US3508184A - Fuse having improved means for reducing the force applied to a fusible means - Google Patents

Description

F. L; CAMERON E AL FUSE HAVING IMPROVED MEANS FOR REDUCING THE FORCE APPLIED TO A FUSIBLE MEANS Filed. Jan. 27, 1969 A ril 21, 1970.

FIG.2.

2 Sheets-Sheet -1 Filed Jan. 27, 1969 EDUCING MEANS April .1, 1970 L. CAMERON ET AL 3,508,184

FUSE HAVING IMPROVE EANS FORCE APPLIED A FU m f: 1 if;

' v/ FIG.5. 372 I42 H I 36 E I l 321 I 1 r 1+1 T 1 450 l4zc 2 Sheets-Sheet 2 United States Patent US. Cl. 337150 8 Claims ABSTRACT OF THE DISCLOSURE A fuse structure having a fusible means disposed inside an electrically insulating casing and electrically connected in series with an elongated electrically conducting member which is axially movable through a body of arcextinguishing material and which is biased in one axial direction by a spring means, the fusible means being operatively connected to the elongated conducting member by means for reducing the force applied to the fusible means to less than the force applied to the elongated conducting member by the spring means.

BACKGROUND OF THE INVENTION In the construction of certain types of high voltage fuses which are employed in electrical systems at voltages, such as 7.2 kv. and higher voltages, an electrically conducting member is connected in series with a fusible means and is axially movable through a body of are extinguishing material under the influence of a biasing spring upon the blowing or melting of the fusible means which normally restrains the movement of the conducting member. Where such fuses are employed to protect loads, such as instrument transformers, whose normal load current is relatively low, such as 7 amperes or less, a problem arises due to the relatively small cross-section of the fusible means required and the mechanical forces which the fusible means must withstand during long periods of service. In order to reduce the force applied to the fusible means in such a fuse structure by the spring which biases the associated electrically conducting member to a value less than the force applied to the conducting member by said spring, various fuse constructions have been proposed in the past such as disclosed in US. Patents 1,779,929, 1,907,581, 2,084,495 and 3,118,992. These known fuse structures have certain disadvantages in applications where a relatively smaller cross-section is desired for the overall fuse structure.

SUMMARY OF THE INVENTION In accordance with the invention, a fuse structure includes an electrically insulating casing having a pair of spaced terminals disposed thereon. A body of are extinguishing material is disposed in the insulating casing and includes a passageway extending axially therethrough. An elongated electrically conducting member extends axially through the passageway in the associated body of arc extinguishing material and is biased by an associated spring means toward one of the terminals and away from the other of the terminals on the associated casing. A fusible means is electrically connected in series with the elongated conducting member through a means which reduces the force exerted on the fusible means by the associated spring means to a value less than that of the force exerted on the elongated conducting member by said spring means. The force reducing means includes first and second latching members which are normally operatively connected to one end of the elongated conducting member and to one end of the fusible means,

3,508,184 Patented Apr. 21, 1970 respectively, and a lever member which is normally disposed between and which engages both of said latching members. The first latching member includes a portion which normally extends generally parallel to the elongated conducting member, while the second latching member extends axially away from the associated fusible means to provide a very compact construction. In the specific embodiment disclosed, the latching members and the associated lever member may be pivotally supported between a pair of spaced electrically insulating plates.

It is therefore an object of this invention to provide a fuse structure including an improved means for reducing the force applied to a fusible means by an associated biasing spring.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a side elevational view of a high voltage power fuse structure which embodies the principles of the present invention and which is shown in the normally closed operating condition;

FIG. 32 is an enlarged, longitudinal sectional view of a fuse unit which forms part of the fuse structure shown in FIG. 1 with portions of the end fittings of the fuse unit omitted;

FIG. 3 is an enlarged, partial, side elevational view, in section, of a portion of the fuse unit shown in FIG. 2 taken along the line III-III in FIG. 2;

FIG. 4 is an enlarged, partial bottom view, in section, of a portion of the fuse structure shown in FIG. 2, taken along the line IVIV in FIG. 2;

FIG. 5 is an enlarged, partial, side elevational view, partly in section, of an alternative construction for a 7 portion of the fuse unit shown in FIG. 2; and

FIG. 6 is an enlarged partial bottom view, in section, of the alternative construction shown in FIG. 5, taken along the line VIVI in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIGURE 1 in particular, the structure shown comprises a power fuse structure of the high voltage drop-out type, the general arrangement of which is set forth more fully in copending application Ser. No. 663,020, filed Aug. 24, 1967 by R. E. Frink and C. T. Walker which issued May 27, 1969 as US. Patent 3,447,114 which is assigned to the same assignee as the present application. As illustrated in FIG. 1, the fuse structure 10 includes a base (not shown) formed of sheet metal and a pair of outwardly extending insulator supports 272 and 282. The upper insulator support 272 fixedly supports in position a latching assembly 250 which includes a relatively stationary break contact 252, as described in greater detail in the copending application just mentioned. The lower insulator support 282 supports a hinge assembly 260. which, in turn, pivotally supports a fuse unit and which includes a relatively stationary hinge contact 262, as described in the copending application just mentioned. As illustrated in FIG. 1, the fuse unit 100 serves to electrically bridge the upper break contact 252 and the lower hinge contact 262 so that electric current will normally pass therebetween by way of terminal pads (not shown) to which an external electrical circuit may be connected.

The fuse unit 100 includes a generally tubular fuse holder or casing 32 which is formed from a suitable weather-proof, electrically insulating material, such as a filament wound, glass-epoxy material or the like, and a pair of upper and lower end fittings or terminals 34 and 36, respectively, which are disposed at the opposite ends of the casing 32 and which are formed from an electrically conducting material. The upper and lower end fittings or terminals 34 or 36, respectively, are securely fastened to the opposite ends of the associated holder or casing 32 by suitable means, such as cement or a bonding material, and a plurality of pins (not shown) which pass transversely through both the terminals 34 and 3-6 and the associated casing 32. As illustrated, the fuse unit 100 also includes a hook-eye 274 which is pivotally mounted on a laterally projecting portion 34A of the terminal 34, as shown in FIG. 2, and which may be utilized for effecting opening and closing movements of the fuse unit 100 by means of a conventional hook-stick. The terminal 36 includes a hinge lifting eye 284 which may be formed integrally with the terminal 36 and which may be employed in conjunction with a conventional hook-stick to effect physical removal of the fuse unit 100 from the hinge assembly 260 for replacement or insertion of the fuse unit 100. The terminal 36 also includes an inwardly projecting flange portion 36B against which the lower end of the casing 32 bears, as shown in FIG. 2.

