US3437971A

US3437971A - Current limiting fuse

Info

Publication number: US3437971A
Application number: US652395A
Authority: US
Inventors: Harvey W Mikulecky
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1967-06-26
Filing date: 1967-06-26
Publication date: 1969-04-08
Anticipated expiration: 1986-04-08

Description

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H. W. MIKULECKY CURRENT LIMITING FUSE Flled June 26 1967 ...IUKIIN ,I mnllhglllLllm Vl Im I wm.. @m1 km, RN NQ QN United States Patent O US. Cl. 337--158 3 Claims ABSTRACT F THE DISCLOSURE This disclosure relates to a current limiting fuse wherein a fusible element is wound on a support and both are arranged within a fuse casing and embedded in an inert granular material of high dielectric strength. The support is made of gas evolving material and includes radial portions which engage the fusible element. The fusible element is helically wound on the support and is engaged only by the radial portions at spaced points along the length thereof. The portions of the fusible element between the points of contact are separated from the spider and surrounded by the inert granular material so that the spider is exposed to the are drawn only at the points of engagement with the fusible element to thereby achieve a limited, controlled amount of gas evolution sufficient to extinguish the arc and prevent restriking but wholly within the fuse. Engagement between the fusible element and support is limited generally to less than 50% of the fusible element and preferably is within a range having a lower limit on the order of %-8% and an upper limit on the order of 30%.

This application is a continuation-in-part of application Ser. No. 408,017, led Nov. 2, 1964, land which was a continuationinpart of application Ser. No. 313,640, filed Oct. 3, 1963, now abandoned.

In accordance with other aspects of this disclosure, the support is cut-out between the points of engagement with the fusible element to afford depressions in the support. These depressions are filled with the inert granular material and prevent axial flashover of the arc between the points of engagement. The support is made of a material capable of evolving a gas in the presence of an arc, possesses anti-tracking properties and is capable of withstanding temperatures up to 250 F. continuously without degradation and up to 500 F. for periods up to one hour without excessive degradation or decomposition. Preferably the support material is a molded thermosetting composition comprising a water-insoluble binder and an anti-tracking substance selected from the class consisting of the hydrates and oxides of aluminum and magnesium. An M effect element is provided on the fusible element to assist in initiating low fault current interruption.

BACKGROUND OF INVENTION Field 0f invention This invention relates to fuses and, more particularly, to fuses of the current limiting type which limit the flow of current in a circuit to a substantially smaller magnitude than the available fault current of the circuit.

Description of prior art 3,437,971 Patented Apr. 8, 1969 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. When subjected to short circuit current, the fusible element attains fusing temperature and vaporizes, whereupon 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 physical contact between the hot arc and the relatively cool granules causes a rapid transfer of heat from the arc to the granules, thereby dissipating most 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 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 and become partial conductors at the high temperature of the arc. The fused 4particles 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 4become insulators as they cool.

Current limitation begins only at a current level above approximately thirty times rated current where melting of the fusible element occurs prior to the peak value of current. Conventional current limiting fuses are not entirely satisfactory for interrupting fault currents of relatively low magnitude below the level at which current limitation begins, i.e. currents that produce melting times of three minutes and longer, particularly in high voltage circuits. A number of cycles may be required for vaporization of sufficient length of the fusible element to initiate the arc quenching process, and apparently such excessive duration of arcing destroys the insulating properties of a portion of the fulgurite with the result that the restriking voltage causes re-ignition and continued current flow.

SUMMARY O'F INVENTION In accordance with this invention a fuse is provided which effectively interrupts 4fault currents of relatively low magnitude as well as fault currents of relatively high magnitude. A fusible element is wound on a support with the support engaging the element only at spaced points along the length of the fusible element. The fusible element and support are embedded in an inert granular material, which material preferably surrounds the fusible element between its points of engagement with the support. At least the portions of the support engaging the fusible element are made of a gas evolving material so that when an arc is drawn, upon fusion of the element, gas is evolved directly into the arc. This arrangement of limited engagement of the fusible element with the support, or the gas evolving material, results in a limited, controlled amount of gas evolution sufficient to extinguish the arc but wholly within the fuse casing, i.e. without producing a potentially hazardous condition externally of the casing. Preferably, the amount 0f contact between the fusible element and support is within a range having a lower limit on the order of 5%-8% and an upper limit on the order of 30% of the length of the fusible element; more generally, such contact is not to exceed 50% of the length of the fusible element. The gas evolving material of the support also possesses anti-tracking properties and is capable of withstanding temperatures of up to 250 F. continuously and of up to 500 F. for up to an hour without degradation and decomposition.

