US2647970A

US2647970A - Current-limiting fusible protective device

Info

Publication number: US2647970A
Application number: US252676A
Authority: US
Inventors: William S Edsall; Kenneth W Swain
Original assignee: Chase Shawmut Co
Current assignee: Chase Shawmut Co
Priority date: 1951-10-23
Filing date: 1951-10-23
Publication date: 1953-08-04
Anticipated expiration: 1970-08-04

Description

Aug. 4, 1953 Filed Oct. 25, 1951 W. S. EDSALL ETAL CURRENT-LIMITING FUSIBLE PROTECTIVE DEVICE 2 Sheets-Sheet l A g- 3 w. s. EDSALL ETAL 2,647,970

- CURRENT-LIMITING FUSIBLE PROTECTIVE DEVICE Filed Oct. 25, 1951 '2 Sheets-Sheet 2 4| II III Patented Aug. 4, 1953 UNITED ES:

2,647,970 CURRENT LIMITIWG FUSIBEEiRGTEETI V DEV ICE

William S, EdsalL-Boston, Mass;,and Ke;nnetlr-W= Swain, Hampton, N. H., assignors to- 'lhe- Chaseshawmut'c mpany, Newbur'yport, Mass a corporatio'n of Massachusetts Application cetober 23-, 1951, Serial No. 252,676

25Clainis. (01. 260 -12031 1' This invention relates to improvements in current-limiting :fusible protective devices, i. -e.,- interrupting devices which operateso rapidlyvthat the maximum current which can occur in caseof overloads in the nature of a short-circuit is; 5 limited to values substantially less than the avail-v able short-circuit current-of the circuit or system. Such devices, if used in an-A. C. circuit;

vices which moreefiectively can accomplish-current-limitingcircuit interruption of circuits as compared with current-limiting devices hereto fore available.

Another object is to provide curre'ntqiniitirig devices wherein individual current-limiting units of predetermined current-limiting characteristicssu are combined in parallel or inseries in a plural or multiple unit device Whose individual units'are protectively surrounded by a dielectric "medium within which the individual units operate and in the midst of which"gaseousproducts' m'aybe""35 vented from the individual units."

A furtherobject is to provide current limitin'g devices wherein individual current-limiting uni-ts' are "connected 'in parallel inorder to produce plurality of parallel unstable arcs-and more' par ticularly to provide a current-limiting :fuse: wherein the number "of I'parallel unstable arcswhich are formed when the fuse blows exceeds the number of current-limiting units of-which-- the fuse is made up.

Another object of the invention is to provide a fuse comprising a plurality of individual-currentlimiting units of which each has a fuse link vary-- ing in cross section along its length for" causing 1 gradual or progressive insertion of arc resistance into the circuit'in order to limit'volta'ge surges" incident upon too rapid interruption.

Another object of the invention 'is'to provide a fuse comprising a plu'r'alityof individual current-limitinaunits of which eachhas a"fuse" link "adaptedto-form sequentially .a plurality of points'ofbreak'for causing g adual or progressive insertion ofarc resistance into the circuit. Still 'another object is to provide current-limiting devices wherein a plurality or multiplicity of fusible elements are individually encased to constitute individualcurrent-limiting units; A and A wherein-the individual units are combined in parallel or in series-within an enclosure -which,-=

or means withinwhich-forms a dielectric barrier between the-opposite terminalsot each unit and v between the terminals of-units--which-are in spaced relation-to each othe-r.

Yet anotherobject is to provide current-limit ing devices wherein a plurality-cor a multiplicity of individual tubular units-have fusible elements separately enclosed within-the respective units witheach fusible element surrounded by .a filler material with-inits unit-enclosure, allof the said 2 "fusible elements being electricallyconnected to- 1 'gether in parallel or in series, or some' in parallel and-others in series, and achtubular unit bin g closely surrounded by .=dielectric' material which? resists interior pressures tending to rupturein dividual units.

pulverulent dielectric material which cioseiyeur 'rounds the; individual? units?- A still'furtlier object is to provide "a current limiting device wherein separately encased cur-' rent-limiting .un its' ar'e electricallyicoimcted in parallel Within-a m'ass of dielectric'm'aterial and wherein inresponseto short-circuit-or thelike' currents,--- interruptionis, sequentially initiated within the units;

determined eharacteristies are combirretl to pro- "duce devices of desiredpluralqinit' and" multiple uni-tinterrupting capacities-1:: 1

In theaccompanying drawings;, Fi1-1 i5 2, 'pla ii" viewers multrple enit current-limiting-eleotricalprotective devioe embody 'ing features'= of- 'the inv'entiom a portio bfi 'theifi outer casing being broken away, and the outermost unit being partly in medial cross-section;

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

Fig. 3 is a medial cross-sectional view of a modified form of multiple-unit device, the crosssection being approximately on line 3-3 of Fig. 4;

Fig. 4 is an end elevation of the device of Fig. 3, with a portion of the end wall broken away;

Fig. 5 is a medial cross-sectional view of another modified form having two units connected in series, the cross-section being approximately on line 55 of Fig. 6, but the units being shown in elevation;

Fig. 6 is a cross-sectional view on line 6-6 of Fig. 5;

Fig. '7 is a medial longitudinal cross-sectional view of one of the units of Figs. 1 and 2, on a larger scale;

Fig. 8 is a medial longitudinal cross-sectional view of a current-limiting device having three individual units in tandem-series relation within a capped outer tube, the individual units being in elevation;

Fig. 9 is a side elevation of a multiple-unit device comparable to that of Figs. 1 and 2 but having generally rectangular shape and having individual units of a modified form; and

Fig. 10 is a diagrammatic showing of currentlimiting units having two pairs of series connected units connected in parallel.

