Note: Descriptions are shown in the official language in which they were submitted.
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HIGH SPEED FUSE
This invention relates in general to fuses and more
particularly to high speed fuses.
High speed fuses have been used for a number of years for
the protection or isolation of semiconductor devices such as
diodes and thyristors. There is very little safety factor in
these semiconductor devices and they can fail quickly when
subjected to overcurrents. Therefore, a fuse designed to protect
semiconductor devices must open quickly. High speed fuses have
very little thermal capacity, and in general open in the order of
.001 to .004 seconds when interrupting short circuits.
Problems exist with high speed fuses currently on the
market because these fuses have been developed over time to meet
specific applications, resulting in a large number of different
fuses made in different sizes and shapes to satisfy the voltage
and amperage ranges expected to be encountered. Several hundred
different parts and subassemblies for these fuses may be
required. Thus, it would be desirable to be able to manufacture
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'uses having standardized parts to reduce the total number of
parts that need to be stocked in order to manufacture a complete
line of high speed fuses.
Many applications for high speed fuses require the use of a
plurality of fuses usually mounted side by side in close
proximity of one another. Many users wish to mount multiple
fuses as close together as possible. Prior art high speed fuses
have metal end bells which are mechanically and thus electrically
connected to the mounting terminals held to the insulating tube
with metal pins which are exposed flush with the tube surface and
are not sealed. Consequently, when in use in an electrical
circuit, the pins are at the same electrical potential as the
terminals and end bells. Typically, three phase electrical
applications use a fuse in each phase mounted adjacent to each
other and as close as possible to conserve space within the
equipment. Industrial standards govern minimum spacing between
electrically hot parts and dictate the minimum distance between
live or " electrically hot" parts through air as a function of
stand-off voltage. Since the pins are electrically hot and
exposed to the tube surface, this minimum distance is measured
between adjacent tube surfaces, as opposed to terminal distances.
Yet another difficulty is encountered in manufacturing high
speed fuses in that the end bell must be joined to the terminal
for mechanical strength of the fuse package and, in most designs,
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for the electrical connection between the current carrying
fusible elements within the fuse and the mounting terminal.
Prior art high speed fuses accomplished this by brazing, welding
or soldering the terminal to the end bell or machining the end
bell and terminal from a solid piece of metal or by pressing the
metal pins through the tube and end bell and into the mounting
terminal. All these techniques are labor intensive.
A further problem is encountered with the end bells in that
these circular pieces of metal are most often forged or machine
from rod stock and coined, drilled, and sized. This again
requires extra time and additional labor and is thus more
expensive.
Yet another manufacturing problem is encountered in making
high speed fuses. These fuses, in general, are filled with sand
or other arc quenching materials. This material is added through
a hole in the end bell after the end bell is assembled to the
fuse tube. Various methods of plugging the hole have been used,
but all suffer from various limitations. For example, costly
knurled plugs are used which require excessive pressure to insert
the plug into the hole in the end bell. A more economical means
to close the sand hole is required.
The present invention employs stamped end bells and
terminals rather than forged or machined parts. A slot for the
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-erminal is punched through the end Dell. The terminal is
inserted into the slot in the end bell and staked in position.
This insures a strong tight fit without requiring welding or
soldering. One end of each terminal has coined ridges to
facilitate automatic welding of the fuse link to the terminals.
Round balls are used to seal the fill holes for the arc quenching
material. Alternatively, the end bells may be molded of plastic.
Metal pins secure the end bells within the fuse housing but are
not electrically connected to the terminals because of the
insulative end bells.
