Abstract:
A current-limiting fuse including a fusible element that comprises a relatively long portion of sheet copper and one or two end portions of sheet silver. The portions of sheet silver are each shorter than the portion of sheet copper. The portion of sheet copper is perforated, establishing points of reduced cross-section. The portion or portions of sheet silver are likewise perforated. The points of reduced cross-section established by the perforation, or perforations, in the portion or portions of sheet silver have a considerably smaller cross-section than the points of reduced cross-section in the portion of sheet copper.

Description:
BACKGROUND OF THE INVENTION 
     The closest prior art known is U.S. Pat. No. 2,653,203; 09/22/53 to F. J. Kozacka for CURRENT-LIMITING FUSE. This patent discloses a fuse having one or more fusible elements of a singel metal, rather than a fuse having a composite fusible element made of two different metals. While the fuse according to the above patent and the fuse according to this invention are based on the same principle, the instant fuse has great advantages over the above prior art fuse--particularly as current-carrying capacity is concerned--as will become more apparent from what follows. 
     SUMMARY OF THE INVENTION 
     A current-limiting fuse according to this invention includes a casing of electric insulating material, preferably of a synthetic-resin glass-cloth laminate. There is a pulverulent arc-quenching filler inside the casing and the latter is closed on both ends thereof by a pair of electro-conductive terminal elements. A fusible element is embedded in said arc-quenching filler and conductively interconnects said pair of terminal elements. The fusible element includes a relatively long portion of sheet copper having at least one point of reduced cross-section. Said fusible element further includes at least one relatively short end portion of sheet silver conductively interconnecting one of said pair of terminal elements and one end of said portion of sheet copper. Said portion of sheet silver has a single point of reduced cross-section, and said single point has a smaller cross-section than any point of reduced cross-section in said portion of sheet copper. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top-plan view of a fusible element according to the present invention; 
     FIG. 2 is a side elevation of the fusible element shown in FIG. 1; 
     FIG. 3 is a longitudinal section of a fuse including a fusible element as shown in FIGS. 1 and 2; 
     FIG. 4 is a cross-section of the fuse shown in FIG. 3 along IV--IV of FIG. 3; 
     FIG. 5 is a modification of the fusible element of FIG. 1 shown in top-plan view; 
     FIG. 6 is a side elevation of the fusible element shown in FIG. 5; 
     FIG. 7 is a longitudinal section of a fuse including the fusible element shown in FIGS. 5 and 6; and 
     FIG. 8 is a cross-section along VII--VII of the fuse shown in FIG. 7. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 to 4, numeral 1 has been applied to indicate a tubular casing of electric insulating material, preferably a synthetic resin glass-cloth laminate which is filled with a granular arc-quenching filler 2, e.g. quartz sand. A pair of electro-conductive terminal elements 3 close the ends of casing 1. Terminal elements 3 may, for instance, be in the shape of caps mounted on the outer ends of casing 1. A fusible element generally indicated at 4 embedded in arc-quenching filler 2 conductively interconnects said pair of terminal elements. Fusible element 4 includes a relatively long portion 4a of sheet copper having at least one point of reduced cross-section. As shown in the drawings, sheet copper portion 4a has four serially arranged points of reduced cross-section 4a&#39; to generate a relatively high arc voltage upon fusion thereof. Fusible element 4 further includes at least one relatively short end portion 4b of sheet silver conductively connecting one of said pair of terminal elements 3 and one end of said portion of sheet copper 4a. The end portion 4b of sheet silver has one single point of reduced cross-section 4b. The provision of an overlay 5 of an M-effect metal is optional. As generally known in the art, M-effect overlays are used either when it is desired to limit the highest temperature which a fuse may reach, or when it is desired to provide overload protection in addition to short-circuit protection. The portions 4a, 4b of the fusible element 4 overlap at 6 and are conductively bonded together at the point where they overlap as, for instance, by spot welding. The portion of sheet silver 4b  is inserted into a slot in a washer 7 of fibrous material, bent a first time 90 deg. at the center of cap 3, then bent a second time 90 deg. about the rim of casing 1 and bent at its end to the outer surface of cap 3. The left end of sheet silver portion 4b is spot-welded at 3a to the left terminal cap 3. This mode of conductively connecting portion 4b of fusible element 4 to the left cap 3 is, however, not the most economical since it requires a relatively long length for silver portion 4b of fusible element 4. 