Referring to the drawings, the current-limiting devices of Figs. 1-4 and 9 are representative of multiple-unit devices in which any desired number of individual current-limiting units may be associated together in parallel to provide an assembly having current-limiting capacity and characteristics predetermined as essential or desirable for any particular circuit.

In the device of Figs. 1 and 2, thirty-four individual units ID are mounted in spaced parallelism between the metallic terminal members I2, l4, each unit ID, in and of itself, being a complete current-limiting device of predetermined current carrying and interrupting capacity. Each unit It] might be used alone for service in accordance with its rating. If the number of units 10 is multiplied, as needed, and if the units are electrically connected in parallel, a composite current-limiting device of any desired total current carrying and interrupting capacity may be produced.

Each unit It, as embodied in the multiple unit device of Figs. 1 and 2, has a tubular insulating casing N5, of any suitable material, capped at its opposite ends by the metallic end caps I8, 20. However, as seen in the larger scale unit ll] of Fig. '7, the end caps I8, are initially applied to a casing 16 by merely placing them over the casing ends, with a snug but not tight fit of the cap flanges around the casing walls. In a plural-unit or multiple-unit device, the relatively loosely applied caps become secured on the casings I6 during the assembly procedure, as later will appear. It should be understood, however, that the caps may be crimped or be otherwise secured on the unit casings.

The casing [6 of a unit l0 may be made of any suitable material capable of withstanding extreme heat and pressure, such as porcelain and known special composition materials but, in the devices of the invention, circuit interruption is effected in a manner which reduces substantially the amount of heat and pressure to which individual units are subjected, as compared with prior comparable units, making it practicable and safe to employ relatively inexpensive unit casings [6. Actually, casings of vulcanized fibre of the varieties conventionally used for cartridge fuse casings have proven entirely adequate.

Each unit II! has a fusible strip or link 28 extending within its casing It, with its opposite ends electrically connected to the opposite end caps I8, 20 as by high melting temperature solder, or the like. The strips 28 may be of any suitable fusible conducting material, such as silver or zinc, or the end portions may be of one material with a central portion of a lower melding temperature material. For example, in Fig. '7, the end portions 28' may be of silver and the central portion 28 of zinc. Each fusible strip 28 has a series of holes 30 therein, each hole leaving a pair of intact necks 32 of reduced cross-section. The holes 30 are spaced apart enough to leave substantial portions of the strip material intervening between the respective pairs of reduced necks 32, these intervening portions being adapted, in large part, to remain unfused when fusing at the reduced necks 32 occurs. The provision of two separate current paths at the point of each hole results in the formation of two separate arclets at each pair of reduced necks 32 when the link is subjected to a shortcircuit or the like current. This promotes rapid sequential extinction of the arclets, since arcs in parallel are unstable, as is well known in the art. The number of pairs of reduced necks 32, with the required spacing, will be predetermined in accordance with the desired or contemplated voltage rating of any individual unit, and then the number of the units required to be associated together will be selected in accordance with the particular current rating desired to be give to the composite structure. Inasmuch as each arclet in any particular unit will provide an arc reignition voltage in the order of several hundred volts, the number of holes in the fusible strips 28 preferably will be selected to provide a total or cumulative arc voltage in any particular device at least equal to the circuit voltage plus the estimate inductive voltage of the circuit to be protected, so that there will be equilibrium between the sum of the arc voltages at all points of break and the sum of the circuit and inductive voltage.

An arc extinguishing filler material surrounds the fusible strip or link 28 in each unit l0 throughout the extent of the strip in the casing, but the filler material 34 at the opposite end portions of the casing, as best seen in Fig. '7, is different from the filler material 35 at the central region. Inert granular quartz is preferred for the material 3.4 in the opposite end portions of the casing because of its high heat-absorbing and heat-conducting qualities.

The material 36 in the central region of the casing preferably will be a thermally insulating material such as gypsum, for example, while the arc extinguishing filler on the axially outer ends of the casing is a good conductor of heat as, for instance, quartz sand. When a current of shortcircuit magnitude causes the fuse to blow, two fulgurites are formed at the axially outer ends of each casing. These fulgurites ar separated by a ypsum zone in the center portion of the fuse. The use of a thermally poor conductor in the center portions of the fuse units as filling material' tends itocprevent.sexcessiveuheatingi o charring .of the center; portions f theicasing Since a. there .is an impediment againstlthe. flows. of heatgenerated at the-axially innerspcrtionsn;

of thefuse units. in radial.tdirection, .ithe heat1 i there. generated will :tendxito flotv.'.toward=. zth'ez-ig axially: outer endsof the fuse.units,. .where itcan;

be most-readily dissipated;

Inxfuseiunits according to-the present invention ylayersof fulgurite, formingarc quenching: materiahalternatewith a layer of non-fulgurite forming arc .quenching material, .1. e.,

gypsum. It has been foundethat;

multielayer fillers ofthis kindreduce .theztend-' 1 eney, of current-limiting fuses-to permit .follcwea currents to persist; i. e.,. smalllcurrents to gflo'wz.

through the fuse for ,an

appreciableltime after interruption, of the circuit :has beensubstantially .1: achieved. The persistence 'of -.-follow.-currentsy is dueto vthe {fact that. fulgurites: are moderately a:

good conductors of electricity :before.1they;:are

cooleolldown belowa certaimtemperature... If,":.

however,- a fulgurite .is more portions. by a gap sub-divided intotwo. or; formedby: a .non-ful-r.

gurite-forming arc quenching material.aas,:..for=-.-'

instance, gypsum, whichhas a smaller COI'ldUC:

tivity at elevated temperature. than quartz' sand,

thenthe tendency for the new of follow cur 1 T- Subdivision of .fulgurites rents is much reduced. by substances having ,a lesserltendencyxati ele.-. vated temperatures to permit the passage of cur.-.

rent than quartz is particularly .desirablewhere the fuses are intended-to'interruptD. C. circuitsm {with a material. as. gyp- To fill. a casing entirely sumis not indicated since it has. but limited heat.

dissipating ability and. amounts of gas underrthe tends; to evolve: large;

heat. of thearc which;

in turn, tend to create sufficiently: high pressures to break the casings.

