Document b5mg8jaR879yeoZJq1rMMgRr0

FILE NAME: McGraw-Edison (ME) DATE: 1965 Feb 26 DOC#: ME011 DOCUMENT DESCRIPTION: Patent - Terminal means for fusible element of current limiting fuse [Approved - 1966 Nov 22] BC Note: Discloses asbestos as a preferred material in spiders for current limiting fuse Nov. 22, 1966 H, W. MIKULECKY 3,287,525 terminal means for fusible element of current limiting fuse Filed Feb. 26, 1965 INVENTOR. ftarvey C c/f' o m e y U nited States Patent Office Patented Nov3.,22827, ,1592656 1 2 nals of the fuse and to merge together adjacent the end 3 287 525 terminals because the path from conductor-to-conduetor TERMINAL MEANS FOR FUSIBLE ELEMENT in an axial direction is shorter than the helical path along OF CURRENT LIMITING FUSE the conductor. As a consequence, the individual arcs do Harvey W. Mikulecky, Racine, Wis., assignor to McGrawEdison Company, Milwaukee, Wis., a corporation of _ not continue along the entire length of the individual con ductors but rather concentrate in a single fulgurite adja Filed Feb. 26,1965, Ser. No. 435,436 7 Claims. (Cl. 200-- 120) cent the fuse end terminals. This heavy concentration of arc heating in a single fulgurite adjacent the fuse end This invention relates to electrical fuses and, more par terminals often scorches the fuse casing and can result in Telease of hot gases, flashover of the fuse, or mechanical ticularly, to fuses of the current limiting type which limit failure of the fuse. the flow of current in a circuit to a substantially smaller It is an object of the invention to provide a high am magnitude than the available fault current of the circuit. perage current limiting fuse construction which prevents Current limiting fuses usually open with such speed scorching of the fuse casing, flashover of the fuse, and that the fuse arc resistance is introduced during the first 15 mechanical failure of the fuse caused by excessive arc 'half cycle of the short circuit current, and the actual "let- heating. through" current is reduced to much less than the avail This and other objects and advantages of the invention able current which would flow in the circuit if the fusible will be more readily apparent from the following detailed element were intact. Current limiting fuses conventional description when taken in conjunction with the accom ly comprise a fusible element embedded in a granular inert 20 panying drawing wherein: material of high dielectric strength such as sand or finely FIG. 1 is a partial longitudinal view through a current divided quartz. Usually the fusible element is in the limiting fuse embodying the invention; and form of one or more thin conductors of silver wound on FIG. 2 is a view taken on line 2--2 of FIG. 1. a supporting core, or spider, of high temperature resist Referring to the drawing, a tubular enclosing casing 10 ant ceramic material. As fault current begins fe> flow 25 for a current limiting fuse is constructed of suitable in the element melts and an arc is formed. When subjected sulating material such as glass, fiber, or glass fiber im to current of high magnitude, the fusible element attains pregnated with epoxy resin. A metallic end piece 11R fusing temperature and vaporizes almost instantaneously, may be secured to the right end of casing 10 by means whereupon arcing occurs along the entire length of the of any suitable seal such as epoxy cement, and a metallic element and the metal vapors rapidly expand to many 30 end piece 11L may have external threads engaging in times the volume occupied by the fusible element and are ternal threads near the left end of casing 10 to affix end thrown into the spaces between the granules of inert filler piece 11L to casing 10. A metallic hinge assembly 12 material where they condense and are no longer available for current conduction. The physical contact between may be secured to the end piece 11R at the right end of fuse casing 10 by screws 14 engaged within threaded aper the hot arc and the relatively cool granules causes a rapid 35 tures in the end piece 11R. The end piece 11L has a transfer of heat from the arc to the granules, thereby dis smaller diameter tubular portion 15 and an axial bore sipating most of the arc energy with very little pressure 16. A tubular metallic terminal member 17 extends into buildup within the fuse enclosure. Consequently, a high axial bore 16 with a force fit and is rigidly secured to end resistance is, in effect, inserted into the path of the cur piece 11L by pins 18 extending radially through terminal rent and initially limits the current to a magnitude which 40 member 17 and smaller diameter portion 15 of end piece is only a small fraction of that available in the circuit. 