US2599646A - Current-limiting fuse - Google Patents

Description

June 10, 1952 F. J. KozAcKA v vCURRENT-Ll/'IING FUSE Filed May 17 1951 iwi.

"s A. `l` Engen@ Patented June l; 195.2

UNITED STATES PATENT OFFICE CURRENT-LIMITING FUSE Frederick J. Kozacka, Amesbury, Mass., assignor to The Chase-Shawmut Company, Newburyport, Mass., a corporation of Massachusetts Application May 17, 1951, Serial No. 226,755

(Cl. ZOO- 120) 21 Claims. 1

This invention relates to fuses for the protection of electric circuits, and more particularly to current-limiting fuses, i. e., fuses which do not permit the current in the circuit into which they are placed to reach the available shortcircuit current of the circuit.

It is an object of this invention to provide a fuse wherein the internal volume of the casing, the range of grain size of the arc quenching filler and the maximum continuous current of the fuse link are correlated to enable the fuse to effectively interrupt both currents of shortcircuit current proportions and small fault currents of inadmissible duration.

Another object of this invention is to provide a composite current-limiting fuse structure consisting of identical fuse units jointly capable of effectively controlling currents over a very wide range.

Still another object of this invention is to provide an extremely compact fuse which drastically limits arc energy on interrupting fault currents of load proportions and on interrupting fault currents of short-circuit current proportions.

A further object of the invention is to provide an extremely compact current-limiting fuse of the type comprising a link of silver and a ller of quartz sand capable of effectively interrupting both currents of short-circuit current proportions and small fault currents of inadmissible duration.

It is well known that the grain size of arc quenching fuse filler, and more particularly the size of the grains of a quartz sand ller in a fuse having a link of silver may have an influence upon the maximum current interrupting ability of a fuse. I have discovered that the grain size of the 'are quenching filler also has an important effect, jointly with other factors, upon the operation of current-limiting fuses in the small overload range.

In testing commercial current-limiting fuses in the power laboratory I have found that many fuses which operated very satisfactorily on large fault currents in the nature of short-circuit currents failed when subjected to protracted currents slightly exceeding their current rating. Such fuses would sometimes interrupt the small excess current in an apparently satisfactory manner, but a restrike would tend to occur as late as many minutes after initial circuit interruption. The rekindling of the arc concomitant with the restrike tends to result in destruction of the fuse casing. This constitutes a fire haz-` ard and impairs the safe operation of fused distribution systems. It is therefore a further object of my invention to provide a currentlimiting fuse which is not subject to the danger of post-interruption restrikes.

Interruption of any current, Whether high or low, in a current-limiting fuse of the silver link quartz filler type results in the formation of a body of fused sand which is in the nature of a semi conductor as long as hot. This fused sand `body is converted into an insulator as it cools down. Formation of the semi conductor results in almost instantaneous reduction of the flow of power current to a very small current which may be referred to as the leakage current of the fuse. Protracted ow of that leakage current on account of the potential prevailing across the terminals of the fuse may cause the 12.1 losses in the fused quartz sand body to become so high as to cause a restrike and rekindling of the arc after initial operation of the fuse and consequent total loss of all of its interrupting ability.

It is a still further object of my invention to provide a current-limiting fuse in which the post interruption 12J losses ocurring in the fused sand body are so low that no restrike can take place irrespective of how long the fuse remains in the faulted circuit after initial interruption thereof by the fuse. I

I have found that restrikes can be effectively precluded by combining three means, i. e., by drastic reduction of the internal volume of the casing of the fuse to the order of several thousandths of a cubic inch per kva. capacity, by the use of a pulverulent quartz filler larger than No. but smaller than No. 18 U. S. Standdard Sieve Number and by a limitation with regard to current, i. e., by the provision of a fuse link which limits the maximum continuous current of the fuse to less than 36 amperes, and preferably even to less than 32 amperes.

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

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

Fig. 2 is a cross-section along 2-2 of Fig. 1;

Fig. 3 is a view, partly in longitudinal section, of a modification of the structure shown in Figs. 1 and 2;

Fig. 4 is a view, partly in longitudinal section, of a multi-element fuse designed for very high currents, embodying my invention; and

Fig. 5 is a cross-section along 5 5 of Fig. 4.

