Arc-quenching filler for high voltage current limiting fuses and circuit interrupters

A high voltage circuit interrupter has a surface modified pulverulent arc-quenching filler composition, with gas-evolving material is bound to the surfaces of the arc-quenching filler by a binder. The pulverulent arc-quenching filler can be selected from the group of silicas and silicates, preferably sand, mica or quartz. The gas-evolving materials can be selected from the group of melamine, cyanuric acid, melamine cyanurate, guanidine, guanidine carbonate, guanidine acetate, 1,3-diphenylguanidine, guanine, urea, urea phosphate, hydantoin, allantoin, and mixtures and derivatives thereof. The device has a generally tubular casing of electrically insulating material, terminal elements closing the opposite ends of the casing, at least one fuse element conductively interconnecting the terminal elements, a core for supporting the fuse element, extending parallel to the longitudinal axis, and a modified pulverulent arc-quenching filler material inside the casing, in close proximity to the fuse element. The modified pulverulent arc-quenching filler material includes a pulverulent arc-quenching filler, a binder, and a gas-evolving material, and the gas-evolving material is bound to the surfaces of the arc-quenching filler.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The invention relates to the field of high voltage circuit interruption in 
electrical devices such as switchgear, transformers, and the like, and in 
particular concerns high voltage current limiting fuses or expulsion 
fuses, circuit breakers, circuit interrupters, separable cable connectors, 
or the like, including a pulverulent arc-quenching filler material of high 
dielectric strength that is adapted to aid in arc extinction, and to 
quickly and effectively to break the circuit. More particularly, the 
invention is directed to an arc-quenching filler material encased within a 
high voltage current limiting device that is surface modified with a 
gas-evolving composition to provide improved arc-quenching properties 
without impairing the free flowing and compacting properties of the 
arc-quenching filler material. The invention also concerns a method of 
making the same. 
Current limiting power interruption in high voltage circuits requires a 
current interruption device that rapidly and effectively brings the 
current to a zero value upon the occurrence of a line fault. The fuse 
devices generally considered herein are those employed in electrical 
circuits typically at voltages of a thousand or more volts. Electrical 
circuits operating at such high levels of voltage can cause extensive 
damage to circuit components, machinery connected to the circuit, or the 
like if the current interruption is not accomplished positively in a short 
period following the occurrence of fault or overload conditions. 
Expulsion fuses or gas-evolving fuses in particular have been used 
extensively for high voltage circuit interruption in switchgear, 
transformers, and other electrical equipment. It is generally known that 
arc-quenching and gas-evolving materials in such a circuit interruption 
device, positioned in contact with the fuse element, aid in, inter alia, 
deionizing, cooling, and thereby quenching the electric arc created under 
fault or overload current conditions. 
It is known to provide a pulverulent (powder) arc-quenching filler 
material, for example sand, inside the, casing of a fuse to absorb the 
energy of a burning or fusing fuse element during the fusing process so 
that the fuse will not explode when interrupting the circuit. The 
conventional arc-quenching filler material tends to confine the arc 
radially and thus to sustain its current limiting voltage, in addition to 
absorbing the energy of the arc. However, such fuse when operating under 
low current conditions may arc for an extended period if time during which 
the sand or powdered arc-quenching filler may be heated sufficiently to be 
fused. In the fused state, the conventional arc-quenching filler suffers a 
loss in insulation properties which can be sufficient to prevent 
interruption of the current or to allow a restrike after a temporary 
interruption. It has been difficult to obtain, however, an arc-quenching 
filler material that is substantially resistant to a fused state, thereby 
forming fulgurites. 
It is also known to provide mandrels or cores of gas-evolving materials to 
evolve an arc-quenching gas during the fusing operation. To avoid 
excessive pressures against the inside of the fuse housing and ferrules 
which may lead to rupture of the fuse housing or blow off the ferrules, 
the amount of evolved gas can be reduced by locally positioning 
gas-evolving materials in controlled small quantities along the core. The 
pressure within the fuse housing does not, therefore, increase unduly, and 
the positive effects of the presence of arc suppressing gas are generally 
maintained. It has been difficult to obtain, however, a gas-evolving 
material whose solid residue in the fused state is relatively 
non-conductive, so as to prevent restriking or tracking of the arc by 
conductance through the fused compound, and a tendency to reestablish a 
current flow through the material after interruption. 
