Field of the Invention
[0001] The present invention relates to voltage surge protection devices and, more particularly,
to a voltage surge protection device including a varistor member.
Background of the Invention
[0002] Frequently, excessive voltage is applied across service lines that deliver power
to residences and commercial and institutional facilities. Such excess voltage or
voltage spikes may result from lightning strikes, for example. The voltage surges
are of particular concern in telecommunications distribution centers, hospitals and
other facilities where equipment damage caused by voltage surges and resulting down
time may be very costly.
[0003] Typically, one or more varistors (
i.e., voltage dependent resistors) are used to protect a facility from voltage surges.
Generally, the varistor is connected directly across an AC input and in parallel with
the protected circuit. The varistor has a characteristic clamping voltage such that,
responsive to a voltage increase beyond a prescribed voltage, the varistor forms a
low resistance shunt path for the overvoltage current that reduces the potential for
damage to the sensitive components. Typically, a line fuse may be provided in the
protective circuit and this line fuse may be blown or weakened by the surge current
or the failure of the varistor element.
[0004] Varistors have been constructed according to several designs for different applications.
For heavy-duty applications (
e.g., surge current capability in the range of from about 60 to 200 kA) such as protection
of telecommunications facilities, block varistors are commonly employed. A block varistor
typically includes a disk-shaped varistor element potted in an epoxy or plastic housing.
The varistor disk is formed by pressure casting a metal oxide material, such as zinc
oxide, or other suitable material such as silicon carbide. Copper, or other electrically
conductive material, is flame sprayed onto the opposed surfaces of the disk. Ring-shaped
electrodes are bonded to the coated opposed surfaces and the disk and electrode assembly
is enclosed within the plastic housing. Examples of such block varistors include Product
No. SIOV-B860K250, available from Siemens Matsushita Components GmbH & Co. KG and
Product No. V271BA60, available from Harris Corporation.
[0005] Another varistor design includes a high-energy varistor disk housed in a disk diode
case. The diode case has opposed electrode plates and the varistor disk is positioned
therebetween. One or both of the electrodes include a spring member disposed between
the electrode plate and the varistor disk to hold the varistor disk in place. The
spring member or members provide only a relatively small area of contact with the
varistor disk.
[0006] Another type of overvoltage protection device employing a varistor wafer is the Strikesorb™
surge protection module available from Raycap Corporation of Greece, which may form
a part of a Rayvoss™ transient voltage surge suppression system. (
See, for example,
U.S. Patent No. 6,038,119,
U.S. Patent No. 6,430,020 and
U.S. Patent No. 7,433,169.
[0007] Varistor-based overvoltage protection devices (
e.g., of the epoxy-shielded type) are commonly designed with an open circuit failure
mode using an internal thermal disconnector or overcurrent disconnector to disconnect
the device in case of failure. Other varistor-based overvoltage protection devices
have a short circuit as a failure mode. For example, some epoxy-shielded devices use
a thermal disconnector to switch to a short circuit path. However, many of these devices
have very limited short circuit current withstand capabilities.
Summary of the Invention
[0008] According to embodiments of the present invention, an overvoltage protection device
includes first and second electrically conductive electrode members and a varistor
member formed of a varistor material and electrically connected with each of the first
and second electrode members. The overvoltage protection device has an integral fail-safe
mechanism operative to electrically short circuit the first and second electrode members
about the varistor member by fusing first and second metal surfaces in the overvoltage
protection device to one another using an electric arc.
[0009] According to some embodiments, the fail-safe mechanism is operative to electrically
short circuit the first and second electrode members about the varistor member by
fusing the first and second metal surfaces in response to a short circuit failure
of the varistor member.
[0010] In some embodiments, the first and second metal surfaces are separated by a gap having
a width in the range of from about 0.2 mm to 1 mm, and the electric arc extends across
the gap to fuse the first and second metal surfaces.
[0011] According to some embodiments, the first and second metal surfaces are separated
by a gap, the overvoltage protection device further includes an electrically insulating
spacer member electrically isolating the first and second metal surfaces from one
another, and the electric arc disintegrates the spacer member and extends across the
gap to fuse the first and second metal surfaces. In some embodiments, the spacer member
is formed of a polymeric material having a thickness in the range of from about 0.1
mm to 0.5 mm.
[0012] According to some embodiments, the first metal surface is a surface of the first
electrode member and the second metal surface is a surface of the second electrode
member. In some embodiments, the first electrode includes a housing having a metal
housing sidewall and defining a housing chamber, the varistor member and at least
a portion of the second electrode are disposed in the housing chamber, and the first
metal surface is a surface of the housing sidewall. In some embodiments, the varistor
member has first and second opposed, generally planar varistor contact surfaces, the
housing includes an electrode wall having a first electrode contact surface engaging
the first varistor contact surface, the second electrode includes a head positioned
in the housing chamber, the head including a second electrode contact surface engaging
the second varistor contact surface and a head peripheral surface surrounding the
second electrode contact surface, and the second metal surface is located on the head
peripheral surface. The overvoltage protection device may include a buffer chamber
on a side of the head opposite the second electrode contact surface, wherein the buffer
chamber is configured to limit propagation of electric arc away from the head.
[0013] The overvoltage protection device may include a biasing device biasing at least one
of the first and second electrode members against the varistor member.
[0014] In some embodiments, the fail-safe mechanism is a first fail-safe mechanism and the
overvoltage protection device further includes an integral second fail-safe mechanism.
The second fail-safe mechanism includes an electrically conductive meltable member.
The meltable member is responsive to heat in the overvoltage protection device to
melt and form a current flow path between the first and second electrode members through
the meltable member. In some embodiments, the overvoltage protection device further
includes an electrically insulating spacer member electrically isolating the first
and second metal surfaces from one another, and the meltable member has a greater
melting point temperature than a melting point temperature of the spacer member. According
to some embodiments, the first fail-safe mechanism is operative to fuse the first
and second metal surfaces at a prescribed region, and the overvoltage protection device
includes a sealing member between the prescribed region and the meltable member. According
to some embodiments, the first fail-safe mechanism is operative to electrically short
circuit the first and second electrode members about the varistor member by fusing
the first and second metal surfaces in response to a short circuit failure of the
varistor member sufficient to generate an arc, and the second fail-safe mechanism
is operative to electrically short circuit the first and second electrode members
about the varistor member in response to a short circuit failure of the varistor member
not sufficient to generate an arc.
[0015] According to some embodiments, the fail-safe mechanism is a first fail-safe mechanism,
the first fail-safe mechanism is operative to electrically short circuit the first
and second electrode members about the varistor member by fusing the first and second
metal surfaces in response to a short circuit failure of the varistor member, the
overvoltage protection device further includes an integral second fail-safe mechanism,
the second fail-safe mechanism including an electrically conductive meltable member,
wherein the meltable member is responsive to heat in the overvoltage protection device
to melt and form a current flow path between the first and second electrode members
through the meltable member, the first fail-safe mechanism is operative to electrically
short circuit the first and second electrode members about the varistor member by
fusing the first and second metal surfaces in response to a short circuit failure
of the varistor member sufficient to generate an arc, the second fail-safe mechanism
is operative to electrically short circuit the first and second electrode members
about the varistor member in response to a short circuit failure of the varistor member
that is not sufficient to generate an arc, the varistor member has first and second
opposed, generally planar varistor contact surfaces, the first electrode includes
a housing defining a housing chamber and having a metal housing sidewall and an electrode
wall, the electrode wall having a first electrode contact surface engaging the first
varistor contact surface, the varistor member is disposed in the housing chamber,
the second electrode includes a head positioned in the housing chamber, the head including
a second electrode contact surface engaging the second varistor contact surface and
a head peripheral surface surrounding the second electrode contact surface, the first
metal surface is a surface of the housing sidewall, the second metal surface is located
on the head peripheral surface, the first and second metal surfaces are separated
by a gap having a width in the range of from about 0.2 mm to 1 mm, the overvoltage
protection device further includes an electrically insulating spacer member electrically
isolating the first and second metal surfaces from one another, the electric arc disintegrates
the spacer member and extends across the gap to fuse the first and second metal surfaces,
and the spacer member is formed of a polymeric material having a thickness in the
range of from about 0.1 mm to 0.5 mm.
