Note: Descriptions are shown in the official language in which they were submitted.
2~3~7~
NON-FRAGMENTING ARRESTER WITH STAGE~ PRESSURE RELIEF MECHANISM
The present invention relates generally to apparatus
for protecting electrical equipment frorn damage or destruction
due to the presence of electrical overvoltages, such apparatus
commonly referred to as a surge arrester. More particularly,
the invention relates to a non-fragmenting, surge arrester.
Still more particularly, the invention relates to an elastorner
housed distribution arrester having a staged pressure relief
system which, in the unlikely event of failure, safely vents
ionized gases generated by internal arcing outside the
arrester, thereby preventing what otherwise could be a
catastrophic failure of the arrester.
A surge arrester is commonly connected in parallel
with a comparatively expensive piece of electrical equipment in
order to shunt overvoltage surges, such as those caused by
lightning strikes, to ground thereby protecting the equipment
and circuit from damage or destruction. A modern surge
arrester typically includes an elongated enclosure made of an
electrically insulating makerial, a series of voltage dependent
nonlinear resistive elements retained within the housing, and a
pair of electrical terminals at opposite ends of the housing
for connecting the arrester between line and ground. The
voltage dependent nonlinear resistive elements employed are
typically, but not restricted to, metal o~ide varistor elements
formed into relatively shork cylindrical disks which are
stacked one atop the other within the enclosure. Other shapes
and configurations may also be used for the varistor elements.
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The varistor elements provide either a high or a low impedance
current path between the arrester terminals depending on the
voltage appearing across the varistor elements themselves.
More specifically, at the power syst0m's steady sta~e or norrnal
operating voltage, the varistor elements have a relatively high
impedance. As the applied voltage is increased, gradually or
abruptly, the varistor elements' impedance progressively
decreases until the voltage appearing across the varistors
reaches the elements' breakdown voltage, at which point their
impedance dramatically decreases and the varistor elements
become highly conductive. Accordingly, if the arrester is
subjected to an abnormally high transient overvoltage, such as
resulting from a lightning strike or power frequency
overvoltage for example, the varistor elements become highly
conductive and serve to conduct the resulting transient current
to ground. As the transient overvoltage and resultant current
dissipate, the varistor elements' impedance once again
increases, restoring the arrester and electrical system to
their normal~ steady--state condition.
Occasionally, the transient condition may cause some
degree of damage to one or more of the varistor elements.
Damage of sufficient severity can result in arcing within the
arrester enclosure, leading to e~treme heat generation and gas
evolution as the internal components in contact with the arc
are vaporizedO This gas evolution causes the pressure within
the arrester to increase rapidly until it is relieved by either
a pressure relief means or by the rupture of the arrester
enclosure. The failure mode of arresters under such
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conditions may include the e~pulsion of components or
component fragments in all directions. Such 4ailures pose
potential risks to personnel and equipment in the vicinity.
Equipment may be especially at risk when the arrester i~ housed
within the equipment it is meant to protect, as in the tank of
a transformer for example.
Attempts havP been made to design and construct
arresters which will not catastrophically fail with the
expulsion of components or component fragments. One such
arrester is described in U.S. Patent No. 4,404,614 which
discloses an arrester having a non-fragmenting liner and outer
housing, and a pressure relie diaphragm located at its lower
end. A shatterproof arrester housing is also disdlosed in
U.S. Patent No. 4,656,555. Arresters having pressure relief
means formed in their ends for ventin~ ionized gas in a
longitudinal direction are described in U.S. Patent Nos.
3,727,108, 4,001,651 and 4,240,124.
Despite such advances, however, state of the art
arresters may still fail with e~pulsion of components or
fragments of components. This may in part be due to the fact
that once the internal components in these arresters fail, the
resulting arc vaporizes the components and generates gas at a
rate that can not be vented ~uickly enough to prevent rupture
of the arrester. Accordin~ly, there e~ists a need in the art
for an arrester which, upon failure, will fail in a
non-fragmentin~ manner. Prefera~ly, such an arrester would
eliminate the possibility of catastrophic failures by
transferring t~he failure-causing arc away from the înternal
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components, thereby preventing the generation of any additional
pressure. ~ne means by which this may be accornplished is to
design an improved arrester which would safely vent the ionized
gases formed by the internal arc outside the arrester, thereby
forming a lower impedance path to ground for the arc~ It is
further desirable that such an improved arrester would vent the
gases in a staged or controlled Imanner BO as to prevent
fracturing the arrester by an abrupt change in internal
arrester pressure.