A typical high voltage fuse can include a generally tubular casing of 
electrically insulating material; a pair of terminal elements closing each 
of the opposite ends of the casing; a pulverulent arc-quenching filler 
material of high dielectric strength inside the casing, such as sand, mica 
beads, or finely divided quartz; a fuse element or elements made of a 
highly conductive material, such as silver, submersed in the filler and 
conductively interconnecting the terminal elements, the fuse element or 
elements typically being wound in a parallel-connected relationship along 
the length of a supporting mandrel or core; a core of high dielectric 
strength electrically insulating high temperature material, such as 
ceramic, the core providing support for the fuse element or elements and 
having longitudinally and radially extending fins of a cross-shaped, 
star-shaped or like cross-section, along the longitudinal axis of the 
casing; and a gas-evolving material regionally distributed along the 
length of the core in contact with the fuse element or elements. 
In operation, when the high voltage current limiting fuse is subjected to 
an applied current that exceeds the rated current carrying capability of 
the fuse element, the excessive current causes sufficient resistive 
heating that the fuse element attains a fusion temperature. Melting and 
vaporization of the fuse element occur at one or more predetermined 
locations along its length, whereupon an electrical arc is established in 
each region where the fuse element melts. A plurality of series connected 
arcs can be formed along the fuse element. Current limitation occurs when 
the sum of the individual arc voltages reaches the voltage applied to the 
fuse. Thus, the current limiting effect results from the introduction of 
arc resistance in series with the circuit. 
When electrical arcing occurs, the fuse element and/or its metal vapors 
rapidly expand to many times the volume originally occupied by the fuse 
element. These metal vapors expand into the spaces between portions of the 
arc-quenching filler material where they condense through heat transfer 
into the arc-quenching filler, and consequently are no longer positioned 
for current conduction. The physical contact between the hot arc and the 
relatively cool arc-quenching filler granules causes a rapid transfer of 
heat from the arc to the granules to dissipate most of the arc energy 
without substantial pressure buildup within the fuse casing. A material 
that rapidly evolves a deionizing gas may be distributed along the length 
of the core to reduce conduction through gas that may be ionized by the 
arc and to cool the arc, which facilitates arc extinction under low 
current conditions. 
However, after this fusing operation occurs, fulgurites are formed in the 
pulverulent arc-quenching filler material. That is, the pulverulent 
arc-quenching filler material is fused or sintered in the hot arcing 
regions into a glass-like body defining a path of relatively lower 
resistance than the surrounding pulverulent material. The fulgurites 
provide a path along which restrike of the arc current can occur. There is 
a need to provide a high voltage current limiting device that uses the 
beneficial properties of energy-absorbing pulverulent arc-quenching filler 
material and localized evolvement of arc-suppressing gas while at the same 
time reducing the tendency to form conductive fulgurites in the fusing 
region. 
A typical arc-extinguishing gas-evolving material may comprise a 
combination of a gas-evolving material and a thermoplastic or 
thermosetting polymeric structural binder. Such material generally is 
highly carbonizing and therefore conductive. Upon gas evolution, the 
organic binder decomposes, leaving conductive carbon residues. There is a 
need to provide a high voltage current limiting device that uses the 
properties of energy-absorbing pulverulent arc-quenching filler material 
and localized evolvement of arc-suppressing gas while reducing the 
tendency to form carbon residues in the fusing region. Carbon residue 
likewise enhances the opportunity for a restrike of the arc, which is 
undesirable. 
U.S. Pat. No. 4,099,153 (Cameron) teaches a high voltage current limiting 
fuse comprising a fuse element wrapped about an electrically insulating 
support mandrel or core along the core length, the fuse element being held 
in position on the core by gas-evolving C-clamps locally distributed along 
the length of the core. The core, fuse element, and gas-evolving clamps 
are embedded in a pulverulent arc-quenching filler inside a casing. 
Cameron teaches positioning the gas-evolving clamps in contact with the 
fuse element in localized regions. Upon fusing and arcing, the pressure of 
the evolved gas forces the arc-quenching filler away form the restricted 
arcing regions. Cameron claims that this reduces formation of fulgurites 
in those regions during fusing, so that undesirable restriking of the arc 
will not occur. 