[0016] According to method embodiments of the present invention, a method for providing
overvoltage protection includes providing an overvoltage protection device including:
first and second electrically conductive electrode members; a varistor member formed
of a varistor material and electrically connected with each of the first and second
electrode members; and an integral fail-safe mechanism operative to electrically short
circuit the first and second electrode members about the varistor member by fusing
the first and second metal surfaces in the overvoltage protection device to one another
using an electric arc. The method further includes directing current between the first
and second electrode members through the varistor member during an overvoltage event.
[0017] According to embodiments of the present invention, an overvoltage protection device
includes first and second electrically conductive electrode members and a varistor
member formed of a varistor material and electrically connected with each of the first
and second electrode members. The overvoltage protection device has an integral first
fail-safe mechanism configured to electrically short circuit the first and second
electrode members about the varistor member when triggered by a first set of operating
conditions. The overvoltage protection device also has an integral second fail-safe
mechanism configured to electrically short circuit the first and second electrode
members about the varistor member when triggered by a second set of operating conditions
different from the first set of operating conditions.
[0018] According to some embodiments, the first and second sets of operating conditions
each include at least one of an overheating event and an arcing event. In some embodiments,
the first set of operating conditions includes an arcing event, and the second set
of operating conditions includes an overheating event.
[0019] According to method embodiments of the present invention, a method for providing
overvoltage protection includes providing an overvoltage protection device including:
first and second electrically conductive electrode members; a varistor member formed
of a varistor material and electrically connected with each of the first and second
electrode members; an integral first fail-safe mechanism configured to electrically
short circuit the first and second electrode members about the varistor member when
triggered by a first set of operating conditions; and an integral second fail-safe
mechanism configured to electrically short circuit the first and second electrode
members about the varistor member when triggered by a second set of operating conditions
different from the first set of operating conditions. The method further includes
directing current between the first and second electrode members through the varistor
member during an overvoltage event.
[0020] Further features, advantages and details of the present invention will be appreciated
by those of ordinary skill in the art from a reading of the figures and the detailed
description of the preferred embodiments that follow, such description being merely
illustrative of the present invention.
Brief Description of the Drawings
[0021] The accompanying drawings, which form a part of the specification, illustrate embodiments
of the present invention.
Figure 1 is an exploded, perspective view of an overvoltage protection device according to
embodiments of the present invention.
Figure 2 is a perspective view of the overvoltage protection device of Figure 1.
Figure 3 is a cross-sectional view of the overvoltage protection device of Figure 1 taken along the line 3-3 of Figure 2.
Figure 4 is a cross-sectional view of the overvoltage protection device of Figure 1 taken along the line 3-3 of Figure 2, wherein a meltable member of the overvoltage protection device has been reconfigured
by melting to bypass the varistor wafer.
Figure 5 is an enlarged, fragmentary, cross-sectional view of the overvoltage protection device
of Figure 1 taken along the line 3-3 illustrating a failure site in a varistor wafer and arcs propagating through the
overvoltage protection device.
Figure 6 is an enlarged, fragmentary, cross-sectional view of the overvoltage protection device
of Figure 1 taken along the line 3-3, wherein a fused interface has been formed by the arcing of Figure 5 to bypass the varistor wafer.
Figure 7 is a schematic diagram representing a circuit including the overvoltage protection
device of Figure 1 according to embodiments of the present invention.
Detailed Description of Embodiments of the Invention
[0022] The present invention now will be described more fully hereinafter with reference
to the accompanying drawings, in which illustrative embodiments of the invention are
shown. In the drawings, the relative sizes of regions or features may be exaggerated
for clarity. This invention may, however, be embodied in many different forms and
should not be construed as limited to the embodiments set forth herein; rather, these
embodiments are provided so that this disclosure will be thorough and complete, and
will fully convey the scope of the invention to those skilled in the art.
[0023] It will be understood that when an element is referred to as being "coupled" or "connected"
to another element, it can be directly coupled or connected to the other element or
intervening elements may also be present. In contrast, when an element is referred
to as being "directly coupled" or "directly connected" to another element, there are
no intervening elements present. Like numbers refer to like elements throughout.
[0024] In addition, spatially relative terms, such as "under", "below", "lower", "over",
"upper" and the like, may be used herein for ease of description to describe one element
or feature's relationship to another element(s) or feature(s) as illustrated in the
figures. It will be understood that the spatially relative terms are intended to encompass
different orientations of the device in use or operation in addition to the orientation
depicted in the figures. For example, if the device in the figures is turned over,
elements described as "under" or "beneath" other elements or features would then be
oriented "over" the other elements or features. Thus, the exemplary term "under" can
encompass both an orientation of over and under. The device may be otherwise oriented
(rotated 90 degrees or at other orientations) and the spatially relative descriptors
used herein interpreted accordingly.
[0025] Well-known functions or constructions may not be described in detail for brevity
and/or clarity.
[0026] As used herein the expression "and/or" includes any and all combinations of one or
more of the associated listed items.
[0027] The terminology used herein is for the purpose of describing particular embodiments
only and is not intended to be limiting of the invention. As used herein, the singular
forms "a", "an" and "the" are intended to include the plural forms as well, unless
the context clearly indicates otherwise. It will be further understood that the terms
"comprises" and/or "comprising," when used in this specification, specify the presence
of stated features, integers, steps, operations, elements, and/or components, but
do not preclude the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
[0028] Unless otherwise defined, all terms (including technical and scientific terms) used
herein have the same meaning as commonly understood by one of ordinary skill in the
art to which this invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be interpreted as having a
meaning that is consistent with their meaning in the context of the relevant art and
will not be interpreted in an idealized or overly formal sense unless expressly so
defined herein.
[0029] As used herein, "monolithic" means an object that is a single, unitary piece formed
or composed of a material without joints or seams.
[0030] As used herein, the term "wafer" means a substrate having a thickness which is relatively
small compared to its diameter, length or width dimensions.
[0031] With reference to
Figures 1-7, an overvoltage protection device according to embodiments of the present invention
is shown therein and designated
100. The device
100 has a lengthwise axis
A-A (
Figure 2). The device
100 includes a first electrode or housing
120, a piston-shaped second electrode
130, a varistor member (herein, "the varistor wafer")
110 between the housing
120 and the electrode
130, and other components as discussed in more detail below. The device
100 further includes an integral first fail-safe mechanism, arrangement, feature or system
161 and an integral second fail-safe mechanism, arrangement, feature or system
141. The fail-safe systems
141, 161 are each adapted to prevent or inhibit overheating or thermal runaway of the overvoltage
protection device, as discussed in more detail below.
[0032] With reference to
Figures 1-3, the housing
120 has an end electrode wall
122 (
Figure 3) and a cylindrical sidewall
124 extending from the electrode wall
122. The sidewall
124 and the electrode wall
122 form a chamber or cavity
121 communicating with an opening
126. A threaded post or stud
129 extends outwardly from housing
120. The electrode
130 has a head
132 disposed in the cavity
121 and an integral shaft
134 that projects outwardly through the opening
126. The varistor wafer
110 is disposed in the cavity
121 between and in contact with each of the electrode wall
122 and the head
132.
