Surge arrester having controlled multiple current paths

An improved surge arrester includes metal oxide varistors in series with spark gap assemblies arranged such that the MOV elements conduct the low magnitude, steady-state current through the arrester along a path that is separate and distinct from the path through which impulse current is conducted during an overvoltage.

BACKGROUND OF THE INVENTION 
The present invention relates generally to surge arresters. More 
particularly, the invention relates to a new design for the internal 
components of surge arresters. Still more particularly, the invention 
relates to a new combination of metal oxide varistors (MOV's) and spark 
gap assemblies in which the MOV elements conduct low magnitude, 
steady-state current through the MOV along a first current path, and 
conduct the higher magnitude impulse or surge currents through the MOV 
along a separate and distinct path. 
Under normal operating conditions, electrical transmission and distribution 
equipment is subject to voltages within a fairly narrow range. Due to 
lightning strikes, switching surges or other system disturbances, portions 
of the electric system may experience momentary or transient voltage 
levels that greatly exceed the levels experienced by the equipment during 
normal operating conditions. Left unprotected, critical and costly 
equipment such as transformers, switching apparatus, and electrical 
machinery may be damaged or destroyed by such overvoltages and the 
resultant current surges. Accordingly, it is routine practice within the 
electrical industry to protect such apparatus from dangerous overvoltages 
through the use of surge arresters. 
A surge arrester is commonly connected in parallel with a comparatively 
expensive piece of electrical equipment so as to shunt or divert the 
overvoltage-induced current surges safely around the equipment, thereby 
protecting the equipment and its internal circuitry from damage. When 
caused to operate, a surge arrester forms a current path to ground having 
a very low impedance relative to the impedance of the equipment that it is 
protecting. In this way, current surges which would otherwise be conducted 
through the equipment are instead diverted through the arrester to ground. 
Once the transient condition has passed, the arrester must operate to open 
the recently-formed current path to ground and again isolate or "reseal" 
the distribution or transmission circuit in order to prevent the 
nontransient current of the system frequency from "following" the surge 
current to ground, such system frequency current being known as "power 
follow current." If the arrester did not have this ability to interrupt 
the flow of power follow current, the arrester would operate as a short 
circuit to ground, forcing protective relays and circuit breaker devices 
to open or isolate the now-shorted circuit from the electrical 
distribution system, thus causing inconvenient and costly outages. 
Conventional surge arresters typically include an elongated enclosure or 
housing made of an electrically insulating material, a stack of 
voltage-dependent, nonlinear resistive elements retained within the 
enclosure, and a pair of electrical terminals at opposite ends of the 
enclosure for connecting the arrester between a line-potential conductor 
and ground. The nonlinear resistive elements are chosen to have a higher 
resistance at the normal steady-state voltage and a much lower resistance 
when the arrester is subjected to high magnitude transient overvoltages. 
Depending on the type of arrester, it may also include one or more spark 
gap assemblies housed within the insulative enclosure and electrically 
connected in series with the nonlinear resistive elements. 
Present-day surge arresters are typically one of two basic types and are 
generally classified according to the type of nonlinear resistive elements 
they contain. The first type of conventional arrester is commonly referred 
to as the series gapped silicon carbide (SiC) arrester. The nonlinear 
resistive elements in this arrester are relatively short cylindrical 
blocks of silicon carbide which are stacked one atop the other within the 
arrester housing in series with spark gap assemblies which are generally 
resistance graded gap assemblies. A resistance graded gap assembly 
comprises a resistor electrically in parallel with the spark gap and 
usually includes one or more resistors in series with the gap. This 
network of resistors is employed to control the voltage level at which the 
spark gap will begin to conduct. The second type of arrester commonly used 
today is know as the gapless metal-oxide varistor (MOV) arrester. In this 
type of arrester, the nonlinear resistive elements comprise disks formed 
of a metal oxide compound which are again stacked within the arrester 
housing in series. 
In both types of prior art arresters, the voltage-current relationship for 
the nonlinear elements is expressed as I=kE.sup.n, where I is arrester 
current, k is a constant, E is the arrester voltage, and n is the 
nonlinear exponent or coefficient. The older series gapped SiC arrester 
uses low exponent silicon carbide blocks in series with low exponent 
nonlinear graded gaps, the exponent n of both elements being less than 10 
and typically being within the range of 4 to 5 at the operating or steady 
state voltage. The more modern MOV arrester typically uses only high 
exponent nonlinear elements of the metal-oxide variety and, as described 
below, does not require series gap assemblies to operate properly as is 
the case of SiC arresters. In the case of MOV arresters, the exponent n is 
usually greater than 10 and typically about 20 or greater at the 
steady-state system voltage. 
Because of the different degrees of nonlinearity of the resistive elements 
employed in silicon carbide and MOV arresters, these arresters differ in 
structure and operation. The silicon carbide blocks are designed to 
provide a very low resistance to surge currents, but a higher resistance 
to the 60 hertz power-follow current which continues to flow through the 
arrester after the transient condition has passed. Despite the higher 
resistance, the silicon carbide blocks will still conduct large currents 
at the normal, steady-state line-to-ground voltage. Accordingly, gap 
assemblies are employed in series with the silicon carbide blocks. As a 
transient overvoltage condition ceases, the resistance of the silicon 
carbide blocks increases so as to limit the magnitude of the power follow 
current. The reduced current flow and the corresponding decrease in the 
voltage across the spark gaps provide the gap assemblies the opportunities 
to open the current path to ground and thus "reseal" the power circuit 
after the surge has passed. This type arrester has been in use for many 
years and is described in many earlier patents, such as U.S. Pat. Nos. 
4,161,763 and 4,174,530. 
With an MOV arrester, the MOV 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 system's steady-state or normal operating 
voltage, the varistors have a relatively high impedance. As the applied 
voltage is increased, gradually or abruptly, the varistors' impedance 
progressively decreases. When the voltage appearing across each varistor 
reaches the elements' breakdown voltage, the varistor impedance 
dramatically decreases, and the varistors become highly conductive. 
Accordingly, if the arrester is subjected to an abnormally high transient 
overvoltage, such as may result from a lightning strike, 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 to a very high value, thereby reducing the current through the 
MOV arrester to a negligible flow and restoring the arrester and 
electrical system to their normal, steady-state condition. A variety of 
MOV arresters have been described in many earlier patents, such as U.S. 
Pat. Nos. 4,930,039 and 4,240,124. 
The series gapped SiC arresters suffer from a variety of undesirable 
traits. First, because the SiC elements are highly conductive at normal 
operating voltages, the gap assemblies are required to support the full 
system line-to-ground voltage over the life of the arrester, the SiC 
elements being used only to limit current which, in turn, assists the gaps 
in returning to their non-conductive mode during a discharge operation as 
described above. Because the gap assemblies must support the full 
line-to-ground voltage, SiC arresters are typically comprised of many such 
assemblies, each of which must withstand its proportionate share of the 
voltage. This type of construction results in more consistent impulse or 
spark-over characteristics than can be achieved through the use of MOV 
arresters; however, the undesirable result is that the design yields 
higher than desired impulse protective characteristics. 