The fuse unit 100 further includes a renewable or re fillable unit 20 which is mounted or disposed within the holder structure which includes the casing 32 and the upper and lower terminals 34 and 36, respectively. The renewable unit 20 includes its own supporting tube or insulating casing 110 which is formed from a suitable electrically insulating material having sufficient strength to withstand the internal gas pressures and intense heat which results during an interrupting operation of the fuse unit 100, such as a filament wound, glass-epoxy material. A body of gas evolving arc extinguishing material, such as 'boric acid, is disposed inside the casing 110 and axially spaced from the ends thereof. The body of gas evolving material may include a plurality of generally annular blocks 122, 124, 126 and 128 each of which include a relatively larger central opening, as indicated at 125 for the block 128 in FIG. 2, and a relatively smaller opening at one side thereof, as indicated at 127 for the block 128 in FIG. 2. Both of the openings in each of the above blocks extend axially through the respective blocks. When the blocks 122, 124, 126 and 128 are axially stacked in end-to-end relation, as shown in FIG. 2, with the respective larger and smaller openings thereof substantially aligned, a main bore 130 is formed through the body of gas evolving arc extinguishing material which includes said blocks and a relatively smaller auxiliary bore 192 is formed through the body of gas evolving arc extinguishing material which includes said blocks.

In order to prevent the travel of ionized gases between the main bore 130 and the auxiliary bore 192 during an interrupting operation of the fuse unit 100, as described in greater detail in US. Patent No. 3,401,243 which issued Sept. 10, 1968 to C. W. Upton, Jr. and I. A. Sensue and which is assigned to the same assignee as the present application, the meeting surfaces of the blocks 122, 124, 126 and 128 are structurally joined to one another around the relatively smaller openings of said blocks which form the auxiliary bore 192 by a sealing and bonding material having a relatively high dielectric strength, such as an epoxy resin. More specifically, as explained in the lastmentioned patent, the meeting surfaces of the blocks 122, 124, 126 and 128 each includes a groove or recess which extends substantially around and is spaced from the relatively smaller opening in each of said blocks and forms with the corresponding recess in the end of the adjacent blocks a larger passageway which is substantially filled with the sealing and bonding material, as indicated at 132 in FIG. 2. It is to be noted that the manner in which the blocks 122, 124, 126 and 128 are bonded to one another around the auxiliary bore 192 substantially prevents the entrance of the sealing and bonding material employed in either the auxiliary bore 192 or into the main bore 130.

In order to limit the gas pressures which result during an interrupting operation of the fuse unit 100 inside the casing 110 to a value within the rupture strength of the casing 110, each of the blocks 126 and 128 includes a generally C-shaped recess, as indicated at 129 in FIG. 2, which extends axially from one end of each of said blocks to a point which is adjacent to and axially spaced from the other end of the respective blocks with each of the recesses terminating peripherally short of the portion of each of said blocks which includes the relatively smaller openings which form part of the auxiliary bore 192. Each of the blocks 126 and 128 therefore includes around a major portion of its inner periphery a frangible inner wall, as indicated 126A and 128A, respectively, which is arranged to disintegrate when the fuse unit 100 is called upon to interrupt relatively large currents and when the intense heat results Within the main bore 130 and the gas pressure within the main bore 130 exceeds a predetermined value, as disclosed in greater detail in US. Patent No. 3,401,246 which issued Sept. 10, 1968 to C. C. Patterson and which is assigned to the same assignee as the present application. During such an interrupting operation, the size or cross-section of the main bore 130 through theh blocks 126 and 128 is effectively increased by the disintegration of the inner walls 126A and 128A, respectively, to thereby increase the size of the gas passageway or chamber and to decrease or limit the gas pressure which would otherwise result.

In order to retain the blocks 122, 124, 126 and 128 in assembled relationship with the associated casing 110 as shown in FIG. 2, the outer surfaces of said blocks may be coated with a suitable cement such as an epoxy bonding material prior to the assembly of said blocks inside the casing 110', said cement serving to bond said blocks to the casing 110. In addition, a generally annular retaining member or plug member 189 may be disposed at the upper end of the blocks 122, 124, 126 and 128 with the major portion of the retaining member 189 extending axially inside the casing 110 as shown in FIG. 2.

The retaining member 189 may be formed or molded from a suitable electrically insulating material having sufiicient strength to assist in retaining said blocks in assembled relationship with the casing 110 during an interrupting operation of the fuse unit 100, such as a glass-polyester material. A washer 183 formed from the same or a similar material may be disposed between the retaining member 189 and the uppermost block 122, as viewed in FIG. 2, and may be employed duringthe assembly and bonding of the blocks 122, 124, 126 and 128 together prior to the assembly of said blocks inside the casing 110. It is to be noted that the retaining member 189, as Well as the washer 183, includes a relatively larger central opening which forms an extension of the main bore 130 and a relatively smaller opening which forms an extension of the auxiliary bore 192.

In order to retain the member 189 in assembled relation with the associated casing 110, during an interrupting operation of the fuse unit 100, the outer surface of the retaining member 189 and the inner surface of the casing 110 at the upper end of said casing include adjacent helical grooves which together form a passageway in which a helical wire 181 is disposed to firmly secure the retaining member 189 in assembled relation with the casing 110. The retaining member 189 may be assembled with the upper end of the casing by first assembling the helical wire 181 in the groove around the outer surface of the retaining member 189 and then screwing the retaining member 189 into the upper end of the casing 110 to the final position shown in FIG. 2. Where desired, the outer surface of the retaining member 189 may also be coated with a suitable cement or bonding material, such as an epoxy bonding material, to additionally secure the retaining member 189 to the casing 110.