[0033] In use, the device
100 may be connected directly across an AC or DC input (for example, in an electrical
service utility box). Service lines are connected directly or indirectly to each of
the electrode shaft
134 and the housing post
129 such that an electrical flow path is provided through the electrode
130, the varistor wafer
110, the housing electrode wall
122 and the housing post
129. Ordinarily, in the absence of an overvoltage condition, the varistor wafer
110 provides high electrical resistance such that no significant current flows through
the device
100 as it appears electrically as an open circuit. In the event of an overvoltage condition
(relative to the design voltage of the device), the resistance of the varistor wafer
110 decreases rapidly, allowing current to flow through the device
100 and create a shunt path for current flow to protect other components of an associated
electrical system. The general use and application of overvoltage protectors such
as varistor devices is well known to those of skill in the art and, accordingly, will
not be further detailed herein.
[0034] Turning to the construction of the device
100 in greater detail, the first fail-safe system
161 includes an electrically insulating spacer member or membrane
160 disposed in the cavity
121. The second fail-safe system
141 includes an electrically conductive meltable member
140 and a meltable member insulating ring
144 disposed in the cavity
121. The device
100 further includes spring washers
146, a flat washer
148, an insulating member
150, an end cap
152, a clip
154, an O-ring
155, an O-ring
156, an O-ring
157, and a cover
158 over the cavity
121. Each of these components is described more fully below.
[0035] The electrode wall
122 of the housing
120 has an inwardly facing, substantially planar contact surface
122A. An annular slot
123 is formed in the inner surface of the sidewall
124. According to some embodiments, the housing
120 is formed of aluminum. However, any suitable electrically conductive metal may be
used. According to some embodiments, the housing
120 is unitary and, in some embodiments, monolithic. The housing
120 as illustrated is cylindrically shaped, but may be shaped differently.
[0036] As best seen in
Figure 3, the head
132 of the electrode
130 has a substantially planar contact surface
132A that faces the contact surface
122A of the electrode wall
122. A circumferential peripheral sidewall surface
132C surrounds the contact surface
132A. The shaft
134 has a lower portion
134A, an intermediate portion
134B, and an upper portion
134C. According to some embodiments, each shaft portion
134A, 134B, 134C has a diameter of from about 0.79 to 1 inch. An integral, annular, lower flange
136 extends radially outwardly from the shaft
134 between the shaft portions
134A and
134B. An integral, annular, upper flange
138 extends radially outwardly from the shaft
134 between the shaft portions
134B and
134C. An annular, sidewardly opening groove
136A is defined in the peripheral sidewall of the flange
136. A threaded bore
135 is formed in the end of the shaft
134 to receive a bolt for securing a bus bar or other electrical connector to the electrode
130. An annular, sidewardly opening groove
134D is defined in the shaft portion
134C.
[0037] According to some embodiments, the electrode
130 is formed of aluminum and, in some embodiments, the housing sidewall
124 and the electrode
130 are both formed of aluminum. However, any suitable electrically conductive metal
may be used. According to some embodiments, the electrode
130 is unitary and, in some embodiments, monolithic.
[0038] An annular gap
G1 is defined between the head peripheral sidewall surface
132C and the nearest adjacent surface of the sidewall
124. A gap
G2 is defined between the varistor
110 and the nearest adjacent surface of the sidewall
124. A gap
G3 is defined between the lower flange
136 and the nearest adjacent surface of the sidewall
124. There may be a gap defined between the membrane
160 and the surface
132C and/or the sidewall
124, as shown for example. Alternatively, the membrane
160 may be substantially in contact with surface
132C and the sidewall
124 (
i.e., the gaps
G1,
G2, G3 may be substantially completely filled by the membrane
160).
[0039] According to some embodiments, each gap
G1,
G2, G3 has a width
W1 (Figure 3) in the range of from about 0.2 to 1 mm and, in some embodiments, in the range of
from about 0.5 to 0.75 mm.
[0040] With reference to
Figure 3, the housing
120 and the end cap
152 collectively define an enclosed device chamber
170. A varistor subchamber
172 is defined by the head
132, the electrode wall
122 and a portion of the sidewall
124. An extinguishing or buffer subchamber
174 is defined by the head
132, the flange
136 and a portion of the sidewall
124. A meltable member subchamber
176 is defined by the flange
136, the flange
138 and a portion of the sidewall
124.
[0041] The membrane
160 is mounted around the electrode
130 between the electrode
130 and the sidewall
124 in the chamber
170. The membrane
160 is annular and surrounds the varistor
110, the head
132, the lower shaft portion
134A, and the lower flange
136. In some embodiments and as shown, the membrane
160 is a relatively thin, cylindrical, tubular piece or sleeve. The membrane
160 is interposed radially between the sidewall
124 and each of the varistor
110, the head peripheral sidewall surface
132C, and the flange
136. Except as discussed below, the membrane
160 electrically isolates the electrode
130 from the housing
120. According to some embodiments, the membrane
160 contacts the sidewall
124.
[0042] The membrane
160 is formed of a dielectric or electrically insulating material having high melting
and combustion temperatures, but which can be disintegrated (such as by melting, burning,
combusting or vaporizing) when subjected to an electric arc or the high temperatures
created by an electric arc. According to some embodiments, the membrane
160 is formed of a high temperature polymer and, in some embodiments, a high temperature
thermoplastic. In some embodiments, the membrane
160 is formed of polyetherimide (PEI), such as ULTEM™ thermoplastic available from SABIC
of Saudi Arabia. In some embodiments, the membrane
160 is formed of non-reinforced polyetherimide.
[0043] According to some embodiments, the membrane
160 is formed of a material having a melting point greater than the melting point of
the meltable member
140. According to some embodiments, the membrane
160 is formed of a material having a melting point in the range of from about 120 to
200 °C and, according to some embodiments, in the range of from about 140 to 160 °C.
[0044] According to some embodiments, the membrane
160 material can withstand a voltage of 25 kV per mm of thickness.
[0045] According to some embodiments, the membrane
160 has a thickness
T1 (Figure 3) in the range of from about 0.1 to 0.5 mm and, in some embodiments, in the range of
from about 0.3 to 0.4 mm.
[0046] The meltable member
140 is mounted on the electrode
130 in the subchamber
176. The meltable member
140 is annular and surrounds the intermediate shaft portion
134B, which is disposed in a central passage of the meltable member
140. In some embodiments and as shown, the meltable member
140 is a cylindrical, tubular piece or sleeve. According to some embodiments, the meltable
member
140 contacts the intermediate shaft portion
134B and, according to some embodiments, the meltable member
140 contacts the intermediate shaft portion
134B along substantially the full length of the intermediate shaft portion
134B and the full length of the meltable member
140. The meltable member
140 also engages the lower surface of the flange
138 and the top surface of the flange
136. The meltable member
140 is spaced apart from the sidewall
124 a distance sufficient to electrically isolate the meltable member
140 from the sidewall
124.
[0047] The meltable member
140 is formed of a heat-meltable, electrically conductive material. According to some
embodiments, the meltable member
140 is formed of metal. According to some embodiments, the meltable member
140 is formed of an electrically conductive metal alloy. According to some embodiments,
the meltable member
140 is formed of a metal alloy from the group consisting of aluminum alloy, zinc alloy,
and/or tin alloy. However, any suitable electrically conductive metal may be used.
[0048] According to some embodiments, the meltable member
140 is selected such that its melting point is greater than a prescribed maximum standard
operating temperature. The maximum standard operating temperature may be the greatest
temperature expected in the meltable member
140 during normal operation (including handling overvoltage surges within the designed
for range of the device
100) but not during operation which, if left unchecked, would result in thermal runaway.