Suitable disks 31, slotted to .fit :overthe fusible element, may be insertedi.-at.thetimezoffilling. of the units, to. ferent filler materials.

According .tothe invention,..a plurality. orta maintain segregation of the dif -f multiplicity of current-limiting individual .units. i0 areassociated together to providerplural-unit and :multiple.-unit devices :having desired L'CUI'F In: a.

rent-limiting. capacities and, characteristics.

the.,Figs,.;1 and .2 embodimentxthepthirtyefounm units it are clamped endwise betweeni-the,ter-x minal, members, i 2, I4, within-1' an; outer insulating 38 which is .securedto theate .t minal. members, aslby theuscrews=39.;1. Quartz' tubular casing sand 40, or a comparable .filler;material,sur-r. rounds theunits fur-Within. the. outerncasingr38i' si effectively insulating the .unitsagainst flash overs" and. bee.

between terminals of individual units:

tween terminals of adjacent :unitsx. Also, if any. particular unit") should become.ruptured itherupture will occur in the :midst.

cent units and without materially.

of the filler material 40 without anyharmful effects .on adjam affecting. the; circuit interrupting and. currentslimiting. ability:

of the plural-unit or.multiple-unitdevice as-a A feature of importance resides in theprovisi'on. for restricted venting of. individualunits 10 into" of the insulating-filler material, 5; products; escaping. from units I I] are. deionized in a most effectiverwayanmq thus precluded from causingflashovers or doing I the midst Where; are cooled and harm of-any other kind. As herein represented,

each unit lll has itsend caps. l8, Zfliinitially merely. pressed over. the ends of the unit. casing IS without any crimpingtof the caps.- onthe cas ing However,v each of the ipterminal lmembersa I 2,2! 4: includes a circularibodtcpart 2 -"l 4". whose opposed .1 faceszihave slightly :tapering sockets .42 ,i

.the .unit. endtcapsul8 a or 20sis seatedduringassemblyer After a desired I number of units II! have been arrangedlbetween l in teachuof swhichr. one..of-

thezpressure within thel'unitsxexceeds. a predetermined-rvaluew In other Words, :.units. :In rare but:

semisealed;which;permitszwpressure to be. built up -,within.;units 40 up, to. a predetermined vpres-- surelevel; Building'upaof pressure. results in an increase q of- Within units circuit.- When the pressure reaches a dangerous level close to k the, point; wh'ereuthe individual fuse casings 3 might burst,-.the semi-seals, of the fuse:

units operatefiller material since the gases :are vented .into the insulating nllermaterial Mi -which surrounds the units:

Under:- particular:condition-s, especially where. only a fewunits- H) are associated together, .filler the units;

mater-ial;.-may bepmitted exteriorly of in which casaarc productsventing'from the unit casings may discharge-into air or other gas enclosed withinthe outer casing .38;

Figs. 3 andt illustrate terminalebod-ies; 112 7, 14' The material nlay be introduced: as a liquid through an. opening 45 in the outer casing 38' after the device hasbeen assembled-,-=;solid-ifying within the outer-casing, or

a solid body of material-44maybe preliminarily formedwithholes dfi for; snugly receiving the unitqcasings-- prior to the capping thereof, the solid body; in the latter case, being clamped betweenethe terminaI 'bOdies-IZ"; I4; when the unitc-aps-are-bemg deformed as described in connection"with Figsrl and"2." In each case, howevergit will be apparent that the material 44 positively supports and; restrains the unit casing l6 against rupture and in additionthereto isolates 4 their-aeapn-xterminals so.as to preclude possibility of arcpover or fiash'over.

Fig. 9 showsa multiple+unitdevice which is generally similar'to the :Figs: 1 and 2 device but having longereindividual units i 0.',:which may be of higher voltage rating, clamped "between terminal bodies, i2 --M of rectangular shape. The 1 united-0 areymounted in the samemanner-as the :il and2 form, butthe outer plates 38 which are secured in place by the screws 69% Quartz: sand; for .the11ike';:. maybe 1 poured into? the outer;-- casing, around- .the units i 0 through an open'side'after three of the'plates- 38 -have .-be.en-secured sin place the fourth plate being. secured following the filling operation;

Whilez'the devices -of-Figs- 1-4 andg9 allhave mult-iplex units' electrically connectedin parallel into; clamping engagement :with the This clamps the; dielectric strengthof the'gas iii, thus aiding interruption ofthe in safety; valve. like fashion. Vent- :ingofrthe are. products and'of gases evolved from within the fuse units is harmless an embodiment of the inventicn genera-11y similar to .the'F'igs. 1 and 2 form gbutsha-ving a solidabody; 44;-of:dielectric materialsurrounding:the units it, between the 7 to provide large current-carrying capacity, it is desirable in some cases to combine the units in serially related groups in order to be able to give higher voltage rating-s to plural-unit and multiple-unit devices.