11L. Terminal member 17 is adapted to fit within a The inserted resistance increases rapidly and results in stationary jaw contact (not shown) of an electrical switch, rapid decay of current and subsequent interruption of and an insulating member 19 provided with an eye (not current with negligible generation of gas and noise. The shown) for receiving a hook-stick may be secured in the sand particles in the immediate vicinity of the arc fuse 40 end of tubular member 17 by suitable means such as and become partial conductors at the high temperature epoxy cement. An arc extinguishing sleeve member 20 of the arc. The fused particles cool upon extinction of slidably fitting over casing 10 may have an inner tubular the arc and solidify into a "fulgurite," which is in the portion 21 of insulating material telescoped over terminal nature of a glass body, and lose their conductivity and member 17 and be urged axially into covering relation become insulators as they cool. ' with terminal member 17 by a helical spring 22 com Unlike expulsion fuses, current limiting fuses are de pressed between end piece 11L and the end portion 24 of signed to operate within a specific range of voltages. arc extinguishing member 20 for the purpose of inter Each voltage size of current limiting fuse is designed to rupting any arc formed between terminal member 17 and operate over its entire fault capacity range with a "per- gg the switch jaw contact as described in my copending ap missible" transient voltage surge. Each size of current plication Serial No. 298,882 filed July 31, 1963 and limiting fuse must be designed so that this "permissible" having the same assignee as this invention. peak arc voltage stays well below the basic impulse level Metallic end plates 25 are disposed against the internal of the power system and below the sparkover potential of surface of end pieces 11L and 11R and are secured there lightning arresters on the power system, and careful g0 to by screws 26. Each end plate 25 has a plurality of choice must be made of the number and size of the silver radially extending tabs 27 adjacent its outer periphery wires or silver ribbons which comprise the fusible ele which may be bent down and form terminals to which the ment to assure proper clearing of the fuse from minimum fusible conductors may be electrically connected. An melting current to maxium rating thereof. elongated insulating core, or spider, 28 is axially mount High amperage current limiting fuses often comprise a gg ed within casing 10. The ends of spider 28 are affixed number of conductors such as silver ribbons helically to metallic end plates 25 by suitable means such as epoxy wound in parallel on the insulating spider and electrically cement. connected between the end terminals of the fuse. Each Spider 28 is of generally star-shaped cross section and , silver conductor rapidly vaporizes under high fault cur has a plurality of radially protruding, peripherally spaced rent, and a fulgurite is formed along the entire length of apart, longitudinally extending fins 32. Each fin 32 has the conductor. The arcs within the individual fulgurites a plurality of depressions 33 spaced apart longitudinally tend to assume the shortest path between the end termi- of spider 28 and forming longitudinally spaced apart, 3,287,525 3 4 raised shoulders 34 on each fin. The depressions 33 of arrangement of the silver ribbons 38 results in the path peripherally successive fins are progressively staggered from ribbon-to-ribbon in an axial direction being shorter in a longitudinal direction of spider 28 so that the periph than the helical path along the ribbon. Consequently, erally successive depressions 33 define a continuous heli the individual arcs do not continue along the entire cal path and the peripherally successive raised shoulders 5 length of the individual ribbons 38 but rather concentrate 34 form support means of helical configuration for a in a single fulgurite adjacent the metallic end plates 25. helically wound circuit interrupting element 36 intercon This heavy concentration of arc heating in a single ful necting the end pieces 11L and HR. A fusible wire 37 gurite often scorched the casing 10 of prior art fuses and for indicating operation of the fuse may be disposed in can burn through the casing and release hot gases which, the continuous helical path defined by the depression 33. 10 in turn, can ignite the casing 10 or bridge between the Spider 28 may be of a heat resistant ceramic insulat end pieces 11L and 11R and flashover the fuse. In em ing material such as porcelain, but in the preferred em bodiments wherein casing 10 is of glass or ceramic ma bodiment of the invention spider 28 is of an electrical in terial, the intense concentration of arc heating occasion sulating material adapted to evolve gas in the presence ally cracked the fuse casing and caused mechanical failure of an arc and may be of a molded thermosetting composi 15 of the fuse. tion comprising a water insoluble binder and an anti tracking substance selected from the class consisting of In accordance with the invention, an elongated metallic clip 43 of U-shaped cross section extending in a direction the hydrates and oxides of aluminum and magnesium as parallel to the axis of the fuse is disposed over a raised disclosed in my copending application Serial No. 313,640 shoulder 34 at each end of spider 28 in abutting relation filed October 3, 1963 entitled Current Limiting Fuse and 20 to end plate 25 and in a position wherein the three having the same assignee as the subject invention. The helical silver ribbons 38 are wound in axially spaced apart composition of spider 28 may also include other fillers relation over the longitudinally extending clip 