US2832868A - Fillerless one-time national electrical code fuses - Google Patents

Description

April 29, 1958 F. J. KozAcKA FILLERLESS ONE-TIME. NATIONAL ELECTRIbAL CODE FUSES Filed Sept. 6, 1955 2 Sheets-Sheet l H u r e N u D.

N EC fuse f 30 amps.

250 volts Ill/11AM 1 III] 1 IIL-LQIIQ No pulverulent April 29, 1958 F. J. KOZACKA FILLERLESS ONE-TIME NATIONAL ELECTRICAL CODE FUSES Filed Sept. 6, 1955 2 Sheets-Sheet 2 xx S w 0mm 255 0 w m 5 c I t O m 2 I WNWNNNNNNNNNNNN NWNNNNNNNNW MW mWNMmAumWNNmWmWWQMMWNNMMM mNNNWMm WW I NHWM WNWNNMMm m-WNNNNMWNNNNNMA H w |M NH 000 8 m Gm @588 =2 ANN Q :2 6.58

E y & 00w maEu o m 3 O m 2 Gm United States Patent 0 FILLERLESS ONE-TIIVIE NATIONAL ELECTRICAL CODE FUSES Frederick J. Kozacka, South Hampton, N. H., assignor to The Chase-Shawmut Company, Newburyport, Mass.

Application September 6, 1955, Serial No. 532,506

14 Claims. (Cl. 200-131) This invention relates to fuses for the protection of electric circuits and for the protection of electrical devices connected into electric circuits, and more particularly to so-called one-time fuses.

Fuses are either one-time or renewable fuses. This distinction emphasizes the fact that fuses of the first mentioned type can only be used once, whereas fuses of the second mentioned type may repeatedly be used upon renewal of the fuse link, or fusible element, previously destroyed by fusion and arcing. The design principles underlying both types of fuses are very different. As far as renewable fuses are concerned, it is imperative to dis pense with pulverulent arc-quenching fillers. Such fillers leave residues in the casing after blowing which make it difficult, if not impossible, to renew the fuse link in the lield, i. e. without relying on special shop equipment and on labor skilled for this particular type of work. The required elimination in renewable fuses of the pulverulent arc-quenching fillers generally used in National Electrical Code one-time fusesalso known as NEC onetime fusesgave rise to special design problems in regard to renewable fuses. A pulverulent arc-quenching filler absorbs large amounts of arc energy and is conducive to a substantial reduction of pressure within the casing, or cartridge, of a fuse. Presence of pulverulent arc-quenching fillers reduces also the pressure upon the terminal clements by which the casing or cartridge is closed. If the pulverulent arc-quenching fillers are omitted, as in case of the type of fuses referred to as renewable fuses, both the casing of the fuse and the terminal elements thereof must have a considerably higher mechanical strength than required for the type of fuses referred to as one-time fuses, wherein the presence of the pulverulent arc-quenching filler tends to limit, or reduce, pressure build-up. Inasmuch as the casing and the terminal elements of a renewable fuse can be used for long periods of time, the fuse link or fusible element being renewable, there is no particular objection against the relatively high cost of material and labor involved in manufacturing the casings and the terminal elements of this type of fuse.

The provision of pulverulent arc-quenching fillers in one-time fuses is a serious drawback as far as the manufacturing process of this type of fuse is concerned. It requires a certain amount of skill to properly fill a fuse. The fuse, once filled, ought to be vibrated to cause the filler to settle and to establish predetermined standards of uniformity of filling, upon which uniformity of fuse performance depends. The vibrating operation requires some skill as well as additional equipment and fioor space. Presence of a pulverulent arc-quenching filler stands also in the way of automation of fuse production inasmuch as there is a tendency on the part of pulverulent or granular filler substances to get into and jam and otherwise impair automatic machinery. This drawback is particularly serious where quartz with its high abrasive action is used as arc-quenching medium. For these and other reasons it has been a desideratum for many years to evolve one ICC time power fuses having the size and the interrupting capacity of National Electrical Code or NEC fuses which fuses do not require, and do not possess, a pulverulent arc-quenching filler, and do not require and do not possess expensive casings and terminal elements of increased mechanical strength as the fillerless fuses, known as re newable fuses, require and possess.

it is, therefore, one object of this invention to provide fillerless one-time power fuses having the dimensions and the ratings of National Electrical Code or NEC fuses, yet requiring casings having as little bursting strength and terminal elements having as little mechanical strength as those heretofore used for one-time fuses which comprise pulverulent arc-quenching fillers.