U.S. Pat. No. 4,319,212 (Leach) teaches a high voltage current limiting 
fuse comprising a fuse element wrapped about a finned core with cutouts 
along its length, and with gas-evolving materials positioned in the 
cutouts. The core, fuse element, and the gas-evolving material are 
surrounded by a granular arc-quenching filler material inside a casing. 
Leach teaches positioning the arc-quenching pulverulent filler in the 
immediate vicinity of the arc-initiating fuse element. The filler absorbs 
the arc energy as the fuse element melts, and forms fulgurites which Leach 
claims are cooled and rendered insulating, rather than conductive, by 
evolved gases also in close proximity to the arc-initiating fuse element 
melts and the arc-quenching filler material. 
U.S. Pat. No. 3,582,586 (Jones) teaches a gas-evolving material comprising 
melamine and a thermoplastic or thermosetting organic binder. As discussed 
above, such gas-evolving material has a tendency to carbonize in air under 
arcing conditions to form conductive carbon residues which enhances arc 
restriking and tracking. 
U.S. Pat. No. 3,761,660 (Jones) teaches a gas-evolving material comprising 
melamine, hydrated alumina and a thermoplastic or thermosetting organic 
binder. The hydrated alumina is provided to release the water of its 
hydration to enhance arc-quenching properties and to catalyze the 
oxidation of carbonaceous materials to reduce carbon residue formation. A 
drawback of hydrated materials in a current limiting device is the 
tendency to cause corrosion as a result of evolution of water from the 
hydrated material, and ionization during arcing. 
U.S. Pat. No. 4,975,551 (Syverston) teaches a gas-evolving material 
comprising of melamine or other related compounds containing carboxylic 
reactive groups, such as amine, hydroxyl, epoxy, aziridine, or thiol 
groups, and a thermoplastic polymer containing carboxylic acid moieties 
which chemically bond to the melamine or related compounds carboxylic acid 
reactive group. Carboxylic acid moieties are highly carbonizing in their 
fused state and, consequently, have a tendency to track the arc. 
It would be desirable to provide a pulverulent arc-quenching filler 
material that has its surfaces modified with a relatively non-carbonizing 
gas-evolving material that can be used in a high temperature current 
limiting device to rapidly and effectively quench an arc. It would be 
further desirable to provide a pulverulent arc-quenching filler material 
modified with a relatively non-carbonizing gas-evolving material that 
maintains the free flowing and compacting characteristics of the 
pulverulent arc-quenching filler material. It would also be desirable to 
provide a pulverulent arc-quenching filler material modified with a 
relatively non-carbonizing gas-evolving material that tends to quench the 
follow current, i.e., the current which flows through the hot fulgurite 
after a fusing operation, through cooling of the fulgurites by the evolved 
gas. The evolved gas of such gas-evolving material on the surface of the 
arc-quenching filler material advantageously produces a deionizing action 
on the arc initiated by vaporization of the fuse element, and reduces the 
tendency for a restrike or track of the arc by reducing fulgurite 
formation and/or cooling the fulgurite formed to a more insulating and 
less conductive body. Such modified arc-quenching filler material can be 
provided in direct contact with the fuse element. 
SUMMARY OF THE INVENTION 
It is an object of the invention to modify pulverulent arc-quenching filler 
material surfaces with gas-evolving materials for use in high voltage 
current limiting devices, and thereby improve their operational 
characteristics. 
It is another object of the invention to extinguish an electric arc in a 
high voltage current limiting device efficiently and effectively, with an 
arc-quenching filler material having a surface coating of a gas-evolving 
material. 
It is an advantage of the invention that tracking or restriking of an arc 
is less likely. 
It is another advantage of the invention that fulgurite formation in the 
arc-quenching filler material is reduced. 
It is a further advantage of the invention that compacting and free-flowing 
properties of the arc-quenching filler material are maintained. 
These and other objects and advantages are accomplished according to the 
invention by providing a surface modified pulverulent arc-quenching filler 
composition, including a pulverulent arc-quenching filler material, a 
binder, and a gas-evolving material, wherein the gas-evolving material is 
bound to the surface of said arc-quenching filler. The pulverulent 
arc-quenching filler can be selected from the group of silicas and 
silicates, preferably sand, mica or quartz. The gas-evolving materials can 
be selected from the group of melamine, cyanuric acid, melamine cyanurate, 
guanidine, guanidine carbonate, guanidine acetate, 1,3-diphenylguanidine, 
guanine, urea, urea phosphate, hydantoin, allantoin, or the like and 
mixtures and derivatives thereof. 