Provided herein is a non-fragmenting surge arrester
having a staged pressure relief system that is structured to
safely vent ionized gases formed by an internal arc outside the
arrester and thereby prevent catastrophic arrester failures.
The arrester of the present invention includes a shatterproof,
insulative housing, a rigid liner within the housing for
retaining the operative components of the arrester in a fixed
relationship, and one or more vents or outlets formed in the
liner for venting gases therethrough. The outlets may
comprise one or more elongate slots or slits formed in the wall
of the liner. Alternatively, the outlets may include an array
of such slots or slits or one or more rows of aligned
perforations formed parallel to the a~is of the liner.
In addition to these outlets, the invention employs
thin or weakened-wall segments formed within the housing as
part of the pressure relief system. In the preferred
embodiment, the thin wall segments are positioned adjacent to
the outlets in the liner. In this configuration, gas vented
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from within the liner through the outlets will break through
the housing at designed locations. The thin or weakened-wall
segments may comprise a number of discrete weakened segments
aligned parallel to the housing's, axis, such as may be formed
between the rain skirts or ribs of the housing, or may instead
comprise a continuous longitudinal channel formed in the
housing.
The invention ~urther includes an e~pansion chamber
within the liner as part of its pressure relief system. The
chamber is de~ined by the inner surface of the liner and the
surface of the electrical components contained therein, such
components typically comprising metal o~ide varistors. The
position of the varistors is maintained within the liner ~y
insulative standoffs that are positioned between the varistors
and the liner. The volume of the chamber acts as a huffer and
momentarily lessens the forces that would otherwise be applied
to the inside of the liner during an arrester failure so as to
allow the generated gas to be safely vented through the liner
outlets and the weakened-wall segments of the housing without
fracturing the liner.
Thus, the present invention compris~s a combination of
features and advantages which enable it to substantially
advance arrester technology by providing a non-fragmenting, and
thus fail-safe, arrester for use in a variety of insulatin~
media. These and various other characteristics and advantages
of the present invention will be readily apparent to those
skilled in the art upon reading the following detailed
description and referring to the accompanying drawings.
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For an introduction to the detailed description of the
preferred ernbodiment of the invention, reference will now be
made to the accompanying drawings, wherein:
Figure 1 shows an elevation view, partly in cross section,
of the surge arrester of the pre,sent invention;
Figure ~ shows a cross section of the surge arrester shown
in Figure 1 taken above line 2--2;
Figure 3 shows a perspective view of the subassembly liner
of the surge arrester shown in Figure 1;
Figures 3A and 3B show perspective views of alternative
embodiments of the subassembly liner shown in Figure 3;
Figure 9 shows, in cross section, an alternative
embodiment th~ surge arrester shown in Figure l;
Figure 5 shows, in cross section, another alternative
embodiment of the surge arrester shown in Figure l;
Figure 6 shows, in cross section, another alternative
embodiment of the surge arrester shown in Figure l; and
Figure 7 shows, in a cross section, a further embodiment of
the surge arrester shown in Figure 1.
Figure 8 shows an elevation view, in cross section, of the
surge arrester shown in Figure 7.
Surge arresters are installed in electrical systems
for the purpose of diverting dangerous overvoltage surges to
ground before such surges can damage e~pensive electrical
equipment. Even current, state-of-the-art arresters will
sometimes fai3., however, and ma~ fail in catastrophic,
e~plosive fashion. When a catastrophic failure occurs,
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shrapnel-like arrester fragments may damage equipment and
endanger personnel. Thus, it is desirable that a surge
arrester be designed and constructed to have a predictable,
controlled, and non-fragmenting failure mode.
Referring initially to Figure 1, there is shown a
non-fragmenting surge arrester lO structured in accordance with
the principles of the present invention. Arrester lO generally
comprises an insulative and protective housing 12, an inner
arrester subassembly 40, a pressure relief system 60 and line
and ground terminals 20 and 24, respectively. Housing 12 is
made of a non-fragmenting, shatterproof material and physically
covers, protects and electrically insulates the subassembly
40. Subassembly 40, in turn, houses the operative components
of arrester lO and, together with housing 12, forms pressure
relief system 60. Terminals 20 and 24 electrically connect
arrester 10 between a line voltage and ground.