Another deficiency characteristic of the series gapped SiC arrester is that 
its high current discharge characteristic and the power follow current 
levels are both controlled by the same nonlinear elements, i.e., the SiC 
elements. To achieve lower high current discharge voltages, it is 
desirable to have silicon carbide elements with a low resistance. Yet, to 
provide lower levels of power-follow current, it is desirable to have 
silicon carbide elements with a high resistance. Due to these 
diametrically opposed requirements of the same components, design 
compromises have resulted in less than desirable protective 
characteristics. Still another inherent problem with the series gapped SiC 
arrester is its comparatively large size and weight. 
The MOV arrester was developed to eliminate the undesirable impulse 
characteristic of the series gapped silicon carbide arrester. In the MOV 
arrester, the nonlinear MOV elements eliminate the need for a series gap 
by remaining highly non-conductive at normal, steady state system 
voltages. As the voltage applied to the arrester is increased, the MOV 
element, which is a semiconductor, gradually begins to conduct, without a 
disruptive discharge as is characteristic with the series gapped SiC 
arrester. This switch-like characteristic enables the MOV arrester to 
shunt all fundamental transient overvoltage energies to ground. The 
inherent problem with this type of arrester, however, is that both the 
turn-on or breakdown voltage and the high current discharge voltage are 
controlled or dictated by the same nonlinear elements. It is desirable for 
the arrester to have higher breakdown voltages so that transient 
overvoltages having a lower, nondestructive magnitude do not result in 
conduction through the MOV elements. At the same time, it is also 
desirable for the arrester to have lower high current discharge voltages 
to provide better equipment protection. Again, as with the series gapped 
SiC arrester, two diametrically opposed requirements of the same component 
result in a compromise of characteristics. In some cases the discharge 
voltage capability of the MOV arrester is compromised. In other instances, 
the arrester's ability to withstand a temporary, relatively low 
overvoltage condition, defined as the arrester's temporary overvoltage 
capability, is reduced. 
More recently, a hybrid arrester has been developed which combines the gap 
assemblies previously used in the silicon carbide gapped arresters with 
the MOV elements of the metal oxide varistor arrester. Such hybrid 
arrester is described in co-pending U.S. patent application, Ser. No. 
07/420,069, and in the publication entitled New Surge Arrester Technology 
Offers Substantial Improvement in Protection and Reliability as presented 
to the SEE Overhead Committee, Annapolis, Md., May 10, 1990, such written 
disclosures being incorporated herein by reference. The hybrid arrester 
has been shown to have superior performance characteristics as compared to 
both the SiC gapped arresters and the MOV arresters. 
Despite the advances made by the hybrid arrester, further technological 
advances would be welcomed by the industry. Specifically, the resistance 
graded gap structures used in the silicon carbide gapped arrester and in 
the hybrid arrester must be precisely matched. Further, assembly of the 
complicated gap structures in the arrester is tedious and thus costly. An 
arrester having similar or improved characteristics as compared to the 
hybrid arrester, but without the disadvantages associated with the 
resistance graded gap structures would be a welcomed addition to the art. 
It would further be desirable to decrease the volume of expensive MOV 
material currently required in the MOV and hybrid arresters. 
SUMMARY OF THE INVENTION 
Accordingly, there is provided a surge arrester structured to have improved 
performance characteristics, as compared to conventional MOV and series 
gapped silicon carbide arresters, and to have fewer and less complicated 
internal components which require less materials and time to manufacture. 
The arrester of the present invention includes a resistive element, which 
in the preferred embodiment comprises a metal oxide varistor, having a 
steady-state current path and a separately defined impulse current path. 
The resistive element includes a central core portion surrounded by an 
outer portion. The core portion comprises the impulse current path which 
is formed parallel to and along the longitudinal axis of the arrester. The 
steady-state current path may be formed at an angle relative to the 
longitudinal axis, or may, alternatively, be formed coaxially with the 
impulse current path through the outer portion of the resistive element. 
The arrester may further include any of a variety of conductive contact 
surfaces formed on the faces of the resistive element and provided both to 
create a more uniform current density for impulse current and to provide 
additional resistance to steady-state current flow in order to direct the 
steady-state current to flow in the desired steady-state path. The contact 
surface may comprise an annular contact, formed about the periphery of the 
face of the resistive element, and a central circular contact formed 
within the annular contact. Alternatively, the contact surface may 
comprise a series of concentric rings formed on the face of the resistive 
element or may comprise an array of spot contacts. To further increase the 
resistance to steady-state current flow in a path other than the desired 
path, grooves, channels or notches may be formed in the faces of the 
resistive element. 
The invention further includes a simplified spark gap assembly which 
directs the steady-state current to flow in a path that is coaxial to the 
impulse current path and which also simplifies the construction of the 
spark gap assembly. In one embodiment of the invention, the arrester 
includes a conductive ring disposed between two adjacent varistors at 
unmetalized surfaces on the periphery of the varistors, the unmetalized 
surfaces creating an additional resistance to the flow of steady-state 
current. The invention may alternatively include a varistor having a base 
portion and a crown portion formed on the upper surface of the base 
whereby the crown portion physically supports the adjacent varistor and 
provides the additional resistance that is desirable in the steady-state 
current path. In this embodiment, the spark gap is formed between the 
opposing faces of the adjacent varistors. The arrester may also include an 
electrode disposed between the crown of one varistor and the base of the 
adjacent varistor, the spark gap being formed between the central portion 
of the electrode and the adjacent face of the varistor. To increase the 
resistance in the coaxial steady-state current path, the invention may 
include a crown portion formulated from a material having electrical 
characteristics which differ from the material forming the base portion of 
the varistor. Alternatively, or to create still additional resistance in 
the steady-state current path, a semi-conductive coating may be applied 
between the crown of the resistive element and the electrode or between 
the crown and the adjacent varistor being supported by the crown. 
The invention also includes an arrester having voltage dependent, nonlinear 
resistive material disposed about a central substrate core, the resistive 
material including a channel spirally formed through the material creating 
spark gaps across the channel between adjacent portions of the resistive 
material. In this embodiment, the arrester includes a current path for the 
steady-state current which is spirally directed about the longitudinal 
axis of the arrester, and a more direct path for the impulse current which 
is directed through the arrester across the spark gaps in a multitude of 
paths which are parallel to the longitudinal axis of the arrester. 