The high voltage current limiting device according to the invention 
includes a generally tubular casing of electrically insulating material; a 
pair of terminal elements closing each of the opposite ends of the tubular 
casing; at least one fuse element conductively interconnecting the pair of 
terminal elements; a core for supporting at least one fuse element, 
longitudinally extending parallel to the longitudinal axis of the tubular 
casing; a modified pulverulent arc-quenching filler material inside the 
tubular casing in close proximity to the fuse element, wherein the 
modified pulverulent arc-quenching filler material comprises a pulverulent 
arc-quenching filler material, a binder, and a gas-evolving material, 
wherein the gas-evolving material is bound to the surface of said 
arc-quenching filler material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view of a high voltage current limiting fuse 1, 
according to the present invention. FIG. 2 is a cross-sectional view of 
the high voltage current limiting fuse of FIG. 1. Generally, the high 
voltage current limiting fuse 1 includes a mandrel or core 10 about which 
is wound a conductive fuse element 20. The core 10 and the fuse element 20 
are typically located in a tubular insulating housing or casing 30, having 
electrical terminals or ferrules 32 at the opposite ends of the tubular 
casing 30 to close each of the opposite ends and to provide an electric 
circuit with the fuse element 20 serially connecting the ferrules 32. A 
single fuse element 20 is shown wrapped about the core 10 for purposes of 
illustration. It should be understood that a fuse construction can also 
include a plurality of fuse elements 20, electrically connected in 
parallel, wrapped about the core 10 and interconnect the terminals or 
ferrules 32 of the fuse. 
The core 10 typically comprises a high dielectric strength, 
electrically-insulating high temperature material such as, for example, 
ceramic. The core 10 is further typically formed to have a cross-shaped, 
star-shaped, or the like cross-section and includes generally radially 
projecting fins 12 that extend longitudinally along the length of the fuse 
casing 30. Such a fin design is known, and is desirable in that it reduces 
the contact area between the fuse element 20 and the core 10. By reducing 
the contact area between core 10 and fuse element 20, the performance of 
the high voltage fuse is improved as compared to a cylindrical core. 
The fuse element 20 typically has a ribbon-type form and is made of a high 
conductivity material, such as, for example, silver. Preferably, the fuse 
element 20 is spirally or helically wound about the core 10 such that 
successive wraps are spaced-apart along the core axis. The fuse element 20 
can also be made of aluminum, copper, tin, zinc, cadmium, or an alloy, 
although silver is a preferred material. The fuse element 20 may comprise 
a plurality of conductors, electrically connected in parallel and wrapped 
about the core 10. 
The fuse element 20 further has a plurality of circular perforations 22, 
spaced longitudinally to define reduced cross-sections which facilitate 
vaporization of the fuse element 20 under fault current conditions, 
resulting in formation of a number of arcs in series. The perforations 22 
are shown in FIG. 1 as being circular in shape, however, some or all of 
the perforations may also be formed in other appropriate shapes, for 
example, ovals, rectangles, etc. Furthermore, the reduced cross-section 
can be formed by employing notches in the sides of the fuse element as 
well as perforations in the middle portion as shown. The fuse element 20 
is wound about core 10 in the desired pattern, preferably spirally or 
helically, and the end potions of the fuse element 20 are then affixed at 
their final or terminal position to the terminals or ferrules 32 of the 
fuse. 
To initiate fuse operation at relatively low level overload currents, it is 
known to provide a fuse element with a conventional tinned portion or 
overlay, such as tinned portions or overlays 24, with each overlay 
disposed adjacent to one of the perforations 22. When a fuse element 20 is 
heated by an overload current that persists for a predetermined duration, 
overlays 24 begin to melt and to alloy with the underlying material the 
fuse element. The overlay when alloyed with the material of the fuse 
element increases the local electrical resistance of the fuse element 
where alloying takes place. The increased resistance dissipates additional 
heat energy and accelerates melting or vaporization of the fuse elements 
20 at these locations. This reduces the time required to form associated 
arcs at the various locations along the fuse element. 
In order to improve the ability of the core 10 to withstand voltages 
applied along its length, notches or cut-outs 14 are provided in the 
radially outer edges of the fins 12 of the core 10. The dielectric 
breakdown along the solid surface of a core 10, for example, ceramic, is 
typically less than that through a similar distance of a pulverulent 
arc-quenching filler medium, for example, sand, mica or quartz. The 
dielectric breakdown between two points on the core 10 may be improved by 
increasing the distance along the surface of core 10 between the points. 
