US3294936A - Current limiting fuse - Google Patents

Description

Dec. 27, 1966 H. w. MIKULECKY CURRENT LIMIT ING FUSE Filed Sept. 8, 1964 ATTORNEY CURRENT 1N AMPERES United States Patent Oiiiice 3,294,936 Patented Dec. 27, 1966 3,294,936 CURRENT LIMITiNG FUSE Harvey W. Mikulecky, Racine, Wis., assigner to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware Filed Sept. 8, 1964, Ser. No. 394,949 7 Claims. (Cl. 20D-120) This invention relates to fuses and, more particularly, to fuses of the current limiting type which limit the ow of current in a circuit under short circuit conditions to a substantially smaller value than the available short circuit current of the circuit.

Current limiting fuses conventionally comprise a fusible element embedded in a granular inert material of high dielectric strength such as sand or finely divided quartz. Usually the fusible element is in the form of one or more thin conductors of silver wound on a supporting core, or spider, of high temperature resistant ceramic material. The fusible element attains fusing temperature and vaporizes when subjected to current of fault magnitude, whereby arcing occurs and the metal vapors rapidly expand to many times the volume originally occupied by the fusible element and are thrown into the spaces between the granules of inert filler material where they condense and are no longer available for current conduction. The current limiting eiect results from the interaction of the metal vapors and the inert granular material surrounding the fusible element. The physical contact between the hot arc and the relatively cool granules causes a rapid transfer of heat lfrom the arc to the granules, thereby dissipating most of the arc energy with very little pressure buildup within the fuse enclosure. As a consequence of the substantially simultaneous volatilization of all portions of the length of the fusible element, the sudden removal of the metal vapors from the arc path, and the extremely rapid cooling of the arc,. a high resistance is, in effect, inserted into the path of the current and initially limits the current to a magnitude which is only a small fraction of 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 of gas and noise. The sand particles in the immediate vicinity of the arc fuse become partial conductors at the high temperature of the arc. The fused particles cool upon extinction of the arc and solidify into a fulgurite, which is in the nature of a glass body, and lose their conductivity and become insulators as they cool.

In some applications current limiting fuses are utilized to interrupt only fault currents of short circuit proportions, and the interruption of relatively small overload currents is accomplished by a serially related device such as an automatic circuit breaker or an expulsion fuse. However, current limiting fuses are often required to control currents of overload proportions as well as larger magnitude currents of short circuit proportions. Most effective interruption of relatively low magnitude fault current of overload proportions is obtained when the fusible element of the current limiting fuse comprises conductors of thin uniform cross section, but current interruption by a fuse having such thin conductors or wires may give rise to transient voltage surges which reach undesirably high peak magnitude and endanger the insulation of the connected apparatus. In vorder to insure against surge voltage peaks in excess of predetermined limits, it is known to use current limiting fuses provided with fusible elements having a variable cross sectional area with a relatively large number of serially related portions of reduced cross sectional area and intermediate portions of relatively large cross sectional area, for example, a silver ribbon with a plurality of longitudinally spaced circular perforations which determine the points where fusion of the element is initiated when the fault current and its rate of rise are high. The perforations form portions of reduced cross sectional area which vaporize almost instantly upon short circuit currents, thereby resulting in the formation of arclets in series, distributing the thermal duty of the arc quenching filler relatively evenly over the entire filler body, and controlling the transient voltages across the fuse. The purpose of the portions of reduced cross section is to cause progressive insertion of arc resistance into the circuit during the entire arcing time in order to keep the rate of change of current, afi/dt, relatively low. This, in turn, tends to limit the inductive surge voltage which may occur when a current limiting fuse is caused to operate and which may present a serious danger to circuit insulation. However, when interrupting protracted currents of relatively small magnitude, appreciable intervals of time may occur between initiation of arcs at various small cross sectional area portions, and generally the arc gap first formed is progressively enlarged by vaporization of the fusible element and the fulgurite is generally continuous. The ar-cing may last during a plurality of half cycles when interrupting fault currents of relatively small magnitude and result in the release of excessively high arc energy and intense heating of the ller adjacent to the arc region before completion of the interrupting process. This intense heating and the excessive arc energy tend to delay cooling of the fulgurite and to allow flow of follow, or leakage, current through the fulgurite and may result in a restrike and rekindling of the arc after initial interruption of the fuse. The additional 12R losses due to the flow of follow current through the fulgurite and the re-ignition of the arc concomitant with the restrike will result in dissipation of additional amounts of heat which may exceed the heat absorbing capacity of the fuse structure and result in destruction of the fuse casing.

