Full-range high voltage current limiting fuse

Information

  • Patent Grant
  • 6614340
  • Patent Number
    6,614,340
  • Date Filed
    Monday, February 11, 2002
    22 years ago
  • Date Issued
    Tuesday, September 2, 2003
    21 years ago
Abstract
A Full-Range fuse element assembly includes an insulative former having opposite first and second ends and electrically conducting connectors coupled to ends of the former. A plurality of fuse elements extend between the first connector and the second connector about the insulative former, and each of the fuse elements include a low current interrupting fuse element portion extending from the first connector and a high current limiting fuse element portion extending from the second connector. An insulative sleeve surrounds each of the low current interrupting fuse element portions, and each sleeve includes an end adjacent a respective one of the high current limiting fuse element portions. Each of the low current interrupting fuse element portions includes a weak spot located proximate the second end of a respective one of the sleeves.
Description




This application claims the benefit of United Kingdom Patent Application Number 0103541.9, filed Feb. 13, 2001.




BACKGROUND OF THE INVENTION




This invention relates generally to fuse element or fuse link assemblies, and, more particularly, to fuse element assemblies for General Purpose or Full-Range fuses.




Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current through the fuse exceeds a predetermined limit, the fusible elements melt and opens one or more circuit through the fuses to prevent electrical component damage.




General Purpose or Full-Range type high voltage, current-limiting fuses are operable to safely interrupt both relatively high fault currents and relatively low fault currents with equal effectiveness. At least one type of General Purpose or Full-Range type fuses employs a fuse element assembly having two distinct portions. One portion is configured for opening of an electrical circuit under relatively low fault current conditions and a second portion is configured for opening of an electrical circuit under relatively high fault current conditions. The first portion includes a plurality of fuse elements contained in respective insulating sleeves and including a weak spot and/or low melting alloy spot located approximately at the center or midpoint of each of the fuse elements. The second portion includes a plurality of fuse elements fabricated from a high conductivity metal and connected in parallel with one another. The first and second fuse element portions are serially wound onto an insulating former and embedded in a arc-extinguishing material within a fuse body.




Under high fault current conditions, the second portion of the fuse element assembly partially vaporizes, and the arc extinguishing material absorbs energy and attains a high electrical resistance to safely and effectively interrupt current through the fuse. Under low fault current conditions, the first portion of the fuse element assembly interrupts current by melting of a fuse element within one or more of the insulated sleeves. The resultant arc within the sleeves generates ionized gas which is expelled from the open ends of the sleeves.




In elevated voltage and current applications, however, such as for protection of increasingly common 12 kV transformers with ratings as high as 1000 kVA, conventional Full-Range fuses have been found deficient. As current ratings and voltage ratings of Full-Range fuses are increased, the fuse is prone to undesirable internal and external damage from resultant increased energy of ionized gas blasts in operation of the fuse. While reinforcement of the insulating sleeves of the first portion of the fuse element assembly is of some use in producing higher current ratings and voltage ratings of Full-Range fuses, reinforcement of the sleeves tends to complicate assembly and increase manufacturing costs of the fuses without overcoming problematic excessive ionized gas blasts and resultant damage during operation of the fuse.




In addition, while voltage and current ratings of Full-Range fuses may be increased by using fuse elements and fuse constructions of greater cross sectional area and capacity, this increases the physical size of the Full-Range fuse. Especially when a large number of fuses are employed, increasing the size of the fuses is problematic.




BRIEF SUMMARY OF THE INVENTION




In an exemplary embodiment of the invention, a fuse element assembly for a Full-Range fuse includes an insulative former having opposite first and second ends. A first electrically conducting connector is coupled to the first end of the former and a second electrically conducting connector is coupled to the second end of the former. At least one fuse element extends between the first connector and the second connector about the insulative former. The fuse element includes a low current interrupting fuse element portion extending from the first connector, a high current limiting fuse element portion extending from the second connector, and the low current interrupting fuse element portion and the high current limiting fuse element portion coupled to one another intermediate the first and second connector. An insulative sleeve surrounds the low current interrupting fuse element portion, and each sleeve includes a first end adjacent the first connector and a second end adjacent the high current limiting fuse element portions. The low current interrupting fuse element portion includes a weak spot located adjacent to but within the second end of a respective one of the sleeves. Alternatively, the weak spot is located in a range from 0 to 25% of the length of the sleeve as measured from the second end of the sleeve.