It is preferred that housing 12 be made from elastomeric
materials such as ethylene propylene bssed Monomers or silicone
based rubbers, silicone based rubbers being currently
preferred. These materials are shatterproof and provide
superior outdoor insulating properties although other
polymeric materials may be employed. Housing 12 substantially
envelopes and houses subassembly 40 and hermetically seals
subassembly 40 from the ambient environment. Housing 12,
which includes spaced-apart rain skirts or ribs 13 formed about
the length of housing 12, is sealingly attached to the lower
end of subassembly 40 by a compression cap 54. Cap 54, which
may be made of brass, copper or other conducting ma~erial,
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provides an axial clamping force when compressed about the
lower end of housing 12 and seals housing 12 to the lower end
of subassembly 40. At the top of arrester 10, housing 12 is
sealed against stud 78 of line terminal 20 b~ washer 73 and nut
75. As shown in Figure 1, stud 78, which includes a shoulder
76, is brazed to upper electrode 46. Washer 73 is seated on
shoulder 76. In this confi.guration, when nut 75 is tightened,
the elastomer of housing 12 is compressed between washers 73
and nut 75, thereby extruding housing 12 into sealing contact
with stud 78.
Arrester 10 is supported by an insulative hanger 30, shown
in Figure 1, which preferably is manufactured of glass filled
polyester, although other polymeric materials may be employed.
A terminal stud (not shown) is brazed to cap 54 and e~tends
through an aperture in hanger 30 and engages conventional
ground lead disconnector 28, electrically connecting cap 54 to
disconnector 28. Disconnector 28, also known as an isolator,
is connected to ground terminal 24 and employed to physically
disconnect the ground lead 26 from the arrester 10 by the
ignition of an explosive char~e when the disconnector 28
reaches a predetermined temperature~ This may occur, for
e~ample, when the arrester 10 has 4ailed to prevent the flow of
the steady state power-fre~uency curre~t after a surge, and is
therefore acting as a short circuit to ground.
Referring still to Figure 1, subassembly 40 gen~rally
comprises subassembly sleeve or liner 42, non-linear resistors
44, and top and bottom electrodes 46 and 48, respectively.
Liner 42 retains non-linear resistors 44, electrodes 46 and 48
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g
and other components in a series or stacked r21ationship within
subassembly 40 and provides rigidity to the arrester housing
12. Liner 42 is preferably an insulative conduit manufactured
of flberglass, although other insulative materials may be
employed. Liner 42 may be tubular or have any of a number of
other shaped cross sections. A tubular liner 42 having wall
thickness of approximately 0.090 inches has pro~en satisfactory
in many applications.
Non-linear resistors 44 are preferably Metal oxide
varistors that have been formed into short cylindrical blocks
or disks. Varistors 44 are retained within liner 42 between
top and bottom el~ctrodes 46 and 48, which are made of brass,
copper or other conductive material~ Top electrode 46 forms a
closure at the top of liner 42, and is attached to liner 42 for
e~ample by nylon pins 47. As shown in Figure l, a compression
spring 50 is biased between bottom electrode 48 and a retaining
yoke 52 which is formed of a fiberglass or other insulative or
conductive material and positioned in slots 53 formed within
liner 42. Bottom electrode 48, spring 50 and yoke 52 cooperate
to provide an axial load against ths stack of varistor elements
44 sufficient to maintain varistor el~ments 44 in intimate
contact with one another and with electrodes 46 and 48, which
is necessary for good electrical contact and for the arrester
to function properly. A conductor 59 is electrically
connected to bottom electrode 48 and to conducting cap 54 and
completes the series circuit between line and ground terminals
20, 24, the circuit including top electrode 46, varistors 44
and bottom electrode 48. Although not shown, one or more
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conductive plates may be positioned between some or all of
varistors 44 to serve as heat sinks and help dissipate heat
generated within arrester 10 when dissipating surge energy and
to provide a good conductive interface between varistors 44.
Pressure relief means 60, best described with reference to
Figures I and 2, generally compr.ises an annular chamber 58,
formed between varistors 44 and liner 42, outlets or ports 62
of liner 42, and weakened-wall rlegions or segments 64 of
housing 12. Together, annular chamber 58, outlets 62 and
weakened-wall regions 64 cooperate to provide for the
controlled or staged relief of the internal pressure produced
by an internal arc during an arrester failure.
Annular chamber 58 is iest shown in Figure 2. As shown,
varistors 44 have a diameter smaller than the inside diameter
of liner 42. Varistors 44 are retained in a stacked
relationship within liner 42 and are spaced apart from the
walls 43 of liner 42 by insulative standoffs 56 which are
positioned within subassembly 40 during manufacture.