An M" eect element is provided on the lfusible element to assist in interrupting low fault currents.

The support is cut-out between the points of engagement with the fusible element to form depressions, the depressions being filled by inert granular material to prevent flashover of the arc between the points of engagement.

DESCRIPTION OF DRAWING FIG. 1 is a longitudinal cross sectional view through a current limiting fuse embodying the invention; and

FIG. 2 is a view taken on line 2-2 of FIG. 1.

DISCUSSION OF PREFERRED EMBODIMENT Referring to the drawing, a tubular enclosing casing for a current limiting .fuse is constructed of suitable insulating material such as glass, liber, 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 atlix end piece 1'1L to casing 10. A metallic hinge assembly 12 may be secured to the end piece 11R at the right end of the fuse casing 10 by screws 14 engaged within threaded apertures in the end piece 11R. The end piece 11L at the left end of fuse casing 10 has a smaller diameter tubular portion 1S and an axial bore 1-6. A tubular metallic terminal member 17 extends into axial bore 16 with a force tit and is rigidly secured to end piece l11L by pins 18 extending radially through terminal member 17 and smaller diameter portion 15 of end piece 11L. Terminal member 17 is adapted to tit 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 hookstick may be secured in the end of tubular member 17 by suitable means such as epoxy cement. An arc extinguishing sleeve member 20 slidalbly fitting 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 11L and the end portion 24 of arc extinguishing member 20 for the purpose of interrupting any are formed between terminal member 17 and the switch jaw contact as described in detail in my co-pending application Ser. No. 298,882, filed July 31, 1963 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 26. =Each end plate 26 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 electrically connected. An elongated insulating core, or spider, 28 is axially mounted within casing 10. The ends of spider 28 are aflixed 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 ns 32. -Each iin has a plurality of depressions 33 spaced apart longitudinally of spider 28 and forming longitudinally spaced apart, raised shoulders 34 on each iin. The depressions 33 of peripherally successive ns are progressively staggered in a direction longitudinal 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 path defined by the depressions 33.

Fusible element 38 preferably has a maximum melting temperature on protracted low magnitude fault current of approximately 600 F. Preferably fusible element 36 comprises a pluality of thin wires 38 of high conductivity material such as silver or copper having M effect causing means thereon as described hereinafter, but alternatively fusible element 36 may be of suitable low melting temperature conductive material. The silver Wires 38 may be helically wound in parallel, spaced apart relation on the raised shoulders 34 so as to be in approximate line contact with fins 32 and touch only peripherally spaced apart points on spider 28. Wires 38 may be secured at their ends by suitable means such as solder to bent-down tabs 27 on the end plates 25 affixed to

end pieces

11L and 11R. Although two fusible wires 38 are shown n the drawing, it should be understood that only a single fusible wire, a single silver ribbon, or a plurality of silver ribbons in parallel may be employed for the fusible element 36 depending upon the current rating of the fuse. The reason for utilizing the plurality of fusible wires in the preferred embodiment is that most effective current limiting action is obtained when the conductors are of very thin cross section, and consequently where current ratingrequires a relatively large cross section, it is desiraable to divide the total cross section into a number of parallel conductors having a thin cross section. Beads 44 of low melting temperature alloy such a tin-lead solder are in intimate contact with each silver wire 38, preferably adjacent the midpoint of the wire. The tin-lead alloy bead 44 enables the wire 38 to melt, or fuse, during protracted, relatively low magnitude overload current conditions with a much lower melting temperature than would otherwise be possible. At melting temperatures the fusible wires 38 ybecome hot enough to melt the beads 44, and the amalgamation of the silver and alloy causes l a hot spot with high enough resistance to melt the wire 38 at this point. This construction, known as the M effect, allows the fusible wires 38 to melt at a temperature in the 400-600 F. range as compared to 1760" F. melting temperature for pure silver. An are is formed and the wire 38 burns back. The casing 10 is filled with a body of suitable pulverulent refractory arc quenching material such as quartz and 46 so that the spider 28 and the fusible element 36 are directly embedded in the quartz sand filler.

