt range; for which fuses of the present kind are primarily intended. Where a plurality of fuse structures of the present invention are arranged in parallel in a low Voltage circuit, the danger of inductive voltage surges is minimized on account of sequential blowing of the fuses and that danger hardly needs any consideration.

The operation of a fuse of the type shown in Figs. 1 and 2 is in substance as follows: On the occurrence of a small but protracted overload say, in the order of 8 to 10 times the rated current of the fuse, a considerable time is required for increasing the temperature of the portions da of reduced cross-section to melting temperature. The long time element involved permits a thorough heat exchange between the portions 4a of reduced cross-section and the portions 4b of relatively larger cross-section. Because of that thorough heat exchange, the temperature distribution along the entire length of the fuse link 4 will -be relatively even, i. e. the difference between the hottest portions of the link and the coolest portions of the link will be relatively small. In other words, on the occurrence of a relatively small but protracted overload the fuse link 4 will be thoroughly pre-heated along its entire length before melting and arc initiation occurs at the points of reduced cross-section. Because of the preheating of the portions 4b of relatively largercross-section, a relatively small amount of energy is required after arc initiation at the reduced cross-section portions 4a, for converting a certain percentage of the portions 411 into vapor and thus producing the required interrupting gap. The amount of heat stored in the portions 4h prior to arc initiation asaaaea 9.j greatly reducestheamountof.heat that must be generatedatthe Vportions 4b afterarc initiation to produce the requiredY interrupting Vgap and hence greatly reducesthearcing. time and the arc energy involvedinthe interruption of overload currents r of relatively small magnitude.

For. a better understanding of4 the operationzof fuses according to the present.inventionreierence is made to-Fig. 3.- Figirefers to ay prior art current-limiting low voltage fuseV andshows the voltage gradientatithepointlof arc initiation plotted against time.4 The diagram of.,..Fig. 3 was obtained by impulse testswith substantially constant current. The record starts with a voltage gradient ofV 300 voltsr perrcentimeter and drops within-40 milliseconds to less Ythan.15.volts per centimeter. The decrease of the voltage gradient and of the resistance of thearc gap at the point of arc initiation with arcing time is due to progressive heating of the pulverulent: arc extinguishing filler at this particular point. Such-a decrease of arc resistance at-the point of arc initiation must be over-compensated by `an increase of the resistance at other points, i. e. by progressiveV gap elongation resulting from progressive vaporization of the fuserlink.v That process of gap elongation involves relatively long arcingA times and relatively large amountsof arc energy'if relatively cold link metal must be vaporized bythe heatof the arc. The process of producing arelatively large arc gap involves also -the generation of-relatively large amounts of metal vapor-adverse tO-arc extinction.A Providing a plurality of portions of reducedcross-section along the fuse link and evenly pre-heating of the entire fuselink before the time 4of arc initiation permits a reductionof both arcing time and arc energy. Reduction of the arcingtime,V in'turn, means that the circuit is interrupted Ibefore a serious decay of the voltage gradient at the point of arc Vinitiation has vtaken place.

The amount of metal which is-vaporized in a current-limiting fuse depends,f aside fromthe nature of the pulverulent arc-extinguishing filler, upon the current density in the fuse-link. The speedatwhich the length ofthe arc gap increases may be expressed by theV equation'- wherein@ isthe speedof Agrowth of the'gap length, I the rate of current flow, f the crosssectional area of thelink and C a constanthaving the dimensionrcmA peramp. Thevaluesofthe constant C have been determined with various test arrangements andtit was` found that they varied over a wide range. Large values oiC. are

obtained where the link is pre-heatedalong.its-

entire length before arc initiation occurs; .whereas small values of C are obtained Where theportions of the link immediately. adjacent thepoint of arc initiation are relativelyY cold. This makes it clear why under otherwise similar conditions a link having a plurality of points of reduced cross-section uniformly distributed substantially along the entirelength of the link so as to cause an even pre-heating of the entire link in case of relatively small protractedoverloads is con ducive to a more rapid growth of the total gap length than a link in which relatively cold link metal must be brought up to vaporizationtemperature to produce, thegap length requiredfor final interruption of the circuit. The plurality of portions of reduced cross-sectionthus have the cumulative effect of limiting the decay of the Kw. seconds Energy' 'required `fcr raising fthe temperature from()r deg; centtto thefusing point 226 Latent heat' oi melting 1.2 Energy' required for" raising temperature from fusing point' to the boiling point.; 3.7 Latentheat'cfvaporization' 23;'0

Total 30:5

Where-small currents are involved, a currentlimiting fuse may be expectedto work the better, the larger the. portion of the total energy requiredfor vaporizing'the link is supplied by the circuit prior to arc initiation and the smaller the portion-of the total energy required for va'- porizing'i-.thelink is suppliedl by the circuit-in formiof are energy. The geometryY of the fuse link shown in Fig. 1 complies evidently well with this requirement.

On the occurrence. of' afault current in the nature of a short-circuitfcurrent, arc initiation is eiected' inv an f extremely Yshort time after inception of the fault; That time is much:l too short to permit any appreciable pre-heatingv by heatl exchange of portionsA lb by heat generated at the portions 4a. Consequently allthe energy that is neededvfor producing. the required interrupting gap must be furnished in formof arc energy by the circuit under interruption. The portions b serve on the occurrenceofffault currents in the nature of short-circuit currents as a means for absorbing large amounts of energy at an extremely rapid rate. TheyV and-the ller 6 enable to absorb inmuch less than half a cycle of the current wave'all the magnetic en-i ergy inherent in the circuit at the time-of arc initiation.

It appears from the foregoing that the-relef tively'large cross-sectionportions 4b pre-store heat in case of low current circuit interruptions, thus limiting the amount of arc .energy-.1 that mustbe furnished by the circuit to effectv alcom'zplete interruptionV thereon while vthese, portions help to rabsorbthe `large amounts of energy that are present inthe circuit'at Athe time of'arcLinitia'- tion in the: case of faults0 in theznatureiof'sh'ortcircuits.

Fig. 4 shows,.diagrainmatically the', behavior of prior art current-limiting fuses and' of 'current-limiting fuses of the present' invention under'l short-circuit conditions. V represents they generated voltage plotted versus time. The short'- circuit is assumed to occur at the time t1. The time elapsingbetween the time of Voltage zero to and the time t1 of fault inception isfoftenA expressed in electricalv degrees and-referred toas fault starting angle 0. The fault starting angle 0 hasanimportant eiect upon severity ofv interruption, since itl determines. theVl displacement of the lshort-circuit current. The displacement will behigh, i. e. the short-circuit current will: be highly asymmetrical, if` the fault starting angle 0 is close to zero. Fig. e refers toa case wherethe short-circuit tends to` ber-followed byai major current loop indicated by the referenceV sign I. Where the fault is initiated at the peak of. vthe voltage wave, i. e. where thefaultstartin'g angle 1l is 90, the short-circuit current is fully symmetrical. When interrupting short-circuit currents maximum severity obtains at such a fault starting angle as to cause melting of the fuse link and initiation of arcing shortly prior to, or at the time when the voltage wave V passes through its peak. Maximum severity obtains if arcing is initiated between 70 and 80 degrees on the voltage wave. Fig. 4 clearly refers to two cases where arcing is initiated shortly .prior to the time when the voltage wave 'passes through its peak. Incase of the prior art current-limiting fuse the current rises up to the peak i max. of the letthrough current. That peak occurs a few electrical degrees prior to the peak of the voltage wave V. The peak i max. of the let-through current'is reached at the time ofv arc initiation. In the time between arc initiation and the subsequent zero of the voltage Wave enough arc resistance is established in the fuse to force the current down to zero. The are energy released during the time of initiation of the arc and the ti-me of its permanent extinction is an important 'severity factor. The total arc energy is determined by the equation wherein Vs is the arc voltage and is the arc current. The total arc energy may be determined from oscillograms by integrating the product of arc voltage Va and arc current is over the entire arcing period.