According to some embodiments, the meltable member
140 is formed of a material having a melting point in the range of from about 80 to 160
°C and, according to some embodiments, in the range of from about 80 to 120 °C. According
to some embodiments, the melting point of the meltable member
140 is at least 20 °C less than the melting points of the housing
120, the electrode
130, the insulator ring
150, and the membrane
160 and, according to some embodiments, at least 40 °C less than the melting points of
those components.
[0049] According to some embodiments, the meltable member
140 has an electrical conductivity in the range of from about 0.5 x 10
6 Siemens/meter (S/m) to 4 x 10
7 S/m and, according to some embodiments, in the range of from about 1 x 10
6 S/m to 3 x 10
6 S/m.
[0050] The meltable member
140 can be mounted on the electrode
130 in any suitable manner. According to some embodiments, the meltable member
140 is cast or molded onto the electrode
130. According to some embodiments, the meltable member
140 is mechanically secured onto the electrode
130. According to some embodiments, the meltable member
140 is unitary and, in some embodiments, monolithic.
[0051] The varistor wafer
110 has first and second opposed, substantially planar contact surfaces
112. The varistor wafer
110 is interposed between the contact surfaces
122A and
132A. As described in more detail below, the head
132 and the wall
122 are mechanically loaded against the varistor wafer
110 to ensure firm and uniform engagement between the surfaces
132A, 122A and the respective opposed surfaces
112, 114 of the varistor wafer
110.
[0052] The varistor wafer
110 has a circumferential, peripheral sidewall surface
116. According to some embodiments, the varistor wafer
110 is disk-shaped. However, the varistor wafer
110 may be formed in other shapes. The thickness and the diameter of the varistor wafer
110 will depend on the varistor characteristics desired for the particular application.
The varistor wafer
110 may include a wafer of varistor material coated on either side with a conductive
coating so that the exposed surfaces of the coatings serve as the contact surfaces.
The coatings can be formed of aluminum, copper or silver, for example.
[0053] The varistor material may be any suitable material conventionally used for varistors,
namely, a material exhibiting a nonlinear resistance characteristic with applied voltage.
Preferably, the resistance becomes very low when a prescribed voltage is exceeded.
The varistor material may be a doped metal oxide or silicon carbide, for example.
Suitable metal oxides include zinc oxide compounds.
[0054] The spring washers
146 surround the upper shaft portion
134B and engage the upper surface of the flange
138. Each spring washer
146 includes a hole that receives the upper shaft portion
134C of the electrode
130. The lowermost spring washer
146 abuts the top face of the flange
138. According to some embodiments, the clearance between the spring washer hole and the
shaft portion
134C is in the range of from about 0.015 to 0.035 inch. The spring washers
146 may be formed of a resilient material. According to some embodiments and as illustrated,
the spring washers
146 are Belleville washers formed of spring steel. While two spring washers
146 are shown, more or fewer may be used.
[0055] The flat metal washer
148 is interposed between the spring washer
146 and the insulator ring
150 with the shaft portion
134C extending through a hole formed in the washer
148. The washer
148 serves to distribute the mechanical load of the spring washer
146 to prevent the spring washer from cutting into the insulator ring
150.
[0056] The insulator ring
150 overlies and abuts the washer
148. The insulator ring
150 has a main body ring
150A, a cylindrical upper flange or collar
150B extending upwardly from the main body ring
150A, and a cylindrical lower flange or collar
150C extending downwardly from the main body ring
150A. A hole
150D receives the shaft portion
134B. According to some embodiments, the clearance between the hole
150D and the shaft portion
134B is in range of from about 0.025 to 0.065 inch. The main body ring
150A and the collars
150B, 150C may be bonded or integrally molded. An upwardly and outwardly opening peripheral
groove
150E is formed in the top corner of the main body ring
150A.
[0057] The insulator ring
150 is preferably formed of a dielectric or electrically insulating material having high
melting and combustion temperatures. The insulator ring
150 may be formed of polycarbonate, ceramic or a high temperature polymer, for example.
According to some embodiments, the insulator ring
150 is formed of a material having a melting point greater than the melting point of
the meltable member
140.
[0058] The end cap
152 overlies and abuts the insulator ring
150. The end cap
152 has a hole
152A that receives the shaft portion
134C. According to some embodiments, the clearance between the hole
152A and the shaft portion
134C is in the range of from about 0.1 to 0.2 inch. The end cap
152 may be formed of aluminum, for example.
[0059] The clip
154 is resilient and truncated ring shaped. The clip
154 is partly received in the slot
123 and partly extends radially inwardly from the inner wall of the housing
120 to limit outward axial displacement of the end cap
152. The clip
154 may be formed of spring steel.
[0060] The cover
158 is configured to cover the end cap
152 and has a hole
158A through which the shaft
134 extends. The cover
158 is formed of an electrically insulating material and serves to insure a desired creepage
distance between the electrode
130 and the end cap
152 or housing
120.
[0061] The cover
158 may be formed of any suitable electrically insulating material having a sufficiently
high melting temperature. The cover
158 may be formed of polycarbonate, ceramic or a high temperature polymer, for example.
According to some embodiments, the cover
158 is formed of a material having a melting point greater than the melting point of
the meltable member
140.
[0062] The O-ring
155 is positioned in the groove
134D so that it is captured between the shaft
134 and the insulator ring
150. The O-ring
156 is positioned in the groove
159 such that it is captured between the end cap
152 and the insulator ring
150. The O-ring
157 is positioned in the groove
136A such that it is captured between the flange
136 and the sidewall
124. When installed, the O-rings
156, 157 are compressed so that they are biased against and form a seal between the adjacent
interfacing surfaces. In an overvoltage event, byproducts such as hot gases and fragments
from the wafer
110 may fill or scatter into the cavity chamber
170. These byproducts may be constrained or prevented by the O-rings
156,
157 from escaping the overvoltage protection device
100 through the housing opening
126.
[0063] The O-rings
155,
156,
157 may be formed of the same or different materials. According to some embodiments,
the O-rings
155,
156,
157 are formed of a resilient material, such as an elastomer. According to some embodiments,
the O-rings
155, 156, 157 are formed of rubber. The O-rings
155, 156, 157 may be formed of a fluorocarbon rubber such as VITON™ available from DuPont. Other
rubbers such as butyl rubber may also be used. According to some embodiments, the
rubber has a durometer of between about 60 and 100 Shore A. According to some embodiments,
the melting point of each of the O-rings
155, 156, 157 is greater than the melting point of the meltable member
140.
[0064] When assembled as shown in
Figures 2 and
3, the O-ring
157 seals the subchambers
172, 174 containing the varistor
110, and the O-rings
155,
156 and
157 seal the subchamber
174 containing the meltable member
140.