By locating the weak spot of the low current interrupting fuse element at an end of the insulating sleeve opposite the connector from which the low current interrupting fuse elements extend, ionized gas blasts generated in operation of a fuse is directed predominately toward a center of the fuse rather then the ends of the fuse near the end-caps. Therefore, by more efficiently and effectively expelling ionized gas from the insulative sleeve, the fuse element assembly avoids damage to the fuse body and end-caps that has been observed in conventional fuses, and higher voltage and current ratings are facilitated without increasing dimensions of fuse components. Thus, a superior performing Full-Range fuse is provided in a compact, space-saving construction in comparison to known Full-Range fuses.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is sectional schematic of a first embodiment of a Full-Range fuse; and





FIG. 2

is a sectional schematic of a second embodiment of a Full-Range fuse.











DETAILED DESCRIPTION OF THE INVENTION





FIG. 1

illustrates a Full-Range fuse


10


including an insulative fuse body


12


, a fuse element assembly


14


within body


12


, electrically conductive end-caps


16


coupled to and enclosing body


12


and electrically connected to fuse element assembly


14


, and an arc quenching material


18


surrounding fuse element assembly


14


within body


12


. Thus, when end-caps


16


are connected to an energized electrical circuit (not shown), a circuit is completed through fuse


10


via fuse element assembly


14


. When current flowing through fuse


10


approaches unacceptable levels, dependent upon characteristics of fuse element assembly


14


and hence the current rating of fuse


10


, fuse element assembly


14


at least partially operates, melts, vaporizes or otherwise opens, as explained more fully below, to limit current flow and interrupt damaging current flow through fuse


10


. Thus, line-side electrical circuits and equipment may be electrically isolated from malfunctioning load-side electrical circuits and equipment to prevent costly damage to the load and line-side circuits and equipment.




In one embodiment, body


12


is fabricated from a known insulative, i.e., non-conductive material, such as ceramic materials, and extends substantially cylindrically between end-caps


16


. It is contemplated, however, that the benefits of the invention may be realized in fuses employing non-cylindrical bodies and fabricated from other materials. In addition, in an exemplary embodiment arc extinguishing medium


18


is granular pure silica sand or powdered quartz that completely surrounds fuse element assembly


14


and substantially eliminates air gaps around fuse element assembly


14


within body


12


. In alternative, embodiments, however, other known arc extinguishing materials and mediums are employed in fuse


10


in lieu of pure silica sand or powdered quartz.




Fuse element assembly


14


includes an insulated former


20


having a first portion


22


and a second portion


24


having a greater relative cross sectional area than first portion


22


. More specifically, in an exemplary embodiment, former


20


is integrally formed and extends substantially cylindrically with a step increase


26


in diameter that delineates former first portion


22


and former second portion


24


into relatively narrow and relatively wide portions, respectively. In alternative embodiments, however, separate narrow and wide portions


22


and


24


are secured to one another in fabrication of former


20


. In addition, it is contemplated that the benefits of the invention may be realized using alternative shapes, i.e., non cylindrical shapes, of former


22


, including but not limited to elliptical cross-sectional shapes, polygonal, ribbed or star cross-sectional shapes. Still further, it will be apparent further below that the invention may be employed on a former


22


having a substantially constant or uniform cross-sectional area, although it is noted that a substantially non-uniform clearance between fuse element assembly


14


and body


12


may result unless body


12


is modified accordingly.




Electrically conductive connectors


28


,


30


are oppositely coupled to former


20


at either end of former


20


, i.e., at respective ends of former first portion


22


and former second portion


24


located away from step diameter increase


26


. Each connector


28


,


30


may include extensions


31


that establish electrical contact with end-caps


16


. Thus, an electrical circuit may be established through fuse elements, explained further below, that are wound about former


20


and electrically coupled to connectors


28


,


30


.