Referring now to Figs. 1 and 2 of the drawing, I have shown -a current-limiting fuse comprising a casing I of considerable mechanical strength capable of withstanding high internal pressures. In order to minimize the adverse action of 12J losses of the post interruption leakage current the material of which casing I is made should be of a heat resistant type. Materials complying with the requirements of high mechanical strength and high heat resistance are glass-cloth-silicon-resm laminates and glasscloth-melamine-resin laminates and I prefer such laminates for making the casings of fuses according to `my invention. To impart the required mechanical strength to casing I its wall thickness is preferably in the order of half the external radius thereof. This figure is based on a considerable safety factor. Terminal caps 2a, 2b are provided at the ends of casing I and these caps are particularly designed to withstand the action of high internal pressures within casing I. To this end casing I is provided with a pair of circular recesses Ia, Ib, one on each end thereof, and caps 2a, 2b are rolled into these recesses. It will be apparent from Fig. 1 and particularly from Fig. 2 that the volume occupied by the material of which casing I is made is large compared to the internal volume of casing I. The internal volume of casing I is in the order of but several thousandths of a cubic inch per kva. capacity. In prior lart National Electrical Code cartridge fuses the internal volume of the casing per kva. is in the order of .026 to .088 for the 250 volt rating and in the order of .073 to .065 for the G00 volt rating. In other words, the ratio of capacity in kva. to internal volume of the casing is considerably more than ten times higher in fuses according to my invention than in National Electrical Code cartridge fuses. The fuse link 3 is of the multiperforated ribbon type and is attached to caps 2a and 2b by means of solder joints 4a, and lb. The extremely compact space of chamber Ic inside of casing I is filled with quartz sand larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number. Quartz sand of this grain range has occasionally been used in prior art fuses but the general trend was toward finer grained quartz sand because ner sand such as sand of 100 to 150 U. S. Standard Sieve Number is conducive to increased interrupting ability. I have found that the ability of a current-limiting fuse to interrupt high current increases generally if the neness of the grains of the quartz sand filler is larger than No. '70 or No. 100 U. S. Standard Sieve Number but that the ability of such a fuse to effectively interrupt small excess currents of inadmissible duration, particularly fault currents of less than two times the rated current, may be considerably increased if the quartz sand ller is relatively coarser. It follows from tests which 1 have conducted that fuses designed'for interrupting small overloads should have a coarser ller than that which would be indicated from'the viewpoint of maximum interrupting abilityand that effective interruption of small currents is a matter of case volume, grain size and link size coordination rather than a matter of grain size per se. To

achieve such a coordination the fuse shown in Figs. 1 and 2 is provided with a link designed to limit the maximum continuous current of the fuse to less than 36 amperes. This limitation of the continuous maximum current of the fuse is achieved by the provision of a plurality of equidistant circular perforations or holes 3a arranged along the axis of link 3 and provision of a tin rivet 5 in one of these perforations or holes. On overloads the tin of rivet 5 alloys with the silver of link 3 and the latter diiuses into the tin. The resulting alloy has a lower melting point and a higher resistance than silver and permits formation of an initial break at overload currents in the order of 30% less than would be required to form an initial break in an all silver fuse link. Alloy formation involves a predetermined time and this results in a time-lag characteristic of the fuse for the low overload current range. The rivet 5 is ineffective as far as interruption of currents of short-circuit current proportions is concerned. Such currents are interrupted before they can reach the available short-circuit current which the circuit is capable of producing. Reduction of the fusing temperature of a link metal by alloy formation with another is frequently referred to as Meffect and 'I use this term since it has been accepted in the trade literature.

The following table refers to a series of tests which I carried out with fuses which had a casing the internal volume of which was in the order of several thousandths of a cubic inch per kva. capacity and which hada silver link surrounded by a quartz filler. The link wasl designed to limit the maximum continuous current of the fuse to less than 36 amperes. The only variable in these tests was the fineness of the grains of the quartz sand filler.

U. S. Standard Opnng Tylr Te t Micron Opening s Sieve N umbcr Designation In Inches Results No filler failures. 3.5 5, 560 221 D0. 130.

