Patent Publication Number: US-5296832-A

Title: Current limiting fuse

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to current limiting fuses. 
     Current limiting fuses typically have one or more fusible elements connecting two conducting terminals within an insulative housing. 
     One type of fuse construction employs a housing made of a tubular casing of melamine glass, cardboard, or thermoset polymer resins in a matrix with glass or papers. The ends of the tubes are typically closed with end caps, which go around the ends of the tube, or end blocks of brass or copper, which are inside of the tube at the ends. When end blocks are employed, there often are terminal blades that are located on the outer surfaces of the end blocks (being either integral with or attached such as by welding or brazing to the end blocks), and fusible elements are connected, e.g., by welding in grooves, to the inside surfaces of the end blocks. 
     Barricklow U.S. Pat. No. 973,250 describes a different type of fuse construction in which the insulative housing is made of two pieces that are bolted together. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features, in general, a fuse which includes a tubular member made of insulative material and having two open ends, two end block portions made of insulative material and located at each end of the tubular member, and a spanner joining together the end block portions inside of the tubular member. Terminals extend through openings in the end block portions, and a fusible element located inside the tubular member has ends connected to each of the terminals. The spanner provides structural integrity to the fuse, facilitates modular assembly, and accurately spaces end block portions from each other. 
     In preferred embodiments, a second spanner can be provided to increase the strength of the end block/spanner assembly. The end block portions and the spanner can comprise a single housing piece that is made of plastic; this approach permits reducing the number of parts and simplifies the assembly and manufacture procedure. Alternatively, the end block portions and the spanner can comprise separate pieces that have been connected together. The spanners can be made of metal. 
     The tubular member is cylindrical and the end block portions are circular. The outside surface of the spanner abuts the inside surface of the tubular member, holding the spanner in place during assembly and increasing the strength of the completed fuse. The terminals can be insert molded in the end block, or other techniques can be used to retain the terminals in the end block portions. For enduring retention, the terminals are ultrasonically welded to the end block portions. Likewise, the tubular member and the end block portions can be ultrasonically welded to each other. 
     In one embodiment, one open end of the tubular member has a lip that makes the open area of that open end smaller than the open area of the other open end. One of the end block portions has two sections sized so that one of the sections fits in the open area defined by the lip and the other section is larger than the open area defined by the lip. This allows that end block portion to be inserted through the lipless end of the tubular member and secured by interference at the lipped end of the tubular member. The other end block portion also has two sections. For this end block, the smaller section is sized so that it fits into the open area at the lipless end of the tubular member, and the larger section is sized to not fit into this open area. The end block portions and the tubular member are joined by welding, and the surfaces at which they contact can have projections of triangular cross section that serve to direct welding energy. 
     In all embodiments, suitable methods of attaching the fusible element to the terminals include spot welding and ultrasonic welding. The fusible element is preferably corrugated and attached to the long sides of the terminals. It should be understood that multiple fusible elements can be used. 
     Preferably, the housing contains arc-quenching fill, in particular a solid fill. 
     Other advantages and features of the invention will be apparent from the following description of particular embodiments thereof and from the claims. 
     DESCRIPTION OF PARTICULAR EMBODIMENTS 
     Particular embodiments of the invention will now be described. 
    
    
     DRAWINGS 
     FIG. 1 is an exploded perspective view of a fuse according to the invention. 
     FIG. 2 is a side view of the spanner/end block assembly of the FIG. 1 fuse. 
     FIG. 3 is a top view of the FIG. 2 spanner/end block assembly. 
     FIG. 4 is a sectional view of a fuse according to an alternative embodiment of the invention. 
    
