Document kmq563kMkn28jD8NjLOXkZyvn

FILE NAME: McGraw-Edison (ME) DATE: 1964 Sept 8 DOC#: ME015 DOCUMENT DESCRIPTION: Patent - Current limiting fuse [Approved -1966 Mar 29] March 29, 1966 F ile d Sept, a 1964 H. W. M IK U LE C K Y CURRENT LIMITING FUSE 3,243,552 2 Sheets-Sheet I Hari/ey 2//. e/fi-HulecHy ATTORNEY March 29, 1966 F ile d Sept. 8, 1964 H. W. M IK U L E C K Y CURRENT LIMITING FUSE 3,243,552 2 Sheets-Sheet 3 4 3 L ^ 58 r<y.46 .<= SB 4 3 L' 58 4-3L ' _ J z2 c y 04 * 4 C7 t-- V S 4 -3 y -- 4e t 4-31' 7 3 " t_ 40 37 -4-- -43R 11-- K . '4 / 74 7 37 -J-- 7 4/ 4 3 R ' J \ SS> L4 7 * 37-, 3-3 37 -J-- 7 cy s-^ f-- 3 A 6-3/? 37 I -43 R jo 0 0 0 0 0 0 0 t 66 63 >! _ x : * 40 o'o O 6Z 39 O X r70 37 O4*' O y ~) r 67 )o o 74 39 4 b 7 >T 00 74--^ _ ^ Z r? 00 o@ 4 --7 S ~r 72 .G 40 o 00 3 7 39 _Z ' o 7 o 6 ^ Vb o o j ? ^ \ tr . ............ --/ 4 7 7 ______ r - 7 3 ~7jS<>. - IN V EN TO R %ari/e)T f. c/fiK ulecfty BY iUAU.J ATTORNEY United States Patent Office 3,243,552 Patented Mar. 29, 1966 1 2 3,243,552 CURRENT LIMITING FUSE Harvey W. Mikulecky, Racine, Wis., assignor to McGrawEdison Company, Milwaukee, Wis., a corporation of Delaware Filed Sept. 8,196 4 , Ser. N o . 394,957 13 Claims. (Cl. 200-- 120) This invention relates to fuses and, more particularly, to fuses of the current limiting type which limit the flow o f current in an electrical circuit under short circuit con ditions to a substantially smaller value than the available short circuit current o f the circuit. Current limiting fuses conventionally comprise a fusible element embedded in a granular inert material o f 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, o f high temperature resistant ceramic material. When subjected to current of fault magnitude, the fusible element attains fusing temperature and vaporizes, where by 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 effect results from the interaction o f 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 from the arc to the granules, thereby dissipating most of the arc energy with very little pres sure buildup within the fuse enclosure. The vapors o f silver have relatively low conductivity unless their tem perature is particularly high, and the temperature o f the silver vapors is rapidly reduced by the quartz sand filler until the vapors will not support a flow of current. Con sequently, 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 o f that available in the circuit. The quartz sand particles in the immediate vicinity o f the arc fuse and become partial conductors at the high temperature of the arc and form a fulgurite, or semiconductor. The fulgurite resulting from fusion and sintering of the quartz sand particles is in the nature o f a glass body, and as it cools it loses its conductivity and becomes an insulator. High voltage, high amperage current limiting fuses used to control fault currents of high magnitude conven tionally employ fusible elements of silver ribbon having serially related portions of relatively small cross sectional area and intermediate portions o f relaitvely large cross sectional area, for example, a silver ribbon provided with a plurality of circular spaced apart perforations which determine the portions where fusion of the fusible ele ment is initiated on currents o f short circuit magnitude. The perforations form portions of reduced cross sectional area which limit the peak arc voltage and make it possible to distribute the thermal duty o f the arc quenching granular material relatively evenly over the entire filler body. If such a fuse is subject to fault current of high mag nitude, all of the portions o f small cross sectional area fuse and vaporize almost simultaneously, resulting in the formation o f arclets in series and controlling the trans ient voltage across the fuse. When subjected to small protracted fault currents, the arc gap first formed is gen erally progressively enlarged by vaporization of the silver element until the gap is o f sufficient length to effect final interruption of the circuit and consequently the fulgurite 5 is generally continuous. When interruption o f small overload currents results in arcing over a plurality o f cycles, the arc energy tends to be large. The relatively large arc energy and the dissipation of additional heat resulting from I2R losses caused by the flow of follow cur 10 rent through the fulgurite combine to delay the cooling of the central portion o f the fulgurite which remains partially conductive, and only the end portions o f the fulgurite, where the arc contacts the relatively cool filler particles, tend to interrupt the arc. Most o f the voltage appears across the ends o f the fulgurite, which are of higher resistance than the hot central portions thereof, and tends to flashover the hot gases, and consequently re ignition of the fusible element and post interruption fail ure frequently occur when current limiting fuses control 20 overload currents of small magnitude. It is an object o f the invention to provide a current limiting fuse for controlling both currents of overload proportions and of short circuit