In accordance with the invention, the spider 28 is of an electrical insulating material adapted to evolve gas in the presence of an arc, having suicient 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 up to one hour without excessive degradation or decomposition. In the preferred embodiment the spider 28 is of a molded thermosetting composition comprising a water-insoluble binder and an anti-tracking substance selected from the class consisting of the hydrates and oxides of the aluminum and magnesium. The composition may also include other fillers such as mica, glass ber, asbestos, or silica. One material suitable for the invention comprises approximately 75% aluminum hydrate filler, 20% polyester resin binder, and approximately 5% glass fiber. 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 or in place of a polyester resin, other resins may be employed as the binder of the molding composition, for example, phenolic, urea, melamine, or silicone resins.

The silver wires 38 have only approximately line contact with peripherally spaced apart portions of spider 28 at the tins 32. Gas is generated by the material of spider 28 at these points of contact with the fusible element 36 which occur directly at the center of the fulgurite being formed by the arc. It is not essential to the invention that the entire support means for the fusible element be of the gas generating material, and in other embodiments the radially outermost portions of the ns 32y in contact with the silver wires 38 are of the disclosed material, or the material may be coated or laminated on the portions of the support means which contact the fusible element 36. The gas generated by the material of spider 28 is particularly helpful in clearing fault current of relatively low magnitude. In one test a current limiting fuse of 12 ampere, 20.0 kilovolt rating having a conventional spider of ceramic material failed to clear a fault current of 23 amperes, whereas an identical fuse having a spider of the above-described gas generating material cleared the same low fault current in 43 arc cycles (approximately 0.7 second). After 43 arc cycles the reignition voltage for the fuse with the ceramic spider was only 2.4 kilovolts, .whereas the reigntion voltage for the fuse having a spider of the abovedescribed gas generating materialwas approximately 28 kilovolts. Apparently the gas generated by the disclosed material produces a very intensive de-ionizing action on the arc products as well as a cooling of the fulgurite in such a. manner as to inhibit the restriking of the arc. The inert granules 46 become molten and semiconductive under the excessive duration of arcing on low magnitude faults and tend to re-ignite the arc, but the gas evolved by the material of spider 28 is blown into the arc stream and cools the fulgurite so that the inert particles lose their conductivity and become insulators.

In another test of current limiting fuses rated at 12 amperes and 8.3 kilovolts, a fuse with a ceramic spider and parallel fusible elements spaced 3&6 inch apart did not clear a fault of 29.4 amperes in 218 arc cycles (3.63 seconds) and after 68.8 kilowatt-seconds of arc energy before the circuit was intentionally interrupated by a circuit breaker, whereas a similar fuse in accordance with the invention cleared the fault after only 21 cycles and 4.85 kilowatt-seconds of arc energy and after the fusible element had burned back approximately 41/2 inches. In still another test, a fuse of this rating having a ceramic spider was provided with silver wire fusible elements spaced 1A; inch apart, and this fuse required 63 cycles to clear the 29.4 ampere fault after 16.8 kilowatt-seconds of arc energy had been dissipated and with approximately 161/2 inches of element burn back.

It has been found preferable on higher amperage fuses to utilize a gas generating and anti-tracking composition of approximately 60% aluminum hydrate, 20% asbestos, and 20% melamine resin binder. The hydrates are more effective are quenching materials than the oxides, and the aluminum compounds appear to give better results with water-insolu-ble organic binders such as polyester, urea, or melamine resin than with inorganic binders, although the latter may be used in accomplishing the results of the invention. In general, gas generating materials provided with organic lbinders have higher structural strength than those with inorganic binders. The binder may comprise from %40% by weight of the composition, but superior interrupting characteristics are obtained with lower percentages of the binder and higher percentages of the gas generating and anti-tracking material.

