CA1189117A - Electric fuse having high ambient temperature interrupting capability - Google Patents

Abstract

ELECTRIC FUSE HAVING HIGH AMBIENT TEMPERATURE INTERRUPTING CAPABILITY

Abstract of the Disclosure A low voltage cartridge fuse having the capabillty of protecting an electric circuit from both major fault currents and overload currents, as well as opening the electric circuit when the ambient temperature surrounding the fuse exceeds a predetermined value. The fuse has a tubular casing enclosed by a pair of end terminals. The current path through the fuse includes a short circuit interrupting element having notched restrictions connected to one of the end terminals by a first relatively high temperature melting point solder joint. The other end of the short circuit element is connected to a plunger contact by a first relatively low temperature melting point solder joint. The plunger is positioned within a metal sleeve to which it is connected by a second relatively low temperature melting point solder joint. The sleeve is in turn connected at one end to the other end terminal of the fuse by a second relatively high temperature melting point solder joint. A spring element positioned inside the metal sleeve cooperates with the sleeve and the plunger to bias the plunger in a manner tending to break both of the low melting point solder joints.

Description

2--Background of the Invention . .
The objec-t of the present invention is to provide an elec-tric fuse that is not only capable of interrupting a circuit subjec-ted to major fault currents and overload curren-ts bu-t also capable of interrupting the circui-t it is pro-tecting when -the ambient temperature surrounding -the fuse rises -to an unsafe level. The fuse is also required to be a rela-tively fas-t acting non-time delay fuse. Such requiremen-ts are se-t forth by -the Canadian S-tandards ~ssociation Specification 59 for "P" type fuses~
The need for an electric fuse having such character-istics came about in an effor-t to prevent fires in domes-tic panel boards, particularly in homes having electric heating sys-tems. The cyclic nature of such heating devices caused deterioration of the standard zinc-element fuses previously used in ~uch applications, as well as deterioration of panel board connectors -thus resulting in panel board Eires.
At that time type "D" fuses were available that had a feature incorporated in their design which would allow a fuse to respond to elevated ambient temperatures. However, as this type of fuse was about three to four times the cost of zinc link fuses, consumers continued to use -the cheaper zinc link products. As a result of this, regulating authorities banned the use of zinc link fuses in certain ratings and required that fuses in these ra-tings mus-t have the capabili-ty of interrupting the elec-tric circuit when the ambien-t temper-atures surrounding the Euse rises to an unsafe level.
The type "D" f-uses described above had the capability of interrupting elevated ambient -temperatures, however, such Euses also incorporated a built in time-delay response to over-~2a load currents~ I-t was es-tablished that the degree of overload found in such fuses could cause other problems in house wiring systems and it was also found that such time-delay was no-t necessary for the cycling of heating loads. Accordingly it is toward the provision of an electric fuse capable of responding -to interruption at elevated ambient tempera-tures but not i.ncorporating time~delay during overload current to which the present invention is directed.

