Electric fuse for elevated circuit voltages
United States Patent Kozacka 1 Apr. 29, 1975 1 ELECTRIC FUSE FOR ELEVATED CIRCUIT VOLTAGES [75] Inventor: Frederick J. Kozacka, South Hampton. NH.  
 [73] Assignee: The Chase-Shawmut Company,  
 Newburyport. Mass.  
  221 Filed: Jan. 3, 1974 [21] App1.No.:430,477  
 [521 U.S. Cl. 337/159; 337/161; 337/295 [51] Int. Cl. H0lh 85/14 [58] Field of Search 337/161. 231. 295, 159  
 [56] References Cited UNITED STATES PATENTS 3.573.699 4/1971 Salzer 337/161 3.810.061 5/1974 Salzer 337/295 X Prinmry Examiner-R. N. Envall. Jr. Attorney, Agent. or FirmErwin Salzer [57] ABSTRACT In a fuse for elevated circuit voltages having a tubular casing closed by a pair of terminal plugs the latter are tied together by a plurality of parallel rods of a gas evolving insulating material These rods form a support for a fusible element which is wound helically around the rods. The rods are braced by an annular member tending to preclude bending thereof in radially inward direction.  
 1 Claim, 2 Drawing Figures BACKGROUND OF THE INVENTION In recent years there has been a tendency of substituting for the conventional supports for helically wound fusible elements in the form of mandrels which are star-shaped in cross-section other types of supports which are less expensive, less bulky and have better electrical performance characteristics. U.S. Pat. No. 3,680,019 to Frederick J. Kozacka, issued July 25, 1972 for HIGH-VOLTAGE FUSE HAVING A PLU- RALITY OF FUSE LINKS WOUND HELICALLY AROUND AN INSULATING MANDREL shows a fuse wherein the conventional fuse link supporting mandrel which is star-shaped in cross-section is replaced by four separate plates of gas evolving material. The cost of four separate plates for supporting a helically wound fusible element is less than that of a single supporting mandrel which is star-shaped in crosssection, and four such plates tend to occupy a smaller volume than a single supporting mandrel which is starshaped in cross-section. Therefore the provision of fuse link supporting mandrels formed of a plurality of separate fuse-link-supporting plates makes it possible to increase the content of arc-quenching filler in a high voltage fuse, all other parameters remaining unchanged. Using link-supporting mandrels formed by separate plates rather than such mandrels formed by a single solid column which is star-shaped in crosssection also reduces the danger of formation of conductive surface layers by metal deposition incident to blowing of the fuse. It is the principal object of this invention to further improve the mandrel structure shown in the aforementioned U.S. Pat. No. 3,680,019.  
  In the structure shown in the above patent the gasevolving insulating plates must have good dimensional stability. To obtain the required degree of dimensional stability, the plates are rectangular in cross-section, and their wide sides are oriented radially in regard to the casing or fuse tube, and in regard to its cylindrical plug terminals. One limitation of this structure lies in the fact that the ratio of the long side to the short side of the rectangular cross-section of the mandrel-forming plates must be large or, in other words, that the mandrel-forming plates must be relatively wide in order to achieve the required dimensional stability. This is a serious limitation because of the relatively high price of high-grade gas evolving insulating materials, e.g. highgrade glass-cloth melamine laminates. Another limitation of the structure of the above referred-to U.S. Pat. No. 3,680,019 resides in the fact that it has a length ceiling and consequently a voltage ceiling, i.e. it is inapplicable, or impractical, if the length of the casing exceeds certain values necessary to achieve a predetermined voltage rating. Still another limitation of the structure of the above referred-to U.S. Pat. No. 3,680,019 lies in the fact that&#39;each of its plug terminals must be provided on the inner end surface thereof with a pair of plate-receiving grooves intersecting at right angles, which requires relatively expensive machining operations.  
  It is the prime object of this invention to improve fuse structures of the kind disclosed in the above referred-to U.S. Pat. No. 3,680,019, and more particularly to remove by appropriate changes or modifications the above referred-to limitations which are inherent in the original form of such structures.  
 SUMMARY OF THE INVENTION Fuses embodying this invention include a tubular casing of electric insulating material and a pair of cylindrical plug terminals plugging the ends of said casing and being affixed to said casing.  
  The axially inner end surfaces of said pair of plug terminals are provided with registering cylindrical blind bores arranged to form substantially circular patterns coaxially related to said pair of plug terminals. A plurality of parallel rods of gas evolving material have ends fitting into and inserted into said blind bores. These rods form jointly with said pair of plug terminals a squirrel-cage-like structure. A fusible element is wound helically around said plurality of rods embedded in a pulverulent arc-quenching filler, and the ends of the fusible element are conductively connected to said pair of plug terminals.  
  Fuses embodying this invention further include at least one annular member of electric insulating material arranged in coaxial relation to said pair of plug terminals at a point intermediate the ends of said plurality of rods and forming a brace for said plurality of rods tending to preclude bending of said plurality of rods in radially inward direction. Said annular brace member is provided with a plurality of angularly displaced grooves at the radially outer periphery thereof and each of said plurality of grooves is engaged by one of said plurality of rods.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a fuse embodying this invention and shows the constituent parts thereof partly in elevation and partly in section; and  
  FIG. 2 is a cross-section of the fuse structure shown in FIG. 1.  
 DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, numeral 1 has been applied to indicate a tubular casing of electric insulating material which is plugged on both ends thereof by a pair of plug terminals 2. Plug terminals 2 are affixed to casing l by steel pins 3 projecting through casing 1 into plug terminals 2. Reference numeral 4 has been applied to indicate a plurality of parallel rods of gasevolving insulating material having ends affixed to juxtaposed end surfaces of terminal plugs 2. Rods 4 are preferably of glass-cloth melamine. The number of rods 4 is preferably four. The axially inner end surfaces of plug terminals 2 are provided with bores or blind holes 5 arranged in circular patterns coaxial with casing l and plug terminals 2. The ends of rods 4 are press-fitted into blind bores 5. If desired some cement may be placed into blind holes 5 to more firmly affix the ends of rods 4 to plug terminals 2. Blind holes 5 will normally be circular in cross-section and rods 4 may be square in cross-section. Then each rod 4 may frictionally engage each blind hole 5 along four generatrices of the latter. Rods 4 and plug terminals 2 jointly form a squirrel-cage-like structure. Reference numeral 6 has been applied to indicate a fusible element formed by a ribbon of sheet silver wound helically around rods 4. The ends of fusible element 6 are conductively connected to terminal plugs 2. This may be achieved by the means which are described in detail in U.S. Pat. No.  
 3,571,775 to Frederick J. Kozacka et al., Mar. 23, 1971 for HIGH VOLTAGE FUSE HAVING A PLURALITY OF HELICALLY WOUND RIBBON FUSE LINKS. Casing l is filled with a pulverulent arc-quenching filler 7, preferably quartz sand, which embeds fusible element 6.  
  Reference character 8 has been applied to indicate an annular member of electric insulating material arranged in coaxial relation to plug terminals 2 at a point intermediate the ends of rods 4. Annular member 8 forms a brace for rods 4 tending to preclude bending of rods 4 in radially inward direction indicated by arrows R in FIG. 2. Since ribbon fuse link 6 is tightly wound around rods 4 there is a tendency for rods 4 to bend radially inwardly, which tendency is counteracted by member 8. Even though the stiffness of rods 4 in the absence of member 8 may be small, bending of rods 4 is effectively prevented by member 8.  
  It will be understood that the number of annular braces 8 to be arranged between the ends of rods 4, or inside of the space bounded by plug terminals 2, may be increased as present circumstances may require.  
  It will be further apparent from FIG. 2 that annular bracing member 8 is provided with a plurality of angularly displaced grooves 9 at the radially outer periphery thereof, each of these grooves 9 being engaged by one of rods 4. Annular bracing member 8 may be formed, and is preferably formed, by a stamping in which case annular member 8 is substantially planar.  
  Reference character s has been applied to indicate the spacing of the ends of rods 4 from the centers of the juxtaposed end surfaces of plug terminals 2, and reference character S has been applied to indicate the spacing of the points of engagement of rods 4 and bracing member 8 from the center of the latter.  
 As a result, annular bracing member 8 imparts a radially outwardly directed thrust to rods 4, causing a slight bending thereof in directions opposite to the directions of arrows R of FIG. 2 or in radially outward directions. The spring bias thus imparted to rods 4 tends to significantly increase the dimensional stability of squirrelcage-like structure 2,4,6,8 prior to being press-fitted into casing 1.  
  It will be understood that the cross-section of rods 4 does not need to be square, but might be round, or might be rectangular as shown in the above referred-to US. Pat. No. 3,680,019. The structure according to the present invention allows to use rods of rectangular cross-section which as such have a smaller dimensional stability or stiffness than the rods of US. Pat. No.  
 3,680,019 yet, as a part of the composite structure 2,4,8 the same rods 4 will have a greater dimensional stability than in their prior art environment.  
  Blind bores 5 into which rods 4 are fitted are easier to machine than the plate-receiving grooves of US. Pat. No. 3,680,019.  
  The central opening 8a in the annular members 8 is necessary in order to make it possible to fill casing l with a pulverulent arc-quenching filler 9. Opening 8a must not necessarily be circular, but may have any other desired shape. The fuse shown is not provided with a blown fuse indicator. If provided with such an indicator the fusible element for controlling the latter is threaded through opening 8a.  
 I claim as my invention:  
  1. An electric fuse for elevated circuit voltages including a. a tubular casing of electric insulating material;  
 b. a pair of cylindrical plug terminals plugging the ends of said casing, the axially inner end surface of each of said pair of plug terminals being provided with registering blind bores arranged to form substantially circular patterns coaxially related to said pair of plug terminals;  
 c. a plurality of parallel rods of gas-evolving insulating material having ends fitting into and inserted into said blind bores and forming jointly with said pair of plug terminals a squirrel-cage-like structure;  
 d. a fusible element wound helically around said plurality of rods embedded in a pulverulent arcquenching filler and having ends conductively connected to said pair of plug terminals, the axial ends of said plurality of rods having a predetermined spacing from the centers of said axially inner end surfaces of said pair of plug terminals; and  
 e. a planar annular brace member of electric insulating material arranged in coaxial relation to said pair of plug terminals at a point intermediate said ends of said plurality of rods, said annular brace member being provided with a plurality of angularly displaced grooves at the radially outer periphery thereof, each of said plurality of grooves being engaged by one of said plurality of rods, the points of engagement of said plurality of rods and of said annular brace member having a larger spacing from the center of said annular member than said predetermined spacing so that said annular member imparts a radially outwardly directed thrust to said plurality of rods.