proportions which has improved means to prevent re-ignition of the arc when 2 clearing relatively low magnitude currents. It is a further object o f the invention to provide a cur rent limiting fuse for interrupting both large and small fault currents which has means to form a series of arc gaps or fulgurites rather than a continuous fulgurite when 30 clearing prolonged, low magnitude overload currents. These and other objects and advantages of the inven tion will be more readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein: 35 FIG. 1 is a sectional view through a current limiting fuse embodying the invention; FIG. 2 is a view taken on line 2-- 2 o f FIG. 1; FIG. 3 is a view taken on line 3-- 3 o f FIG. 1; FIGS. Aa-Af are schematic views illustrating the se 40 quential operation of the fuse of FIG. 1 when interrupt ing small magnitude fault currents; FIGS. 5 and 6 are schematic views of alternative em bodiments of the invention illustrating the main and ._ auxiliary fusible elements in linear form rather than hel 45 ically wound as in the actual construction and as shown in FIG. 1; and FIG. 7 is a perspective view o f the arcing electrodes utilized in the embodiment o f FIG. 1. r,, Referring to the drawing, a tubular enclosing casing 10 for a current limiting fuse is constructed o f suitable insulating material such as glass fiber impregnated with epoxy resin, glass, or fiber. A metallic end piece H R is secured on the right end of casing 10 by means of any suitable seal such as epoxy cement 12 and radially ex 55 tending pins 14 (see FIG. 3) protruding through the casing 10 and into holes in end piece 11R. A metallic end piece 11L may have external threads engaging in ternal threads formed near the left end o f casing 10. A fin metallic hinge assembly 16 may be secured to end piece b0 H R shown at the right end o f fuse casing 10 by screws 17 engaged within threaded apertures in the end piece 11R. The end piece 11L shown at the left end o f fuse casing 10 has a smaller diameter portion 18 and an axial bore 19. A tubular metallic terminal member 20 extends into axial bore 19 with a force fit and is rigidly secured to end piece 11L by pins 21 extending radially 3,243,552 3 4 through tubular member 20 and smaller diameter por 400-600 F. range when subjected over a long period tion 18 of end piece 11L. Terminal member 20 may be o f time to low magnitude currents as compared to the adapted to fit within a stationary contact jaw o f an elec 1760 F. melting temperature for pure silver. It will be trical switch, and an insulating member 22 provided with appreciated that on currents of short circuit magnitude, an eye (not shown) for receiving a hookstick may be 5 the alloy element 40 has little or no effect and the silver secured in the end of terminal member 20 by suitable elements 37 vaporize at the fusion temperature for the means such as epoxy cement. End plates 23 are dis silver. posed against the internal surface o f end pieces 11L and An auxiliary fusible element 41 is wound on spider H R and are secured thereto by screws 24. Each end 26 in radially inward spaced relation from the main ele plate 23 has a plurality of radially extending tabs 25 ad 10 ment 36 and in the helical path defined by the depres jacent its outer periphery which may be bent down and sions 33. Auxiliary element 41 may be a copper or a form terminals to which the fusible conductors can be silver ribbon, a copper or a silver wire, a plurality of connected. An elongated insulating core, or spider 26, silver wires 42, as shown in the drawing, or a resistance is axially mounted within casing 10. The ends o f spider wire, and is electrically connected at its ends by suitable 26 are affixed to metallic end plates 23 by suitable means 15 means such as solder to metallic clip, arc gap electrodes such as epoxy cement. 43 each of which fits over one o f the raised shoulders 34 Spider 26 is o f generally star-shaped cross section o f spider 26. The ends o f clip electrodes 43 are resiliently having a plurality o f radially protruding, peripherally urged against the raised shoulder 34 to prevent move spaced apart, longitudinally extending fins 32. Each fin ment of the electrode 43 relative to spider 26. The arc 32 has a plurality o f depressions 33 of semi-cruciform 20 gap electrodes 43 are located at points approximately configuration spaced apart longitudinally o f spider 26 midway between the low melting temperature alloy bead forming longitudinally spaced apart raised shoulders 34. 