Another object of this invention is to provide one-time power fuses of equal sizes and ratings as National Electrical Code or =NEC fuses which fuses have casings of laminated paper or cardboard and simple terminal ele ments in the form of caps of sheet metal in spite of the complete absence of pulverulent arc-quenching fillers.

Another object of the invention is to provide one-time power fuses of equal sizes and ratings as National Electrical Code or NEC fuses, which fuses can be manufactured at considerably smaller cost and which fuses lend themselves to production by partly, or fully, automatic means.

It is in itself not particularly difiicult to achieve interruption of low voltage circuits by fusible means withotit the aid of pulverulent arc-quenching fillers. Seemingly unsurmountable difliculties arise, however, when the conditions are being imposed that the fuse must have a predetermined voltage rating and a predetermined current rating as specified in the National Electrical Code, that the casing of the fuse must not exceed the standard dimensions of NEC cartridge fuses of that particular voltage rating and current rating, and that in spite of the elimination of a pulverulent arc-quenching filler the casing of the fuse and the terminal elements thereof must not be more elaborate and/or expensive than the casing and terminal elements of NEC fuses including pulverulent arc-quenching fillers. Heretofore it has been the consensus of opinion among designers of fuses that the task of evolving such a fuse is impossible of achievement; all that could be achieved seemed to be elimination of pulverulent arc-quenching fillers coupled with increased mechanical strength, counteracting the tendency toward increased pressures incident to blowing of fillerless fuses.

It is one object of this invention to provide a family of National Electrical Code fuses having all the character istic features of such fuses, including the required size and interrupting ability, which fuses have casings and terminal elements of the kind normally provided for National Electrical Code fuses including pulverulent arcquenching fillers, and in which fuses such fillers are entirely dispensed with.

The ratings and dimensions of the family of National Electrical Code fuses are tabulated below, and wherever reference is being made hereinafter to ratings and dimensions of National Electrical Code fuses, such reference is meant to imply the specific figures given in the table below.

In order to satisfy the National Electrical Code fuses must comply with the Standard for Fuses of Underwriters Laboratories, Inc., i. e. they must be able to interrupt a D.-C. circuit having an available short-circuit current of 10 kiloamps. and a rate of rise of about 2,000 kiloamps. per see.

I have found that by providing a ribbon-type fuse link for an NBC fuse with a critical number of portions of reduced cross-sectional area, the are energy and the concomitant pressure within the NEC fuse incident to blowing thereof can be critically reduced close to the point, or to the point, where a pulverulent arc-quenching filler can be dispensed with. Such a ribbon-type fuse link should preferably be made of a material having a high conductivity, thus making it possible to minimize the amount of metal needed to carry continually a given current and thus tending to minimize the amount of metal vapor generated within the casing when the fuse blows. Both silver and copper have a relatively high conductivity, yet copper is preferable for the contemplated interrupting capacity range.

The critical number of portions of reduced cross-sectional area is eight. It is possible to design the eight portions of reduced cross-sectional area for an optimal rate of build-up of arc voltage, and if the eight portions of reduced cross-sectional area are so'designed, pulverulent arc-quenching fillers may be omitted and the fuse link may be housed in a casing having but relatively little mechanical strength, e. g. a-casing of laminated paper, which casing may be closed by terminal elements having likewise but relatively little mechanical strength, e. g. simple caps of sheet metal mounted on and crimped upon the ends of the casing.

While it is feasible to design the eight portions of reduced cross-sectional area in such a way that pulverulent arc-quenching fillers may be entirely dispensed with and yet inexpensive casings and terminal elements of NEC one-time fuses retained, production of fuse links making this possible involves some difficulties. If the eight portions of reduced cross-sectional area of the fuse link are not designed for optimal build-up of arc voltage, production of the fuse link is considerably facilitated. If such a fuse link is then mounted in the casing of an NBC one-time fuse and the casing closed by sheet metal caps as generally used for this type of fuses and the pulverulent arc-quenching filler omitted, the fuse tends to operate close to the margin of the interrupting capacity thereof. In other words, successful interruptions alternate with failures. The overall performance of such a fuse is erratic, yet suggests that the fuse can be made fully operative and provided with a sufficient margin of safety by use of deionizing and cooling means considerably less effective than pulverulent arc-quenching fillers. The erratic behavior can be completely eliminated and the required safety margin reached by mounting on the fuse link a loosely woven sleeve of a silicious inorganic material such as, for instance, fiber glass, in such a way that the sleeve covers fully the eight portions of reduced cross-sectional area of the link. Such a sleeve operates similar to a quartz sand filler yet is free from many, or most, of the disadvantages of quartz sand fillers. The woven silicious sleeve shares with the quartz sand filler the ability of relatively high absorption of are energy resulting from the sameness of the mechanism of energy absorption, i. e. high latent heat of fusion. Though, due to the relatively small mass thereof, the are energy absorbing capacity of a woven silicious sleeve is less than that of a pulverulent arc-quenching filler, the are energy absorbing capacity of a woven silicious sleeve is sufficient to make up for the margin of required interrupting capacity that must be added where the eight portions of reduced cross-sectional area of the fuse link are not designed for build-up of arc voltage at an optimal rate.