It is an object of the invention to provide a current limiting fuse capable of interrupting fault current of short circuit proportions without danger of occurrence of surge voltage peaks in excess of predetermined limits and which has excellent arc'clearing characteristics for low magnitude fault currents.

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 longitudinal cross sectional view through a current limiting fuse embodying the invention;

FIG. 2 is a view taken on line 2 2 of FIG. l; and

FIG. 3 is a typical time-current characteristic curve for the current limit-ing fuse of the invention.

Referring to the drawing, a tubular enclosing casing Ii) for a current limiting fuse is constructed of suitable insulating material such as glass, fiber, or glass iiber impregnated with epoxy resin. A metallic end piece IIR may be secured on the right end of casing i@ by means of any suitable seal such as epoxy cement, and a metallic end piece HL may have external threads engaging internal threads near the left end of casing 1t) to aiiix end piece i11L to casing 10. A metallic hinge assembly 12 may be secured to the end piece 11R at the right end of the fuse casing l() by screws 14 engaged within threaded apertures in the end piece HR. The end piece 11L at the left end of fuse casing 1d has a smaller diameter portion 15 and an axial bore 16. A tubular metallic terminal member 17 extends into axial bore 16 with a force fit and is rigidly secured to end piece 11L by pins 13 extending radially through tubular member 17 and smaller D diameter portion of end piece 11L. Terminal member 17 is adapted to ht within a stationary contact jaw (not shown) of an electrical switch, and an insulating member 19 provided with an eye (not shown) for receiving a hoolrstick may be secured in the end of tubular member by suitable means such as epoxy cement. An arc extinguishing sleeve member 2d slidably tting over casing 10 may have an inner tubular portion 21 of insulating material telescoped over terminal member 17 and be urged axially into covering relation with terminal member 17 by a helical spring 22 compressed `between end piece 111. and the end portion 24 of arc extinguishing member for the purpose of interrupting any arc formed between terminal member 17 and the switch jaw contact as described in detail in my copending application Serial No. 298,882, filed July 31, 1963J and having the same assignee as this invention.

Metallic end plates 25 are disposed against the internal surface of end pieces 11L and 11R and are secured thereto by screws 25. Each end piece 25 has a plurality of radially extending tabs 27 adjacent its outer periphery which may be bent down and form terminals to which the fusible conductors may be connected. An elongated insulating core, or spider, 28 is axially mounted within casing 10. The ends of spider 28 are affixed to metallic end plates 25 by suitable means such as epoxy cement.

Spider 28 is of generally star-shaped cross section and has a plurality of radially protruding, peripherally spaced apart, longitudinally extending fins 32. Each n 32 has -a plurality lof depressions 33 of semi-cruciform configuration spaced apart longitudinally of spider 2S and forming longitudinally spaced apart raised shoulders 34. The depressions 33 of peripherally successive fins 32 are progressively staggered in a direction longitudinal of spider 2S 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 fusible element 36 interconnecting the end pieces 111, and 11R. A fusible wire (not shown) for indicating operation of the fuse may be disposed in the continuous helical path defined by the depressions 33.

