US3471818A - Unitary full-range current-clearing fusible element - Google Patents

Description

Oct. 7, 1969 R. E. KOCH 3,471,818

UNITARY FULL-RANGE CURRENT-CLEARING FUSIBLE ELEMENT Filed Dec. 13. 1967 United States Patent 3,471,818 UNITARY FULL-RANGE -(lURRENT-CLEARING FUSIBLE ELEMENT Robert E. Koch, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Filed Dec. 13, 1967, Ser. No. 690,158 Int. Cl. Hlllh 85/04 U.S. Cl. 337-296 6 Claims ABSTRACT OF THE DISCLOSURE A plurality of continuous electrically and physically parallel fusible filaments are respectively laterally separated and laterally contiguous in longitudinally contiguous series sections. In one form, a section of a ribbon of fusible material is slit or punched out to separate the filaments which are integral in the remaining part of the ribbon. In another form, laterally separated fusible wires are twisted together or braided to form the section of contiguous filaments. The separated filaments are preferably provided with low melting alloy beads.

This invention relates to electric fuses and more particularly to improved full-range current clearing fusible elements for current fuses.

An electric fuse is a protective device for automatically interrupting excess current flow. The principal part of a fuse is a fusible element which melts when heated by excess current flow through it. Ina current fuse, the fusible element is embedded in granular material such as sand so that an are or arcs formed when the element melts is rapidly cooled and the arc rapidly extinguished thus producing rapid current interruption or clearing. In fact, such fuses can interrupt current too fast and thus produce an objectionable momentary or transient inductive voltage rise in the circuit.

This transient voltage is usually controlled byhaving the cross sectional area of the fusible element nonuniform so that it melts first in the smallest cross section area and then burns back at a controlled rate as the cross sectional area increases. For example, the element may be a tapered or stepped diameter rod or wire or a perforated or notched ribbon. While this is quite satisfactory for interrupting very high currents such as fault currents, it is diflicult to secure a fast enough rate of fusible element burnback to reliably interrupt currents slightly in excess of the rating, such as small overload currents.

Reliable overload clearing is usually accomplished by having two or more parallel spaced fusible elements. In such an arrangement, one element will always melt first thus increasing the current in the remaining elements so that another one will melt neat thus producing a cumulative cascading action until all have melted. Rapid burnb ack then takes place and results in interruption of the overload current. Often a low melting alloy bead is placed around each wire to control the melting of the first wire on very small overloads. Upon melting, the bead material will alloy with the wire to form a lower melting point eutectic mixture. However, when subjected to fault currents such as may be produced by short circuits the multiple parallel wire construction burns back too fast and thus can produce objectionable voltage rise.

An obvious way to reduce over-voltages yet obtain positive full current range clearing is to aggregate a conventional weakened ribbon fuse and a conventional multiple parallel wire fuse by connecting them in series either in separate casings or cartridges or in the same casing or cartridge. However, this is expensive.

In accordance with this invention there is provided a novel unitary fusible element of continuous filaments See which has positive, controlled voltage full range currentclearing ability. The filaments may be integral parts of a ribbon which in a certain longitudinal section or sections are later-ally separated by slitting or punching the ribbon or they may be separated parallel wires which in a certain longitudinal section or sections are consolidated into contiguous contact by twisting or braiding.

An object of the invention is to provide a new and improved fusible element for electric fuses.

Another object of the invention is to provide a new and improved fullarange currentclearing fusible element for current limiting fuses.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing,

FIG. 1 is a plan view of -'a ribbon type fusible element embodying the invention,

FIG. 2 is a modification of FIG. 1,

FIG. 3 is a similar view of a multiple wire type fusible element embodying the invention, and

FIG. 4 another modified ribbon type fusible element embodying the invention.

Referring now to the drawing and more particularly to FIG. 1, there is shown therein a length of ribbon conductor 1 composed of suitable good electrically conductive material such as silver. It is longitudinally divided into a set of sections 2 which are similar to each other alternated with sections 3 which are different from sections 2 but are similar to each other. The sections 2 have an overall generally circular shape such as would be formed by punching a circular hole in the ribbon 1. However, they actually comprise la plurality of filaments 4, 5, 6, 7, 8 and 9 formed by punching or cutting out intermediate portions 10, =11, 12, 13 and 14. The central cutout 12 is the longest, the outer cutouts 10 and 14 are the shortest and the intermediate cutouts 1 1 and 13 are of intermediate length. The ends of the cutouts are shown square but this is not essential and they can be curved or rounded. Preferably surrounding each of the six filaments 49, in one of the sections 2 which is located in the center of the ribbon longitudinally, is a bead or beads 15 of low melting alloy.

The operation of FIG. 1 is as follows: On the occurrence of a high fault current, most if not all of the slotted sections 2 melt before the merged sections 3 melt and burnback proceeds from the slotted sections into the merged or solid sections. This controls the voltage surge by controlling the rate at which resistance is introduced into the circuit. The solder bead 15 does not enter into the operation on high fault current clearing.

When subjected to low current faults, melting takes place first in the area of the solder bead 15. It is not important that one filament melt first and several can melt at the same instant. It is important for the individual filaments to be small enough or that there be a high enough number in parallel so that the last one to melt will have a high enough current flowing through it to effect rapid burnback. The individual filaments in the slotted sections 2 do not immediately begin to burnback upon melting but every current carrying filament separates slightly as it melts until the last parallel filament melts and then burnback proceeds. Current will then transfer from one filament to the other or to several in parallel, and burn them back, until complete arc extinguishing takes place.

