Directionally vented underground distribution surge arrester

The invention is directed to the provision of an external shield about the periphery of the housing of a surge arrester containing one or more arrester devices to provide an expansion limiting device which permits the housing to expand to a degree necessary to permit the expulsion of arc-generated gases within the housing while serving to help to limit the total expansion to a level below the elastic limit of the housing to prevent its destruction and permit its recovery after the gases have been dissipated. The shield may be made of metal, elastomeric materials, reinforced elastomeric materials or plastic, and the shield may be supported about the housing by an external strap of metal, plastic or elastomeric materials or may be supported by a plurality of fins arranged along the periphery of the housing and engaging an edge of the shield on its internal surface.

CO-PENDING APPLICATIONS 
U.S. patent application No. 07/483,656 filed Feb. 23, 1990 for Surge 
Arrester With Rigid Insulation Housing by Harry G. Yaworski and Larry N. 
Siebens and assigned to the Assignee of the instant application. 
FIELD OF THE INVENTION 
The invention is directed to the field of surge arresters to protect high 
voltage systems from the effects of overvoltage incidents created by 
lightening strikes and, more particularly, to the construction of such 
surge arresters to minimize the chance of injury to personnel or equipment 
due to the catastrophic failure of such surge arresters during overvoltage 
incidents. 
DESCRIPTION OF THE PRIOR ART 
Surge arresters used to protect underground and overhead high voltage 
electrical systems widely employ metal oxide varistor elements to 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 particularly, at the system's steady state or normal 
operating voltage, the varistor elements have a relatively high impedance. 
As the applied voltage is increased, as in response to a lightening 
strike, their impedance decreases until the voltage appearing across the 
elements reaches their breakdown voltage, at which point their impedance 
rapidly decreases towards zero and the varistor elements become highly 
conductive. In this highly conductive condition, the varistor elements 
serve to conduct the resulting transient follow-on current to ground. As 
the transient overvoltage due to the strike and the follow-on current 
dissipate, the varistor elements, impedance increases effectively removing 
the short to ground and restoring the varistor elements and electrical 
system to their normal steady state condition. 
Occasionally, the transient condition or a succession of such transient 
conditions within a short time may cause some level of injury or damage to 
one or more of the varistor elements. Damaging of sufficient severity can 
result in thermal runaway and subsequent arcing within the arrester 
enclosure, leading to extreme heat generation and gas evolution as the 
internal components in contact with the arc are vaporized. 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 catastrophic failure mode of arresters 
under such conditions may include the expulsion of components or component 
fragments in all directions. Such failures may pose a potential risks to 
personnel and equipment in the vicinity. Equipment may be particularly at 
risk when the arrester is housed within the equipment it is meant to 
protect as in the tank of a transformer for example. Personnel may be at 
risk if in a cable vault or equipment room where they may be in close and 
confined proximity to an exploding arrester. 
Different efforts made to date to minimize the possibility for injury have 
generally dealt with techniques to strengthen the arrester by providing a 
non-fragmenting liner and outer housing and a pressure relief diaphragm 
located at its lower end as in U.S. Pat. No. 4,404,614, or a shatterproof 
arrester housing as in U.S. Pat. No. 4,656,555. In U.S. Pat. No. 
4,910,632, gas passages are provided which end in their wall sections 
which are melted to allow the gases to escape and reduce the internal 
pressure on the remainder of the insulating housing. U.S. Pat. No. 
4,930,039 provides a liner having outlets formed in the walls thereof for 
venting ionized gases generated within the liner by internal arcing. This 
prevents the generation of internal pressure which could otherwise cause a 
fragmenting failure mode of the arrester. 
Because of the placement of dead front underground distribution surge 
arresters in cable vaults or equipment rooms, the industry has determined 
that in the event of an arrester failure it would be safest and thus most 
desirable to have any arrester components or component fragments exit the 
arrester housing through the bottom and strike the vault or room floor. 
Also, any hot gases generated within the arrester housing should be 
depressured, vented and directed-downwardly so as to minimize the 
potential for injury to any workmen present or to the equipment contained 
therein. Such a requirement is more stringent than those applied to the 
devices described above which are for overhead use and are far above the 
ground, and their venting will have little effect on persons on the 
ground. 
One attempt to control the direction of movement of component fragments 
exiting a dead front underground distribution arrester under catastrophic 
failure is shown, described and claimed in the above-identified co-pending 
application and by this reference made a part hereof. 