    
     STRUCTURE, MANUFACTURE, AND OPERATION 
     Referring to FIGS. 1-3, fuse 10 includes tubular member 12 made of insulative material, spanner/end block assembly 14, consisting of spanners 30 and end block portions 32 and made of insulative material, terminals 16 made of conducting material, and fusible elements 18 made of conducting material. Spanner/end block assembly 14 is sized to fit within tubular member 12, and includes slots 20 and fill holes 22 located on opposite ends of spanner/end block assembly 14. Terminals 16 include external portions 24, internal portions 26, and middle portions 28. External portions 24 have holes 34. Fusible elements 18 are attached to opposite surfaces 36 of internal portions 26. Fusible elements 18 have current limiting notch sections 38 defined by rows of holes and are generally corrugated to provide a relatively larger number of notch sections 38 for a given length of housing than would be permitted if fusible elements 18 were straight. 
     In manufacture, the ends of fusible elements 18 are attached to surfaces 36 by resistance (spot or continuous) welding or ultrasonic welding, and terminals 16 (with elements 18 attached therebetween) are insert molded within spanner/end block assembly 14 during molding of assembly 14. Alternatively, terminals 16 could be held in place in spanner/end block assembly 14 by welding that would cause melting and reflow of the plastic material of the end block portion; e.g., ultrasonic welding could be employed. The subassembly of terminals 16, attached fusible elements 18, and spanner/end block assembly 14 is then inserted in tubular member 12 and joined to tubular member 12. 
     When tubular member 12 and spanner/end block assembly 14 are made of thermoplastic material, they can be joined together by ultrasonic welding. During such welding, one piece is held fixed, while the other is vibrated at 20 KHz and moved toward the first, causing frictional heating and melting. The thermoplastic material has the capability to be melted and reformed while retaining its properties when cooled below its melt point; this is desirable to permit joinder of preformed housing pieces by welding and to avoid the use of adhesives. The material should also have a sufficiently high continuous use temperature so as to maintain structural integrity at elevated temperatures resulting from heating when operating at rated current conditions. Preferably the continuous use temperature (UL746C, 100,000 hour test) is greater than 120° C. Fillers are preferably added to the thermoplastic resins to reduce the cost of the material and to improve the mechanical properties of the plastic by forming a support matrix within the plastic. Fillers tend to increase the continuous use temperature of the thermoplastic material, thereby providing improved structural integrity at elevated temperatures. However, depending on the resin and filler material, increasing filler concentration beyond a certain amount tends to reduce the strength; also, increasing the concentration beyond a certain amount may tend to negatively affect the ability to create strong bonds using ultrasonic welding. It accordingly is desirable to increase the continuous use temperature as much as possible while still achieving good bond strength using ultrasonic welding. Suitable filler materials include fiber glass, calcium carbonate, carbon fiber, cellulose, and graphite fiber. In general, thermoplastic materials with a continuous use temperature above 120° C. and a filler concentration between 20% and 40% (most preferably between 30% and 35%) provide necessary strength at elevated temperature while still permitting processing by ultrasonic welding. The thermoplastic material also preferably includes a flame retardant, is nontoxic (not give off toxins when at elevated temperature), and has high dielectric strength (above 400 volts/mil). 
     A suitable material for the thermoplastic material is glass reinforced polyphthalamide semicrystalline resin containing 33% glass filler available under the Amodel AF-133 VO trade designation from Amoco Performance Products, Inc., Atlanta, GA. This material includes a flame retardant and has a continuous use temperature of 125° C. per UL746C. 
     Other suitable materials include a highly crystalline Nylon 4.6, having 30% glass filler, and available from DSM Corp. under the Stanyl trade designation; polyphenylene sulfide having 30% glass filler and available from Phillips Corp. under the Ryton trade designation; and glass-filled liquid crystal polymers such as Xydar from Amoco, Supec from General Electric, and Vectra from Hoechst Celanese. 
     Another technique for joining tubular member 12 and spanner/end block assembly 14 together is by adhesive bonding, e.g., when the material is thermoset or thermoplastic material. 
     After tubular member 12 and spanner/end block assembly 14 are bonded together, the resulting void space in tubular member 12 is filled with a granular arc-quenching fill material (e.g., 50/70 or 40/60 quartz; not shown) through fill holes 22 located in end block portions 32. When the fuse employs a solid fill, sodium silicate binder solution is introduced through a hole 22 and coats the quartz granules. After the binder solution is cured, fill holes 22 are then sealed with preformed metal plugs 27 or non-conductive potting. 
     Referring to FIG. 4, fuse 40, an alternative embodiment of the invention, includes tubular member 42 made of insulative material and having lip 44, end blocks 46, 48 made of insulative material, spanners 50 preferably made of metal (though other materials can be used), terminals 52 made of conducting material, and fusible elements 54 made of conducting material. End block 46 has an exterior section 56, sized to fit within an area 58 defined by lip 44, and an interior section 60 sized to be larger than area 58 and to fit within the interior of tubular member 42. End block 48 has an interior section 62, sized to fit within the interior of tubular member 42, and an exterior section 64, sized to be larger than the interior of tubular member 42. 
     In manufacture, spanners 50 are connected to end blocks 46, 48 at holes 66. Holes 66 are sized to hold spanners 50 in place by an interference fit; alternatively, spanners 50 could be affixed by ultrasonic or other welding or other means. The spanners could also be provided with shoulders to prevent slippage of the spanners relative to an end block during attachment of the end block to the tubular member. Terminals 52 are inserted into end blocks 46, 48 through slots 68, and fusible elements 54 are attached to opposite surfaces 70 of terminals 52. (Alternatively, the fusible elements could be attached to the terminals first, and the end blocks could be insert molded with the terminals and spanners in place.) Next, the terminal/end block/spanner assembly is inserted into tubular member 42 via open end 72 until interior section 60 of end block 46 contacts lip 44. The terminal/end block/spanner assembly is attached to tubular member 42 using ultrasonic welding. To facilitate the welding process, exterior section 64 of end block 48 and lip 44 can have triangular cross section projections 74 that serve to direct welding energy. The spanners may or may not be removed after the end blocks have been secured to tubular member 42; if they are removed, the holes could be used for introducing fill material. 
     Other embodiments of the invention are within the scope of the following claims. E.g., in addition to circular cross section tubes, other shapes such as squares and hexagons can be used for tubular member 12. Also, other techniques can be used to secure end block portions to tubular casings; e.g,, pins through the casing and end blocks, internal C-rings, external rings, fingers and detents.