Note: Descriptions are shown in the official language in which they were submitted.
DP 26.4.76
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1038~Z7 :
This invention concerns fuse elements of the kind .
which are i~corporated in so-called cartridge fuses. .
It is advantageous, in fu~e practice, to design a
fu3e that can withstand relati~ely high values of current :
: surge in the one to ten second region of the fuse time/
current characteristic, and yet which has a relati~ely ~
low value of minimum fusing currentO - .
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Such a fuse can be used in the protection of
electrical apparatus (such as an electric motor) which
in its normal operation, produce~ non~injurious surges :
of current of short duration, without unwanted fuse .
blowing. A fuse of this type avoids the need for ex- .
cessire over-fusing, and enables close protection to be
given to the associated cables supplying the apparatu .
: A~ object of the in~ention is to provide such a fuse.
With this ob~ect in view, the present invention provides
a fuse element comprising a strip o~ silver, copper or
-other suitable electrically-conductive metal, said strip
ha~ing a substantial mass of a lower melting point ~etal
or metal alloy (for example, tin or a tin/lead al~oy)
: located in a trough~ formed in the strip,and located at
any ~uitable point along the length of the strip but
preferably approximately at the centre, the ~trip beine
reduced in section, to create a relatively high resistance
zone, i~mediately adjacent to one or both of the bound-
aries of the trough.
With such an element, at minimum fusing current
suff~cient heat is produced in t~e reduced section or
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sections, if the current i~ su~tained e~entually to osuse
melting of the low melting point metal or alloy in the
~' ad~ac0nt troughO The metal of the strip~ in contact
with this molten metal, then diffuseq into the molten
metal and the strip becomes severed in this region of ;:
reduced section~ thereby to interrupt the electric circuit
in which the element i8 di8poged.
At currentq in the region of two to four times the
fu~e'~ minimum fusing current, the reduoed sections again
produce heat 9 but the quantity of metal or alloy in the
trough immediately adjacent to the reduced section or
oections provides a ready means o~ absorbing the heat,
and the meltlng of the trough metal and of the adjacent
reduced section i9 delayed ~or a significant period of
time, so that fusing does not occur i~ such currents are
maintained only ~or a short period of ti~e. -
The in~ention will be described ~urther, by way of
example, with reference to the accompanying drawings, in
which:
j 20 Fig. 1 i9 an enlarged plan view o~ a first embodiment
o~ the ~use element of the in~ention;
Fig. 2 i8 an ele~ation of the fuse element of Fig. 1;
Fig. 3 i8 sectional elevation, to a reduced scale
compared with Figs. 1 and 2, illustrating a cartridge fuse
incorporating the fuse element o~ Figs. 1 and 2;
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l DP 2704.76
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1038427
Figs. 4 and 5 are respectively a plan view and a
cor:responding elevation, illustrating a second embodiment
of the fuse element of the invention;
Fi~o 6 is a sectional elevation, to a reduced ~cale
compared with ~igs. 4 and 5, illu~trating a cartrldge fuse
lnoorporating two of the fu~e elements of Figs. 4 and 5;
. Fig~o 7 and 8 are respectively a fragmentary plan and
¦ a oorre3ponding elevation, showin~ a third embodiment of
the fuse element of the invention;
~ig~. 9 and 10 are views ~imilar to Fig~. 7 and 8
but showine a fourth embodiment;
.. Figs. 11 and 12 are view-R similar to Figs. 7 and 8
but ~howin~ a flrth embodiment, - -
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. Fi~so 13 and 14 are views si~ilar to Figs. 7 and 8
but showing a si~th embodiment; and
Fies. 15 and 16 are views ~i~ilar to Figs. 7 and 8
but showing a seventh embodiment.
In the various figures, similar reYerence numeral~
have been allocated to similar parts.
1 20 The practical~embodiment of the fuse element illustrated
in Figs. 1 and 2 comprises. a strip 10 of electrically-
conductive material, conventionally silver, whose thick-
ness t may be from one-and-a-half thousandths to ten-
thousandths of an inch, and whose width 1~ may be, for
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1 example, from one-sixtee~h to o~e-~uarter of an inch. Approx-
imately at its middle, the strip 10 has a mass 11 of low melting
point metal or alloy, e.g. a tin/lead alloy, located thereo~ in
a trough 12 formed in the strip 10 by bending the latter.
At each side of the trough 12, immediatel~ adjacent
thereto, the strip 10 is reduced in section to create relatively
high resistance zones, by aligned notches having been cut into
the strip 10 from opposite longitudinal edges thereof, these
notches being of width x and leaving narrow necks 13 of width b
approximately one third of the width W of the strip 10.
In addition to the notches defining the necks 13
immediately adjacent the mass 11, further notches, of width y,
are formed in the strip 10 to define necks 15, 15 and 16, 16, of
width a, which are successively spaced away from the mass 11~ - -
Fig. 3 shows a cartridge fuse incorporating one of the
elements of Figs. 1 and 2. The element, indicated generally by
the reference numeral 17, is enclosed within a ceramic tube 18
having a particulate arc-quenching filling 19, for example of
iron-free silica sand, and inner end caps 20 with which the
respective ends of the element 17 are connected. Respective
outer end caps 21 fit snugly over the inner end caps 20 so as to
be electrically-conducting contact therewith whilst at the same
time retaining respective protective discs 22, for example of
asbestos, across the respective inner end caps 20.
