US3571776A

US3571776A - High voltage fuse having helically wound ribbon fuse link

Info

Publication number: US3571776A
Application number: US879090A
Authority: US
Inventors: Frederick J Kozacka; Richard A Belcher
Original assignee: Chase Shawmut Co
Current assignee: GOLUD INC A DE CORP; Chase Shawmut Co; Gould Inc
Priority date: 1969-11-24
Filing date: 1969-11-24
Publication date: 1971-03-23
Anticipated expiration: 1988-03-23

Abstract

A high-voltage fuse having a helically wound ribbon fuse link forming an extension-springlike structure. The ribbon fuse link is entirely self-supporting, not requiring any support between the ends thereof other than a pulverulent arc-quenching filler.

Description

United States Patent Frederick J. Kozacka South Hampton;

Richard A. Belcher, Hampton Falls, N.H. 879,090

Nov. 24, 1969 Mar. 23, 1971 The Chase-Shawmut Company Newburyport, Mass.

Inventors App]. No. Filed Patented Assignee HIGH VOLTAGE FUSE HAVING HELICALLY WOUND RIBBON FUSE LINK 7 Claims, 7 Drawing Figs.

U.S. Cl. 337/16], 3 1.22 Int. Cl. I-I0lh 85/08, 7

H0 1 h 8 5/12 FieldofSearch 337/158,

[5 6] References Cited UNITED STATES PATENTS 3,460,085 8/1969 McAllister 337/295(X) 3,374,328 3/1968 Cameron 337/293(X) 2,5 99,646 6/1952 Kozocka 337/ -l 60 FOREIGN PATENTS 807,347 l/l959 Great Britain 337/295 Primary Examiner-Barnard A. Gilheany Assistant Examiner-Dewitt M. Morgan Attorney-Erwin Salzer ABSTRACT: A high-voltage fuse having a helically wound ribbon fuse link forming an extension-springlike structure. The ribbon fuse link is entirely self-supporting, not requiring any support between the ends thereof other than a pulverulent arc-quenching filler.

PATENTEUHARZSIQ?! SHEET 1 BF 2 FIG. I

A 2% m A m m K v mJ K m R E D E R F RICHARD A. BELCHER WIM/m ATTY.

PATEN-TE'U mes I97! SHEET 2 0F 2 INVENTORS:

llillfillil VOLTAGE FUSE HAVKNG HEHCAILLY WOUND REBBON FUSE LllNK BACKGROUND OF INVENTION High-voltage fuses often call for fusible elements whose length exceeds that of the casing of the fuse or fuse tube. in such instances as a general rule the fusible elements are would helically around a mandrel of ceramic or other heat resistant material, the mandrel being substantially star-shaped in cross section. This conventional design is subject to many drawbacks and limitations, and it has been the desire for many years of many designers of high-voltage fuses to provide highvoltage fuses which do not require a mandrel for supporting helically wound fusible elements.

The problem of providing high-voltage fuses having a helically wound fusible element, or several helically wound fusible elements, differs depending upon whether the fusible element or elements are formed of a silver wire, or a silver ribbon, or of a material other than silver. In this context consideration will be given only to the use of ribbon fuse links of sheet silver whose use is mandatory as far as current-limiting high-voltage fuses are concerned.

it is possible to wind a silver ribbon link helically around a temporary mandrel and remove or withdraw the mandrel from the casing of the fuse during the process of filling the casing of the fuse with a pulveru'lent arc-quenching filler. In that particular instance the arc-quenching filler provides a gradually extending lateral support for those portions of the helically wound fuse link from which the temporary link-supporting mandrel is being gradually withdrawn. This process of making high-voltage fuses whose helically wound ribbon fuse links do not require a supporting mandrel when the fuse is fully assembled calls for a great deal of manual dexterity and the cost of manufacture of such fuses are, therefore, relatively high.

British Pat. No. 807,347 refers to one of the early attempts to design high-voltage fuses whose ribbon fuse links are wound helically and do not require a link-supporting mandrel. The helically wound fuse link disclosed in the above patent is channel-shaped in cross section, or has a similar shape, tending to greatly increase its rigidity. This avoids, or is supposed to avoid, serious distortions of the helically wound fuse link or fusible element by the impact of pulverulent arc-quenching filler that normally occurs in the process of filling such fuses.

The configuration of a fusible element of silver as disclosed in the above British Fatent is relatively complex and tends to increase the required amount of silver beyond that needed to comply with current-carrying requirements. Such an increase of the requirements of silver tends, in turn, to adversely affect the interrupting capacity of fuses designed according to the above British patent and reduces their cost-effectiveness to impracticability. The offcenter ends of the helical fuse link according to the above British Patent are subject to considerable stresses so that the link, if considerably extended over and above its original length, tends to become overstressed.

it is possible to design high-voltage fuses having helically wound ribbon fuse links which are laterally supported at a few discrete points, particularly the center thereof, rather than at a large number of points, as in case of use of a mandrel arranged inside of a helical fuse link. The above is a step in the right direction, but still far away from the ultimate goal of dispensing entirely with additional structures providing lateral support for helically wound fuse links.