Thus, the present invention comprises a combination of features and 
advantages which enable it to substantially advance arrester technology by 
providing a surge arrester having resistive elements and separately 
defined current paths within the resistive elements for the steady-state 
and impulse currents. Controlling the current paths through the arrester 
in this manner simplifies and reduces the cost of manufacturing of the 
arrester and provides improved performance characteristics as compared to 
conventional MOV and series gapped SiC arresters. For example, the 
arrester of the present invention will have a temporary overvoltage 
capability equal to twice the maximum continuous overvoltage rating of the 
arrester and will have discharge voltages that are significantly lower 
than similarly rated conventional MOV and SiC arresters. Further, the 
arrester made in accordance with the invention may be smaller and lighter 
than conventional MOV arresters since the MOV elements may be shorter in 
height than those required in conventional gapless MOV arresters. 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.

DESCRIPTION OF PREFERRED EMBODIMENT 
Surge arresters are installed in electrical systems for the purpose of 
diverting dangerous overvoltage-induced surges to ground and preventing 
such surges from damaging costly or critical electrical equipment. The 
present invention relates in general to any type of electrical apparatus 
which may be protected by surge arresters, such apparatus including 
transformers and electrical switching devices. For example, the arrester 
and arrester subassemblies of the present invention may be employed in low 
voltage, distribution class and station class arrester applications. 
Referring to FIG. 1, there is shown a cross-sectional view of an arrester 
subassembly 10 structured in accordance with the present invention. As 
shown, subassembly 10 generally comprises a nonlinear metal oxide varistor 
(MOV) 20 and upper and lower electrodes 40 and 50, respectively. MOV 20 is 
made of metal oxide and preferably is formed into a short cylindrical disk 
having an upper face 22 and a lower face 24. Disposed circumferentially 
about the outer surface of MOV disk 20 is a dielectric collar or sleeve 34 
preferably made of epoxy. MOV 20 must be capable of withstanding high 
energy surge currents. The metal oxide for MOV 20 may be of the same 
material used for any high energy, high voltage MOV disk, and is 
preferably made of a formulation of zinc oxide. See, for example, U.S. 
Pat. No. 3,778,743 of the Matsushita Electric Industrial Co. Ltd., Osaka, 
Japan, incorporated herein by reference. In the preferred embodiment, MOV 
20 will have a uniform microstructure throughout the MOV disk and the 
exponent n for the zinc oxide formulation of MOV 20 will be in the range 
of about 10-25 at the steady state system voltage. An exponent n of 
approximately 20 is most preferred. 
MOV 20 must be capable of discharging the high energy surge currents caused 
by lightning strikes and then thermally recovering so as to be capable of 
enduring repetitive high surge currents. It is desirable for MOV 20 to be 
able to thermally recover from a high energy surge current while it is 
energized at the power system's maximum continuous operating voltage 
(MCOV). MOV 20 of the present invention is capable of conducting lightning 
surge currents of up to 100,000 amps. MOV 20 will recover from a 100,000 
amp surge current of a short duration such as a 4/10 wave (four 
microseconds to crest and decaying to half crest in 10 microseconds). The 
cross-sectional area of MOV 20 will partially dictate its durability and 
recoverability from high surge currents. It is preferred that the circular 
cross-section of MOV 20 have a diameter between approximately 1 to 3 
inches to insure that there is sufficient surface area of between about 
0.785 and 7.07 square inches to maintain the desired durability and 
recoverability. At the same time, it is also desirable that MOV 20 have as 
small a cross-sectional area as possible in order to reduce the size, 
weight and cost of the arrester. As size is reduced, however, the 
durability and recoverability of the disk is decreased. Given these 
considerations, a diameter of approximately 2 inches is the most 
preferred. The thickness of MOV 20 as measured between faces 22 and 24 is 
preferably about 0.75 inches. As understood by those skilled in the art, 
given a particular metal oxide formulation and a uniform or consistent 
microstructure throughout the MOV disk, the thickness of the MOV disk 
determines the operating voltage level. 
Referring still to FIG. 1, MOV 20 also includes metalized contact surfaces 
26 and 28 formed on upper face 22 and metalized contact surfaces 30 and 32 
formed on its lower face 24. In the preferred embodiment, contact surfaces 
26, 28, 30 and 32 are sprayed-on metalized coatings of molten aluminum 
having a thickness approximately equal to 0.002 to 0.010 inches. As shown 
in FIG. 1, contact surface 26 is an annular contact formed along the 
periphery of upper face 22 of MOV disk 20. Contact surface 28 is centrally 
disposed on the upper face 22 of MOV disk 20 concentrically within contact 
surface 26. An unmetalized portion 27 is thus formed between contact 
surfaces 26 and 28 on upper face 22. Contact surface 30 comprises an 
annular contact formed on lower face 24 of MOV disk 20. Contact surface 32 
is located in substantially the center of lower face 24 and concentrically 
within annular contact surface 30. An unmetalized portion 31 is thereby 
formed between contact surfaces 30 and 32 on lower face 24. It is 
preferable that unmetalized portions 27 and 31 have a width approximately 
equal to 0.1875 inches. 
Electrodes 40 and 50 are preferably formed of brass, although copper, 
aluminum, tin-plated steel or other electrically-conducting material may 
be employed. Upper electrode 40 comprises a circular plate 41 having a 
raised rim or lip 42 disposed about its periphery, lip 42 being positioned 
in electrical contact with contact surface 26 of MOV disk 20. Lower 
electrode 50 includes a circular plate 51 having a centrally located 
contact 52 which is in electrical contact with contact surface 32 of MOV 
disk 20. Upper and lower conductors 54 and 56 are electrically connected 
to contacts 40 and 50, respectively, and are employed to connect the 
arrester subassembly 10 to a voltage source. 
Referring still to FIG. 1, arrester subassembly 10 further includes a first 
spark gap 12 which is formed between electrode 40 and contact surface 28 
at the upper face 22 of MOV disk 20. Similarly, a second spark gap 14 is 
formed between electrode 50 and annular contact surface 30 at the lower 
face 24 of MOV disk 20. In the preferred embodiment, spark gaps 12 and 14 
are within the range of approximately 0.01-0.10 inches. As explained more 
fully below, this structure of arrester subassembly 10 creates separate 
and distinct controlled current paths through MOV 20, such paths being 
represented by arrows P.sub.1 and P.sub.2 as shown in FIG. 1. As shown, 
arrow P.sub.1 represents a controlled current path from contact surface 26 
on the outer periphery of upper face 22 to the lower central contact 
surface 32 on lower face 24. Although only two such paths P.sub.1 are 
shown in FIG. 1, it should be understood that a multitude of similar 
current paths are created between the annular contact surface 26 to lower 
central contact surface 32. A second controlled current path represented 
by arrow P.sub.2 is formed substantially parallel to the longitudinal axis 
of MOV disk 20 and substantially perpendicular to upper and lower faces 22 
and 24. Again, although only a single arrow P.sub.2 is shown, a multitude 
of parallel paths P.sub.2 exist through substantially the entire cross 
section of MOV 20 from upper face 22 to lower face 24. The paths shown by 
arrows P.sub.3, P.sub.4 and P.sub.5 in FIG. 1 represent some of the myriad 
of other potential current paths between metalized contact surfaces 26, 
28, 30 and 32. 