[0065] As noted above and as best shown in
Figure 3, the electrode head
132 and the electrode wall
122 are persistently biased or loaded against the varistor wafer
110 to ensure firm and uniform engagement between the wafer surfaces
112, 114 and the surfaces
122A, 132A. This aspect of the device
100 may be appreciated by considering a method according to the present invention for
assembling the device
100. The O-rings
156, 157 are installed in the grooves
150E, 136A. The meltable member
140 is mounted on the shaft portion
134B using any suitable technique (e.g., casting). The varistor wafer
110 is placed in the cavity
121 such that the wafer surface
112 engages the contact surface
122A. The electrode
130 is inserted into the cavity
121 such that the contact surface
132A engages the varistor wafer surface
114. The spring washers
146 are slid down the shaft portion
134C and placed over the flange
138. The washer
148, the insulator ring
150, and the end cap
152 are slid down the shaft portion
134C and over the spring washers
146. A jig (not shown) or other suitable device is used to force the end cap
152 down, in turn deflecting the spring washers
146. While the end cap
152 is still under the load of the jig, the clip
154 is compressed and inserted into the slot
123. The clip
154 is then released and allowed to return to its original diameter, whereupon it partly
fills the slot and partly extends radially inward into the cavity
121 from the slot
123. The clip
154 and the slot
123 thereby serve to maintain the load on the end cap
152 to partially deflect the spring washers
146. The loading of the end cap
152 onto the insulator ring
150 and from the insulator ring onto the spring washer
146 is in turn transferred to the head
132. In this way, the varistor wafer
110 is sandwiched (clamped) between the head
132 and the electrode wall
122. The cover
158 is installed over the end cap
152.
[0066] The varistor
110 may be constructed and operate in conventional or known, or similar, manner. As is
well known, a varistor has an innate nominal clamping voltage VNOM (sometimes referred
to as the "breakdown voltage" or simply the "varistor voltage") at which the varistor
begins to conduct current. Below the VNOM, the varistor will not pass current. Above
the VNOM, the varistor will conduct a current (
i.e., a leakage current or a surge current). The VNOM of a varistor is typically specified
as the measured voltage across the varistor with a DC current of 1mA.
[0067] As is well known, a varistor has three modes of operation. In a first normal mode,
up to a nominal voltage, the varistor is practically an electrical insulator. In a
second normal mode, when the varistor is subjected to an overvoltage, the varistor
temporarily and reversibly becomes an electrical conductor during the overvoltage
condition and returns to the first mode thereafter. In a third mode (the so-called
end of life mode), the varistor is effectively depleted and becomes a permanent, non-reversible
electrical conductor.
[0068] The varistor
110 also has an innate maximum clamping voltage VC (sometimes referred to as simply the
"clamping voltage"). The maximum clamping voltage VC is defined as the maximum voltage
measured across the varistor when a specified current is applied to the varistor over
time according to a standard protocol.
[0069] As discussed above, in the absence of an overvoltage condition, the varistor wafer
110 provides high resistance such that no current flows through the device
100 as it appears electrically as an open circuit. That is, ordinarily the varistor
110 passes no current. The electrodes
130 and the housing are electrically isolated from one another by the varistor
110, the gaps
G1,
G2,
G3, the membrane
160, the insulator ring
150 and the cover
158. In the event of an overcurrent surge event (typically transient;
e.g., lightning strike) or an overvoltage condition or event (typically longer in duration
than an overcurrent surge event) exceeding VNOM, the resistance of the varistor wafer
decreases rapidly, allowing current to flow through the device
100 and create a shunt path for current flow to protect other components of an associated
electrical system. Normally, the varistor
110 recovers from these events without significant overheating of the device
100.
[0070] The VNOM of a given varistor begins at a certain value and over time could degrade
to a lower effective VNOM value as a result of varistor aging. Typically, a varistor
is initially rated for a "maximum continuous operating voltage" (MCOV), indicating
that the VNOM of the varistor exceeds the rated MCOV when first placed in service.
For example, the rated MCOV of a selected varistor may be 1500V, but may drop to 1300V
due to aging.
[0071] Varistor aging (
i.e., degradation resulting in reduction of the VNOM) can be caused by surge currents
or continuous leakage currents (during continuous overvoltage events) applied to the
varistor in service, as well as by passage of time with the nominal voltage applied
on the varistor (rare case, typically caused by low quality varistors). Aging degradation
is generally thermally induced.
[0072] Varistors have multiple failure modes. These failure modes may be caused by aging
or a surge of sufficient magnitude and duration. The failure modes include: 1) the
varistor
110 fails as a short circuit; and 2) the varistor
110 fails as a linear resistance (aging of the varistor). A short circuit failure typically
manifests as a localized pinhole or puncture site (herein, "the failure site") extending
through the thickness of the varistor
110. This failure site creates a path for current flow between the two electrodes of a
low resistance, but high enough to generate ohmic losses and cause overheating of
the device
100 even at low fault currents. Sufficiently large fault current through the varistor
110 can melt the varistor in the region of the failure site and generate an electric
arc. A varistor failure as a linear resistance will cause the conduction of a limited
current through the varistor that will result in a buildup of heat. This heat buildup
may result in catastrophic thermal runaway and the device temperature may exceed a
prescribed maximum temperature. For example, the maximum allowable temperature for
the exterior surfaces of the device may be set by code or standard to prevent combustion
of adjacent components. In some cases, the current through the failed varistor could
also be limited by the power system itself (
e.g., ground resistance in the system or in photo-voltaic (PV) power source applications
where the fault current depends on the power generation capability of the system at
the time of the failure) resulting in a progressive build up of temperature, even
if the varistor failure is a short circuit. There are cases where there is a limited
leakage current flow through the varistor due to extended in time overvoltage conditions
due to power system failures, for example. These conditions may lead to temperature
build up in the device, such as when the varistor has failed as a linear resistance
and could possibly lead to the failure of the varistor either as a linear resistance
or as a short circuit as described above.
[0073] One way to avoid such thermal runaway is to interrupt the current through the device
100 using a fuse that blows prior to the occurrence of overheat in the device
100. However, as discussed below, in some cases this approach is undesirable as it may
cause damage to other important components in an associated circuit or leave the load
unprotected after disconnecting the surge protective device. In addition, in some
applications the current could be lower than the rating of the fuse.
[0074] In some cases, the device
100 may assume an "end of life" mode in which the varistor wafer is depleted in full
or in part (
i.e., in an "end of life" state), leading to an end of life failure. When the varistor
110 of the device
100 reaches its end of life, the device
100 will become substantially a short circuit with a very low but non-zero ohmic resistance.
[0075] As a result, in an end of life condition, a fault current will continuously flow
through the varistor
110 even in the absence of an overvoltage condition. As a result, notwithstanding the
short circuit provided by the end of life device
100, the fault current may be insufficient to trip or blow an associated breaker or fuse.
In this case, the current may continue to flow through the varistor
110, thereby generating heat from ohmic losses in the varistor
110. If the condition is permitted to persist, the heat generated in the device
100 may build up until the housing
120 melts or explodes. Such an event may be regarded as catastrophic. If the fault current
is of sufficient magnitude, the fault current will induce or generate electric arcing
through and around the varistor
110 (herein, an "arcing event"). Such an arcing event may rapidly generate additional
heat in the device
100 and/or may cause localized damage to other components of the device
100.
[0076] The first fail-safe system
161 and the second fail-safe system
141 are each adapted and configured to electrically short circuit the current applied
to the device
100 around the varistor
110 to prevent or reduce the generation of heat in the varistor. In this way, the fail-safe
systems
141, 161 can operate as switches to bypass the varistor
110 and prevent overheating and catastrophic failure as described above. According to
embodiments of the invention, the fail-safe systems
141, 161 operate independently of one another. More particularly, in some embodiments, the
fail-safe system
161 will operate to short circuit the device
100 when a first type or set of operating conditions are experienced by the device
100 and the fail-safe system
141 will operate to short circuit the device
100 when a second type or set of operating conditions, different from the first, are
experienced by the device
100. That is, under different circumstances, the fail-safe system
161 may operate or execute first or the fail-safe system
141 may operate or execute first. Ordinarily, though not necessarily, only one of the
fail-safe systems will execute, whereupon the conditions necessary to invoke the other
fail-safe system will be prevented from arising.