The test results tabulated above were-.obtained from data accumulated from tests with single rfuses having a current rating of 30 amperes and composite fuses having current ratings of :60 and amperes at 1000 volts. The design of .these composite fuses is described below. The 'tests were conducted over a widecurrent range from currents slightly exceeding the rated current up to 80,000 amperes available short-circuit current: The line potential was 1048 D. C.

Any test has been deemed a failure if the fuse failed either at a high current or a low current test. The failures of fuses filled with U. S. Standard Number 3.5 to 18 quartz sand occurred when the fuses were subjected to currents of short-circuit currrent proportions. The failures of fuses lled with U. S. Standard Sieve Number 80 to 325 quartz sand occurred when the fuses were subjected to small protracted overloads. It appears from the above table that the fuses which passed successively both high current and low current interrupting tests were those which had a quartz filler larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number. The fuses of the filler range enabling them to pass successfully both high current and low currrent interrupting tests were subsequently radiographed and it was then found from analysis of the radiographs that quartz sand having a grain size Ibetween No. 30 and No. 40 U. S. Standard Sieve Number had a high ability to interrupt small overload currents as well as very high currents. It is true that sand of that preferred iineness has not the highest short-circuit current interrupting ability but it has sufficient, or even ample, high current interrupting ability to effect current-limiting interruption of the test circuit.

Successfully operating fuses may be filled with a quartz filler which is a mixture of particles of various U. S. Standard Sieve Numbers, provided that the grain sizes lie within the limits suggested by the above table. In other words, satisfactory operation may be expected from mixtures that will pass through an 18 mesh screen and be held on an 80 mesh screen.

The table below refers to a new line of current-limiting fuses having current ratings varying between 30 and 200 amperes at 1000 volts filled with the preferred 30/40 grain size quartz sand. The 30/40 filler grain has a maximum diameter of .0232" and a minimum diameter of .0164".

Internal Volume (Cu. In.)

Kva. per

Current Rating amps. Cu. IIL

Kva.

The fuse rated at 60 amperes was made up of two fuse units of the kind shown in Figs. 1 and 2 arranged in parallel. Similarly the fuse rated at 100 amperes consisted of three and the fuse rated at 200 amperes of seven fuse units of the type shown in Figs. 1 and 2. These composite structures for fuses exceeding the 30 ampere rating are a consequence of the requirement that the maximum continuous current of -a single currentlimiting fuse be less than 36 amperes if the fuse is to interrupt low currents satisfactorily without resorting to special relatively complicated W current interrupting means as, for instance, a pair of contacts spring-biased to the open position thereof and adapted to break small fault currents upon fusion of a eutectic alloy normally holding the contacts in the closed position.

The multiperforated ribbon-type fuse link applied in the structure of Figs. 1 and 2 has a large surface to volume ratio and other properties conducive to effective interruption of short-circuit currents. These properties are enhanced by `assigning the relatively small continuous current carrying duty of less than 36 amperes to the link making it possible to minimize the amount of link metal. The rated current of such a small current carrying duty and high current interrupting capacity link :arranged in a fuse structure of the kind under consideration is less than 40 amperes. Low fault current interruption is aided by the limitation of the maximum continuous current to less than 36 amperes and the use of an Meffect causing means on the link. Both these features concur in limiting the operating temperature of the fuse and make it possible to effect interruption of low currents with, and in spite of, a drastic reduction of the amount of available granular quenching medium. It is believed that the operation of Paaschens law has also an important bearing on the operation of the fuse since the pressures observed therein are in excess of the pressure range observed in prior art current-limiting fuses. There is evidence tending to show that the oscillatory pressures occurring within the casing at the interruption of small protracted overloads coact critically with the magnitude of the surfaces of the arc quenching ller granules and the sizes of the voids between the ller granules. This coaction seems to effect a maximum of deionization by turbulent flow of gas-diluted metal vapors through the voids between the quartz granules and of deionization at the surfaces of the quartz granules.