The peak current at the time of arc initiation is one of the most characteristic data determining the behavior of a current-limiting fuse under short-circuit` conditions. The maximum letthrough current i max. of a fuse of the present invention is shown in Fig. 4 to be quite smaller than the maximum let-through current i max. of a prior art current-limiting fuse having the same current and Voltage ratings. The arc extinguishing capacity of the arc-extinguishing means, including that of the ller 6 of the fuse of the present invention, are so related to the mass of metal of which the fuse link consists that the arcing time is limited-even under the most onerous short-circuit interrupting conditions to which the fuse may .be subjected in the circuit with which it is associated-to the range of l to 2 times the time elapsing between initiation of the short-circuit and initiation of arcing. In other words, even if the fault occurs at such a fault starting angle 0 as to cause arc initiation close to the peak of the generated voltage, the 4time required for permanent extinction of the arc will be within the range of l to 2 times the time elapsing between initiation of the fault and initiation of arcing.

The drastic reduction of the peak of the letthrough current i max., and the consequent drastic reduction of the a-rcing time and of the arc energy has been achieved in the fuse of the present invention by drastically reducing the total amount of link metal and particularly the amount of link metal that must be heated up to the time of melting and arc initiation. This drastic reduction in the amount oi metal that must be heated to the melting point to initiate arcing is achieved by the provision of the extremely short portions 4a. of reduced cross-section of the link d. These portions do not, in spite of the smallness of their cross-section, result in la, relatively low current rating of the fuse because they are eiectively cooled during normal operation of the fuse by the large masses of metal constituted by the immediately adjacent portion 4b of relatively larger cross-section. In other words, the reduced cross-section portions greatly reduce the peak of the let-through current z" max. without aiecting the normal current carrying capacity of the fuse. It is possible to minimize the total mass of metal of the link and that present at the reduced cross-section portions 4a to such an extent that the resultant reduction of the peak of the let-through current i max. causes a limitation of the arcing time to 1 to 1.5 times the time elapsing between initiation of a short-circuit and initiation of arcing. By suiiciently reducing the total mass of link metal and particularly the mass of metal at the points 4a such a limitation may be caused even where the short-circuit is initiated at such a lead angle that initiation of arcing occurs substantially at, or just before, the peak of the voltage wave V.

When reviewing random oscillograms showing the operation of prior art current-limiting fuses having conventional fuse links it is possible to discover some interruptions wherein the arc lasts not longer than l to 1.5 times the time elapsing between fault initiation and are initiation. In the prior art current-limiting fuses such short arcing times can only be observed where the interrupting conditions are not severe, i. e. when arc initiation occurs relatively close to the zero of the voltage wave V. In fuses of the present invention the arcing time is as short under the most onerous or severe interrupting conditions as with prior art current-limiting fuses under most favorable or relatively favorable interrupting conditions. This is due, as previously mentioned, to a proper balance between the arc extinguishing capacity of the arc extinguishing means, particularly the pulverulent ller within the fuse casing, and the peak of the let-through current. Since the arc extinguishing capacity of the arc extinguishing means cannot be greatly changed-it is mainly determined by the thermal characteristics of the best grade of quartz sand used as a fuse filler-the peak of the let-through current had to be brought down by appropriate reduction of the mass of metal melted prior to initiation of arcing under short-circuit conditions to enable shortening of the arcing time and reduction of the are energy beyond the limits attained in prior art fuses. By properly designing the restricted cross-section portions 4a of the link 4 the arcing time may consistently be limited to 1 to 1.5 times the time elapsing between fault initiation and arc initiation even where the fault is initiated at such a lead angle that initiation of arcing occurs within the range of 20 degrees prior and 10 degrees after the peak of the voltage Wave V.

In order to control the let-through current to an extent which will effect a consistent reduction of the arcing time of a high current carrying capacity current-limiting low voltage fuse, the amount of metal in the fuse link must be reduced to a minimum by reducing the length of the fuse link as much as possible. There is a maximum critical link length for fuses having a voltage rating Within the limits of 600 and 1000 volts. That maximum critical link length is about 2 inches.

In the modiiication of the structure of Figs. 1 and 2 which is shown in Fig. 5, each of the reduced cross-section portions is constituted by a pair of parallel

current paths

4a and 4a. Be-

13 cause of the subdivision of the portions of reduced cross-section intoV twov parallel current paths 4a. and 4a", upon melting of thecurrent paths 4a. and 4a two parallel arclets will be initiated. It is well known that arcs in parallel are unstable and this tends to facilitate arc extinction. Because of small differences dueto manufacturing tolerances between the current paths 4a. and 4a, there tends to be a slight difference between the time when arclets are initiated atv these two points. Similarly. extinction of the arclets formed at these two points occurs sequentially rather than simultaneously. Sequential initiation and sequential `extinction of the pair or" arclets formedfat each point of restricted cross-section of the fuse link'r tends to limit the rate of change of current andY therefore to limit inductive voltage surges incident upon interruption of relatively highly inductive circuits.

As shown in Fig. 5, the fuse link` 4 has a plurality of rectangular perforations 8 arranged along the center line thereof. These perforations are spaced substantially equidistantly and extend substantially along the entire length of the link. Each perforation 8 is laterally bounded by a pair of conductive elements forming two separate parallel current paths 4a', 4a. The cross-section of these parallel current paths` 4a' and 4a" is much less than the cross-section of the portions 4b of the link arranged intermediate the perforated portions thereof. The length of each intermediate portion 4b is a multiple of the length of each perforated portion. The total length of the perforated portions and of the intermediate non-perforatedportionsexceeds the length of the link which is vaporized under the most onerous circuit interrupting conditions to which the fuse may besubjected in the circuit with which it is associated. The portions of the link 4 which remain substantially intact, i. e. which are not vaporizedeven if the fuse is subjected to the most onerous interrupting conditions which the circuitcan produce which the fuse is called to protect are indicated by cross-hatching in Fig.f5. The abovemay also be expressed by saying that the serially related arclets which form at the reduced cross-section or reduced width portions of the ribbon-type fuse link never merge into one single long arc, this being due to the relative length of the larger cross-section or larger Width portions of the link.

As previously mentioned, upon blowing of the fuse under short-circuit conditions, the pulverulent arc extinguishing ller 6 forms a fulgurite, or rather a plurality of aligned fulgurties. In order to obtain a fuse structure which is as compact as possible, the diameter of the casing 3 should be substantially equal to the diameter of the fulgurite or fulgurites formed by the filler E upon vaporization of the link under the action of a current of short-circuit current proportions. In prior art current-limiting fuses the diameter of the casing is generally much larger than the diameter of the fulgurite or fulgurites formed Within the casing, since it was thought, heretofore, that it is necessary to provide a large excess of ller, for cooling sufficiently the gaseous products of arcing, i. e. a large amount ofV filler not melted under short-circuit conditions and not converted into a fulgurite. It has notonly been found that thisV is noti correct, but that it. is desirable, for reasons other than that of compactness of design, to limiti the diameter of the casing substantially to that of the. fulgurite or fulgurites formed upon vaporization of the fuse link under Short-circuit conditions. The larger the diameter of the casing in excess of the diameter of the fulgurite, the larger the heat dissipating ability of the fuse structure, the larger the amount of heat which must begenerated by 121' losses within the fuse to obtain the desired current-time characteristic. Therefore, in order to minimize 121' losses, the diameter of the casing 3 should only slightly exceed the diameter of the fulgurite or fulgurites which form when a current-limiting fuse is subjected to a shortcircuit current.