Spider 2S may be of inert ceramic material such as porcelain, but it preferably is of an electrical insulating material adapted to evolve gas in the presence of an arc, as disclosed in my copending application Serial No. 313,640, filed October 10, 1963, having sufficient mechanical strength to be self supporting, and being capable of withstanding temperatures up to 250 F. continuously without degradation and temperatures up to 500 F. for periods of up to one hour without excessive degradation or decomposition. Spider 28 may be of a 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. The composition may also include other fillers such as mica, glass, fiber, asbestos or silica, :and one suitable material comprises approximately 75 percent aluminum hydrate filler, 20 percent polyester resin, and approximately 5 percent glass fiber.

Fusible element 36 wound on the helical support means 34 preferably comprises a ribbon 38 of high conductivity material such as silver having a plurality of circular perforations 39 spaced apart longitudinally thereof and being secured adjacent one end by suitable means such as solder to one or a plurality of thin wires 41 of high conductivity material such as silver. Silver ribbon 38 may extend somewhat less than half the length of fusible element 36 and be affixed at its other end to a tab 27 on metall-ic end plate 25 affixed to end piece IIL. The thin silver wires 41 are helically wound in spaced apart relation on the raised shoulders 34 and are secured by suitable means such as solder to a tab 27 on end plate 25 affixed to end piece 11R.

Beads 44 of low melting temperature alloy such as tin-lead solder are in intimate contact with each silver wire 41, preferably adjacent the midpoint of the wire. The lead-tin alloy bead 44 enables the wire 41 to melt, or fuse, during protracted, relatively low magnitude overload current conditions with a much lower melting temperature than would otherwise be possible. This construction, known as the M effect, allows the fusible wires 41 to melt at a temperature in the 400-600 F. range when subjected over a long period of time to low magnitude overload currents, as compared to the 1760 F. melting temperature for pure silver. The M arises from thc fact that the lead-tin bead 44 when heated by protracted overload currents forms with the silver an alloy having a melting point lower than the melting point of silver alone.

The casing 10 is filled with a body of suitable pulverulent refractory arc quenching material, such as quartz sand 46 so that spider 28 and the lfusible element 36 are directly embedded in the quartz sand ller 46.

The number and size of the thin silver wires 41 are selected so that the wires 41 will not vaporize prior to the silver ribbon 38 on large magnitude fault currents such as those of short circuit proportions. When clearing such large magnitude fault currents, the portions of ribbon 38 of minimum cross sectional area formed by the holes 39 vaporize almost instantaneously, resulting in the formation of arclets in series, and cause progressive insertion of arc resistance into the circuit during the initial arcing period and thus limit the inductive voltage surges which may occur.

When the fuse is subjected to relatively small protracted overload currents, the arc quenching filler 46 and the thin silver wires 41 are heated prior to melting of the fusible element, and the beads 44 alloy with the silver of Wires 41. The resulting alloy has a lower melting point and a higher resistance that silver, and consequently the silver wires 41 fuse at the alloy `beads 44 on protracted, relatively low magnitude overload currents.

The wire section of the fusible element 36, i.e., the number and size of silver wires 41, is selected to provide the proper time current characteristic for the particular fuse rating. A plurality of silver wires are conventionally used, in preference to a single wire, to obtain more efficient current clearing operation when interrupting low magnitude currents, and further the silver wire section is normally of a minimum length required to develop a suciently high arc voltage to maintain the current limiting function. l have found that a single wire element will operate satisfactorily to clear low magnitude fault currents when the spider 28 is of gas evolving material as discussed hereinbefore, and further I have discovered that the aforementioned minimum length of fusible ele ment to produce a sufficiently high arc voltage to maintain the current limiting function is not necessary when clearing low magnitude currents, particularly when the spider 28 is of gas evolving material. When the fuse of the invention is subjected to relatively small magnitude protracted overload currents, the granular filler 46 and the si-lver wires are heated prior to melting of any of the portions of the fusible element, and the beads 44 form an alloy with the silver wires 41 which has a lower melting point and a higher resistance than silver. Consequently, the silver wires 41 fuse at the alloy beads 44 on protracted, relatively low magnitude overload currents, and the short wire section accomplishes all of the clearing of low magnitude currents.

