Note: Descriptions are shown in the official language in which they were submitted.
CA 02579932 2010-02-24
FUSE PROVIDING CIRCUIT ISOLATION
AND VISUAL INTERRUPTION INDICATION
FIELD OF THE INVENTION
The present invention relates generally to electric current interruption
devices
and, more particularly, to a low voltage fuse having the ability to provide
circuit
isolation and indication of operation in circuits presenting little or no
voltage across
the open fuse.
BACKGROUND OF THE INVENTION
A fuse is a protective device for electrical circuits which has a fusible
element
that melts and opens to interrupt the circuit when subjected to excessive
currents. The
melting occurs, in large part, due to 12 R heating of the fusible element. For
many
types of fuses, melting at relatively high currents (i.e., currents that
produce melting
in less than about I second) typically occurs at one or more reduced cross-
sectional
areas of the fusible element, so designed as to control the melting time
versus current
characteristic of the fuse (melting time-current characteristic or TCC). The
TCC is an
important characteristic of the fuse that enables it to provide appropriate
system
protection and coordination with other devices. Favorable ratios of rated
continuous
current to short time melting current frequently require the use of relatively
high
melting point materials for the fusible element, such as silver and copper. At
longer
melting times, a common practice is to employ an additional means to initiate
melting
and arcing, using a lower melting point material. This is done in order to
prevent
excessive fuse component temperatures before circuit interruption, and to
provide
suitable graphical curves of TCC for typical applications.
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The low melting point material is typically implemented in one of three ways.
The first, termed the "M" effect (after its discoverer, Metcalf), attaches the
low
melting point material to the high melting point element in such a way as to
cause the
high melting point material to dissolve into the lower melting point material
when the
latter becomes molten. Thus, after a period of time, the element is severed
and an arc
is initiated at the melted open point.
A second method, disclosed in U.S. Patent No. 5,604,474 to Leach and
Bennett, employs a tin low melting point material as a "bridge" between high
melting
point elements, wherein an arc is initiated when the tin melts. This method
does not
suffer from the potential deterioration problems inherent in the first method,
but is
harder to manufacture.
A third method known in the art employs a spring loaded joint, termed a
"mouse trap," made between high melting point components using a low melting
point solder. When the melting point of the solder is reached, the spring
causes
separation of the contacts thereby producing an arc. This method allows
considerable
flexibility in the design of the fuse's TCC, since the mass of the components
and
melting point of the solder can be used to control the melting
characteristics,
frequently allowing superior surge withstand for the fuse. However, in order
to allow
the physical movement that initiates circuit interruption, the joint has to be
surrounded
by a fluid (usually air) rather than by sand, which is the preferred medium to
surround
a fuse element because it gives improved interrupting capability to the fuse.
Whatever method is used to initiate arcing at longer melting times, a common
requirement for such fuses is the provision of some form of indication that
the fuse
has indeed operated. This makes finding the "blown" fuse much easier.
The most common method of indication is to run a small conductive wire in
parallel with the main element(s). When the main element melts, system voltage
causes current to flow through the indicator wire and to melt it. The current
quickly
switches back to the main elements, which then arc and interrupt the
overcurrent. The
melting of the indicator wire provides indication through a variety of means.
Most
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commonly, the indicator wire is arranged to release a spring loaded pin, or
ignite a
small explosive charge to move a striker, when the indicator wire melts.
Obviously a
minimum voltage, sufficient to drive enough current through the indicator wire
to
cause it to melt, is necessary for this indication method to work. Typically
this
requires at least 5-10 volts.
Another means of indication has been to connect, in parallel with the fuse, a
circuit containing a light emitting device, such as a neon, LED or lamp.
Again,
system voltage across the indication circuit after the fuse has operated is
necessary for
this method to work.
For most conventional low voltage fuse applications, the techniques described
above are sufficient to successfully interrupt fault currents, provide
indication and
then withstand system voltage for very long periods of time. However, there
are some
applications for which these conventional low voltage fuses are not suitable.