Figs. and 6 illustrate a two-unit device whose two units It) are connected in series between terminal bodies 48 and within the outer insulating casing 50. Each terminal body 48 may have a socket 52 for receiving a unit end cap, as previously described, and may have a larger recess 54' for receiving a block 56 of insulating material in which a socket 58 is formed, to receive a unit end cap. A small insulating tube 6% extends between the two insulating blocks 56, and a conductor 62 extends through the tube and has one end connected to the unit cap which is seated in one block 56, and has its other end connected to the unit cap which is seated in the other block 56. Filler material 64 preferably surrounds the units H), as in the earlier described embodiments. It will be obvious that the device of Figs. 5 and 6 will have a voltage rating substantially equal to the sum of the voltage ratings of the individual units 10.

In the Figs. 1-4, 9 and 5 and 6 embodiments, the terminal members l2, l4, terminal parts or bodies I2, I4, l2", 44', 12 M and 48 are shown as relatively massive terminals which have large capacity for absorption and extraction of heat from the interior units, and they are in good heat-receiving relation to the end caps of the interior units, whereby relatively large and rapid transfer of heat to the terminals occurs in response to any substantial heat generation within the interior units. This avoids unnecessary operations of the protective devices in response to moderate overload currents which are of a harmless nature, and rapidly removes heat from the interior units when fusing of the units occurs, thereby permitting use of fuse links of smaller cross-section, with smaller reduced cross-section portions, than otherwise would be feasible, and minimizing any tendency toward build up of dangerous temperatures and pressures within the individual units.

Fig. 8 shows a tandem series arrangement of units 10, comparable to the units H) of Figs. 1-4 and 7, within an outer insulating tube 38 in each end of which an exteriorly threaded sleeve 38" is fixed for reception of a metallic end cap 38". lfhree of the units H! are shown clamped endwise between the end caps 38", with arc extinguishing filler material 40 surrounding the units l0. Here again the device will have a voltage rating substantially equal to the sum of the voltage ratings of the individual units Ill.

In special cases, it may be desirable to assemble multiple-unit devices with some of the units in parallel and others in series to attain both an increased voltage rating and an increased ourrent-carrying and interrupting capacity for a device as well as required current-limiting characteristics. This may be achieved by combining the features of the Figs. 1 and 2 and Figs. 5 and 6 forms within a single outer enclosure. Fig. diagrammatically illustrates such a series-parallel relation of units [0.

Devices embodying the disclosed features of the invention have proven exceptionally efficient in the performance of current-limiting protection against short-circuit and the like currents, both in A. C. and in D. C. circuits and in both low and relatively high voltage service. The invention makes it practicable to assemble current-limiting units in parallel or series or some in parallel and some in series relation to produce composite devices of any of a wide variety of voltage and current ratings. Individual unit constructions may be varied to provide different unit characteristics. If desired, units of different current carrying and interrupting capacity may be connected in parallel. In a multiple-unit device embodying the present invention, progressive insertion of arc resistance into the circuit and consequent avoidance of voltage surges is due to the coaction of several causes. Small tolerances or differences of the cross-sectional areas at the various points of break are in part responsible for the fact that are initiation at the various points of break occurs in rapid sequence rather than simultaneously. Another cause responsible for gradual insertion of arc resistance into the circuit resides in the fact that the cross-section of each link increases from each point of arc initiation in a direction longitudinally of the link. The cross-section at the points of minimum cross-section is evidently equal to the diiference between the width of the link and the diameter of the hole times the thickness of the link, whereas the cross-section at the points of maximum cross-section is evidently equal to the width of the link times the thickness of the link. In composite current-limiting fuses according to the present invention, magnetic effects and particularly the skin effect cause a larger portion of the current to flow through some units than others, and therefore units having an equal rating fuse before and after others. This triggering off of certain units in advance of others tends to reduce the rate of change of current and therefore to limit the inductive surge voltage. When the fusible elements have portions of different melting temperatures as herein disclosed, the portions of lower melting temperature are converted into arcs before the portions of higher melting temperature fuse.

Also, the unit construction wherein a fusible strip or link has a series of holes providing pairs of fusible conducting necks with substantial bodies of metal between adjacent pairs of the necks, permits interruption of circuits in the shortest time compatible with prevention of voltage surges and, on account of this fact, such units volatilize less metal and generate less heat than prior comparable units.

The provision for venting of units into the midst of surrounding dielectric material relieves internal pressures of units while the said surrounding dielectric material, especially in its solid form of Figs. 3 and 4, resists internal unit pressures which otherwise might be capable of rupturing the unit casings. But, according to the invention, actual rupture of a unit casing, if it should occur, will be in the midst of insulating dielectric material where it can have no appreciable ill effect upon adjacent units or upon the over-all efliciency of the device.

It should be understood that the particular embodiments of the invention illustrated in the drawings are representative rather than limiting and it is intended that the patent shall cover such modifications as fall within the scope of the appended claims.

We claim as our invention:

1. A current-limiting device comprising a pair of electrically conducting terminal members, a plurality of electrically conducting fusible elements electrically connected together with at least one element electrically connected to one of said terminal members and at least one other element-electrically connected to theotherof said --terminalmembers, each said fusible element hav- =ing at-least two portions of reduced cross-section,

and a portion 'of'relatively much greater cross metal whichpin substantial part; continues-unf. fused by extension of the arcs at said reduced cross-section portions, a separate enclosure of insulating material for each of said fusible ele- 'ments; arc "quenching filler material surrounding g each fusible element within its .said enclosure; a "co'm'mon insulating housing for said plurality of -fusible elements and the enclosures thereof, :said housing extending between said terminal 'm'em bers and maintaining them in a' predetermined "spaced relationship, and'dielectric material with in" said housing between said terminal members and surrounding each of said separate en- -'c losures.