In interrupting short-circuit currents the fuse operates entirely differently without the pressure oscillations typical for low current interruptions and without the accompanying flows of diluted metal vapors back and forth through the voids or interstices between the quartz granules. This difference in the mode of operation is apparently the reason why the optimum interrupting ability of quartz sand of given grain size is different for high current and low current interruptions and why a judicious compromise as to the neness of the quartz grains must be sought if a currentlimiting fuse is required to interrupt Without restriking small protracted overload currents in addition to short-circuit currents.

The same reference signs have been used in Fig. 3 as in Figs. 1 and 2 to indicate like parts. The structure shown in Fig. 3 diifers from that of Figs. 1 and 2 in that the right end of the fuse housing contains a ller of chalk which surrounds the Meffect causing portion of the fuse link. At low current interruptions arcing is initiated within that chalk filler and this results in the formation of carbon dioxide according to the equation CaCOaCOz-l-CaO. The quartz sand filler and the chalk ller are separated from each other by a washer 1, which is loosely fitted into the casing I. Because of this loose fit the carbon dioxide gas generated at the point of arc initiation diluting the metal vapors there formed is free to flow to the left portion of the fuse, i. e., into the voids or interstices between the quartz particles of critical size. The provision of a limited amount of chalk within the fuse casing I tends to increase the pressure prevailing therein at the interruption of low currents Without raising the pressure beyond the limit of the mechanical strength of casing l The aver-age heat conductivity of carbon dioxide between 0 and 6000 degrees absolute is about 2.5 times the average heat conductivity of air and the interrupting ability of a gaseous medium increases generally in proportion to its heat conductivity. Therefore the provision of a small amount of chalk limited to the point of arc initiation has a plurality of desirable effects including the formation of an effective scavenging gas flow away from the point accadde of .are initiation towards the 'highly vleffective quartz sand deionizen the addition of a highly `neat conductive gas :to the gasesand. nietalvapoi's in the ,fuse casing, and an increase .of the pressure level prevailing therein With an attending lincrease .of the dielectric Strength of the gas .and vapor -rnixture within the lcasing -i-n conformity with Paaschens law.

The Vlink ofthe fuse of Fig. k3 is provided with a neck Vor reduced cross-sectionportion Sli-Which limits the let-through current of the fuse-Without affecting its maximum continuous current and rated current. The operation of this particular neck or reduced cross-section portion .3b is more fully described in mv copending patent application Ser-idalNo. 298,548,1iled January 30, 1951, for Current Limiting Fuses, assigned to the saine .aS- signee as the present patent application. The circuit into which the -fuse is inserted has been diagrammatically indicated by leads 8a, 8b in Fig. 3. In actual practice leads 8a, 8b will be lconnected to the terminal caps 2a, 2b by means of a fuse holder (not shown in Fig. 3).

The composite high capacity fuse shown in Figs. 4 and 5 is made up of a plurality .of single units of the type shown in Figs. 1 .to 3 of which each unit comprises a separate casing I. The left terminal caps 2a of the individual -fuse units are inserted into cylindrical recesses 9' formed in a heavy metal block 9a. Similarly the right terminal caps 2b of the individual fuse units are inserted into cylindrical recesses yEl formed in the heavy metal block 9b. The space between the individual fuse units is filled with a granular material I3, such as quartz sand, adapted to Acope `with the arc products formed in the individual fuse units in case that one of these units should burst under the action vof ,excess pressure. The blade contacts lila, IGb form integral parts of metal blocks 9a, 9b and are intended `for inserting the composite fuse into a fuse holder and an electric circuit, respectively. Ihe cylindrical shell I I made of insulating material encloses the individual fuse units, the granular material I3 and the `metal blocks 9a, Sb and issecured to the latter `by means of vtransverse studs I4. Outer `ferrules I5a, i511 arerarranged kon opposite `ends of shell II and overlap the cylindrical side Walls thereof.

A composite fuse which is similar in sonne respects to that shown in Figs. 4 and 5 is disclosed in the copending patent application of William S. Edsall and Kenneth W. Swain, Serial No. 252.676, filed October 23,. 1.951. for Current- Limiting lFusible Protective pevices, assigned to the same assignee as the present patent Aapplication, and I do not clairn the basic structures disclosed and .claimed .in tlieabove patent application.