[0077] The operation of the fail-safe systems
141, 161 will be described in more detail hereinbelow. As used herein, a fail-safe system
is "triggered" upon occurrence of the conditions necessary to cause the fail-safe
system to operate as described to short circuit the electrodes
120, 130.
[0078] Turning to the fail-safe system
141 in more detail, when heated to a threshold temperature, the meltable member
140 will flow to bridge and electrically connect the electrodes
120, 130. The meltable member
140 thereby redirects the current applied to the device
100 to bypass the varistor
110 so that the current induced heating of the varistor
110 ceases. The fail-safe system
141 may thereby serve to prevent or inhibit thermal runaway without requiring that the
current through the device
100 be interrupted.
[0079] More particularly, the meltable member
140 initially has a first configuration as shown in
Figures 1 and
3 such that it does not electrically couple the electrode
130 and the housing
120 except through the head
132. Upon the occurrence of a heat buildup event, the electrode
130 is thereby heated. The meltable member
140 is also heated directly and/or by the electrode
130. During normal operation, the temperature in the meltable member
140 remains below its melting point so that the meltable member
140 remains in solid form. However, when the temperature of the meltable member
140 exceeds its melting point, the meltable member
140 melts (in full or in part) and flows by force of gravity into a second configuration
different from the first configuration. When the device
100 is vertically oriented, the melted meltable member
140 accumulates in the lower portion of the subchamber
176 as a reconfigured meltable member
140A (which may be molten in whole or in part) as shown in
Figure 4. The meltable member
140A bridges or short circuits the electrode
130 to the housing
120 to bypass the varistor
110. That is, a new direct flow path or paths are provided from the surface of the electrode
portion
134B to the surfaces of the housing sidewall
124 through the meltable member
140A. According to some embodiments, at least some of these flow paths do not include the
varistor wafer
110.
[0080] The reconfigured meltable member
140A is typically contained in the chamber
176. The insulating ring
144 is positioned in the chamber
176 to provide a better flow path for the meltable member
140. More particularly, the insulating ring
144 serves as a spacer to direct the flow of the molten member
140 over the upper end section of the membrane
160. If the molten member
140 were permitted to flow directly into the membrane
160, the membrane
160 may prevent or jeopardize a quick and reliable engagement between the member
140 and the sidewall
124. The membrane
160 may extend above the top edge of the flange
136 in order to provide sufficient electrical creepage distance between the flange
136 and the sidewall
124.
[0081] According to some embodiments, the fail-safe system
141 can be triggered by at least two alternative triggering sets of operating conditions,
as follows.
[0082] The fail-safe system
141 can be triggered by heat generated in the varistor
110 by a leakage current. More particularly, when the voltage across the varistor
110 exceeds the nominal clamping voltage VNOM, a leakage current will pass through the
varistor
110 and generate heat therein from ohmic losses. This may occur because the VNOM has
dropped due to varistor
110 aging and/or because the voltage applied by the circuit across the device
100 has increased.
[0083] The fail-safe system
141 can also be triggered when the varistor
110 fails as a short circuit. In this case, the varistor
110 will generate heat from a fault current through the short circuit failure site (
e.g., a pinhole
118 as illustrated in
Figure 5). The fault current generates heat (from high localized ohmic loss heating at the pinhole)
in and adjacent the varistor
110. As discussed below, a fail-short varistor may trigger the first fail-safe system
161 instead of the second fail-safe system
141, depending on the magnitude of the fault current and other conditions.
[0084] With reference to
Figures 3, 5 and
6, the first fail-safe system
161 can be triggered when the varistor
110 fails as a short circuit. In this case, arcing will occur adjacent and within a short
circuit failure site
118. More particularly, the arcing will occur between the varistor
110 and one or both of the electrodes
120, 130 at the varistor-electrode contact interfaces
112/122A, 114/132A. The arcing will propagate radially outwardly toward the housing sidewall
124. The arcing may travel from the electrode wall
122 of the housing
120 up the housing sidewall
120 (
i.e., with the arc extending between the varistor sidewall
116 and the housing sidewall
124) and/or may travel from the varistor upper contact face
114 to the sidewall
132C of the electrode head
132. Ultimately, the arcing propagates up the housing sidewall
124 such that arcing occurs directly between the outer peripheral sidewall
132C of the electrode head
132 and the adjacent surface of the housing sidewall
124. This latter arcing causes a metal surface portion
137 of the head sidewall
132C and a metal surface portion
127 of the housing sidewall
124 to fuse or bond directly to one another in a prescribed region at a bonding or fusing
site
164 to form a bonded or fused interface portion, or region
162 (Figure 6). For example,
Figure 5 shows an exemplary varistor short circuit failure site or pinhole
118 wherein an electric arc
A1 has been generated by a fault current. The arc
A1 may propagate as an arc
A2 along the housing contact surface
122A and up the sidewall
124 as an arc
A3 (across the gap
G1 from the varistor sidewall surface
116 to the sidewall
124 surface) to ultimately form an arc
A4 at the fusing site
164. The arc
A4 fuses or bonds the surfaces and portions
127, 137. Alternatively or additionally, the arc
A1 may propagate as an arc
A5 along the electrode contact surface
132A to ultimately form the arc
A4 at the fusing site
164. In some embodiments, the electrodes are both formed of aluminum or aluminum alloy,
so that the bond is direct aluminum-to-aluminum, which can provide particularly low
ohmic resistance. The fusing or bonding may occur by welding induced by the arc. In
this way, the electrodes
120, 130 are shorted at the interface
162 to bypass the varistor
110 so that the current induced heating of the varistor
110 ceases.
[0085] The electrical insulation membrane
160 is provided between the housing sidewall
124 and the electrode head
132 and the varistor
110 to provide electrical isolation in normal operation. However, the membrane
160 is formed of a material that is quickly melted or vaporized by the arcing so that
the membrane
160 does not unduly impede the propagation of the arc or the bonding of the electrodes
120, 130 as described.
[0086] The chamber
174 provides a break between the adjacent surfaces of the electrode
130 and the housing
120 to extinguish the electric arc (
i.e., to prevent the arc from continuing up the sidewall
124). The chamber
174 reduces the time required to terminate the arc and facilitates more rapid formation
of the bonded interface
162.
[0087] In the event of a fail-short varistor, either of the first and second fail-safe systems
161, 141 may be triggered or activated, in which case it is unlikely that the other will be.
The first fail-safe system
161 requires a fault current sufficient to create the arcing, whereas the second fail-safe
system
141 does not. When sufficient fault current is present to create the arcing, the first
fail-safe system
161 will typically execute and form the electrode short circuit before the second fail-safe
system
141 can form the meltable member short. However, if the applied current is insufficient
to generate the arcing, the fault current will continue to heat the device
100 until the second fail-safe system
141 is activated. Thus, where a fail-short varistor is the trigger, the second fail-safe
system
141 will operate for relatively low current and the first fail-safe system
161 will operate for relatively high current.
[0088] Thus, the meltable member
140A and the fused interface
162 each provide a direct electrical contact surface or a low resistance bridge between
the electrode
130 and the housing
120 and an enlarged current flow path (
i.e., a lower resistance short circuit) via the meltable member
140A or the fused site
162. In this way, the fault or leakage current is directed away from the varistor
110. The arcing, ohmic heating and/or other phenomena inducing heat generation are diminished
or eliminated, and thermal runaway and/or excessive overheat of the device
100 can be prevented. The device
100 may thereby convert to a relatively low resistance element capable of maintaining
a relatively high current safely (
i.e., without catastrophic destruction of the device). The fail-safe systems
141, 161 can thus serve to protect the device
100 from catastrophic failure during its end of life mode. The present invention can
provide a safe end of life mechanism for a varistor-based overvoltage device. It will
be appreciated that the device
100 may be rendered unusable thereafter as an overvoltage protection device, but catastrophic
destruction (
e.g., resulting in combustion temperature, explosion, or release of materials from the
device
100) is avoided.