2.IA current limiting device comprising a pair "of electrically conducting terminals members, a i pluralityofelectrically"conducting fusible elem'ents'" electrically "connected together-with at least one" element electrically connected to one I of said te'rminalmembersand at least one'other elementplectrically connected to the other of "said terminalmemb'ers; a separate enclosure of insulating material for eachofsaid fusible elements; are quenching fill'er material surrounding each=ffusib1e element within its said enclosure, a c'ommon insulatinghousing for said plurality of fusible elements and the enclosures thereof, said housing extending "betweensaid terminal members and maintaining them inapredetermined spaced 'r'elationship; dielectric granular material within said housing surrounding each "of said separate enclosures throughoutthe ex- "tents th'ereofbe'tween the terminal'members; and restricted-passage means permitting gases'un- -der: pressure "to 'vent from -saidseparate=enclosures into contactwith said dielectric'granular material, each ofsaid' plurality of fusible *elemerits having a: plurality'of portions of reduced cross-section spaced substantially equidistantly along substantially the entire length of-said' link and a plurality of-sportions of relatively larger I? cross-sectionintermediate said "reducedcrosssection portions, "e'achsaidreduced cross-section por'tiorr having a 'sufii'ci'ently small cross-sectional area to i'nitiate arcing at instantaneous currents far: below" the available short-circuit current of a cir cuit' in which "the device is'to serve; and

each said "relatively larger cross section portion -having alength'being a multipleofthe len'gth of said reduced cross-section portions.

I 3." A curre'nt-limiti'ng device comprising a' pair of electrically conducting terminal members, a

' plurality of individual current-limiting units arranged between saidterminalmembers, each said unit having'an insulating casing and an elecw trically conducting fusible element wi-thin -the casing-surrQundedbyarc quenching filler mar terial,-and the fusible elements of all of said units being electrically connected together with at leastone-fusi-ble element electrically connected .:to' one oftheterminal members andat least: one

(intervening portion of each fusible element providingamass offfusible metali'which, in sub- I stanuai part,; continues unfused' by extension of lithe arcs'fat saidreduced' cross-section portions,

jin'sulatingmeans extending between the terminal members and constituting, with said terminal members, an enclosure around said-individual vunits, andldielectric ,material within said enlclosur'e surroun'ding:said individual units and l1substantiallyffilling, 'all spaces Within the enclosure notfoccupied by said individual units.

I 4.,AT current-limiting device comprising a pair of .electrically conducting terminal, members, insulating walls"rigidlymaintaining said terminal members in spaced relation, individual current-limiting units clampedendwise between'said I terminaliinembe'rs and thereby electrically c'onnectedfin, parallel between. said terminal members and surrounded by :said insulating walls,

granular inorganic insulating material surroundingj said individual units'iwithinsaidsurrounding swan, each of said 'unitsfcornprising an insulating tube, an electrically conducting fusible element arranged therein'and' having one end'electrically i connected tolioneiof said terminal members and QltheTother end electrically connectedto the'other lof saidltermin'almembers, and an arc quenching substance surrounding the fusible element and filling the tube, and restricted,passagemeans for venting arc products, under pressure formed within each tube intoisaid granular inorganic insulating material, each said fusible element having a' plurality of portions of reduced crosssection spaced substantially, equidistantly 'along substantiallythe entire length, of said link and a plurality of; portions of relatively larger crosssection intermediate said reduced. cross=section pOttiorisfeach'said reduced cross-section portion having a sufficiently small cross-sectional area to initiate arcingjat instantaneous currents-far below. the available short-circuit current" of a (circuit in 'whichsthe' device; is to serve, and each saidrelatively largercross-section portion "hav- Zing, a, length being av multiple of the length of i said reduced cross-section portions.

5. A current-limiting device comprising a pair of electrically 'condu'ctingterminal members,

each said, terminal member having a facedisposed in spaced general parallelism with, asim- "ilar face'of the other terminal member, a multipIicityof insulating tubes clamped endwiseibetween said terminalm'embers and disposed in spaced general parallelismwbetween said termiinall members, an electrically conducting fusible =element' Within e eachfltube having its opposite ends electrically connected to different. ones of said terminal members, each said fusible element l having a'plurality'of portions of reduced crosssection spaced substantially equidistantly'ialong substantially theentire length of said linkand a plurality of portionsfof relatively largerflcross- I section'intermediate saidl'reduced cross section portions," each said reduced cross-section portion having a sufficiently small cross-sectional area to initiate arcing at instantaneous currents far below the available short-circuit current of a circuit in which the device is to serve, and each said relatively larger cross-section portion having a length being a multiple of the length of said reduced cross-section portions, are quenching filler material surrounding the fusible element within each said tube, insulating walls rigidly maintaining said faces of the terminal members in their said spaced relation and surrounding said multiplicity of tubes, granular dielectric material surrounding each of said tubes within said surrounding insulating walls, and restricted passage means at opposite end regions of each said tube whereby gases under pressure evolved within the tubes can escape into contact with said granular dielectric material surrounding the tube.

6. A current-limiting device comprising a pair of electrically conducting terminal members, each said terminal member having a face disposed in spaced general parallelism with a similar face of the other terminal member, each said face having a multiplicity of cavities therein, a multiplicity of insulating tubes disposed in spaced general parallelism between said faces of the terminal members, a metallic end cap on each end of each of said tubes, one end cap of each of said tubes being seated in one of said cavities in the face of one terminal member and the opposite end cap of each of said tubes being seated in one of said cavities in the face of the other terminal member, an electrically conducting fusible element within each of said tubes connected to the opposite end caps thereof, arc quenching filler material surrounding the fusible element within each of said tubes, and insulating means rigidly connecting said terminal members toether.