The choice of a metal suitable for making the fuse links of fast acting fuses is a compromise between vari- 4 ous properties of metals. Properties of prime importance are the energy required to increase the temperature from ambient to the fusing point (which energy depends upon the specific heat and upon the fusing point of the particular metal), the latent heat of fusion of the metal and the heat of vaporization thereof. The smaller these quantities, the faster the operation of the fuse. Another very important property of a metal in regard to its fitness as a material for fuse links is its current-carrying ability expressed in terms of resistivity or conductivity at ambient temperature. The energy required to heat a metal to the fusing point, the required latent heat of fusion, the required heat of vaporization and the total heat required for rupturing a conductor can be expressed in terms of fi -dt.

If certain alloy-forming metals are brought into physical contact with each other and heated to the fusing point of the one of the two metals which has the lower fusing point, the high fusing point metal diffuses into the low fusing point metal, or both metals interdiffuse, resulting in a solution of one metal in the other. If, for instance, a link of copper is covered with a globule of tin solder and caused to carry a current sufficiently high to cause fusion of the tin, copper-tin-alloys are formed having a relatively high resistivity. This causes increased heat generation, increased diffusion at the boundary area of the two metals and ultimate destruction of the copper link. Tin may be used as link-destroying alloy-forming agent on copper as well as on silver links. Similarly lead-tin alloys, or cadmium may be used as ink-destroying alloyforming agents. To destroy silver links at relatively low temperatures overlays of indium or alloys of indium may be used as disclosed in detail in United States Patent 2,703,352 to Frederick J. Kozacka, Fuse and Fuse Link of the Time Lag Type, issued March 1, 1955.

For reasons which will be set forth below more in detail preferably the entire fuse link ought to be covered with a thin coat of link-destroying alloy-forming metal, preferably in the order of of an inch thickness, if thicker, not exceeding 5 of an inch.

Where fuse links made of a metal having a relatively high fusing point have an overlay of a metal having a relatively low fusing point to effect destruction of the link at temperatures below the fusing point of the metal having the higher fusing point by alloy-formation between the two metals, the mode in which both metals are associated, or brought together, is of crucial importance with regard to the behavior of the link and of the entire fuse structure.

Generally an overlay of low fusing point metal is formed on a link ribbon of high fusing point metal by coating the surface of the latter with low fusing point metal in melted form. In this way a ribbon-type link of silver may be provided with an overlay of tin, or of tin alloys. The method of formation of overlays at elevated temperatures results in a tendency of initiating the alloying process between the two metals at the very instant in which they are brought together. As the link is more or less heated during its normal operation, the alloying process started during manufacturing the link further progresses, resulting in a progressive increase of the resistance of the link. This progressive increase of resistance causes increased generation of heat, in turn causing continued alloy-formation between the two metals. This undesirable gradual self-destruction of fuse links is known as ageing. Ageing involves the presence of particularly brittle alloys formed by diffusion of the high fusing point metal into the low fusing point metal impairing the mechanical integrity of the fuse link.

Ageing can be minimized or avoided by arranging a protective layer between the low fusing point metal and the high fusing point metal. The high fusing point metal in the shape of a ribbon may have a perforation into which the low fusing point metal in form of a rivet may be inserted. Such a rivet, e. g. a tin rivet, is normally covered with an oxide layer which forms a skin containing the tin after fusion thereof. The protective skin effect of the oxide layer may be increased by coating the tin rivet with a flux, such as rosin.- Boiling of the melted tin within its oxide and flux skin causes bursting of the skin and spilling of the liquid tin over the adjacent hot portions of silver or copper of the link. This results in an accelerated metallurgical reaction by which the link is rapidly destroyed.

Bursting of the oxide skin occurs, however, rather erratically. It may not occur at all, i. e. the oxide skin may contain the melted low fusing point metal for an undetermined period of time. The low fusing point metal may never reach its boiling point and break through its skin or container of oxide.