Referring briefly to FIG. 2, there is shown a simplified schematic diagram 
of the arrester subassembly 10 shown in FIG. 1 connected to line-potential 
conductor 11. The resistances R.sub.p1, R.sub.p2, R.sub.p3, R.sub.p4, and 
R.sub.p5 shown in FIG. 2 represent the impedance of paths P.sub.1, 
P.sub.2, P.sub.3, P.sub.4 and P.sub.5 respectively, as shown in FIG. 1. 
R.sub.26 and R.sub.32 represent the impedance formed by contact surfaces 
26 and 32, respectively. For simplicity, the impedances of contact 
surfaces 28 and 30 (both to current flow in a direction perpendicular to 
and parallel to upper and lower faces 22, 24) have not been shown in the 
schematic diagram. Likewise, spark gaps which exist across unmetalized 
surface 27 (between contact surfaces 26 and 28) and unmetalized surface 31 
(between contact surface 30 and 32) are not shown in FIG. 2. G.sub.12 and 
G.sub.14 represent gaps 12 and 14 respectively. The impedances of 
conductors 54, 56, plates 41, 51, electrode lip 42 and electrode contact 
52 are each represented by an "R" with a subscript having the 
corresponding reference numeral. 
The operation of arrester subassembly 10 will now be described with 
reference to FIGS. 1 and 2. During steady-state operation, when no 
transient overvoltage is present on the electrical system to which 
subassembly 10 is connected, the resistance of MOV disk 20 is relatively 
high such that the system line-to-ground voltage is shared by MOV 20 and 
by gaps 12 and 14. In this instance, the voltages across gaps 12 and 14 
are not of a magnitude high enough to cause the gaps to conduct. 
Accordingly, the steady-state current at the system frequency (typically 
60 hertz in the United States) flows through arrester subassembly 10 along 
the path formed by upper electrode 40, contact surface 26, diagonal path 
P.sub.1 and finally to ground via contact surface 32 and electrode 50. 
Under impulse or surge conditions, the resistance of MOV disk 20 will 
decrease dramatically. As this occurs, more and more of the voltage that 
is applied to subassembly 10 appears across the series of resistances 
which are in parallel with gaps 12 and 14. More specifically, as the 
applied voltage increases, the voltage across gaps 12 increases due to the 
resistances R.sub.26, R.sub.41 and R.sub.42. Likewise, the voltage across 
gap 14 will increase as the transient voltage is applied due to the 
resistances R.sub.32, R.sub.51 and R.sub.52. When the voltage across gaps 
12, 14 reaches the gap's spark-over voltage, current will be conducted 
across the gap. Depending on the magnitude of the transient overvoltage, 
one or both gaps 12, 14 may sparkover. In instances where both gaps 12, 14 
sparkover, the high magnitude surge or impulse current is conducted 
through arrester subassembly 10 via electrode 40, across gap 12 to upper 
central contact surface 28, through the entire central or core portion of 
MOV 20 along path P.sub.2 and the many corresponding parallel paths (not 
shown) to lower peripheral contact surface 30, and across gap 14 to ground 
through electrode 50. As understood by those skilled in the art, a certain 
portion of the impulse current conducted through MOV 20 along path P.sub.2 
will be conducted directly to lower electrode 50 through contact surface 
32 and contact 52, rather than across gap 14. 
The control of the current through MOV 20 via alternate controlled current 
paths P.sub.1 and P.sub.2 is accomplished by controlling the dimensions of 
contact surfaces 26, 28, 30 and 32 and the dimensions of gaps 12 and 14. 
Controlling these dimensions dictates the resistance to the current flow 
which, in turn, causes the current to be conducted primarily through one 
path or another. Referring to FIGS. 1 and 2, the resistances across gaps 
12 and 14 are comparatively very large under steady-state conditions such 
that gaps 12 and 14 will not conduct. Contact surfaces 26, 28, 30 and 32 
are dimensioned such that unmetalized portion 27 on upper face 22 and 
unmetalized portion 31 on lower face 24 have a high resistance to current 
conducted along the paths denoted by arrows P.sub.3 and P.sub.4. 
Unmetalized portions 27 and 31 ensure that the resistance of path P.sub.1 
is much less than the sum of resistances R.sub.p3 plus R.sub.p2 and much 
less than the sum of resistances R.sub.p4 and R.sub.p5, thus the 
steady-state current flows through MOV 20 along paths P.sub.1. In the 
impulse or surge conduction mode, gap 12 or gap 14 or both will spark over 
and begin to conduct, allowing the surge current to be conducted through 
MOV 20 along the shorter and more direct paths P.sub.2. 
As understood by those skilled in the art, the "discharge voltage" of an 
arrester is the voltage appearing across the arrester terminals when the 
arrester is functioning to dissipate the potentially damaging energy 
associated with transient overvoltages. The discharge voltage is the 
product of the transient or impulse current flowing through the arrester 
multiplied by the resistance of the arrester during the surge, and is the 
voltage that the equipment being safeguarded by the arrester will see. To 
provide greater margins of protection for the equipment, it is desirable 
for arresters to have low discharge voltages. At the same time, it is 
desirable for arresters to have high temporary overvoltage (TOV) 
capabilities to allow temporary voltage excursions of system 
frequency-above the nominal system operating voltage. Accordingly, it is 
desirable for the arrester to "turn on" at voltages as near to the 
discharge voltage as possible. With conventional MOV and SiC arresters, 
these two characteristics are achieved such that one of the desirable 
characteristics must be sacrificed in order to optimize the other. By 
contrast, the present invention provides greatly improved (lower) 
discharge voltages than the conventional arresters while, at the same 
time, maintaining relatively high TOV capabilities. The present invention 
thus reduces the separation between the "turn on" or "break down" voltage 
of the arrester and the discharge voltage of the arrester. This is 
achieved by controlling the path taken by the steady state current and 
requiring the steady state current to take a path P.sub.1 through MOV 20 
that is longer than the height of MOV 20 (longer than P.sub.2 and P.sub.5 
shown in FIG. 1). Thus, providing multiple controlled current paths 
through MOV disks 20 provides improved protective margins (lower discharge 
voltages) without sacrificing TOV capability as is required with 
conventional arresters. 
As also understood by those skilled in the art, a surge arrester typically 
includes a number of MOV's 20 stacked in series relationship inside an 
arrester housing, the number of MOV's 20 being dependent upon the 
formulation of the metal oxides and the doping materials employed in 
manufacturing the MOV's, the thickness of the MOV elements, and the 
voltage rating required of the arrester in the given application. 