The casing 3 is preferably made of an organic combustible material, such as hard fiber. The diameter of the casing 3 should slightly exceed the diameter of the fulgurite or fulguritcs formed upon blowing of the fuse, not more than necessary to preclude substantial scorching ofthe material of which-the casing 3 is made by the'fulgurite or fulgurites which form when the fuse blows. Hard fibre and other organic materials evolve gases under the action of heat and gases which may evolve from casing 3 tend to provide a protective layer between thev casing and the fulgurite formed therein, reducing the tendency of the former to bedestroyed by the heat of the latter.

In a fuse of the type shown in Figs. 1, 2-and 5, where the fuse link 4 is provided with a plurality of reduced cross-section portions, the diameter of the fulgurite or fulgurites is inthe order of the distance between the reduced cross-section portions of the link. Hence the diameter of casing 3 should be in the order of the distance between the reduced cross-section portions.

Fig. 6 illustrates the features of the invention embodied in a current-limiting fuse having a relatively small current rating. The fusible element has but three portions 4a of restricted crosssection. These portions are formed by circular perforations rather than by rectangular perforations, as shown in Fig. 5.

The fuse illustrated in Figs. 7 andr 8 comprises a link 4 having veportions 4a.', 4a. of restricted cross-section formed by provision of iive circular perforations in the fuse link; The perforation situated in the center of the link 4 is provided with a rivet-like insert I0 as can best be seen in Fig. 8. Insert l0 consists of a metal having a lower fusing point than the metal of which the fuse link 4 is made and adapted to form anV alloy with the metal of which the fuse link 4 is made having a lower fusing point than the metal of which the fuse link 4 is made.

In a fuse as shown in Figs. l, 2, `5 and 6 the point having the highest temperature is the zone of restricted cross-section situated midway between the ends of the fuse link 4. It is at this point that arcing is initiated'at the occurrence of very small overloads of inadmissible duration. Insert l0, owing to its heat absorbing capacity, delays the rate of increase in temperature of the middle portion of the fuse link. When insert i0 melts at a given temperature, its melting results in the formation with metal of'which link 4 is made of an alloy having a relatively low melting temperature. This is conducive to rapid fusion of the middle portion ofthe link 4, once insert I0 has reached its melting point.

The insert I0 may consistof asuitableeutectic alloy to achieve interruption'of' the circuit at a temperature which can be predetermined with a high degree of accuracy. However, satisfactory results may also be obtained with a tin insert in a link of silver.

In prior art fuses comprising a metal having a relatively low fusing point intended to form upon fusion thereof with a high fusing point metal a low fusing point alloy, formation of the alloy took generally piace during normal operation of the fuse rather than at the time when the fusing temperature of the low fusing point metal was reached. Such a gradual conversion of two metals having different fusing points into an alloy having a relatively low fusing point is highly objectionable since it results in a gradual change of the current-time characteristic of the fuse. An objectionable change of that kind of the current-time characteristic of a fuse is often referred-to as aging The structure of Figs. '7 and 8 is characterized in that it minimizes, or completely avoids, aging of the fuse. This is probably due to the fact that the surface along which there is immediate contact between the two metals is minimized as long as the rivet-like insert l is unfused. The shank a of rivet-like insert Hl` is probably covered with an oxide film tending to preclude the insert from reacting With the surrounding metal of which the fuse link 4 is made. This tendency is increased by the formation of a small gap between the shank Illa of the rivet-like insert lil and the surrounding fuse metal. The upset head lb of rivet-like insert l can effectively be separated and isolated from the adjacent metal of which the fuse link i is made by means of a layer il of a suitable fluxing agent, such as rosin. That layer Il prevents interaction of the two metals and formation of an alloy only as long as fusion of insert I has Vnot taken place, but is conducive to extremely rapid formation of an alloy when insert Il! ultimatelymelts during a period of a small but protracted overload.

Sound design of a fuse requires that the amount of heat generated therein while it is carrying current of permissible magnitude be minimized, and that whatever heat is'generated therein be dissipated in such a way as to keep the fuse-reasonably cool while it is carrying current. In high capacity and particularly current-limiting fuses an additional problem of a thermal nature arises. When such a fuse is caused to blow by a fault current of short-circuit current proportions, a relatively large amount of heat is generated within the fuse at an almost explosive rate, and therefore the temperature prevailing within the fuse tends to be high. If blowing of a fuse by a fault current of short-circuit current proportions is preceded by a` smalll pro.- tracted overload, the temperature of the portion of the fuse tube or casing midway between its ends tends to be already relatively high before an additional large amount of heat is released in explosion-like fashion. There isa general tendency, particularly under the above described conditions, that .fuse `tubes or casings of current-limiting fuses become damaged, i. e. charred, shortly after successfulvv circuit interruption has occurred, when the heat generated during the interrupting process atv the arcing zone is transferred to the axially inner .portion of the fuse tube or` casing, where the temperature always tends to be highest. Such damage can be preventedby providing a good thermal insulation between the axially inner portion of the link and the fuse tube or casing, thus diverting the destructive radial heat flow that tends 'to occur after blowing of a fuse in a direction longitudinally of the link and precluding overheating of the center portion of the fuse tube or casing. This is achieved in the embodiment of the Ipresent invention shown in Fig. 9 by providing different arc-extinguishing pulverulent fillers I2 and 6 Within the fuse or casing 3i. e. gypsum at the axially inner portion and quartz sand at the axially outer portion. Gypsum has a much smaller thermal conductivity than quartz sand and thus protects effectively the most endangered portion of the fuse tube or casing 3 against excessive heating. Quartz sand is a far more effective cooling and deionizin'g medium than gypsum and ensures the currentlimiting action of the fuse. Quartz sand is a fulgurite-forming substance which evolves virtually no gas under the heat of the arc, while gypsum is not fulgurite-forming and evolves relatively large amounts of gas under the heat of the arc. The combination of quartz sand and gypsum or similar gas evolving substances evolving relatively large amounts of gas under the heat of the arc renders it possible to control with great accuracy the amount of gas generated as well as the amount of gas pressure Within the casing.

It was believed, heretofore, that currentlimiting fuses should preferably not be provided with gas evolving fillers, i. e. fillers which evolve appreciable amounts of gas under the heat of the arc, because of the difficulties involved in limiting the pressure within the casing below dangerous pressure levels, i. e. pressures which involve the danger of bursting of the casing or fuse tube. To provide current-limiting fuses only with quartz sand or similar non-gas-evolving fillers means to foregothe advantages resulting from the increased dielectric strength of gases under pressure. The structure of Fig. 9 permits to combine effective current-limiting action with controlled generation of gas Within the casing or fuse tube 3. The pulverulent fulgurite-forming arc-extinguishing ller 6 surrounds all the reduced cross-section portions 4a of the link but one. The pulverulent gas evolving non-fulgurite forming ller I2 surrounds but one reduced cross-section portion of the link. Preferably llers 6 and l2 are separated by a pair of insulating barriers I2a.