When referring to a quartz filler larger than No. 8D but smaller than No. 18 U. S. Standard Sieve Number I mean to refer to a mixture of grain sizes that will pass through a No. 18 `and be held on a No. 80 U. S. Standard Sieve Number screen. The same language applied to a -nar- -rowcr band of grain sizes lying Within .the alcove broad band is to .be construed in the saine Wav, i. e., as referring to a mixture of grain sizes that will pass through a screen of given maximum mesh size and be held on a screen .of given minimum mesh size.

`While the data which have been Agiven above refer to the behavior of lthe fuses in l). circuits, fuses according to this invention are ap- .pucab1em A. c. circuits as wen as in D. o. circuits.

The .behavior of a pulverulent quartz nller in a .currentf-limiting silver link fuse depends also to some extent upon the .crosssectional area and upon `the configuration of the .cross-sectional area of the fuse link. The critical grain size range which I :have discovered was found to exist with regard to -fuses the links of which were designed to limit the maximum .continuous current of the fuseto less than 36 amperes andrnore particular- Vly with regard te vfuses having multiperforated ribbcn-typelfuse links designed for such low currents .and yprovided with a .Meffect causing metal-element.

In thefusesshown in Figs. 1 130,3 the perfora- .tions 3.a of .the `fuse link 3 are sufliciently large to Vcause such heating Aof the linl; by 12.1" losses that .the Mefiect ,occurs at continuous currents of less than 36 amperes. Since the M- effect causing metal element is arranged away from the hottest point of the link, a portion of the link will be hotter at the time the M- eiect occurs than the point where the M- `effect occurs. The relatively high temperature of the link enables to rapidly forrna Wide .gap bv vaporizationof link rnetal immediately upon .occurrence .of the M-eiec.t. .thus minimizing the .time required on interruption of lovv currents to forni the gap length needed `vfor permanent circuit interruption and simultaneously minimizing. aro v,duration .and arc energy- ,If vthe `capacity oi a circuit to loe protected lov the composite fuse structure of Figs. Il and 5 is N kva. and the number of fuse units which go .into the composite `structure is then .each .fuse unit should nave .an approximate .capacity y kva.

and the internal volume of each fuse unit should be in the order of several thousandths, or 1/400- 1/soo of a cubic inch per N kva.

capacity.

In practice magnetic effects cause the interrupting duty not to be equally divided among the individual fuse units which go into the composite fuse structure, but for the purpose of determining the required internal volume for the .casings -of the individual fuse units the above :simplied .calculation has proved to be permissible.

While it is not possible to state a detailed theory `of the operation of the type of fuses which I have discovered, it is apparent that the selection of .the range of grain size is predicated upcn the diierence in the physics of high current vinterruption and low current interruption. High current linterruption is in the nature oi an explosion, involving a single pressure wave. This intense pressure Wave is best deionized by a relatively densely packed -filler of a high degrec .of 4iineness. But the, periodic, oscillatory, gradually increasing pressures that occur at interruption of low currents and the resulting oscillatorv new of ionized products oi arcing reduire another type ci granule size and .interstice size for .most effective kcooling and deionization.

I am aware that quartz sand coarser than mesh has been used heretofore for lling current-limiting `fuses and I do not make any claim to `the use of such sand in fuses of this type. The prior art fuses which contained this type of filler had a relatively limited short-circuit current interrupting ability and were unreliable on interrupting low currents. I was able by proper coordination of case size, ller size and maximum continuous current to obtain a structure which outperforms the prior art currentlimiting fuses with coarser than 30 mesh filler as well as any other prior art current-limiting fuse for moderate voltages with which I am familiar.

The term capacity in terms of kva. as used in the present context refers to the product of rated current and rated voltage, i. e. to the power carrying and power transmitting capacity of a fuse.

Having `disclosed several preferred embodiments of my invention, it is desired that the same be not limited to the particular structures disclosed. It will be obvious to any person skilled in the art that many modifications and changes may be made without departing from the broad spirit and scope of my invention. Therefore it is desired that the invention be interpreted as broadly as possible and that it be limited only as required by the prior state of the art.

I claim as my invention:

1. A current-limiting fuse comprising a casing having an internal volume in the order of several thousandths of a cubic inch per kva. capacity, a pulverulent ller larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number in said casing, and a fuse link adapted to limit the maximum continuous current of said fuse to less than 36 amperes arranged in said casing in contact with said filler.