TECHNICAL BACKGROUND ln high-voltage fuses the dielectric strength of the arc gap immediately following arc-extinction is a matter of crucial im- We have found that it is possible to make helical fuse links of critically small diameter which do not require any lateral support to prevent serious distortions thereof during the process of filling the fuses with a pulverulent arc-quenching filler. We have further found that if a helically wound fuse link of critically small diameter is housed in a tubular casing whose inner diameter is a small multiple of that critically small diameter of the fuse link, the electrical performance of the fuse is greatly boosted. This is attributed to the increase in pressure inside the casing incident to blowing of the fuse which occurs when the inner diameter of the casing is limited to a small multiple of the critical diameter of the helically wound fuse link.

A helically wound fuse link of critically small diameter such as three-sixteenths inch has inherently a relatively small current-carrying capacity. Since the inner diameter of the casing must be limited for best electrical performance to a small multiple of the diameter of the helically wound ribbon fuse link, fuse units embodying this invention are limited in regard to their current-carrying capacity. Their current-carrying capacity can, however, be increased to comply with all existing needs or requirements in regard to current-carrying capacity by paralleling several fuse units embodying this invention. Such a multiple fuse embodying the present invention compares in many respects favorably with conventional high-voltage fuses including a common mandrel of relatively large diameter supporting a plurality of spaced helically wound ribbon fuse links. One of these favorable distinctions resides in the absence of a link-supporting mandrel. Another less obvious favorable distinction relates to the L/l ratio, wherein L is the length of the helically wound ribbon fuse link and l the useful length of the casing of the fuse. In high-voltage fuses including a common mandrel supporting a plurality of spaced helically wound ribbon fuse links of relatively large diameter the LH ratio decreases as the number of fuse links on the common mandrel increases and under such circumstances the L/l ratio may be increased or improved by providing module type fuses including a plurality of units each having a helically wound ribbon fuse link of critically small diameter and a casing whose internal diameter is also critically small. In any fuse the casing area which is exposed to pressure incident to blowing of the fuse decreases as the diameter of the fuse tube decreases, and it is possible to admit in fuses embodying this invention extremely high pressures without resorting to casings having an excessive wall thickness.

Helically wound fuse links embodying this invention must have certain elastic properties, i.e. they must operate like extension springs. To this end they must be made of silver having a high elastic or Youngs modulus. This is, in other words, relatively hard silver. Silver known as hard silver is preferred. It is, however, possible to wind ribbons of half hard silver into coil springs satisfactory for the purposes of this invention particularly since the process of coil winding involves work hardening. I

As used in this context the term helical fuse link of critically small diameter means a helically wound fuse link whose diameter ranges from about three-sixteenths of an inch to less than one-half inch. The diameter can hardly be significantly less than three-sixteenths of an inch, but the latitude in regard to an increase of diameter is larger than in regard to a decrease in diameter.

The statement that the diameter of the casing ought to be but a small multiple of the diameter of the helically wound fuse link means that the diameter of the casing ought to be less than about three times to about four times the diameter of the helically wound ribbon fuse link. Best results under severe short circuit conditions have been achieved with casings having an inner diameter of about 2.6 times the diameter of the helically wound ribbon fuse link.

The silver ribbons to be wound helically ought to be rectangular in cross section and have serially related points of reduced cross-sectional area. Such ribbons are wound with the wide side thereof parallel to the axis of the helix, or spring axis. The silver ribbon is originally wound upon a temporary mandrel. Contiguous turns of the winding should be narrowly spaced, e.g. one thirty-second of an inch, or. their spacing may even be virtually reduced to zero (closed winding). When arranged within the casing of the fuse the helically wound ribbon fuse link of silver is stretched to such a degree that the deflection per turn exceeds the diameter of the helix. When a helically wound ribbon fuse link of relatively hard sheet silver having a diameter of about three-sixteenths to about one-half inch is loaded, or stretched, to such a degree that the deflection per turn exceeds significantly the diameter of the helix, e.g. is twice the diameter of the helix, the helically wound silver link is rendered capable of carrying predetermined lateral loads without buckling or losing its helical shape. To be more specific, the process of filling of a cartridge fuse having an extensionspring-type ribbon fuse link with a pulverulent arcquenching filler involves a lateral loading of the fuse link tending to cause buckling and more or less serious distortions of the fuse link. The danger of buckling under relatively small lateral loads peculiar to extension springs of silver can be avoided by limiting the diameter of such springs and by axially loading, or preloading, such springs to such an extent that the deflection per turn exceeds significantly the diameter of the helix.