In order to substantially prevent the escape of ionized gases from the upper end of the renewable unit 20 around the elongated electrically conducting member 83 which extends through the main bore a generally tubular member 185 is disposed in concentric or nested relation with the retaining member 189 as shown in FIG. 2 and is preferably formed from an electrically insulating 'material having a relatively low coefiicient of friction, such as polytetrafluoroethylene, which is sold under the trademark Teflon. A shoulder portion 185A is provided at the upper end of the tubular member 185 and includes a central opening of reduced cross-section or size through which the conducting member 83 passes and which forms a substantially gas-tight seal with the conducting member 83 during an interrupting operation of the fuse unit 100 when the conducting member 183 is actuated to move axially upwardly, as viewed in FIG. 2, under the influence of a biasing spring 76, as will be described in greater detail hereinafter. The tubular member 185 also acts as a bearing to guide the axial movement of the conducting member 83. In order to prevent the tubular member 185 from being blown out of the upper end of the casing 110 during an interrupting operation of the fuse unit 100, the retaining member 189 includes an inner shoulder portion against which the upper end of the tubular member 185 bears, as shown in FIG. 2. The escape of ionized gases from the upper end of the renewable unit 20- through the auxiliary bore 192 may be adequately prevented by reducing the size of the relatively smaller opening through the retaining member 189 through which the auxiliary conductor 182 passes so that the cross-section of the auxiliary conductor 182 substantially fills the relatively smaller opening through the retaining member 189.

In order to further assist in retaining the blocks 122, 124, 126 and 128 in assembled relation with the casing 110 during an interrupting operation of the fuse unit 100, a generally tubular or annular retaining member 142 is disposed inside the casing 110 at the lower end of said blocks, as shown in FIG. 2, and is formed or molded from an electrically insulating material having sufficient strength to assist in retaining said blocks inside the casing 110 during such an interrupting operation, such as a glass-polyester material. The outer surface of the retaining member 142 is preferably coated with a bonding material, such as an epoxy material, prior to the assembly of the retaining member 142 inside the casing 110. This bonding material serves to secure the retaining member 142 to the inside of the casing 110. The retaining member 142 includes a relatively larger opening 142A which extends axially therethrough, into which the lower end of the main bore 130 opens and which may serve as an exhaust passageway for high pressure gases which result during the operation of the fuse unit 100. The opening 142A also serves as a chamber in which the fusible means 160 is disposed along with the means 300 which operatively connects the elongated conducting member 183 to the fusible means 160, as will be described hereinafter. The retaining member 142 also includes a relatively smaller opening 1428 which extends axially therethrough. The lower end of the auxiliary bore 192 opens into the opening 142B and the lower end of the auxiliary conductor 182 extends or projects into the same opening. The insulating wall or partition 142C which is formed integrally with the retaining member 142 around the relatively smaller opening 142B assists in preventing certain arc products which may result during the operation of the fuse unit 100 in the relatively smaller opening 142B from being deflected into the relatively larger opening 142A and impinging on the fusible means 160 or parts of the means 300. The retaining member 142 also includes an upwardly projecting tubular portion 142D adjacent to the relatively smaller opening 142B with the projecting portion 142D being structurally joined to the adjacent block 128 around a recess in the block 128 which is adapted to receive the projecting portion 142D by a flexible bonding material, such as silicone rubber, which is also an electrically insulating material. This joint between the retaining member 142 and the block 128 around the auxiliary bore 192 assists in preventing the travel or escape of ionized gases between the auxiliary bore 192 and the main bore and between the auxiliary bore 192 and the relatively larger opening 142B of the retaining member 142 during an interrupting operation of the fuse unit 100.

The elongated electrically conducting member or rod 83 of the refillable unit 20 is normally disposed, as shown in FIG. 2, to extend through the main bore 130 with the upper end of the conducting member 83 projecting axially beyond the upper end of the casing 110 and with the upper portion of the conducting member 83 being externally threaded as indicated at 83A. The conducting member 83 is normally held in the position shown in FIG. 2 by an operative connection through the connecting means or force reducing means 300 and the fusible means 160 to the generally annular or tubular lower electrically conducting member or contact 150. In general, the fusible means 160 exerts a restraining force on the conducting member 83 through the connecting means 300 which opposes the biasing force exerted on the conducting member 83 by the biasing spring 86 which would otherwise tend to actuate the conducting member 83 axially upward, as will be ex plained hereinafter. In order to permit a releasable operative connection between the connecting means 300 and the conducting member 83, the lower end of the conducting member 83 includes an axially extending opening 83B as best shown in FIG. 3. As shown in FIG. 3, a connecting link 302 extends axially into the opening 83B and is secured therein by suitable means, such as brazing. The connecting link 302 is formed from an electrically conducting material, such as copper, and includes an opening 302A.

More specifically, the connecting means or force reducing means 300 comprises a first generally L-shaped latching member or bell-crank lever 322 which normally engages the lower end of the conducting member 83 through the connecting link 302, a second latching member 326 which is normally connected to the upper end of the fusible element or means 160 and is laterally spaced from the first latching member 322 on the other side of the longitudinal axis of the casing 110, and an intermediate lever member 324 which is normally disposed between said latching members and engaging both of said latching members to transmit forces therebetween. The latching members 322 and 326 as well as the lever member 324 are all formed from an electrically conducting material such as copper in order to normally provide an electrically conducting path between the conducting member 83 and the fusible element 160 and are pivotally supported by the pivot pins 332, 334 and 336, respectively. The pivot pins 332, 334 and 36, in turn, are supported by a pair of generally parallel, spaced, electrically insulating plates or members 312 and 314 with each of said pivot pins passing through corresponding substantially aligned openings in the respective plates. The insulating plates 312 and 314 are, in turn, supported by a stationary terminal member 342 to which the plates 312 and 314 are secured by suitable means, such as a plurality of screws 316 which pass through substantially aligned openings in the plates 312 and 314 and a spacer member 352, which is disposed between said plates, to engage internally threaded openings provided in the stationary terminal member 342. The stationary terminal member 342 which is formed from electrically conducting material, such as copper, is, in turn, secured to the generally tubular conducting member or lower contact adjacent to the upper end thereof by suitable means, such as brazing. The spacer member 352 is also formed from electrically conducting material and has mounted thereon or formed integrally therewith a terminal post 354 on a portion of the spacer member 352 which projects to the left of the insulating plates 312 and 314, as viewed in FIG. 3. In order to provide an electrically conducting path between the spacer member 352 and the stationary terminal member 342, an electrically conducting strip member or contact 313 may be secured to the assembly just described with the first portion of said strip member disposed to engage the stationary terminal member 342 and a second portion of said strip member disposed to engage the electrically conducting spacer memher 352.