For
example, in some applications, where step-down transformers are used to supply
a
low voltage distribution network consisting of many parallel conductors fed
from
many transformers at different points in the system, it is common practice to
fuse
individual cables to prevent them from being overloaded. These cables
typically use
conventional current-limiting fuses or fusible limiters, designed to open when
excessive current flows. However, it is possible in such applications for
little or no
voltage to appear across the limiter after the circuit is opened (only the IR
drop in the
parallel cables will appear across the open point), because an overloaded
cable can
often have other cables connected in parallel.
This leads to two potential problems. The first is that with little voltage
across
the limiter when melting occurs, there is little arcing. This can lead to a
relatively
high resistance open point, sufficient to prevent current flow in the
overloaded cable,
but which is not high enough to enable conventional fault finding equipment to
be
effective at finding the open point, as compared with a fuse which arcs and
which
would normally have a resistance of many millions of ohms. This delays the
finding
and replacing of the operated device.
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Visual indication of the operated limiter would obviously be desirable to
speed
up the process. However, the second problem is that the lack of recovery
voltage also
prevents conventional indicators from working.
Accordingly, there is a need for a fuse capable of interrupting current
effectively for conditions where the voltage can vary from rated voltage, down
to very
low values (possibly as low as 1 V) whilst providing indication of its
operation in a
manner that allows visual indication at the fuse, together with remote
indication if
desired. Such indication is needed with whatever current causes the fuse to
melt
open, and with what ever degree of arcing that occurs.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fuse which is designed
to
provide visual indication, and a distinct open section, with relatively high
currents that
cause the melting of an element made from a relatively high melting point
material
(>500 C), independent of the voltage that can cause arcing.
It is another object of this invention to provide a fuse which is designed to
provide visual indication, and a distinct open section, with relatively low
currents that
cause the melting of a joint made with a relatively low melting point material
(<500 C).
It is still another object of this invention to provide a fuse with the
previously
described characteristics in which the open section(s) are surrounded by
granular
dielectric filler, the purpose of which is to improve the interrupting
capability of the
fuse, and to provide a better dielectric withstand after current interruption.
In the efficient attainment of these and other objects, the present invention
provides a fuse with interruption indication. The fuse generally includes a
fuse
housing having first and second electrical terminals, a fusible structure
disposed
within the fuse housing and defining an electrical path between the first and
second
terminals, a biasing element acting on the fusible structure and an indicator
rod
connected to the fusible structure for providing visual indication when the
fuse has
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blown. The fusible structure includes a movable portion and a meltable
portion,
wherein the meltable portion melts when subjected to a threshold current
flowing
therethrough. The biasing element acts on the movable portion of the fusible
structure
for moving the movable portion upon melting of the meltable portion. The
indicator
is connected to the movable portion of the fusible structure and is driven by
the
biasing element for providing visual indication of an interrupted condition of
the fuse
upon melting of the meltable portion.
In a preferred embodiment, the fusible structure includes a high fault current
interrupting element responsive to high currents and a low fault current
interrupting
element responsive to low fault currents connected between the high fault
current
interrupting element and one of the first and second electrical terminals of
the
housing. The low fault current interrupting element preferably has a low
melting
point solder joint portion, while the high fault current interrupting element
preferably
has one or more reduced cross-section neck portions.
The fuse of the present invention further preferably includes a fixed
partition
disposed within and fixed to the housing and a movable partition connected
between
the fusible structure and the indicator rod, wherein the biasing element acts
between
the fixed partition and the movable partition. Also, the indicator preferably
includes
an indicator rod disposed within the housing, which is driven by the biasing
element
to protrude out of the housing upon melting of the fusible structure to
provide the
visual indication.
