Note: Descriptions are shown in the official language in which they were submitted.
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FUSE TUBE AND METHOD OF MANUFACTURE THEREOF
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to arc-quenching compositions and
articles formed
therefrom and more particularly to a fuse tube construction which is easily
manufactured utilizing
inexpensive materials and that exhibits excellent strength and arc-quenching
properties for
operation over a wide range of currents.
Description of the Related Art
Fuse tubes for medium and high-voltage electrical fuses wherein circuit
interruption takes
place within the fuse tube requires high strength and arc-quenching
properties. Examples of prior
art fuse tubes are found in U.S. Patent Nos. 3,911,385, 3,979,709, 3,984,800,
4,313,100,
4,349,803, 4,373,555, 4,373,556, 4,564,830, 4,808,963, 5,015,514 and
5,119,060. Fuse tubes
for operation to interrupt currents over a wide range, e.g. 100-10,000
amperes, require especially
high strength and arc-quenching properties that are difficult to obtain. In
conventional fuse
cutouts utilized in electrical power distribution systems, the fuse tube is
fabricated by winding a
filament-wound glass-epoxy outer tube over an inner tube of wlcanized fiber
that provides the
arc-quenching properties. Various alternatives have been proposed to fabricate
fuse tubes with
overall high strength and a bore of suitable arc-quenching properties. Some of
these alternatives
include various fabrication techniques utilizing fiber supported in epoxy
resin mixtures to provide
an inner layer having arc-quenching properties over which is formed an outer
layer providing
strength. In the aforementioned Patent Nos. 4,373,555 and 4,373,556, the inner
arc-quenching
layer of the fuse tube is fabricated from polyester fiber and epoxy resin
mixture. In Patent No.
5,015,514, approximately 55-60% aluminum trihydrate is included by weight in
an inner arc-
quenching layer along with organic fiber and epoxy resin. In the
aforementioned Patent No.
4,349,803, a molded tube includes a layer of porous fiberglass cloth with an
inner layer of
thermosetting material having an arc-quenching material therein and an outer
weather resistant
portion being formed about the intermediate porous fiberglass cloth.
While the prior art arrangements may be generally usefirl to provide fuse
tubes for
electrical fuses, the prior arrangements involve either relatively expensive
natural resources or
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complex processing and fabrication. Further, most do not provide a fuse tube
which is capable of
withstanding the pressures of high-current interruptions and also being
capable of interrupting
low current faults.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to provide an
arc-quenching
composition that is useful for forming articles thereof and particularly the
inner arc-quenching
layer of a high-strength layered fuse tube.
It is another object of the present invention to provide a fuse tube for
electrical fuses that
exhibits excellent strength and arc-quenching characteristics while also being
easily fabricated
from resins, fiber and fillers.
It is a further object of the present invention to provide a fuse tube which
is fabricated by
forming an inner tube having a bore with arc-quenching properties and before
the curing of the
inner tube forming an outer tube over the inner tube such that the inner and
outer tubes cure as a
single structure and avoid any dielectric joint or interface therebetween.
In one aspect of the present invention there is provided a fuse tube having a
multiple
layered laminate construction including an inner arc-quenching surface layer
comprised of a
wound filamentous fiber material supported in a matrix comprising a
thermosetting resin and
melamine, and also including at least one outer layer of filament wound glass
fiber reinforced
thermosetting resin, said outer layer being bonded to said inner arc-quenching
surface layer
whereby no dielectric or mechanical interface is present between said inner
and outer layers, said
inner arc-quenching surface layer comprising at least 10% by weight melamine
and being at least
70% by weight organic material. In a further aspect of the present invention
there is provided a
fuse tube having a multiple layered laminate construction including an inner
arc-quenching
surface layer comprised of a filament-wound fiber-reinforced matrix comprising
a thermosetting
resin and melamine, and also including at least one outer layer of filament-
wound, glass fiber
reinforced thermosetting resin, said outer layer being bonded to said inner
arc-quenching surface
layer whereby no dielectric or mechanical interface is present between said
inner and outer
layers, said inner arc-quenching surface layer comprising at least 10% by
weight melamine, at
least 10% by weight of said filamentous fiber material and at least 40% by
weight of said
thermosetting resin.
