US3287525A - Terminal means for fusible element of current limiting fuse - Google Patents

Description

NOV. 22, 1966 H, W, MlKULECKY 3,287,525

TERMINAL MEANS FOR FUSIBLE ELEMENT OF CURRENT LIMITING FUSE Filed Feb. 26, 1965 IlIIllIlllll/VIIIII 36 I N VEN TOR.

3 287 525 TERMINAL MEANS FOR FUSIBLE ELEMENT F CURRENT LIMITING FUSE Harvey W. Mikulecky, Racine, Wis., assignor to McGraw-j Edison Company, Milwaukee, Wis., a corporation of Delaware Filed Feb. 26, 1965, Ser. No. 435,436 7 Claims. (Cl. 20D-120) This invention relates to electrical Lfuses and, more particularly, to 'fuses of the current limiting type which limit the flow of current in a circuit to` a substantially smaller m-agnitude than the available 4fault current of the circuit.

Current limiting fuses usually open with such speed that the fuse arc resistance is introduced during the rst |half cycle ol? the short circuit current, and the actual letthrough current is reduced to much less than the available current which would llow in the circuit if the fusible element were intact. Current limiting fuses conventionally comprise a fusible element embed-ded in a granular inert material of high dielectric strength such as sand or iine'ly divided quartz. Usual-1y the fusible element i-s in the form of one or more thin conductors o'f silver Wound on a supporting core, or spider, of high temperature resistant ceramic material. As fault current begins to ilow the element melts and an arc 'is formed. When subjected to current of high magnitude, the fusible element attains fusing temperature and vaporizes almost instantaneously, whereupon -arcing occurs along the entire length ofthe element and the metal vapors rapidly expand to many times the volume occupied by the fusible element and are thrown into the spaces 'between the Igranules of inert filler y material where they condense and are no longer available for current conduction. The physical contact between the hot arc and the relatively cool granules causes a rapid trans-fer of heat from the arc to the granules, thereby dissipating Imost of the arc energy with very little pressure buildup within the fuse enclosure. Consequently, a high resistance is, in effect, inserted into the path of the current and initial-ly limits the -current to a magnitude which is only a small fraction o'f that available in the circuit. The inserted resistance increases rapidly and results in rapid decay of current and subsequent interruption of current with negligible `generation o'f gas and noise. The sand pa-rticles in the immediate vicinity lof the arc fuse and become partial conductors at the high temperature of the arc. The fused particles cool upon extinction of the arc and solidify i-nto a ifulguritef which is in the n-ature of a glass body, and lose their conductivity and become insulators a-s they cool.

Unlike expulsion fuses, current limiting fuses are designed to operate within a specific range of voltages. Each voltage lsize of current limiting fuse is designed to operate over its entire fault capacity range with a permissible transient voltage surge. Each size of current limiting fuse must be designed so that this permissible peak arc voltage stays well below the basic impulse level of the power system and below the sparkover potential of lightning arresters on the power system, and careful choice must be made of the number and size of the silver wires or silver ribbons which comprise the Ifusible element to assure proper clearing of the fuse from minimum melting current to maxiu-m rating thereof.

High amperage current limiting fuses often comprise a number o'f conductor-s such as silver ribbons helically wound in parallel on the insulating spider and electrically connected between the end terminals of the fuse. Each -silver conductor yrapidly vaporizes under high fault current, and a tulgu-rite is formed along the entire length of the conductor. The arcs within the individual fulgurites tend to assume the shortest path between the end termi- 3,287,525 Patented Nov. 22, 1966 nals of the fuse and to merge together adjacent the end termin-als #because the path from conductor-to-conductor in an axial 4direction is shorter than the helical path along the con-ductor. As a consequence, the individual arcs do not continue along the entire length of the individual conductors but rather concentrate in a single tulgurite adjacent the fuse end terminals. This heavy concentration of arc heating in a single fulgurite adjacent the fuse endA terminals often scorches the fuse casing and can result in release of hot gases, ashover o'f the fuse, or mechanical failure o'f the fuse.

It is an o'bject of the invention to provide a high arnperage current limiting fuse construction which [prevents scorching of the fuse casing, ila-shover of the lfuse, and mechanica-l failure of the fuse caused by excessive arc heating.

This and other objects and advantages of the invention will be more readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein:

FIG. l is a partial longitudinal view through a current limiting .fuse embodying the invention; and

FIG. 2 is a view taken on line 2-2 of FIG. l.