The cut-outs 14 are placed in the outer surface areas of the fins of the 
core 10 to increase the surface length along core 10 between two given 
points, and therefore to improve its dielectric breakdown characteristic. 
The surface distance of particular interest that is increased, is the 
distance between the locations at which the fins 12 are contacted by 
adjacent turns of the fuse element 20, so as to increase the voltage 
necessary to cause a dielectric breakdown between adjacent turns of the 
fuse element 20. This aspect, wherein the breakdown voltage needed to 
overcome the dielectric strength of the core along its surface is 
increased, is commonly referred to as an increase in the creepage between 
adjacent turns of the fuse element. 
As further shown in FIG. 1, a pair of electrically conductive terminal 
rings 34 are attached to the opposite ends of the core 10. The fuse 
element 20 is electrically coupled to the terminal rings 34 by suitable 
means. The terminal rings 34 further contain electrically conductive tabs 
36 and 38 that are conductively attached to the terminals or ferrules 32 
on the tubular casing 30 to provide an electrical interconnection between 
the fuse element 20 and the ferrules 32. The tubular casing 30 is 
typically made of an insulated material, for example, glass reinforced 
epoxy. The pair of terminals or ferrules 32 are attached to the opposite 
ends of a tubular casing 30 by suitable means closing each of the opposite 
ends of the tubular casing 30, and are typically made of an electrically 
conductive material, such as, for example, copper. The ferrules 32 provide 
the electrical interconnection means between the fuse element 20 and an 
external circuit (not shown). Other interconnection means can be used to 
electrically interconnect the fuse element to the ferrules, as are known 
in the art. 
Also shown in FIG. 1, according to the invention, the tubular insulating 
casing 30 is filled with a modified pulverulent arc-quenching filler 
material 40, especially in the immediate vicinity of the arc-initiating 
fuse element. According to the invention, the modified pulverulent 
arc-quenching filler material 40 at its surface is bonded to and thus 
modified by an arc-quenching gas-evolving material as shown in FIG. 3. In 
conventional high voltage current limiting fuses, an arc quenching filler 
material such as, for example, sand, occupies substantially all of the 
space within the tubular casing that is not occupied by the core and the 
fuse element. The typical arc-quenching filler material serves in a 
conventional manner to cool arcing, and thereby to assist in extinguishing 
the arcs that are developed when the fuse element is vaporized under fault 
current conditions, to complete the current interruption process. However, 
by providing arc-quenching filler particles 42 with a surface modification 
of gas-evolving material 46 according to the invention, some important 
results are obtained. The modified pulverulent arc-quenching filler 
material 40 assists rapidly and effectively to quench the arc during fault 
current conditions, while also reducing fulgurite formation within the 
arc-quenching filler material. These results are achieved while 
maintaining advantageous free-flowing and compacting characteristics of 
the pulverulent arc-quenching filler material. 
Referring to FIG. 3, the invention is particularly directed to providing a 
pulverulent arc-quenching filler material 40 having its surface modified 
or coated with a gas-evolving material 46. The surface modified or coated 
pulverulent arc-quenching filler material 40 reduces the tendency for 
restriking or tracking of the arc during arcing conditions. The 
free-flowing and compacting characteristics assist in the ability to 
position the coated pulverulent arc-quenching filler 40 locally in the 
immediate vicinity of the arc-initiating fuse element 20, where the arcing 
occurs. These characteristics prevent the modified arc-quenching filler 40 
from settling and moving outside of the arcing region at one or more 
points along the length of the fuse element. 
It has been found particularly advantageous to fill the tubular casing 30 
with layers of coated and uncoated arc-quenching filler material, by 
conventional compacting and vibrating techniques, in order to provide the 
coated pulverulent arc-quenching filler 40 only in the localized arcing 
regions. This further reduces pressure build-up in the fuse upon gas 
evolution. 
The surface modified or coated pulverulent arc-quenching filler 40 provides 
a gas-evolving surface that improves the arc-quenching characteristics and 
effectiveness of the arc-quenching filler material per se. The surface 
modified or coated pulverulent arc-quenching filler 40 minimizes fulgurite 
formation in the fusing region and/or fulgurite conductivity upon 
fulgurite formation, both of which help to minimize the opportunity for 
the arc to restrike along a path other than along the fuse element. 
