Patent Publication Number: US-4059816-A

Title: Electrical loadbreak fuse and canister assembly

Description:
This is a continuation of application Ser. No. 633,692 filed Nov. 20, 1975, which is a continuation of application Ser. No. 545,782 filed Jan. 31, 1975, both abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to the combination of a current-limiting fuse and a loadbreak device for protecting an electrical apparatus such as a distribution transformer. 
     II. Description of the Prior Art 
     In the past various arrangements have been provided for utilizing protective fuses with transformers in order to protect the transformer from destruction by large fault currents. At one point in time the protective arrangement included a protective fuse link and a separate loadbreaking switch connected in series with a transformer winding. As the capacities of electrical distribution systems were increased, to deliver higher levels of power, increased power became available at a fault, and current-limiting fuses were provided for interrupting these large fault current. More recently the current limiting and loadbreaking functions have been combined in a single assembly such as disclosed in U.S. Pat. No. 3,628,092, issued Dec. 14, 1971. Further improvements on the Electrical Inductive Apparatus with Removable Protective Fuse, disclosed in U.S. Pat. No. 3,628,092, are claimed to be shown in U.S. Pat. No. 3,732,517 issued May 8, 1973 and in U.S. Pat. No. 3,792,215 issued Feb. 12, 1974. Both of the latter mentioned patents are directed toward improving the performance of the device with respect to interrupting the current and extinguishing the arc which develops upon interruption of the current. However, it has been found that there is still a need for further improvement in reliability of the current interrupting and arc extinguishing abilities of such combination current-limiting and loadbreaking devices. Further, it would be desirable to provide these improved performances with a simplified structure utilizing a minimum number of components. 
     Therefore, it is accordingly an object of this invention to provide a combination current-limiting and loadbreaking device which more reliably extinguishes the arc generated upon loadbreaking. Further, it is another object of the invention to provide the combined functions of current-limiting and loadbreaking with a mininum number of components in a device which is readily assembled but which still provides the desired improvement in operating performance. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are accomplished in accordance with this invention, in one form thereof, by providing a combined current-limiting and loadbreaking device housed within a canister formed of an insulating tube. A flange or mounting means is provided at one end of the tube for mounting the canister on a wall of the housing enclosing the electrical apparatus to be protected by the device. The canister, which extends into the housing, is closed at the inner end by an assembly which forms a loadbreak contact member of the combination fuse and loadbreak device. This loadbreak contact member extends into the insulating tube and is surrounded by a sleeve formed of an insulating arc extinguishing material. An insulating spacer and a non-loadbreak contact assembly are secured to one end of a current-limiting fuse, while a loadbreak contact probe having an insulating tip is secured to the opposite end of the fuse. The end of the insulating spacer remote from the fuse is provided with a cap which covers the external end of the canister and with a means for removing the fuse and loadbreak contact member from the canister. A non-loadbreak contact ring is provided inside the canister for engaging the non-loadbreak contact assembly connected to the fuse. An electrical connection portion extends through the canister for making electrical connection to the non-loadbreak contacts external to the canister within the housing. A barrier of an insulating and arc extinguishing material closely spaced from the inner wall of the canister is supported between the loadbreak contact probe and fuse. This barrier, along with an enlarged portion of the sleeve closely spaced from the inner wall of the canister, confine the hot gases resulting from the arcing which may occur between the loadbreak contacts and serve to cool and defuse the ionized gases generated by the arc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention itself, however, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an elevation view, partially in cross-section, showing an electrical loadbreak fuse and canister assembly constructed in accordance with the preferred embodiment of this invention; 
     FIG. 2 is a cross-sectional view of the nonloadbreak contacts taken along the line 2--2 of FIG. 1; 
     FIG. 3 is an exploded perspective view of the loadbreak contacts shown in FIG. 1; 
     FIG. 4 is an enlarged elevation view partially in cross-section with the loadbreak contacts shown in FIG. 1 in the engaged position; 
     FIG. 5 is an enlarged elevation view partially in cross-section with the loadbreak contacts shown in FIG. 1 in the disengaged position; 
     FIG. 6 is an enlarged elevation view, partially in cross-section of the loadbreak contacts shown in FIG. 1 in the position just prior to engagement or just after disengagement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, and more particularly to FIGS. 1, 2 and 3, a loadbreak fuse and canister assembly 2, constructed in accordance with the preferred embodiment of this invention is shown. The canister is in part formed of a cylindrical insulating tube 4 which is in the preferred embodiment a filament-wound oil-proof, glass-filled epoxy tube, the outer periphery of which has a resin rich surface. A first or outer end 6 and a second or inner end 8 of the tube 4 are each provided with external threads. The tube 4 is supported by a mounting means, such as a flange 10 on a wall 12 of a sealed enclosure housing an electrical device to be electrically protected by the assembly 2. In the preferred embodiment flange 10 is formed as a cast aluminum member with internal threads for receiving the external threads on the outer end 6 of the tube. The flange 10 is provided with space apertures, only one of which 14 is shown for receiving studs 16 which are used to secure the flange to the wall 12. 