Figure lA shows a perspective view partially cut away of a
prior art fuse;
Figure 1B shows a perceptive view partially in section and
exploded of a fuse according to the present invention.;
Figure 2 shows a top view of a terminal of the fuse shown
in Figure 1;
Figure 3 shows a front view along the lines 3-3 of the
terminal shown in Figure 2;
Figure 4 shows a complete end bell assembly;
Figure S shows a side cross-sectional view of the assembled
fuse of Figure 1;
Figure 6 is a top cross-sectional view of the fuse shown in
Figure 1;
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Figure 7 is a partial cross-sectional -riew of the
installation of the ball in a sand hole as shown in Figure S;
Figure 8 is a cross-sectional view along the lines 8-8 in
Figure 7; and
Figure 9 shows the cross section of two adjacent fuses
illustrating positioning distance.
Figure lA shows a prior art high speed fuse 9. Metal end
bells 13 with the terminals 15 are housed within the ends of an
insulating tube 40. It is seen that the terminal 15 is welded or
brazed at 17 onto the metal end bell 13. Thus the end bell 13 is
electrically hot when the fuse is mounted in an electrically
energized circuit. The end bell is held in place in insulating
tube 40 by metal pins 41 which are also at the same voltage level
as the end bell 13. Thus, as shown in Figure 9, the minimum
distance " F" that prior art high speed fuses can be placed
adjacent to each other, as dictated by industrial standards, is
governed by the distance between the electrically hot pins 41 of
adjacent fuses.
In the fuse according to the present invention shown in
Figure 1B, 5 and 6 and referred to generally by numeral 8, an end
bell assembly of an end bell 12 and terminal 14 are adapted to be
received in each end of insulating housing or tube 40. The end
bell 12 and terminal 14 are stamped from a piece of material
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and a slot 16 is punched in the end bell 12. The terminal i4 is
inserted into the slot 16 until shoulders 48 engage the end bell
12. The terminal 14 is then staked at 46 or coined or
mechanically upset in position as shown in Figure 4 to attach
terminal 14 to end bell 12 . Thus the term; r,a ~ any rr,o o.,a ~..., ,
are joined without brazing, welding or soldering, and without
complicated mechanical assembly using additional components. It
is more cost efficient to produce the parts by stamping rather
than by forging. Stamping provides a scrap reduction over making
the parts from forging and also allows the selection of
appropriate materials for the end bell 12 and terminal 14 which
can be of dissimilar materials such as plastic and metal.
As shown in Figures 2 and 3 , a series of small rectangular
cross-sectioned ridges, weld pads or projections 44 are embossed
on the inner end of terminal 14 during fabrication for welding
terminal 14 to one end of a fuse element 30, preferably by
resistance welding. These weld projections 44 may be coined or
machined into terminal 14. The height of each of weld
projections 44 is the same over the entire weld area. The number
of projections 44 is determined by the width of terminal 14. The
size and shape of the pads may vary from terminal to terminal
depending upon the size of fuse element 30 and end bell 12. A
saw tooth pattern may also be used in some applications.
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The pitch and depth of the saw tooth will -.~ary with link
thickness.
An advantage to using weld projections 44 on the terminal
14 is that it improves the welding of the fusible element 30 to
the terminal 14. Weld projections 44 provide consistent weld
quality and welded surface area resulting in consistent heat
transfer and electrical conductivity through the welded joint
between the fusible element 30 and the terminal 14 of fuse 8.
This results in more reliable fuse performance and reduction in
costs because all welds can be made simultaneously. This
procedure also reduces maintenance of the weld electrodes because
both electrodes are flat blades as compared to small pointed
electrodes. This type of construction is also very useful for
automating welding and results in a more consistent weld than
that afforded by prior art spot welding techniques. Direct labor
is reduced because multiple welds can be made with each electrode
closure. Thus, the time required to weld the strip to the
terminal is reduced, the consistency of the weld area is
improved, and the electrical conductivity and heat transfer from
terminal strip to fuse element is consistent.
The fuse element 30 is preferably of a standardized planar
design using an accordion shape which allows for the use of an
element having a substantially longer overall effective length
than can be achieved with a straight through element as in most
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prior art high speed fuses. 'i'he increase in effective length
enhances the ability of the fuse to clear lower level overcurrent
situations, especially on DC circuits. As shown in Figures 5 and
6, a fuse element 30 may be welded to both sides of terminals 14
to provide a multiple element fuse.
