US2847537A - Modular low impedance fuse - Google Patents

Description

Aug. 12, 1958 F. J. KozAcKA 2,847,537

MODULAR Low IMPEDANCE FUSE:

Filed July 22, 1955 Ila a l /mmj MWU Invezsozd: 11a E4 ,z'c JKozdy MODULAR LOW IMPEDANCE FUSE Frederick J. Kozacka, South Hampton, N. H., assigner to The Chase-Shawmut Company, Newburyport, Mass.

Application July 22, 1955, Serial No. 523,832

3 Claims. (Cl. 200-135) This invention relates to fuses, and more particularly to very fast fuses having a relatively small current rating.

It is one object of this invention to provide fuses operating sufficiently fast to effectively protect electric devices which can carry only relatively small overloads for only relatively short periods of time, which devices are subject to thermal destruction by relatively narrow margins of excess heat above the heat generated therein when carrying thier normal or rated current.

Another object of the invention is to provide improved fuses having fuse links of which at least one portion has a very small cross-sectional area to obtain ultra-rapid response at relatively small overcurrents, said portion being relatively short to limit the amount of heat generated therein and to facilitate heat dissipation by conduction in a direction longitudinally of the links.

Another object of the invention is to provide vfuses comprising fuse links having a portion whose cross-sectional area is smaller than the cross-sectional area of any conductor that can readily be produced from a metal ribbon by removal of portions thereof by stamping or blanking operations, said fuse links having other portions whose cross-sectional area and surface are relatively large to enable the fuse links to carry their rated current without signicant increase in temperature and to vabsorb and dissipate the heat generated in the portion whose cross-sectional area is small.

Another object of the invention is to provide fuses wherein on the occurrence of overcurrents link destruction is accelerated by the formation of link-destroying alloys.

Another object of the invention is to provide currentlimiting fuses for instrument protection and like applications which have a smaller resistance and/ or impedance than comparable prior art fuses.

Another object of the invention is to provide modular fuses for instrument protection and like lapplications which are easier to manufacture than comparable prior art fuses.

Another object of the invention is to provide modular fuse links comprising subassemblies in the nature of building blocks which can readily be arranged in series in any desired number to form multibreak fuses, said sub-assemblies lending themselves to the manufacture of fuses having different voltage ratings depending upon the number of sub-assemblies serially integrated into them.

Another object of the invention is to provide a method for mass producing instrument fuses which is more eco nomical than yother methods for producing fuses of this kind.

Still another object of the invention is to provide cur* rent-limiting fuses which operate without the presence of a conventional pulverulent arc-quenching tiller, the arc kindled in the fuses upon blowing thereof being quenched by the dynamic action of arc-generated blasts of gas.

The foregoing and other general and special objects l United States Patent O ICC of the invention and advantages thereof will more clearly appear from the ensuing particular description thereof, as illustrated in the drawing wherein Fig. l is a section of a sub-assembly used to make fuses embodying the invention;

Fig. 2 is a side elevation of the structure shown in Fig. l;

Fig. 3 is substantially a longitudinal section of a fuse embodying the invention;

Fig. 4 is substantially a longitudinal section of another use embodying the invention;

Fig. 5 is substantially a longitudinal section of the fuse shown in Fig. 4 taken at 90 degrees to the plane of Fig. 4;

Fig. 6 is substantially a longitudinal section of a fuse vhaving a relatively high voltage rating embodying the invention;

Fig. 7 is a front elevation of a piecetof material used for manufacturing the sub-assembly shown in Figs. 1 and 2, and

Fig. 8 is a cross-section along 8 8 of Fig. 7 showing the piece of material in another stage of the manufac* turing process.

All iigures are drawn on very much enlarged scales, the scale of Figs. l and 2 being larger than that of the other figures.

Referring now to Figs. l and 2, reference character 1 has been applied to generally indicate a laminated disc or block. This disc or block comprises outer laminations 2 of a metal having a high conductivity, e. g. copper, and inner lamination 3 of a relatively heat resistant insulating material. Laminations 2, 3, 2 are bonded together to form a self-sustained structural unit. Passage 4 having a relatively small diameter is provided transversely across outer laminations 2 and inner lamina tion 3. Wire type fuse link S preferably of a high fusing point metal such as silver or copper is threaded through passage fi and conductively connected at each end thereof, e. g. by means of joints 6 of soft solder, to one of the outer laminations 2. Wire 5 is preferably tin plated for reasons which will hereinafter be set forth in detail.

