Note: Descriptions are shown in the official language in which they were submitted.
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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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q~
- 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
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^ 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,
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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
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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
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- 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
.
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~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
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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:
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