In addition to precluding charring of the center portion of the fuse tube or casing 3 and of causing a limited rise of pressure therein, the arrangement shown in Fig. 9 comprising arcextinguishing fillers I2 having different properties, at different points of the fuse tube or casing 3, has a further important advantage. The fulgurite or fulgurites formed in currentlimiting fuses are relatively good conductors of electricity. They lose their conductivity relatively rapidly with decreasing temperature.v If the rate of decrease of temperature of a fulgurite is not sufficiently rapid, a small current will continue to flow through the fulgurite for a short time after extinction of the arc and interruption of the circuit have been achieved. That small, undesirable current is often referred to as the follow current. 'Ihe gypsum filler l2 in the structure of Fig. 9 has a relatively small energy absorbing capacity, but it provides an effective insulating barrier between the fulgurites formed on both sides thereof and thus precludes the flow of a follow curren while the fulgurites are still relatively hot and semiconductive.

It is Well known in the art of current-limiting fuses to surround the middle or center portion of the .fuse link with a thermal insulator, e. g. glass beads. This is, however, done for controlling the fusing characteristics of the link rather than for any of the aforementioned purposes.

The fuse units shown in Figs. 1, 2, 5 and 6 to 9 have a relatively small current carrying capacity. These fuse units are intended to be combined into a composite current-limiting fuse Where a relatively high current-carrying capacity is required. Such a composite high current-carrying capacity current-limiting fuse is formed by an assembly of a plurality of identical separate fuse units adapted to be arranged in parallel in an electric circuit. An embodiment of such an assembly or composite fuse is shown in Figs. .and l1. A structure of the kind shown in these figures is capable of carrying continuously 4,000 R. M. S. amps., and if desired even much higher currents.

Referring now to Figs. 10 and l1, the composite fuse comprises two terminal elements I5 arranged on opposite ends of an outer shell or housing I6. The terminal elements I5 are in the form of cylindrical 'caps integral with 'knife blade contacts a adapted vto lbe inserted in a suitable fuse holder `(not shown). Shell l5 is secured to terminal elements i5 by screws i8 or the like. A plurality of fuse uni-ts of the type shown in Figs. l, 2, 5 Vand 6 to 9 is arranged within the shell vor housing I6. These units are adapted te bridge or inter-connect terminal elements l5,

-thus bei-ng Aarranged in parallel in the electric circuit to -be protected. Each said units 20 comprises a casing ymember 3, terminals caps I, 2 on opposite -ends thereof, a fuse link 4 having a plurality `of portions 4a of restricted crosssectionakl -area arranged within said casing member I3 and a .pulverulent arc-extinguishing filler .1.6 -likewise arranged .within casing member 3 and surrounding link '4. The upper terminal caps 2 0f the fuse units 20 are in .contact With .the upper, and the lower terminal caps l of the fuse units 2-0 are in contact with the lower, Itermi-nal element l5. These fuse units .2B are :grouped in yspaced relation to form a .pattern of concentric shells, I1, Ila, I'lb, etc., to cause the total current flowing through said -fuse :units 2-0 to be unequally distributed among fuse units 20 pertaining to different shells I'l, Ila, -I'I-b, etc. This, in turn, causes fusion of the portions a of restricted cross-sectional :area of links of Afuses 2d pertaining to different shells Il, I'a, Iib, etc. to occur at slightly different points of time. These time differentials are helpful in limiting the rate of change di dt of the total current at the time the peak of the let-through current is reached, thus limiting the magnitude of possible inductive voltage surges. The arrangement of individual fuse units 2'0 in a pattern of concentric shells tends 'also to establish a magnetic blowout effect by which the arc stream formed in each individual fuse unit 2i) is impelled in a lateral direction and its `rate of deionization is increased. This magnetic 'blow-out effect and the unequal distribution of the total current among the individual 'fuse 'units 20 'by which the composite assembly ls constituted may be further increased if the currentpath throughthe holder for the fuse and the fuse is substantially U-shaped, resulting in a magnetic loop action upon the arc current in each individual fuse unit 20. The spaces between the casings 3 of the individual fuse units 20 are 'lled with a granular inorganic cooling substance I9, such as quartz sand, tending to maintain the fuses in their shell pattern and providing a safety feature in case that the casing 3 of any of the individual fuse units 20 should accidentally burst on account of an excess of pressure generated therein.

As lpreviously mentioned, current-limiting fuses are generally filled with quartz sand to the exclusion ,of any arc-extinguishing substance which evolves substantial amounts ofgas under the heat of the arc as, for instance, gypsum ror marble powder. The `arrangement shown in Fig. 9 of a quartz filler at the axially outer ends of the casing and of a gas-evolving filler midway between the ends thereof is a first step toward an vaccurately pressure-controlled current-'limiting fuse. The second step consists in providing venting means in the nature of a safety valve of utmost simplicity and reliability in combination Awith means for precluding any damage, such as 0ccurrence of an electric breakdown, resulting from the egress of lextremely hotarc products from the casing of the individual fuse units 20. In the structure of Figs. 1.0 and 11 the terminal caps I and 2 of the individual fuse units 20 are but relatively loosely secured to the casings .3 of the individual fuse units. The casing 3 and either one. but preferably both, terminal caps I and2 of each fuse unit 20 define a small leakage gap therebetween. vThis leakage path permits a restricted escape of hot gases evolved within any casing 3 to the body of sand I0 by which the individual fuse units are surrounded. .Such ,a Venting arrangement is obviously much less vexpensive lto provide than the conventional prior art venting arrangements in non current-limiting fuses which comprise small venting holes in their terminal caps. The labyrinth leakage path resulting from relatively loosely mounting rthe terminal caps I and 2 on 'the casings 3 ,is much more effective in cooling of hot gases and in preventing loss of pulverulent arc-extinguishing filler than Yany prior art venting means has been. The provision of the body of Sand I 9 immediately adjacent the circular outlet of the safety leakage gap formed between the caps vI and 2 and the casings 3 is of vital importance as evidenced by the occasional formation of a ring of melted or chemcally affected sand immediately adjacent the circular outlet of the safety leakage gap.

Reference is made to copending application Serial No. 60,328, lfiled November 16, 1948, for a more detailed disclosure of the composite structure of Figs. 10 Aand 1-1.

It willbe understood that, although but a few embodiments `of this invention have been shown and described in detail, the invention is not limited thereto and that the illustrated embodiments may vbe 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:

l. In combination an electric circuit having a circuit voltage not exceeding the 600 volt range, a current-limiting fuse arranged in said circuit, said fuse comprising a casing, terminal elements on opposite ends of said casing, a fuse link interconnecting said terminal elements and having a plurality cf portions of reduced cross-section 19 spaced substantially equidistantly along substantially the entire length of said link and a plurality of portions of relatively larger cross-section intermediate said reduced cross-section portions, each said reduced cross-section portion having a sufliciently small cross-sectional area to initiate arcing at instantaneous currents far below the available short-circuiting current of said circuit, each said relatively larger crosssection portion having a length being a multiple of the length of said reduced cross-section portions, a pulverulent arc-extinguishing ller Within said casing surrounding said links, said iller having a sufficiently high arc-extinguishing capacity in relation to the number of points of arc initiation to effect extinction of any shortvcircuit arc that said circuit is capable of producing in less than 1/120 of a second after the time of fault inception, the total length of said reduced cross-section portions and said relatively larger cross-section portions exceeding the length circuit, said fuse comprising a casing, terminal elements on opposite ends of said casing, a fuse link interconnecting said terminal elements and having a plurality of portions of reduced crosssection spaced substantially equidistantly along substantially the entire length of said link and a plurality of portions of relatively larger crosssection intermediate said reduced cross-section portions, each said reduced cross-section portion having a sufiiciently small cross-sectional area to initiate arcing at instantaneous currents far below the available short-circuit current of said circuit, each said relatively larger crosssection portion having a length being a multiple of the length of said reduced cross-section portions,l a pulverulent arc-extinguishing ller Within said casing surrounding said link, said filler having a sufficiently high arc-extinguishing capacity in relation to the arcs initiated at said portions of reduced cross-section to eiTect complete interruption of any short-circuit current that said circuit is capable of producing in less than one half cycle of the current wave after the ltime of fault inception, the total length of said reduced cross-section portions and of said relatively larger cross-section portions exceeding the length of said link vaporized under the most onerous circuit interrupting conditions to which said fuse may be subjected in said circuit.