As best shown in FIG. 3, the first latching member or bell-crank lever 322 includes a relatively shorter arm 322A which normally projects into the opening 302A of the connecting link 302 and is engaged by the lower end of the conducting member 83 through the connecting link 302. The latching member 322 also includes a relatively longer arm 322B which is normally disposed generally parallel to the conducting member 83 and which extends axially away from the conducting member 83, as shown in FIG. 3. The intermediate lever member 324 is generally arcuate or curved in configuration and includes a shoulder adjacent to the pivot pin 334 on which the lever member 324 is supported which is normally engaged by the lower end of the relatively longer arm 322B of the first latching member 322 and includes a portion 325 which extends axially upwardly from the pivot pin 334 toward the conducting member 83. The secand latching member 326 includes a shoulder portion adjacent to the pivot pin 336 on which the latching member 326 is pivotally supported and which is normally engaged by the upper portion 325 of the lever member 324, as shown in FIG. 3, and has mounted thereon or formed integrally therewith a terminal post or pin 328 to which the upper end of the fusible element 160 is normally secured. It is to be noted that the force which is normally exerted on the first latching member 322 by the conducting member 83 through the connecting link 302 acts through a relatively shorter radius with respect to the pivot pin 332 and that the force transmitted by the first latching member 322 to the intermediate lever member 324 acts through a relatively greater radius with respect to the pivot pin 332. The force exerted by the first latching member 322 on the lever member 324 is therefore less than the force exerted on the first latching member 322 by the conducting member 83 through the connecting link 302 in accordance with the ratio of the radii through which the force is received and transmitted by the first latching member 322. Similarly, the force exerted by the first latching member 322 on the lever member 324 acts through a relatively shorter radius with respect to the pivot pin 334 and is transmitted by the lever member 324 to the second latching member 326 through a relatively longer radius with respect to the pivot pin 334. Finally, the force exerted by the lever member 324 on the second latching member 326 acts through a relatively shorter radius with respect to the pivot pin 336 and is transmitted to the fusible element 160 through a relatively longer radius with respect to the pivot pin 336. In other words because of the cumulative mechanical advantage provided by the lever system of the means 300, the force exerted on the fusible element 160 through the first latching member 322, the lever member 324 and the second latching member 326 may be reduced by a factor or ratio of the order of 50 compared with the force exerted on the conducting member 83 by the biasing spring 76.

In order to restrain the movement or rotation of the second latching member 326 under the influence of the forces transmitted from the biasing spring 76 through the conducting member 83, the first latching member 322 and the lever member 324, the fusible element 160 is secured at its upper end to the terminal post or pin 328 on the second latching member 326 and is secured at its lower end to the terminal post or pin 354 on the spacer member 352. More specifically, the terminal posts or pins 328 and 354 include grooves around the peripheries thereof and the opposite ends of the fusible element 160 are wrapped around said terminal posts and coiled back on the intermediate .portion of the fusible element 160. The fusible element 160 is formed from an electrically conducting alloy material, such as nickel-copper wire or nickel- 8 chromium Wire, having a relatively greater mechanical or tensile strength than other fusible alloys, such as silvertin. In order that the fusible element 160 be adapted to melt or fuse at relatively low currents such as lto 7 amperes, the cross-section or size of the fusible element is relatively small, such for example as 0.005 inch. It is to be noted that the fusible element 160 normally completes an electrically conducting circuit at the lower end of the renewable unit 20 which extends from the lower end of the conducting member 83 through the connecting link 302, the first latching member 322, the lever member 324, and the second latching member 326 to the upper end of the fusible element 160. As mentioned previously, the electrically conducting circuit just described is continued through the fusible element 160, the spacer member 352, an electrically conducting strip member 313 to the stationary terminal member 342 and then to the lower contact 150, as previously indicated.

The auxiliary conductor 182 which is of a relatively smaller cross-section or size than the conducting member 83 normally extends through the auxiliary bore 192 with the upper end of the auxiliary conductor 182 extending axially beyond the upper end of the auxiliary bore 192 and being both mechanically and electrically connected to the upper portion of the conducting member 83 by a transversely extending spring pin 184. The pin 184 is disposed in a transversely'extending recess or opening provided at the upper end of the retaining member 189 to prevent rotation of the conducting member 83 after assembly of the conducting member 83 in the renewable unit 20. The upper end of the auxiliary conductor 182 may be formed as a loop Which is assembled over the electrically conducting spring pin 184 and retained thereon by the head 186 of the spring pin 184. The lower end of the auxiliary conductor 182 extends or projects into the relatively smaller opening 142B of the retaining member 142, as best shown in FIG. 2, and is electrically connected to a helical conducting wire of reduced cross section, as indicated at 194. The upper end of the helical wire 194 which is disposed inside the smaller opening 142B of the retaining member 142 is secured to the lower end of the auxiliary conductor 182 by suitable means, such as brazing. The lower end of the helical wire 194 is coated or covered with electrically insulating material 199, which may be tubular in form, with the electrically insulated lower end of the helical wire 194 being secured to the stationary terminal 342 by suitable means, such as crimping the insulated lower end of the helical wire 194 in a slot, as indicated at 349 in FIG. 4.

The lower contact or conducting member also includes an elongated arcing terminal 158, which is of the type disclosed in greater detail in US. Patent No. 3,401,247 which issued Sept. 10, 1968 to C. W. Upton, C. C. Patterson and F. L. Cameron and which is assigned to the same assignee as the present application. The arcing terminal 158 projects upwardly from the upper end of the contact 150 into the relatively smaller opening 142B of the retaining member 142 to axially overlap the lower end of the auxiliary conducting member 182 with the lower portion of the arcing terminal 158 being disposed adjacent to and generally parallel to the axis of the helical Wire 194. The arcing terminal 158 is electrically insulated along its length by a coating or film of electrically insulating material, such as an insulating enamel, which is provided on the arcing terminal 158 to prevent the electrical shorting out of the helical wire 194. The arcing terminal 158 which is formed from an electrically conducting material may be structurally secured to the upper side of the contact 150 at the inner periphery thereof by suitable means, such as brazing, or may be formed integrally with the stationary terminal 342 where desired in the particular application. It is to be noted that so long as the electrically conducting path which includes the conducting member 83, the connecting means 300 and the fusible element 160 remains intact, substantially no electrical current will flow through the auxiliary current path which includes the auxiliary conductor 182, the helical wire 342 and the arcing terminal 158 because of the electrical insulation which is provided on the arcing terminal 158 and on the lower end of the helical wire 194, as just described.