7. A current-limiting device comprising a pair of electrically conducting terminal members, each said terminal member having a face disposed in spaced general parallelism with a similar face of the other terminal member, each said face having a multiplicity of cavities therein, a multiplicity of insulating tubes disposed in spaced general parallelism between said faces of the terminal members, a metallic end cap on each end of each of said tubes, one end cap of each of said tubes being seated in one of said cavities in the face of one terminal member and the opposite end cap of each of said tubes being seated in one of said cavities in the face of the other terminal member, an electrically conducing fusible element in each of said tubes connected to the opposite end caps thereof, filler material surrounding the fusible element within each of said tubes, insulating walls rigidly connecting said terminal members together, said insulating walls and terminal members definining an enclosure around said multiplicity of tubes, and porous dielectric filler material surrounding each of the multiplicity of tubes within the said enclosure.

8. A current-limiting device comprising a plurality of insulating tubes, an electrically conducing fusible strip extending within each tube, each strip having a plurality of portions of reduced cross-section spaced substantially equidistantly along substantially the entire length of said link and a plurality of portions of relatively larger cross-section intermediate said reduced crosssection 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 a circuit in which the device is to serve, and each said relatively larger cross-section portion having a length being a multiple of the length of said reduced cross-section portions, are extinguishin filler material surrounding the strip in each tube, metallic terminal caps on the opposite: ends of each tube and connected to the opposite ends of the fusible strip therein, means electri-- cally connecting at least one terminal cap of each tube to a terminal cap of at least one other tube, pulverulent dielectric material closely surrounding each tube throughout its extent between its: terminal caps, and means at the capped ends of the tubes providing restricted passages for venting pressure gases from the tubes into said pulverulent dielectric material.

9. A current-limiting device comprising a plurality of insulating tubes, an electrically conducting fusible strip extending within each of said tubes, each strip being composite of materials of. different melting temperatures and havin the lower melting temperature material constituting a mid-portion of the length of the strip, and each of said strips having a series of holes substantially equally spaced apart along the strip with at least one of the holes in said mid-portion, an arc extinguishing filler surrounding the entire length of the strip in each of said tubes, electrically conducting terminal means at the opposite. ends of each of said tubes and connected to the opposite ends of the fusible strip therein, means. electrically connecting at least one terminal of each tube to a terminal of at least one other tube, and dielectric material closely surrounding each of said tubes throughout its extent between its terminal means.

10. A current-limiting device comprising a plurality of insulating tubes, an electrically conducting fusible strip extending within each of said tubes, each strip having a series of holes substantially equally spaced apart along the strip providing 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 sufficiently small cross-sectional area to initiate arcing at instantaneous currents far below the available short-circuit current of a circuit in which the device is to serve, and each said relatively larger cross-section portion having a length being a multiple of the length of said reduced cross-section portions, a highly heatabsorbent pulverulent arc extinguishing filler surrounding opposite end portions of the strip in each of said tubes, a thermally and electrically insulating pulverulent material surrounding the central portion of the strip in each of said tubes, electrically conducting terminal means on opposite ends of each of said tubes and connected to opposite ends of the fusible strip therein, means electrically connecting at least one terminal of each of said tubes to a terminal of at least one other of said tubes, and dielectric material closely surrounding each of said tubes throughout its extent between its terminal means.

11. A current-limiting fuse comprising a radially outer electrically insulating casing, a radially inner electrical insulating casing arranged in spaced relation within said radially outer casing, electrically conducting terminal elements arranged on opposite ends of said casings, an electrically conducting fuse link within said radially cross-sectional area to initiate arcingi-atinstantaneous currents far belowthe available:short circuitcurrent of a circuit in Which the;.-device is to serve, and each said relativelylarger crosssection portion having alength being amultiple "of thelength of said reduced'cross sectionportions, a first pulverulent.electrically. -insulating. filler Within said radially innencasing surrounding said fuse link; a second pulverulentfelectrically insulating filler within the space defined between said radially outercasing'and said radi- "ally inner casing, and-restricted passag 111163115 permitting a limited leakage of gasesiunder pressure from said radially inner casing .intosaid radially outer casing to .-providef for: progressive cooling ofarc gases underpressure in twozconsecutive steps.

12. A current-limiting fuse comprising a:plu-

' rality of electrically insulating casings arranged 1 in parallel spaced relation, a common housing. for

said casings, said housing having its lateral :Wall

formed of electrically insulating :material .and

arranged in spaced relation witherespect to said casings, an electrically conductingfuse link in each of said plurality ofcasings, each said fuse link having a plurality of portions of reduced cross-section spaced substantially equidistantly' along substantially .the entire length of saidlink and a.-plurality.of portions 10f relatively larger cross-section intermediate said. reduced crosssection portions; each said reduced cross-section portion having a sufficiently small cross-sectional rarea to initiate arcing at instantaneous currents far belovv'the available short-circuit current of a circuit in which the device. is to-serve, and each said relatively larger cross-section portion having a length being a multiple of the length of said 4 reduced cross-section portions, common electri-' cally conducting terminal elements for thefuse link in each ofsaid pluralityof casings arranged on opposite ends of :said housing, a pulverulent electrically insulating'filler in eachof :said plu rality of casings, apulverulent electrically in-' sulating fillerin the space defined betweensaid pluralityof casings and said lateral wall of-said housing, and restricted passage means permitting. a limited leakageiof gases undervpressure 1 from said plurality of casings. into said housing to. provide for progressive cooling of arc products under pressure in twoconsecutive stages.