Fuses constructed in accordance with this invention possess the ability to interrupt fault currents, of both high and low magnitude, wholly within the fuse casing. As a result, such fuses can be used safetly in enclosed areas without fear of damage tothe enclosure or adjacent equipment upon yfuse operation. It is ybelieved that this results from the controlled, limited amount of gas evolved upon fusion of the fusible element. More specifically, the volume of gas evolved is sufficient to effectively extinguish the arc but is not such as to rupture the fuse casing or result in the expulsion of extremely high temperature gases or elements from the fuse. The volume of gas evolved is controlled and limited by the arrangement of the fusible element on the gas evolving spider as is illustrated in the drawing. The gas evolving spider has limited engagement with and exposure to the fusible element. Generally, prior accepted practice has been to expose substantially the full length of the fusible element to the gas evolving material, where such material was used, to insure maximum gas evolution. Such designs have a number of shortcomings, one of which is excessive gas evolution which presents a problem of casing rupture and another is that the continuous exposure increases the susceptibility to carbonizing a leakage path which can then lead to complete fuse failure, restriking the arc or heating the fuse to complete burn up. In `accordance with this invention preferably the amount of contact between the .spider and fusible element falls generally within a range having a lower limit on the order of 5%-8% and an upper limit on the order of 30% of the length of the fusible element. More generally, this contact should not exceed 50% of the length ofthe fusible element. These percentages of contact are clearly evident in the drawing. In an -arrangement which has given satisfactory results, the amount of the fusible element contacting the gas evolving material at any point did not exceed approximately 1A; inch and adjacent points of contact were spaced sufficiently so -that each was beyond the influence of the other; again this structural arrangement is evident from the drawing. These dimensions can be determined from the full size representation of a fuse in FIGS. 1 and 2 where it can also be seen that this invention permits use of a relatively thin wall fuse casing. AS illustrated, for a fuse casing of 2 inch diameter the wall thickness is 1/16 inch.

As stated above, the continuous exposure of fusible element to its support presents the problem of resulting in a continuous leakage path. This problem is further aggravated Where that support is a gas evolving member. This obtains from the fact that the residual heat of the fulgurite continues to work on the gas evolving member. Condensation of the evolved gases has a tendency to form a conductive path within the fuse. With the full fuse element length, and corresponding full fulgurite length Ibeing exposed to the gas evolving material, excessive gas evolves increasing the conductive condensation and, with a continuous path being provided, the likelihood of fuse failure is increased correspondingly. This fuse failure has been observed in actual tests of fuses with full exposure of the fusible element to the gas evolving support. These test fuses failed to clear the test fault as restriking of the arc occurred with the entire fuse then being completely engulfed in flame. Under identical test conditions fuses having the limited, controlled gas evolution afforded by this invention cleared the fault with virtually no external evidence of fuse operation and with the capability of maintaining circuit voltage indeiinitely. Thus, violent destruction of the fuse, and of a nature which cannot tbe tolerated in enclosed fuse installations, has been found to occur as a result of excess exposure of fusi-ble element and gas evolving material, this evidencing itself -both in actual rupture of the fuse casing or leakage current ultimately resulting in fuse burn up.

The disclosed material `decomposes along the -ns 32 of spider 28 and evolves cooling gas into the arc stream under the extremely high temperatures which exist after the lfusible element 36 vaporizes. I have found that fuses of certain current ratings having spiders with continuous fins 32 fail to operate properly because the hot gases generated by the arc along the surface of the spider 28 flow along the ns 32 and cause ashover between adjacent helical turns of the fusible element 36 and consequent failure of the fuse. The depressions 33 spaced apart longitudinally of the fins 32 occur -between adjacent helical turns of the fusible element 36 and prevent flow of the hot gases in a direct line Ibetween adjacent turns of the fusible element, thereby preventing ashover between adjacent helical turns.