Referring to FIG. 3, there is shown a plurality of arrester subassemblies 
60 combined in series to form a surge arrester 70 structured in accordance 
with the present invention. 
Arrester 70 generally comprises insulative housing 72, a plurality of 
arrester subassemblies 60, upper and lower closures 80, 82 and upper and 
lower terminals 74, 76, respectively. Housing 72 is generally cylindrical 
in shape and made from an insulative material such as porcelain or a 
polymer. A central cavity 84 is formed through housing 72 substantially 
along the longitudinal axis of the housing. Arrester subassemblies 60 are 
stacked in series relationship within cavity 84 along with conductive 
spacer 86, conductive plates 88, 90, conductive strap 94 and coil spring 
92. Upper and lower closures 80, 82, respectively, are disposed about the 
ends of housing 72 and hermetically seal the components within cavity 84 
from the ambient environment. Closures 80 and 82 are formed of brass, 
copper or similar conductive material and are electrically connected to 
terminals 74 and 76 which, in turn, are employed to electrically connect 
arrester 70 between and voltage source and ground. Conductive spacer 86 
engages and makes electrical contact between upper closure 80 and 
conductive plate 88. Coil spring 92 impacts a compressive force on 
subassemblies 60, plates 88 and 90, and spacer 86 as is necessary for good 
electrical contact. Conductive strap 94 completes the electrical path 
between conductive plate 90 and lower closure 82. An isolator 78 is 
connected between lower closure 82 and lower terminal 76 and employed to 
explosively disconnect arrester 70 from the ground connection (not shown) 
in the event that the arrester fails to reseal after the arrester has 
operated to divert a surge to ground. 
Subassemblies 60 are similar in structure to subassembly 10 previously 
described with respect to FIGS. 1 and 2. In describing subassembly 60, 
like reference numerals have been used where the elements correspond to 
the elements previously described with respect to subassembly 10 in FIG. 
1. As shown in FIG. 3, subassembly 60 generally comprises MOV disk 20, an 
electrode 96 and an insulative ring 102. MOV disk 20 includes upper face 
22, lower face 24, upper peripheral contact surface 26, upper central 
contact surface 28, lower peripheral contact surface 30 and lower central 
contact surface 32 all as previously described. Electrode 96 comprises a 
relatively flat base portion 100 formed along its periphery and includes a 
raised central portion 98. Electrode 96 is preferably comprised of brass, 
but may also be made from copper, aluminum or other conductive materials. 
Electrode 96 is disposed on upper face 22 of MOV 20 such that electrode 
base portion 100 is in electrically contact with peripheral contact 
surface 26. Upper spark gap 12 is thereby formed between the raised 
portion 98 of electrode 96 and central contact surface 28 of MOV 20. 
As shown in FIG. 3, adjacent subassemblies 60 are separated within housing 
72 by insulative rings 102. Insulative rings 102, which are preferably 
formed of porcelain or insulative polymer, are employed to separate lower 
face 24 of MOV 20 from electrode 96 of an adjacent subassembly 60. In the 
preferred embodiment, insulative ring 102 has a height substantially equal 
to 0.1 inch, which is the distance between base portion 100 and the peak 
of raised portion 98 of electrode 96. In this configuration, insulative 
rings 102 define and maintain spark gap 14 which is formed between 
electrode 96 and contact surface 30 on lower face 24 of MOV 20. 
In operation, arrester 70 conducts current through a series path defined by 
upper terminal 74, closure 80, spacer 86, conductive plate 88, 
subassemblies 60, conductive plate 90, conductive strap 94, lower closure 
82 and lower terminal 76. In the steady-state mode, the voltage will be 
shared equally by subassemblies 60 and, within each subassembly 60, will 
be shared by MOV 20 and gaps 12 and 14. The voltage appearing across each 
subassembly 60 in such steady-state mode will not be of a magnitude great 
enough to cause spark gaps 12 and 14 to conduct. Accordingly, the 
steady-state current is conducted through each subassembly 60 along the 
path defined by electrodes 96, upper peripheral contact surface 26, the 
internal path denoted by arrow P.sub.1 and lower central contact surface 
32. 
When arrester 70 experiences a transient of a magnitude high enough to 
cause spark gaps 12 and 14 to conduct, the resulting surge current is 
conducted through each subassembly 60 along the path formed by spark gap 
12, upper central contact surface 28, internal MOV paths P.sub.2. The 
current is then conducted through the next adjacent subassembly 60 via 
spark gap 14 formed between contact surface 30 and the adjacent electrode 
96. In this embodiment too, a certain portion of the surge current 
conducted through MOV 20 along paths P.sub.2 will be conducted directly to 
electrode 96 through raised central portion 98, rather than across gap 14. 
Shown in FIGS. 4 and 5 is an alternative embodiment of an arrester 
subassembly of the present invention designed to increase the length, and 
thus the resistance, of the steady-state current path P.sub.1 through the 
MOV material. Referring to FIG. 4, there is shown an arrester subassembly 
110 generally comprising MOV 120, upper electrode 112 and lower electrode 
114. MOV disk 120 is comprised of zinc oxide or other suitable metal oxide 
material as previously described with respect to MOV 20 shown in FIGS. 1 
and 3. MOV 120 includes upper and lower faces 122, 124, respectively. 
Formed on upper face 122 are peripheral contact surface 126 and central 
contact surface 128 best shown in FIG. 5. Lower face 124 includes central 
contact surface 132 and peripheral contact surface 130. Surfaces 126, 128, 
130 and 132 are similar in structure and function as surfaces 26, 28 and 
32 previously described with reference to FIG. 1. Grooves 140 and 142 are 
machined in upper face 122 and lower face 124, respectively, of MOV disk 
120. Alternatively, MOV disk 120 may be originally formed and sintered to 
include grooves 140 and 142. In either event, grooves 140 and 142 
penetrate into MOV disk 120 to a depth at least equal to one-half the 
thickness of MOV 120 and preferably to a depth equal to approximately 
three fourths of the thickness of MOV 120. As described more fully below, 
grooves 140 and 142 extend the length, and thus increase the resistance, 
of steady-state current path P.sub.1 through MOV disk 120. 
Electrode 112 comprises a circular plate made of conducting material, 
preferably brass. Disposed between contact 112 and upper face 22 of MOV 
120 is a conductive ring 118 made of brass, copper, aluminum or other 
conductive material. Ring 118 provides a series path for current between 
electrode 112 and contact surface 126 and, together with electrode 112, 
comprises an alternative electrode structure to the rimmed electrode 40 
previously described with reference to FIG. 1. Lower electrode 114 
includes central contact 116 which provides electrical contact with 
contact surface 132. Electrode 114 is identical to electrode 50 previously 
described with respect to FIG. 1. Series gap 134 is formed between upper 
electrode 112 and contact surface 128. Gap 136 is defined by lower 
electrode 114 and lower peripheral contact 130. 