Referring to the drawing, a tubular enclosing casing 10 for a current limiting fuse is constructed of suitable insulating material such as glass, fiber, or glass fiber irnpregnated with epoxy resin. A metallic end piece 11R may be secured to the right end of casing 10 by means of any suitable seal such as epoxy cement, and a metallic end piece 11L may have external threads engaging internal threads near the left end of casing 10 to aix end piece 11L to casing 10. A metallic'hinge assembly 12- may be secured to the end piece 11R at the right end of fuse casing 10 by screws 1'4 engaged within threaded apertures in the end piece 11R. The end piece 11L has a smaller diameter tubular portion 15 and an axial bore 16. A tubular metallic terminal member -17 extends into axial bore 16 with a torce lit and is rigidly secured to end piece 11L 'by pins 18 extending radially through terminal member 17 and smaller 4diameter portion 15 of end piece 11L. Terminal member 17 i-s yadapted to lit within a stationary jaw contact (not shown) of an electrical switch, and .an insulating member 19 provided with an eye (not shown) for receiving a hook-stick may be secured in the end `of tubular member 17 lby suitable means suc'h asv An arc extinguishing sleeve member 20 epoxy cement. slidably -fitting over casing 10 m-ay have an inner tubular portion 21 of insulating material telescoped over terminal member 17 and be urged axially into covering relationwith termin-al member 17 by a helical spring 22 compressed between end piece 11L and the end portion 24 of larc extinguishing member 20 for the purpose of interrupting any arc formed between terminal member 17 and the switch jaw contact as described in my c-opending application Serial No. 298,882 led July 31, 1963 andv Spider 28 is of generally star-shaped cross section and has a plurality of radially protruding, peripherally spaced apart, longitudinally extending tins 32. Each iin 32 has a plurality of `depressions 33 spaced apart longitudinally of spider '28 and forming longitudinally spaced apart,

raised shoulders 34 on each n. The depressions 33 of peripherally successive ns are progressively staggered in a longitudinal direction of spider 28 so that the peripherally successive depressions 33 define a continuous helical path and the peripherally successive raised shoulders 34 form support means of helical configuration for a helically Wound circuit .interrupting element 36 interconnecting the end pieces 11L and 11R. A fusible wire 37 for indicating operation of the fuse may be disposed in the continuous helical lpath defined by the depression 33.

Spider 28 may be of a heat resistant ceramic insulat-` ing material such as porcelain, but in the preferred embodiment of the invention spider 28 is of an electrical insulating material adapted to evolve gas in the presence of an arc and may be of la molded thermosetting composition comprising a water insoluble binder and an antitracking substance selected from the class consisting of the hydrates and oxides of aluminum and magnesium as disclosed in my copending application Serial No. 313,640 led October 3, 1963 entitled Current Limiting Fuse and having the same assignee as the subject invention. The composition of spider 28 may also include other fillers such as mica, glass liber, asbestos, or silica, and one suitable material comprises approximately 60 percent aluminum hydrate filler, 20 percent melamine resin binder, and approximately 20 percent asbestos. The active gas generating and anti-tracking ingredient may be commercial grade aluminum hydrate Al (OH)3, magnesium hydrate Mg(OH)2, an oxide of aluminum such as alumina A1203, or magnesium oxide. In addition to orin -place of the melamine resin, other resins may be employed as the binder of the molding composition, for example, phenolic, urea, polyester or silicone resin.

The casing is lilled with a body of suitable pulverulent refractory arc quenching Imaterial 46 such as quartz sand so that the spider 28 and the fusible element 36 are directly embedded in the quartz sand filler.

Fusible element 36 is illustrated in the high amperage current limiting fuse shown in the drawing as comprising three silver ribbons 38 helically wound in parallel, spaced apart relation on the raised shoulders 34 so as to be in approximate line contact with tins 32 and touch only peripherally spaced apart points on spider 28. Ribbons 38 may be secured at their ends by suitable means such as solder to Ibent-down tabs 27 on the end plates 25 which are aixed to the end pieces 11L and 11R. The silver ribbons 38 may have .a plurality of circular perforations, or holes, 39 spaced apart along the length thereof which determine the points Where fusion of the ribbon is initiated when the fault current and its rate of rise are high. The perforations 39 form portions of reduced cross sectional area so that each ribbon 38 has a number of serially related portions of relatively small cross sectional area and intermediate portions of relatively large cross sectional area. Beads 44 of low melting temperature alloy such as tin-lead solder are in intimate contact with the silver ribbons 38 preferably adjacent the midpoint thereof. At melting currents flowing for prolonged periods, the fusible ribbons 38 become hot enough to melt the alloy bodies 44, and the amalgamation of the silver and alloy causes a hot spot with high enough resistance to melt the ribbons 38 at this point. This construction, known as the M effect, allows the fusible ribbons 38 to melt at a temperature in the 400-600 F. range, when subjected over a long period of time to low l magnitude currents, as compared to the 1760 F. melting temperature for pure silver.