[0089] According to some embodiments, the meltable member
140A bypass and the fused interface
162 bypass each have an ohmic resistance of less than about 1 mOhm.
[0090] In some embodiments, the device
100 may be effectively employed in any orientation. For example, the device
100 may be deployed in a vertical orientation or a horizontal orientation. When the meltable
member
140 is melted by an overheat generation event, the meltable member
140 will flow to the lower portion of the chamber
176 where it forms a reconfigured meltable member (which may be molten in whole or in
part) that bridges the electrode
130 and the housing
120 as discussed above. The chamber
176 is sealed so that the molten meltable member
140 does not flow out of the chamber
176.
[0091] With reference to
Figure 7, an electrical circuit
30 according to embodiments of the present invention is shown schematically therein.
The circuit
30 includes a power supply
32, an overcurrent protection device
(e.g., a circuit breaker
34), a protected load
36, ground
40, and the overvoltage protection device
100. The device
100 may be mounted in an electrical service utility box, for example. The power supply
32 may be an AC or DC supply and provides power to the load
36. The load
36 may be any suitable device, system, equipment or the like (
e.g., an electrical appliance, a cellular communications transmission tower, etc.). The
device
100 is connected in parallel with the load
36. In normal use, the device
100 will operate as an open circuit so that current is directed to the load
36. In an overvoltage event, the resistance of the varistor wafer will drop rapidly so
that overcurrent is prevented from damaging the load
36. The circuit breaker
34 may trip open. However, in some cases, the device
100 may be subjected to a current exceeding the capacity of the varistor wafer
110, causing excessive heat to be generating by arcing, etc. as described above. The fail-safe
system
141 or the fail-safe system
161 will actuate to short circuit the device
100 as discussed above. The short circuiting of the device
100 will in turn trip the circuit breaker
34 to open. In this manner, the load
36 may be protected from a power surge or overcurrent event. Additionally, the device
100 may safely conduct a continuous current.
[0092] Notably, the device
100 will continue to short circuit the circuit
30 following the overcurrent event. As a result, the circuit breaker
34 cannot be reset, which notifies an operator that the device
100 must be repaired or replaced. If, alternatively, the branch of the device
100 were interrupted rather than short circuited, the circuit breaker
34 could be closed and the operator may be unaware that the load
36 is no longer protected by a functional overvoltage protection device.
[0093] Overvoltage protection devices according to embodiments of the present invention
(
e.g., the device
100) may provide a number of advantages in addition to those mentioned above. The devices
may be formed so to have a relatively compact form factor. The devices may be retrofittable
for installation in place of similar type overvoltage protection devices not having
a meltable member as described herein. In particular, the present devices may have
the same length dimension, as such previous devices.
[0094] According to some embodiments, overvoltage protection devices of the present invention
(
e.g., the device
100) are adapted such that when the fail-safe system
141 or the fail-safe system
161 is triggered to short circuit the overvoltage protection device, the conductivity
of the overvoltage protection device is at least as great as the conductivity of the
feed and exit cables connected to the device.
[0095] According to some embodiments, overvoltage protection devices of the present invention
(
e.g., the device
100) are adapted to sustain a current of 1000 amps for at least seven hours without occurrence
of a breach of the housing (
e.g., the housing
120 or
220) or achieving an external surface temperature in excess of 80 degrees Kelvin.
[0096] Overvoltage protection devices (
e.g., the device
100) as disclosed herein can be particularly well-suited or advantageous when employed
in a direct current (DC) circuit or system where the current conducted by the varistor
110 is very high. According to some embodiments, the device
100 is configured such that, when the fail-safe system
161 is triggered, the device
100 can withstand a short circuit current of at least 2 kA for more than 200 ms, and
a permanent current flow of at least 700 A without overheating. The maximum temperature
rise should not be more than 120 degrees Kelvin and the temperature rise five minutes
after the failure of the device should not exceed 80 degrees Kelvin during the permanent
current flow.
[0097] While meltable member
140 as described above is mounted so that it surrounds and is in contact with the electrode
130, according to other embodiments of the present invention, a meltable member may instead
or additionally be mounted elsewhere in a device. For example, a meltable member (
e.g., a sleeve or liner of the meltable material) may be mounted on the inner surface
of the sidewall
124 and/or the underside of the flange
138. Likewise, the meltable member may be shaped differently in accordance with some embodiments
of the invention. For example, according to some embodiments, the meltable member
is not tubular and/or symmetric with respect to the chamber, the electrode, and/or
the housing.
[0098] According to some embodiments, the areas of engagement between each of the contact
surfaces (
e.g., the contact surfaces
122A, 132A) and the varistor wafer surfaces (
e.g., the wafer surfaces
112, 114) is at least 0.5 square inches.
[0099] According to some embodiments, the biased electrodes
120,130 apply a load to the varistor
110 in the range of from 100 lbf and 1000 lbf depending on its surface area.
[0100] According to some embodiments, the combined thermal mass of the housing
120 and the electrode
130 is substantially greater than the thermal mass of the varistor wafer
110. As used herein, the term "thermal mass" means the product of the specific heat of
the material or materials of the object (
e.g., the varistor wafer
110) multiplied by the mass or masses of the material or materials of the object. That
is, the thermal mass is the quantity of energy required to raise one gram of the material
or materials of the object by one degree centigrade times the mass or masses of the
material or materials in the object. According to some embodiments, the thermal mass
of at least one of the electrode head
132 and the electrode wall
122 is substantially greater than the thermal mass of the varistor wafer
110. According to some embodiments, the thermal mass of at least one of the electrode
head
132 and the electrode wall
122 is at least two times the thermal mass of the varistor wafer
110, and, according to some embodiments, at least ten times as great. According to some
embodiments, the combined thermal masses of the head
132 and the wall
122 are substantially greater than the thermal mass of the varistor wafer
110, according to some embodiments at least two times the thermal mass of the wafer
110 and, according to some embodiments, at least ten times as great.
[0101] Methods for forming the several components of the overvoltage protection devices
of the present invention will be apparent to those of skill in the art in view of
the foregoing description. For example, the housing
120, the electrode
130, and the end cap
152 may be formed by machining, casting or impact molding. Each of these elements may
be unitarily formed or formed of multiple components fixedly joined, by welding, for
example.
[0102] Multiple varistor wafers (not shown) may be stacked and sandwiched between the electrode
head and the center wall. The outer surfaces of the uppermost and lowermost varistor
wafers would serve as the wafer contact surfaces. However, the properties of the varistor
wafer are preferably modified by changing the thickness of a single varistor wafer
rather than stacking a plurality of varistor wafers.
[0103] As discussed above, the spring washers
146 are Belleville washers. Belleville washers may be used to apply relatively high loading
without requiring substantial axial space. However, other types of biasing means may
be used in addition to or in place of the Belleville washer or washers. Suitable alternative
biasing means include one or more coil springs, wave washers or spiral washers.
[0104] Many alterations and modifications may be made by those having ordinary skill in
the art, given the benefit of present disclosure, without departing from the spirit
and scope of the invention. Therefore, it must be understood that the illustrated
embodiments have been set forth only for the purposes of example, and that it should
not be taken as limiting the invention as defined by the following claims. The following
claims, therefore, are to be read to include not only the combination of elements
which are literally set forth but all equivalent elements for performing substantially
the same function in substantially the same way to obtain substantially the same result.