A plurality of low current interrupting fuse elements


32


are wound about former first portion


22


and extend longitudinally from connector


28


toward former step increase


26


in a helical fashion. Each low current interrupting fuse element


32


is fabricated from a relatively low-melting point alloy or metal such as tin, or alternatively, for example, from a silver or copper element having an M effect overlay (low melting alloy spot)


34


or M spot thereon and located intermediate connector


28


and former step diameter increase


26


.




More specifically, in an exemplary embodiment, each low current interrupting fuse element


32


is at least partially coated with an overlay


34


of a conductive metal that is different from a composition of fuse element


32


. In one illustrative embodiment, for example, fuse elements


32


are fabricated from copper or silver and overlay


34


is fabricated from tin. As tin has a lower melting temperature than copper or silver, overlay


34


is heated to a melting temperature in an overcurrent condition before copper fuse element


32


. The melted overly then reacts with copper or silver fuse element


32


and forms a tin-copper alloy that has a lower melting temperature than either metal by itself. As such, an operating temperature of fuse element


32


is lowered in an overcurrent condition, and each fuse element


32


is prevented from reaching the higher melting point of silver or copper. Thus, conductive characteristics and advantages of copper or silver are utilized while avoiding undesirable operating temperatures. In alternative embodiments, other conductive materials may be used to fabricate fuse elements


32


and overlay


34


, including but not limited to copper and silver alloys and tin alloys, respectively, to achieve similar benefits. In further alternative embodiments, overlay


34


is fabricated from antimony or indium.




Overlay


34


is applied to respective fuse elements


32


using known techniques, including for example, gas flame and soldering techniques. Alternatively, other methods, including but not limited to electrolytic plating baths, thin film deposition techniques, and vapor deposition processes may be employed. Using these techniques, in various embodiments overlay


34


is applied to some or all of fuse elements


32


. For example, in one embodiment, only a central portion of a fuse element


32


includes overlay


34


, while in another embodiment, an entire surface area of a fuse element


32


includes overlay


34


. In a further embodiment, overlay


34


is applied on one side only of a fuse element


32


, while in a different embodiment, both sides of a fuse element


32


include M effect overlay


34


.




Each low current interrupting fuse element


32


further includes a narrowed portion, or weak spot


36


, of reduced cross sectional area in which fuse element


32


is designed to melt, open, or otherwise break an electrical connection through fuse


10


. Because of the reduced cross-sectional area of weak spot


36


relative to a remainder of fuse element


32


, weak spot


36


is heated to a higher temperature as current flows therethrough than through a remainder of fuse element


32


, and hence reaches the melting point of fuse element


32


before the remainder of fuse element


32


. Thus, fuse element


32


predictably opens in the area of weak spot


36


before other portions of fuse element


32


. It will be appreciated by those in the art that weak spots


36


could alternatively be formed according to other known methods and techniques known in the art, such as, for example, forming holes in fuse elements


32


rather than narrowed regions.




Each low current interrupting fuse element


32


is further encased in a flexible thermally insulative sleeve


38


of slightly greater dimension than a width of each fuse element


32


. Insulative sleeves


38


are fabricated from materials capable of withstanding high temperatures when fuse


10


is operated and also has a sufficient electrical resistance for insulative purposes. In an exemplary embodiment, sleeves


38


are fabricated from silicon rubber. In alternative embodiments, other known materials are used in lieu of silicone rubber for fabricating sleeves


38


. In further embodiments, inserts (not shown) of, for example, silicon grease, are positioned in respective ends of open sleeves


38


adjacent connector


28


and former step diameter increase


26


to prevent arc extinguishing medium


18


from entering sleeves


38


, yet while allowing ionized gas to escape sleeves


38


as fuse


10


is operated.