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It is yet another object of the present invention to provide an arc-quenching
bore for a
fuse tube that is capable of interrupting a wide range of currents via the
combination of an arc-
quenching compound in the bore and a predetermined taper in the bore.
These and other objects of the present invention are efficiently achieved by
the provision
of an arc-quenching composition including a filler, a fiber and a binder.
Preferably, the filler
includes an arc-quenching compound such as melamine. The binder includes a
thermosetting
resin to facilitate forming of the arc-quenching composition into an arc-
quenching fuse tube. In a
preferred arrangement, an outer tube is formed over the arc-quenching fuse
tube to provide an
overall high-strength fuse tube. Also in a preferred arrangement, in order to
provide a fuse tube
that is capable of operation over an extremely wide current range, the inner
arc-quenching tube
includes a tapered bore. According to one fabrication technique, the outer
tube is formed over the
arc-quenching inner tube before the curing of the inner tube such that a
single structure results.
BRIEF DESCRIPTION OF THE DRAWING
The invention, both as to its organization and method of operation, together
with further
objects and advantages thereof, will best be understood by reference to the
specification taken in
conjunction with the accompanying drawing in which:
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FIG. 1 is a perspective view of an arc-quenching tube in accordance with the
present
invention;
FIGS. 2 is a perspective view of a layered fuse tube in accordance with the
present
invention;
FIGS. 3 and 4 are enlarged views of a section of the wall of the fuse tube of
FIGS. 1 and
2; and
FIG. 5 is an enlarged sectional view of a preferred embodiment of the fuse
tube of FIGS. 1
and 2 illustrating the taper in the bore of the fuse tube.
DETAILED DESCRIPTION
It has been found that an arc-quenching composition can be unexpectedly
utilized to form
an inner layer of a fuse tube, e.g. via filament winding, injection molding,
or an extrusion process
such as pultrusion, while exhibiting improved arc-quenching properties, the
arc-quenching
composition including a fiber, a filler, and a binder such as a thermosetting
resin. The filler
preferably includes an arc-quenching compound such as melamine. Additional
fillers are also
included in specific embodiments. The arc-quenching compound can also be
characterized as an
arc-quenching material or mixture. While a fuse tube of this fabrication may
be suitable for some
purposes, for the fabrication of high-strength fuse tubes which are required
to also provide
suitable arc-quenching bore characteristics for a wide range of arcing
currents initiated therein, it
has unexpectedly been found possible to provide such a high-strength tube by
providing a high-
strength fiber-reinforced layer over an inner layer of the arc-quenching
composition. For fuse
tubes which are required to provide an extremely wide range of arc-quenching
properties
including extremely high currents at medium voltages, the inner arc-quenching
layer is provided
with a taper so as to be larger at the exhaust opening of the fuse tube to
alleviate stagnation of
gases evolved within the bore arising from a high rate of gas generation at
the extremely high
currents. In a preferred embodiment, the mild taper is provided by a stepped
counterbore
structure.
While articles in accordance with the present invention can be fabricated
utilizing a variety
of conventional extrusion, winding, injection molding and other processes, in
a preferred form of
the invention, fuse tubes are fabricated via a filament winding process. In
accordance with
important aspects of the present invention, to achieve high-strength fuse
tubes, a layered filament
winding process is utilized wherein a high-strength layer is wound over an
inner arc-quenching
layer or tube. Alternatively, the inner arc-quenching layer is fabricated by
filament winding and
the outer layer is molded over the inner arc-quenching layer. In other
embodiments, the inner arc-
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quenching layer is formed via injection molding or pultrusion, and the outer
layer is formed
thereon either via filament winding or an over-molding process. In the
arrangements where the
inner arc-quenching layer is formed via an injection molding or extrusion
process, the fiber is in
the form of chopped, short fibers rather than a filament form which is
suitable for fabrication via
winding.