Referring to FIG. 1, which is FIG. 2 of the co-pending application, it can 
be seen that the reinforced surge arrester assembly 72 consists of a 
number of metal oxide varistor (MOV) blocks 74 and end fittings 60 and 70 
arranged in a stack and glued to one another by a silver epoxy adhesive 
surrounded by a preformed rigid tube 78, and the interstices are filled 
with a filler layer 84. Tube 78 is offset upwardly above the end fitting 
60 to provide a downward preferred direction of failure. The presence of 
the rigid tube 78 acts to create a pressure vessel to not only contain 
block fragments of blocks that catastrophically fail, but also retain the 
gases that are evolved by the internal arcing. As a result, the pressure 
builds within the arrester housing 30 until the entire assembly 72 is 
ejected from the end of vertical leg 18. The opposing forces generated by 
the assembly 72 ejection can cause the body 30 to move upwardly in FIG. 1 
causing horizontal leg 12 to move free of the bushing insert into which 
arrester 10 is inserted or to rotate about said bushing insert destroying 
the insert and the bushing well into which the bushing insert is placed. 
SUMMARY OF THE INVENTION 
The present invention overcomes the difficulties noted above with respect 
to prior art devices and seeks to provide protection up to and exceeding 
the level provided by the co-pending application and without the possible 
undesired side effects that can be created by extreme gas build-up and 
catastrophic block failure. The stack of MOV blocks and end fittings 
suitably joined at their abutting end faces by a silver epoxy paste may be 
substantially surrounded by a dielectric insulating material to provide an 
air-free, non-electrically ionizable environment and to rigidify the stack 
of components by engulfing the blocks and end fittings to form a unitary 
assembly. The glued block stack may also be used without such 
pre-insulation. 
This assembly may then be inserted into one leg of a dielectric insulating 
housing with a conductive molded outer shield in the general configuration 
of the well known cable elbow. The receiving leg having a bore of a 
diameter less than the diameter of the block assembly and dilatable to 
receive the assembly therein and thereafter return to its former size to 
grasp the block assembly in a generally void free interface. 
Alternatively, as is also well known in the prior art, the block assembly 
may be placed in a similar housing by molding same about the block 
assembly. In using a pre-molded housing, the blocks may also be inserted 
without pre-molding a dielectric insulation about them or even gluing the 
blocks together. The blocks may be individually inserted and the end 
spring used to press the mating surfaces into electrical engagement with 
one another. 
Regardless of the manner of inserting or press fitting the block assembly 
into a housing or molding the housing about the block assembly, the 
housing will have sufficient resiliency and flexibility so that it can 
expand and contract in response to gases generated within the arrester. 
Further, the dielectric insulating material about the block assembly does 
not adhere to the ceramic coating on the block periphery and is also 
resilient and flexible enough to expand and contract in the presence of 
arc-generated gases. The presence of such gases within the arrester may be 
expelled by the expansion of the assembly insulation layer and the 
arrester housing allowing such gases to pass out of the arrester between 
the block peripheries and the insulation layer in both the press fit and 
molded versions and between the insulation layer and the housing in the 
press fit version only. 
To prevent the arrestor housing from expanding to too great an extent, a 
restraining, retaining and reinforcing expansion tube is employed. The 
expansion tube is placed about the outside of the leg containing the block 
assembly and spaced apart from it by sufficient distance to permit a 
limited outward expansion of the housing so that the volume of the 
pressure vessel within the housing can expand to decrease the pressure of 
the arc-generated gases within the housing and to permit the escape of 
such gases and to act as a restricting mechanism for the housing itself so 
that the elastic limits of the EPDM rubber housing is not exceeded. The 
expansion tube also serves as a protective shield to retain any block 
fragments from exploding blocks, prevent their sideways travel, and help 
direct them downwardly and out of the housing. It is an object of this 
invention to provide an improved surge arrester. 
It is an object of this invention to provide an improved surge arrester 
employing an expansion tube. 
It is still another object of this invention to provide an improved surge 
arrester employing an external expansion tube. 
It is another object of this invention to provide an improved surge 
arrester which can selectively employ an external expansion tube. 
It is still another object of this invention to provide an improved surge 
arrester employing an external expansion tube which is affixed to the 
exterior of a surge arrester housing permitting limited expansion of such 
housing in response to the presence of arc-generated internal gas. 
It is another object of this invention to provide an improved surge 
arrester having an expandable housing about the internal arrester elements 
to vent arc-generated gases within the arrester housing. 
It is still another object of this invention to provide an improved surge 
arrester having an internal expandable housing about the arrester elements 
to vent arc-generated gases within the arrester housing and an external 
expansion tube to limit the expansion of the arrester housing during such 
gas venting while reinforcing the arrester housing. 
Other objects and features of the invention will be pointed out in the 
following description and claims and illustrated in the accompanying 
drawings which disclose, by way of example, the principles of the 
invention and the best modes which have been contemplated for carrying 
them out.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to FIG. 2, there is shown a first embodiment of a surge 
arrester 100 constructed in accordance with the concepts of the invention. 
Although the surge arrester construction is shown housed in an elbow 
configuration as used in the underground distribution of high voltage 
currents, it is equally applicable to terminations and transmission line 
supports and protectors for above-ground transmission or distribution 
lines and circuits. 