13..A current-limiting fuse comprising a radially outer electrically insulating casing, a radially inner electrically insulating casingarranged in spaced relation within said radially outer casing, electrically conducting terminal elements arranged on opposite ends of said casings, an electrically conducting fuse link within said radi ally inner casing interconnecting: said terminal elements, said fuse link having a plurality of por- ItlOl'lS of reduced cross-section spacedsubstantially equidistantly along substantially the-entire length ofsaid link and a plurality of portions of relatively larger cross-section intermediate said reduced crosssection:portions, each said reduced cross-section portion having asufliciently small cross-sectional area to initiate arcing at instantaneous currents...far below; the available 14 ishort-circuit current of" a circuit iniwhichithe izdevicejs to serve, and: each said relativelyylarger cross-sectionportion' having. a. length? being a multiple of the length -ofisaid" reducedcross-section portions, a pulver'ulent electrically. insulat- 'ing gas evolving filler-within. said radiallyginner casing surroundingone'portion of said fuse-link, a pulverulent, electrically insulating non .:gas evolving filler within. said radially inner casing surrounding another portion of said fuse link, a pulverulent .electrically insulating nonzgas evolvzingifiller'within' the space. defined between said .radiallyoutercasing andtsaid radially inner casling, and a -restrictedpassage meansbetween said radially inner casing and :said space to permit a limit'edafiowof gas evolved from said gas evolving f filler through said non'gas evolving fillerwithin saidradiallyinner-casing into said non gas evolving filler Within said space.

Y 14; A current-limiting fuse comprising a radially outer electrically insulating. casing".- arradially inner electrically insulatingcasingearranged spaced relation. withinsaid-radially -outer casing, electrically conducting terminal elements arranged on opposite'ends of said casings,

an.electricallyconducting fuse link within: said radially inner casing interconnecting saidi-ter- "minal'elements, a pulverulent electrically insulating gas evolving filler Withinsaid: radially inner casing surrounding the axially. inner por- 'tion of .said fuse link, quartz sand fillerswithin said radially inner casing surrounding the axially outer portions of said fuse link, a pulverulent electrically insulating non gas evolving filler within. the space. definedlbetween said radially outer casing and said radially inner casing,iandrestricted passage'means .on both ends "of said radially'inner casing to permit. limited fiowsof gas'evolved from said gas-ev'olving fillerin opposite directions through said quartz sandfillers into said non gas evolvingfiller.

I 15.. A current-limiting fuse comprising a :radially outer electricall insulating casing, a radially inner electrically insulating casing arranged: in spaced'relation within said radially outer .:casing,: a 'pairof electrically conducting -.'terminal:b1ocks arranged on opposite ends of said radially outer. casing anddefining recessesfor receivingopposite ends of said radially inner Jcasing,.anelectrically conducting fuse link Within said radiallyfinner casing, a'first pulverulent arc-extinguishing 'Ifiller surrounding said fuse link,; a second pulverulent electrically insulating filler'within' the space defined between said radially. outer casing. andsaid radially inner casing, and a pair of electrically conducting termi- 1 nal caps on opposite. ends of said radially inner casing electrically. connected to the opposite ends of-said-fuse link and arranged-within said re- -cessesgdefined by said terminal'blocks, said terminal caps and said radially; inner casing being adapted-to defineupon a predeterminedincrease in.pressure within said'radially. inner. casing a 'restricted-circulargap permitting a limited escape of gas under pressure from said radially inner easing into said radiallyouter casing,-said terminal .caps extending beyondsaid recess to cause 'directdis'charge of gas escaping from said casing into saidsecond pulverulent filler.

'16. A current-limiting fuse comprising a plu- .ralitylof electrically insulating casingsarranged in parallel relation, a common electrically insulatingihou'sing for said casings, electrically conducting terminal blocks arranged on oppositeends .ofa;:said:; housing and defining recesses for -receiving the axially outer ends of said plurality of casings, an electrically conducting fuse link in each of said plurality of casings, a filler of quartz sand in each of said plurality of casings surrounding said fuse link, a pulverulent electrically insulating filler having a relatively high heat absorbing capacity arranged in the space defined between said plurality of casings and the lateral wall of said housing, and a pair of electrically conducting terminal caps on the axially outer ends of each said casing electrically connected to the opposite ends of said fuse link therein and arranged within said recesses defined by said terminal blocks, said terminal caps and said casings being adapted to define upon a predetermined increase in pressure within said casings restricted circular gaps permitting a limited escape of gas under pressure from said casings into said housing, said terminal caps extending beyond said recesses to cause direct discharge of gas escaping from said casings into said second pulverulent filler.

17. A current-limiting fuse comprising a radially outer electrically insulating casing, a radially inner electrically insulating casing arranged in spaced relation within said radially outer casing, relatively massive metallic terminal elements arranged on opposite ends of said casings in intimate heat-receiving relation to the opposite ends of said radially inner casing, an electrically conducting fuse link within said radially inner casing interconnecting said terminal elements, each said terminal element having relatively large heat-absorbing capacity for large and rapid absorption and extraction of heat from said radially inner casing in response to sub stantially generation of heat within said radially inner casing, a first pulverulent electrically insulating filler within said radially inner casing surrounding said fuse link, and a second pulverulent electrically insulating filler within the space defined between said radially outer casing and said radially inner casing.