To achieve the optimum in arc-quenching performance over a wide range of
currents
including high currents, the bore of the inner arc-quenching layer is tapered
or counterbored in
steps to alleviate gas stagnation at high currents. In this regard and in
accordance with important
aspects of the present invention, the arc-quenching layer has a predetermined
uniformity of
distribution of the fiber and a mixture of the filler and binder such that the
outer surface of the
arc-quenching layer is provided with a predetermined uniformity to permit
maximum tapering into
the bore of the arc-quenching layer, i.e. minimizing the thickness of the
layer and utilizing the
maximum extent thereof without exposing the bore to the outer high-strength
layer or increasing
the thickness of the inner layer. Any unnecessary increase in the diameter of
the arc-quenching
1 S layer is undesirable since the overall strength of the fuse tube is
reduced thereby. The taper is
formed either during fabrication or thereafter by machining. Similarly, the
outer layer also has a
predetermined uniformity of distribution to achieve desirable uniformity.
In accordance with other important aspects of the present invention, the
relationship
between the arc quenching compound and the amount of the taper in the bore
provides optimum
performance over a wide current range. It has been found that a general
relationship exists
between the amount of arc quenching compound and the degree of bore tapering
such that low
current performance is improved and the low current range is extended by a
relatively higher
percentage of arc-quenching compound while high current performance is
improved and the high
current range is extended by increasing the amount of bore tapering. That is,
a sufficient amount
of arc quenching compound is required with a given binder and fiber in order
to achieve low
current performance such that sufficient quenching gases are generated. Also
when sufficient
amounts of arc quenching material are present in the arc-quenching layer, the
maximum high
current performance is achieved by providing relatively larger tapering of the
bore so that high
amounts of generated gases at the high currents do not cause stagnation. With
the arc-quenching
characteristics established by the inner arc quenching layer, the outer layer
or layers of the fuse
tube are formulated to achieve high-strength of the overall fuse tube. For
example the outer layer
in a preferred embodiment utilizes a high strength fiber such as fiberglass
supported in epoxy resin
to achieve high-strength.
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Considering now the fabrication of an illustrative embodiment of the present
invention to
provide a high strength fiase tube with an arc-quenching bore to interrupt
currents over a wide
current range, e.g. 100 amperes to 10,000 amperes, a curing agent, a filler
including an arc-
quenching compound, a thermosetting resin, and an accelerator are suitably
combined and mixed.
In this mixture, which may also be characterized as a matrix, where melamine
is used for the arc-
quenching compound, the composition by weight of the melamine is in the
approximate range of
10-30% depending on the particular properties to be achieved, as will be
explained in more detail
hereinafter. The fiber, in the form of a filament or strand, is then drawn
through the mixture and
then wound around a mandrel coated with a mold release agent, with one or more
winding passes
being made to accomplish an inner layer of predetermined thickness. The payout
of the fiber and
the winding speed are arranged such that the individual fiber strands lay flat
and do not overlap on
the mandrel during each winding pass of the inner layer. Also, the winding
tension is arranged to
achieve a desirable relationship between mixture and fiber in the wound inner
layer. The inner
arc-quenching layer is wound to be oversized relative to the final desired
diameter to allow for
compression due to the tension of the winding of the outer layer. The outer
layer is then
immediately applied over the wound inner, arc-quenching layer, again one or
more winding passes
being made to achieve an outer layer of predetermined thickness. The winding
tension is arranged
such that the inner layer is not compressed too much so as to squeeze out too
much mixture.