The aforementioned degree of stretching, or deflection, is not only required for mechanical reasons, i.e. for the purpose of preventing buckling or distortion under the lateral loads involved in the process of filling of the casing with a pulverulent arc-quenching filler, but also for electrical reasons. There would be a tendency upon blowing of the fuse of short-circuiting of the fulgurites taking the place of contiguous turns of a helical ribbon fuse link having a diameter of as little as about three-sixteenths to one-half of an inch if the deflection or spacing between contiguous turns were not significantly larger than the diameter thereof.

In a fuse complying with the above requirements the danger of buckling and distortion of the helical ribbon fuse link during the filling process is eliminated. l-Ience any lateral support for helical ribbon fuse link between the terminal elements of the fuse may be dispensed with. Once the casing of a fuse which includes a helically wound ribbon fuse link is filled with a pulverulent arc-quenching filler, the latter establishes lateral support for the helical fuse link, provided that the filling operation is properly performed, and particularly that no voids are left in the pulverulent arc-quenching filler that might result in an inadmissible lateral loading of its helical fuse link.

The coil radius, or winding radius, of any extension spring decreases when the spring is loaded and its deflection increased. ln springs according to the present invention used as fuse links this decrease of coil radius, or winding radius, is insignificant and may be neglected.

The ribbon fuse links embodying this invention formed into a helical extension-springlike structure have overcenter ends at the ends of the helical portion thereof.

Summarizing the above, we have discovered that it is possible to avoid the danger of distortion of helically wound ribbon fuse links of high-voltage fuses without providing any lateral supports between the ends of the fuse links other than a pulverulent arc-quenching filler, and to arrive at high-voltage fuses having superior electrical performance characteristics if all of the following conditions are met concurrently:

1. The ribbon fuse link must be of relatively hard resilient sheet silver;

2. The ribbon fuse link must be of rectangular cross section;

3. The ribbon fuse link must have a plurality of serially related points of reduced cross-sectional area;

4. The ribbon fuse link must be wound helically with the wide side thereof parallel to an axis thus forming an extensionspringlike structure if sufficiently hard and resilient sheet silver is being used;

5. The diameter of said cxtension-springlike structure must range from about three-sixteenths inch to less than one-half inch;

6. Contiguous windings of the extension-springlike structure must be narrowly spaced and their spacing must be substantially less than the diameter thereof;

7. The aforementioned extension-springlike structure must be axially stretched to such a degree that the deflection per turn exceeds the diameter thereof resulting in an extension to a predetermined length;

8. The aforementioned extension-springlike structure must have overcenter ends at the ends of the helical portion thereof;

9. The aforementioned extension-springlike structure must be arranged in coaxial relation inside of a cylindrical passage defined by a receptacle having the same length as said springlike structure when the latter is in axially stretched condition and the aforementioned passage must have an inner diameter ranging from less than about three times to four times the diameter of the aforementioned extension-springlike structure.

10. The receptacle must be closed at the ends thereof by a pair of terminal elements conductively interconnected by the aforementioned extension-springlike structure and the latter must be supported laterally by a pulverulent arc-quenching filler only (dispensing with any other means of support).

Normally points of reduced cross-sectional area may be established in ribbon fiise links either by perforations or by lateral incisions beginning at the edges and extending toward the center line of the fuse link. In the case of the present invention it is, however, highly desirable that all points of reduced cross section be formed by circular perforations rather than by lateral incisions.

SUMMARY OF INVENTION High-voltage fuses embodying this invention include a ribbon of relatively hard silver having a rectangular cross section and a plurality of serially related points of reduced cross-sectional area which is wound helically and has the physical characteristics of an extension spring. The silver ribbon is wound with the wide side thereof parallel to the spring axis along a helical path ranging from about three-sixteenths inches to less than one-half of an inch in diameter. This extension-springlike structure has an initial deflection per turn of substantially less than the diameter of the helical path along which it is wound, and said initial deflection varies preferably between zero and one thirty-second inches. The extensionspringlike structure is stretched axially to minimize the tendency of buckling and distortion under lateral loads. The axial stretching or loading of the extension-springlike structure is effected to such an extent that the deflection per turn of said structure significantly exceeds its diameter. The aforementioned ribbon link in spring form has overcenter ends at the ends of the helical portion thereof. Fuses embodying this invention further include means defining a cylindrical passage having substantially the length of the aforementioned extension-springlike structure and having an inner diameter ranging from less than about three times to four times the diameter of the helical silver ribbon spring. The latter is arranged inside said passage in coaxial relation thereto and the overcenter ends of the helical silver ribbon spring are substantially coextensive with the axis of the passage. A pair of terminal elements is arranged at the ends of the passage each conductively connected to one of the overcenter ends of the helical silver ribbon spring. The passage is filled with a pulverulent arcquenching filler submersing the silver ribbon spring and said filler is the only lateral support of said spring between said terminal elements of the fuse.