18. A current-limiting fuse comprising a plurality of electrically insulating casings arranged in parallel spaced relation, a common electrically insulating housing for said casings with its lateral wall arranged in spaced relation with respect to said casings, an electrically conductive fuse link in each of said plurality of casings, relatively massive metallic terminal elements secured to the opposite ends of said common housing and maintained thereby in electrical connection with the opposite ends of all of said fuse links, said terminal elements connecting said fuse links in parallel, and each terminal element having relatively large heat-absorbing capacity for large and rapid absorption and extraction of heat from said casings in response to substantial generation of heat within said casings, pulverulent electrically insulating filler material within said casings surrounding said fuse links, and pulverulent electrically insulating filler material within the space defined between said common housing and said casings.

19. A current-limiting fuse comprising a plurality of electrically insulating casings arranged in parallel relation, a common electrically insulating housing for said casings, relatively massive metallic terminals elements arranged on opposite ends of said housing and defining recesses for receiving the opposite ends of said casings, an electrically conducting fuse link in each of said casings, pulverulent electrically insulating filler material in each of said casings surrounding the fuse link therein, a pulverulent electrically insulating filler material in the space defined between said casings and said common housings, and a pair of electrically conducting terminal caps on the opposite ends of each of said casings and arranged within said recesses defined by said terminal elements, each said terminal element having relatively large heat-absorbing capacity for large and rapid absorption and extraction of heat from said casings in response to substantial generation of heat within said casings.

20. A current-limiting device comprising a pair of relatively massive metallic terminal members, each said terminal member having a face disposed in spaced general parallelism with a similar face of the other terminal member, each said face having a plurality of cavities therein, a plurality of electrically insulating tubes disposed in spaced general parallelism between said faces of the terminal members, a metallic end cap on each end of each of said tubes, one end cap of each of said tubes being seated in one of said cavities in the face of one terminal member and the opposite end cap of each of said tubes being seated in one of said cavities in the face of the other terminal member, an electrically conducting fusible element within each of said tubes connected to the opposite end caps thereof, and electrically insulating filler material surrounding the fusible element within each of said tubes, said terminal members having relatively large heat-absorbing capacity for large and rapid absorption and dissipation of heat from said tubes and end caps in response to substantial generation of heat within said tubes.

21. A composite current-limiting fuse formed by an assembly of a plurality of identical separate fuse units adapted to be arranged in parallel in an electric circuit, each said fuse units comprising an electrically insulating casing member, electrically conducting terminal elements on opposite ends thereof, an electrically conducting fuse link having a plurality of portions of restricted cross-sectional area within said casing member and electrically connected to the 0pposite terminal elements thereof, and a pulverulent arc-extinguishing filler surrounding said link, said fuse units being arranged in spaced relation to form a pattern of concentric shells to cause the total current through said fuse units to be unequally distributed among fuse units pertaining to different shells to cause fusion of the portions of restricted cross-sectional area of links of fuse units pertaining to different shells to occur at slightly different points of time.

22. A composite current-limiting fuse formed by an assembly of a plurality of identical separate fuse units adapted to be arranged in parallel in an electric circuit, each said units consisting of a separate electrically insulating tubular casing member having separate electrically conducting terminal elements on opposite ends thereof and an electrically conducting fuse link electrically connected to said separate terminal elements and a pulverulent arc-extinguishing filler therein, said fuse units being arranged in spaced relation to form a pattern of concentric shells, and a pulverulent electrically insulating substance provided in the spaces formed between said fuse units tending to maintain said fuse units in their shell-pattern position.

23. A current-limiting low-voltage fuse comprising a tubular casin of insulating material, a ribbon-type fuse link within said casing, a

pulverulent arc-extinguishing filler within said casing surrounding said link, and terminal caps supported by said casing and conductively connected to said link, said caps having cylindrical lateral portions engaging the axially outer ends of said casing under sufiieiently high pressure to normally provide gas-tight seals for said casing, and said cylindrical portions of said caps engaging said axially outer ends of said casing under sufficiently low pressure to permit egress of gaseous products therefrom on the occurrence of excess pressures therein through leakage paths then forming between said cylindrical portions of said caps and said axially outer ends of said casing.

24. A current-limiting low-voltage fuse comprising a casing of insulating material, a ribbontype fuse link within said casing, said link having a plurality of portions of greatly reduced width and intermediate portions of much larger width, the length of said reduced width portions being small in comparison to the length of said larger width portions, a pulverulent arc-extinguishing filler within said casing surrounding said link, and terminal caps mounted on the axially outer ends of said casing and conductively connected to said link, said caps bein so loosely mounted on said casing as to permit egress of gaseous products therefrom on the occurence of excess pressures therein through leakage paths then forming between said casing and said caps.

25. A current-limiting low-voltage fuse comprising a casing of insulating material, a ribbontype fuse link within said casing, said link having a plurality of portions of greatly reduced width and intermediate portions of much larger width, said reduced width portions being short in comparison to said larger width portions and arranged substantially along the entire length of said link, a pulverulent arc-extinguishing filler within said casing surrounding said link, and terminal caps supported by said casing and conductively connected to said link, said caps havin cylindrical lateral portions engaging the axially outer ends of said casing under sufficiently high pressure to normally provide gastight seals for said casing, and said cylindrical portions of said caps engaging said axially outer ends of said casing under sufficiently low pressure to permit egress of gaseous products therefrom on the occurrence of excess pressures therein through leakage paths then formin between said cylindrical portions of said caps and said axially outer ends of said casing.

WILLIAM S. EDSALL. KENNETH W. SWAIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,180,975 Brown Nov. 21, 1939 2,523,139 Owens Sept. 19, 1950 2,592,399 Edsall et a1. Apr. 8, 1952 2,594,315 Kozacka Apr. 29, 1952 FOREIGN PATENTS Number Country Date 480,044 Great Britain Feb. 16, 1938