The biasing feature of the present invention can also be utilized in high
current
fuses with or without the indicator. Such a fuse would simply include a fuse
housing
having first and second electrical terminals, a high fault current
interrupting element
responsive to high currents disposed within the fuse housing and defining an
electrical
path between the first and second terminals and a biasing element. The high
fault
current interrupting element has a meltable portion which melts when subject
to a
high threshold current flowing therethrough to create a gap in the electrical
path and
the biasing element acts on the high fault current interrupting element for
elongating
the gap in the electrical path upon melting of the fusible element. Moreover,
with
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such high current fuses, the fuse housing preferably contains a granular
dielectric
medium therein, and the fusible structure preferably has a sheath surrounding
the
movable portion of the fusible structure to prevent the granular dielectric
medium
from interfering with the movement of the movable portion.
As a result of the present invention, a low voltage interrupting device is
provided which fulfills the above stated need and includes a housing having a
conductive terminal at each end of the housing, and a fusible element
electrically
connected to the terminals. In one embodiment of the invention, the element is
surrounded by a granular dielectric medium such as sand. The fusible element
consists of one or more elements arranged to melt, at relatively high currents
(that is
currents typically causing melting at times less than approximately ten
seconds) at one
or more reduced cross-sectional points on the element, and at lower currents
(typically
causing melting at times longer than ten seconds) at a solder joint. It should
be
realized that the specified melting times for the two parts of the element are
approximate and may be varied considerably, should the protection requirements
so
dictate.
Both the high current element and the solder joint are held in tension such
that
the melting action causes a physical movement of the element. This physical
movement assists in the initiation of an arc, and therefore the current
interruption, and
provides a high resistance "gap" that is independent of the amount of arcing
and
therefore recovery voltage. This movement further forces movement of an
indicating
plunger that can provide either visual indication of the fuse's operation or
can
complete an external circuit to provide remote indication.
A preferred form of the fuse, as well as other embodiments, objects, features
and advantages of this invention, will be apparent from the following detailed
description of illustrative embodiments thereof, which is to be read in
conjunction
with the accompanying drawings.
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In accordance with one aspect of the present invention, there is provided a
fuse
comprising: a fuse housing having first and second electrical terminals; a
movable
partition movably disposed within the fuse housing; a fusible structure
connected
between the first electrical terminal and the movable partition, the fusible
structure
melting when subjected to a threshold current flowing therethrough; a
conductor
connected between the movable partition and the second electrical terminal,
the
conductor, the movable partition and the fusible structure defining an
electrical path
between the first and second electrical terminals; a biasing element contained
within the
housing and connected to the movable partition, the biasing element acting on
the
movable partition to maintain the fusible structure in tension and for moving
the movable
partition upon melting of the fusible structure; an indicator connected
between the
movable partition and the second electrical terminal, the indicator protruding
out of the
second electrical terminal of the fuse housing upon melting of the fusible
structure for
providing visual indication of an interrupted condition of the fuse; and a
fixed partition
fixed within the fuse housing, the fixed partition and the movable partition
defining three
interior housing sections and the biasing element being retained between the
fixed
partition and the movable partition, wherein the fixed partition includes an
aperture and
the fusible structure is slidably received in the aperture, and wherein the
fixed partition
further includes a sleeve fixed in the aperture, the sleeve including a
central bore for
slidably receiving the fusible structure.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional view of a preferred embodiment of a low voltage
fuse of the present invention.
Figure 2 is a cross-sectional view of the preferred embodiment of the low
voltage fuse shown in Figure 1 rotated 90 on its axis as compared to Figure
1.
Figure 3 is a cross-sectional view of the low voltage fuse shown in Figure 1,
after a relatively high current has melted its element.
Figure 4 is a cross-sectional view of the low voltage fuse shown in Figure 1,
after a relatively low current has melted its element.
Figure 5 is a partial cross-sectional view of an alternative embodiment of the
low voltage fuse in accordance with the present invention.
Figure 6 is a partial cross-sectional view of another alternative embodiment
of
the low voltage fuse in accordance with the present invention, before the fuse
has
operated.
Figure 7 is a partial cross-sectional view of the fuse shown in Figure 6,
after
the fuse has operated.