It is, therefore, another object of this invention to provide fuse links comprising a base portion of a metal having a relatively high fusion point which base portion has an overlay portion of a metal having a relatively low fusing point causing rupture of said base portion by a metallurgical reaction between said relatively high fusing point metal and said relatively low fusing point metal occurring at temperatures below said relatively high fusing point, which fuse links are not subject, or less subject, to ageing than links of prior art fuses and which fuse links are not subject to erratic behavior resulting from the aforementioned oxide skin effect.

This particular object can be achieved by providing the high fusing point base portion of the fuse link with a low fusing point overlay electroplated on the base portion, e. g. a base portion of copper may be provided with an overlay of tin which is electroplated thereon. A bond between two metals produced by electroplating is free from any initial alloy-formation since such a bond is formed at ambient rather than at elevated temperatures. The low fusing point metal may be deposited on the high fusing point metal in a state of considerable chemical purity, oxide layers between the two metals being entirely absent if the metals are joined together by electroplating. The chemical purity of the overlay combined with the lack of an impurity barrier between the two metals tends to. greatly accelerate diffusion of the high fusing point metal into the low fusing point metal once the fusing point of the latter has been reached. This, in turn, increases the tendency of complete rupture, rather than mere ageing, of the high fusing point base portion of the fuse link once the low fusing point overlay portion has reached its fusing point.

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

For a better understanding of the invention reference may be had to the accompanying drawing illustrating the invention, wherein Pig. 1 is a side elevation of a fuse embodying the invention;

Fig. 2 is substantially a longitudinal section of the fuse shown in Fig. 1 with the fuse link shown in side elevation;

Fig. 3 is a section along 3-3 of Pig. 2, i. e. substantially a longitudinal section of the fuse shown in Fig. 1 taken at right angles to the section shown in Fig. 2 with the fuse link shown in top plan view;

Fig. 4 shows a portion of Fig. 2 on a larger scale;

Fig. 5 is substantially a longitudinal section of another fuse structure embodying the invention with the fuse link shown in side elevation;

1 Fig. 6 is substantially a longitudinal section of the fuse shown in Fig. 5 taken at right angles to the plane of Fig. 5 with the fuse link shown in top plan view;

Fig. 7 is an exploded view of the parts which go into th sinister-sot Figs. .5 and 6;

Fig. 8 is substantially a longitudinal section of still another fuse structure embodying the invention with the fuse link shown in side elevation;

Fig. 9 is substantially a longitudinal section of the fuse shown in Fig. 8 taken at right angles to the section shown in Fig. 8 with the fuse link shown in top plan view;

Fig. 10 is a longitudinal section on a larger scale of a link of the type used in fuses as shown in Figs. 5 and 6;

Fig. 11 is substantially a longitudinal section of another fuse structure embodying the invention with the fuse links shown in side elevation, and

Fig. l2. is substantially a section of the fuse shown in Fig. 11 taken at right angles to the plane of Fig. 11 with the fuse links shown in top plan view.

Referring now to the drawing, and more particularly to Figs. 1 to 4 thereof, reference numeral 2 has been applied to indicate a tubular casing of insulating material. Any desired kind of insulating material can be used for easing 2, including the least expensive material and the material having the smallest bursting strength, i. e. laminated paper or cardboard. Ribbon-type fuse link 3 is diagonally arranged within casing 2. The length of link 3 exceeds the diagonal length of easing 2 and link 3 is bent with the outer ends thereof around the edges 2a of easing 2. Link 3 consists of a relatively resilient material, preferably copper of the hard temper to spring temper type. Fuse link 3 is preferably tin plated, the thickness of the plating not exceeding A of an inch, preferably being in the order of of an inch. T he tin plating of link 3 performs several functions. One of these functions is to establish in a simple and efficient manner a highly conductive connection or solder joint between link 3 and a pair of terminal caps I mounted on the ends of easing 2. Each cap 1 consists of sheet metal and is crirnped at several points in, i. e. inwardly folded at several points, into the body of the casing 2. After insertion of the link 3 into casing 1 and bending of the ends of link 3 around the edges of casing 2, the