In order to assist in retaining the blocks 122, 124, 126 and 128 as well as the retaining member 142 in assembled relationship inside the casing 110 as well as for another purpose which will be explained hereinafter, the lower tubular conducting member or contact 150 includes a main portion 152 which extends axially inwardly from the lower end of the casing 110 to bear against the lower end of the retaining member 142. The lower contact 150 as shown in FIG. 2 also includes a flange portion 154 at the lower end thereof against which the lower end of the casing 110 bears when the conducting member 150 is assembled with the casing 110.

In order to retain the lower contact 150 as well as other parts of the renewable unit 20 is assembled relationship with the casing 110 during an interrupting operation of the fuse unit 100, a generally tubular external terminal member or ferrule 172 is disposed to telescope over the lower end of the casing 110. In order to firmly secure the external terminal member 172 to the lower end of the casing 110, the internal surface of the external terminal member 172 and the external surface of the portion of the casing 110 adjacent the member 172 include helical grooves which when the parts are assembled form a combined helical passageway in which a helical wire 173 is disposed. In the assembly of the external terminal member 172 on the lower end of the casing 110, the helical wire 173 may be first assembled in the helical groove on the lower end of the casing 110 and the terminal member 172 may be then screwed onto the lower end of the casing 110 until the parts reach their final positions as shown in FIG. 2. In order to additionally assist in retaining the external terminal member 172 on the lower end of the casing 110, the outer surface of the casing 110 and the inner surface of the terminal member 172 may be coated with a bonding material, such as an epoxy material, prior to the assembly of the terminal member on the casing 110. It is to be noted that the terminal member 172 also includes an inwardly projecting flange portion 172A adjacent the flange portion 154 of the conducting member 150 to assist in retaining the conducting member 150 in assembled relation with the other parts of the renewable unit 20.

In order to form a current carrying path which extends between the lower end fitting 36 and the lower contact 150 of the renewable unit 20, the terminal member 172 also includes an external flange portion 172C which bears against the inwardly projecting flange portion 36B of the lower end fitting 36. The electrically conducting path thus formed extends from the lower contact 150 through the inwardly projecting flange portion 172A and the flange portion 172C of the terminal 172 to the flange portion 36C of the lower end fitting 36. The renewable unit 20 may be additionally held in position by an electrically conducting contact ring 195 which is disposed to threadedly engage an internally threaded opening at the lower end of the end fitting 36 and to bear against the terminal mem-,

ber 172 as shown in FIG. 2.

It is important to note that in order to prevent the concentration of relatively high potential stresses adjacent to the terminal member 172 during an interrupting operation of the fuse unit 100 at relatively high voltages, the upper end of the lower contact 150 extends axially beyond-the upper end of the terminal member 172 toward the other end of the casing 110 a minimum distance to prevent such a concentration of relatively high potential stresses externally of the casing 110 and adjacent to the terminal member 172, as disclosed in greater detail in US. Patent No. 3,401,245 which issued Sept. 10, 1968 to F. L. Cam- 10 eron and which is assigned to the same assignee as the present application.

In order to actuate the axial movement of the conducting member 83 as well as that of the auxiliary conductor 182 during an interrupting operation of the fuse unit and to electrically connect the renewable unit 20- to the upper end fitting or terminal 34, a spring and cable assembly 30 is disposed inside the outer casing 32 between the renewable unit 20 and the upper end fitting 34, as disclosed in greater detail in US. Patent No. 3,401,244 which issued Sept. 10, 1968 to C. C. Patterson and which is assigned to the same assignee as the present application. The spring and cable assembly 30 includes at its lower end a generally tubular electrically conducting member 84 having. an internally threaded central opening as indicated at 84A which is adapted to receive the upper threaded end 83A of the conducting member 83. The lower spring seat member 86 is fixedly mounted on the tubular member or socket 84 for movement therewith by assembling the spring seat 86 over the outer periphery of the socket 84 with the lower end of the spring seat 86 bearing against the shoulder provided on the socket 84 and with the upper end of the spring seat 86 being engaged by a plurality of portions of the socket 84 at the upper end of the socket 84 which serve to secure the spring seat 86 on the socket 84. The spring and cable assembly 30 also includes an upper spring seat 74 which is slidably disposed over the lower portion 60A of a generally cylindrical electrically conducting member 60 whose integral upper portion 60B extends axially through an opening 34B in the upper terminal 34 and is externally threaded at the upper end thereof as indicated at 60C. As illustrated, the conducting member 60 may be secured to the upper terminal 34 by an internally threaded end cap 44 which may be screwed down on the upper threaded portion 60C of the conducting member 60 until the flange portion 44A of the end cap 44 bears against the upper end fitting 34 around a flange or shoulder portion, as indicated at 34C in FIG. 2. A helical tension spring 76 is secured at its upper end to the external helically threaded portion of 'the upper spring seat 74, while the lower end of the spring 76 is secured to the external, helically threaded portion of the lower spring seat 86 to bias the conducting member 83, as well as the auxiliary conductor 182, in a generally upward direction as viewed in FIG. 2 away from the lower contact 150. It is important to note that the turns of the spring 76 are generally rectangular in cross section to substantially prevent any overlapping of the turns of the spring 76 and the consequent damage to the spring 76 that might otherwise result during an interrupting operation of the fuse unit 100, as explained in greater detail in US. Patent No. 3,401,244 just mentioned.

In order to electrically connect the renewable unit 20 and more specifically the conducting member 83 to the upper terminal 34 both prior to and during an interrupting operation of the fuse unit 100, a plurality of helically coiled flexible cables or conductors 82 are electrically and structurally connected at the bottom ends thereof to the socket 84 into separate openings (not shown) provided in the socket 84 by suitable means, such as brazing, and at the upper ends thereof are secured to the conducting member 60 in separate openings provided in said conducting member by suitable means, such as brazing. In order to increase the effective current transfer area between the conducting member 60 and the upper terminal 34, a washer 54 formed of electrically conducting material may be disposed between the shoulder which is formed at the intersection of the upper and lower portions 60A and 60B, respectively, of the conducting member 60 and the shoulder which is formed inside the upper terminal 34 as indicated at 34D, around the central opening 34B.

In order to facilitate the assembly of the renewable unit 20 and the associated spring and cable assembly 30 inside the outer casing 32, a pair of spring pins 58 may be disposed in associated openings provided at the opposite sides of the upper portion 60B of the conducting member 60 to be positioned finally within an enlarged central opening or recess 34E in the upper terminal 34 as shown in FIG. 2.