2. A current-limiting fuse comprising a casing having an internal volume in the order of several thousandths of a cubic inch per kva. capacity, a. pulverulent quartz i-lller larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number in said casing, and a fuse link adapted to limit the maximum continuous current to said fuse to less than 36 amperes arranged in said casing in contact with said filler.

3. A current-limiting fuse comprising a casing of a glass cloth base having relatively thick walls and defining a chamber having an internal volume in the order of several thousandths of a cubic inch per kva. capacity, a pulverulent quartz ller larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number within said chamber, a ribbon-type fuse link having a plurality of circular perforations arranged within said chamber in contact with said filler, a, metal adapted to form 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 in contact with said link, said plurality of perforations of said link being sufficiently large to cause alloy formation between said metal and said link at continuous currents of less than 36 amperes.

4. A current-limiting fuse comprising a casing of fiber-glass-cloth-resin-laminate having a wall thickness in the order of half the radius thereof and defining a chamber having an internal volume in the order of several thousandths of a cubic inch per kva. capacity, a pulverulent quartz ller larger than No. 60 but smaller than No. 20 U. S. Standard Sieve Number within said chamber, and a multiperforated ribbon-type fuse link arranged within said chamber in contact with said ller, said link having Meifect causing means thereon adapted to limit the maximum continuous current of said fuse to less than 36 amperes.

5. A current-limiting fuse comprising a casing dening a chamber having an internal volume in the order of several thousandths of a cubic inch per kva. capacity, a 'pulverulent quartz ller larger than No. 60 but smaller than No. 20 U. S. Standard Sieve Number within said chamber, and a ribbon-type fuse link having a plurality of perforations along the center line thereof adapted to limit the maximum continuous current ofv said fuse to less than 32 amperes arranged within said chamber in contact with said ller.

6. A current-limiting fuse comprising a casing of considerable mechanical strength having a wall thickness in the order of half the radius thereof and defining a chamber having an internal volume in the order of several thousandths of a cubic inch per kva. capacity, a pulverulent quartz filler in said chamber consisting of a mixture of grain sizes that will pass through a No. 30 U. S. Standard Sieve Number screen and be held on a No. 40 U. S. Standard Sieve Number screen, and a fuse link in said chamber in contact with said filler adapted to limit the continuous current of said fuse to less than 32 amperes.

7. A current-limiting fuse comprising a casing adapted to withstand considerable internal pressures defining a chamber in the order of several thousandths of a cubic inch per kva. capacity, a granular quartz ller within said chamber consisting of a mixture of grain sizes that will pass through a No. 30 U. S. Standard Sieve Number screen and be held on a No. 40 U. S. Standard Sieve Number screen, a ribbon-type fuse link having a plurality of aligned perforations arranged within said chamber in contact with said ller, and an Meffect causing means on said link adapted to become operative at continuous currents of less than 32 amperes.

8. A current-limiting fuse comprising a casing adapted to withstand considerable internal pressures deiining a chamber in the order of QO to 1500 cubic inch per kva. capacity, a pulverulent quartz filler in said chamber consisting of a mixture of grain sizes that will pass through a No. 18 and be held on a No. 80 U. S. Standard Sieve Number screen, and a fuse link of silver within said chamber in contact with said ller adapted to limit the maximum continuous current of said fuse to less than 32 amperes.

9. A current-limiting fuse comprising a glass cloth base casing having a wall thickness in the order of half the radius thereof and defining a chamber having an internal volume in the order of 1/400 to %00 cubic inch per kva. capacity, a pulverulent quartz filler within said chamber including a mixture of grain sizes that will pass through a No. 30 and will be held on a No. 40 U. S. Standard Sieve Number screen, a ribbontype fuse link having a plurality of serially related portions of reduced cross-section within said chamber in contact with said ller, a metal element on said link adapted to form 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, and a gas evolving pulverulent ller within said chamber surrounding said alloy forming metal element, said alloy forming metal element and said link being adapted to alloy and fuse at continuous currents of less than 35 amperes.