Insulative standoffs 56 may be, or example, elongate members
55 made from nylon or other insulative material and attacbed to
lin~r 42 by any of a variety of insulating adhesives. In the
preferred embodiment, varistors 44 are coaxially al;gned and
conc~ntrically positioned within a tubular liner 42 and spaced
apart from walls 43 of liner 42 by a gap ranging ro~ 1~16 to
1/4 inches depending upon the size and rating of the arrester.
In this configuration, annular chamber 58 is formed between
varistors 44 and liner 42. The volume of annular chamber 58
provides an e~pansion chamb~r within liner 42 to momentarily
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retain the ionized gas generated by an internal arc before the
gas pressure is relieved outside the arrester 10.
Pressure relief means fiO furt:her comprises one or more
rents or outlets 62 formed in the walls 43 of liner 42. In the
preferred embodiment as shown in Figure 3, a single elongate
outlet or slot 62 is formed through the entire length of liner
42; however, a variety of other configurations can be employed
as described below. The outlet 62 should exten~ the length of
the stack of varistors 49 to enable venting to occur along the
entire axial length of the varistor stack.
Referring again to Figures 1 and 2, pressure relief means
60 further comprises weakened or thin walled regions 64 formed
in housing 12. Weakened-wall sections 64 ma~ be molded or cut
into housing 12. In the preferred embodiment, weakened-wall
sections 64 are preferably formed along the inside of housing
12 between each skirt or rib 13. Housing 12 may have, for
e~ample, a wall thickness equal to approsimately 0.15 inches
and weakened-wall sections 64 formed within housing 12 to a
depth of approximately one-half the wall thickness, in this
e~ample 0.075 inches, although other thicknesses of housing
walls and weakened-wall sect;ons may be employed. Weakened-wall
sections 64 could likewise be formed on the outside of housing
12. By forming the weakened-wall sections 64 between ribs 13
and not reducing the thickness of the housin~ walls at ribs 13,
the a2îal strength of housin~ 12 is not compromised and the
radial or hoop strength of the housing is maintained at each
rib location. ,hs shown, weakened-wall s~ctions 64 are aligned
in housin~ 12 and positioned so as to be adjacent to outlet 62
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formed in liner 42. Alternatively, as shown in Figure 7 and
Figure 8, rather than having discrete weakened secjments formed
in housing 12 between ribs 13, housing 12 ina~ employ a
continuous longitudinal groove or channel 69 along the inside
surface of housing 12, the channel having a length
substantially equal to the height of the stack of var;stors 4
and the length of outlet 62.
With reference to Figure 1, the operation of arrester 10
will now be explained. In operation, the arrester 10 of the
present invention is installed in parallel with the electrical
equipment it is intended to protect by connecting line lead 22
to a power carrying conductor, and connecting ground lead 26 to
ground. After installation, if any of the varistor elements 44
in arrester 10 should e~perience a dielectric breakdown or fail
for other reasons during operation, the voltage which builds
across the defective varistor element or elements 44 will cause
an internal arc to form a~ross the failed element or elements
as the current continues to be conducted through the arrester.
The arc, which may burn at a temperature of several thousand
degrees, will vaporize the internal components of subassembly
40 that are ;n contact with the arc. As the arc continues to
burn, a large volume of ionized gas is generated within
subassembly 40. As described below, pressure relief means 60
radially vents the generated gas outside housing 12 in a
controlled manner so as to prevent the violent failure of the
arrester.
The ionized gas generated during an arrester failure first
pressurizes annular chamber 58 which surrounds varistors 44.
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Annular chamber 58 provides an e~pansion chamber for the gas so
as to reduce the ~hock that would otherwise be exparienced by
liner 42 if no such chamber were provided. After annular
chamber 58 is pressurized, the ionized gas is vented in a
radial direction through the walls 43 of liner 42 via the
outlets 62. When the ionized gas is vented through the outlets
62, housing 12 may initially stretch to accommodate the
increased volume but will then rupture along the weakened-wall
regions 64 due to the increased internal pressure. Once housing
12 ruptures, the ionized gas, now outside arrester 10, forms a
lower impedance path for the current than the parallel path
existing inside subassembly 40. Thus, the current being
conducted by arrester 10 diverts to the lower impedance
alternate path formed by the ionized gas, and an e~ternal arc
is ~ormed around the failed internal elements. When this
occurs, the internal arc is effectively transferred to the
alternate path. Since the internal arc has been diverted from
the failed elements, the generation of further pressure within
arrester 10 is prevented. Annular chamber 5B, outlets 62 and
weakened-wall regions 64 limit the arrester's internal pressure
to a pressure below the bursting pressure of the subassembly
40, thereby preventing an~ fracture of the arrester 10 and the
e~pulsion of components or component fragments.