40 and the ends of the main fusible ribbons 37, and the The depressions 33 o f peripherally successive fins 32 arc gap electrodes 43 are spaced radially inward from are progressively staggered in a direction longitudinal the main ribbon 37 so that an air gap of approximately of the spider 26 so that the peripherally successive de 25 %4 inch to Ys2 inch exists between the electrodes 43 and pressions 33 define a continuous helical path and the pe the main ribbons 37. The arc gap electrodes 43 have ripherally successive raised shoulders 34 form support sufficient mass to withstand arcing without deformation means of helical configuration for a circuit interrupting or melting over prolonged periods until the auxiliary main fusible element 36 which is usually o f silver or element 41 clears the fault. Depending tab portions 45 copper and normally carries all o f the current and as dis 30 extending in a direction away from the main ribbons 37 closed in FIGS. 1 and 2 comprises a pair of parallel rib are provided on the arc gap electrodes 43 to which the bons 37 of suitable fusible material such as silver helically ends o f the auxiliary element 41 are attached by suitable wound so as to contact only the peripherally spaced apart means such as solder so that the arc cannot easily con raised shoulders 34 of spider 26 and being electrically tact the auxiliary element 41 and cause premature melt connected in parallel between metallic end pieces 11. The 35 ing thereof. ends of the ribbons 37 are soldered to tabs 25 on end plates 23. Auxiliary fusible element 41 is selected so that the ratio o f the average 100 second melting current of the main Spider 26 may be o f inert ceramic material such as element 36 to the average 100 second melting current for porcelain, but it preferably is o f an electrical insulating the auxiliary element 41 is in the range of 2 to 6, and material adapted to evolve gas in the presence of an arc, 40 preferably this ratio is approximately four. Stated in as disclosed in my copending application Serial N o. 313, another manner, auxiliary fusible element 41 is selected 640 filed October 10, 1963, having sufficient mechanical to have a minimum melting current sufficiently less than strength to be self supporting, and being capable of with that o f the main element 36 so that, when the minimum standing temperatures up to 250 F. continuously with melt current is reached for the main element 36, good out degradation and temperatures up to 500 F. for 45 low current clearing characteristics exist for the auxiliary periods up to one hour without excessive degradation or element 41. decomposition. Spider 26 may be o f a molded thermo An indicator wire 47 of suitable material having high setting composition comprising a water insoluble binder er resistance than silver such as tungsten or Nichrome and an anti-tracking substance selected from the class is connected at one end to the arc gap electrode 43R ad consisting o f the hydrates and oxides o f aluminum and 50 jacent hinge assembly 16 at the right end o f the fuse as magnesium. The composition may also include other shown in the drawing. The indicator wire 47 is wound fillers such as mica, glass, fiber, asbestos, or silica, and in the helical path defined by the depressions 33 and ex one suitable material comprises approximately 60 percent aluminum hydrate filler, 20 percent melamine resin tends through an opening 48 in the right end o f spider 26, through axial opening in an insulating disc 49, the end binder, and approximately 20 percent asbestos. 55 plate 23, and the end piece 11R, and is secured by suit When the fuse is required to control currents of short able means such as solder to the bottom wall 50 of circuit proportions, the silver ribbons 37 o f the main a cup shaped indicator 51. Cup shaped indicator 51 is fusible element 36 may be provided with a plurality of normally disposed within an axial bore 52 in end piece circular perforations, or holes, 39 spaced apart along the 11R and is urged outward therefrom to a visible po length thereof which determine the points where fusion 60 sition by a compression spring 54. Indicator wire 47 of the element is initiated when the fault current and normally holds indicator 51 in the inward position wherein its rate of rise are high. The perforations 39 form por spring 54 is compressed between the bottom wall 50 of tions of reduced cross sectional area so that each ribbon indicator 51 and the end plate 23. Indicator 51 has a 37 has a number of serially related portions o f relatively circumferential flange 56 adjacent its open end, and an small cross sectional and intermediate areas o f rela 65 eyelet 57 secured by suitable means such as staking to tively large cross sectional area. Beads 40 o f low melt the end piece 11R adjacent the margin defining axial bore ing temperature alloy such as tin-lead solder are in inti 52 interferes with flange 56 and prevents indicator 51 from mate contact with the main silver ribbons 37 preferably being removed from end piece H R . adjacent the midpoint thereof. A t melting currents flow Disposed within the interior o f casing 10 and embed ing for prolonged periods, the fusible ribbons 37 become 70 ding the spider 26, the main ribbons 37, the auxiliary ele hot enough to melt the alloy bodies 40, and the amalga ment 41 and the indicator wire 47 is a body o f granular mation of the silver and alloy causes a hot spot with inert or refractory material 55 of high dielectric strength high enough resistance to melt the ribbon 37 at this such as sand or finely divided quartz. point. This construction, known as the "M " effect, allows Under currents o f short circuit magnitude, the