In order to actuate the release of the latching assembly 250 shown in FIG. 1 following an interrupting operationby the fuse unit 100 as disclosed in US. Patent No. 3,401,244 previously mentioned, a tripping rod 52 is slidably disposed inside a central opening 72 which is provided in the conducting member '60 with the upper end of the tripping rod 52 being normally positioned below the top of the end cap 44 as shown in FIG. 2. The lower end of the tripping rod 52 is fixedly coupled to the upper spring seat 74 for axial movement therewith by the cross pin 56 which passes laterally through aligned transverse openings in the tripping rod 52 and the upper spring seat 74 and through a pair of elongated slots 62 provided at the opposite sides of the conducting member 60 with the cross pin 56 being normally positioned at the lower ends of the slots 62 as shown in FIG. 2. In order to permit the axial movement of the tripping rod 52 upwardly through the end cap 44 following an interrupting operation by the fuse unit 100, the top of the end cap 44 includes a central opening 46 through which the tripping rod 52 may pass to actuate the release of the latching assembly 250 shown in FIG. 1. When the latching assembly 250 is released by the movement of the tripping rod 52, the upper end of the fuse unit 100 will be actuated to rotate in a clockwise direction, as viewed in FIG. 1, about the lower hinge assembly 260 to thereby provide an electrically insulating gap between the upper break contact 252 and the lower stationary hinge contact 62 by such drop-out action.

In order to assemble the renewable unit 20 and the associated spring and cable assembly 30 into the outer casing 32, the threaded end of the conducting rod 83 is first screwed into the socket 84 at the lower end of the spring and cable assembly 30. A refill fusing tool (not shown) is then screwed into the internally threaded central opening 72 at the other end of the spring and cable assembly 30. The spring and cable assembly 30 is then inserted into the outer casing with the upper end of the spring and cable assembly 30 being inserted first into the lower end of the casing 32 as viewed in FIG. 2 until the refill tool (not shown) passes through the central opening 34B of the upper terminal 34. By the use of the refill tool, the spring 76 is stretched and placed in tension until the cross pins 58 mounted at the sides of the conducting member 60 are drawn upwardly through a pair of radial slots (not shown) provided in the upper terminal 34 around the central opening 34B. The conducting member 60 and the spring and cable assembly 30 are then rotated until the pins 58 rest on the shoulder provided at the bottom of the enlarged opening 34E in the upper terminal 34. The end cap 44 may then be screwed down on the upper threaded portion 60C of the conducting member 60 to further stretch the spring 76 to the final condition or position shown in FIG. 2 in which the cross pins 58 are drawn upwardly away from the shoulder in the upper terminal 34 at the bottom of the enlarged opening 34E. It is to be noted that when the spring and cable assembly 30 and the renewable unit 20 are assembled inside the casing 32 as just described, the cross pin 56 which couples the upper spring seat 74 to the rod 52 is disposed at the bottom of the slot 62 at the opposite sides of the conducting member 60 to permit limited upward travel of the upper spring seat 74 along with the cross pin 56 and the rod 52 to a final position of the rod 52 in which the rod 52 projects beyond the end cap 44 axially to release the latching assembly 250, as previously mentioned. The washer 54 also acts as a stop surface against which the upper end of the spring seat 74 bears to limit the upward travel of the rod 52, the cross pin 56 and the sprlng seat 74.

In considering the operation of the fuse unit 100, 1t

is to be noted that normally an electrically conducting path extends from the upper terminal 34 to the lower terminal 36 through an electrical circuit which includes the washer 54, the conducting member 60, the flexible conductors 82, the socket 84, the conducting member 83, the connecting link 302, the first latching member 322, the lever member 324, the second latching member 326, the fusible element 160, the spacer member 352, the electrically conducting strip member 313, the stationary terminal 342, the lower contact 150, and the terminal 172 which engages the lower terminal 36. As mentioned previously, substantially no electrical current flows through the electric circuit which includes the auxiliary conductor 182 and the helical wall 194 due to the presence of the electrical insulation on the arcing terminal 158 and the electrical insulation which is provided at the lower end of the helical wire 194. This construction is necessary since in the absence of the electrical insulation just mentioned, the resistance of the electrical circuit which includes the auxiliary conductor 182 would be comparable to the electrical resistance of the circuit which includes the conducting member 83, the connecting means 300 and the fusible element 160 due to the relatively small cross-section of the fusible element 160 and the alloy material from which the fusible element 160 is formed in order to provide increased mechanical or tensile strength in the fusible element 160. The force exerted on the conducting member 83 by the biasing spring 76 is normally of the order of 40 to 50 pounds in order to accelerate the mass of the spring 76 and the mass of the conducting member 83 sufiiciently during an interrupting operation of the fuse unit to the velocity which is necessary to interrupt relatively high fault currents or short circuit currents. As previously explained, the force exerted on the fusible element 160 by the spring 76 through the conducting member 83 and the connecting means 300 is arranged to be relatively small due to the overall reduction in force which is accomplished through the connecting means 300 as previously described.

When the current which is flowing through the fuse unit 100 increases to a value which is of sufficient magnitude and duration to melt or blow the fuse element 160, the conducting member 83 is no longer restrained from upward movement under the influence of the biasing spring 76 and the conducting member 83, as well as the auxiliary conductor 182, will start to move upwardly to thereby stretch the helical wire 194 which is connected to the lower end of the auxiliary conductor 182. More specifically, when the fuse element 160 melts, the second latching member 326 is free to rotate in a clockwise direction about the pivot pin 336 and the lever member 324 which is normally restrained by the second latching member 326 will then be free to rotate in a counterclockwise direction about its pivot pin 334. The first latching member 322 is released by the latter movement of the lever 324 and is free to rotate in a clockwise direction about its pivot pin 332 to thereby release the connecting link 302 and the conducting member 83. During the initial movement of the conducting member 83 and the auxiliary conductor 182 following the melting of the fusible element 160, the stretching of the helical wire 194 will permit the formation of an electrically insulating gap in the main bore which will gradually increase as the conducting member 83 moves upwardly and the voltage across the insulating gap in the main bore 130 would increase until the voltage is sufficient to break down the electrical insulation provided at the lower end of the helical wire 194. When the electrical insulation breaks down at the lower end of the helical wire 194 arcing will be initiated in the circuit which includes the auxiliary conductor 182 and the helical wire 194. The arcing in the auxiliary circuit will then burn through the electrical insulation on the arcing terminal 158. The current in the helical wire 194 will then be sufiicient to melt the elical wire 194 or the helical wire 194 will be St e h to its limit and the helical wire 194 will fracture mechanically. Even after the helical Wire 194 melts or is broken as just described, the formation of a significant electrically insulating gap in the auxiliary bore 192 is further delayed by the overlapping of the auxiliary conductor 182 by the arcing terminal 158 until the retreating free end of either the wire 194 or the conductor 182 passes the upper end of the arcing terminal 158 whose insulation will have burned through by this time. It is important to note that the insulating gap in the main bore 130 between the separated ends of the conducting parts will increase at a faster rate than the formation of an insulating gap in the auxiliary bore 192 due to both the delay in the formation of an arc in the auxiliary bore 192 because of the presence of the helical wire 194 and due to the overlapping of the auxiliary conductor 192 by the arcing terminal 158. It is also important to note that the arcing which takes place in the fuse unit 100 during an interrupting operation will always be established in the auxiliary bore 192, as just explained. When the retreating end of either the helical wire 194 or the auxiliary conductor 192 passes the upper end of the arcing terminal 158, the arcing which takes place in the auxiliary bore 192 will cause gases to be evolved from the gas evolving material around the auxiliary bore 192 which will not be ionized.