Moreover, the gas-evolving material 46 is particularly selected to be made 
from relatively non-carbonizing materials to minimize carbon residue 
tracking of the arc upon arcing conditions. 
Preferably, the pulverulent arc-quenching filler material to be modified 
has a high dielectric strength. Appropriate pulverulent arc-quenching 
filler material preferably is selected generally from the group of silica 
and silicates, and more particularly from one or more of sand, mica, 
quartz or the like. Other arc-quenching fillers which can be used include 
glass, fiber, asbestos and the like. The arc-quenching filler is 
preferably provided in a granular, free-flowing form, preferably bead 
granules. Even more preferably, the arc-quenching filler material is a 
silica having consistent particle size distribution, such as GRANUSIL, 
sand sold by Unimin Corporation. 
As shown in FIG. 3, the surface of arc-quenching filler particles 42 are 
coated with gas-evolving material 46. The gas-evolving material 46 is 
attached physically and/or chemically to the surface of the arc-quenching 
filler particles 42 by a binder material 44 to form the modified or coated 
arc-quenching filler 40 according to the invention. Preferably, each of 
the primary arc-quenching filler particles 42 are coated with a 
gas-evolving compound 46. The binder 44 is selected from the group of 
relatively non-tracking adhesives such as acrylics, urethanes, melamines, 
epoxies and polyesters or the like, acrylics being preferred. The binder 
44 attaches the gas-evolving compound 46 to the surface of the 
arc-quenching filler particles 42. Even though an acrylic binder is high 
in carbon content, the acrylic upon arcing conditions decomposes to its 
monomer structures, with minimal adverse carbonizing properties and carbon 
residues. Consequently, minimal restriking or tracking of the arc occurs. 
The gas-evolving material 46 is preferably selected from compounds 
possessing rapid gas-evolving properties, minimal tracking properties, 
high electrically non-conductive properties, high insulating properties 
and high thermal properties. The gas-evolving material is preferably 
selected from a compound high in nitrogen content and low in carbon 
content, minimize tracking from carbon (graphite) residues formed in the 
circuit interruption device when exposed to arcing conditions and high 
temperatures. More preferably, the gas-evolving material is a nitrogen 
heterocyclic compound. Even more preferably, carbonates, acetates, 
phosphates salts or the like derived from a nitrogen heterocyclic compound 
are particularly desirable because of their high thermal stability. 
The gas-evolving material 46 that is applied to the surface of the 
arc-quenching filler include materials which evolve a gas in the presence 
of an arc, such as, for example, guanidine carbonate, guanidine acetate, 
guanidine, 1,3-diphenyl guanidine, guanine, cyanuric acid, melamine, 
melamine cyanurate, urea, urea-phosphate, hydantoin, allantoin, and the 
like, and/or derivatives and mixtures thereof. Even more preferably, the 
gas-evolving materials are selected from the group of guanidine carbonate, 
hydantoin, and urea-phosphate. The gas-evolving material loading in the 
modified arc-quenching filler is preferably 2 to 70% by weight of the 
modified arc-quenching filler material, even more preferably 5 to 40% by 
weight, and most preferably up to 20% by weight of the modified 
pulverulent arc-quenching filler material. 
The current limiting fuse can also contain separate gas-evolving members 
(not shown) which evolve a gas in the presence of an arc. The evolved gas 
further aids in the extinction of the arc conditions within the fuse 
housing which occurs when a fuse element is subjected to overload or fault 
current conditions. The gas-evolving members can be positioned within 
cut-outs on the fins of a core, integrally formed from the core, coated 
onto the core, fuse element or casing, or secured to the fuse element. A 
detailed description of the construction and operation of high voltage 
current limiting fuses and of localized placement of separate gas-evolving 
structures is taught, inter alia, in U.S. Pat. Nos. 4,319,212 (Leach); 
4,339,742 (Leach, et al.); and, 4,099,153 (Cameron), each of which is 
incorporated by reference herein. 