main the fusible ribbon 37 to melt at a temperature in the 75 element 36 and the indicator wire 47 vaporize almost 3,243,552 5 6 instantaneously, thereby permitting spring 54 to urge indi regions rather than at a single fulgurite, the burning back cator 51 to a visible position as an indication that the of the main ribbon 37 will occur at a much faster rate fuse has operated. The disclosed fuse is particularly and the arc energy is dissipated in three regions rather adapted to clear relatively small magnitude currents of than at a single fulgurite, thereby permitting the central overload proportions, and FIGS. 4 a -4 / schematically 5 portion of the fulgurites to cool and become a noncon show the sequence o f operation under these conditions ductor. Further, the fuse will thus have the two end and illustrate the main and auxiliary elements as a single portions with good arc extinguishing characteristics at ribbon 37 and as single wire 41 respectively to facilitate each of the three fulgurites, and consequently the dis the understanding of the invention. Under relatively closed fuse rapidly clears small overload currents. small but prolonged overloads, the main ribbon 37 melts 10 If the cross section of auxiliary element 41 is too first at the low melting temperature alloy bead 40 near large as compared to main ribbon 37, the arc at the air the center of the ribbon, and the main ribbon 37 begins gaps 58 and 59 may exist for a sufficient length o f time to burn back under the initial arc formed at this point to damage that section of the fuse. If, on the other hand, as shown schematically in FIG. 4b. As discussed here the cross sectional area of auxiliary element 41 is insuffi inbefore, even when the main ribbon 37 has perfora 15 cient, the time of arcing at the air gaps may be too short tions 39 forming portions of reduced cross sectional to completely burn the main ribbon 37 open at this point area, the arc gap first formed at the center of the ribbon and consequently only one opening will be provided in is being progressively enlarged by the vaporization of the main ribbon if re-ignition occurs. the ribbon rather than forming arclets in series. The FIGS. 5 and 6 illustrate embodiments wherein the full line potential appears across the glass-like fulgurite 20 main fusible element is caused .to bum back at a still resulting from the fusion of the quartz filler 55 imme greater number of points when subjected to small pro diately adjacent the initial arc, and the relatively large longed overload currents. The main and auxiliary fusible arc energy, the excessive duration of arcing, and the F R elements of the embodiments of FIGS. 5 and 6 are pref losses due to flow of follow current when clearing pro erably wound helically on a support spider and embedded longed low magnitude current delay the cooling of the 25 within a granular inert filler in the same manner as in the central portion of the fulgurite so that it remains semi preferred embodiment, but in order to simplify the de conducting. Consequently, only the two end portions of scription and facilitate the understanding of the invention, the fulgurite where the arc contacts the relatively cool the main and auxiliary elements are shown in linear form quartz sand tend to extinguish the arc. in the drawing and the arc gap electrodes are designated A s soon as the voltage rises sufficiently across the 30 as arrowheads. The main fusible element of the embodi arc formed at bead 40 adjacent the center of the main ment of FIG. 5 is a silver ribbon 37 connected between element ribbon 37, the air gap 58 between the clip elec the fuse end pieces (not shown) and having a plurality trode 43L shown at the left o f the drawing and the main of spaced apart circular perforations 39 and a bead 40 ribbon 37 sparks over, thereby causing current flow o f low melting temperature alloy adjacent its center. The through the indicator wire 47. Since the portion of in 35 serial arrangement of three silver wire auxiliary elements dicator wire 47 within casing 10 and embedded in the 61, 62, 63 is parallel to but electrically isolated from the quartz sand filler 55 has better heat dissipation charac main ribbon element 37. The outer auxiliary elements 61 teristics than the portion of indicator wire surrounded by and 63 may be of the same wire size and are of smaller air within axial bore 52 in end cap H R , the latter portion wire size than the central auxiliary element 62. Elec surrounded by air will melt first under low fault condi 40 trodes 66 and 67 at the outer ends of auxiliary elements tions and assure movement of indicator 51 to visible 61 and 63 (which may be similar to clip electrodes 43 position. o f the preferred embodiment) form air gaps with the Since the indicator wire 47 is o f relatively high main electrode 37 which are smaller than the air gaps resistance, it will melt almost immediately at a number formed between main ribbon 37 and electrodes 69 and o f points and impose the arc voltage across the air gaps 45 70 connected respectively at the junctions o f auxiliary 58 and 59 in series