Electric fuse having plug terminals
BACKGROUND OF THE INVENTION 
     This invention relates to electric fuses having terminal plugs inside the casing and closing the ends thereof. Plug type terminal fuses are distinguished from ferrule type terminal fuses by their large dimensional stability. Such fuses are disclosed, for instance, in U.S. Pat. No. 2,658,974; Nov. 10, 1953 to Frederick J. Kozacka for HIGH CURRENT CARRYING CAPACITY CURRENT LIMITING FUSES, and in many other patents. 
     One of the disadvantages of prior art plug terminal fuses consists in that the fusible elements thereof were easily distorted by bending or twisting forces. 
     Another of the disadvantages of prior art plug terminal fuses consists in that the positioning of the fusible elements thereof was determined by the positioning of the grooves in the axially inner end surfaces of the plug terminals. This made it difficult to position the fusible elements in the best way required for any particular application. 
     Another limitation of prior art plug terminal fuses resides in the relatively large cross-section and length of the blade contacts which are in excess of the cross-section and length required to reduce the voltage drop to a predetermined minimal value and thus constitute a waste of material, or an over design. 
     It is the principal object of the present invention to provide plug terminal fuses which are not subject to any of the above limitations. 
     SUMMARY OF THE INVENTION 
     Electric low-voltage fuses embodying the present invention include a tubular casing of electric insulating material; a pulverulent arc-quenching filler inside said casing; a pair of plug terminals arranged inside said casing and plugging the ends thereof; grooves in the axially inner end surfaces of said pair of plug terminals; and a fusible element inside said casing, embedded in said arc-quenching filler and forming a portion of a current path conductively interconnecting said pair of plug terminals. 
     The novel feature in fuses according to the present invention consists in that the ends of said fusible element are conductively connected to said pair of plug terminals by a pair of connectors formed by bent metal strips of which the axially inner ends are conductively connected to said fusible element, of which the axially outer ends are inserted into said grooves of said plug terminals and conductively connected to said pair of plug terminals. Said axially outer ends and said axially inner ends of each said metal strips are arranged in parallel, spaced planes, and each said metal strips further includes a transversely arranged intermediate portion conductively connecting said axially inner ends and said axially outer ends of each of said strips. The ends of said strips conductively connected to said plug terminals are preferably much longer than the transversely arranged portions thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is substantially a longitudinal section of a fuse embodying this invention having a channel-shaped fusible element whose web portion is situated to the right of FIG. 1; 
     FIG. 2 is substantially a longitudinal section taken along II--II of FIG. 1; 
     FIG. 3 shows the same structure as FIGS. 1 and 2 taken along III--III of FIG. 1; 
     FIG. 4 is substantially a longitudinal section of a fuse embodying the present invention, but having a higher current-carrying capacity than the fuse shown in FIGS. 1--3; 
     FIG. 5 is a longitudinal section of the fuse shown in FIG. 4 taken along a plane at right angles to the plane along which FIG. 4 is taken; 
     FIG. 6 is a section along VI--VI of FIG. 5; 
     FIG. 7 is a longitudinal section of a multifuse element fuse wherein each of the fusible elements are arranged to cooperate with a large amount of arc-quenching filler; 
     FIG. 8 is a longitudinal section of the fuse shown in FIG. 7 sectioned by a plane at right angles to the plane that sectioned FIG. 7; 
     FIG. 9 is a section along IX--IX of FIG. 7; and 
     FIG. 10 is a section along X--X of FIG. 7. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, and more particularly to FIGS. 1-3 thereof, reference character 1 has been applied to indicate a tubular casing of electric insulating material such as, e.g., a laminate of glass-cloth and a synthetic resin. Casing 1 is filled with a pulverulent arc-quenching filler 2 such as, for instance, quartz sand. A pair of plug terminals 3 is arranged inside of casing 1 and plugs the ends thereof. The axially inner end surfaces of plug terminals 3 are provided with a pair of grooves 4. Plug terminals 3 and casing 1 may be connected by fasteners, such as steel pins. This is well known in the art and shown, for instance, in U.S. Pat. No. 2,740,187; 04/03/56 to P. C. Jacobs, Jr. et al for METHODS OF ASSEMBLING FUSES. Fusible element 5 is arranged inside of casing 1 and submersed in arc-quenching filler 2. Fusible element 5 and terminals 3 are conductively connected by connectors or metal strips generally indicated by reference numeral 6. Each of connectors 6 is formed by a metal strip that is bent at 90 degrees. The axially inner ends 6a of metal strip 6 are conductively connected to fusible element 5. In the embodiment shown, fusible element 5 has multiple perforations and is channel-shaped. It may, however, take other shapes, e.g. it may be planar, or L-shaped. The axially outer ends 6b of connector-forming metal strips 6 are inserted into grooves 4 of plugs 3, and conductively connected to plugs 3. This may be effected by solder joints (not shown). The axially outer ends 6b and the axially inner ends 6a of each metal strip 6 are arranged in parallel spaced planes which makes it possible to arrange fusible elements in a pattern other than that defined by grooves 4, as shown more specifically in FIGS. 4-10. Metal strips 6 further include intermediate portions 6c arranged transversely to the portions 6a and 6b and conductively interconnecting portions 6a and 6b. The fusible element proper is perforated and may be provided with an M-effect causing overlay 7 of a metal having a lower fusing point than the base metal, e.g. silver or copper, which supports the overlay metal. Fusion of the overlay metal initiates a metallurgical reaction, or diffusion, which results in severing of fusible element 5 and of the current-path through the fuse. If desired, each plug terminal 3 may be provided with a groove at the axially outer end surfaces thereof for receiving a blade contact 8. The axially inner ends of blade contacts 8 may be inserted into said grooves and affixed to terminal plugs 3 by hard solder joints. 