Referring now to Figure 9, since the terminal 14 projects
through the front face and back face of the end bell 12, the
fusible element 30 may be electrically connected directly to the
terminal 14. Thus, the end bell 12 does not need to be made of
electrically conducting material, and may be made of plastic or
other non-electrically conductive materials. End bells 12 of
insulating material, such as plastic, are less expensive than
metal end bells and can be molded instead of machined allowing
for the use of more intricate shapes when necessary. The
insulating end bells 12 not only serve their normal function of
closing off the ends of the insulating tube 40 of fuse 8 and
provide the required structural integrity to the fuse package,
their use results in the metal pins 41, which secure the end bell
12 within the tube 40, being insulated from the electrical
circuit passing through terminal 14 and fusible element 30.
An advantage of using plastic or other non-electrically
conductive material for the end bell 12 is that pins 41, designed
to project through the insulating tube 40 into the end bell 12,
are not energized since the end bell is not electrically
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conductive. Since holding pins 41 are not " electrically hot"
when mounted side by side, fuses can be positioned closer
together thus conserving panel board space in equipment. In the
prior art fuse where the metal pin is electrically hot, the
adjacent fuses cannot be mounted side by side in close proximity
more than the distance " F" shown in Figure 9. The separation
between the fuses is governed by the distance between the
terminals which are electrically hot. However, when mounted in
an electrical circuit, high speed fuses manufactured according to
the present invention may be positioned closer to one another
since the minimum distance " F" is measured between the
electrically hot terminals 14 and not the pins 41 as shown in
Figure 9.
As the fuses are assembled, a first terminal is joined to a
first end bell a second terminal is joined to a second end bell,
and the fusible element 30 is welded between the two terminals.
Because of the weld projections 44 on the terminal 14, the
welding of the fusible element may be done by projection welding.
Referring now to Figures 5-8, the assembly of the end bell
assemblies welded to the ends of the fusible element 30 is
slipped into the insulating tube 40 and the end bells are held in
position within tube 40 by pins 41 which are inserted into
aligned apertures in tune 40 and end bells 12. Aligned
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apertures are placed in the end bells i2 and tube 40 by drilling
just prior to insertion of pins 41. An arc quenching material
43, shown in Figures 5 and 6, typically special sand, is poured
into sand holes 20 in the end bell 12 and the high speed fuse 8
is filled with sand, the holes 20 are closed using a solid
spherical ball 18. These round balls 18 may be steel or other
material and are slightly larger than the hole 20 in the end bell
12. The ball 18 is prevented from misalignment because it is
guided by its own natural radius into the hole 20. The balls 18
are thus self-centering and are held in place by frictional
force. Alternately, particularly where end bells 12 are metal,
such as brass, the sand hole 20 may be coined after insertion of
the ball 18 to hold the ball in. The balls 18 are forced or
pressed into the end bell 12 by an insertion tool 45. The ball
18 is trapped between three small pins 47 which displace the
metal 49 of the metal end bell 12 over the ball 18 while the
final insertion is being accomplished. The flat bottom of the
insertion tool 45 provides automatic insertion depth. Over
insertion is prevented by the design of the insertion tool 45.
Using balls 18 has several advantages. The hardened steel ball
18 provides a low cost, self-aligning, easily inserted means of
plugging the fill hole 20 in the end bell 12 of a fuse a . The
steel ball 18 requires less force to insert and tends to be self-
locking. This is significantly easier than prior art processes
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which often used pins, hollow closed-end cylinders, or screws to
seal the holes.
It is seen that high speed fuses manufactured according to
the present invention are easier to construct, require less labor
and are consequently less expensive to manufacture and, in one
embodiment, can be used closer together, when mounted adjacent to
one another, with reduced danger of shorting from fuse to fuse.
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