On fault currents of high magnitude, the alloy element 44 has little or no effect, and the silver ribbons 38 vaporize almost instantaneously at the fusion temperature for silver and form fulgurites along the entire length of the ribbons 38. The arcs within the individual 4fulgurites tend to assume the shortest path between the metallic end plates 25, and the fulgurites tend to merge together adjacent the end plates because the parallel, side-by-side from ribbon-to-ribbon in an axial direction being shorter than the helical path along the ribbon. Consequently, the individual arcs do not continue along the entire length of the individual ribbons 38 but rather concentrate in a single fulgurite adjacent the metallic end plates 25. This heavy concentration of arc heating in a single fulgurite often scorched the casing 10 of prior art fuses and can burn through the casing and release hot gases which, in turn, can ignite the casing 10 or bridge between the end pieces 11L and 11R and flash-over the fuse. In embodiments wherein casing 10 is of glass or ceramic material, the intense concentration of arc heating occasionally cracked the fuse casing and caused mechanical failure of the fuse.

In accordance With the invention, an elongated metallic clip 43 of U-shaped cross section extending in a direction parallel to the axis of the fuse is disposed over a raised shoulder 34 at each end of spider 28 in abutting relation to end plate 25 and in a position wherein the three helical silver ribbons 38 are wound in axially spaced apart relation over the longitudinally extending clip 43. One of the radially extending tabs 27A on end plate 25 is bent down and soldered to clip 43 so that clip 43 is connected electrically to the end plate 25 and end piece 11. When a high magnitude fault current vaporizes the silver ribbons 38, the arcs formed along the ribbons terminate on the longitudinally extending metallic clips 43 rather than on the end plates 25. Inasmuch as clips 43 extend -in an axial direction, no fulgurite-to-fulgurite pat'h in an axial direction exists adjacent the fuse terminals which is shorter than the-path along the ribbons, and the individual ribbon -fulgurites thus terminate on the clips 43 and have no tendency to merge together or concentrate at a single point. The disclosed construction prevents concentration of the arc and excessive arc heating which in prior art `fuses often scorched the fusetube and released hot gases that flashed over the fuse tube.

On low magnitude faults, the gas generated by the material of spider 28 at the points of contact with fusible ribbons 38 is blown into the are stream and cools the fulgurite and produces a deionizing' action on the arc products so that the inert sand particles quickly lose their conductivity and become conductors.

While only a single embodiment of the invention has been illustrated and described, many modifications and variations thereof will be readily apparent to those skilled in the art, and consequently it is intended to cover in the appended claims all such modifications and variations which fall within the true spirit and scope of the invention.

I claim:

1. In a high amperage current limiting fuse, a tubular insulating casing, an inert granular material of high dielectric strength within said casing, an insulating spider extending parallel to the axis of said casing and embedded in said inert granular material and having peripherally spaced apart, radially protruding portions extending longitudinally thereof, a plurality of silver ribbons helically wound in parallel spaced apart relation on said radially protruding portions of said spider and being embedded in said granular inert material, and a metallic terminal on each end of said casing, said metallic terminals each including a portion extending axially with respect to said casing and engaging said ribbons, said ribbons engaging and electrically connected to said axially extending portions at points spaced axially thereon with respect to said casing so that all of the arcs formed upon vaporization of said ribbons, incident to high magnitude fault current carried by said fuse, terminate on said axially extending terminal portions andl do not tend to merge together and cause excessive archeating.