The silver ribbon section 3S is selected to coordinate properly with the silver wire section 41 so that the ribbon 38 will not melt under low magnitude overcurrents but will Vaporize before the wire under large magnitude currents such as those of short circuit proportions. FlG. 3 illustrates a typical time-current characteristic curve 50 for a current limiting fuse embodying the invention which is a composite of the characteristic 52 for the silver wires intervaligreater than approximately 0.1 second.

shown in dashed lines and the characteristic 51 for the silver ribbon 38 shown in dash-dot lines. All fault currents of a magnitude greater than approximately 120 amperes will produce initial melting and arcing of the silver ribbon 38 of this embodiment, Whereas all fault currents of a magnitude smaller than approximately 120 amperes will be interrupted by the silver wires 41 in an In this embodiment the silverribbon 38 will not melt on currents up to the value corresponding to the 0.1 second point on the characteristic 50 and will melt before the wire 41 at high currents, Abut it will Ibe appreciated that the time interval at which the wire and ribbon characteristics cross .is not critical and, for example, can 'be anywhere in the range from .005 to 0.1 second.

When clearing large magnitude faults, usua-lly the entire fusible element 36 including wires 41 and ribbons 38 will become fulgurite. The portions of ribbon 38 of minimum cross sectional area formed by the holes 39 vaporize almost instantaneously, resulting in the formation of arclets in series, and cause progressive insertion of arc resistance into the circuit during the initial arcing period and thus limit the inductive voltage surges which may occur. Since the ribbon 38 controls arc voltage rise as the current limiting action begins7 the peak let-through current and the magnitude of current are reduced, because of the current limiting action, when the wire section 41 melts. This reduction in current magnitude when the wire section 41 melts reduces the transient voltage surges across the wire section 41, since the number of series arclets .in the wire is related to the current density at the time the wire melts. Further, the peak arc voltage across the wire section 41 is reduced because the wire section 41 is shorter than that minimum length of fusible element required to develop sufficient arc voltage to maintain current limiting action.

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 in the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.

I claim:

1. In an electric current limiting fuse, an enclosing casing provided with a pair of spaced apart electrically conductive terminals, an elongated fusible element mounted in said casing between said terminals and having a continuous length of silver ribbon section provided with a plurality of longitudinally discrete portions of maximum cross sectional area and a plurality of intermediate portions of reduced cross sectional area in series with a silver wire section having a metal element thereon adapted to form an alloy with silver having a lower fusing temperature than silver, said Wire section having a cross sectional area less than the cross sectional area of said discrete ribbon portions continuously along the length of said wire, said portions of reduced cross sectional area of said ribbon section being adapted to melt before said wire section on fault currents of large magnitude, and a body of granular inert material of high dielectric strength filling said casing and embedding said fusible element.

2. In a current limiting fuse, an enclosing casing or" insulation, electrically conductive terminals on the ends of said casing, a core of insulating material extending axially within said casing, an elongated fusible element supported on said core within said casing and interconnecting said terminals and comprising a continuous length of ribbon of electrically conductive material having a plurality of longitudinally discrete portions of maximum cross sectional area and a plurality of intermediate portions of reduced cross sectional area in series with at least one wire of conductive material having Mefiect causing means thereon, said wire section having a cross sectional area less than the cross sectional area of said discrete ribbon portions continuously along the length of said wire, and a body of granular inert material of high dielectric strength lling said casing and embedding said fusible element, the time-current characteristic of said ribbon crossing the time-current characteristic of said wire intermediate the ends of the current range of said fuse, whereby said ribbon controls arc voltage buildup on large magnitude fault currents without generation of surge voltage peaks in excess of predetermined limits and said wire clears low magnitude fault currents.

3. In an enclosed current limiting fuse having a filler of inert granular material of high dielectric strength, an elongated fusible element embedded in said ller having a continuous length of ribbon section of electrically conductive material provided with a plurality of longitudinally discrete portions of maximum cross sectional area and a plurality of intermediate portions of reduced cross sectional area in series with a wire section of conductive material having Meffect causing means thereon, said wire section having a cross sectional area less than the cross sectional area of said discrete ribbon portions continuously along the length of said wire section, the timecurrent characteristic of said ribbon section crossing the time-current characteristic of said wire section intermediate the ends of the current range of said fuse, whereby said ribbon section controls arc voltage buildup on large magnitude fault currents without dangerously high surge voltage peaks and the said wire section clears low magnitude fault currents.