ends of link 3 tend to flex away from the outer surface of easing 2 (as shown in Fig. 7). Upon mounting of caps 1 upon casing 2 and upon the portions of fuse link 3 situated outside of easing 1, the ends of fuse link 3 abut, due to their inherent resiliency, under slight pressure against the inner surfaces of caps 1. As a result of this pressure and of the tin plating of fuse link 3 it is possible to blind-solder link 3 to the insides of caps 1, i. e. to establish a highly conductive mechanically sound solder connection between the insides of caps 1 and link 3. In order to cause fusion of the tin plating on link 3 to establish blind solder joints between link 3 and caps I. the latter are preferably introduced into a high frequency electric field which causes virtually instantaneous local fusion of the tin plating of the portions of link 3 which rest under slight spring pressure against the inside of caps 1. Link 3 is provided with eight points 3a of reduced cross-sectional area or necks. These points 3.! of reduced cross-sectional area are equidistantly spaced from each other. Each of the eight points 3a of reduced cross-sectional area is formed by a pair of lateral incisions leaving therebetween narrow short bridgesthe necks proper-each connecting two portions of link 3 the cross-sectional areas of which are relatively large. This particular geometry of the link, i. e. the narrowness and length of the necks proper and their number combinml with the suddenness of change of cross-sectional are-.1 is conducive to a rapid and high build-up of arc volt sufficient to interrupt a 250 volt A.-C. circuit at the occurrence of a severe short-circuit without the presence of a pulverulent arc-quenching filler within casing 2, and without resorting to any other arc-quenching means. Since it is relatively diflicult to manufacture the type of fuse Jinks shown in Figs. 1 to 4 whose geometry can best be seen in Fig. 3, the structure shown in Figs. 5 and 6 has been evolved which can be manufactured without resorting to very small, dimensionally relatively unstable nssks 3 Referring now to Figs. and 6, the NEC fuse rated 30 amps. at 250 volts shown therein comprises a casing 2 preferably made of laminated paper or cardboard. The terminal caps 1 are mounted upon casing 2' and crirnped as described in connection with Figs. 1 to 4. The outer ends of tin plated link 3' which is diagonally arranged in casing 2 are bent around the edges of casing 2 and conductively connected by means of blind solder joints 1]) to caps 1'. Link 3 is provided with eight, i. c. with at least eight, perforations 3a in the form of spaced circular holes aligned along the longitudinal axis of symmetry of link 3. The interrupting chamber 212 formed or defined by easing 2 does not contain any pulverulent arc-quenching medium such as, for instance, chalk or quartz sand. Instead fuse link 3' is covered with a loosely woven sleeve 4 of an inorganic silicious material such as, for instance, fiber quartz or fiber glass. Combined with the particular geometry of the fuse link shown in Figs. 5 and 6 which is conducive to a substantial, yet smaller build-up of arc voltage than the geometry of the link shown in Figs. 2 and 3, sleeve 4 provides that safety margin of interrupting capacity required at the occurrence of severe faults for enabling NEC fuses whose voltage rating is 250 volts to consist ently interrupt the faulted circuit without the presence of a potent deionizer in the form of a puverulent arccxtinguishing filler.

In the embodiments of the invention of Figs. 1 to 6, inclusive, the ribbon-type fuse link is adapted to carry continually currents in the order of amps. The rating of a ribbontype fuse link of given length and crosssection having eight or more serially arranged portions of reduced cross-sectional area or necks is mainly determined by the size of the portions of reduced cross-sectional area or necks situated close to the center of the link. The eflect upon rating or current-carrying capacity of the portions of reduced cross-sectional area or necks situated axially relatively far outwardly is relatively small. 1 have found that there is no need for increasing the number of portions of reduced cross-sectional area or holes for NEC fuses having a voltage rating of 600 volts rather than of 250 volts. This is surprising to some extent yet can readily be explained by the fact that the ratio of power to internal volume of the 600 volt line of NEC fuses does not exceed the ratio of power to internal volume of the 250 volt line of NEC fuses. This ratio is indicative of the energy absorbing capacity required per unit of volume of the casing of the fuse. Specific figures for this important ratio have been included in the above tabulation of characteristic dimensions of NEC fuses.

Where it is desired to produce fuses having a relatively high current rating or a relatively high current-carrying capacity a plurality of ribbon-type fuse links of which each has a current-carrying capacity in the order of 30 amperes is arranged in a common casing in such a way as to form parallel current paths. An arrangement of this kind has been shown in Figs. ll and 12, referring to an NEC fuse having a voltage rating of 250 volts and a current rating of amperes.

The fuse shown in Figs. 8 and 9 has a voltage rating of 600 volts and a current rating of 30 amperes. It comprises casing 2" of cardboard or laminated paper accommodating the tin-plated copper link 3" having at least eight holes or perforations 30. It is desirable not to exceed the number of eight holes or perforations 3a" because an increase in number tends to cause excessive voltages incident to blowing of the fuse on the occurrence of severe short circuit currents. Link 3" is bent around the edges 2a of easing 2" and blindly spot-soldered at 1b" to the insides of caps It. The space 211" within casing 2" does not contain any pulverulent arc-quenching filler or deionizing means of equivalent effectiveness. The sleeve 4 of woven fiber quartz or fiberglass mounted on link 3" and covering all the holes 3a" thereof pro- Sit 8,. vides the deionizing action required for consistent interrupting performance at low and high fault currents.