Notably, and unlike conventional Full-Range fuses, weak spot


36


of each low current interrupting fuse element


32


is located proximally to step diameter increase


26


of fuse assembly former


14


, or toward a center of fuse


10


. In other words, in one embodiment weak spots


36


of low current interrupting fuse elements


32


are located, to the extent possible, as far away from connector


18


and end-cap


16


as is practicable but still within respective sleeves


38


. As fuse elements


32


open near weak spots


36


, an electrical arc is generated across the break in weak spot


36


within sleeves


38


. The resultant blast of ionized gas is expelled from sleeve


38


predominately through the closer end of sleeve


38


located opposite from connector


28


and toward a center of fuse


10


, i.e., proximal to former step diameter increase


26


in the illustrated embodiment. Therefore, only a small portion of ionized gas travels through sleeves


38


to their ends adjacent connector


28


, and excessive exhaust pressure generated in sleeves


38


is primarily, and harmlessly, dissipated in arc extinguishing medium


18


surrounding fuse element assembly


14


away from connector


28


and end-cap


16


, or adjacent former step diameter increase


26


in the illustrated embodiment. Only a small portion of exhaust pressure travels longitudinally through sleeves


38


and exits sleeves


38


adjacent connector


28


and end-cap


16


. Thus, unlike known Full-Range fuses, increased energy of ionized gas blasts from elements


32


operating at higher currents, i.e., up to 100 A, and high voltages, i.e., 12 kV to 38 kV may be safely and effectively dissipated without rupturing fuse body


12


near end-cap


16


adjacent connector


28


and without damaging or displacing end-cap


16


.




It is contemplated that the benefits of the invention could be attained in alternative embodiments by locating weak spot


36


of each low current interrupting fuse element


32


in a range of positions toward a center of fuse


10


and away from a central region of respective low current interrupting fuse elements


32


. More specifically, some or all of the above-described advantages accrue to fuse elements


32


having weak spots


36


located up to about 25% of the total length of a sleeve


38


as measured from the end of the sleeve opposite connector


28


, i.e., the end of a sleeve


38


located closest to the center of fuse


10


.




In the illustrated embodiment, a reinforcing medium


40


is employed over insulating sleeves


38


to prevent damage to sleeves


38


from exhaust pressure in sleeves


38


when fuse


10


operates. In one embodiment, reinforcing medium is glass-fiber tape, although in alternative embodiments other known reinforcing media known in the art is employed to accomplish similar objectives. It is appreciated, however, that positioning weak spots


36


of each low current interrupting fuse element


32


away from connector


38


and toward a center of fuse


10


may obviate the need for reinforcing media


40


in certain fuse ratings by more efficiently dissipating exhaust pressure in sleeves


38


away from connector


28


and end-cap


16


where fuse


10


is less susceptible to damage, thereby simplifying manufacturing of fuse


10


and reducing manufacturing costs.




A plurality of high current limiting current fuse elements


44


are wound around former second portion


24


and are electrically coupled to connector


30


on an end of former


20


opposite connector


28


. Each high current limiting fuse element


44


is fabricated from a relatively high-melting point material, such as silver or copper, and extends in a helical fashion from connector


30


toward step diameter increase


26


of fuse element assembly former


22


. Each high current limiting fuse element is connected in parallel via connector


30


and includes a plurality of weak spots


46


or narrowed regions of reduced cross sectional area located at spaced intervals between connector


30


and low current interrupting fuse elements


32


. It will be appreciated by those in the art that weak spots


46


could alternatively be formed according to other methods and techniques known in the art, such as, for example, forming holes in fuse elements


44


rather than narrowed regions.




Each high current limiting fuse element


44


is coupled to a respective one of low current interrupting fuse elements


32


to form a plurality of continuously extending fuse elements that are partly high current limiting fuse elements


24


and partly low current interrupting fuse elements


32


. The continuously extending fuse elements are wound about former


22


in a helical fashion and are connected in parallel with one another between connectors


28


,


30


.




In an alternative embodiment, low current interrupting fuse elements


32


and high current limiting fuse elements


44


are connected to an interconnector member (not shown) disposed between low current interrupting fuse elements


32


and high current limiting fuse elements


24


in the vicinity of former step diameter increase


26


. As such, different numbers of low current interrupting fuse elements


32


relative to high current limiting fuse elements


44


may be employed to vary voltage and current ratings of fuse


10


. As will be appreciated by those in the art, actual voltage and current ratings of fuse


10


may be further manipulated by altering dimensional characteristics of low current interrupting fuse elements


32


and high current limiting fuse elements


44


.