A body 102 of resilient, non-tracking, insulating material such as EPDM 
rubber or butyl rubber is formed in a generally L-shape with a horizontal 
leg 104 and a vertical leg 106. A shielding layer 108 of conductive 
material such as semi-conductive EPDM rubber or butyl rubber is placed 
over a major portion of body 102. The interior of the cavity within leg 
104 is tapered to form a receptacle 110 to receive therein the interface 
of a bushing insert (not shown) and probe 112 is arranged to engage with 
the female contacts thereof (not shown) in a known manner. The arrester 
100 is locked to the bushing insert by engagement of annular detent rib 
114 with an annular recess in the bushing insert (not shown). 
Inserted in vertical leg 106 is a stack of three metal oxide varistor (MOV) 
blocks 116 of the type commercially available from Meidensha or General 
Electric Company, for example, and preferably comprise zinc oxide 
non-linear resistor material. Although three blocks 116 are shown, the 
number and size of the blocks employed will depend upon the circuit rating 
as is well known. The blocks 116 generally have a ceramic collar around 
the peripheral surfaces thereof to insulate such surfaces and, if desired, 
the individual blocks may be joined to one another and to any end fittings 
by a highly electrically conductive silver epoxy paste. 
The upper block 116 is brought into contact with a compression spring 118 
and shunt 120 to join blocks 116 to probe 112 by means of metal coupling 
122 into which probe 112 has been threaded. The lower block 116 is in 
contact with a further compression spring 124 and shunt 126 held in 
position at the open end of vertical leg 106 by cap 128. With such an 
arrangement, once metal coupling 122 and probe 112 are joined within body 
102, the blocks 116 can be inserted individually without being glued 
together or inserted as a group having been previously joined at their 
interfaces by silver epoxy paste. The completion of the assembly by using 
cap 128 assures proper electrical assembly by means of compression springs 
128 and 118 and shunts 126 and 120 regardless of whether the blocks 116 
were previously glued together. A further layer of semi-conductive EPDM 
130 surrounds metal coupling 122, spring 118, shunt 120, the top of the 
upper block 116 and the end of receptacle 110 as is well known in the art. 
Connected to cap 128 is a threaded stud 132 to which a ground strap 134 
can be coupled by means of nuts 136 and 138. 
Arranged about the exterior of vertical leg 106 is an expansion tube 140 
which may be supported by a series of fins 142 extending outwardly from 
the exterior of vertical leg 106. Alternatively, as is shown in FIG. 3, 
expansion tube 140 may be supported by a strap 144 extending over 
horizontal leg 104. Virtually any arrangement may be used which will 
permit the leg 106 to expand to the extent of the interior diameter of the 
expansion tube 140. As is shown in FIG. 2, a series of external fins 142 
are employed. Although only two fins 142 are shown, in practice three fins 
at a 120.degree. spacing about the housing periphery are used. Internal 
fins may also be used and these may be placed at the lower end of vertical 
leg 106 as is true of fins 142 or at the top of leg 106 near its joinder 
to horizontal leg 104, or both, as long as the fins do not prevent the 
desired expansion of the vertical leg 106. 
In practice, when arc-generated gases are present within the bore of 
vertical leg 106, the leg 106 expands to create a spacing between the 
peripheral edges of blocks 116 and the inner surface of vertical leg 106 
that defines the bore. This expansion has two desirable results. The first 
is that it increases the size of the vessel containing the gases which 
decreases the pressure exerted by such gas and provides a path for 
expulsion of the gases from the cavity. The gases may be vented at the 
interface between the cap 128 and the end of vertical leg 106 to the 
outside, and thus dissipated. If additional venting is required, vent 
ports with appropriate unidirectional seals as is well know in the art may 
be placed in cap 128. 
Expansion of the vertical leg 106 is permitted to continue until the outer 
surface of leg 106 contacts the inner surface of expansion tube 140 at 
which time expansion of vertical leg 106 terminates. The spacing between 
the outer surface of vertical leg 106 in its normal condition and its 
expanded condition is sufficient to allow the expected maximum volume of 
gas to be expended within a reasonable period of time while keeping the 
vertical leg 106 within the elastic limits of the material employed to 
fabricate the housing 102 and shield 108. The fins 142 or strap 144 will 
not interfere with the return of vertical leg 106 to its normal condition 
and size once the gases have been dissipated. The expansion tube 140 has 
the additional advantage of providing yet another shield, one which has 
not been softened by the hot gases within leg 106 and which can help to 
restrain any fragments of an exploding block 116 and help direct such 
fragments down and out of the leg 106. 