10. A current-limiting device for a circuit requiring a capacity of N kva. including a plurality of n basic units adapted to carry current in parallel and combined to form a composite selfsustained structure, each of said 11. units consisting of a current-limiting fuse comprising a casing having an internal volume in the order of several thousandths of a cubic inch per N/n. capacity, a pulverulent quartz ller larger than No.' 80 but smaller than No. 18 U. S. Standard Sieve Number in said casing, and a fuse link adapted to limit the maximum continuous current of said fuse to less than 36 amperes arranged ink said casing in contact with said ller.

11. A current-limiting device for a circuit requiring a capacity of N kva. including a plurality of n basic units adapted-to carry current in parallel and combined to form a composite selisustained structure, each of said 11; units consisting of a current-limiting fuse comprising a casing of a glass cloth base having relatively thick Walls and renng a chamber having an internal volume in the order of several thousandths of a cubic inch per N/n capacity, a pulverulent quartz ller larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number within said chamber,A aribbon-type fuse link having a plurality of' circular perforations arranged within said chamber in contact with said ller, and a metal adapted to form an alloy with the metal of which said link is made having a lower fusing point than the metal of which said link '1s made in contact with said link, said plurality of perforations of' said link being suiiiciently large to cause alloy formation between said metal and said link at continuous currentsl of` less than 36 amperes.

12. A current-limitingv device for a circuit requiring a capacityv of N kva. including a plurality of n basic units adapted to carry current in parallel and combined toA form a composite selfsustained structure, each of said 'n units consisting of a current-limiting fuse comprising a casing of ber-glass-cloth-melamine-resn-laminate having a wall thickness in the order of half the radius thereof and defining a chamber having. an internal volume in the order of several thousandths of a cubic inch per N/n capacity, a pulverulent quartz ller larger than No. 60`but smaller than No. U. S'. Standard Sieve Number Within said chamber, and a multiperforated ribbon-type fuse link arranged within said chamberv in contact with said filler, said link having M-effect causing means thereon adapted to limit the maximum continuous current of said fuse to less than 36 amperes.

13. Ay current-limiting device for a circuit requiring a capacity of N kva. including a plurality of n basic units adapted to carry current in parallel and combined to form a composite selfsustained structure, each of said n units consisting of a current-limiting fuse comprising a casing dening a chamber having an internal Volume in the order of several thousandths of a cubic inch per N/n capacity, apulverulent quartz Y iiller vlarger than No. but smaller than No. 20 U. S.,Standard Sieve Number within said chamber, and a ribbon-type fuse having a plurality of perforations along the center 'line thereof adapt-v ednto.` limit the maximum' continuous currentof said fuse to less than BZ/.amperes arranged within said vchamber in' contact with said iiller.

14. Av current-limiting device for a circuit requiring a capacity of N kva. including a plurality ofrn basic units adapted to carry current in parallel and combined to form a composite selisustained structure, each of said n units consisting of a current-limiting fuse comprising a casing of considerable mechanicalstrength having a Wall thickness in the. order of half the radius thereofv and defining a chamber having an internal volume in the order of several thousandths of a cubic inch per N/w capacity, a pulverulent quartz filler insaid chamber consisting of a mixture of grain sizes that will pass through a No. 30 U. S. Standard Sieve Number screen and be held on a No. 40' U. S. Standard Sieve Number screen, and a fuse link in said chamber in contact with said filler adapted to limit the continuous current of said fuse to less than 32 amperes.

15. A current-limiting device for a circuit requiring a capacity of N kva. including a plurality of n basic units adapted to carry' current in parallel and combined to form a composite selfsustained structure, each of said n units consisting of a current-limiting fuse comprising a casing adapted to withstand considerable internal pressures dening a chamber in the order of several thousandths of a cubic inch per N/n capacity, a granular quartz filler within said chamber consisting of a mixture of grain sizes that will pass through a No. 30 U. S. Standard Sieve Number screen and be held on a No. 40 U. S. Standard Sieve Number screen, a ribbon-type fuse link having a plurality of aligned perforations arranged Within said chamber in contact with said iiller, and an M-effect causing means on said link adapted to become operative at continuous currents of less than 32 amperes.