43. One such as mica, glass fiber, asbestos, or silica, and one suit of the radially extending tabs 27A on end plate 25 is bent able material comprises approximately 60 percent alumi down and soldered to clip 43 so that clip 43 is connected num hydrate filler, 20 percent melamine resin binder, 25 electrically to the end plate 25 and end piece 11. When and approximately 20 percent asbestos. The active gas generating and anti-tracking ingredient may be com a high magnitude fault current vaporizes the silver rib bons 38, the arcs formed along the ribbons terminate mercial grade aluminum hydrate A1 (O H )3, magnesium on the longitudinally extending metallic clips 43 rather hydrate Mg(OH)2, an oxide of aluminum such as alu than on the end plates 25. Inasmuch as clips 43 extend mina A120 3, or magnesium oxide. In addition to or in 30 in an axial direction, no fulgurite-to-fulgurite path in an place of the melamine resin, other resins may be em axial direction exists adjacent the fuse terminals which ployed as the binder of the molding composition, for is shorter than the path along the ribbons, and the indi example, phenolic, urea, polyester or silicone resin. vidual ribbon fulgurites thus terminate on the clips 43 The casing 10 is filled with a body of suitable pulver and have no tendency to merge together or concentrate ulent refractory arc quenching material 46 such as quartz 35 at a single point. The disclosed construction prevents sand so that the spider 28 and the fusible element 36 are concentration of the arc and excessive arc heating which directly embedded in the quartz sand filler. in prior art fuses often scorched the fuse tube and re Fusible element 36 is illustrated in the high amperage leased hot gases that flashed over the fuse tube. current limiting fuse shown in the drawing as comprising On low magnitude faults, the gas generated by the three silver ribbons 38 helically wound in parallel, spaced 40 material of spider 28 at the points of contact with fusible apart relation on the raised shoulders 34 so as to be in ribbons 38 is blown into the arc stream and cools the approximate line contact with fins 32 and touch only fulgurite and produces a deionizing action on the arc peripherally spaced apart points on spider 28. Ribbons products so that the inert sand particles quickly lose their 38 may be secured at their ends by suitable means such conductivity and become conductors. as solder to Ibent-down tabs 27 on the end plates 25 45 While only a single embodiment of the invention has which are affixed to the end pieces 11L and 11R. The been illustrated and described, many modifications and silver ribbons 38 may have a plurality of circular perfora variations thereof will be readily apparent to those skilled tions, or holes, 39 spaced apart along the length thereof in the art, and consequently it is intended to cover in which determine the points where fusion of the ribbon the appended claims all such modifications and variations is initiated when the fault current and its rate of rise are 50 which fall within the true spirit and scope of the in high. The perforations 39 form portions of reduced cross vention. sectional area so that each ribbon 38 has a number of I claim: serially related portions of relatively small cross sec _ I. In a high amperage current limiting fuse, a tubular tional area and intermediate portions of relatively large insulating casing, an inert granular material of high di cross sectional area. Beads 44 of low melting tempera 55 electric strength within said casing, an insulating spider ture alloy such as tin-lead solder are in intimate contact extending parallel to the axis of said casing and em with the silver ribbons 38 preferably adjacent the mid bedded in said inert granular material and having periph point thereof. At melting currents flowing for prolonged erally spaced apart, radially protruding portions extend periods, the fusible ribbons 38 become hot enough to ing longitudinally thereof, a plurality of silver ribbons melt the alloy bodies 44, and the amalgamation of the 60 helically wound in parallel spaced apart relation on said silver and alloy causes a hot spot with high enough re radially protruding portions of said spider and being sistance to melt the ribbons 38 at this point. This con embedded in said granular inert material, and a metallic struction, known as the "M" effect, allows the fusible terminal on each end of said casing, said metallic ter ribbons 38 to melt at a temperature in the 400-600 F. minals each including a portion extending axially with range, when subjected over a long period of time to low 65 respect to said casing and engaging said ribbons, said magnitude currents, as compared to the 1760 F. melt ribbons engaging and electrically connected to said axial ing temperature for pure silver. ly extending portions at points spaced axially thereon Oh fault currents of high magnitude, the alloy element with respect to said casing so that all of the arcs formed 44 has little or no effect, and the silver ribbons 38 vapor upon vaporization of said ribbons, incident to high mag ize almost instantaneously at the fusion temperature for 70 nitude fault current carried by said fuse, terminate on silver