The claims are thus to be understood to include what is specifically illustrated and
described above, what is conceptually equivalent, and also what incorporates the essential
idea of the invention.
[0105] A set of numbered clauses setting out feature of the invention is provided below.
- 1. An overvoltage protection device comprising:
first and second electrically conductive electrode members;
a varistor member formed of a varistor material and electrically connected with each
of the first and second electrode members; and
an integral fail-safe mechanism operative to electrically short circuit the first
and second electrode members about the varistor member by fusing first and second
metal surfaces in the overvoltage protection device to one another using an electric
arc.
- 2. The overvoltage protection device of clause 1 wherein the fail-safe mechanism is
operative to electrically short circuit the first and second electrode members about
the varistor member by fusing the first and second metal surfaces in response to a
short circuit failure of the varistor member.
- 3. The overvoltage protection device of clause 1 wherein:
the first and second metal surfaces are separated by a gap having a width in the range
of from about 0.2 mm to 1 mm; and
the electric arc extends across the gap to fuse the first and second metal surfaces.
- 4. The overvoltage protection device of clause 1 wherein:
the first and second metal surfaces are separated by a gap;
the overvoltage protection device further includes an electrically insulating spacer
member electrically isolating the first and second metal surfaces from one another;
and
the electric arc disintegrates the spacer member and extends across the gap to fuse
the first and second metal surfaces.
- 5. The overvoltage protection device of clause 4 wherein the spacer member is formed
of a polymeric material having a thickness in the range of from about 0.1 mm to 0.5
mm.
- 6. The overvoltage protection device of clause 1 wherein the first metal surface is
a surface of the first electrode member and the second metal surface is a surface
of the second electrode member.
- 7. The overvoltage protection device of clause 6 wherein:
the first electrode includes a housing having a metal housing sidewall and defining
a housing chamber;
the varistor member and at least a portion of the second electrode are disposed in
the housing chamber; and
the first metal surface is a surface of the housing sidewall.
- 8. The overvoltage protection device of clause 7 wherein:
the varistor member has first and second opposed, generally planar varistor contact
surfaces;
the housing includes an electrode wall having a first electrode contact surface engaging
the first varistor contact surface;
the second electrode includes a head positioned in the housing chamber, the head including
a second electrode contact surface engaging the second varistor contact surface and
a head peripheral surface surrounding the second electrode contact surface; and
the second metal surface is located on the head peripheral surface.
- 9. The overvoltage protection device of clause 8 including a buffer chamber on a side
of the head opposite the second electrode contact surface, wherein the buffer chamber
is configured to limit propagation of electric arc away from the head.
- 10. The overvoltage protection device of clause 1 including a biasing device biasing
at least one of the first and second electrode members against the varistor member.
- 11. The overvoltage protection device of clause 1 wherein the fail-safe mechanism
is a first fail-safe mechanism and further including an integral second fail-safe
mechanism, the second fail-safe mechanism including an electrically conductive meltable
member, wherein the meltable member is responsive to heat in the overvoltage protection
device to melt and form a current flow path between the first and second electrode
members through the meltable member.
- 12. The overvoltage protection device of clause 11 wherein:
the overvoltage protection device further includes an electrically insulating spacer
member electrically isolating the first and second metal surfaces from one another;
and
the meltable member has a greater melting point temperature than a melting point temperature
of the spacer member.
- 13. The overvoltage protection device of clause 11 wherein:
the first fail-safe mechanism is operative to fuse the first and second metal surfaces
at a prescribed region; and
the overvoltage protection device includes a sealing member between the prescribed
region and the meltable member.
- 14. The overvoltage protection device of clause 11 wherein:
the first fail-safe mechanism is operative to electrically short circuit the first
and second electrode members about the varistor member by fusing the first and second
metal surfaces in response to a short circuit failure of the varistor member sufficient
to generate an arc; and
the second fail-safe mechanism is operative to electrically short circuit the first
and second electrode members about the varistor member in response to a short circuit
failure of the varistor member not sufficient to generate an arc.
- 15. The overvoltage protection device of clause 1 wherein:
the fail-safe mechanism is a first fail-safe mechanism;
the first fail-safe mechanism is operative to electrically short circuit the first
and second electrode members about the varistor member by fusing the first and second
metal surfaces in response to a short circuit failure of the varistor member;
the overvoltage protection device further includes an integral second fail-safe mechanism,
the second fail-safe mechanism including an electrically conductive meltable member,
wherein the meltable member is responsive to heat in the overvoltage protection device
to melt and form a current flow path between the first and second electrode members
through the meltable member;
the first fail-safe mechanism is operative to electrically short circuit the first
and second electrode members about the varistor member by fusing the first and second
metal surfaces in response to a short circuit failure of the varistor member sufficient
to generate an arc;
the second fail-safe mechanism is operative to electrically short circuit the first
and second electrode members about the varistor member in response to a short circuit
failure of the varistor member that is not sufficient to generate an arc;
the varistor member has first and second opposed, generally planar varistor contact
surfaces;
the first electrode includes a housing defining a housing chamber and having a metal
housing sidewall and an electrode wall, the electrode wall having a first electrode
contact surface engaging the first varistor contact surface;
the varistor member is disposed in the housing chamber;
the second electrode includes a head positioned in the housing chamber, the head including
a second electrode contact surface engaging the second varistor contact surface and
a head peripheral surface surrounding the second electrode contact surface;
the first metal surface is a surface of the housing sidewall;
the second metal surface is located on the head peripheral surface;
the first and second metal surfaces are separated by a gap having a width in the range
of from about 0.2 mm to 1 mm;
the overvoltage protection device further includes an electrically insulating spacer
member electrically isolating the first and second metal surfaces from one another;
the electric arc disintegrates the spacer member and extends across the gap to fuse
the first and second metal surfaces; and
the spacer member is formed of a polymeric material having a thickness in the range
of from about 0.1 mm to 0.5 mm.
- 16. A method for providing overvoltage protection, the method comprising:
providing an overvoltage protection device including:
first and second electrically conductive electrode members;
a varistor member formed of a varistor material and electrically connected with each
of the first and second electrode members; and
an integral fail-safe mechanism operative to electrically short circuit the first
and second electrode members about the varistor member by fusing the first and second
metal surfaces in the overvoltage protection device to one another using an electric
arc; and
directing current between the first and second electrode members through the varistor
member during an overvoltage event.
- 17. An overvoltage protection device comprising:
first and second electrically conductive electrode members;
a varistor member formed of a varistor material and electrically connected with each
of the first and second electrode members;
an integral first fail-safe mechanism configured to electrically short circuit the
first and second electrode members about the varistor member when triggered by a first
set of operating conditions; and
an integral second fail-safe mechanism configured to electrically short circuit the
first and second electrode members about the varistor member when triggered by a second
set of operating conditions different from the first set of operating conditions.
- 18. The overvoltage protection device of clause 17 wherein the first and second sets
of operating conditions each include at least one of an overheating event and an arcing
event.
- 19. The overvoltage protection device of clause 18 wherein:
the first set of operating conditions includes an arcing event; and
the second set of operating conditions includes an overheating event.
- 20. A method for providing overvoltage protection, the method comprising:
providing an overvoltage protection device including:
first and second electrically conductive electrode members;
a varistor member formed of a varistor material and electrically connected with each
of the first and second electrode members;
an integral first fail-safe mechanism configured to electrically short circuit the
first and second electrode members about the varistor member when triggered by a first
set of operating conditions; and
an integral second fail-safe mechanism configured to electrically short circuit the
first and second electrode members about the varistor member when triggered by a second
set of operating conditions different from the first set of operating conditions;
and
directing current between the first and second electrode members through the varistor
member during an overvoltage event.