and flashover air gap 59 as shown elements 61 and 62 and o f auxiliary elements 62 and 63. in FIG. 4c and thus connect auxiliary element 41 in par On small overload currents, the main ribbon 37 will first allel with main ribbon 37. The intense heat of the arc melt at the bead 40 at the center of the ribbon 37 and ing to the main element 37 at the air gaps 58 and 59 the ribbon will bum back from this point. After the quickly burns open sections of the main ribbon 37 50 arc voltage across the fulgurite at the center o f ribbon adjacent the clip electrodes 43L and 43R as shown in 37 rises to a sufficiently high magnitude, the air gaps FIG. 4d. Auxiliary element 41 has been selected so between ribbon 37 and electrodes 66 and 67 flashover and that its minimum melt current is approximately one- continue to burn back the ribbon 37 at these points dur fourth that of the main fusible element 37, and conse ing the period required to melt the auxiliary elements 61 quently several half cycles will be required to vaporize the 55 and 63 and thus clear the overload current. If the main auxiliary element 41 on the relatively low fault current. ribbon 37 is re-ignited, the voltage across the arc at the During the time required to melt the auxiliary element fulgurite adjacent the center o f the main element 37 will 41, arcing will continue at the air gaps 58 and 59 be rise sufficiently to flashover the air gaps between the main tween the arc gap electrodes 43 and the main element ribbon 37 and the electrodes 69 and 79, and the arcing at ribbons 37 and will destroy a small section of each main 60 these air gaps during the period required to melt the ribbon 37 adjacent each of the air gaps. During the plu auxiliary element 62 will destroy sections o f the main rality of cycles the auxiliary element 41 is melting, the ribbon 37 adjacent the electrodes 69 and 70. If the main fulgurite at the center of the main ribbon 37 has had time ribbon element 37 is now re-ignited, five arcs are con to cool and lose its conductivity. Interruption of the fault nected in series throughout the length of main element current by the auxiliary element 41 as schematically 65 37, and the main element will be burning back at ten dif illustrated in FIG. 4f is particularly easy when the spider ferent places, thereby providing ten fulgurite end por 24 is o f gas evolving material which cools the inert gran tions with good arc extinguishing characteristics which ules 55 and helps prevent reignition of the arc as dis rapidly clear the overload current. closed in my aforementioned application S.N. 313,640. In the embodiment o f FIG. 6, three auxiliary silver However, if insufficient length of the main silver ribbon 70 wire elements 71, 72 and 73 are disposed parallel to 37 has been consumed, re-ignition of the main element main ribbon 37 but isolated therefrom by air gaps formed 37 may occur. As illustrated in FIG. 4/, the main ribbon between main element 37 and electrodes 74 and 75 con 37 will now be burning back at the fulgurite at the nected to the ends of auxiliary element 71, electrodes 76 center of the ribbon and also at the fulgurites adjacent and 77 connected to the ends of auxiliary element 72, and the air gaps 58 and 59. Since arcing is occurring at three 75 electrodes 78 and 79 connected to the ends of auxiliary 3 ,2 4 3 ,5 5 2 7 8 element 73. The center auxiliary element 72 is not joined gaps from portions of said main element on opposite sides to the outer auxiliary elements 71 and 73 as in the em of said alloy element, means disposed adjacent each said bodiment of FIG. 5, and the center auxiliary element 72 air gap and responsive to arcing in said gap for burning crosses the outer auxiliary elements 71 and 73 so that completely through a portion of said main element adja electrodes 76 and 77 are closer to the ends of main ribbon 5 cent said gap, and granular arc quenching material em 37 than electrodes 75 and 78. When the main element bedding said main and auxiliary fusible elements and said 37 melts adjacent bead 40 on a small fault current, the last-named means. arc voltage appears across only electrodes 76 and 77, 4. In a high voltage fuse of the current limiting type, thereby causing the air gaps between these electrodes and a tubular insulating casing, terminals on the ends o f said the ribbon 37 to flashover. The arcing at the air gaps 10 casing, a main fusible element within said casing inter causes the main ribbon to melt adjacent electrodes 76 connecting said terminals and having a maximum long and 77 during the interval required to melt the center aux time melting temperature of approximately 600 F. iliary element 72. If current flow should re-ignite the when subjected to prolonged fault currents o f rela main element 37 which has been burned open at three tively small magnitude, an auxiliary fusible element with places, a portion of the 'arc voltage will now appear across 15 in said casing having its ends spaced slightly by air electrodes 74 and 75 and a portion of the arc voltage gaps from longitudinally spaced apart portions of said w ill also appear