3. In combination an electric circuit, a currentlimiting fuse arranged in said circuit, said fusev tions Ibeing constituted by a pair of conductive elements forming separate parallel current paths, each said relatively larger cross-section portions having a length being a multiple of the length of said reduced cross-section portions, a pulverulent arc-extinguishing nller within said casing surrounding said link, said filler having a suiciently high arc-extinguishing capacity in relation to the arcs initiated at said portions of reduced cross-section to effect permanent interruption of any short-circuit current that said circuit is capable of producing in less than 1/120 of a second after the time of fault inception, the total length of said reduced cross-section portions and of said relatively larger cross-section portions exceeding the length of said link vaporized under the most onerous circuit interrupting conditions to which said fuse may be subjected in said circuit.

4. In combination an electric circuit, a current-limiting fuse arranged in said circuit, said fuse comprising a casing, terminal elements on opposite ends of said casing, a fuse link interconnecting said terminal elements, said link having along the center line thereof a plurality of rectangular perforations spaced substantially equidistantly and extending substantially along the entire length of the link, each said perforations being laterally bounded by a pair of conductive elements forming separate parallel current paths, said elements having a sufciently small cross-sectional area to initiate arcing before a short-circuit current reaches the available short-circuit current of said circuit, the portions of said link situated intermediate said perforated portions thereof having a length being a multiple of the length ci said perforated link portions, a pulverulent arc-extinguishing filler Within said casing surrounding said link, said ller having a suiilciently high arc-extinguishing capacity in relation to the arcs initiated at said portions of reduced cross-section to eiiect permanent interruption or any short-circuit current that said circuit is capable of producing in less than 1/320 of a second after the time of fault inception, the total length of said perforated portions and of said intermediate portions exceeding the length of said link vaporized under the most onerous circuit interrupting conditions to which said fuse may be subjected in said circuit.

5. A current-limiting fuse comprising a casing, terminal elements on opposite ends of said casing, a pulverulent arc-extinguishing filler Within said casing, a fuse link Within said casing interconnecting said terminal elements and being surrounded by said iiller, said fuse link having a plurality of portions of reduced cross-section and a plurality of portions of relatively large crosssection intermediate said reduced cross-section portions, each said relatively larger cross-section portions having a length being a multiple of the length of said reduced cross-sections portions, and each said reduced cross-section portions being constituted by a pair of conductive elements forming separate parallel current paths.

6. A high current carrying capacity currentlimiting fuse having a voltage rating of 600 to 1000 volts comprising a casing, terminal elements on opposite ends of said casing, a pulverulent arc-extinguishing iiller within said casing, a fuse link within said casing interconnecting said terminal elements and .beng surrounded by said ller, said fuse link comprising a plurality of portions of reduced cross-section each being constituted by a pair of conductive elements forming separatev parallel current paths, said link comprising at lea-st one portion of relatively larger cross-section intermediate said reduced crosssection portions, said relatively larger cross-section portion having a length being a multiple of the length of said reduced cross-section portions.

atea-see 21 thetotal length vof 'sa-id link being approximately two inches.

"7. -A 'high Acurrent carrying vcapaci-ty cur-rentliiniting fuse having a voltage rati-ng--of-76U0 to "1000 volts ycomprising-'a casing, terminal elements on opposite 'ends of said casing, -'a 'pulverulent arc-extinguishing iil-ler within said-casing, a fuse link lwithin 'said casing interconnecting said terminal elements and being surrounded by `'said filler, 'said link comprising at least-two portions having rectangular perorations and at least 'one portion of relatively larger cross-'sectionbetween said Vperforated portions, the length -of said relatively larger cross-section `port-ion #being 1a -multiple 'of the length of each said `perforated porftions 'andthe tota-l length of said link being no vgreater than two inches.

-8. A high --current :carrying capacity currentllimiting fuse having a -voltage rating 'between 600 and 1000vol-ts comprising-'a casing, terminal elements on opposite fend-s of said -casing,a pulverulent arc-extinguishing filler within said -casing, a -f-use link-Within said casing-interconnecting "said terminal elements and `being surrounded '-by said ii-ller, said link having along the center than two inches, andthe mass of said"1inkbe`ing lsufficiently small and the arc-extinguishing l-capacity-'of said filler being -suiiic-iently'large to vlimit 'the -a-rcing time to 1 -to 2 times vthe ltime -e'l'apsin'g between -fault inception and arc initiation when said fuse is subjected to Athe-most severe interrupting conditions of the -circuit for Which-said fuse is intended.

9. A high current carrying capacity currentlimiting fuse having a voltage rating between 600 and 1000 volts comprising a casing, rterminal Velements on vopposite `ends-o1 `said casing, a pulverulent arc-extinguishing filler -wi-thin said -casing, a fuse link Within said casing interconnecting vsaid terminal elements and being sur-rounded 'by said iiller, said link having a plurality of por- -i tions of uniformly reduced cross-section 'and lat least one intermediate portion of relatively vlar-ger cross-section, said reduced cross-section -portions having a length being a fraction ofthe length of said relatively larger cross-section portion, the

l0. In combination an elect-ric circuit, .a current-limiting fuse arranged in 'said circuit, said 'fuse comprising Aa casing, terminal relements on opposite ends of said casing, :a Vfuse link elnterconnecting said terminal elements 'and havinga Aplurality of portions of reduced :cross-section, arc-extinguishing `means of predetermined 'arcextinguishing capacity including 'a pulverulent filler within said casing surrounding :said link,

l1522 1arc-extinguisliing means `being -suinciently .large to limi-tithe farcing time `ito l Vto f2 Ltimes fthe-time -elaps'ing between Ifault inception and :.arcsinitiaf-ti'on AWhen-said -fuse is subjected tov the :most severe interrupting conditions which can be `pro'dusaed 'fby ysaid Acircuit.

i111. current-ilimiting *fuse comprising a icasing, terminal elements on opposite ends -oftsaid casing, `a 1fuselink within said casing interconnecting `'said :terminal elements, :said nfuse 'lin'k having fa plurality of portions .of reduced crossfsec'tion Vand va plurality of 'portions of relatively :large -cross-'section intermediate said VYreduced cross-'section por-tions, arc-extinguishing Lmeans vof predetermined arc #extinguishing .capacity )including a pulverulent iiller within 'fsa'id fca'sing usurrounding said link, the lm'a'ss `of lmetal -.of '.said yli-n'k being Vsuiliciently small "to 'limit the'peak of -the let-throughl 'current to lsufiiciently flow 'values to ena-ble "said arc-@extinguishing means itc 'effect com-plete are extinction in 1 1501.5 times the-time vfelapsi-ng between initiation of 'a short-circuit land -i-ritiationfef arcin'g -when the short-'circuit lis ini-tiat'ed at .such a 'lead ang-le that initiation vof arc-ing A`occurs substantially 'at or just `befoifethe rpeak `of vtlre 'voltage' l-wave.

