Electric all purpose fuse
BACKGROUND OF THE INVENTION 
     This invention relates to electric fuses having an elevated voltage rating, e.g. a rating of 2.8 to 23 Kv. It relates more specifically to fuses whose dimensions are drastically reduced as set forth below in more detail. 
     The invention solves the problem of providing all purpose fuses having extremely small dimensions, i.e. fuses capable of interrupting very small overloads requiring 1 hr to cause fusion of the fusible element or elements thereof (ANSI Standard C 37.40-2.2.2.2). 
     A solution to the problem of enabling fuses to interrupt the entire wide spectrum of currents which may occur is disclosed in my U.S. Pat. No. 3,810,062, May 7, 1974 for HIGH-VOLTAGE FUSE HAVING FULL RANGE CLEARING ABILITY. The solution shown in that patent has limitations. The fuses cannot be miniaturized to the required extent. The beads of melamine resin wth inorganic fillers are relatively bulky and expensive and it requires particular care to maintain equidistance between the turns of the windings of the fusible elements if they are carrying beads and if their number exceeds two. 
     In order to adapt fuses for the interruption of very small currents it is necessary to cause series miltibreaks in each of the fusible elements. Fuses wherein small currents are caused to be interrupted by series multibreaks in the fusible element, or elements thereof are disclosed in U.S. Pat. No. 3,735,317, 05/22/73 to P. C. Jacobs for ELECTRIC MULTIBREAK FORMING CARTRIDGE FUSE, and in my patent 3,969,694, 7/13/76 for ELECTRIC FUSE FOR ELEVATED CIRCUIT VOLTAGES CAPABLE OF INTERRUPTING SMALL OVERLOAD CURRENTS, and in other printed publications. These solutions require auxiliary circuitry for severing the fusible element, or elements, at several points and are, for this reason, not sufficiently cost-effective. 
     The present invention eliminates the drawbacks inherent in prior art solutions to the problem at hand. 
     SUMMARY OF THE INVENTION 
     Fuses embodying this invention include a tubular casing of electric insulation smaller than normal, i.e. having a length of 8-15 inches depending upon the voltage rating thereof. The ends of said casing are closed by a pair of terminal elements and filled with a granular quartz filler. A plurality of equal, wire-like fusible elements whose cross-sectional area changes periodically conductively interconnects said pair of terminal elements. The wire-like fusible elements are stampings of sheet metal. By subdividing the current path into wire-like strips of sheet metal the density of metal vapor that may occur at any point of the fuse is greatly reduced. Said plurality of fusible elements is wound substantially helically around a fusible element support which comprises rods of gas evolving insulating material, i.e. material evolving a gas under the action of an electric arc. 
     Fusion near the center of each conductor causes formation of an initial break in each of them, the fusion of the individual conductors occuring at a relatively rapid sequence. The formation of each break is accompanied by a significant voltage surge or voltage kick equal to -L (di/dt). This voltage kick is significant because it results from a significant rate of change of current flow in the presence of more or less inductance. The axial spacing of said plurality of fusible elements is so narrow that said voltage kicks or spikes cause electric breakdowns between contiguous windings of said fusible element and formation of series breaks therein. On the other hand, the pitch or axial spacing between the conductors is sufficiently wide to preclude self-sustaining discharges between the constituent elements of the winding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS. 
     FIG. 1 is substantially an elevational view of a fuse embodying this invention showing portions thereof in cross-section rather than in elevation: and 
     FIG. 2 is a section along 2--2 of FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS. 
     In the drawings numeral 1 has been applied to indicate a tubular casing of electric insulation, e.g. glass-cloth-melamin laminate. The casing has a length of 8-15 inches depending upon the voltage rating of the particular fuse. The table below indicates approximate casing length depending upon voltage rating. 
     
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Max kV      Approximate casing length                                     
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5.5         8.15&#34;                                                         
8.3         8.15&#34;                                                         
15.5        12.41&#34;                                                        
23.0        15.36&#34;                                                        
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     Reference numeral 2 has been applied to indicate a pair of plug terminals closing the ends of casing 1. The O.D. of plug terminals is somewhat less than the I.D. of casing 2. O-rings 3 of an elastomer are inserted between casing 1 and plug terminals 2. The I.D. of casing 1 changes with the current-rating of the fuse. It is, for instance, 2 inches for a fuse rated at 15 kV and 200 A, and decreases with decreasing current rating. Four wire-like fusible elements 4 inter-connect plugs 2, thus maximizing the interface between the metal vapors resulting from the vaporization of elements 4 and the surrounding quartz filler 4a. The cross-sectional area of fusible elements 4 changes periodically to limit voltage surges resulting from their vaporization. Elements 4 are sheet metal stampings which are much easier to manufacture than wires. 
     It is important that the current be carried by a relatively large number of wire-like current paths, as contrasted with fusible elements in the shape of a ribbon. In the 15 kV embodiment of the invention shown, the fusible elements vary between 0.031 inch and 0.023 inch and have 101 points of narrowest cross-section. The fusible elements are blanked or stamped out of sheet metal having a thickness of 0.012 inch. If the fuse rating is decreased, a thinner sheet of silver may be used for stamping out the fusible elements. 
