US 3913050 A
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' United States Patent Mikulecky Oct. 14, 1975 FUSE ASSEMBLY FOR CURRENT LIMITING FUSES Harvey W. Mikulecky, Racine, Wis. Assignee: RTE Corporation, Waukesha, Wis.
Filed: June 3, 1974 Appl. No.: 475,440
U.S. Cl. 337/159; 337/202; 337/280 Int. Cl. HOIH 85/04 Field of Search 337/158, 159, 160, 202,
References Cited UNITED STATES PATENTS 5/1973 Mikulecky 337/159 X 4/1974 Howard 337/159 3,846,728 11/1974 Salzer 337/159 Primary Examiner-14.- T. Hix
Assistant ExaminerFred E. Bell Attorney, Agent, or FirmR0nald E. Barry ABSTRACT A high voltage fuse of the current limiting type including a fusible element spirally wrapped around a spider provided within the casing of the fuse and embedded with a granular inert refractory arc-quenching material, the fusible element having a plurality of reduced area sections located at spaced intervals and mounted on said spider so that the ribs of the spider contact the fusible element at points between the reduced area sections. The spider can be made of a high temperature ceramic or a gas evolving non-tracking material.
10 Claims, 3 Drawing Figures US. Patent 0612. 14, 1975 FUSE ASSEMBLY FOR CUR-RENT LIMITING FUSES BACKGROUND OF THE INVENTION Current limiting fuses conventionally include -a fusible element embedded in a granular inert material of high dielectric strength such as sand or finely divided quartz. The fusible element-is generally in the form of a thin conductor or ribbon of silver having a plurality of reduced area sections at spaced intervals. Theribbon is spirally wound on a supporting member commonly called a spider. The supporting member includes a number of ribs extending radially outward at spaced intervals and is made of a high temperature resistant ceramic materiaL'When the fusible element attains fusing temperature and vaporizes, arcing occurs and the metal vapors rapidly expand to many times the volume originally occupied by the fusible element. The vapors are blown into the spaces between the granules of inert 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 are energy with very little pressure build up within the fuse enclosure. The sand particles n the immediate vicinity of the fuse arc 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, lose their conductivity and become" insulators as they cool.
It has been found that when the reduced area sections fall right upon the ribs of the supporting member or spider, the chances of flashover to adjacent turns is greatly increased. Under fault current operation the silver ribbon melts at all of the reduced sections first and thus these sections encounter the longest period of arcing and the greatest degree of element vaporization. The vaporized gases thus can be blasted the farthest distance from that area. Furthermore, the gases will tend to follow a smooth surface, such as that of the supporting rib, easier than the irregular path through the sand filler material.
SUMMARY OF THE INVENTION In the high voltage fuse of the present invention, a fuse assembly has been provided which reduces the possibility of flashover under fault current conditions. This assembly includes a spider having a spider width which corresponds to the spacing of the reduced area sections of the silver ribbon fuse element. The fuse element has a winding pitch such that the reduced sections automatically fall midway between the supporting ribs. Although a single reduced section is shown between each spider, more than one reduced area section can be provided between the spiders.
Other objects and advantages of the present invention will become more apparent from the following description when considered in conjunction with the drawings in which:
FIG. 1 is a sectional side view in elevation of a high voltage fuse showing the fuse assembly of the present invention;
FIG. 2 is an enlarged view taken on line 2-2 of FIG. 1 showing the spacing of the reduced area sections of the fuse between the ribs of the spider; and
FIG. 3 is a perspective view of the fuse assembly.
DESCRIPTION OF THE INVENTION Referring to the drawing, the fuse assembly 10 of the present invention is shown positioned within a tubular casing 12 constructed of a dielectric material such as a ceramic. In this regard, the casing 12 is shown positioned within the opening 14 in an enclosure 16 for an electrical apparatus such as a transformer. The casing is retained in the opening by means of a hold down assembly 18 of conventional type.
The casing 12 includes an axial passage 20 and is closed at each end by electric terminals 22 and 24. As seen in the drawings, the lower terminal 22 is clamped to the lower end of the casing and is sealed by means of a seal 26. The upper terminal 24 is in the form of a terminal casing which is threadedly received on a hold down bolt 28 provided in an insulating bushing 30. The terminal 24 is sealed in the end of the casing by a gasket 32.