Referring now to Fig. 10, this figure shows on a larger scale a link 3 of the same type as shown in Figs. 8 and 9. Link 3" comprises a ribbon of a metal having arelatrvely high fusing point, e. g. copper, which is provided w1th a plurality of serially related circular perforations 3c forming areas of reduced cross-section as well as increased edge areas. The inner layer of copper, or other high fusi oint metal, is sandwiched between two outer layers of tin about inch thick. The tin layers are produced by electroplating, which method enables to produce tin overlays to close tolerances. The tin layers cover not only the surfaces of the base portion of copper, or other high fusing point metal, but also the insides of the holes 3a which have been referred to above as 1ncrcased edge areas. It is of considerable importance that the overlay of low fusing point metal extend over the increased edge areas produced by stamping, or blanking, holes, or other reduced cross-section portions, into the fuse link. Such extension of the overlay greatly increases the area of metallurgical interaction between the base metal and the overlay metal of the link at the particular points thereof where its cross-sectional area is smallest and its temperatures highest. This tends to cause complete rupture, rather than mere ageing, of the high fusing point base or inner portion of the fuse link once the low fusing point overlay portion has reached its fusing point.

in a ribbon-type fuse link such as shown in Figs. 6, 9 and 10, i. e. in a multiperforated fuse link, preferably of copper, designed to carry continually currents in the order of 30 amperes and intended to be used in a NEC cartridge without presence of any pulverulent arc-quenching filler, the ratio of the diameter of the perforations to the width of the link should be kept within a certain range to assure in addition to required current carrying capacity generation of sufficiently high are voltages, on the one hand and to preclude generation of dangerous inductive surge voltages, on the other hand. This requirement can be met by keeping the ratio of the diameter of the eight perforations of the fuse link to the width of the fuse link within the range of l/1.4 to l/ 1.6. The tinned link is treated with a water dip lacquer before being inserted into the casting of a fuse. Such a lacquer forms a fiuxing agent needed for establishing blind solder joints 1b" between the link'3" and the terminal caps 1" of the fuse of which the link forms an integral part. In addition thereto such a lacquer prevents oxidation and thus increases the shelf-life of the fuse links.

It will be apparent from Figs. 8 and 9 that the fuse link 3" extends directly from one terminal cap 1 to the other, thus conductively interconnecting both terminal caps 1". The basic temperature distribution along link 3 is, in substance, in the shape of an inverted parabola with the peak temperature at the center of the link and the lowest temperature at the axially outer ends thereof. A number of sub-peaks is superimposed upon this basic temperature distribution curve, each of these sub-peaks being produced by one of the perforations 3a". There is a point along link 3 near to the center thereof where the temperature of copper strip 3" tends to be highest and where copper strip 3 is being ruptured on the occurrence of relatively small overloads, e. g. overloads of 300% of the rated current of the fuse. The overlay of tin or other link-destroying alloy-forming metal having a relatively small conductivity and relatively low fusing point is electroplated on the portion of link 3" where the temperature tends to be highest. As a result of this, the inner copper layer of link 3" will be rapidly destroyed by alloy-formation on the occurrence of relatively small overloads. It has been found that rapid link-destruction by alloy-formation can be effected with link-destroying alloy-forming overlays even substantially less than $5 of an inch thick, provided that the asaasas overlays extend over the entire perforated area of link 3. To make a blind solder connection such as that indicated at 1b" in Fig. 8, the overlay ought to be thicker than the minimum thickness required for link-destruction by alloy-formation. Both purposes can be achieved if the thickness of the tin plating is less than 17500 of an inch, but not substantially less than of an inch.

The link-destroying alloy-forming overlay does not have to be of tin. Any low fusing point metal or combination of low fusing point metals capable of the de sired link-destroying action may be electroplated on the base portion of a link made of a suitable high fusing point metal. Where it is desiredto effect link-destruction at particularly low temperatures, the link should be made up of indium plated silver.

Referring now to Figs. 11 and 12 illustrating an NBC fuse rated 60 amps. at 250 volts, this fuse comprises a casing 2' closed by terminal caps 1" mounted thereon and crimped at la. Two ribbon-type fuse links 3' extend diagonally across casing 2" each having eight perforations 3a'. Both fuse links 3 are arranged in a common length of woven tubing 4 of fiber glass or equivalent silicious material.