BRIEF DESCRIPTlON OF DRAWINGS FIG. 1 is a longitudinal section of a modular multiunit highvoltage fuse with the center portion of the fuse being broken away to reduce the size of F 16. ll;

FIG. 2 is a cross section taken along ll-ll of FIG. 1;

FIG. 3 is an isometric view of a detail of the structure of FIG. 4;

FIG. 41 is a longitudinal section of a single fuse unit, five such units being included in the modular structure of FlGS. l and 2;

FIG. is a section along V-V of FIG. 4;

H6. s is a longitudinal section of a modification of the structure of FlGSfi and 2; and

FIG. 7 is a section along Vll-Vll of FIG. 6.

DESCRlPTIQN OF PREFERRED EMBODIMENTS OF THE INVENTlON Referring to FIGS. l and 2, numeral 1 has been applied to indicate a tubular casing of electric insulating material housing five fuse units 2 of the general character illustrated in FIGS. 3-5. Casing l is closed on the ends thereof by electroconductive terminal plugs 3. Each of the axially inner end surfaces of terminal plugs 3 is provided with five cylindrical recesses 3a. The recesses 3a in the right terminal plug 3 are arranged in registry with the recesses 3a in the left terminal plug 3. Juxtaposed recesses 30 in plugs 3 receive the terminal caps 2a of fuse units 2. Plugs 3 are secured to casing l by transverse steel pins 4 and the space inside casing 1 between fuse units 2 may be filled with a body of quartz sand 5.

Each fuse unit 2 includes a casing 2b of electric insulating material housing a helical extension-springlike structure formed by a ribbon of relatively hard silver having a rectangular cross section as indicated in FIGS. 2 and 5. The aforementioned ribbon has serially related points of reduced cross-sectional area formed by spaced circular perforations 20 as best shown in FIG. 3. The ribbon forming the extension-springlike structure 20 is wound with the wide side thereof parallel to the spring axis along a helical path of about three-sixteenths inch diameter. To be more specific, the silver ribbon of which spring 20 is formed is initially wound on a mandrel having a diameter of less than three-sixteenths of an inch and expands to a diameter of about three-sixteenths of an inch when the helical spring 20 is removed from its cylindrical forming mandrel. Helical spring 20 has an initial deflection per turn substantially less than the diameter of its helical path. When placed inside casing 2b spring 20 is stretched axially to such an extent that the deflection per turn or winding significantly exceeds the diameter thereof. The ratio of the diameter d of spring 20 to the deflection Abetween contiguous turns, or windings, may be about 1:2. This degree of axial loading of spring 20 coupled with its critically small diameter make it possible for spring 20 to be subjected to a certain degree of lateral loading without buckling, such lateral loading being inherent in the process of filling the casing of a fuse with a pulverulent arc-quenching filler. Spring 20 has overcenter ends 2c" betweenthe helical portion thereof, as best shown in FIG. 4. The inner diameter of tubular casing 217 ranges from less than about three times to four times the diameter of spring 2c. This limitation in regard to the size of easing 2b has two important implications. The smaller the diameter of a fuse tube or casing relative to that of a helically wound coaxially arranged ribbon fuse link housed therein, the smaller the tendency of distortion or buckling of the fuse link during the process of filling of the fuse tube or casing with a pulverulent arcquenching filler, all other conditions being equal. Furthermore, the smaller the diameter of a fuse casing relative to that of a helically wound fuse link coaxially arranged therein, the larger the pressure generated inside the casing incident to blowing of the fuse all other conditions remaining equal, and the smaller the diameter of a fuse casing relative to that of a helically wound coaxial ribbon fuse link arranged therein, the smaller the mechanical strength requirements imposed upon the casing when a given pressure is generated therein incident to blowing of the fuse. Thus benefits of an electrical as well as of a mechanical nature ensue from the above relation between the critically small diameter of the helically wound ribbon fuse link and that of the casing housing the same.

The terminal caps 2a are mounted on the axially outer ends of casing 2b and are perforated at the center thereof. The outer end surfaces of caps 2a form recesses in the shape of truncated cones. The overcenter ends 20" of spring 20 are threaded through the perforations in caps 2a and the axially outer ends of springs 20 are soldered at S to caps 2a.

Casing 21: may be filled with one or several pulverulent arcquenching fillers, depending upon the desired performance characteristics of fuse unit 2. As shown in FIG. 45 fuse unit 2 is filled at the left side thereof with a relatively large body of quartz sand 6 and on the right side thereof with a relatively small body of a low thermal conductivity filler 7. Fillers 6 and 7 are sufficiently compacted to avoid intermixing at their interface 8. The portion of spring 20 embedded in the low thermal conductivity arc-quenching filler 7 is provided with a linksevering overlay 9 of a metal having a considerably lower fusing point than silver, e.g. tin. The low thermal conductivity arc-quenching tiller 7 is preferably a mixture including about 50 percent of silica, and 20 percent of iron oxide and aluminum oxide. The merits of such a composite pulverulent low thermal conductivity arc-quenching filler are set forth in greater detail in the copending patent application of Frederick J. Kozacka, filed Nov. 13, 1969, Ser. No. 876,445 for HIGH- VOLTAGE FUSE. Quartz sand may be substituted for the body of low thermal conductivity tiller 7 if the fuse is not intended to interrupt overload currents but only intended to interrupt major faults currents.