     The fusible element 5 is of relatively thin, low resistivity metal, and said metal strips are of a relatively thick metal having a relatively low resistivity. This allows with a fuse closed by a pair of plug terminals to achieve all the advantages of a fuse with cap terminals or ferrules as disclosed in the co-pending patent application of Robert J. Panaro; 08/30/79, Ser. No. 073,079 for ELECTRIC FUSE HAVING COMPOSITE FUSIBLE ELEMENT. Fusible element 5 may be made of relatively thin sheet silver and metal strips 6 may be made of relatively thick copper having a thickness in the order of at least twice but less than five times the thickness of the fusible element 5. This means that strips 6 must have a much lower resistance than the fusible element 5, or that the lower resistivity of silver per unit of length greatly increased by the perforation therein is by far overcompensated by the higher thickness of the copper strip 6. In other words, the resistance of fusible element 5 exceeds the resistance of both metal strips 6. 
     It is also possible to make both the fusible element 5 and strip 6 of the same metal, particularly copper. In that instance the fusible element 5 is of relatively thin sheet copper, or of a small resistivity alloy thereof, and metal strips 6 are of relatively thick copper having a thickness of twice, or more than twice, but less than five times the thickness of fusible element 5. The reasons for these critical values are more fully explained in the above patent application of R. J. Panaro. The thickness of metal strips 6 must be at least in the order of two times the thickness of fusible element 5 to minimize the voltage drop across the fuse, and hence the energy consumed by the fuse. The thickness of metal strips 6 must be less than five times that of the fusible element 5 because the reduction in voltage drop beyond that level is not worth the additional amount of copper for strips 6 needed to achieve it. In other words, it is a matter of decreasing return. 
     The perforated fusible element 5 does not necessarily have to be channel-shaped. It may, for instance, be planar. This allows a relatively high heat transfer from perforated fusible element 5 into the surrounding arc-quenching filler 2, and a relatively small increase of the thickness of strips 6 relative to that of fusible element 5, e.g. a thickness ratio of about 2:1. On the other hand a channel-shaped configuration of the fusible element may require a higher thickness ratio such as, e.g., a thickness ratio of 4.5:1. 
     Referring now to FIGS. 4 to 6, in these figures an M-effect overlay on the fusible elements 5&#39; has been deleted, but such an overlay may be applied where necessary, or desirable, e.g. to limit the maximal rise in temperature of fusible elements 5&#39;. 
     For reasons of greater clarity the fastener means tying together plug terminals 3&#39; and casing 1&#39; have likewise been deleted. 
     Like parts as in FIGS. 1-3 have been designated in FIGS. 4-6 by the same reference characters with a prime added, and have been designated in FIGS. 7-10 by the same reference characters with two primes added. 
     Referring now more specifically to FIGS. 4-6, numeral 1&#39; has been applied to indicate a tubular casing of electric insulating material filled with a pulverulent arc-quenching filler 2&#39; and plugged by a pair of terminal plugs 3&#39; inside of casing 1&#39;. Grooves 4&#39; in the axially inner end surfaces of plug terminals 3&#39; receive the axially outer ends of metal strips 6&#39; which may be connected to grooves 4&#39; of plug terminals by soft solder joints. The fusible element proper 5&#39; may be either of silver or of copper, or of a low resistivity alloy thereof. The portions of casing 1&#39; not occupied by other parts are filled with an arc-quenching filler 2&#39;, preferably quartz sand. The fusible elements 5&#39; are arranged in two parallel planes having a given spacing 5&#39;. The spacing of the grooves 4&#39; is much smaller than that of fusible elements 5&#39;. If, as in the instant case, the fusible elements are channel-shaped, the spacing 5&#39; is the spacing between their web portions. If the fusible elements 5&#39; are planar, their spacing is the shortest distance between the planes thereof. Blade contacts 8&#39; may project from plug terminals 3&#39; as in the embodiment of the invention shown in FIGS. 1-3. The planes defined by grooves 4&#39;, and the planes defined by fusible elements 5&#39;, or the web portions thereof, are all parallel, but s&#39;&gt;S&#39;. Furthermore, the length L of arms 6b&#39; by far exceeds the length o of arms 6c&#39;. These proportions result in a great flexibility of units 5&#39; and 6&#39;, and in a favorable spacing of fusible elements 5&#39; which, in turn, results in an efficient use of the heat absorbing capacity of arc-quenching filler, or quartz-sand 2&#39;. 