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- Summar oF the Invention The present invention relates to an electric fuse capable of lnterrupting an electrlc circuit ~hen subjected to a major fault current and overload currents ln the electrlc circuit, and, for also interruptlng the flow of electricity through the fuse when the ambient temperature surrounding the fus~
exceeds a predetermined value. The structure of the fuse includes a casl~g of electrlc insulating materiDl and a palr of terminal caps closing the ends of the casing. An elongated fusible element hav~ng a plur~lity of serially arranged points of reduced cross-section for interruptlng major fault or short clrcuit currents i9 provided inslde the casing with one end coslductlvely connected to the lnside of one oE the terminal caps by a first rela~ively high temperature melting polnt ~older ~oint. An axially movable plunger is located inside the caslng ln a~ial end to end rela~lonshlp with ~he elongated fusible element and i9 conductively connected to the other end of the fusible elemen~ by a flrs~ relatively low temperature melting po-lnt solder joint. A
metal sleeve is pro~ided inside the casing to partially surround at least a portion of the plunger to deflne an annular cavlty therebetween. One end of the metal sleeve ~s conductively connected to the other terminal cap of the fuse by a second relatively high temperature meltlng point solder joint.
2Q A helical spring ls dlsposed within the annular cavity defined b~tween the metal sleeve and ths plunger. The spring operably engages both the plunger and the interior of the matal sleeve in a manner such that it i9 held in a compressed condition. The force of the compressed spring reacts upon the plunger ~nd ~he sleeve ln a manne.r tendlng to break the first low temperature meltlng point solder joint. A second low temperature melting point solder joint i6 provlded to conductively interconnect the end of the metal sleeve opposite that which is connec~ed to the ~erminal cap to the outer surface o~ the plungerO The force of ~he compressed spring ac~ing upon the plunger and the sleeve also tends to break this second low tempera~ure melting polnt solder joint. A pulverulent arc-quenching Eiller ls provided to fill ~he remainder of the irl~erior of the fuse caRing.
In a preferred embodiment the metal sleeve is provided with a plurality of radially e~tendlng tabs or feet at the end which i~ connected to one of ~he ^ terminal caps. Tha radially ext:end:Lng feet are of sufflcient length that ~hey ex~end into a space defined between the axlal end of the fuse caslng whlch the termlnal cap closes and the lnterlor surface of the end cap thereby positively mechanically retainlng the metal sleeve between these two elements.
Brief_Description of the Drawin~s The novel features tha~ are consldered characteristic of the inventlon are set forth with particularity in the appended claims. The invention i~gelf 9 however, both as to its organization and its method oE operation, toge~her wlth addltlonal obJects and advalltagea ~hereof, will kest be under-stood from ~he fol:Lowlng description of the preferred embodlment when read ln connectiorl with the accompanying drawings wherein like numbers have been employed in the different flgures ~o denote the same parts and whereln:
Figure 1 is a longitudinal section of a complete fuse as~embly according to ~his invention in its orlglnal state, i.e., it has not interrupted on low current, high curren~ or high temperature conditlons;
Flgure 2 is a viaw simllar to Figure 1 showlng the fuse ln the condition followin~ interruption on si~her a low current overload or hlgh amblent temperature condi~lon, Figure 3 i~ a cross-section taken along the line 3-3 of Figure l;
Figure 4 is a cross-section taken along the line 4-4 of F-lgure l;
Flgure 5 is an alternate ~hor~ circuit fusible element to that ~hown in the fuse of Figure 1, and Flgure 6 is another alternate fusible element.
Descr~ption of the Preferred ~mbodiment Referrlng now to the drawings, a lo~ voltage electric cartridge fuse incorporatlng the improvement of the present invention is generally desig-nated by ~he reference numeral 10. The fuse includes a tubular fuse body 12 made from an electric insulating material such as, a convolute or spiral wound Kraft paper9 a synthetic-resin-glass cloth lamlnate or vulcanized rag pulp fiber body. The right end 14 and the left end 16 of the fuse body are closed by a pair of s~lhs~antially identical metal terminal caps 18, 20 respectively. The end caps 18, 20 are positively attached to the fu~e body ~ by crimping at leas~ a portion of each of the end caps as shown a~ 22 and 24, EF'~20 respectlvely, into ~q positlve retainlng relat:Lonstlip with at least a portion of the fuse body.
Looking now to the interior of the fuse, reference number 26 has been used to designate a fusible element designed to interrupt the electric circ.uit in wh:Lch the fuse is used upon the occurrence of a maJor fault current. The short circult element 26 is formed from a ~iheet of metal and is provided with a plurali~y of serially arranged points of reduced cross-sectlon 28 as i8 well known in the prior art for short circu:Lt protection elements in electric Euses.
The right hand end 30 of the fuslble element 26 i8 conduc~ively connected to the lnner 6urface 32 of ~he rlght end cap 18 by a ~older ~oin~ 34 formed from a solder having a predetermlned relatively high temperature meltlng poin~.
In the illustrated embodiment a flat circular washer 36 is posltioned in parallel relationship with the inside surface 32 of the end cap 18. The washer 1~ sized such ~hat it is in overlapp~ng relationshlp with the right hand axial end 38 of the right hand end 14 of ~he fuse body 1~. The washer is preferably fabricated from a me~allls co~ductive materlal and include~ a slot (not ~hown) extending from it8 ou~er periphery into the central portion thereof through whlch the rlght hand end 30 oE the short circuit element 26 may pa~8~ This washer services to properly posit1on ~he short circult element wi~h respect to the fuse body 12 as well as to assure that a good solder Joint is formed between the end 30 of the short circuit elemen~ and the end cap llmer surface 32. ~The wash~r also serves to assure tha~ a pulverulent arc-quenching flller materlal 42, preferably Fullers ~arth,-which fills the inside oE the fuse body which is not occupied by the fuslble element,does no~ escape the confines of the tube body 12 and get into the reglon between the rlght hand end cap 18 and the outslde of the fuse body. As be~t shown in Flgure 2 i~ will be noted that the left hand end of the short circui~ element 26 is provided with an axlally extendlng tapered tab 44 which i9 adapted to be operatively connected to another element of the fuse a9 will hereinafter be described.