     In a typical installation of the assembly 2, the wall 12 would be a part of the enclosure for a distribution transformer. The enclosure of which the wall 10 is a part, would contain the transformer and would be substantially filled with oil in which the transformer core and coil are emersed. The enclosure would be sealed so as to prevent atmospheric contamination of the oil. In order to provide a sealing arrangement between the flange 10 and the wall 12, an annual groove 18 is provided in the flange and an annular resilient washer 20 is placed therein. The flange 10 is secured to the side wall 12 of the enclosure by the studs 16 (one of which is shown), and nuts 22 (one of which is shown). 
     Secured to the inner end of the tube 4 is a closure and loadbreak contact assembly 24. In the preferred embodiment of this invention a hub 26 is formed as a cast aluminum member which is internally threaded to receive the threaded inner end 8 of tube 4. A threaded bore 28 is provided on the inner surface of the hub 26 for threadedly receiving and supporting load break contact 30. As is best seen in the exploded prospective view of FIG. 3, the loadbreak contact 30 is a tulip type contact having a plurality of fingers 32 extending from a cylindrical externally threaded base 34. This contact may, for instance, be machined from a hard tempered copper rod. The fingers 32 are provided with a groove 36 near their free end for receiving a coil spring 38 which exerts an inward force on the fingers. Threaded base 34 of the contact 30 is received in the threaded bore 28 in the hub 26. 
     An insulating sleeve 40 having a stepped internal diameter is provided with internal threads 42 at one end for engagement with the threaded-base 34 of the contact 30. The free or inner end of the sleeve 40 is provided with a conical opening 44, the smaller end of which opens into a bore of reduced cross-section 46 in sleeve 40. The portion of the sleeve 40 surrounding the finger 32 of contact 30 has a bore of enlarged cross-section 48. The outer wall of the sleeve 40 is provided intermediate its length with a portion 50 of enlarged cross-section. The perimeter of this enlarged portion substantially conforms to the inner wall of the tube 4, being spaced apart only sufficiently to provide clearance for assembly. 
     A non-loadbreak contact ring 52 is formed of a resilient conductive material such that when placed in the tube 4 it expands to engage the inner wall of the tube. The contact ring 52 is preferably formed of a brush alloy beryillium copper, the natural diameter of which is larger than the internal diameter of the tube 4, such that spring pressure will be exerted against the tube by the contact ring. A connector stud 54 extends from the contact ring 52 through an aperture 56 provided in the wall of the tube 4. The inner surface of the tube 4 may be roughened in the area of the contact ring 52 and an adhesive provided between the inner wall of the tube and the contact ring to assure the permanent placement of the contact ring. 
     In order to obtain a positive seal between the threads on flange 10 and hub 26 and the threaded ends of tube 4, the threads are coated with a room temperature vulcanizing material prior to assembly. A room temperature vulcanizing material is aslo applied around the connector stud 54 to provide a seal between the stud and the tube. Finally, a semi-conductive coating is applied to the outer surface of the tube 4, adjacent the hub 26 as shown at 58 and in the area overlaying the contact ring 52 as shown at 60 to prevent or minimize corona discharges from the electrical connections in these areas. 
     Having thus described the canister assembly, the fuse and movable contact assembly that is received therein will now be described. A load limiting fuse 62 is provided with mounting studs 64 and 66 which also serve as electrical connections for the fusible element of the fuse. Secured to mounting stud 66 is a movable loadbreak contact 68 which mates with the loadbreak contact 30. Loadbreak contact 68 includes a cylindrical contact engaging portion 70 which is formed from a conductive material such as brass. The conductive portion of the loadbreak contact, which may be formed as a machined part, is provided with an enlarged cylindrical portion 72 which has a bore therein for receiving the contact stud 66 of the fuse 62. Threaded holes 74 are provided in the enlarged portion 72 for receiving set screws 76 which securing the stud 66 of the fuse in the bore of enlarged portion 72 of the contact 68. 
     The stud 66 of the fuse is provided with a shoulder 78, against which is placed a disc-shaped barrier 80 formed of an insulating arc suppression material such as acetal homopolymer, acetal copolymer or an equivalent. This barrier, the outer periphery of which is closely spaced from the inner surface of the tube 4, is provided with a bore 82 through which passes the stud 66. The barrier is held in place between the shoulder 78 and the enlarged cylindric portion 72 of the contact. 