However, sufficient tension is utilized such that the inner layer is not
oversized which could result
in reduced strength of the overall fuse tube. The fuse tube is then heat
cured, e.g. at
approximately 300 degrees F. In a preferred embodiment to provide a suitable
high-strength fuse
tube for a wide range of current interruption, the inner layer as fabricated
includes a composition
by weight of approximately 25% fiber, approximately 20% melamine, and
approximately 55%
thermosetting resin with curing agent. The fuse tube in specific embodiments
is provided with a
tapered bore, either via the winding process or after curing via suitable
machining, e.g.
counterboring or the like.
In an illustrative embodiment of the inner tube, the fiber is an organic fiber
such as acrylic
yarn. Other suitable fibers include polyester, nylon, rayon, cotton, cellulose
and mixtures thereof.
The fiber for the inner tube is preferably in the form of yarn, cloth, veils
or tapes rather than
particles or individual fibrous material. Considering an illustrative
embodiment, the arc-quenching
compound is melamine. Other suitable compounds include fluorine containing
polymers like
Teflon, boric acid, aluminum trihydrate, magnesium hydroxide, silicones,
polyesters,
polyurethanes, zinc borate, benzoguanamine, dithioammelide, ammeline, and a
cyanuric halide and
mixtures thereof. Additionally considering an illustrative embodiment, the
thermosetting resin is
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bisphenyl-A epoxy resin while suitable thermosetting resins include
cycloaliphatic epoxy resin or
mixtures of bisphenyl-A and cycloaliphatic epoxy resins, phenolics,
polyurethanes, polyesters,
silicones, or urea-formaldehydes. However, it has been found expedient to
avoid any structural
problems by using bisphenyl-A epoxy resin for both the inner and outer layers.
In a specific
embodiment, BDMA has been found to be a suitable accelerator agent and
Lindride 32 has been
found to be a suitable anhydride curing agent. A suspension aid, such as
Bentone, has also been
found useful to avoid any settling problems in the mixture of the resin and
arc-quenching
compound. While not required for the practice of the present invention to
achieve the desired
arc-quenching properties and strength of the fuse tube, additional fillers may
also be utilized in
IO specific embodiments, e.g. silica, hydrated alumina, aluminum fluoride,
bentonite, that either
augment or do not affect the properties obtained by the combination of the
fiber, the arc-
quenching compound and the thermosetting resin.
With reference to FIG. l, a fuse tube 10 in accordance with the present
invention is
formed as an elongated tubular body, e.g. via winding, molding or extrusion,
for use as a fuse
tube or as an arc-quenching liner. Referring now to FIG. 2, a fuse tube 16 in
the form of an
elongated tubular body includes an inner tube 12 and an outer tube 14. The
inner tube 12 is
fabricated with a central bore 18 and from the arc-quenching composition to
provide desired arc-
quenching properties. The outer tube 14 is then formed about the inner tube
12, e.g. via winding
or molding, to provide additional strength to the fuse tube 16. The material
of the inner tube 12 is
a thermosetting resin with suitable arc-quenching fillers and fiber content to
allow fabrication in a
winding, molding or extrusion process, e.g. such that the material has
suitable viscosity and
workability. Essentially immediately thereafter, or preferably before the
inner tube 12 cures, the
outer tube 14 is formed about the inner tube 12 such that any dielectric or
mechanical interface is
avoided and the materials of the inner and outer tubes 12, 14 bond or cross
link at their interface.
In one example, the outer tube 14 is fabricated by filament winding of a
thermosetting resin
including suitable fiber content such as fiberglass to achieve the desired
structural strength.
In one illustrative embodiment, the inner tube 12 and the outer tube 14 are
wound
essentially simultaneously, i.e. concurrently, in tandem or sequentially, on a
continuous filament
winding machine. In other illustrative embodiments, the inner tube 12 is
extruded or molded and
the outer tube 14 is wound or molded over the inner tube 12 preferably such
that both tubes cure
as a single structure. The materials of the inner and outer tubes 12, 14 are
selected such that they
cure as a single structure via bonding and/or crosslinking. This eliminates
any dielectric or
mechanical interfaces or boundaries between the tubes 12, 14 and ensures that
the inner and outer
tubes 12, 14 remain an integral mechanical structure and are highly resistant
to any separation,
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e.g. "push-out" forces to which the inner and outer tubes 12, 14 are subjected
during the
interruption of electrical arcs.