Reference numeral 46 ha~ been used to generally designate the overload/
high ambient temperature point interrupting mechanism of th~ fuse of the invention. This mechanism comprises a hollow substantially cyllndrical metal sleeve 48 fabricated from a good electrically conductive materlal. The leEt ~5--~-20 hand end oE the cyllndrical Rleeve 48 iR flared radlally outwardly and has been stamped to remove a portion of material so as to form four radially e~tending mountlng feet 50 as best shown in Figure 4.
The f~ur radially extending feet 50 are of such a length that they extend at least ~n partial overlapping relatlonsh:Lp with the axial end 52 of the left hand end 16 of the fuse body 12. The feet 50 are conductively connected to the lnner surface 54 of the left hand end cap 20 by a solder Jolnt 56 formed from a solder having a predetermined relatlve:'y high ~emperature melting polnt substantially iden~ical to ~hat of the solcler ~oint 34 attaching the short circuit e:Lement 26 to the righ~ hand end cap. The spaces 58 formed between the four radially ex~ending mounting feet 50 provide for an even flow of solder during the soldering operation of the sleeve 48 to the inner surface 54 of left hand termlnal cap and further provides for a release of soldering flux gases from within the sleeve to insure that the feet sit flush to the inside of the termi~al cap.
Mounted for axi~l movement withln the metallic ~leeve 4~ i8 a metallic plunger 60 having an enlarged diame~er section 62 at its left hand end ~hereby forming an annular shoulder 64 facing to the right with respect to the fuses longitudinal axis, as shown in Figure 1. The plunger 60 further includes a main, substantiAlly tubular9 portion 66 extendlng from the enlarged diameter portLon 62 in ~ slightly tapered fashion terminati~g at 1~ righ~ hand end 6 with it~ portion of smalle~t cross-sectional area. Looking at the cutaway sectlon of Figure 2 it will be seen ~hat the right hand end 68 of ~he plunger i~ provLded with a cavity 70 into which the tapered tab 44 formed at ~he left hand and of the short clrcult element 26 may be matingly received. Such a relatioll8hip i9 sho~n :Ln Figure l, wherein ~he short clrcllit element 26 i8 attached to the plunger 60 by a solder ~oint 72 retaining the tab 44 in the above described mating relation wlth the cavity 70. The solder Joint 72 is formed from a solder having a predetermined relatlvely low temperature melting point.
Looking IIOW again at the ou~er metal sleeve 4B in which the plunger 60 is partially retained it will be noted that the sleeve abruptly tapers down to a smaller diame~er section 74 at its right hand end thereby defi~ing a shoulder RF ~0 q~