It is apparent from FIG. 4 that fillers 6,7 inside casing 2b submerse spring 2:: and that fillers 6,7 are the only lateral support of spring 2c between terminal elements or terminal caps 2a.

The ratio of fillers 6 and 7 has important implications regarding the operation of the fuse. If the ratio of fillers 6 and 7 is percent to 25 percent, breaks will form at overlay 9 up to currents of about 78 times the rated current. At higher overload currents the initial point of break will shift from overlay 9 in filler 7 to a point of link 2c situated in filler 6. Such operation is satisfactory since overload currents as high as 78 times the rated current can readily be interrupted in quartz sand filler 6. This is not true of overload currents of smaller magnitude. It is, therefore, important that the magnitude of the transfer current, i.e. the current at which the initial point of break transfers from overlay 9 inside filler 7 to a point remote from overlay 9 inside filler 6 be sufficiently high, or of such magnitude as to allow rapid arc-quenching by an arcquenching filler of quartz sand.

Fuses or fuse units according to FIGS. 4 and 5 have been manufactured for a wide variety of voltage ratings having springs 20 and casings 2b of widely varying length. These fuses were X-rayed prior to performing electrical tests thereon. N0 significant distortion could ever be observed of the helical spring fuse links 20, though some of the fuses were of considerable length, e.g. 15 inches, and though the spring fuse links 20 were not supported between the ends thereof by any other means than the bodies of pulverulent arc-quenching filler 6 and 7.

If the inner diameter D of casing 212 is increased above four times the diameter d of coaxial spring 2c, casing 217 may still be filled with pulverulent arc-quenching fillers without buckling or distortion of spring 20. The danger of buckling or distortion of spring 2c during the filling process of the fuse is increased if the spring 20 is not arranged in coaxial relation to fuse tube or casing 212. Arranging two or more helical springs 2c inside of a casing in noncoaxial relation increases the danger of buckling and/or distortion of the springs 2c during the process of filling the casing with a pulverulent arc-quenching filler. While an increase of the inner diameter D of casing 2b beyond four times the diameter d of spring 2c does not significantly increase the danger of buckling and distortion of fuse link or spring 2c, it was found that that such an increase has an adverse effect on the electric performance of the fuse unit 2. Short circuit tests conducted in the power laboratory have shown that an increase of the inner diameterl) of easing 2b and a concomitant increase of the volume of fillers 6 and 7 may even result in complete failure under otherwise identical conditions.

in FIGS. 3 and 4- the reference character A has been applied to indicate the deflection per turn when spring 2c is loaded or stretched. it is evident that A d.

FIGS. 3-5 refer to a fuse having a voltage rating of 4160 volts and a current carrying capacity of 15 amps. The voltage rating may be increased to 15 KV by increasing the length L of spring 20 and the length l of easing 2b.

Very satisfactory test results were obtained with hard silver ribbon 0.0035 inches thick and 0.086 inches wide. The points of reduced cross-sectional area were established by circular perforations having a diameter of 0.046 inches spaced 0.203 inches. This material was wound upon a mandrel having a diameter of one-eighth inch, and a clearance of three thirtyseconds inch was left between juxtaposed edges of the helically wound silver ribbon. Upon removal of spring 20 from its forming mandrel the diameter of the former expanded to three-sixteenths inches. The silver ribbon wound on the forming mandrel included 40 perforations or a length of ribbon of about 40 X 0.203 8.12 inches. The length of the relaxed spring prior to its axial stretching within fuse tube or casing 2b was about 3 inches. When stretched the length of spring 2c was equal to the length l of casing 2b which was 6% inches. The overcenter ends 20 "of spring 2c are not perforated since this would adversely affect the mechanical strength or integrity of the spring 20.

It will be noted that the ratio of the length of the unloaded spring to the length of the loaded and extended or deflected spring is 3:6 amabout 1:2 which accounts for the magnitude of the ultimate deflection A per turn or winding.

The above FIGS. or data are guidelines. They can be varied within limits, but a drastic departure therefrom is not feasible. For instance, a drastic reduction of the cross section of the ribbon fuse link 20 of fuse unit 2 would adversely affect its current-carrying capacity which should not be less than amps. in order to make it possible to integrate a plurality of fuse units 2 in modular fashion into a composite fuse structure of relatively large current-carrying capacity. A drastic reduction of the cross section of the ribbon fuse link of fuse unit 2 is also incompatible with the required spring action thereof.

lt will be apparent from the above that fuse unit 2 is designed to operate singly, i.e. without being combined with other such fuse units, or to operate in modular combinations having a current-carrying capacity which is an integer multiple of the current-carrying capacity of a single fuse unit.