     The perforated channel-shaped center portions or fusible element portions 5&#39; may be of silver, and each of the non-perforated end portions 6&#39; may be of copper having a thickness of more than twice, e.g. 4.5 times, the thickness of the perforated center portions 5&#39;. As an alternative, the perforated center portions 5&#39; and the non-perforated strips, or ends 6&#39;, may be both of copper, or a relatively low resistivity alloy thereof, the thickness of the latter being e.g. about 4.5 times that of the former. 
     Referring now to FIGS. 7-10, casing 1&#34; is filled with a pulverulent arc-quenching filler 2&#34; and plugged by plug terminals 3&#34;. It houses six fusible elements 5&#34; which are arranged in such a way as to maximize the amount of arc-quenching filler 2&#34; active in solidifying the gases resulting from vaporization of fusible elements 5&#34;. The fusible elements 5&#34; are provided with strips 6a&#34;, 6b&#34;, 6c&#34; of which sections 6b&#34; project into grooves 4&#34; in plugs 3&#34;, sections 6a&#34; are conductively connected to fusible elements 5&#34; and are arranged in spaced parallel relations from sections 6b&#34;, and sections 6a&#34; are arranged at right angles to sections 6b&#34; and 6c&#34;. There are but four grooves 4&#34; in terminal plugs 3&#34;, each of which is arranged in one of four parallel planes A,B,C,D. Yet the spacing of the planes of fusible elements 5&#34; from that of sections 6b&#34; allows a favorable configuration of fusible elements 5&#34; in regard to casing 1&#34;, in addition to increasing the flexibility of units 5&#34;, 6b&#34;, 6c&#34;, 6a&#34; and in addition to reducing the voltage drop across the fuse. 
     In the structure shown in FIGS. 7-10 there are two channel-shaped fusible elements 5&#34; whose axially outer strips 6b&#34; are located in plane D and whose flange portions are directed upwardly (as seen in FIG. 10). There is one channel-shaped fusible element 5&#34; whose axially outer strips 6b&#34; are located in plane C and whose flange portions are directed downwardly (as seen in FIG. 10). There are two channel-shaped fusible elements 5&#34; whose strip portions 6b&#34; are located in plane B and whose flange portions are directed upwardly (as seen in FIG. 10). And there is one channel-shaped fusible element 5&#34; whose strip portions 6b&#34; are located in plane A and whose flange portions are directed downwardly (as seen in FIG. 10). 
     Such a geometry provides a large interface between the fusible elements 5&#34; and the arc-quenching filler 2&#34; into which the gases resulting from vaporization of fusible elements 5&#34; escape. The spacing of the fusible elements 5&#34; of FIGS. 7-10 has proven desirable, though the upper and the lower portions of FIG. 10 are not symmetrical. 
     It will be observed that all the grooves in all the embodiments of the invention are parallel. Hence the grooves 4,4&#39; and 4&#34; in each plug terminal can be produced by a single operation of a gang of cutters. 
     In the structure of FIGS. 7-10 there is a pair of grooves 4&#34; in each the opposite axially end surfaces of terminal plugs 3&#34;. Said pair of grooves 4&#34; is situated at opposite sides of a plane at 90 degrees to said end surfaces. A plurality of fusible elements 5&#34; is arranged to different sides of, and in spaced relation from, said common plane. Said plurality of fusible elements is connected by a plurality of metal strips 6&#34; to the grooves 4&#34; in plug terminals 3&#34;. Said metal strips 6&#34; have axially outer ends 6b&#34; engaging said pair of grooves 4&#34; and conductively connected to said pair of plug terminals 3&#34;. Said metal strips 6&#34; have axially inner ends 6a&#34; conductively connected to the axially outer ends of said plurality of fusible elements 5&#34;, and said metal strips have intermediate portions 6c&#34; angularly related to and conductively interconnecting said outer ends 6b&#34; and said inner ends 6a&#34; of said plurality of metal strips. Some of said intermediate portions 6 c&#34; are arranged to one side and others of said intermediate portions 6c&#34; are arranged to the other side of the aforementioned plane. 
     To maximize flexibility the length relation between the strip portions 6b&#34; and 6c&#34; of the structure of FIGS. 7-10 ought to be the same as in the structure of FIGS. 4-6. 
     The ratio of thickness between strips and fusible elements set forth in connection with FIGS. 4-6 applies also to the embodiment of the invention shown in FIGS. 7-10. 
     The term low fusing i 2  ·t metal is applied to encompass both silver, copper and alloys thereof that have substantially the same conductivity as silver and copper.