According to the method of making the modified pulverulent arc-quenching 
filler 40 according to the invention, it has been found particularly 
advantageous first to suspend a supply of arc-quenching filler particles, 
for example, sand, preferably rounded sand and having a uniform particle 
size distribution, in a binder solution to provide a surface coating of 
the binder on the arc-quenching filler particles, particularly the primary 
particles. The binder solution can include binder in a liquid carrier 
selected from the group of toluene, xylene, methyl ethyl ketone, methyl 
iso-butyl ketone or the like and mixtures thereof. The binder coated 
arc-quenching filler particles are then brought into contact with the 
gas-evolving materials, preferably in powdered form. By this method, the 
powdered gas-evolving material readily attaches itself to the 
arc-quenching filler particles and forms a layer of gas-evolving materials 
around the arc-quenching filler particles. The amount of gas-evolving 
materials attached to the surface of the arc-quenching filler particles is 
a function of the amount of binder, the particle size of the gas-evolving 
compound, and the amount of gas-evolving compound. Loadings of the 
gas-evolving material of up to 20% by weight of the modified arc-quenching 
filler are especially preferred. Once the arc-quenching filler is modified 
with the gas-evolving compounds, the modified arc-quenching filler 
exhibits normal free-flow characteristics with minimal clumping and 
agglomeration, because the binder is no longer exposed on the surfaces. 
Other methods of coating, such as spraying or the like, can also be used. 
Thus, the method of modifying the surface of the filler material with a 
gas-evolving compound comprises the steps of providing a supply of 
pulverulent arc-quenching filler material; suspending the pulverulent 
arc-quenching filler material in a binder solution; drying the pulverulent 
arc-quenching filler and binder to tackiness; applying a gas-evolving 
compound to the binder coated arc-quenching filler particles; and, drying 
the resulting surface modified pulverulent arc-quenching filler material. 
The modified pulverulent arc-quenching filler material is loaded into the 
space within the tubular casing 30 that is not occupied by the fuse 
element and core. It has been found particularly advantageous to position 
the modified pulverulent arc-quenching filler material locally, in areas 
of the fuse housing where arcing will occur. 
During the operation of the high voltage current limiting fuse device 1, 
when the current applied to the fuse element 20 exceeds the current 
carrying capability of the fuse element 20, the excessive current produces 
resistive heating that initiates melting of the fuse element 20. When the 
fuse element 20 is subjected to this fault magnitude current, the fuse 
element quickly attains fusing temperatures and vaporizes. Arcing occurs 
and the metal vapor rapidly expands to many times the volume originally 
occupied by the fuse element 20. These vapors are emitted into the spaces 
between grains of the modified pulverulent arc-quenching filler material 
40, where they condense through heat transfer into the modified 
arc-quenching filler, and are no longer disposed in a condition for 
current conduction. The current limiting effect of the fuse as a whole 
results from the introduction of arc resistance into the circuit. During 
arcing conditions, the gas-evolving compounds 46 attached to the surface 
of the modified pulverulent arc-quenching filler rapidly evolve a 
deionizing gas, thereby reducing free ions available for conduction along 
the arc, damping the arcing as well as reducing the incidence of tracking 
or restriking of the arc. 
It is desirable that the physical contact between the hot arc initiated by 
the melting of the fuse element 20 and the relatively cooler modified 
filler granules 40 cause a rapid transfer of heat from the fuse element to 
the granules, thereby dissipating most of the arc energy with little 
pressure build-up within the fusing casing 30. It is also desirable that 
the modified arc-quenching filler material 40 is disposed in the immediate 
vicinity of the arc-initiating fuse element 20 as it melts and absorbs arc 
energy. The modified arc-quenching filler 40 is preferably locally 
positioned only in areas where arcing occurs by layering modified and 
unmodified filler inside the casing. Any resulting fulgurite from the 
fusing and sintering of the arc-quenching filler particles provides a 
semiconducting glass body which would enhance restriking of the arc. 
However, the gas-evolving materials 46 attached to the filler particle 
expel their gas during arcing conditions which not only provides a 
deionizing action on the arc but it is believed to also provide a cooling 
action on the fulgurites formed. The cooled fulgurites become insulating 
upon cooling, and the deionizing action reduces fulgurite formation in the 
first place. The gas-evolving compounds positioned on the surface of the 
arc-quenched filler are provided in such an amount that only slight 
pressure build-up within the fuse enclosure results as the evolved gas 
forms. 