The structure of FIGS. 6 and 7 includes a cylindrical block 10 of a refractory ceramic material having five bores llla. Each bore lilla has end portions of increased diameter and a shoulder lllb is formed at each of the end portions. The ends of each bore 1011 are closed by terminal caps Ill whose axially inner ends abut against shoulders 10b. The axially outer end surfaces of terminal caps ll project beyond the end surfaces We of ceramic block ill and engage recesses 12a provided in the axially inner end surfaces of terminal plugs 12. Terminal plugs 12. close the axially outer ends of a relatively large tubular casing 13 of insulating material housing ceramic block 10. Plugs 12 are secured to casing 13 by transverse steel pins 1d, i.e. steel pins projecting transversely through casing 13 into plugs 12.

The bores lllla of block 10 house exactly the same structure as the tubular casing 21) of FIGS. 4 and 5, and the same reference characters have been applied in FIGS. 4, 5, 6 and 7 to indicate like parts. Hence there is no need to describe in detail the structures housed in bores 10a of block 10.

The control of melting F't values in a current-limiting fuse depends primarily upon the selection of the minimum crosssectional of the fusible element, but the control of arcing F-t values depends on a large number of parameters and is relatively difficult to achieve and to explain. Fuse units according to FIGS. 3-5 have relatively small arcing F1 values which may be explained as follows:

investigations of electric arcs in current-limiting fuses revealed that it is necessary to distinguish between (a) the case where the arc is surrounded by a body of gas and (b) the case where the arc is surrounded by a body of pulverulent arcquenching medium.

it can be shown that if a wire inside of a tube not containing any filler is caused to explode under the impact of an electric current having a high rate of rise, the are energy depends upon the diameter of the tube and may be decreased by decreasing the diameter of the tube. The explosion of the wire produces a shock wave expanding radially from the wire. The front of this shock wave is followed by a rarefaction, or region of relatively low pressure. if the pressure is sufficiently low the arc is rekindied following a temporary interruption of the circuit known as dark pause.

If the casing of a fuse contains a pulverulent arc-quenching filler, such a filler obstructs the aforementioned pressure wave.

and results in much smaller pressures at the wall of the casing than those occurring in the absence of a pulverulent arcquenching filler. The critically small diameter of the casing makes it possible to achieve the desired pressure incident to blowing of the fuse without imposing undue stresses upon its casing. An increase of the inner diameter of the casing above the data which have been given above results in an increase of the volume of the casing and of the mass of pulverulent arcquenching filler inside the casing. The former increase tends to preclude the desired rise in pressure and the latter is superfluous because the contribution to the arc-quenching action of particles of quartz sand or other pulverulent arc-quenching filler decreases inversely with the spacing of the particles from the arc path.

The coaxial arrangement of the fuse tube or casing and of the fusible element is a matter of considerable importance. if a fusible element is arranged eccentrically inside the fuse casing or fuse tube the pressure at the points of the casing nearest to the fusible element occurring incident to blowing of the fuse will be much higher than at points of the casing more remote from the fusible element. Such asymmetrical pressure patterns should be avoided to avoid overstressing of the material of which the casing of the fuse is made.

The pressure which is generated inside of the casing of a fuse depends not only upon the design of the fuse but also upon the parameters and the topology of the circuit into which the fuse is inserted. Our above f ndings refer to fuses in industrial circuits having circuit voltages from about 4 KV to about 15 RV. The pressure inside the casing of a fuse tends to increase if the circuit inductance is increased.

The required length of a fusible element and the number and spacing of its points of reduced cross-sectional area depend primarily upon the voltage of the circuit for which the particular fuse if intended, and the number of fusible elements which must be connected in parallel depends primarily upon the required current-carrying capacity of the fuse. With these data determined by any particular application, it is easier to comply with them by resorting tothe multiple fuse structures of FIGS. l-2 and 6'/' than by resorting to the conventional high-voltage fuse structures including one single mandrel and a plurality of fuse links helically wound around the same. This is particularly true if the number of helical windings is relatively large to achieve a relatively large current-carrying capacity.