across electrodes 78 and 79, thereby main element, the ratio of the 100 second melting cur causing the air gaps between these electrodes and the rent of said main element to the 100 second melting cur main fusible element 37 to flashover. The subsequent rent o f said auxiliary element being in the range from arcing at these gaps causes the main ribbon 37 to melt 20 2 to 6, and granular inert refractory arc quenching ma adjacent arc gap electrodes 74, 75, 78 and 79 during the terial within said casing embedding said main and auxil interval required to melt the auxiliary elements 71 and iary elements, whereby flashover of said air gaps connects 73. If current now re-ignites the main element 37, seven said auxiliary element in parallel with said main element arcs will be connected in series throughout the length of and burns away portions of said main element adjacent main element 37, and fourteen fulgurite end portions hav 25 said air gaps, while said auxiliary element is melting, and ing good arc extinguishing characteristics tend to interrupt provides a plurality o f arcing points in series along the the overload current. length of said main element. While only a few embodiments of the invention have 5. In a high voltage fuse of the current limiting type, been illustrated and described, many modifications and a tubular insulating casing, terminals on the ends o f said variations thereof will be readily apparent to those skilled 30 casing, a main fusible element within said casing inter in the art, and consequently it is intended in the appended connecting said terminals, a body of low melting tempera claims to cover all such modifications and variations which fall within the true spirit and scope of the invention. I claim: 1. In a high voltage fuse o f the current limiting type, 35 a tubular insulating casing, metallic terminals on the ends of said casing, a main fusible element within said casing interconnecting said metallic terminals, a body o f low melting temperature alloy in intimate contact with said main element, an auxiliary fusible element within said 40 casing having its ends spaced slightly from portions of said main element on opposite sides of said body of alloy and forming air gaps with said main element, the ratio o f the 100 second melting current of said main element to the 100 second melting current of said auxiliary element being in the range of from 2 to 6, and granular inert re 45 fractory arc quenching material within said casing em bedding said main and auxiliary elements, whereby flash over of said air gaps connects said auxiliary element in parallel with said main element and burns away portions o f said main element adjacent said air gaps while said 50 auxiliary element is melting and provides a plurality of arcing points in series along the length of said main ele ment. . 2 - In. an electric circuit interrupter capable o f substan tially limiting the magnitude of fault current flowing 55 therethrough including a fusible main element having a body of low melting temperature alloy in intimate con tact therewith, means for causing said main element to burn back in a plurality of serially related points when interrupting low magnitude fault current including an aux 60 iliary fusible element spaced apart at its ends by air gaps from portions of said main element on opposite sides of said body of alloy, the ratio of the 100 second melting current of said main element to the 100 second melting current o f said auxiliary element being in the range from 65 2 to 6, and granular inert arc quenching material em bedding said main and auxiliary fusible elements. 3. In an electric circuit interrupter capable of substan tially limiting the magnitude of fault current flowing therethrough including a fusible main element having a 70 low melting temperature alloy element in intimate con tact therewith, means for causing said main element to burn back in a plurality o f serially related points when interrupting low magnitude fault current including an auxiliary fusible element spaced apart at its ends by air 75 ture alloy in intimate contact with said main element, an auxiliary fusible element within said casing having its ends spaced slightly from portions o f said main element on opposite sides of said body o f alloy forming air gaps with said main element, the ratio of the 100 second melting current of said main element to the 100 second melting current of said auxiliary element being in the range from 2 to 6, insulating support means within said casing for said main and fusible elements and being in contact with said fusible elements at only spaced apart points along the length o f said main and auxiliary ele ments and being adapted to evolve gas in the presence of an arc, and granular inert refractory arc quenching material within said casing embedding said support means and said main and auxiliary elements. 