With additional reference now to FIG. 3, if suitable uniformity in the winding
of the inner
tube 12 is not achieved relative to the amount of bore taper (e.g. due to
overlapping of the
winding strands such that the individual fiber strands do not lay flat), when
the arc-quenching
composition of the bore 18 is removed to provide the bore taper as illustrated
by the line 20, this
actually extends beyond the inner tube 12 and into the outer tube 14 exposing
the tapered bore 20
to the material of the outer tube 14 as illustrated at 22. This, of course, is
extremely undesirable
in a fuse tube application where the bore is required to have arc-quenching
properties. With
additional reference now to FIG. 4, if the inner arc-quenching tube 12
achieves the predetermined
uniformity as illustrated, then the bore taper with removal of material at 20
does not expose the
tapered bore 20 to the material of the outer tube 14, but instead is entirely
within the inner arc-
quenching layer of the inner tube 12. For example, this attribute can be
characterized as the non-
uniformity or variation referred to at 23 being sufficiently small so as not
to affect the minimum
wall thickness referred to at 25, i.e. such that the non-uniformity does not
significantly impact or
interfere with the desired wall thickness of the tapered bore 20. Of course,
the significance in
particular applications depends on the amount the wall thickness at 25 is
expected to be eroded or
ablated during arc-quenching and over the expected use of the inner tube 12.
In a specific
illustration, this could be described as the non-uniformity or variation
referred to at 23 being
significantly less than the minimum wall thickness referred to at 25. Further,
the uniformity of the
outer tube 14 is also important so as to minimize any machining of the outer
surface to obtain a
suitable surface and also to maximize strength.
In a specific illustrative embodiment, the bore 18 of the fuse tube 16 is .50
of an inch, the
final outer diameter of the inner tube 12 is .72 of an inch, and the outer
diameter of the fuse tube
16 is 1.0 inch. For application with nominal voltage ratings in the range of
7.2 to 25 kv, the
length of the fiase tube 16 is in the range of 9- I 6 inches.
Considering now the tapered bore in more detail and with additional reference
to FIG. S,
the taper of the bore is illustrated on an enlarged scale and the proportions
have been greatly
exaggerated for illustrative purposes. For example, for a fuse having a
nominal voltage rating of
25kv and a 12000 RMS asymmetrical ampere maximum current interrupting rating,
five steps of
taper are utilized as described in more detail in the aforementioned Patent
No. 4,313,100 which
increase the bore opening from .500 of an inch to .656 of an inch over a
length of 3.5 inches.
Further, for a firse having a nominal voltage rating of 25kv and a 8000 RMS
asymmetrical ampere
maximum current interrupting rating, five steps of taper are utilized as
described in more detail in
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the aforementioned Patent No. 4,313,100 which increase the bore opening from
.500 of an inch to
.656 of an inch over a length of 5.5 inches. Specifically, in FIG. 5, three
bore steps 24, 26 and 28
are illustrated which enlarge the bore opening 18 of the fuse tube 16 so as to
define included
angles "a" and "b" which range from 1-3 degrees in accordance with the
dimensions herein above,
the angle "a" being defined between the opening of the bore 20 at the exhaust
end 30 and the
innermost portion of the step 24, while the angle "b" is defined between the
opening of the bore
20 at the exhaust end 30 and the outermost portion of the step 24.
While there have been illustrated and described various embodiments of the
present
invention, it will be apparent that various changes and modifications will
occur to those skilled in
the art. Accordingly, it is intended in the appended claims to cover all such
changes and
modifications that fall within the true spirit and scope of the present
invention.
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