The modified pulverulent arc-quenching filler 40 can occupy approximately 
all of the unoccupied space within the tubular casing 30, which can be 
enhanced with the assistance of a suitable means such as a vibrating or 
shaking of the casing during loading. The modified pulverulent 
arc-quenching filler can also occupy only localized regions of arcing, 
unmodified filler occupying the remainder of the unoccupied space within 
the tubular casino 30. Thus, it is important to maintain free-flowing and 
compacting characteristics of the filler material. 
The invention will be further clarified by a consideration of the following 
example, which is intended to be purely exemplary of the invention. 
EXAMPLE 1 
Preparation Of Surface Modified Arc-Quenching Filler Material 
170 grams of pulverulent arc-quenching filler particles, granular round 
sand (approximately 100 ml volume), were treated in a beaker with a 
diluted solution of an acrylic coating adhesives. The acrylic coating 
adhesive had been diluted with toluene in a ratio of 2:1. The resulting 
slurry was then stirred for approximately five (5) minutes to thoroughly 
suspend and coat the sand filler particles with the adhesive. The 
suspension was then allowed to stand for approximately two (2) minutes to 
allow the sand to settle to the bottom of the beaker. The excess acrylic 
adhesive solution was decanted off and the acrylic treated sand was 
air-dried to tackiness for approximately five (5) minutes to allow excess 
solvent to evaporate, while sitting in an aluminum pan. The dried sand 
(still tacky) was then mixed in four (4) separate aluminum pans with 
different powdered gas-evolving materials: (Sample 1) 19% (by weight) 
guanidine carbonate, (Sample 2) 2% (by weight) guanidine carbonate, 
(Sample 3) 10.5% (by weight) hydantoin, and Sample 4) 19% (by weight) 
urea-phosphate. The filler sand and the gas-evolving powder were mixed 
thoroughly together and then allowed to air dry. After drying, the 
modified sand had very small clumps which could be easily broken up into 
granular form. 
The arc-quenching effectiveness of the four (4) samples was tested using 
the following test procedure. The circuit used to test the fuses 
containing the coated sand is shown in FIG. 4. A high voltage distribution 
transformer was used to provide a realistic recovery voltage across the 
fuse. The circuit parameters were chosen to give a current of 37 A.sub.RMS 
through the fuse under the test. The arcing time was recorded as the time 
of the current flow in the fuse. The fuse used to test the modified sand 
filler was constructed from a 17 inch long insulating tube and a single 
silver fuse element. Prior to assembling the fuse, a drop of tin solder 
was placed on the center of the fuse element to lower the melting point of 
the silver element in the soldered area and thereby assure that arcing 
took place in the center of the fuse. 
The fuse element was fed into the tube and an uncoated round sand (25 ml) 
was poured and compacted into the fuse tube to fill the bottom 1/3 of the 
tube, followed by modified sand (30 ml) according to the (4) samples into 
the center of the tube, and then having the tube topped off with uncoated 
round sand (25 ml). Therefore, only the center part of the tube, where 
arcing was expected, was filled with the coated sand in order to conserve 
the treated sand and also to minimize pressure-buildup in the tube. The 
fuse was then melted at the tinned area by passing 12 A.sub.DC current for 
ten (10) minutes and tested in the circuit as shown in FIG. 4. 
The results obtained with the (4) samples are summarized in Table 1. The 
"arcing timed" values are a measure of the arc-quenching capabilities of 
the various modified pulverulent arc-quenching fillers, the lower the 
value the more effective the material. 
TABLE 1 
______________________________________ 
Arc-Quenching Effectiveness Of Coated Sand Samples 
Sample Gas-Evolving % By Weight Arcing 
No. Additive Used 
in Sand 
(Milliseconds) 
______________________________________ 
Control 
None 0.0 &gt;240* 
1 Guanidine Carbonate 
19.0 68 
2 Guanidine Carbonate 
2.0 204 
3 Hydantoin 
79 
4 Urea-phosphate 
19.0 
97 
______________________________________ 
*The control material (uncoated sand) failed to interrupt the arc, and, 
therefore, the arc was mechanically interrupted after 240 miliseconds. 
The invention having been disclosed in connection with the foregoing 
specification and example, additional variations will now be apparent to 
persons skilled in the art. The invention is not intended to be limited to 
the variations specifically mentioned, and accordingly reference should be 
made to the appended claims rather than the foregoing discussion of the 
specification and example, to assess the true scope and spirit of the 
invention in which exclusive rights are claimed.