Assuming that the current normally carried by wire 5 is exceeded at a given time. If the rate of rise of current from normal is very low, there will be a tendency for the excess heat generated to'be absorbed and dis sipated by the laminations 2. If the rate of rise of the current from normal is not very low, there will be a tendency for thermal destruction of wire 5, as a result of which an arc is kindled in passage 4 in the space thereof situated within insulating lamination 3. This results in a pressure build-up in passage 4 followed by a sudden arc-extinguishing blast of gas out of the ends of passage 4. This blast may be slightly delayed until the ends of the passage 4 normally closed by solder joints 6 are opened by virtue of the heat generated within passage 4, softening of the masses 6 of solder and the pressure exerted upon masses 6 from within passage 4. Considerable blast velocities are being reached as the gases, i. e. the hot products of arcing, escape out of passage 4 through the holes provided in laminations 2. Lamination 3 consists preferably of an organic insulating material evolving relatively large amounts of arc-quneching and arc-diluting gases when subjected to the heat of the arc, which may be in the order of several thousand degrees centigrade.

The structures shown in Figs. 3 to 5 comprise subassemblies identical to that illustrated in Figs. l and 2.

According to Fig. 3 such a sub-assembly 1 is enclosed in the tubular casing 7 of insulating material closed on both ends by caps S of sheet metal kmounted thereon.

The outer diameter of sub-assembly 1 is substantially the same as the inner diameter of casing 7 so that subassembly 1 fits loosely into casing 7. Sub-assembly 1 is held in position by a pair of helical springs 9 of which each rests with one end thereof against one of the outer laminations 2 and with the other end thereof against one of the terminal caps 8. Springs 9 have a relatively low resistance, i. e. the resistance of one unit length of springs 8 is very small compared to the resistance of one unit length of link wire 5. If desired, springs 9 may be silver plated to decrease their ohmic resistance. Caps 8 may be permanently secured to casing 7 by crimping of the axially inner ends thereof.

The fuse structure shown in Figs. 4 and 5 comprises a casing 7 of insulating material supporting a terminal cap or ferrule 8 on each end thereof. Sub-assembly 1 is arranged substantially midway between the ends of insulating casing 7. The solder joints 6 of sub-assembly 1 which connect the fuse wire 5 to metal laminations 2 are also used for conductively connecting a pair of metal ribbons 1t) to the metal laminations 2. Ribbons 10 consist preferably of copper and are bent at the axially outer ends thereof around the edges of of casing 7 as can clearly be seen in Fig. 5. Metal ribbons 10 are prefer'- ably tinned, e. e. by tin plating. This enables to conveniently establish a blind solder joint at the points 10 between the inner surfaces of caps 8' and ribbons 10 by applying heat to the caps S. It will be observed that the space within casing 7 does not contain any pulverulent arc-quenching filler. Quenching of the arc formed upon fusion of link 5 is achieved by blasts of gas evolved from insulating lamination 3 under the heat of the arc and escaping through the outer ends of passage 4 upon being opened by the action of heat and pressure.

Referring now to Fig. 6, the fuse shown therein comprises a casing 7 of insulating material closed by crimped caps 8" and housing a plurality of series connected sub-assemblies 1, i. e. three such sub-assemblies. These sub-assemblies are spaced from each other and conductively interconnected by means of springs 9' and 9. Springs 9 are arranged between the sub-assemblies 1, and springs 9" are arranged between the axially outer sub-assemblies 1 and the caps 8".

Figs. 7 and 8 show a sheet 11 of a material comprising an inner lamination of insulating material sandwiched between two outer sheets of copper. Circular holes 11a in sheet 11 are formed by a stamping operation and the discs 12 resulting from that operation are subsequently used to make up such units as shown in Figs. l and 2. Fig. 8 shows a number of discs 12 upon removal thereof from the parent laminated sheet. Each disc 12 is provided with a central fuse-link-receiving bore which bores may be drilled into sheet 11 before the stampings 12 are being made. It will be apparent from the foregoing that the cylindrical insulating edge faces of stampings 1 are ush with the metallic edge faces thereof.

The preferred material for making the discs 12 is of the type used in the electronic industry for making socalled printed circuits. Such circuits are made from copper-clad sheets comprising thin layers of copper laminated to both sides of a plastic sheet of desired grade and thickness. Not any material applicable for printed circuits can be used for making discs 12`since it may be necessary for discs 12 to withstand higher temperatures than normally necessary for printed circuits, or in the manufacture of printed circuits. A tin coating on the fusible wire 5 permits to associate the wire with plastic materials which have a relatively limited heat withstanding ability. lf it is desired to further reduce the operating temperature of the fusible wire 5, the latter may be made of indium coated silver. This results in a drastic reduction of the operating temperature as more fully shown in United States Patent 2,703,352'to Frederick I. Kozacka, Fuse and Fuse Link of the Time Lag Type, March 1, 1955. If the mass of tin, indium, r

indium alloy, or other low fusing point link-destroying alloy-forming metal coated on wire 5 is relatively small, the link-destroying alloy-forming action of the overlay will occur very rapidly, i. e. not involve any time delay.