6. In a high voltage fuse of the current limiting type, a tubular insulating casing, terminals on the ends of said casings, a spider extending parallel to the axis of said casing and having peripherally spaced apart, radially protruding fins generally longitudinal thereof, a main fusible element within said casing interconnecting said terminals and being wound helically on said spider so that it touches only said fins, a body of low melting tempera ture alloy in intimate contact with said main element, an auxiliary fusible element within said casing wound helically on said spider so that it touches only said fins and having its ends spaced slightly from portions of said main element on opposite sides of said body o f alloy and forming air gaps with said main element, the ratio of the 100 second melting current of said main element to the 100 second melting current o f said auxiliary element being in the range from 2 to 6, the portions of said fins in contact with said main and auxiliary elements being of an insulating material adapted to evolve gas in the presence o f an arc, and granular inert refractory arc quenching material within said casing embedding said spider and said main and auxiliary elements. 7. In a high voltage fuse of the current limiting type, a tubular insulating casing, terminals on the ends of said casing, a spider of star shaped cross section having radially protruding fins extending longitudinally thereof disposed within said casing parallel to the axis thereof, a main fusible element within said casing interconnecting said terminals and being helically wound on said spider and touching only spaced apart portions of the periphery of 3,243,552 9 _ 10 said spider along said fins, a body o f low melting tempera tween said depressions and the depressions of peripheral ture alloy in intimate contact with said main element, an auxiliary fusible element within said casing wound ly successive fins advancing longitudinally of said spider and defining a genrally helical path and said raised portions helically on said spider and touching only spaced apart of peripherally successive fins forming generally helical portions of the periphery o f said spider along said fins and 5 support means radially outward from said helical path, having its ends spaced slightly from portions of said main a main fusible element within said casing interconnect element on opposite sides o f said body of alloy and form ing said terminals and being wound on said helical sup ing air gaps with said main element, the ratio of the 100 port means defined by said raised portions, a bead o f second melting current of said main element to the 100 low melting temperature alloy in intimate contact with second melting current of said auxiliary element being in 10 said main element, arc gap electrodes affixed to individual the range of two to six, granular inert refractory material raised portions o f said spider on opposite sides of said o f high dielectric strength within said casing embedding bead and being spaced slightly inward radially from said said spider and said main and auxiliary elements, said main elements and defining air gaps therewith, an auxil spider being of a molded, thermosetting, electrical insu iary fusible element within said casing disposed in the lating composition, including a water insoluble binder 15 helical path defined by said depressions and being affixed and an anti-tracking material adapted to evolve gas when at its ends to said arc gap electrodes, the ratio of 100 heated by an arc selected from a class consisting o f the second melting current of said main element to the 100 hydrates and oxides o f magnesium and aluminum. second melting current o f said auxiliary element being 8. In a high voltage fuse of the current limiting type, in the range from two to six, and granular inert refrac a tubular insulating casing, terminals on the ends o f said 20 tory arc quenching material within said casing em casing, a spider of heat resistant insulating material ex bedding said spider and said main and auxiliary ele tending parallel to the axis of said casing having periph ments and said arc gap electrodes. erally spaced apart, radially protruding fins generally 11. In an electric circuit having current limiting fuse longitudinal thereof, said fins having depressions spaced means for substantially limiting the magnitude of fault apart longitudinally thereof forming raised portions be 25 current flowing through said circuit including a fusible tween said depressions and the depressions of periph main element having a body of low melting temperature erally successive fins advancing longitudinally o f said alloy in intimate contact therewith and granular inert spider and defining a generally helical path and said raised arc quenching material embedding said main element, portions of peripherally successive fins forming generally means for causing said main element to burn back in a helical support means radially outward from said helical 30 plurality of serially related points when interrupting low path, a main fusible element within said casing intercon magnitude fault current including an auxiliary element necting said terminals and being wound on said helical embedded in said arc quenching material and extending support means defined by said