112. IA current-limiting ruse comprising :a icasing, termina-1 `elements on .opposite ends .-of `said casing, la v-iuse 'link within said 'leasing interconlne'cting ,said terminal elements, Vsaid 'fuse -lin-k having a plurality of portions :of reduced `'crosssec'tion .spaced `substantial-ly lequidistanti-y along substantial-ly itl-ie entire length -fof said link .and va plurality fof portions of relatively larger cross- Jsection vinteri-nediate 'said Areduced cross-'section portions, the length :of 'each said reduced cross- #section portion being a small raction of the length of said 4relativelylarger cross-section portons, arc-extingu'is'hing 4means of predetermined lare#extinguishing capacity including va pulveru- `lent iler '-wi'thin said casi-ng `surrounding 'said Vxlink, the mass el? v`said li-nk b'eing fsunciently small 1ra-nge of 2'0rdegrees prior to and yllltlegne'es after `the 'peak of the generated "voltage,

13. @current-limiting fuse comprising :a tubu. lar casing, terminal elements .on opposite ends 4of-.sai'd acasing, afuse link within fsaid fcasing iintercormecting said terminal elements, fa pulverulent arraextinguish-ing fu'lgurite-forming iller .surrounding lfsai'd link, the diameter of said casing being substantially equal to :the :diameter of tlre .fnlgurite ,formed .by-.said filler upon vapor-ization of 4said lin-k under .the action of fa `current of short-:circuit current proportions.

14. .-A current-,limiting fuse -comprising a tubular teasing fof an .organic combustible material, terminal .elements -on opposite ends eof :said dcaszing. fatfu-se lin-k said .casing interconnecting said .terminal elements, said -link having 'a plu- 4,r-.alityofportions pf reduced :cross-section, a Apulseraient arc-'extinguishingrulguriteeforming :till- :er surrounding `said link, the :diameter nf said casing being substantially equal "to :the vdiameter of th'e ffulgurite .formed zup'on 'interruption :of a

the mass of metal of said linkbeingsu'fliciently y,

"Iz-Situada small 'and the arc -extinguishing capacity `vof said short-'circuit icurrent and just suiciently Ylarge preclude substantial :scorching by y'ful- `gurite Aof the :material of 'which Nsaid casing :is

Y 15. A current-limiting fuse comprising a tubular casing, terminal elements on opposite ends of said casing, a fuse link within said casing interconnecting said terminal elements, saidlink having a plurality of portions of reduced crosssection and a plurality of portions of relativelylarger cross-section intermediate said reduced cross-section portions, a pulverulent arc-extinguishing fulgurite-forming filler surrounding said link, the diameter of said casing being in the order of the distance'between said reduced crosssection portionsof said link to limit the size of said casing to the size required by the formation of avfulgurite upon vaporization of said link under the action of a current of short-circuit current proportions;

16. A current-limiting fuse comprising a tubular casing of an organic combustible material, terminal elements on opposite ends of said casing, a fuse link interconnecting said terminal elements and having a pluralityof portions of reduced cross-section and a plurality of portions of relatively larger cross-section intermediate said reduced cross-section portions, a pulverulent arc-extinguishing fulgurte-forming ller surrounding said link, the diameter of said casing exceeding the diameter of the fulgurite formed by said ller upon vaporization of said link under the action of a short-circuit current only to the extent required for precluding scorching of said casing by said iulgurite.

17. A current-limiting fuse comprising a casing, terminal elements on opposite ends of said casing, a metallic -iuse link interconnectingV said terminal elements and having a plurality of portions of reduced cross-section and a plurality of portions of relatively larger cross-section intermediate said reduced cross-section portions, each said reduced cross-section portions having a hole laterally bounded by a pair of current paths in parallel, the length of each said relatively larger cross-section portions being a multiple of the length of said reduced cross-section portions, a pulverulent arc-extinguishing ller within said casing surrounding said link, and an insert arranged in said hole of at least one of said reduced cross-section portions. said insert consisting of a metal having a lower fusing point than the metal of which said fuse` link is made and being adapted to form an alloy with the metal of which said fuse link is made having a -lower fusing point than the metal of `which said fuse link is made.

18. A current-limiting fuse comprising a casing, terminal elements on opposite ends of said casing, a pulverulent arc-extinguishing filler within said casing, a metallic fuse link within said casing interconnecting said terminal elements and being surrounded by said filler, said link having along the center line thereof a plurality of perforations spaced substantially equidistantly and extending substantially along the entire length of said link, and a rivet-like insert arranged in one of said perforations, said insert consisting of a metal having a lower fusing point than the metal of which said fuse link is made and being adapted to form an alloy with the metal of which said fuse link is made having a lower fusing point than the metal of which said lfuse link is made.

'19;In a current-limiting fuse the combination of a casing, a pair of terminal elements arranged on opposite ends of said casing, a fuse link within said casing interconnecting said terminal elements, and an arc-extinguishing Vller within said casing and surrounding said link, said iiller comprising a pulverulent substance having relatively small heat conductivity arranged in one portion of said casing between said casing and a portion of said link and an inert pulverulent substance having a relatively large heat conductivity arranged in said casing between said casing and the remainder of said link. l

20. In a current-limiting fuse the combination of a casing, a pair of terminal elements arranged on opposite ends of said casing, a ribbon-type multiperforated fuse link within said casing interconnecting said terminal elements, and an arc-extinguishing filler within said casing, said ller consisting of quartz sand at the axially outer ends of said link in immediate contact therewith and of a pulverulent substance havingv a smaller heat conductivity thansaid sand at the axially inner portion of said link.

21. In a current-limiting fuse the combination of a casing, a pair of terminal elements arranged onOpposite ends of said casing, a ribbon-type inultiperforated fuse link within said casing interconnecting said terminal elements, and an arc-extinguishing ller within said casing, said ller consisting of a pulverulent fulgurite-forming substance associated with the axially outer portions of said link and in immediate contact therewith and of a pulverulent substance of smaller heat conductivity but larger gas evolving capacity than said fulgurite-forming substance associated with the axially inner portion of said link.

22. In a current-limiting fuse the combination of a casing, a pair of terminal elements arranged on opposite ends of said casing, a ribbontype fuse link within said casing interconnecting said terminal elements, and an arc-extinguishing ller within said casing, said iiller consisting of quartz sand at the axially outer portions of said link and of gypsum powder at the axially inner portions thereof.

23. In a current-limiting fuse the-combination of a casing, a pair of terminal elements arranged on opposite ends of said casing, a fuse link within said casing interconnecting said terminal elements, vsaid link having a plurality of relatively short reduced cross-section portions and a plurality of relatively long large crosssection portions alternating with said reduced cross-section portions, and an arc-extinguishing nller within said casing, said iiller consisting of a pulverulent fulgurite-forming substance at.the axially outer ends of said link and of a pulverulent substance of smaller heat conductivity but larger gas evolving capacity than said fulgurite forming substance midway between the axially outer ends of said link.

24. In a current-limiting fuse the combination of a casing, a pair of terminal elements arranged on opposite ends of said casing, a fuse link within said casing interconnecting said terminal elements, said link having a plurality of relatively short reduced cross-section portions and a plurality of `relatively long large cross-section portions alternating with said reduced cross-section portions, a pulverulent fulgurite-forming arcextinguishing iiller within said casing surrounding all said reduced cross-section portions of said link but one, and a pulverulent gas-evolving non fulgurite-fcrming arc-extinguishing filler within said casing surrounding but one of said reduced cross-section portions.

' .25. In ,a current-limiting fuse the combination of a casing, a pair of terminal caps arranged on opposite ends of said casing, a fuse link within said casing. interconnecting said terminal caps, a pulverulent fulgurite-forming non gas evolving filler within said casing surroundingV one portion of the length of said link, a pulverulent non fulgurite-forming gas evolving iiller within said casing surrounding another portion of the length of said link, a substantially pulverulent cooling substance outside of and surrounding said casing, and means providing from the inside of said casing a restricted leakage path between said casing and at least one of said terminal caps to said cooling substance outside of said casing.