- surface 76 fac:Lng to the left hand end of the slee~e 48. The inner surface of the sleeve 48 9 the left hand facing shoulder 76, the outer surface of the movable plunger 60, and the right hand facillg shoulcler 64 deflned by the enlarged sectlon of the pl~lnger, all cooperate to define an annular cavity 78 surro~mdlng the main body of the movable plunger. Disposed within this cavity in coaxial arrangement wLtll the sleeve and the plunger is a helical spring 80 which is held in a compressed position between the shoulder 64 on ~he plunger and ~he ~houlder 76 of the metal 01eeve. A second solder Joint 82 forined from a solder having a predetermLned relatively low temperature melting point i8 formed between the extreme right hand and of ~he rnetal Rleeve 48 and the outer surface 84 of the plunger.
It will be apprecia~ed ~rom the foregoing structural deæcrlptio~ that the two low temperature meltlng point solder jointæ, 72 and 82 hold ~he movable plunger 60 in the posi~ion illustrated in Figure 1, and, ~hat ~he helical spring 80 reacts between ~he right hand facing shoulder 64 of the plunger and the left hand faclng shoulder 76 of the metal ~leeve in a manner ~endlng to displace the plunger to the left within the metal sleeve thereby breaking tne two relatively low temperature melting poin~ solder ~oints. It wlll be accordingly further appreciated that immediately upon meltlng of the two rela~ively low temperature melting polnt solder ~olnts 72 and 82, ~he movable plunger 60 will be rapidly displaced to the left to assume the positlon shown in Figure 2 thereby posltively in~errupting the flow of elec~ricity through the fuRe .
It should be appreciated that the helical spring 80 also exerts a Eorce on the shoulder 76 whlch tends to dlsplace the ~leeve 48 to the rlght. Such displacement is however prohibited aæ ~he solder ~oint 56 connectlng the sleeve feet 50 to the end CAp læ made wlth a solder havlng a higher melting poin~ than the solder ~oints 72 and 82. Purthe~, the overlap between the four feet and the end 5Z of the fuse body provides a mechanical anchor for the sleeve 48.
Throughout the speelfica~ion the two solder joints 72 and 82 which keep the sprlng loaded plunger ~0 from moving with respect to the sleev2 48 and ~he sh~rt circuit element 26 have been descrlbed as being formed from a æolder .

~F-20 havlrlg a predetermlned relatively low temperature meltlng polnt. Slmllarlg, the solder ~oints 34 and 56, Jolning the 3hort circuit element 26 and the sleeve feet 50 to thelr respectlve end caps 18 and 20, have been described as being formed from a solder havin~ a predetermined rel~tively high t:emperature melting point~ In the preferred construct:Lon all of these ~oints are formed from what may be categorized as low mel~lng point alloy solders. The Lmpor~ant relationghip i8 that the melting point of t:he ~oints 72 and 82 must be Yuffi clen~ly lower than that of the Joints 34 and 56 to a~sure that 72 and 82 will melt~before 34 and 56. Further9 lt i~ preferred that the ~olnts 72 and 82 be formed from a eutectic alloy. Such a solder has no plastlc range9 rather it has a sharply defined point at which melting occura.
In one specific example9 the ~olnts 72 and 82 are formed from a eutectic alloy comprising 58% bismuth and 42% tln. Such an alloy has a melting poin~
of 281F. The end cap ~olnts 34 and 56 may be ormed from a eute~tic alloy comprislng 63% tin and 37% lead9 whlch has a melting point of 361F.
It will be noted in the illustrated embodlment that an insulatlng sleeve 86 has been provlded surrounding the metallic sleeve. This slee~e 86 is provided on certain ampere ratings oE the fuse design to assist in controlling the thermal transfer between the metallic shell and the fuse filler material.
Further illustrated in this embodiment is a cir~ular washer 87 formed from a suitable insulating material whlch is shown at the left hand end of the fuse lnterposed be~ween the inwardly faclng surfaca of tha four radially extending feet 50 and the left hand end i6 of the fuse body 12. This waæher serves to assist in precluding passage of filler material 42 into ~he interior of the metallic sleeve 48.
Normal current path through the above de~cribed f~e is as follows:
terminal cap 18, solder jolnt 84, ~hor~ clrcui~ element 26, solder ~oint 72, plun~er 60, solder ~Oillt 82, metallic sleeve 48, solder joint 56, and terminal cap 20. In the operation of the fuse the occurrence of a major fault or short clrcuit, causes the rate of rise in ~emper~ture of the reduced cross-~ectional areas 28 of the short circuit element 26 to more or less instantan-eously vaporlze which lnterrupts the faulted current flowing through the fuse. In the case ~f low overload curren~s9 the ra~e of rise in temperature of the reduced cross-sectional area 28 in the fusible element 26 is quite slo~ thus permi-tting the heat generated to con~
ductively flow -through into the other components of the fuse.
When sufficient heat has built up such that the melting temper-ature of the solder joints 72 and 82 is reached, interconnect-ion between the plunger ~0 and the metallic sleeve 48 is bro~sen and the force of the compressed helical spring 80 immediately displaces the plunger from the position shown in Fi~ure 1 to -that shown in Figure 2 thereby interrupting the circuit.
Interruption of the fuse in this manner, i.e. dis-placement of the plunger 60, rnay occur as a result of heat build up from two different sources: 1) heat generated by sources internal to the fuse; and/or 2) heat generated by sources ex-ternal to the fuse. Internal heat is caused in a conventional fashion when an overload current higher than the rated current of the fuse is passed through the elements of the current path as discussed hereinabove. The second heat source is external heat which eventually may cause sufficient heat build up in the fuse to cause interruption of the fuse via the melting of solder joints 72 and 82 in a manner identical to that of an overload interruption. External heat may be caused from man~r sources, such as, for example, poor contacts, poor wire termination in the fuseholder, or a fire in -the region of the fuse panel. Accordingly when the ambient temperature out-side the fuse reaches a sufficient level to drive the solder joints 72 and 82 to their melting point the fuse will interrupt.
Figures 5 and 6 simply depict alternative configur-ations of a short circuit element for use for various ampere rated fuses. The element of Figure 5 is formed from sheet ~1 ~3 -9a~