We claim:

1. A current-limiting high-voltage fuse including:

a. a ribbon of relatively hard resilient sheet silver of rectangular cross section having a plurality of serially related points of reduced crosssectional area wound helically with the wide side thereof parallel to an axis to form an extension-springlike structure, said extension-springlike structure having a diameter ranging from about three-sixteenths of an inch to less than one-half of an inch and having when in unloaded condition a spacing between contiguous windings thereof of substantially less than said diameter, said extension-springlike structure being axially stretched to such a degree that the deflection per turn ex ceeds said diameter resulting in an extension to a predetermined length, said extension-springlike structure having overcenter ends at the ends of the helical portion thereof;

b. receptacle means defining a cylindrical passage having said predetermined length and an inner diameter ranging from lessthan about three times to about four times diameter of said extension-springlike structure, said extension-springlike structure being arranged inside said passage in coaxial relation thereto and said overcenter ends of said extension-springlike structure being substan' tially coextensive with the axis of said passage;

c. a pair of terminal elements arranged at the ends of said passage each conductively connected to one of said overcenter ends of said extension-springlike structure; and

d. compacted pulverulent arc-quenching filler means inside said passage submersing said extension-springlike structure, said filler means being the only lateral support of said extension-springlike structure between said pair of terminal elements of said fuse.

2. A current-limiting high-voltage fuse as specified in claim 1 wherein said spacing in unloaded condition between contiguous turns ranges from near zero to about one thirty-second of an inch.

3. A current-limiting high-voltage fuse as specified in claim 1 wherein said deflection per turn of said extension-springlike structure is about twice the radius thereof.

4. A current-limiting fuse as specified in claim 1 wherein said plurality of serially related points of reduced cross-sectional area of said ribbon is formed by serially related circular perforations arranged along the center of said ribbon and wherein the longitudinal edges of said ribbon are unnotched.

5. A'current-limiting fuse as specified in claim 11 wherein said pulverulent arc-quenching filler consists of a relatively long layer of quartz sand and a relatively short layer including a mixture of silica and oxides of aluminum and iron with a content of about 50 percent of silica and about percent of said oxides, and wherein said extension-springlike structure has a link-severing overlay of a metal having a lower fusing point than silver at a point thereof submersed in said relatively short layer of filler.

6. A current-limiting high-voltage fuse including:

a. a tubular main casing of insulating material having a relatively large diameter;

b. a pair of terminal plugs closing said main casing at the ends thereof, each of said terminal plugs having registering cylindrical recesses at the axially inner and surfaces thereof;

c. a plurality of tubular subcasings having a predetermined length and closed at the ends thereof by terminal caps projecting into said recesses of said pair of terminal plugs, each of said plurality of subcasings having a smaller inner diameter than said main casing, said smaller inner diameter ranging from about one-half of an inch to less than 2 inches;

d. a plurality of extension-springlike structures each formed of a ribbon of relatively hard silver of rectangular cross section having a plurality of serially related circular perforations wound helically with the wide side thereof parallel to an axis to form one of said plurality of extension-springlike structures, each of said plurality of extension-springlike structures having a diameter ranging from about three-sixteenths of an inch to less than one-half of an inch and having when in unloaded condition a length substantially less than said predetermined length of each of said plurality of subcasings, each 'of said plurality of extension-springlike structures being axially stretched to such a degree that the deflection per turn is in the order of twice said diameter and results in an elongation of each of said plurality of extension-springlike structures to equal substantially said predetermined length of each of said plurality of subcasings, each of said extension-springlike structures having overcenter ends at the ends of the helical portion thereof and being arranged inside one of said plurality of subcasings in coaxial relation thereto, said overcenter ends of each of said plurality of extension-springlike structures being threaded through perforations in the end surfaces of said terminal caps of one of said plurality of subcasings and conductively interconnecting said terminal caps; and

. compacted pulverulent arc-quenching filler means inside each of said plurality of subcasings, said filler means being the only lateral support of each of said plurality of extension-springlike structures between said terminal caps of each of said plurality of subcasings.

. A current-limiting high-voltage fuse including: a tubular main casing of insulating material having a relatively large dipmeter;

. a pair of terminal elements closing said main casing at the ends thereof;

. a cylindrical block of a ceramic insulating material ara plurality ofextension-springlike structures each formed of a ribbon of relatively hard silver rectangular in cross section having a plurality of serially related circular perforations would helically with the wide side thereof parallel to an axis to form one of said plurality of extensionspringlike structures, each of said extension-springlike structures having a diameter ranging from about threesixteenths of an inch to less than one'half of an inch and having when in unloaded condition a length substantially less than said predetermined length of said plurality of bores of said cylindrical block, each of said plurality of extension-springlike structures being axially stretched to such a degree that the deflection per turn is in the order of twice said diameter of each of said plurality of extension-springlike structures to equal substantially said predetermined length of each of said plurality of bores, each of said extension-springlike structures having overcenter ends at the end of the helical portion thereof and being arranged inside of one of said plurality of bores in coaxial relation thereto, said overcenter ends of each of said plurality of extension-springlike structures being conductively connected to said pair of terminal elements; and