Figure 8 is a partial cross-sectional view of still another alternative
embodiment of the low voltage fuse in accordance with the present invention,
before
the fuse has operated.
Figure 9 is a partial cross-sectional view of the fuse shown in Figure 8,
after
the fuse has operated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The significance of the present invention will be best understood by a
description of the sequence of events when the device is subjected to both
high and
low currents.
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Referring first to Figures 1 and 2, the fuse 100 of the present invention
generally includes an elongate, insulative, non-conducting housing 1 closed at
its
opposite ends with two conductive terminals 2 and 3, which are electrically
connected
to a power circuit. A movable partition 4 and a fixed partition 5 divide the
interior of
the housing 1 into three sections. First section 6 extends between movable
partition 4
and the first terminal 2, second section 7 extends between the movable
partition 4 and
the fixed partition 5 and third section 8 extends between the fixed partition
5 and the
second terminal 3.
The movable partition 4 is made of an electrically conductive material and is
generally in the form of a piston-type element, which is able to slide within
the
interior space of the housing 1. Fixed to the movable partition 4, and passing
through
the second and third housing interior sections 7 and 8 is a fusible element 9
made
from a high current melting material, such as copper or silver. The fusible
element 9
preferably includes one or more neck portions 10 of reduced cross-sectional
area, as
compared with the rest of the fusible element. As can be appreciated by one
skilled in
the art, such reduced portions 10 are not necessary for the successful
operation of a
fuse made according to the present invention, but are normally preferred in
order to
obtain superior melting characteristics for the fusible elements.
The fixed partition 5 is fixed to the interior wall of the housing 1 and
includes
a slot or aperture 19, through which the fusible element 9 movably extends.
The
fusible element 9 extends from one end fixed to the movable partition 4,
through the
slot 19 of the fixed partition 5, and is attached at its opposite end to the
second
terminal 3. The fusible element 9 is mechanically and electrically attached to
the
second terminal 3 via a fusible joint 11. The fusible joint 11 is preferably a
soldered
joint made from a relatively low melting point material, such as tin. As will
be
discussed in further detail below, the fusible joint 11 is designed to release
the fusible
element 9 from the second terminal 3 when subjected to a low current
threshold.
A biasing element 12, such as a spring, is disposed within the second housing
interior section 7 between the movable partition 4 and the fixed partition 5.
The
biasing element 12 acts between the movable partition 4 and the fixed
partition 5 to
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bias the partitions apart. Such biasing action keeps the fusible element 9 and
the
fusible joint 11 in tension. The electrical circuit is completed by a flexible
conductor
13 electrically connected to the movable partition 4 at one end and
electrically
connected to the first terminal 2 at its opposite end.
Also attached to the movable partition 4 is an indicating rod 14, which
extends
from the movable partition through the first housing interior section 6 and
passes
through an opening 28 formed in the first terminal 2. As will be discussed in
further
detail below, the indicating rod 14 preferably has a length such that the
protrusion end
16 of the rod, opposite the movable partition, is flush or slightly recessed
within the
first terminal 2 when the fuse 100 is in its operating condition, as shown in
Figures 1
and 2, but will extend or protrude from the first terminal when the fuse is in
its
interrupted state, as shown in Figures 3 and 4. The change in the protrusion
of the
indicating rod 14 therefore gives indication of the condition of the fuse 100.
Specifically, as shown in Figures 1 and 2, with its fusible element 9 intact
and
connected to the second terminal 3, the fuse 100 is in its operating
condition, wherein
an electrical path is established between the first terminal 2 and the second
terminal 3
via the flexible conductor 13, the movable partition 4, the fusible element 9
and the
fusible joint 11. By virtue of the length of the fusible element 9, the
housing 1 and the
indicating rod 14, the protrusion end 16 of the indicating rod is disposed
flush or
slightly recessed within the first terminal when the fuse 100 is in its
operating
condition.