me-tal havi.ng serially arranged points of reduced cross-section 88 with a rectangular reduced cross-section 90 located near the tip 92. The advantage of the rectangular notch 90 is -to provide a hot-ter notch than the notches 83. The notch 90 is l.ocated nearest the solder joint 72 between the plunger 60 and the sl.eeve 48 thereby providing a more accurate and concise heating effect to melt .solder joints 72 and 82 during low overload current and high ambient temperature operation of the fuse. This feature is usually used by, but not limited to, fuse si~es of low rated amperage. Fusible element 94 of Figure 6 is of stamped sheet metal and has ,r.~

serially arranged polnts of reduced cross-sectlon 96 and a tapered tip 98 to engage the plunger cavity 70. This fusible element design ls usually used for intermediate fu~e rated current values.
Thus it is apparent that there has been provided, in connection with the invention an elec~ric fu~e havingJ in addition to short circuit and overload current protection capabillty, the capabili~y of reliably interrupting when the ambient temperature surrounding the fuse e~ceeds fl predetermlned value.
This invention may be practiced or embodied in still other ways wlthout departing from the ~plrit or esYential character ~hereof. The preferred embodiment de~cribed herein i8 therefore illu~trative aad not res~ric~lve, the scope of the lnven~ion being lndicated by the appended clalms and all varia-tions which come withln the meaning of the claims are lntended to be embraced therein. We claim as our invention:

Claims

Claim 1 An electric fuse capable of interrupting an electric circuit when sub-jected to major fault currents, and, overload currents in the electric circuit, and, for also interrupting when the ambient temperature in the proximity of the fuse exceeds a predetermined value, said fuse comprising;
(a) a casing of electric insulating material;
(b) a pair of terminal caps including a first terminal cap and a second terminal cap closing the ends of said casing;
(c) an elongated fusible element inside said casing, said element having a plurality of serially arranged points of reduced cross-section, one end of said element being conductively connected to said first terminal cap by a first relatively high temperature melting point solder joint;
(d) an axially movable plunger arranged inside said casing, said plunger being conductively connected by a first relatively low temperature melting point solder joint to the other end of said elongated fusi-ble element;
(e) a metal sleeve, inside said casing, surrounding at least a portion of said plunger and defining an annular cavity therebetween, one end of said sleeve being conductively connected to said second terminal cap by a second relatively high temperature melting point solder joint;
(f) a helical spring disposed within said annular cavity, said spring being held in a compressed condition by operatively engaging said plunger and said metal sleeve, the force of said compressed spring acting upon said plunger in a manner tending to break said first relatively low temperature melting point solder joint;
(g) a second relatively low temperature melting point solder joint conductively interconnecting the other end of said metal sleeve to said plunger; the force of said compressed spring acting upon said plunger also tending to break said second relatively low temperature melting point solder joint and (h) a pulverulent arc-quenching filler filling the remainder of the interior of said casing.
Claim 2 The electric fuse of Claim 1 wherein said metal sleeve is provided with a plurality of radially extending feet at said one end connected to said second terminal cap.
Claim 3 The electric fuse of Claim 2 wherein said feet are of sufficient length that they extend between the end of said second terminal cap and the axial end of said fuse casing which said cap closes.