and form fulgurites along the entire length of the said axially extending terminal portions and do not tend ribbons 38. The arcs within the individual fulgurites to merge together and cause excessive arc heating. tend to assume the shortest path between the metallic 2. In a high amperage current limiting fuse, a tubular end plates 25, and the fulgurites tend to merge together insulating casing, an inert granular material of high di adjacent the end plates because the parallel, side-by-side 75 electric strength within said casing, an insulating spider 3 ,287 ,5 2 5 5 extending parallel to the axis of said casing and em 6 bons at points spaced axially thereon with respect to said bedded in said inert granular material and having periph casing so that, all of the arcs formed upon vaporization of erally spaced apart, radially protruding portions extend said ribbons terminate on said axially extending terminal ing longitudinally thereof, a plurality of silver ribbons portions and do not tend to merge and to cause excessive helically wound in parallel spaced apart relation on said 5 arc heating. radially protruding portions of said spider and being 5. In a fuse in accordance with claim 4 wherein said embedded in said granular inert material, each of said rib spider is of an insulating material capable of generat bons having a body of low melting temperature alloy in ing gas in the presence of an arc and each of said rib contact therewith, said radially protruding portions along bons has a body of low melting temperature alloy in at least the surface thereof in contact with said silver 10 contact therewith. ribbons being of an electrical insulating material adapted 6. In a fuse in accordance with claim 5 wherein said to generate gas in the presence of an arc and being ca insulating material includes a water insoluble binder and pable of continuously withstanding temperatures up to an anti-tracking substance selected from a group consist 250 F. without degradation and up to 500 F. for ing of the hydrates and oxides of aluminum and mag periods of up to one hour without substantial decom 15 nesium. position, and a metallic terminal on each end of said 7. In a high amperage current limiting fuse, a tubular casing and electrically connected to all of said ribbons, insulating casing, an inert granular material within said said metallic terminals including a portion extending casing, an insulating support core extending parallel to axially with respect to said casing across the helical path the axis of said casing and embedded in said inert granu of and electrically connected to all of said ribbons, said 20 lar material, a plurality of fusible elements helically ribbons engaging and electrically connected to said axial wound in generally parallel spaced relation on said sup ly extending portions at points spaced axially thereon port core and embedded in said inert granular material, with respect to said casing so that all of the arcs formed and a metallic terminal at each end of said casing, said upon vaporization of said ribbons, incident to high mag metallic terminals each including a portion extending nitude fault current carried by said fuse, terminate on 25 axially with respect to said casing and engaging said fu said axially extending terminal portions and do not tend sible elements, said fusible elements engaging and elec to merge together and cause excessive arc heating. trically connected to said axially extending portion at 3. In a fuse in accordance with claim 2 wherein said points spaced axially thereon so that all of the arcs insulating material includes a water insoluble binder formed upon vaporization of said fusible elements ter and an anti-tracking substance selected from a group 30 minate on said axially extending terminal portions at said consisting of the hydrates and oxides of aluminum and axially spaced points and do not tend to merge together magnesium. and cause excessive heating. 4. In a high amperage current limiting fuse, a tubular insulating casing, an inert granular material of high di References Cited by the Examiner electric strength within said casing, an insulating spider 35 extending parallel to the axis of said casing and embedded in said inert granular material and having peripherally spaced apart, radially protruding portions extending lon gitudinally thereof, a plurality of silver ribbons helically wound in parallel spaced apart relation on said radially 40 2,230,207 2,667,549 2,768,264 3,138,682 UNITED STATES PATENTS 1/1941 1/1954 10/1956 6/1964 Thommen et al___ 200--120 X Fahnoe et al. Jones et a l . _______ 200--144 Dannenberg et a l . __ 200--120 protruding portions of said spider and being embedded OTHER REFERENCES in said granular inert material, a metallic terminal on each end of said casing having a plurality of circumfer Metals Handbook, 1939 ed., Cleveland, Ohio, Ameri can Society For Metals, p. 1523, TA472, A3-1939. entially spaced apart portions individually affixed to said helically wound ribbons and also having a portion ex 45 BERNARD A. GILHEANY, Primary Examiner. tending axially with respect to said casing across the heli cal path of and electrically connected to all of said rib- H. B. GILSON, Assistant Examiner.