     Attached to the free end of the cylindrical contact engaging portion 70 of contact 68 is an insulating contact tip 84. The contact tip 84 may be formed of the same insulating arc suppression material as barrier 80. The external diameter of the contact tip 81 is the same as that of the cylindrical contact engaging portion 70. The tip 84 is provided at one end with a threaded aperture for receiving a threaded stud 86 of the movable loadbreak contact 68, and at the free end with a conical tip 88 for engagement with the fingers 32 of the fixed loadbreak contact 30 to aid in spreading them upon insertion of the movable loadbreak contact 68. 
     At the outer, or left end of the fuse 62, as viewed in FIG. 1, a non-loadbreak contact 90 is secured to the fuse mounting stud 64. The non-loadbreak contact 90, which is shown in the cross-sectional view of FIG. 2 is formed of a resilient contact material and has an annular base portion 92 from which radially extend a plurality of fingers 94. The fingers 94 are bent back on themselves in a U-shape as shown at 96 in FIG. 1 around a washer 98. The outer surfaces of the bights of the U-shaped fingers engage the contact ring 52 with spring force when the fuse and movable contact assembly is positioned in its operating position in the canister. After the contact 90 is assembled over the stud 61 of the fuse, a bore 100 in an insulator 102 is positioned over the stud 64 and is secured thereto by a screw 104. The annular base portion 92 of the contact 90 is captured between the washer 98 and the end wall of the fuse, while the free ends of the fingers 94 are captured between the washer 98 and the end wall of the insulator 102. 
     Attached to the outer end of the insulator 102 is a cap 106, which may be formed of stainless steel, and which is secured to the insulator 102 by threaded engagement of the eyebolt 108 in a bore provided in the insulator. A felt annular gasket 110 is positioned between the flange 10 and the cap 106 to provide a seal therebetween. The fuse and movable loadbreak contact assembly is held in the operative or engaged position in the canister by clips 112 (one of which is shown) which are secured to the flange 10 by the nuts 22. 
     Having thus described the components and assembly of the loadbreak fuse and canister assembly, the operation will now be considered. The fuse 62, the loadbreak contacts 30 and 68, and the non-loadbreak contacts 52 and 90 are connected in series with the electrical device to be protected through suitable electrical connections within the housing by making electrical connections to connector stud 54 and hub 26. With the fuse and movable contact assembly fully inserted within the canister as shown in FIGS. 1 and 4, a circuit is completed through connector stud 54, non-loadbreak contacts 52 and 90, fuse 62, loadbreak contacts 30 and 68, and hub 26. Should excessive current flow through the device being protected, either due to a fault or an overlaod, the current-limiting fuse 62 will act to interrupt the circuit. 
     When the fuse and movable contact assembly are inserted within the canister under a fault condition, an arc of great magnitude will be established between the loadbreak contacts 30 and 68. An arc is not established between the non-loadbreak contacts 52 and 90 wherein they make contact prior to engagement of the pair of loadbreak contacts. The hot ionized gases generated by the arc are thrust out of the sleeve 40 through the bore 46 toward the fuse 62 at a high rate of velocity. The hot gases impinge upon the barrier 80 which serves as a defuser and cooling media. The hot ionized gases which rebound off of the barrier 80 arc once again defused and cooled by impingement with enlarged portion or collar 50 of the sleeve 40. This path of flow of the hot gases is shown in FIG. 6 by the dashed lines and arrows. It will be observed in FIG. 6 that the cylindrical contact engaging portion 70 and the fingers 30 are spaced apart a short distance when the arcing occurs. Thus, the collar 50 serves to prevent the hot gases from creating a flashover path around the outside of the sleeve 40 to the hub 26. Similarly, the barrier 80 serves to prevent the hot gases from creating a flashover path around the outside of the fuse 62 to the non-loadbreak contacts 52 and 90, and to the cap 106. As previously set forth the barrier 80 and the sleeve 40, including the integral collar 50, may be made of acetal homopolymer, acetal copolymer, or an equivalent material that will gas off and cool the hot ionized gases. It has been found in prior art devices without the barrier and the sleeve 40, and more particularly, the collar 50, that the hot ionized gases will travel the length of the tube and cause a flashover which may result in an explosion, fire or bodily harm to the operator. In FIG. 5 the loadbreak contacts are shown in their spaced apart position as would exist before or after engagement as shown in FIG. 4. 
     It should be apparent to those skilled in the art, that while what has been described is considered at the present to be the preferred embodiment of this invention, in accordance with the patent statutes, changes may be made in the disclosed electricl loadbreak, fuse and canister assembly without actually departing from the true spirit and scope of this invention.