However, when the fuse 100 is subjected to either a high or low threshold
current, the protrusion end 16 of the indicating rod 14 will protrude out from
the end
of the first terminal 2, thereby indicating an interrupted condition of the
fuse. In
particular, when the fuse 100 is subjected to a high threshold current, the
fusible
element 9 will melt. As mentioned above, such melting will probably occur at
one or
more of the reduced cross-section neck portions 10 of the fusible element 9.
As
shown in Figure 3, melting of the neck portion 10 releases the tension in the
fusible
element 9 whereby the biasing element forces the movable partition 4 away from
the
fixed partition 5. Such movement draws the fusible element through the
partition 5,
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thereby elongating the gap 15 created by the melted neck portion 10. By
increasing
the gap 15 in the fusible element 9, the degree of electrical arcing is
significantly
reduced.
At the same time, movement of the movable partition 4 away from the fixed
partition 5 causes the indicator rod 14 to slide through the first terminal 2
whereby the
protrusion end 16 extends out from the first terminal giving a visual
indication that the
fuse 100 has melted. It may be observed that this indication is independent of
the
degree of arcing (if any) taking place at the gap 15. Thus, if the fuse is
part of a
circuit wherein no significant arcing can be sustained (due to a parallel
conductive
path via other cables) the spring loaded action ensures that an adequate gap
exists to
prevent any gap breakdown (re-strike) should system voltage later be imposed
on the
fuse. The gap also enables conventional fault finding equipment to operate,
since the
fuse will have a very high resistance.
On the other hand, when the fuse 100 is subjected to a relatively low
threshold
current, the fusible solder joint 11 will melt, as opposed to the fusible
element 9. As
shown in Figure 4, melting of the fusible joint 11 releases the fusible
element from the
second terminal 2. With the release of the joint tension, the biasing element
12 again
forces the movable partition 4 to move away from the fixed partition 5,
thereby
drawing the fusible element 9 through the slot 19 in the fixed partition,
which in turn
elongates a gap 17 created where the solder joint 11 has melted.
Simultaneously, the
indicator rod 14 is driven through the opening 28 of the first terminal 2,
such that the
protrusion end 16 extends therefrom giving indication that the fuse has
melted. Thus,
under a low current condition, the fuse 100 of the present invention again
reduces the
degree of arcing, while at the same time provides indication of interruption.
The first, second and third interior chambers 6, 7, and 8 of the housing 1 can
be filled with air, but the use of air surrounding a fuse element may reduce
the fuse's
ability to interrupt current, particularly high currents and those in circuits
where
conditions are severe (e.g. high X/R circuits). Accordingly, Figure 5 shows a
preferred embodiment of a fuse 100a, according to the present invention,
wherein the
CA 02579932 2007-02-27
third interior chamber 8, surrounding the fusible element 9, is filled with a
granular
dielectric medium 18, such as sand.
However, with a granular dielectric medium 18, such as sand, occupying the
third chamber 8, it may be necessary to take additional steps to ensure
movement of
the fusible element 9 through the sand. Accordingly, to prevent the sand 18
from
entering the slot 19 formed in the fixed partition 5 and thereby possibly
inhibiting
movement of the fusible element 9 therethrough, the slot 19 is preferably
lined with a
sleeve 20. The sleeve 20 is preferably made from a resilient material able to
withstand high temperatures. Suitable materials include gasket materials made
from
ceramic or glass fiber.
The sleeve 20 is designed to prevent the sand 18 from getting wedged in the
slot 19 of the fixed partition. In this regard, the sleeve 20 has a central
bore having an
inner diameter sized to slidably receive the fusible element 9 therein. The
outer
diameter of the sleeve is sized to be fitted and held within the slot 19
formed in the
fixed partition 5. Also, the sleeve 20 preferably includes opposite outwardly
flared
ends to further facilitate slidable movement of the fusible element 9 in the
sleeve.