However, in the case of both 'high and low fault currents, burnback will proceed through several or many of the merged sections 3 before complete current interruption takes place.

The relative lengths of the filaments in sections 2 are not critical and in the modification shown in FIG. 2 the intermediate cutouts 11' and 13 are made the same length as the central cutout 12. It is also not essential that the solder bead for initiating and localizing low fault current melting be on the filaments and as shown in FIG. 2 a solder bead 15 can be located at the center of one of the solid or merged sections 3'. FIG. 2 is otherwise the same as FIG. 1 and separates essentially in the same manner.

In the modification shown in FIG. 3, the fusible element comprises a plurality of separate wires 16, 17, 18, 19 and 20 which in a section or sections 21 are laterally spaced or separated, each one being provided with a low melting alloy bead or beads 21'. In another section or sections 22 of the fusible element the individual wires are brought together by twisting or braiding them. In other words, the fusible element can 'have two sections as shown in FIG. 3 or there can be a plurality of sections 21 alternated with a plurality of sections 22.

The operation of FIG. 3 is somewhat different from the operation of FIGS. 1 and 2 because the actual current carrying area is the same in both sections 21 and 22. Therefore, high current will melt section 22 first because the smaller surface area in contact with sand will provide less heat loss. However, the large effective diameter of section 22 will provide a lower voltage surge per unit length than the small parallel spaced wires of section 21.

In the modification shown in FIG. 4, a fusible element 1' has a conventional high current clearing section 23 [formed by a series of axially spaced circular 1holes formed therein and it has a low current clearing section 24 made up of subsections 25 in which filamentary fusible elements are formed by cutouts. As a concrete example, the element 1' may be .189" wide and .0033" thick. The holes in the section 23 may be .125" in diameter. The subsections 25 may be .25" long separated by .040" and the conducting filaments formed between cutouts may be .014" in width.

It will, of course, be understood that suitable lengths of the fusible elements shown in FIGS. 1 through 4 may conventionally be spirally wound on a central support or core of any suitable gas evolving material, the whole being embedded in a sand or the equivalent filled casing or cartridge with the ends of the fusible element connected respectively to the external terminals of the fuse. It will likewise be understood that two or more such elements may be arranged in the above described conventional manner in a single fuse, those elements being electrically in parallel.

While there have been shown and described particular embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and therefore it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A one-piece full-range current-clearing fusible element for a current limiting (fuse comprising an elongated flat fusible conductor having a plurality of perforated sections at substantially equally spaced-apart areas along its length and having at least one imperforate section of substantially uniform cross section between each of said perforated sections, each of said perforated sections comprising a plurality of fiat filaments having their respective longitudinal axes generally parallel to the longitudinal axis of said conductor, the respective opposite ends of each of said fiat filaments comprising an integral part of the respective imperforate sections disposed adjacent thereto whereby jointless transition is afforded between said filaments and said imperforate sections, the total cross sectional area of the filaments in each of said perforated sections being less than the cross-sectional area of each of said imperforate sections, thereby to adapt said fusible conductor to afford low overcurrent protection by causing it to fuse at said perforated sections when subjected to a sustained low overcurrent and to afford a controlled rate of burnback due to the heat-sink effect of said imperforate sections when caused to fail under high overcurrent conditions.

2. The invention defined in claim 1 wherein the flat filaments of each perforated section are substantially iden tical in shape and number to the filaments in each of the other perforated sections of the fusible conductor.

3. The invention defined in claim 2 wherein the filaments of each perforated section define a plurality of apertures in the fusible conductor, the apertures in each of said perforated sections defining a plurality of overall generally circular holes in the conductor with the filaments of said sections extending across each of said holes, whereby the current carrying portions of some of the filaments in said perforated sections are of different lengths so their respective burnback times under overcurrent oonditions are substantially different and afford a uniform rate of burnback for the conductor as a whole when it fails under a high overcurrent condition.

4. A fusible element as defined in claim 1 including a bead mounted on at least one of said filaments, said head being formed of an alloy that has a lower melting point than the melting point of said filaments without said bead thereon.

5. A fusible element as defined in claim 1 including a bead mounted on at least one of said imperforate sections, said bead being formed of an alloy that has a lower melting point than the melting point of said imperforate section without said bead thereon.

6. A fusible element as in claim 1 in which said conductor has another portion of its length provided with longitudinally tapering cutout areas that are of a different size than the removed portions of one of the perforated sections thereof, whereby the rate of burnback of said other portion is made different than the rate of burnback of the portion of said fusible conductor containing the perforated sections.

References Cited UNITED STATES PATENTS 569,803 10/1896 Scott 337-296 1,239,876 9/1917 B urnham 337-293 X 1,631,669 6/1927 Brown 337293 1,081,214 12/1913 Conant 337244 1,277,045 8/1918 Cole 337295 1,692,445 ll/1928 Hope 337-293 X 3,138,682 6/1964 Dannenberg et al. 337295 X FOREIGN PATENTS 364,546 11/ 1962 Switzerland.

BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner US. Cl. X.R.

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