26. In a current-limiting fuse the combination of a casing, a pair of terminal' caps arranged on opposite ends of said casing, a fuse link within said casing interconnecting said terminal caps, said link having a plurality of relatively short reduced cross-section portions and a plurality of relatively long large cross-section portions alternating with said reduced cross-section portions, a pulverulent fulgurite-forming arc-extinguishing filler within said casing surrounding all said reduced cross-section portions of saidl link but one, a pulverulent gas-evolving non fulguriteforming arc-extinguishing ller within said casing, surrounding but one of said reduced crosssection portions", a substantially pulverulent inorganic cooling substance outside of and surrounding said casing, said casing and at least one of said terminal caps deiining a small leakage gap therebetween permitting restricted escape ol gases evolved' from said gas evolving ller to said cooling substance.

27. A current-limiting fuse comprising a casing, terminal elements on opposite ends of said casing, a ribbon-type metallic fuse link interconnecting said terminal elements and surrounded by a pulverulent arc-quenching filler of high cooling capacity, said link having a plurality of substantially equidistant perforations arranged along the center line4 thereof extending along a predominant portion of the length of. said link and defining a plurality ofserially related pairs of parallel current paths bounded at each side thereof by one of the lateral edges of said link, and a. metal means arranged in contact with said link at the area` o1 one of said plurality of pairs of parallel current paths, said means being,

of a metal having a lower fusing point than the metal of which said link is made and. adapted toform an alloy with the metal of which said link is made having a lower fusing point than the metal of which said link is made, whereby fusing is initiated on relatively small protracted overloads at a point of said link where one of said pairs of parallel current paths is located.

28; A current-limiting fuse comprising aY casing, terminal elements on opposite ends of said casing, a ribbon-type fuse link of silver interconnecting said terminal elements and surrounded by a pulverulent quartz ller alongv a predominant portion of the length thereof, said link having a plurality of substantially equidistant perforations extending along substantially the entire length of saidlink and denning a plurality of serially related pairs of parallel current paths bounded at each side thereof by one of the lateral edges of said link, and metal means in contact with said link adapted to form an alloy with silver having a lower fusing point than silver, said metal means being arranged at the area of one of said pairs of parallel current paths to cause initial fusion of said. link attheoccurrence of relatively small protracted overloads at said one of said pairs of parallel current paths.

29. A current-limiting fuse comprising a casing, terminal elements on opposite ends of said casing, a ribbon-type fuse link of silver interconnesting said terminal elements and surrounded at least in part by a pulverulent quartz filler, saidlink having a plurality of substantially equidistant perforations-extending along a predominant portion of' the length of said link and deiiningv aplurality of serially related pairs of parallel current paths bounded at each side thereof by one of the lateral edges of said link, and metal'means in contact withsaid link adapted to form a silvertin alloy with the silver of which said link is made, said metal means being arranged at the area of one of said pairs of parallel current paths to causev initial fusion of said link at relatively small protracted overloads at the point thereof Where said one of saidv pairs of parallel current paths-is located;

80. A current-limiting fuse comprising a casing, terminal elements on. opposite ends off saidcasing, a pulverulent arc-quenching lledv Within said casing, a fuse link within said casing interconnecting said terminal elements and being surrounded by said ller, said link having a plurality of portions of minimum cross-section spaced substantially along the entire lengthA of said link, a plurality of portions of maximum cross-section each alternating with one of said pluralityof' minimum cross-section portions, and` a plurality of portions of progressively varying cross-section arranged between said minimum cross-section portions and said maximum crosssection portions, said plurality of minimum cross-section portions and'said plurality of progressively varyingv cross-section portions dening a plurality of serially related pairs of parallel' cur-rent paths, one of said plurality of pairs of parallel current paths being associated with fusing point aiecti-ng metal means for causing initial fusion of said link at theoccurrence of small protracted overloads only at the point of saidlink where said one of said plurality of pairs of parallel current paths is-located.

3l. A high current-carryingl capacity currentlimiting: fuse having a Voltage rating of 600- to 1000 vol-ts comprising a casing, a pulverulent arc-- extinguishing ller Within said casing, a fuse link Within said casing inter-connecting said terminal elements and being surrounded by said ller, said fuse link comprising a plurality of portions of minimum cross-section, a plurality rof portions of maximum cross-section and a plurality of intermediate portions of progressively changing cross-section, said plurality ofv portionsof minimumv cross-section portions and said plurality of progressively changing cross-section portions forminga plurality of serially related pairs of parallel current paths, the sum total of the length of said plurality of minimumcross'- section portions being but a small fraction of the' sum total of the-length of said'plurality of, maximum cross-section portions and of the length of said plurality of progressively changing cross.- section portions, the total length of' said link being approximately two inches, and means for causing initial fusion of said link at a temperature lower than the, fusing. point of the metal of which said link is made, said initial fusion causing means being, associated with only one ofsaid. plurality of pairs of parallel current paths to initiate fusion of said. link at the point. thereof 27 where said one of said plurality of pairs of parallel current paths is located.

32. In combination an electric circuit, a current-limiting fuse arranged in said circuit, said fuse comprising a casing, terminal elements on opposite ends of said casing, a fuse link interconnecting said terminal elements and having a plurality of spaced portions of minimum crosssection, a plurality of spaced portions of maximum cross-section, and a plurality of intermediate portions of varying cross-section, each said minimum cross-section portions having a sufficiently small cross-sectional area to initiate arcing at instantaneous currents far below the available short-circuit current of said circuit and being constitutedby a pair of conductive elements forming separate parallel current paths, the sum total of the length of said plurality of maximum cross-section portions and of the length of said plurality of intermediate cross-section portions being a multiple of the sum total of the length of said plurality of minimum cross-section portions, means associated with only one of said plurality of minimum cross-section portions for causing initial fusing of said link at protracted small overloads at a lower temperature than the fusing point o1" the metal of which said link is made, a pulverulent arc-extinguishing filler within said casing surrounding said link, said rlller having a suiiiciently high arc-extinguishing capacity in relation to the arcs initiated at said portions of minimum cross-section at fault currents of short-circuit current proportions to effect permanent interruption of any short-circuit current that said circuit is capable of producing in less than 1/120 of a second after the time of fault inception, the sum total of the length of said plurality of minimum cross-section portions, said plurality of maximum cross-section portions and said plurality of intermediate portions exceeding the length of said link vaporized under the most onerous circuit interrupting conditions to which said fuse may be subjected in said circuit.

33. In combination an electric circuit, a current-limiting-fuse arranged in said circuit, said fuse comprising a casing, terminal elements on opposite ends of said casing, a fuse link of silver interconnecting said terminal elements and having a plurality of substantially equidistant circular perforations arranged along the center line thereof, said perforations extending along a predominant portion of the length of said link and establishing a plurality of serially related pairs of parallel current paths, each said pairs of parallel current paths having a cross-sectional area suiliciently small to cause arc initiation at instantaneous currents far below the available Short-circuit current of said circuit, alloy-forming means associated with one of said pairs of parallel current paths for lowering the fusing point of said link at the point where said one of said pairs of parallel current paths is located, a pulverulent quartz filler within said casing surrounding a suicient portion of the total length of said link to cause arcs initiated substantially simultaneously at the occurrence of fault-currents of short-circuit'current proportions at said plurality of serially related pairs of current paths to be permanently extinguished in less than 1/120 of ya second after fault inception, the sum total of the perforated and non-perforated portions of said link exceeding the length vaporized under the most onerous'circuit interrupting conditions to to leave some residual non-vaporized link metal within said casing even on the occurrence of said most onerous circuit interrupting conditions.