     In view of the recent steep increase in the price of silver it is desirable to keep the silver portion 4b of fusible element 4 as short as possible. This is not achieved in FIG. 3 of the drawing since the fusible silver section 4b is shown to be bent to outer surface of the casing, bent around the axially inner edge of terminal cap 3 and spot-welded at 3a to cap 3. A more economical or silver-saving way would be to conductively connect the axially outer end of sheet silver portion 4b directly to the inner end surface of cap 3, or to conductively connect a point close to the center of terminal cap 3 to the axially outer end of sheet silver portion 4b of fusible element 4. By so doing, the length of the silver portion 4b of the fusible element between the end surface of cap 3 and weld 3a can be saved. This can be achieved by the process known as blind soldering which relies on solder in paste form for establishing inaccessible solder joints. 
     In FIGS. 5 to 8 the same reference characters as in FIGS. 1-4 have been applied to designate like parts. It is, therefore, sufficient to describe FIGS. 5 to 8 only to the extent that the structure shown in these figures differs from that shown in FIGS. 1 to 4. 
     According to FIGS. 5 to 8 the center portion 4a of sheet copper has two end portions 4b of sheet silver. The center portion 4a of sheet copper may have one or several points of reduced cross-section 4a&#39; whose cross-section is relatively large, while each of the end portions 4b of sheet silver has but one single point of reduced cross-section whose cross-section is relatively small. The length of center portion 4a exceeds the length of each end portion 4b. Portions 4a,4b are spot-welded together at points 6 where they overlap. 
     The structures shown in the drawings operate as follows: 
     Under load-current and overload current conditions points 4b &#39; of sheet silver ends 4b will generate much heat, particularly because they are the points in the fusible element having the smallest cross-section. But since points 4b are so closely positioned to the terminals 3 which, in turn, remain cool because of the fact that they are in contact with a fuse holder having a large heat absorbing and heat dissipating capacity, all the heat generated at points 4b&#39; will readily be dissipated by an axial heat flow toward caps 3 and the fuse holder which is in engagement with caps 3. As a result, the points of reduced cross-section 4b&#39; will have no or very little effect in the load and overload range on the current-carrying capacity of the fusible element and the current rating of the fuse. 
     The situation is very different under short-circuit-current-like conditions. This is due to the simultaneous occurrence of three conditions indicated below: 
     (a) Under short-circuit-current-like conditions the rate of heating of the points of reduced cross-section 4b&#39; is so fast that none of the heat generated therein by i 2  ·r losses can escape prior to melting of point or points 4b&#39; . In other words, the process of melting the points 4b&#39; occurs so fast that there is no heat loss between the calculated and the actual energy required to melt the points of reduced cross-section 4b&#39;,i.e. the melting process of the points of reduced cross-section is an adiabatic process. 
     (b) since the cross-section of points 4b&#39; is less than the cross-section of points 4a&#39;, the former will tend to fuse ahead of the time required for fusing points 4a&#39;. 
     (c) The melting time of points 4b&#39; is much less than the melting time of points 4a&#39;, because the melting i 2  ·t of silver in terms of (amp/cm. 2 ) 2  ·sec. is much less than that of copper. 
     The arc voltage generated at point or points 4b&#39; will keep the arc-current from rising, or rising significantly, during a short while. Thus the arc-voltage generated at the point or points 4b&#39; causes a delay in the rise of the current sufficient for the points of reduced cross-section 4a to melt in accordance with the i 2  ·t of points 4a, and the latter to generate the arc-voltage required to bring the fault current rapidly down to zero. 
     It should be understood that where the term point of reduced cross-section or similar expression is used in this context, this means that the total cross-section at this point is reduced, but not necessarily that the particular point does not include a plurality of parallel current paths.