, Having disclosed preferred embodiments of my invention it is desired that the same be not limited to any 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 low-voltage one-time cartridge fuse having a predetermined voltage rating and a predetermined current rating comprising a tubular casing of laminated paper having a predetermined bursting strength, said casing having the standard dimensions of a NEG cartridge fuse of said predetermined voltage rating and said predetermined current rating, a pair of terminal caps mounted on and crimped upon the ends of said casing, at least one ribbontype fuse link of copper adapted to carry continually currents in the order of 30 amperes conductively interconnecting said pair of caps, a plurality of serially related points of reduced cross-section on said link sufficient in number to limit without presence of a pulverulent arc-quenching filler in said casing are energy and concomitant pressure incident to blowing below said predetermined bursting strength, the number of said points of reduced crosssection being in the order of eight.

2. A low-voltage one-time cartridge fuse having a predetermined voltage rating and a predetermined current rating comprising a tubular casing of laminated paper having a predetermined bursting strength, said casing having the standard dimensions of an NBC cartridge fuse of 5 said predetermined voltage rating and said predetermined current rating, a pair of terminal elements each arranged at one end of said casing, at least one ribbon-type fuse link of copper adapted to carry continually currents in the order of 30 amperes conductively interconnecting said pair of terminal elements, said fuse link having at least eight serially related circular perforations forming points of reduced cross-section limiting without presence of a pulverulent arc-quenching filler in said casing are energy and concomitant pressure incident to blowing below said predetermined bursting strength.

3. A low-voltage one-time cartridge fuse having a predetermined voltage rating and a predetermined current rating comprising a tubular casing of laminated paper having the standard dimensions of NEC cartridge fuses of said predetermined voltage rating and said predetermined current rating, a pair of terminal caps mounted on and crimped upon the ends of said casing, at least one ribbontype fuse link of copper adapted to carry continually currents in the order of SO amperes conductively inter-- connecting said pair of caps, a plurality of serially related points of reduced cross-sectional area on said link sufficient in number to limit arc energy and pressure within said casing incident to blowing to lesser amounts than those requiring presence of a pulverulent arc-extinguishing filler in said casing, the number of said points of reduced cross-sectional area being in the order of eight, and a de-ionizing structure of relatively limited de-ionizing ability substantially coextensive with the portion of said link having said points of reduced cross-sectional area.

4. A low-voltage one-time cartridge fuse having a predetermined voltage rating and a predetermined current rating comprising a tubular casing of insulating material having the standard dimensions of NEC cartridge fuses of said predetermined voltage rating and said predetermined current rating, a pair of terminal elements each arranged at one end of said casing, at least one ribbon-type fuse link of copper adapted to carry continually currents in the order of 30 amperes conductively interconnecting said pair of terminal elements, said link having at least eight serially related circular perforations forming points of reduced cross-sectional area limiting are energy and concomitant pressure withinsaid casing incident to blowing of said fuse to lesser amounts than those requiring presence of a pulverulent arc-quenching filler in said casing, and a de-ioniziug structure of relatively limited deionizing ability substantially coextensive with the portion of said link having said points of reduced cross-sectional area.

5. A low-voltage one-time cartridge fuse having a predetermined voltage rating and a predetermined current rating comprising a tubular casing of laminated paper having a predetermined bursting strength, said casing having the standard dimensions of NEC cartridge fuses of said predetermined voltage rating and said predetermined current rating, a pair of terminal caps mounted on and crimped upon the ends of said casing, a ribbon-type fuse link conductively interconnecting said pair of caps, a plurality of serially related points of reduced crosssectional area on said link, a porous woven sleeve of a silicious inorganic material supported by said link covering said plurality of points of reduced cross-sectional area, the number of said plurality of points of reduced crosssectional area being selected to have a pressure-limiting action which when supplemented by the pressure-limiting action of said sleeve is sutliciently high to limit the pressure in said casing incident to blowing of said fuse below said predetermined bursting strength without presence of a pulverulent arc-extinguishing filler in said casing.

6. A low-voltage one-time cartridge fuse having a predetermined current rating and a predetermined voltage rating comprising a tubular casing of insulating material having the standard dimensions of NEC cartridge fuses of said predetermined voltage rating and said predetermined current rating, a pair of terminal elements each arranged at one end of said casing, a ribbon-type fuse link of copper having a predetermined number of points of reduced cross-section conductively interconnecting said pair of terminal elements, a porous sleeve woven of silicious inorganic fibers mounted on said link and enveloping said predetermined number of points of reduced cross-section thereof, said predetermined number of points of reduced cross-section being sufficiently large when supplemented by the pressure-limiting action of said sleeve to limit the pressure within said casing incident to blowing below the pressure level requiring application of pulverulent arc-quenching fillers.