34. A current-limiting low-voltage fuse comprising a pair of terminals, a ribbon-type multiperforated fuse link connected to and extending between said pair of terminals, a pulverulent fulgurite-forming arc-quenching substance sur.- rounding said link, and a casing substantially of an organic material housing said link and said substance, the distance between said link and the internal wall of said casing being approximately equal to the radius of the fulgurite formed by said substance upon blowing of the fuse.

35. In a current-limiting low-voltage fuse ci the type wherein one single fuse link is arranged in a casing, the combination of a pair of terminals, a ribbon-type fuse link connectedY to and extending between said pair of terminals, said link having a plurality of portions of relatively small width spaced substantially equidistantly and arranged substantially along the entire length of said link and a plurality of portions of relatively larger width intermediate said small width portions, each of said relatively larger width portions having a length being a multiple of the length of one of said small width portions, a pulverulent fulgurite-forming arc-quenching ller 'surrounding said link, a casing substantially of an organic material housing said link and said iller, and the internal diameter of said casing being approximately equal to the diameter of the fulgurite formed by said iller upon blowing of the fuse.

36. A current-limiting fuse having a voltage rating of 600 to 1000 volts, comprising a pair of terminal elements, a ribbon-type fuse link of silver connected to and extending between said pair of elements, said link comprising a plurality of portions of minimum width, a plurality of portions of maximum width, and a plurality of intermediate portions of progressively changingwidth, said plurality of portions of minimum width being spaced substantially equidistantly and arranged along a preponderant portion of the length of said link, said plurality of portions of minimum width and said plurality of progressively changing width portions forming a plurality of serially related pairs of parallel current paths, the sum total of the length of said plurality of minimum width portions being but a small fraction of the sum total of the length of said plurality of maximum width portions and the length of said plurality of progressively changing width portions, a quartz iiller surrounding said link, a casing substantially of organic insulating material housing said link and said quartz filler, and the distance between said link and the internal wall of said casing being approximately equal to the radius of the fulgurite formed by said iiller on occurrence of a short circuit current.

37. A current-limiting fuse having a voltage rating of 600 to 1000 volts comprising a pair of terminal caps, a ribbon-type fuse link conductively connected to and extending between said pair of caps, said link having a plurality of substantially equidistant perforations aligned along the longitudinal axis of said link and arranged along a preponderant portion of the length thereof, a body of quartz sand surrounding said link, and a tubular casing having a coaxial external and internal surface, said internal surface being of an organic insulating material, said casing supporting said caps and housing said link and said body of quartz sand, the internal diameter of said housing being in the order of the spacing between said perforations and not substantially exceeding said spacing.

38. A current-limiting low-voltage fuse of the type wherein one single fuse link is arranged in a tubular insulating element, comprising a, pair of terminal caps, a ribbon-type fuse link conductively connected to and extending between said pair of caps, said link having a plurality of circular substantially equidistant perforations aligned along the longitudinal axis thereof and arranged along a preponderant portion ofV the length thereof, a fusing-point-reducing metal element associated with one of said perforations, a body of quartz sand surrounding said link to form a fulgurite upon vaporization thereof, and a tubular insulating element supporting said caps and having an external diameter substantially equal to the internal diameter of said caps, said tubular element housing said link and said body of quartz sand and having an internal wall consisting of an organic insulating material, the average spacing between said link and said internal wall being approximately equal to the maximum transversal growth of said fulgurite resulting in maximum transversal growth of said fulgurite to immediately adjacent said internal wall.

39. A current-limiting low-voltage fuse comprising a pair of spaced terminal caps, a body of quartz sand, a ribbon-type fuse link conductively connected to and extending between said pair of caps and embedded into said body of quartz sand, said link having a plurality of substantially equidistant perforations aligned along the 1ongitudinal axis thereof and arranged along a preponderant portion of the length thereof to form a plurality of serially related pairs of parallel current paths, the spacing between each of said plurality of perforations being sufficiently large to preclude merger of the serially related pairs of parallel arclets formed at said pairs of parallel current paths on interruption of short-circuit currents into longer arcs, and a tubular insulating element for housing said body of quartz sand and said link and for supporting said caps on the outer surface thereof, the inner surface of said insulating element consisting of an organic iny tively connected to and extending between said pair of caps, and embedded in said body of quartz sand, said link having a plurality of relatively short substantially equidistantly spaced portions of restricted width arranged along substantially the entire length thereof and a plurality of intermediate portions of greatly increased width and of sufficient length to preclude merger of the serially related arclets formed at said restricted width portions on interruption of short-circuit currents into one single long arc, and a tubular element of insulating material for housing said body of quartz sand and said link and for supporting said caps on the external surface thereof, said tubular housing being of an organic nature at the internal surface thereof.

41. In combination, an electric circuit having a circuit voltage not exceeding 1000 volts, a current-limiting fuse arranged in said circuit, said fuse comprising a casing consisting substantially of an organic insulating material, a pulverulent arc-quenching filler in said casing, terminal elements on opposite ends of said casing, a ribbontype fuse link conductively interconnecting said elements and surrounded by said filler, said links having a plurality of substantially equidistantly spaced perforations positioned along the longitudinal axis thereof and provided along a preponderant portion of the length thereof to form a plurality of serially related pairs of parallel current paths, said pairs of parallel current paths having a sufficiently small cross-sectional area to cause initiation of serially related pairs of parallel arclets at the occurrence of a short-circuit current in said circuit before said current can rise to the peak value thereof, and the spacing between said plurality of perforations being sufliciently large to preclude a merger of said serially related pairs of parallel arclets into one single long arc.

42. In combination, an electric circuit having a circuit voltage not exceeding 1000 volts, a current-limiting fuse arranged in said circuit, said fuse comprising an insulating casing, terminal elements on opposite ends of said casing, a ribbon-type fuse link interconnecting said terminal elements and having a plurality of portions of reduced width spaced substantially equidistantly along a preponderant portion of the length of said link and a plurality of portions of relatively larger width intermediate said reduced width portions, each of said plurality of reduced width portions having a sufficiently small cross-sectional area to initiate arcing at the occurrence of a short-circuit current in said circuit before said current can rise to peak value thereof, each of said plurality of relatively larger width portions having a length being a multiple of the length of said reduced width portions, a body of quartz sand surrounding said link, the number of said reduced width portions being suiiciently large and the length of said larger width portions suiiciently high in relation to the arc-quenching-capacity of said body of quartz sand to effect extinction of the serially related arclets formed at said reduced width portions at the occurrence of a short-circuit current in said circuit before said arclets can merge into one single long arc, the mass of said link being so small as to limit the peak of the let-through current of said fuse to sufficiently low values to enable said body of quartz sand to effect extinction of said arclets in 1 to 1.5 times the time elapsing between initiation of a short-circuit in said circuit and initiation of said arclets, even if said short-circuit is initiated at such a lead angle that initiation of said arclets occcurs within the range of 20 degrees prior and 10 degrees after the peak of the circuit voltage.

WILLIAM S. EDSALL. KENNETH W. SWAIN. FREDERICK J. KOZACKA.

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

UNITED STATES PATENTS Number Name Date 1,140,953 Cole May 25, 1915 2,157,906 Lohausen May 9, 1939 2,354,134 Ludwig et al. July 18, 1944 2,496,704 Fahnoe Feb. 7, 1950