The sleeve 20 is fixed in the fixed partition 5 and may have a length
sufficient
to merely line the slot 19 of the partition. Alternatively, the sleeve 20 may
extend
along the length of the fusible element 9 all the way to the second terminal
3. In yet
another alternative embodiment, a second fusible element sleeve 21 can also be
provided in addition to the slot sleeve 20 to form a sheath around all or part
of the
fusible element 9 to assist in its free movement, once melting is initiated.
In still another alternative embodiment of the invention, the fusible element
9
can be pre-coated or enclosed in a sheath of material having a relatively low
coefficient of friction, e.g. PTFE (commonly called "Teflon"), in order to
ease the
movement of the element, while providing containment. If the appropriate
material is
used, this material assists in current interruption. Such a sheath can be used
in
conjunction with, or instead of granular filter material.
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Turning now to Figures 6-9, as described above, the movable partition 4
moves when the fuse operates (melts) so that the indicator rod 14 can be used
to
provide indication that the fuse has operated. The indicating rod 14 can be
used to
provide direct indication by protruding through the terminal, as described
above.
However, in a preferred embodiment, as shown in Figures 6 and 7, the indicator
rod
14 acts upon a separate indicator button 22. In particular, an indicator
button 22 is
retained in a through hole of the first terminal 2, in a non-deployed state,
by a second
biasing element 23, such as a spring. A space 24 is left between the indicator
rod 14
and the button 22 such that the button 22 does not move simultaneously with
rod 14.
This allows for the protrusion of the button 22 not to be affected by
manufacturing
tolerances that affect the position of the end of the rod 14 relative to the
movable
partition 4, and prevents normal thermal expansion and contraction of the fuse
element from being communicated to the indicator button 22. By this means, if
the
indicator button 22 is used to operate a mechanism to signal fuse operation,
or trip a
protective device, it can be made to do so with very little movement.
In another alternative embodiment, as shown in Figures 8 and 9, a third
biasing element 25 is utilized. Such embodiment is beneficial if it is desired
that the
direct force of the main spring 12 not be communicated directly to the
indicator
button 22. Thus, the additional resilient member 25, such as a spring, can be
interposed between the indicator rod 14 and the indicator button 22.
Figures 6-9 also show a stop 30 provided on the movable partition 4 to limit
movement of the movable partition. In particular, the stop 30 may take the
form of a
leg formed on the movable partition and extending in a direction toward the
first end
terminal 2. Upon movement of the movable partition 4, the end of the stop 30
contacts the first end terminal 2 thereby stopping further movement of the
movable
partition and hence the indicator 14. The length of the stop 30 can thus be
chosen to
impart a desired protrusion distance for the indicator 14.
A proposed application for the fuse of the present invention is to protect
cables
from damage caused by overheating with long duration overloads. In a fuse
having a
relatively short body (typically less than about 6 inches) much of the heat
generated
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within the fuse is lost from the conductors connected to its terminals.
Therefore the
temperature of these conductors will significantly affect the temperature of
the fuse
components adjacent to them. Thus, in the preferred embodiment of the present
invention, the low melting point joint 11 is located close to the second end
terminal 3
of the fuse, and the conductor to be protected is connected to this terminal
3. The
conductor's temperature thus has a significant influence on the temperature of
the
joint 11. It is therefore possible to arrange that the joint 11 well models
the desired
protective requirements of the cable, and arrange that this joint will melt
before cable
temperatures reach critical levels. In this regard, Figure 5 shows the fusible
element 9
attached by the joint 11 to an appropriately shaped heat-sink member 26, sized
such
that the combination of components, in conjunction with the attached cable,
provide
the required protection for the cable.
Although the illustrative embodiments of the present invention have been
described herein with reference to the accompanying drawings, it is to be
understood
that the invention is not limited to those precise embodiments, and that
various other
changes and modifications may be effected therein by one skilled in the art
without
departing from the scope or spirit of the invention.
Various changes to the foregoing described and shown structures will now be
evident to those skilled in the art. Accordingly, the particularly disclosed
scope of the
invention is set forth in the following claims.
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