The expansion tube 140 may be made of metal such as stainless steel, 
copper, or aluminum or may be a rigid tube formed of filament windings of 
any suitable continuous fiber such as nylon, rayon, glass and polyethylene 
impregnated with a resinous material which may be natural or synthetic and 
may be in the partially cured or uncured state. A glass filament winding 
with epoxy resins are preferred. The resins are fully cured so that the 
tube is rigid. Hose with tire cord reinforcement may also be employed. The 
tube 140 will have a length approximately equal to the height of block 116 
stack but should not be so long as to restrict the free movement of the 
end vertical leg 106 adjacent cap 128 which will undergo the greatest 
expansion. 
Typical dimensions for tube 140 is thickness in the range of 0.031" to 
0.250", length 3.00" to 8.00", spacing from the outer wall of leg 106 
0.250" to 1.00". 
FIG. 3 illustrates the invention as applied to a molded-in arrester block 
stack. Two MOV blocks 116 are shown in arrester 160 although, as stated 
above, the number and size of the blocks 116 employed will depend upon 
circuit parameters. The blocks 116 are glued together at their interfaces 
and to end fittings 162 and 164 as at 166 with a silver epoxy paste. A 
layer 168 of a suitable dielectric insulating material such as a thermoset 
or thermoplastic resin such as glass-filled nylon is applied by injection 
molding to preassemble the blocks 116 and end fittings 162, 166. The 
insulating material layer 168 is permitted to engulf portions of the ends 
of end fittings 162 and 166 to seal the entire unit. A suitable bonding 
agent is applied to the outer surface of layer 168 prior to its insertion 
into the final mold where EPDM is injection molded about the block stack 
assembly and within EPDM shield layer 108 to form the housing 102. In this 
manner the outer surface of layer 168 is joined to the inner surface of 
housing 102 to form a void-free interface. This bonding also occurs with 
metal coupling 122 and shield 130 which are also coated with a bonding 
agent prior to insertion into the final mold to give a bond between the 
coupling 122 and the end of probe 112 and shield 130, and shield 130 with 
body 102 so that the entire region above end fitting 162 is sealed and gas 
tight. 
End fitting 162 is joined to metal coupling 122 by a suitably double 
threaded metal part 170 which threads into end fitting 162 at a first end 
and metal coupling 122 at a second. A similar double threaded metal part 
172 threads into end fitting 164 at a first end and provides threaded stud 
132 at the other. 
Expansion tube 140 is shown hung by means of strap 144 about leg 106 of 
arrester 160. This strap may be metal, nylon, or other suitable thermoset 
or thermoplastic compositions and suitably contoured to maintain the 
desired position of expansion tube 140. 
When an arc begins and gas is generated it builds up more rapidly in leg 
106 than it did in the press fit version of FIG. 2 due to the seal of the 
metal fitting 122 and shield 130 which prevents any gas from escaping 
along receptacle 110. This has the desirable effect of preventing the 
arrester 160 being blown from the mating bushing insert by such gases. The 
gas will continue to build up until layer 168 is displaced from the 
peripheral surfaces of blocks 116. As stated above, blocks 116 have a 
ceramic or other collar about their peripheral surfaces which prevent the 
layer 168 or the body 102 from adhering to the blocks 116. Thus, as the 
layer 168 expands away from the peripheral edges of blocks 116, there is a 
venting passage created which decreases the pressure of the gas by making 
available a larger volume space and by providing a venting path out of 
body 102. The gases escape along metal part 172 to even larger chamber 174 
and then to the outside of arrester 160 along the joint between cap 128 
and shield 108 and along stud 132. If desired, a series of vent ports can 
be arranged in layer 168 and cap 128 to more rapidly dissipate collecting 
gases. Suitable one way valve arrangements well known in the art can be 
employed as required. 
The separation of layer 168 from the peripheral surfaces of blocks 116 also 
results in the expansion of the vertical leg 106 to the extent permitted 
by expansion tube 140 which retains the material of leg 106 within its 
limit of expansion and elastic limit so that the leg 106 can recover its 
original condition when all the gas has been dissipated and the arrester 
160 returns to its initial condition. The presence of the expansion tube 
140 again is available to help to contain block fragments if the blocks 
were to explode and to help to direct the fragments downwardly as is true 
of the device of the co-pending cited application. 
The mounting of expansion tube 140 by means of an external strap 144 
permits devices which are installed in the field to be retrofitted with a 
containment device not previously available except during the initial 
construction as in the co-pending application. Also, the strap mount and 
fin mount arrangements permit a user to determine the need for such a 
device and to elect whether or not to purchase same. The same basic 
arrester can be used for either configuration and the expansion tube added 
only to those who elect to do so. 
While there have been shown and described and pointed out the fundamental 
novel features of the invention as applied to the preferred embodiments, 
it will be understood that various omissions and substitutions and changes 
of the form and details of the devices illustrated and in its operation 
may be made by those skilled in the art without departing from the spirit 
of the invention.