In addition to evolving gas into the arc stream, the 'anti-tracking material of the preferred embodiment has the advantage of releasing water in the presence of the arc which cools the arc and the fulgurite and, upon further decomposition, is available to react with any free carbon, thus tending to prevent the deposition of a carbon path. The material of the preferred embodiment may be hot-molded or cold-molded to form spider 28 or it can be fabricated yfrom at stock since it is readily machineable. Further, this material facilitates molding the spider 28 in more intricate configurations than porcelain or other ceramic materials and thus permits simplification land reduction in the cost of the fuse.

Use of the above-described composition for the fuse casing provides no advantage in clearing the arc since the sand ller prevents the material from -aiding in interrupting the arc. Further, the anti-tracking and gas generating material in mixture with the inert granular material is of dubious value in clearing an arc, since this material in such mixture generates such a large quantity of gas under large amperage faults as to build up destructive pressure within the fuse casing. inasmuch as the fusible element 36 contacts the spider 28 of the invention only along the radial fins 32, the amount of gas generated on even the highest magnitude fault currents is limited to fuse casing 10.

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 modiiicationspand variations which fall within the true spirit and scope of the invention.

I claim:

1. In 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 within said casing,

a spider of star-shaped cross section having peripherally spaced apart, radially protruding fins extending longitudinally thereof, said spider extending parallel to the axis of said casing and being embedded in said inert granular material,

a fusible element having a maximum melting temperature of approximately 600 F. on protracted low magnitude fault current helically wound on said spider and contactingonly said radially protruding fins of said spider and being embedded in said granular inert material and interconnecting said metallic terminals,

at least the portions of said spider in contact with said fusible element being of an electrical insulating material adapted to generate gas in the presence of an arc,

and said tins having depressions spaced apart longitudinally thereof disposed between adjacent helical turns of said fusible element, said depressions filled with said inert granular material and preventing hot gases generated by said spider from flashing over said adjacent turns of said fusible element.

2. In 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 within said casing,

a spider of star-shaped cross section having peripherally spaced apart, radially protruding iins extending longitudinally thereof, said spider extending parallel to the axis of said casing and being embedded in said inert granular material,

said iins having depressions spaced apart longitudinally thereof and the depressions of peripherally successive fins being progressively staggered in a direction longitudinal of said spider with said inert granular material disposed in said depressions,

a fusible element helically wound on the portions of said fins between said depressions and contacting only said radially protruding fins of said spider and being embedded in said granular inert material and interconnecting said metallic terminals,

said fusible element having M effect causing means thereon,

and at least the portions of said spider in contact with said fusible element being ofn a molded, thermosetting, electrical insulating material including a water-insoluble binder and an anti-tracking substance capable of evolving gas in the presence of an arc selected from a group consisting of the hydrates and oxides of aluminum and magnesium.

3. In a high voltage fuse of the current limiting type,

a tubular insulating casing,

metallic terminals on the ends of said casing,

a fusible element having a maximum melting temperature of approximately `600" F. on protracted low magnitude fault currents disposed within said casing and interconnecting said metallic terminals,

elongated support means Within and extending substantially the length of said casing for supporting said fusible element with said fusible element helically wound thereon, means on, said support means providing limited contact with said fusible element at only spaced apart points along the length of said element, and wherein the contact between said fusible element and said support means is within a range of 5% to 30% of the fusible element length,

the portions of said support means in contact with said fusible element being of an electrical insulating material capable of evolving gas in the presence of an arc and said support means have depressions between said portions which contact said fusible element,

and an inert granular material of high dielectric strength within said casing surrounding said fusible element and said support means and filling said depressions to prevent arc flash-over between adjacent points of contact of said fusible element with said support means,

whereby gas is evolved into the arc stream at the points of contact between said support means and said fusible element and aids in cooling said granular material and in preventing re-ignition of the arc.

References Cited UNITED STATES PATENTS 2,605,371 7/ 1952 Fahnoe 200--120 2,667,549 1/ 1954 Fahnoe et al 20G-120 2,768,264 10/ 1956 Jones etal.

2,917,605 12/1959 Fahnoe 200-120 X 3,012,121 12/1961 Hicks 200-120 3,138,682 6/1964 Dannenberg et al. 20G-120 3,194,923 7/ 1965 Cameron et al. 200-120 2,417,268 3/ 1947 Powell 200--121 BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner.

U.S. C1. XR.