When the current to be interrupted by the fuse unit 100 is relatively low such as 1000 amperes or less and when the gas pressure of the evolved gases in the auxiliary bore 192 increases to thereby increase the dielectric strength in the auxiliary bore 192, the insulating gap which is formed in the auxiliary bore 192 along with the corresponding increased dielectric strength will be suflicient to interrupt the current following a particular current zero if alternating current is being interrupted by the fuse unit 100. The insulating gap which is formed simultaneously in the main bore 130 ofthe fuse unit 100 at a relatively faster rate will have sufficient dielectric strength consider ing the instantaneous potential difference between the separating conducting parts in said main bore to prevent a restrike of the arc in the main bore 130 for such relatively low fault currents. In other words, when any fault current is interrupted by the fuse unit 100, arcing will always be established in the auxiliary bore 192 and for relatively small fault currents, the arcing which results will be finally interrupted in the auxiliary bore 192. One important reason for this is that for relatively low fault currents, the relative dielectric strength of the main bore 130 at the time that the arc is finally interrupted in the auxiliary bore 192 will be relatively higher than that in the auxiliary bore 192 to prevent a restrike or breakdown of the main bore 130 due to the potential difference which results between the separated conducting parts in th main bore 130.

For relatively higher fault currents, the arcing which is initiated in the fuse unit 100 will always be established in the auxiliary bore 192 in the manner just described. For such relatively higher fault currents however, the gas pressure which builds up in the auxiliary bore 192 during an interrupting operation and the burning back of the separated conducting parts in the auxiliary bore 192 will result in a relatively higher dielectric strength in the auxiliary bore 192 compared with that in the main bore 130 between the separated conducting parts in the main bore 130. If the instantaneous potential difference between the separate ends of th conducting part in the main bore 130 is sufficient when the dielectric strength of the bore 130 becomes relatively less than that of the auxiliary bore 192, the arc will restrike in the main bore 130 to threby cause the evolution of gases in the main bore 130 which are not ionized to thereby increase the gas pressure in the main bore 130 as well as the corresponding dielectric strength in the main bore 130. The are which restrikes in the main bore 130 will be elongated both by the upward movement of the conducting member 83 and by the burning back of the separated conducting parts in the main bore to thereby increase the quantity of gases evolved from the gas evolving material around the main bore 130. The are in the main bore 130 will then finally be interrupted following a particular current zero :in the alternating current which is being interrupted when the insulating gap and the corresponding dielectric strength in the main bore 130 is sufiicient to withstand the instantaneous potential difference between the separated conducting parts in the main bore 130.

If the fault current which is being interrupted by the fuse unit is of a relatively still higher magnitude or value, the gas pressure in the main bore along with the intense heat which results will be sufiicient to break up or disintegrate the frangible inner walls 126A and 128A of the blocks 126 and 128, respectively, to thereby limit the gas pressure of the evolved gases because of the increase in the size or volume of the gas passageway or the volume of the gas space inside the refillable unit 20 to thereby limit the gas pressure of evolved gases to a value within the rupture strength of the casing 110. It is to be noted that when the inner walls 126A and 128A break up during the operating condition just mentioned, the outer walls of the blocks 126 and 128 will then be exposed to the arc being interrupted and will continue to evolve gases which will aid in arc extinction of the relatively higher currents which are present in such an interrupting operation. As previously mentioned, the inner walls 126A and 128A will remain intact when the fuse unit 100 is interrupting relatively lower currents tothereby assist in confining the are by maintaining a normal size opening of the main bore 130 to more effectively aid in the arc extinction of such lower currents. When the arc is interrupted in the main bore 130 of the fuse unit 100 or in the main bore 130 as enlarged by the breaking up of the inner walls 126A and 128A as just described to thereby cause the evolution of gas from the gas evolving material in the blocks 122, 124, 126 and 128 which surround the main bore 130, the upward movement of the conducting member 83 along with the upward movement of the auxiliary conductor 182 will be additionally accelerated by the force of the 'gas pressure of such evolved gases in the main bore 30 along with the force exerted on the conducting member 83 by the biasing force of the spring 76.

During an interrupting operation of the fuse unit 100 as just described, when the conducting member 83 is released and moves upwardly under the influence of the spring 76 or under the influence of both the spring 76 and the gas pressure of the evolved gases inside the renewable unit 20, the turns of the springs 76 which are normally held in tension will partially collapse toward the compressed condition but after the turns of the spring 76 collapse to a certain extent, the upper spring seat 74 will slide axially on the lower portion of the conducting member 60 until the upper end of the spring seat 74 bears against the Washer 54 to thereby actuate the rod '52 in an upward direction, as viewed in FIG. 2. The rod 52 will then be actuated from the position shown in FIG. 2 until the upper end of the rod 52 :actuates the release of the latching means 250 as described in US. Patent No. 2,403,121 which issued July 2, 1946 to H. L. Rawlins et al. or in copending application 'Ser. No. 663,020 previously mentioned. It is to be noted that the upward movement of the conducting member 83 and the auxiliary conductor 182 will establish the insulating gaps previously described between the separated ends of the conducting parts inside the renewable unit 20 following an interrupting operation. In addition, the fuse unit 100 will be actuated by the release of the latching means 250 by the rod 52 to rotate in a clockwise direction as viewed in FIG. 1 about the lower hinge assembly 260 in a dropout movement to establish a longer electrically insulating gap between the break contact 252 and the lower stationary hinge contact 262 of the overall fuse structure 10.