Fuse


10


operates as follows. During low overcurrent conditions, e.g., less than six times the current ratings of fuse element assembly


14


, high current limiting fuse elements


44


are cooled by arc extinguishing medium


18


and low current interrupting fuse elements


32


open at M spots


34


within sleeves


38


. Low pressure ionized gas from resultant arcs is expelled from sleeves


38


at either end of sleeve


38


without damaging fuse body


12


or end cap


16


adjacent connector


28


.




At higher current conditions just before the point where high current limiting elements


44


take over the duty of fault interruption, fuse elements


32


open at weak spots


36


within sleeves


38


due to temperature effects from thermally insulating sleeves


38


before M effect spots


34


have sufficient time to operate and interrupt current through fuse elements


32


. The resultant arc when fuse elements


32


open at weak spots


36


is extinguished in sleeves


38


by the above-described expulsion process of ionized gas in sleeves


38


. As gas is predominately dissipated harmlessly into arc quenching medium


18


toward the center of fuse


10


and away from connector


28


and end-cap


16


, damaging effects of high exhaust pressure near connector


28


is avoided. With proper dimensioning of weak spots


36


, it can be ensured that operation of fuse elements


32


occurs at weak spots


36


before opening of fuse element


32


in the vicinity of M spots


38


at predetermined current levels that approach current values sufficient to operate high current limiting fuse elements


44


.




At even higher values of overload current, opening of fuse elements


32


at weak spot


36


and opening of fuse elements


44


at weak spots


46


occurs substantially simultaneously. Consequently, arc energy is dissipated in each of the single weak spots


36


of fuse elements


32


. However, at such higher current, an even greater gas blast may be generated within sleeves


38


. Thus, positioning of weak spot


36


of respective low current interrupting elements


32


closer to center of fuse and in the vicinity of former step diameter increase


26


if of greater significance to direct damaging gas blasts away from connector


28


at the end of fuse


10


.




A fuse


10


is therefore provided that controls ionized gas blasts in sleeves


38


at a full range of fault currents, including takeover current values wherein current interrupting duty is transferred from low current interrupting fuse elements


32


to high current limiting fuse elements


44


. Therefore, fuse


10


is capable of performing at higher voltage and current ratings than known Full-Range fuses. A much wider range of applications is therefore available for using fuse


10


due to controlled ionized gas blast in sleeves


38


. For example, a Full-Range fuse


10


having a voltage rating of 10 kV and a current rating of 100 A may be used to protect a transformer of 1000 kVA or greater. Similarly, Full-Range fuses


10


having voltage ratings as high as 38 kV may be constructed.




In addition, by locating weak spots


36


of low current interrupting fuse elements


32


at an end of insulating sleeves


38


opposite connector


28


and therefore directing ionized gas blasts predominately toward a center of fuse


10


rather then the ends of fuse


10


, fuse


10


is capable of attaining higher voltage and current ratings without increasing dimensions of fuse components. Thus, a superior performing Full-Range fuse


10


is provided in a compact, space-saving construction in comparison to known Full-Range fuses.





FIG. 2

is a sectional schematic of a second embodiment of a Full-Range fuse


60


wherein common features with fuse


10


(shown in FIG.


1


and described above) are indicated with like reference characters. Comparing fuse


10


and fuse


60


, it may be seen that fuse


60


includes an M spot


62


located proximally to weak spot


36


of each low current interrupting fuse element


32


, as opposed to M spot


34


(shown in

FIG. 1

) located in a central portion of each fuse element


32


. Therefore, in addition to the benefits described above when fuse elements


32


open at weak spots


36


, ionized gas generated from operation of fuse elements


32


at M spots


34


also is harmlessly dissipated into arc extinguishing medium through sleeves


38


toward the center of fuse


60


. Fuse


60


otherwise operates substantially as described above with respect to fuse


10


, and the benefits described above in relation to

FIG. 1

are also attained. Positioning of M spot


34


either in a center of respective sleeves


38


(as shown in

FIG. 1

) or proximal to weak spots


36


(as shown in

FIG. 2

) is dictated by thermal parameters of specific materials of the fuse components.