raised portions, a bead of parallel to said main element and spaced apart at its low melting temperature alloy in intimate contact with said main element, an auxiliary fusible element within 35 ends by first and second air gaps from portions o f said main element on opposite sides o f said body o f alloy, said casing,disposed in the helical path defined by said depressions and having its ends spaced slightly from por and a second auxiliary element embedded in said arc tions of said main element on opposite sides of said bead quenching material and extending parallel to said main forming air gaps with said main element, the ratio of element and having its ends spaced slightly from por 110000 sseeccoonndd mmeeltlitninggcucurrrernent tofofsasidaidaumxialiianryeleelmemenent ttboetinhge 40 taiiorngsaopf asnadid fmoramininegletmhierndt aonnd ofpopuorstihteasiridgesaposf wsaitihd sfiarisdt in the range from two to six, and granular inert refractory main element, whereby the arc voltage developed across arc quenching material within said casing embedding said the portion o f said main element burned away by arc spider and said main and auxiliary elements. ing at said first air gap will flashover said third and 9. In a high voltage fuse of the current limiting type, fourth air gaps. a tubular insulating casing, terminals on the ends of 45 12. In combination with an electric circuit having cur said casing, a spider of heat resistant insulating ma rent limiting fuse means for substantially limiting the terial adapted to evolve gas in the presence o f an arc magnitude of fault current flawing through said circuit extending parallel to the axis of said casing and having including a fusible main element having a low melting tem fciarlstpaanthds siencotnhde ecxotnecreionrtrpicersippahceerdy athpearret,ofg, eanemraalilny hfueslii 50 pfoerractuaruesianlgloysaeildemmeanint inelienmtiemnattetocobnutrancttbhaecrkewinitha, pmlueraanls ble element within said casing disposed in said first ity o f serially related points when interrupting low mag helical path and interconnecting said terminals, a body nitude fault current including three serially connected of low melting temperature alloy in intimate contact with auxiliary fusible elements extending parallel to said main wsaiitdhinmasianidfucsaisbinleg edleismpeonset,d ainn asuaixdiliasercyonfudsihbeleliceallempaentht 55 emleinmimenut,mthme elmtiindgdlceuraruexnitliathryaneltehme enout tehravainugxilaiarhyigehleer and having its ends spaced slightly from portions of ments, the junctions of said three auxiliary elements be said main element on opposite sides o f said alloy body ing spaced apart by first and second air gaps from por and forming air gaps with said main element, the ratio tions of said main element on opposite sides of said al toof tthhee 110000 sseeccoonndd mmeellttiinngg ccuurrrreenntt ooff ssaaiidd mauaxiniliaelreymeelnet 60 eloleymeelnetms ebnetinagndsptahceedoufrteormensadisd omf asinaideleomuteenrt abuyxitlhiairrdy ment being in the range of from two to six, said spider and fourth air gaps spaced further from said alloy ele contacting only portions of said main and auxiliary ele ment than said first and said second air gaps and hav ments spaced apart longitudinally o f said elements, and ing lower flashover voltages than said first and second ignrganeumlabredindeirntgrseafridactsopridyeraracnqduesanicdhimngaiwn itahnidn saauixdilicaarsy 65 baierddgianpgs,saainddmgarainnualnard isnaeidrt aaurxcilqiaureyncfuhsinibglemealetemrieanltse.m elements. 13. In a high voltage fuse of the current limiting type, 10. In a high voltage fuse of the current limiting a tubular insulating casing, terminals on the ends of said type, a tubular insulating casing, terminals on the ends casing, a main fusible element within said casing inter o f said casing, a spider o f heat resistant insulating ma 70 connecting said terminals, a body o f low melting tem terial adapted to evolve gas in the presence of an arc perature alloy in intimate contact with said main ele extending parallel to the axis o f said casing having pe ment, an auxiliary fusible element within said casing hav ripherally spaced apart, radially protruding fins general ing its ends spaced slightly from portions of said main ly longitudinal thereof, said fins having depression spaced element on opposite sides of said body of alloy and apart longitudinally thereof forming raised portions be- 75 forming air gaps with said main element, means dis- n 3,343,552 posed adjacent each said air gap and responsive to arc 12 when interrupting low magnitude fault current will flash ing in said air gap for burning through a portion of over said air gaps and burn back said main element said main element adjacent said air gap, land inert adjacent each said air gap. granular refractory arc quenching material within said casing embedding said main and auxiliary elements and 5 N o references cited. said last-named means, whereby the arc voltage across the portion o f said main element at said body of alloy B. A. GILHEANY, Primary Examiner. H. B. GILSON, Examiner.