It is important to note that during an interrupting operation of the fuse unit 100 as previously described for either relatively low fault currents or for relatively high currents, the gas seal and joint structure between the blocks 122, 124, 12 6 and 128 substantially prevents the escape of ionized gases from the auxiliary bore 192 in which all arcing is established to the main bore 130 along the meeting surfaces of the successive blocks in a renewable unit 20. If such ionized gases were permitted to escape to the main bore 130, a restrike might result when the fuse unit 100 is called upon to interrupt relatively low fault currents and the restrike of an arc in the main bore 130 would result in an arcing condition which the fuse unit 100would find difiicult to interrupt because for such relatively low fault currents the quantity of gas evolved in the main bore 130 is not normally sufiicient to establish a dielectric strength in the main bore 130 which would be great enough to interrupt the are which would result for such relatively low fault currents in the main bore 130.

Referring now to FIGS. 5 and 6, there is illustrated an alternative embodiment of the applicants invention in a fuse unit 100 which includes a modified refillable or renewable unit which is structurally the same as the renewable unit previously described except that the renewable unit 20' employs a lower contact ring 450 instead of the generally tubular lower contact 150 as in the renewable unit 20. The contact ring 450 of the renewable unit 20 has formed integrally therewith a stationary terminal 454 which corresponds to the stationary terminal 342 of the renewable unit 20. Otherwise the construction and operation of the fuse unit 200 is the same as previously described in detail in connection with the fuse unit 100. It is to be noted that the construction of the fuse unit 200 is particularly adapted for application at voltages lower than those at which the fuse unit 100 might be applied and where the potential stresses adjacent to the terminal 372 would be relatievly lower during an interrupting operation of the fuse unit 200. In other words, the construction of the fuse unit 200 is adapted to application at voltages at which the tubular conducting member 150 would not be required to prevent the occurrence of relatively high potential stresses adjacent to the external terminal 372.

It is to be understood that the teachings of the applicants invention may be applied to power fuses for high voltage applications which are of the non drop-out type, as disclosed in US. Patent No. 3,401,244 mentioned previously. It is also to be understood that the teachings may be applied to a fuse construction in which a gaseous arc extinguishing medium such as fulfur hexafluoride is provided rather than a body of solid arc extinguishing material as disclosed.

The apparatus embodying the teachings of this invention has several advantages. For example, the mechanical forces applied to the fusible element or link in a construction as disclosed are reduced to a relatively low level while permitting the use of a biasing spring which is necessary to achieve the required acceleration of the separated conducting parts during an interrupting operation. In addition, the force reducing means 300 as disclosed is uniquely adapted to provide a compact construction which substantially minimizes the required cross-section or size of the assembly in a direction transverse to the longitudinal axis of the fuse unit in which the force reducing means 300 is incorporated. In addition, a fuse construction including a connecting means or force reducing means 300 as disclosed readily lends itself to ease of manufacture since the connecting means 300 may be preassembled before assembly with the fuse unit in which the connecting means is incorporated.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What we claim is:

1. A fuse comprising an electrically insulating casing having a pair of spaced terminals thereon, a body of arc extinguishing material disposed inside said casing and having at least one passageway extending axially therethrough, an elongated conducting member disposed to extend axially through said passageway, spring means disposed inside said casing and connected to one end of said conducting member for applying a force to said conducting member which biases said conducting member toward one of said terminals and away from the other of said terminals, fusible means having one end connected to said other of said terminals, and means operatively connected between the other end of said fusible means and the other end of said conducting member for reducing the force normally applied to said fusible means through said conducting member by said spring means to less than the force applied to said conducting member, the last-mentioned means comprising a first latching member disposed to normally engage the other end of said conducting member and having a portion normally extending generally parallel to said conducting member, a second latching member connected to the other end of said fusible means and normally extending axially away from said fusible means toward said conducting member and a lever member normally disposed between and engaging both of said latching members.

2. The combination as claimed in claim 1 Wherein said first and second latching members and said lever member are formed from electrically conducting material to normally form an electrically conducting path between said elongated conducting member and said fusible means.

3. The combination as claimed in claim 1 wherein said first and second latching members and said lever member are all pivotally supported between a pair of spaced electrically insulating plates.

4. The combination as claimed in claim 1 wherein said first latching member is generally L-shaped and includes a relatively shorter portion which normally engages said other end of said conducting member and a relatively longer portion which is normally disposed generally parallel to said elongated conducting member.

5. The combination as claimed in claim 1 wherein said first latching member comprises a bell crank lever which is pivotally supported adjacent to said other end of said conducting member.

6. The combination as claimed in claim 1 wherein said insulating casing is generally tubular with said first latching member and said lever member being pivotally supported on the same side of the axis of said casing and with said second latching member being pivotally supported on the other side of the axis of said casing.

7. A fuse comprising an electrically insulaitng casing having a pair of spaced terminals thereon, an elongated conducting member disposed inside said casing to extend longitudinally with respect to said casing, spring means disposed inside said casing and connected to one of said conducting members for applying a force to said conducting member which biases said conducting member toward one of said terminals and away from the other of said terminals, fusible means having one end connected to said other of said terminals, means operatively connected between the other end of said fusible means and the other end of said conducting member for reducing the force applied to said fusible means through said conducting member by said spring means to less than the force applied to said conducting member, and are extinguishing material surrounding said other end of said conducting member and at least a portion of said conducting member extending axially away from said other end of said conducting member, said force reducing means comprising a first latching member disposed to normally engage said other end of said conducting member and having a portion extending generally parallel to said conducting member, a second latching member connected to the other end of said fusible means and normally extending axially away from said fusible means toward said conducting member and a lever member normally disposed between and engaging both of said latching members.

8. The combination as claimed in claim 7 wherein said first latching member is generally L-shaped and includes a relatively shorter portion which normally engages said other end of said conducting member and a relatively longer portion which is normally disposed generally parallel to said elongated conducting member.

References Cited UNITED STATES PATENTS Hermann 337-l78 Hart 337174 X Schultz 337178 X Gesellschap 337l74 X Thiede et a1 337174 X Barta 337178 Patterson 337-178 H. B. GILSO-N, Primary Examiner US. Cl. X.R.

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