In accordance with the invention the fuse assembly 10 includes asupporting member 34 in the form of a spider having a first element 36 which provides a current limiting function during high fault current interruption and a second element 38 which senses and operates to interrupt low currents, just above the fuse rated current. The element 36 is electrically connected at one end-to the terminal 22 by means of a connecting link 35 and at the other end to the second element 38. The low current sensing element 38 is electrically connected to the terminal 24 through the bolt 28 to provide a series current path through the fuse.
The high fault current sensing element 36 is in the form of a silver wire ribbon having a plurality of reduced diameter sections 40 shown in the form of holes equally spaced in the ribbon 36. It should be noted that the reduced diameter sections can be in the form of depressions or notches rather than holes if desired.
The spider 34 includes a number of ribs 42 which extend radially outwardly and have their outer edges 44 equally spaced from each other. The spider 34 can be formed from a high temperature ceramic or a gas evolving, non-tracking material. This latter material can be a molded thermosetting composition and an anti-tracking substance selected from the class consisting of hydrates and oxides of aluminum and magnesium.
The silver wire ribbon 36 is spirally wrapped around the outer periphery of the spider so that the reduced area sections 40 are located intermediate the ends 44 of the ribs 42 of the spider. In order to accomplish this result the spider width, winding pitch, and spacing between the reduced diameter sections must be predetermined in order to achieve automatic winding. Although a single reduced area section has been shown between the ribs 42, more than one reduced area section can be located between the ribs 42, if desired.
In a specific embodiment, the following has been found to be the proper arrangement for a 20 amp fuse.
Spider Width 0.625 inches Winding Pitch 0.875 inches I-Iole Spacing 0.500 inches (centerline to centerline) After assembling the fuse assembly 10 within the easing 12, the casing is filled with a granular refractory material 43 such as sand. Conductive layers 45 and 46 can be provided on the outside and inside surfaces, re-
spectively, of the housing to reduce corona at the opening 14 as described in application Ser. No. 275,178,
filed on July 26, 1972 and entitled Combination Fuse and Bushing. A thermal insulating layer can be provided on the inside surface of the casing as shown in copending application Ser. No. 385,436, filed Aug. 3, 1973 and entitled A Heat Insulated Fused High Voltage Bushing.
1. A high'voltage fuse of the current limiting type, a tubular insulating casing, terminals on the ends of said casing, a spider of heat resistant insulating material positioned in said casing, a fusible element having a plurality of reduced area sections, said element being helically wrapped around said spider and interconnecting said terminals, said spider contacting only portions of said fusible element at points between reduced area sections of said fusible element, and granular inert refractory, arc-quenching material within said casing embedding said spider and said main fusible element.
2. The fuse according to claim 1 wherein said spider is formed of a gas evolving non-tracking material.
3. In a high voltage fuse of the current limiting type, a tubular insulating casing, terminals on the end of said casing and insulating support means within said casing, a main fusible element spirally wrapped about said support means and including a plurality of reduced cross sectional area sections, said main fusible element contacting said support means only at points between said reduced area sections, and granular inert refractory arc-quenching material within said casing embedding said support means and said main fusible element.
4. The fuse according to claim 3 wherein said reduced area sections comprise holes.
5. The fuse according to claim 3 wherein said support means comprise a spider having equally spaced radially extending ribs, said element being spirally wrapped around the outer periphery of said ribs with said reduced area sections being located between said ribs.
6. The fuse according to claim 3 wherein said reduced area sections are located at equally spaced intervals.
7. The fuse according to claim 3 wherein said spider is formed of a gas evolving non-tracking material.
8. The fuse according to claim 3 wherein said housing includes an electrically conductive layer on the inside and outside surfaces of said housing.
9. A fuse assembly for a sand filled high voltage fuse, said assembly comprising a heat resistant spider having equally spaced ribs, and a ribbon of fusible material having a plurality of equally spaced reduced area sections, said element being spirally wrapped around said spider with said reduced area sections located between and spaced from said ribs.
10. The fuse assembly according to claim 9 wherein said spider is formed of a gas evolving, non-tracking material.