In operation, arrester subassembly 110 functions similarly to the 
subassembly 10 described with reference to FIG. 1. Referring again to FIG. 
4, under normal, non-transient conditions, the steady-state current is 
conducted through electrode 112 and conductive ring 118 and through MOV 
disk 120 along path shown by arrows P.sub.1, from upper contact surface 26 
to lower contact surface 132. Upper and lower grooves 140, 142 provide 
added resistance and thereby prevents the steady-state current from 
flowing substantially diagonally from the contact surface 126 directly to 
lower central contact surface 132 as was permitted in the embodiment of 
FIG. 1. With the added resistance provided by grooves 140 and 142, the 
steady-state current is required to take a more circuitous or meandering 
path as represented by arrow P.sub.1 in FIG. 4. To provide even greater 
resistance than that produced by grooves 140 and 142 alone, grooves 140 
and 142 may be filled with an insulative material such as RTV type 
silicone rubber. When the subassembly 110 experiences an impulse 
condition, gaps 134 and 136 will begin to conduct permitting the surge 
current to flow between electrodes 112 and 114 along shortened and more 
direct paths as represented by arrows P.sub.2 in FIG. 4. 
FIGS. 6 and 7 show an arrester subassembly 150 having multiple current 
paths which is an alternative to subassembly 10 shown in FIG. 1. Referring 
now to FIGS. 6 and 7, subassembly 150 generally comprises MOV disk 160, 
upper electrode 190, lower electrode 192 and conductive ring 194. Upper 
and lower electrodes 190, 192 and ring 194 are identical in structure and 
function as upper electrode 112, lower electrode 114 and conductive ring 
118 described with reference to FIG. 4. MOV disk 160 includes upper face 
162 and lower face 164. Formed or machined in upper face 162 are 
concentric grooves 182, 184 and 186. A bore 188 is formed substantially in 
the center of upper face 162. Bore 188 and grooves 182, 184 and 186 are 
formed to a depth approximately equal to one-eighth to one-fourth of the 
total thickness of MOV disk 160. As best shown in FIG. 7, concentric, 
spaced-apart contact surfaces 172, 174, 176 and 178 are formed on upper 
face 162 between grooves 182, 184, 186 and bore 188. Lower face 164 
includes a central contact surface 196 and peripheral contact surface 198 
which are identical in structure and function as contact surfaces 32 and 
30, respectively, described previously with reference to FIG. 1. A series 
spark gap 152 is defined by upper electrode 190 and contact surfaces 172, 
174, 176, 178. A series gap 154 is formed between lower peripheral contact 
surface 198 and lower electrode 192. 
Under normal, non-transient conditions, steady-state current flows between 
electrodes 190 and 192 through conductive ring 194 and through MOV disk 
160 along the path denoted by arrows P.sub.1. Under surge conditions, 
current is conducted across gaps 152 and 154 and through MOV 160 along 
path denoted by arrows P.sub.2. To provide for the maximum energy handling 
capability of MOV 160, it is desirable to conduct surge current between 
upper face 162 and lower face 164 through the entire cross-sectional area 
of the MOV 160. If that were the only design consideration, the entire 
upper face 162 could include a sprayed-on metallic contact surface; 
however, such a design would permit steady-state current to propagate from 
conductive ring 194 toward the center of MOV block 160 along the metalized 
upper face 162 and then along path denoted by arrow P.sub.2, rather than 
along the desired longer, higher resistance path denoted by arrow P.sub.1. 
Accordingly, to prevent such propagation, grooves 182, 184, 186 and bore 
188 are provided in upper face 162 to increase the path length and, thus, 
the resistance of the path denoted by arrow P.sub.3 shown in FIG. 6. 
Because the combined resistance of the path denoted by arrow P.sub.3 and 
P.sub.2 is much greater than the resistance in path denoted by arrow 
P.sub.1, steady-state current is conducted along the path shown by arrows 
P.sub.1, while surge current may be conducted through the entire 
cross-sectional area of MOV 160 along multiple paths denoted by arrows 
P.sub.2. 
Referring to FIG. 8, there is shown an alternative embodiment for upper 
face 162 of MOV 160 shown in FIGS. 6 and 7. In this embodiment, an array 
200 of spot or point contact surfaces 202 are applied to upper face 162 of 
MOV 160. Each spot contact surface 202, which is formed by arc spraying or 
similar metalizing process, is separated from adjacent contacts 202 by a 
predetermined distance. Providing the array 200 of contacts 202 on the 
entire upper face of MOV 160 enables substantially the entire 
cross-sectional area of MOV 160 to be employed in conducting surge current 
through the MOV. At the same time, the unmetalized or uncoated portions 
203 of upper face 162 between each surface contact 202 increases the 
surface resistance to steady-state current flowing from the periphery to 
the center of upper face 162. In this way, as described above with 
reference to FIG. 6, the steady-state current will be conducted through 
MOV 160 along the diagonal paths noted by arrow P.sub.1 in FIG. 6. 
FIGS. 9 and 10 depict alternative embodiments of the inventive surge 
arrester shown in FIG. 3. For clarity, conventional arrester component 
such as the housing, upper and lower terminals, internal spacers and 
springs are not shown in FIGS. 9 and 10. 
Referring to FIG. 9, arrester assembly 210 generally comprises a series 
combination of MOV disks 212, 214, 216, spark gaps 250, 252, 254 and 256, 
insulative spacers 234, 236 and conducting rings 238, 239. MOV disks 212, 
214, 216 are identical to MOV's 20 previously shown and described with 
reference to FIGS. 1 and 3. Each MOV 212, 214, 216 include an upper face 
218 and a lower face 220. MOV 212 and 216 each includes a peripheral upper 
contact surface 228, an upper central contact surface 230, a lower 
peripheral contact surface 226 and a lower central contact surface 224. 
MOV 214 includes an upper central contact surface 260, an upper peripheral 
contact surface 262, a lower central contact surface 264, and a lower 
peripheral contact surface 266. Contact surfaces 224, 226, 228 and 230 and 
contact surfaces 260, 262, 264 and 266 comprise metallic coatings 
produced, for example, by arc spraying. These contact surfaces are 
identical to surfaces 26, 28, 30 and 32 previously described with 
reference to FIGS. 1 and 3. As shown in FIG. 9, MOV disk 214 is identical 
in structure to MOV's 212, 216, but is stacked in series in an upside down 
relationship as compared to MOV's 212 and 216. 
Arrester assembly 210 includes at its upper end a plate electrode 240. 