7. In a fuse the combination of a tubular casing of insulating material, a pair of terminal elements each arranged at one end of said casing, a ribbon-type fuse link made of a metal having a relatively high conductivity and a relatively high fusing point arranged within said casing conductively interconnecting said pair of elements, said link having a plurality of perforations coextensive with the center region thereof, an overlay of 'a link-destroying alloy-forming solder having a relatively small conductivity and a relatively low fusing point covering the major portion of the surface of said link including said plurality of perforations thereof, said overlay being fused at the axially outer ends of said link thereby establishing solder joints between said pair of elements and said link.

8. In a fuse the combination of a tubular casing of insulating material, a pair of terminal elements each ranged at one end of said casing, a multiperforated tinplated ribbon-type fuse link of copper within said casing conductively interconnecting said pair of elements, the tin plating at the axiallyouter ends of said link being fused thereby establishing current paths of low resistance between said pair of terminal elements and said link.

9. In a fuse the combination of a tubular casing of insulating material, a multiperforated tin-plated ribbontype fuse link of relatively resilient sheet copper diagonally extending across said casing with the ends thereof bent over the edges of said casing and tending to move away from the outer surface thereof, a pair of terminal caps mounted upon said casing tending to clamp said ends of said link against said outer surface of said casing, and the plating of said link at said axially outer ends thereof being fused thereby establishing current paths of low resistance between said link and said pair of caps.

10. In a fuse the'combination of a tubular casing of laminated paper, a ribbon-type fuse link made of relatively resilient metal having a relatively high conductivity and a relatively high fusing point diagonally extending across said casing with the ends thereof bent over the edges of said casing overlapping the outer surface thereof, said link having at least eight perforations and including a portion where the temperature thereof tends to be highest and where rupture occurs at relatively small overloads, a plated overlay of a link-destroying alloy-forming solder having a relatively small conductivity and a relatively low fusing point coextensive with said ends of said link and coextensive with the perforated portion thereof, a pair of terminal caps mounted upon said casing and upon said ends of said link, and said overlay being fused at said ends of said link establishing current paths of relatively low resistance between said link and said pair of caps.

11. A fuse as specified in claim 10 wherein said link is made of copper and said overlay consists of tin and is less than of an inch thick.

12. In a fuse the combination of a tubular casing of insulating material, a ribbon-type fuse link of resilient sheet copper sandwiched between layers of tin arranged within and diagonally extending across said casing, the length of said link exceeding the diagonal length of said casing and the ends of said link being bent over the edges said link.

13. in a fuse the combination of a tubular casing of insulating material, a ribbon-type fuse link of sheet copper sandwiched between layers of soft solder each in the order of of an inch thick, said link being arranged within and extending diagonally across said casing and exceeding in length the diagonal length of said casing, the ends of said link being bent over the edges of said casing toward the outer surface thereof, a pair of terminal caps mounted over the ends of said casing overlapping said ends of said link, each of said caps having a spot solder joint on the inner lateral surface thereof formed by local fusion of one of said layers of soft solder and establishing a highly conductive connection with said link.

14. in a fuse the combination of a tubular casing of insulating material, a ribbon-type fuse link of a high fusing point metal within said casing, layers of a low fusing point metal plated on said link, said link having ends bent over the edges of said casing upon the outer surface thereof, a pair of terminal caps mounted upon said casing clamping said ends of said link against said outer surface of said casing, and connections for transferring current from said link to said caps established by local fusion of said low fusing point metal layers on said link.

References Cited in the file of this patent UNITED STATES PATENTS 1,277,045 Cole Aug. 27, 1918 1,629,266 Feldkamp May 17, 1927 1,856,317 Clark May 3, 1932 1,861,369 Sundt May 31, 1932 1,986,819 Herbst Jan. 8, 1935 2,270,404 Bitter J an. 20, 1 942 2,532,078 Baxter Nov. 28, 1950 2,592,399 Edsall et al. Apr. 8, 1952 2,653,203 Kozacka Sept. 22, 1953- 2,680,173 Hitchcock June 1, 1954 FOREIGN PATENTS 484,782 Great Britain May 10, 1938 580,759 Great Britain Sept. 18, 1946

Images