As arrester 10 is generally installed near electrical
equipment or other structures, it is desirable to directionally
vent the ionized gas and divert the internal arc in a direction
away from such structures and equipment. Accordingly, arrester
10 i5 installed such that the outlets 62 in liner 42 face in a
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direction opposite to that of nearby electrical equipment or
structures. Installed in this manner, directional outlets 62
vent the gas generated within a failed arrester away from the
nearby equipment or structures to ensure that the e~posed arc
doe~ not damage the equipment or structures. It is generally
desirable that line and ground leads 22, 26, outlet 62 and
weakened-wall regions 64 are positioned such that all lie in
the same plane so that the ionized gas generated during a
failure, is vsnted from the arrester 10 between line lead 22
and ground lead 26 so as to create the shortest path to ground
for the arc.
An alternative embodiment of liner 42 and pressure relief
means 60 is depicted in Figure 3A in which an array 66 of
outlets 62 is shown formed within an arcuate segment of liner
42. Like the single elongate outlet 62 shown in Figure 3,
array 66 also provides directional control for transferring the
arc outside the arrester and away from nearby equipment and the
like. While it is not important to the operation of the
arrester 10 that the outlet 62 extend the entire length of the
liner 42 as shown in Figure 3, this design may be more easily
manufactured than that of Figure 3A where the length of outlets
62 is matched to the height of the varistor element stack.
Another alternative embodiment of liner 42 and pressure relief
means 60 is shown in Figure 3B. In this embodiment, pressure
relief means 60 comprises a plurality of aligned perforations
or apertures 68 ormed in a row 70 parallel to the a~is of
liner 42. In the embodiments depicted in Figures 3A and 3B,
the weakened-wall segments 64 of housing 12 would be positioned
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adjacent to array 66 and row 70, respectively.
Where directional venting is not a requirement, a number of
alternative embodiments of the present invention may be
employed. Referring first to Figure 4, there is shown one such
alternative embodiment in which liner 42 is manufactured with
two slots 72 formed through walls 43 at locations 180 degrees
apart. In this embodiment, housing 12 is formed and positioned
about subassembly 40 with alignedl rows of weakened-wall regions
64 adjacent to each slot 72. It is of cvurse understood that a
variety of other configurations of outlets 64 could be
employed. For example, three slots 72 could be provided in
liner 42 at locations 120 degrees apart, ~ach slot ~ being
positioned adjacent to a corresponding set of weakened-wall
regions 64 formed in housing 12. Similarly, as shown in Figure
5, a number of rows 70 of apertures 68 may be formed in walls
43 of liner 42, each row 70 being positioned adjacent to an
aligned set of weakened-wall rPgions 64 in housing 12, six such
sets spaced 60 degrees apart being depicted in Figure 5.
Referring now to ~igure 6, there is depicted another
embodiment of the present invention which provides directional
control of the transferred arc. In this embodiment, varistors
44 are themselves coaxially aligned but are stacked in an
eccentric alignment within liner 42 and retained in this
eccentric or offset position by insulative standoffs 56 such
that the greatest volume of annular, chamber 58 is adjacent to
weakened-wall segments 64 in housing 12. In this
configuration, the gap between varisters 44 and the walls 43 of
liner 42 would approximate 1/2 inch at the point nearest outlet
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62. It is contemplated that this configuration would provide a
highly reliable degree of directional control.
An alternative means for retaining varistors 44 in liner 42
is depicted in Figure 5. As shown, insulative standoffs 56
may comprise pins or rivets 57 made of nylon or other
insulative, material. When rivets 57 are employed to maintain
the desired separation between varistors 44 and liner 42, the
rivets 57 are positioned within apertures 68 shown in Figure 5
an~ Figure 3B at the interface between adjacent varistor blocks
44. It should be understood that rivets 57 may likewise be
employed with the liner 42 depicted in Figures 3 and 3A, riv~t
57 then being fitted through slots 72 formed therein.
While the preferred embodiment of this invention has been
shown and described, modifications thereoE can be made by one
skilled in the art without departing from the spirit of the
invention. The embodiments described herein are e~emplary only
and are not limiting. Many variations and modifications of the
system and apparatus are possible and are within the scope of
the invention. Accordingly, the scope of prot~ction is not
limited by the above description, but is only limited by the
claims which follow, that sco~e including all equivalents of
the suhject matter of the claims.
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