Glass base laminates are particularly suitable for the purpose in hand since glass or quartz has a very high heat absorbing capacity. Thus discs made of glass base laminates are able to absorb considerable amounts of arc energy, in addition to the good dimensional stability, and other desirable properties, which they offer. Various types of synthetic resins are suitable to form the inner layer 3 of the sandwich which is laminated between the outer metal layers 2 such as, for instance, silicone resins, melamine resins, or epoxy type resins. The resins must be selected with due consideration to the object of evolving arc-quenching gases when subjected to the heat of the arc which gases should not contain anything as, for instance, carbon, which would tend to make circuit interruption more onerous. Fiber glass silicon is best suited for relatively high operating temperatures.

The mass of the coating of linkdestroying, alloyforming low fusing point metal, e. g. tin or indium, with which link wire 5 is coated should be compared to the mass of the link wire 5. The tin or indium coating may be less than 1/1000 thick. If the mass of linkde stroying alloy-forming low fusing point metal were only slightly less, of the same order, or larger than the mass of the link wire 5, a time delay or time lag effect would result rather than an acceleration of the interrupting process. If the layer of link-destroying, alloy-forming low fusing point metal is sufficiently thin, the time involved in destruction or disintegration of link wire 5 mainly by dissolution thereof in the fused alloy-forming overlay is less than the time required to thermally destroy link wire 5 without auxiliary link-destroying alloyforming action merely by heating it to the fusing point of the base metal thereof.

It will be understood that I have illustrated and described several embodiments of my invention and that various alterations may be made in the details thereof without departing from the invention as defined in the appended claims.

I claim:

l. An instrument type fuse comprising a stamping formed from metal-clad laminated plastic sheet material of the kind used for printed circuits comprising a pair of layers of sheet metal laminated on both sides of a sheet of a synthetic resin evolving gas when subjected to the heat of an electric arc, the insulating edge face of said stamping being ush with the metallic edge faces thereof, and a short length of fuse wire threaded through a bore projecting transversely across said stamping and conductively connected to the metallic surfaces thereof.

2. An instrument type fuse comprising a stamping formed from metal-clad laminated plastic sheet material comprising a pair of outer laminations of metal and an inner lamination of an insulating material evolving gas when subjected to the heat of an electric arc and capable of withstanding but a relatively low ceiling temperature, the insulating edge face of said stamping being flush with the metallic edge faces thereof, a short length of a fuse wire having a relatively high fusing point threaded through a bore projecting transversely across said stamping and conductively connected to the metallic parts thereof, and an overlay of a relatively low fusing point metal on said fuse wire causing disintegration thereof at temperatures below said relatively high fusing point.

3. An instrument type fuse comprising a stamping formed from a metal-clad laminated plastic sheet material comprising a pair of outer laminations of metal and an inner lamination of an organis insulating material, the insulating edge face of said stamping being flush with the metallic edge faces thereof; a relatively short length of a fuse wire having a relatively high fusing point threaded through a bore projecting trans- 5 6. Versely across said stamping, the ends of said wire being 2,270,404 Bitter -.a.. Jan. 20, 1942 brought out of said bore, bent upon the outer surfaces 2,441,960 Eisler May 25, 1948 of said stamping and soldered thereon; and an overlay of a relatively low fusing point metal on said wire caus- FOREIGN PATENTS ing disintegration thereof at temperatures below said 5 300,161) Great Britain Oct 29, 1923 relatlvely lush fusmg pom 327,518 Great Britain Apr. 1o, 1930 327,961 Great Britain Apr. 11, 1930 References Cited in the le of this patent UNITED STATES PATENTS 10 897,852 Sachs Sept. 1, 1908 UNITED STATES PATENT OFFICE CERTIFICATE OE CORRECTION Patent No, 2,82;7,537 August 12, i958 Frederick JQ Kozaoke s in bhe printed specification It is hereby certified that error appear tion and that the said Letters of' the' above numbered patent requiring correc Patent should read as corrected below.

column A, line 713I Column 3, line'Z, for "eo eq read if e g. mia;

Signed and sealed this 21st dey of October 1958.,

SEAL) 1165i: KARL H.. AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Oicer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Patent Nog 2,8ZJ7,53'7

August l2, 1958 Frederick J1 Kozacka v It is hereby certified that error appears in 'the prin-bed specification t the said Letters of one above numbered patent requiring correction and tha Patent should read as corrected below.

Column 3, line 25, for e eq" read e e., gr de; column A, line "73.5T

Signed and sealed 'this 21st day o' October 1.958a

SEAL) 1168i: KARL-` vI.Al-\XLI1\IF.` l ROBERT C. WATSON Commissioner of Patents Attesting Ofcer`

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