It is contemplated that the benefits of the invention could be achieved at lower fuse ratings using a single low current interrupting element


32


and a single high current limiting member


44


. In addition, in alternative embodiments, low current interrupting elements


32


may employ more than weak spot


36


located toward a center of fuse


10


and away from a central region of fuse elements


32


. Still further, in alternative embodiments, fuses are electrically connected to end-caps


16


without being helically wound about former


20


, such as for example, by employing substantially linear fuse elements between end-caps


16


, with or without former


20


.




While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.



Claims
  • 1. A fuse element assembly for a Full-Range fuse, said fuse element assembly comprising:an insulative former comprising opposite first and second ends; a first electrically conducting connector coupled to said former first end; a second electrically conducting connector coupled to said former second end; at least one fuse element extending between said first connector and said second connector about said insulative former, said at least one fuse element comprising a low current interrupting fuse element portion extending from said first connector, a high current limiting fuse element portion extending from said second connector, and said low current interrupting fuse element portion and said high current limiting fuse element portion coupled to one another intermediate said first and second connector; and an insulative sleeve surrounding said low current interrupting fuse element portion, said sleeve having a first end adjacent said first connector and a second end adjacent said high current limiting fuse element portion, said low current interrupting fuse element portion comprising a weak spot located adjacent said second end of said sleeve.
  • 2. A fuse element assembly in accordance with claim 1, said former comprising a first portion having a first cross-sectional area and a second portion having a second cross sectional area, said second cross sectional area greater than said first cross sectional area.
  • 3. A fuse element assembly in accordance with claim 2, said former further comprising a step increase in cross sectional area between said former first portion and said former second portion.
  • 4. A fuse element assembly in accordance with claim 3 wherein said at least one fuse element extends helically about said former.
  • 5. A fuse element assembly in accordance with claim 1 comprising a plurality of fuse elements, said plurality of fuse elements are connected in parallel.
  • 6. A fuse element assembly in accordance with claim 1 wherein said low current interrupting fuse element portion further comprises an M effect overlay.
  • 7. A fuse element assembly in accordance with claim 6 wherein said M effect overlay is located adjacent said weak spot of each low current interrupting fuse element portion.
  • 8. A fuse element assembly for a Full-Range fuse, said fuse element assembly comprising:an insulative former comprising opposite first and second ends; a first electrically conducting connector coupled to said former first end; a second electrically conducting connector coupled to said former second end; a plurality of low current interrupting fuse elements extending from said first connector toward said second connector; each of said low current interrupting fuse elements comprising a weak spot therein; a plurality of said high current limiting fuse elements extending from said second connector toward said first connector, each of said high current limiting fuse element portions comprising a plurality of weak spots, said low current interrupting fuse element portions and said high current limiting fuse element portions coupled to one another intermediate said first and second connectors; and a plurality of insulative sleeves each surrounding one of said low current interrupting fuse element portions, said sleeves each having a first end adjacent said first connector and a second end opposite said first end, said second end of each sleeve located proximally to a respective said weak spot of a respective one of said low current interrupting fuse elements.
  • 9. A fuse element assembly in accordance with claim 8 wherein each of said low current interrupting fuse elements are connected in parallel.
  • 10. A fuse element assembly in accordance with claim 9 wherein each of said low current interrupting fuse elements extends helically about said former.
  • 11. A fuse element assembly in accordance with claim 8 wherein said former comprises a first portion, a second portion, and a step increase intermediate said first portion and said second portion, said second end of said sleeve positioned adjacent said step increase.
  • 12. A fuse element assembly in accordance with claim 8 wherein each of said low current interrupting fuse elements comprises an M effect overlay.
  • 13. A fuse element assembly in accordance with claim 12 wherein said M effect overlay is located adjacent said weak spot on each of said low current interrupting fuse elements.
Priority Claims (1)
Number Date Country Kind
0103541 Feb 2001 GB
US Referenced Citations (5)
Number Name Date Kind
3287525 Mikulecky Nov 1966 A
3735317 Jacobs, Jr. May 1973 A
4210892 Salzer Jul 1980 A
4308514 Kozacka Dec 1981 A
5355111 Haasler et al. Oct 1994 A
Foreign Referenced Citations (3)
Number Date Country
2126808 Mar 1984 GB
2184301 Jun 1987 GB
216852 May 1997 HU