4. A current limiting fuse comprising, in combination, an enclosing casing of insulation, electrically conductive terminals on the ends of said casing, a support of an insulating material adopted to evolve gas in the presence of an arc extending axially within said casing, an elongated fusible element supported on said support and contacting said support at only points spaced apart longitudinally of said element and interconnecting said terminals and having a ribbon section of electrically conductive material in series with a thin wire section of conductive material having M-efect causing means thereon, and a body of granular inert high dielectric strength material filling said casing and embedding said fusible element, the time-current characteristic of said ribbon section crossing the time-current characteristic of said wire section intermediate the ends of the current range of said fuse and said wire section clearing low magnitude fault currents and said ribbon section controlling arc voltage rise on large magnitude fault currents.

S. A current limiting fuse in accordance with claim 4 wherein said ribbon section has a plurality of longitudinally discrete portions of maximum cross sectional area and a plurality of intermediate portions of reduced cross sectional area and said insulating material of said support is 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.

6. A current limiting fuse in accordance with claim 4 wherein said insulating material of said support includes a water insoluble binder and an anti-tracking substance selected from a group comprising the hydrates and oxides of aluminum and magnesium.

7. ln a high voltage fuse of the current limiting type, a tubular insulating casing, metallic terminals on the ends of said casing, an inert granular material of high dielectric strength in said casing, a spider extending parallel to the axis of said casing and being embedded in said granular material and having peripherally spaced apart, radially protruding portions extending longitudinally thereof, an elongated fusible clement wound helically on said spider and contacting only said radially protruding portions thereof and being embedded in said granular inert material and interconnecting said metallic terminals and having a silver ribbon section provided with a plurality of longitudinally discrete portions of maximum cross sectional area and a plurality of intermediate portions of reduced cross sectional area in series with a silver wire section having Meffect causing means thereon7 said spider being of a material capable of evolving gas in the presence of an arc and including a water insoluble binder and an anti-tracking substance selected from a group consisting of the hydrates and oxides of aluminum and magnesium, the time-current characteristic of said ribbon section crossing the time-current characteristic of said wire section intermediate the ends of the current range of said fuse and said wire section clearing low magnitude fault currents and said ribbon section controlling arc voltage buildup on large magnitude fault currents.

References Cited by the Examiner UNITED STATES PATENTS 4/1948 Schuck 200-120 7/1952 Falinoe 200-120 `lil/1956 JOncS et al. ZOO-144 l0/l96l Kozacka 200-l20 12/1961 Hicks 200--120 6/1964 Dannenberg et al. ZOO- 120 FOREIGN PATENTS 4/1955 Great Britain.

BERNARD A. GILHEANY, Primary Examiner.

l5 H. GILSON, Assistant Examiner.

Claims (1)

1. IN AN ELECTRIC CURRENT LIMITING FUSE, AN ENCLOSING CASING PROVIDED WITH A PAIR OF SPACED APART ELECTRICALLY CONDUCTIVE TERMINALS, AN ELONGATED FUSIBLE ELEMENT MOUNTED IN SAID CASING BETWEEN SAID TERMINALS AND HAVING A CONTINUOUS LENGTH OF SILVER RIBBON SECTION PROVIDED WITH A PLURALITY OF LONGITUDINALLY DISCRETE PORTIONS OF MAXIMUM CROSS SECTIONAL AREA AND A PLURALITY OF INTERMEDIATE PORTIONS OF REDUCED CROSS SECTIONAL AREA IN SERIES WITH A SILVER WIRE SECTION HAVING A METAL ELEMENT THEREON ADAPTED TO FORM AN ALLOY WITH SILVER HAVING A LOWER FUSING

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