2. In a high amperage current limiting fuse, a tubular insulating casing, an inert granular 'material of high dielectric strength within said casing, an insulating spider extending parallel to the axis of said casing and embedded in said inert granular material and having peripherally spaced apart, radially protruding portions extending longitudinally thereof, a plurality of silver ribbons helically wound in parallel spaced apart relation on said radially protruding portions of said spider and being embedded in said granular inert material, each of said ribbons having a body of low melting temperature alloy in contact therewith, said radially protruding portions along at least the surface thereof in contact with said silver ribbons being of an electrical insulating material adapted to generate gas in the presence of an arc and being capable of continuously withstanding temperatures up to 250 F. without degradation and up to 500 F. for periods of up to one hour without substantial decomposition, and a metallic terminal on each end of said casing and electrically connected to all of said ribbons, said metallic terminals including a portion extending axially with respect to said casing across the helical path of and electrically connected to all of said ribbons, said ribbonsv engaging and electrically connected to said axially extending portions at points spaced axially thereon with respect to said casing so that all of the arcs formed upon vaporization of said ribbons, incident to high magnitude fault current carried by said fuse, terminate on said axially extending terminal portions and do not tend to merge together and cause excessive arc heating.

3. In a fuse in accordance with claim 2 wherein said insulating material includes a water insoluble binder and an anti-tracking substance selected from a group consisting of the hydrates and oxides of aluminum and magnesium.

4. In a high amperage current limiting fuse, a tubular insulating casing, an inert granular material of high dielectric strength within said casing, an insulating spider extending parallel to the axis of said casing and embedded in said inert granular material and having peripherally spaced apart, radially protruding portions extending longitudinally thereof, a plurality of silver ribbons helically wound in parallel spaced apart relation on said radially protruding portions of said spider and being embedded in said granular inert material, a metallic terminal on each end of said casing having a plurality of circumferentially spaced apart portions individually affixed to said helically wound ribbons and also having a portion extending axially With respect to said casing across the helical path of and electrically connected to all of said ribbons at points spaced axially thereon with respect to said casing so that, all of the arcs formed upon vaporization of said ribbons terminate on said axially extending terminal portions and do not tend to merge and to cause excessive arc heating.

5. In a f-use in accordance with claim 4 wherein said spider is of an insulating material capable of generating gas in the presence of an arc and each of said ribbons has a body of low melting temperature alloy in -contact therewith.

6. In a fuse in accordance with -claim 5 wherein said insulating material includes a water insoluble binder and an anti-tracking substance selected from a group consisting of the hydrates and oxides of aluminum and magnesium.

7. In a high amperage current limiting fuse, a tubular insulating casing, an inert granular material within said casing, an insulating support core extending parallel to the axis of said casing and embedded in said inert granular material, a plurality of fusible elements helically wound in generally parallel spaced relation on said support core and embedded in said inert granular material, and a metallic terminal at each end of said casing, said metallic terminals each including a portion extending axially with respect to said casing and engaging said fusible elements, said fusible elements engaging and electrically connected to said axially extending portion at points spaced axially thereon so that all of-the arcs formed upon vaporization of said fusible elements terminate on said axially extending terminal portions at said axially spaced points and do not tend to merge together and cause excessive heating.

References Cited by the Examiner UNITED STATES PATENTS 2,230,207 l/ 1941 Thommen et al. 200--120 X 2,667,549 1/ 1954 Fahnoe et al.

2,768,264 l0/l956 Jones et al. 200--144 3,138,682 6/1964 Dannenberg et al. 200-120 OTHER REFERENCES Metals Handbook, 1939 ed., Cleveland, Ohio, American Society For Metals, p. 1523, TA472, A3-1939.

o BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner.

Claims (1)

1. IN A HIGH AMPERAGE CURRENT LIMITING FUSE, A TUBULAR INSULATING CASING, AN INERT GRANULAR MATERIAL OF HIGH DIELECTRIC STRENGTH WITHIN SAID CASING, AN INSULATING SPIDER EXTENDING PARALLEL TO THE AXIS OF SAID CASING AND EMBEDDED IN SAID INERT GRANULAR MATERIAL AND HAVING PERIPHERALLY SPACED APART, RADIALLY PROTRUDING PORTIONS EXTENDING LONGITUDINALLY THEREOF, A PLURALITY OF SILVER RIBBONS HELICALLY WOUND IN PARALLEL SPACED APART RELATIONON SAID RADIALLY PROTRUDING PORTIONS OF SAID SPIDER AND BEING EMBEDDED IN SAID GRANULAR INERT MATERIAL, AND A METALLIC TERMINAL ON EACH END OF SAID CASING, SAID METALLIC TERMINALS EACH INCLUDING A PORTION EXTENDING AXIALLY WITH RESPECT TO SAID CASING AND ENGAGING SAID RIBBONS, SAID RIBBONS ENGAGING AND ELECTRICALLY CONNECTED TO SAID AXIAL-

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