Electrode 240 is spaced apart from upper central contact surface 230 of 
MOV 212 by conductive ring 238 which creates a series electrical path 
between electrode 240 and upper peripheral contact surface 228. Spark gap 
250 is thus formed between plate electrode 240 and upper central contact 
surface 230. MOV's 212 and 214 are spaced apart by insulative spacer ring 
234 and contact 244 which comprises a relatively small cylindrical contact 
of conducting material such as brass, copper or aluminum. Contact 244 has 
a height equal to the thickness of insulative spacer ring 234 when ring 
234 is compressed the desired, predetermined amount when assembled in a 
completed arrester. Insulative ring 234 is preferably made of silicone 
rubber, Buna N or neoprean. As shown, series spark gap 254 is formed 
between lower peripheral contact surface 226 of MOV 212 and upper 
peripheral contact surface 262 of MOV 214. 
Conductive ring 239 is identical to ring 238 previously described and is 
disposed between MOV blocks 214 and 216 and electrically engages the 
adjacent peripheral contact surfaces 266 and 228. Spark gap 254 is thereby 
formed between lower central contact surface 264 of MOV 214 and upper 
central contact surface 230 of MOV 216. 
The lower end of arrester assembly 210 includes plate electrode 242 which 
is identical in structure to plate electrode 240 previously described. 
Plate electrode 242 is spaced apart from lower face 220 of MOV 216 by 
contact 245 and insulative spacer ring 236 which are identical to contact 
244 and spacer ring 234, respectively, described above. As shown, spark 
gap 256 is thereby formed between lower peripheral contact surface 226 of 
MOV 216 and plate electrode 242. 
The multiple controlled current paths of this embodiment are denoted by 
arrows P.sub.1 and P.sub.2 as shown in FIG. 9. More specifically, 
steady-state current is conducted through MOV blocks 212, 214, 216 along 
the paths denoted by arrows P.sub.1. As shown, the steady-state path is 
formed from the periphery of the upper face 218 to the central portion of 
the lower face 220 of MOV disks 212 and 216 and from the central portion 
of upper face 218 to the periphery of the lower face 220 of MOV disk 214. 
By contrast, to quickly dissipate transient surges, the current induced by 
transient overvoltages is conducted along the more direct path across gaps 
250, 252, 254, 256 and through MOV's 212, 214, 216 along the multiple 
paths denoted by arrows P.sub.2. 
Referring now to FIG. 10, a further alternative embodiment of the present 
invention is shown. In FIG. 10, arrester subassembly 280 is shown to 
generally comprise MOV's 212, 214, 216, upper electrode 240 and lower 
electrode 242, all as previously described with reference to FIG. 9. In 
this arrangement, each MOV is separated from an adjacent MOV or adjacent 
electrode by insulative spacers 270 and conductive spacers 272. It is 
preferred that both spacers 270, 272 be formed of arcuate ring segments of 
equal height, although each may also be formed into a cylinder, cube or 
other easy-to-manufacture geometric shape. Insulators 270 are comprised of 
a soft polymer, such as Buna N or Neoprean, or other insulative material 
known to those skilled in the art. Conductive spacers 272 are preferably 
made of brass or aluminum, although other conductive materials may 
similarly be employed. 
Formed on upper and lower faces 218, 220 of MOV's 212, 214, 216 are central 
contact surfaces 276 identical to upper contact surface 28 previously 
described with reference to FIGS. 1 and 3. Additionally, a contact surface 
274 is formed on upper and lower faces 218, 220 of MOV's 212, 214, 216 
adjacent to each conductive spacer 272 so as to create a series electrical 
path between conductive spacers 272 and the adjacent MOV disks. 
In this embodiment, the steady-state current path, denoted by arrows 
P.sub.1 in FIG. 10, passes through MOV disks 212, 214, 216 in a diagonal 
path from a location on the upper periphery of upper face 218 to an 
opposite location on lower face 220. In this embodiment, steady-state path 
denoted by arrow P.sub.1 is longer than the steady-state paths P.sub.1 
shown in FIGS. 3 and 9, for example. Still, however, as with the other 
embodiments described, the current paths for surge current, denoted by 
arrows P.sub.2, is more direct, allowing surge current to pass directly 
through arrester assembly 280 across gaps 250, 252, 254, 256 and between 
contact surfaces 276 in a path substantially parallel to the longitudinal 
axis of MOV's 212, 214, 216. 
Another alternative embodiment of the present invention is shown in FIG. 
11. Referring to FIG. 11, an arrester subassembly 282 is shown. 
Subassembly 282 generally comprises MOV 288, plate electrodes 240, 242, as 
previously described with reference to FIG. 9, and spark gaps 284 and 286. 
MOV 288 includes upper face 290 and lower face 292 and is identical to MOV 
20 previously described with reference to FIGS. 1 and 3. Electrodes 240 
and 242 are spaced apart from upper face 290 and lower face 292, 
respectively, by insulative spacers 270 and conductive spacers 272 which 
have previously been described with reference to FIG. 10. Conductive 
spacer 272 creates a series electrical path between electrodes 240, 242 
and contact surfaces 274, described above with reference to FIG. 10. Upper 
and lower faces 290, 292 include an array of spot contacts 202, previously 
described with reference to FIG. 8. Spot contacts 202 on the upper and 
lower faces of MOV 288 increase the resistance seen by steady-state 
current which might otherwise be propagated between electrodes 240 and 242 
along a different, less resistive path, such as the path denoted by arrows 
P.sub.3, P.sub.2 and P.sub.4, for example. The added surface resistance 
provided by the unmetalized portions 203 ensures that steady-state current 
flows through MOV 288 diagonally from the upper face 290 to the lower face 
292 via the path denoted by arrow P.sub.1 in FIG. 11. While increasing the 
surface resistance in steady-state current, spot contacts 202 also provide 
an improved conductive surface for surge current to flow through MOV 288 
across gaps 284 and 286 in the paths that are substantially parallel to 
the longitudinal axis of MOV 288, as denoted by arrow P.sub.2 in FIG. 11. 
Applying spot contacts 202 about substantially the entire upper and lower 
faces 290, 292 of MOV 288 also serves to create a uniform current density 
for the surge current so that substantially the entire cross-sectional 
area of the MOV disk 288 may be employed to conduct the surge current and 
to dissipate the resultant energy as previously described with reference 
to FIGS. 6 and 7. 
Referring to FIGS. 12 through 14, another alternative embodiment of the 
present invention is shown which includes arrester subassembly 298. As 
shown, subassembly 298 generally comprises MOV 300 having an upper face 
302 and lower face 304. MOV 300 is substantially identical to MOV 20 
previously described with reference to FIGS. 1 and 3. As best shown in 
FIGS. 13 and 14, in this embodiment, upper and lower faces 302, 304 each 
include contact surfaces 308 which cover substantially the entire face, 
with the exception of edge portions 310 which are left unmetalized. Upper 
and lower faces 302, 304 each include a contact surface 306 which is 
located within one of the unmetalized edge portions 310 such that upper 
and lower contacts 306 are positioned diagonally opposite one another on 
MOV 300. 
The operation of subassembly 298 is best described with reference to FIG. 