16. A current-limiting device for a circuit requiring a capacity of N kva. including a plurality of n basic units adapted to carry current in parallel and combined to form a composite selfsustained structure, each of said n units consisting of a` current-limiting fuse comprising a casing adapted to withstand considerable internal pressures dening a chamber in the order of 1/400 to l/auo cubic inch per N/n capacity, a pulverulent quartz ller in said chamber consisting of a mixture of grain sizes that will pass through a No. 18 and be held on a No. 70 U. S. Standard Sieve Number screen, and a fuse link of silver within said chamber in contact with said filler adapted to limit the maximum continuous current of said fuse to less than 32 amperes.

17. Ak current-limiting device for a circuit requiringa capacity of N kva. including a plurality of 'n' basic'units adapted to carry current in parallel and combined to form a composite selfsustained'structure, each of said n units consisting of a `current-limiting fuse comprising a glass cloth base casing having a wall thickness in the order of half the radius thereof and delining a chamber having an internal volume in the order ofOO to 1/500 cubic inch per N/'n capacity, a pulverulent quartz filler Within said chamber including a mixture of grain sizes that will pass through a No. 30 and will be held on -a No. 40 U. S. Standard-Sieve Number screen, a ribbon-type fuse link having a plurality of serially related portions of reduced cross-section within said chamber in contact with said ller, a metal element on said link adapted to form an alloy with the metal of which said link is made having a lower fusing point than the metal ci. which said link is made, and a gas evolving pulverulent llerwithinA said chamber surrounding said alloy forming metal element, said alloy forming metal element and said link being adapted to alloy and fuse at continuous currents of less than 36 amperes.

18. In combination an electric circuit having a predetermined capacity, a current-limiting fuse arranged in said circuit, said fuse comprising a casing having an internal volume in the order of several thousandths of a cubic inch per kva.

of said capacity, a pulverulent quartz ller larger than No. 80 but smaller than No. 18 U. S. Standard Sieve Number Within said casing, and a fuse link adapted to limit the maximum continuous current of said fuse to less than 36 amperes arranged in said casing in contact with said filler.

19. In combination an electric circuit having a predetermined capacity, a current-limiting device including a plurality of n basic units adapted to carry current in parallel and combined to form a composite self-sustained structure arranged in said circuit, each of said n units consisting of a current-limiting fuse having a casing dening a chamber the internal volume of which is in the order of 1/400 to 1/100 cubic inch per kva.

of said capacity, a pulverulent quartz ller in said chamber consisting of a mixture of grain sizes that will pass through a No. 18 and be held on a No. 80 U. S. Standard Sieve Number screen, a ribbon-type fuse link arranged in said chamber in contact with said iiller, and said link having an Meiect causing means thereon limiting the maximum continuous current of said fuse to less than 36 amperes.

20. In combination an electric circuit having a predetermined capacity, a current-limiting device including a plurality of n basic units adapted to carry current in parallel and combined to form a composite self-sustained structure arranged in said circuit, each of said n units consisting of a current-limiting fuse having a casing dening a chamber the internal volume of which is in the order of several thousandths of a cubic inch per kva.

of said capacity, a pulverulent quartz ller within said chamber larger than No. 70 but smaller than No. 20 U. S. Standard Sieve Number, a ribbontype fuse link having a plurality of circular perfora-tions along the center line thereof arranged in said chamber in contact with said filler, and a metal element adapted to form 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 arranged at some point of said link and adapted to cause alloy formation and fusing of said link at continuous currents of less than 36 amperes.

21. In combination an electric circuit having a predetermined capacity, a current-limiting device including a plurality of n basic units adapted to carry current in parallel and combined to form a composite self-sustained structure in said circuit, each of said n units consisting of a currentlimiting fuse having a casing dening a chamber the internal volume of which is in the order of several thousandths of a cubic inch per kva.

of said capacity, a pulverulent quartz iiller within said chamber consisting of a mixture of grain sizes that will pass through a No. 20 and be held on a No. U. S. Standard Sieve Number screen, a ribbon type fuse link arranged within said chamber in contact with said ller, and a metal element 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 in contact with said link, said link and said metal element being adapted to limit the maximum continuous current of said fuse to less than 36 amperes.

- FREDERICK J. KOZACKA.

No references cited.

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