. compacted pulverulent arc-quenching filler means inside

Claims

1. A current-limiting high-voltage fuse including: a. a ribbon of relatively hard resilient sheet silver of rectangular cross section having a plurality of serially related points of reduced cross-sectional area wound helically with the wide side thereof parallel to an axis to form an extension-springlike structure, said extension-springlike structure having a diameter ranging from about three-sixteenths of an inch to less than one-half of an inch and having when in unloaded condition a spacing between contiguous windings thereof of substantially less than said diameter, said extension-springlike structure being axially stretched to such a degree that the deflection per turn exceeds said diameter resulting in an extension to a predetermined length, said extension-springlike structure having overcenter ends at the ends of the helical portion thereof; b. receptacle means defining a cylindrical passage having said predetermined length and an inner diameter ranging from less than about three times to about four times said diameter of said extension-springlike structure, said extension-springlike structure being arranged inside said passage in coaxial relation thereto and said overcenter ends of said extensionspringlike structure being substantially coextensive with the axis of saId passage; c. a pair of terminal elements arranged at the ends of said passage each conductively connected to one of said overcenter ends of said extension-springlike structure; and d. compacted pulverulent arc-quenching filler means inside said passage submersing said extension-springlike structure, said filler means being the only lateral support of said extensionspringlike structure between said pair of terminal elements of said fuse.

5. A current-limiting fuse as specified in claim 1 wherein said pulverulent arc-quenching filler consists of a relatively long layer of quartz sand and a relatively short layer including a mixture of silica and oxides of aluminum and iron with a content of about 50 percent of silica and about 20 percent of said oxides, and wherein said extension-springlike structure has a link-severing overlay of a metal having a lower fusing point than silver at a point thereof submersed in said relatively short layer of filler.

6. A current-limiting high-voltage fuse including: a. a tubular main casing of insulating material having a relatively large diameter; b. a pair of terminal plugs closing said main casing at the ends thereof, each of said terminal plugs having registering cylindrical recesses at the axially inner and surfaces thereof; c. a plurality of tubular subcasings having a predetermined length and closed at the ends thereof by terminal caps projecting into said recesses of said pair of terminal plugs, each of said plurality of subcasings having a smaller inner diameter than said main casing, said smaller inner diameter ranging from about one-half of an inch to less than 2 inches; d. a plurality of extension-springlike structures each formed of a ribbon of relatively hard silver of rectangular cross section having a plurality of serially related circular perforations wound helically with the wide side thereof parallel to an axis to form one of said plurality of extension-springlike structures, each of said plurality of extension-springlike structures having a diameter ranging from about three-sixteenths of an inch to less than one-half of an inch and having when in unloaded condition a length substantially less than said predetermined length of each of said plurality of subcasings, each of said plurality of extension-springlike structures being axially stretched to such a degree that the deflection per turn is in the order of twice said diameter and results in an elongation of each of said plurality of extension-springlike structures to equal substantially said predetermined length of each of said plurality of subcasings, each of said extension-springlike structures having overcenter ends at the ends of the helical portion thereof and being arranged inside one of said plurality of subcasings in coaxial relation thereto, said overcenter ends of each of said plurality of extension-springlike structures being threaded through perforations in the end surfaces of said terminal caps of one of said plurality of subcasings and conductively interconnecting said terminal caps; and e. compacted pulverulent arc-quenching filler means inside each of said plurality of subcasings, said filler means being the only lateral support of each of said plurality of extension-springlike structures between said terminal caps of each of said pluralitY of subcasings.

7. A current-limiting high-voltage fuse including: a. a tubular main casing of insulating material having a relatively large diameter; b. a pair of terminal elements closing said main casing at the ends thereof; c. a cylindrical block of a ceramic insulating material arranged inside of and in coaxial relation to said main casing between the axially inner end surfaces of said pair of terminal elements, said block having a plurality of axially extending bores, each of said plurality of bores having a predetermined length and a diameter ranging from about one-half of an inch to 2 inches; d. a plurality of extension-springlike structures each formed of a ribbon of relatively hard silver rectangular in cross section having a plurality of serially related circular perforations would helically with the wide side thereof parallel to an axis to form one of said plurality of extension-springlike structures, each of said extension-springlike structures having a diameter ranging from about three-sixteenths of an inch to less than one-half of an inch and having when in unloaded condition a length substantially less than said predetermined length of said plurality of bores of said cylindrical block, each of said plurality of extension-springlike structures being axially stretched to such a degree that the deflection per turn is in the order of twice said diameter of each of said plurality of extension-springlike structures to equal substantially said predetermined length of each of said plurality of bores, each of said extension-springlike structures having overcenter ends at the end of the helical portion thereof and being arranged inside of one of said plurality of bores in coaxial relation thereto, said overcenter ends of each of said plurality of extension-springlike structures being conductively connected to said pair of terminal elements; and e. compacted pulverulent arc-quenching filler means inside each of said plurality of bores of said cylindrical block, said filler means being the only lateral support of each of said plurality of extension-springlike structures throughout said predetermined length of each of said plurality of bores of said cylindrical block.