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38427
1 In suc~ a cartridge fuse, a fusible element 17 having the
following dimensions will give a ~use of 20 ampere rating:
a = 0.030" t = O.OOZ8"
b = 0.030" C = 0.085"
y = 0.030~' D = 0.218"
x = 0.030" W = 0.090"
Precisely the same configuration of fusible element,
of substantially similar dimensions, but different thicknesses
can be used to give fuses of diffexent ratings Thus,for a 25
ampere fuse one would, for example, use dimensions as tabulated
above but with the strip thickness altered to t = 0.0048" and
for a 30 ampere fuse one would use a strip of thickness
t = 0~.0065".
In the case of the 30 ampere fuse, the minimum fusing
current is 40 amperes and the ten second current is 72 amperes;
the minimum fusing currents and ten second currents of the 20
and 25 ampere rated fuses are comparable. .
The reduced sections or necks 15, 16 of the element,
having dimensions a and y, constitute short cicuit regions of the
elément and in use deal with higher values of current, in excess
of about four times the fuse's minimum fusing current. The
portion of the strip 10 consisting of the necks 13, trough 12 ::
and mass 11 and having the dimensions x, b, C and D constitutes
an overload region. It will be observed that the reduced-section
: necks 13, of dimensions x and b, are each disposed immediately
adjacent the respective boundary of the trough 12 whose
dimensions are indicated at C and D and which is occupied by the
mass 11 of low melting point metal or alloy which is shaded in /;:
the figures.
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1~38427
t The invention is not restricted to elements of the
form shown in Figs. 1 and 2, and having the precise dimensions
indicated above, which may be varied. The dimension x could also
be made longer or shorter than dimension y and dimension b could
be made wider than dimension a. Also, within practical limits,
it is an advantage to make the dimensions C and D as great as
possible.
Other variations are possible. Thus, as shown in Figs.
4 and 5, just a single neck 13 may be provided to one side only
of the trough 12, and, as shown in Fig. 6, two of the fusible
elements, indicated by the numerals 23, may be incorporated into
a cartridge fuse composed of substantially similar components ;~
to those of the fuse of Fig. 3 so as to be spaced apart and
provide parallel current paths bet~een the two outer end caps 21.
In such a cartridge fuse, two fusible elements 23
having the following dimensions will give a fuse 40 ampere
rating: , :-
a = 0.061" t = 0.0018"
b = 0.080" C = 0.093"
y = 0.030" D = 0.125"
x = 0.125" - W = 0.180"
As already described above in relation to Fig. 3,
higher fuse ratings are achieved simply by using thicker strips,
whilst keeping all the other dimensions the same. This may be
exemplified by the following table:
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Fuse ra~ihg Thickness- t of the two
" fusible elem-ents 23 the-r of
45 amperes 0.0026"
50 amperes 0.003"
60 amperes 0.004"
amperes 0.0045"
amperes 0.0055"
amperes 0.0065"
100 amperes 0.0085"
' lO In the case of the 60 ampere fuse according to the
above table, the minimum fusing current is 78 amperes, with a
ten second current of the order of 200 amperes, and the minimum -~
fusing currents and ten second currents of the other fuses are
comparable.
The reduction in the element section both adjacent
the trough 12 and spaced therefrom may be accomplished by any
, of the means normally used in fuse practice; for example by use
of necks (as in the elements of Figs. 1 and 2 and F}gs.~ ~ and
5~, holes 24 trough the strip 10 (Figs; i and 8~ or ii~denting the
strip,l0-,(as ,at 25 in Figs. 9 and 10). Just as the necks 13 may
be provided either at both side or one side only of the trough 3~2! '~
so can the holes 24 or indentations 25. '
The mass 11 of low melting point metal or alloy may
be contained in a trough or troughs of configuration different , ,~
from those discussed ln relation to Figs. 1 to 10. One such ~
possibility is shown in Figs. 11 and 12 which illustrate a -;
trough 26 of rectangular section. Figs. 13 and 14 show an
arrangement of a circular rod-like mass 11 oE alloy located in a ';
semi-circular trough 12.
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1~38~27
1 In the embodiment of Figs. 15 and 16 of the drawings,
two troughs 27 and 28 are provided, these being spaced apart by
a distance just sufficient to accomodate a single neck 13
therebetween. In this embodiment, of course, further reduced
sections, e.g. in the form of necks 13, holes 17 or indentations ~-
18 may be provided at one or both of the boundaries, remote from
the illustrated neck 13, of the troughs 27, 28.
In the embodiment of Figs. 1 and 2, the necks 15 and
16 spaced away from the trough 12, and providing regions which
melt rapidly upon occurrence of short-circuit currents, may be
more or fewer in number than in the illustrated case and can, of
course, be omitted. Naturally such necks or similar reduced-
section regions may be provided, if desired, in each of the
elements of Figs. 4 to 16.
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