     As mentioned above, fusible elements 4 are wound substantially helically around four rods 5 of a gas-evolving material, e.g. a material including melamin resin. Rods 5 are so thin as to be incapable of withstanding without intolerable bending the lateral thrust R exerted by the fusible elements 4. A plurality of annular braces 6 impart to rods 5 the bending strength, or stiffness, required to support fusible elements 4. The ends of rods 5 are arranged in a circle whose diameter is slightly exceeded by the O.D. of annular braces 6 so that rods 5 are slightly bent outwardly or pre-stressed by braces 6. Braces 6 are cemented to rods 5 so that braces 6 and rods 5 form an integral structure. An important feature of braces 5 resides in the fact that they are cylindrical, their length 1 by far exceeding their radial width, i.e. the difference between their O.D. and their I.D. 
     A plurality of low-fusing point fusible-element-severing overlays 7, e.g. of tin, is arranged approximately at the center region of each of fusible elements 4. These overlays cause at the occurrence of small overload currents by a metallurgical reaction between the base metal and the overlay metal fusion and formation of initial breaks. Formation of these breaks occurs at a relatively rapid sequence. The voltage kick or surge voltage associated with these breaks is sufficiently high to cause electric breakdown between contiguous windings of the fusible element. The breakdowns cause formation of additional breaks in the fusible elements 4, i.e. formation of series breaks in addition to those initially formed at the points 7 where the low fusing point overlays are located. 
     What occurs between contiguous windings of the fusible elements depends inter alia upon the spacing between the windings which can be measured either by the spacing between parallel turns A-A 1  or the axial spacing A-A 2 . If that spacing is relatively large, a phenomenon called &#34;trigging&#34; will occur, i.e. each break formation at an overlay 7 will increase the current to be carried by the parallel fusible elements 4 which are still intact, thus causing formation of one break in each of the fusible elements in a relatively rapid sequence. This will occur when, and as long as, A-A 1  = X or A-A 2  = X&#39;. If the above spacings (or distances) are drastically reduced, contiguous windings of the fuse merge upon blowing thereof into one integral fulgurite, i.e. the fuse fails practically, or entirely. 
     There is a critical spacing where the dielectric breakdown between contiguous windings occurs, but where the breakdown is not followed by an arc discharge or self-sustained discharge across the breakdown gap. The electric breakdown which occurs subsequently at random points of the fusible element 4 however damages contiguous windings to such an extent that the points where the breakdown occurs results in increased i 2  ·r losses, and in formation of breaks at these points. 
     It should be understood that the discovery of a critical distance or spacing between contiguous turns is the most important, but only one, of the prerequisites for success. If, for instance, formation of initial breaks at the point 7 would occur sequentially, and the correct voltage spikes -L (di/dt) generated which cause a non-self-sustained low current discharge, the fuse might fail at high currents if the interface between the fusible elements 4 and their gas-evolving support is too large. 
     Fulgurites resulting from blowing of fuses according to this invention fully confirm the above theory of operation, and in particular the formation of series multibreaks, each fulgurite being separated from the contiguous fulgurite by a layer of sound, i.e. not fused, sand particles. 
     Fuses according to this invention are primarily intended for the protection of distribution transformers. The overlays 7 or &#34;M spots&#34; reduce the fusing current from about 960° C to about 300° C. Once fusion is initiated the sequential formation of additional initial breaks and series breaks occurs in rapid sequence. 
     The current rating of fuses according to this invention is the so-called &#34;C&#34; rating. See American National Standard Distribution Fuse Disconnecting Units C 37.47. To be more specific, according to paragraph 47-3.1 of that standard the fusible or current responsive element of such fuses shall melt in 1000 seconds at an rms current within the range of 170 to 240 percent of the continuous current rating of the fuse unit. 
     Regarding the geometry of the fusible elements, the same conforms to the geometry shown in my U.S. Pat. No. 3,810,062 (see FIG. 3 thereof), 5/7/74 for HIGH-VOLTAGE FUSE HAVING FULL RANGE CLEARING ABILITY, except for the fact that fusible elements according to the present invention have a smaller cross-section so that they appear to be wires rather than ribbons, and perform like wires, rather than ribbons. FIG. 1 clearly shows one edge of eac fusible element to be straight and the opposite edge to be scalloped. 
     It is possible to distinguish between various kinds of electric gas discharges, such as spark-discharges, corona discharges, etc. Gas discharges are generally divided into two groups, those which are self-sustaining and those which are not self-sustaining. A spark discharge is a transition from a non-self-sustaining discharge to a self-sustaining discharge. The electric arc is a self-sustained discharge having a low voltage drop and high current discharge. With this terminology in mind, the discharge which takes place in the subject-matter of the present invention is a spark discharge. An electric arc or high current discharge would result in a short-circuit between contiguous turns of the fusible elements by formation of a fulgurite between contiguous turns. Such fulgurite would short-circuit parallel current paths established by the several fusible elements and be considered not as an embodiment of the present invention, but as a malperforming fuse. 
     For further details in regard to the critical gaps, reference may be had to the book of James D. Cobine, Gaseous Conductors, 5 ed., McGraw Hill Book Company, New York and London 1941, particularly page 143 and following &#34;Field-Intensified Ionization and Breakdown of Gases&#34;. 
     Referring to fuses according to this invention blown on a small currents, such fuses may show four fulgurites extending from the center of the fusible element to the right. These fulgurites are initiated in the center region of the fusible element and formed by arcing from left to right. A separate breakdown may occur immediately adjacent the right terminal. Two other breakdowns may occur to the left of the center of the fusible elements. The breakdown zones are apparent from the fulgurites which formed therein and which are separated from one another by portions of sound fusible elements.