12. As shown, subassembly 298 conducts steady-state current diagonally 
through MOV 300 between contacts 306 along the path denoted by arrow 
P.sub.1. Surge current is conducted through MOV 300 from upper face 302 to 
lower face 304 along the many parallel paths denoted by arrow P.sub.2. As 
shown in FIG. 12, surge current passes directly through MOV 300 
substantially parallel to the longitudinal axis of MOV 300. Contact 
surfaces 308 provide for better conduction of such surge current across 
the adjacent spark gaps (not shown) which, for example, may be formed by 
means of electrodes 240, 242 and spacers 270, 272 as shown in FIG. 11. 
Referring again to FIG. 12, unmetalized edge portion 310 increases the 
resistance to steady-state current being conducted between contacts 306 
and contact surfaces 308 which might otherwise allow the steady-state 
current to be conducted directly through MOV 300 via path P.sub.2. Thus, 
unmetalized portions 310 again ensure that the steady-state current is 
conducted along the longer, higher resistive path denoted by arrow 
P.sub.1. 
FIG. 15 shows a further modification or alternative embodiment of the 
present invention which may be utilized to further lengthen the path 
P.sub.1 for steady-state current. In this embodiment, subassembly 314 
includes MOV 316 having an upper face 318 and a lower face 320. MOV 316 is 
substantially identical to MOV 20 previously described with reference to 
FIGS. 1 and 3. Upper and lower faces 318, 320 of MOV 316 include spot 
contacts 306 positioned diagonally opposite one another on faces 318 and 
320 as described with reference to FIG. 12. Adjacent to spot contacts 306 
are grooves or notches 322 which are formed into faces 318, 320 of MOV 
disk 316. Notches 322 provide an increased resistance to steady-state 
current flowing from contact 306 toward the center of MOV 316. 
Additionally, notches 322 further lengthen the steady-state current path 
P.sub.1 as compared to the more direct diagonal path the steady-state 
current would take if notches 322 were not provided, such as the path 
denoted by arrows P.sub.1 shown in FIG. 12. 
FIGS. 16 through 23 show various embodiments of the present invention which 
include coaxial current paths through the MOV elements and arrester 
assemblies. More specifically, in these embodiments, the steady-state 
current flows through the MOV's and the arrester assemblies along the 
outer periphery of the MOV elements in a path that is parallel to the 
current path of the surge current. The current path for the surge current 
extends substantially through the entire cross-sectional area of the MOV 
disks and is parallel to the longitudinal axis of the MOV's. 
Referring now to FIG. 16, there is shown arrester subassembly 320 which 
generally comprises MOV's 322, 324, upper and lower electrodes 340, 342 
and conducting rings 326. MOV's 322, 324 each include upper and lower 
faces 318 and 320, respectively. MOV's 322, 324 are substantially 
identical to MOV 20 previously shown and described with reference to FIGS. 
1 and 3. Conducting rings 326 are identical to rings 238, 239 previously 
described with reference to FIG. 9. Electrodes 340, 342 are identical to 
electrodes 240, 242, also previously described with reference to FIG. 9. 
As shown in FIG. 16, MOV's 322 and 324 include a central metalized contact 
surface 328 on upper face 318 and on lower face 320. The peripheral 
portions 319 of upper face 318 and lower face 320 are not metalized. 
Conductive rings 326 are disposed on upper and lower faces 318 and 320 on 
unmetalized portions 319 and are used to separate the MOV blocks from 
adjacent MOV's and from electrodes 340 and 342. In this arrangement, a 
spark gap 330 is formed between electrode 340 and MOV 322. Similarly, 
spark gap 344 is formed between lower electrode 342 and MOV 324, and gap 
332 is formed between the adjacent MOV's 322, 324. The steady-state and 
surge current paths through MOV's 322, 324 are represented by arrows 
P.sub.1 and P.sub.2, respectively. 
Referring still to FIG. 16, in a steady-state or normal condition, the 
voltage present across arrester assembly 320 will not be great enough to 
cause the series spark gaps 330, 332, 334 to conduct. Accordingly, the 
steady-state current will flow through assembly 320 between electrodes 340 
and 342 through conductive rings 326 and along the outer periphery of 
MOV's 322, 324 along the path denoted by arrow P.sub.1. The unmetalized 
peripheral portions 319 of upper and lower faces 318 and 320 provide an 
increased resistance to current flowing through conductive rings 326 and 
along the path denoted by arrows P.sub.1 than would be present if portion 
319 included a metalized contact surface. Although the lengths of the 
steady-state current path P.sub.1 and surge current path P.sub.2 are 
substantially the same, the resistance to steady-state current through 
path P.sub.1 is substantially greater than that through path P.sub.2 due 
to additional resistance imposed in path P.sub.1. The additional 
resistance imposed in path P.sub.1 by unmetalized surface 319 controls or 
grades the adjacent parallel spark gap 330 and will determine at what 
voltage conduction will occur across the gap. Additionally, the 
unmetalized portion 319 also provides additional resistance to 
steady-state current which would tend to flow from conductive ring 326 
toward the center of MOV's 322, 324 along the upper and lower faces 318, 
320, thus ensuring that the steady-state current flows through MOV's 322, 
324 along the path denoted by arrows P.sub.1. When subassembly 320 is 
exposed to high transient voltages, spark gaps 330, 332, 334 will conduct 
and permit surge current to flow through assembly 320 along the path 
denoted by arrows P.sub.2. Contact surfaces 328 provide a uniform surge 
current density and allow substantially the entire cross-sectional area of 
MOV disks 322, 324 to be used in dissipating the surge energy. 
While the conductive standoffs between MOV's 322 and 324 and between the 
MOV's and adjacent electrodes 340, 342 have been shown and described as 
conductive rings 326, it will be understood by those skilled in the art 
that a variety of other conductive standoff means may be employed, such as 
a plurality of conductive cylinders, disks, cubes or the like. 
An alternative embodiment of an arrester subassembly having coaxial current 
paths is shown in FIG. 17. As shown in FIG. 17, arrester assembly 350 
comprises a series of MOV's stacked in columnar fashion with upper and 
lower plate electrodes 340, 342 as previously described. Two MOV's 352, 
354 are shown in FIG. 17, although as understood by those skilled in the 
art, several more MOV's may be employed in a given arrester, the total 
number being dependent upon the formulation of the metal oxide, the 
thickness of the MOV disks and the applied voltage level. 
Each MOV 352, 354 generally comprises a cylindrical base portion 358 and a 
crown portion 356. Base portion 358 includes upper face 360 and lower face 
362. Upper and lower faces 360, 362 include a central metalized contact 
surface 370 substantially identical to central contact surface 28 
previously described with reference to FIGS. 1 and 3. Crown 356 is formed 
along the periphery of upper face 360. As shown in FIG. 17, lower plate 
electrode 342 supports MOV 354 and is in electrical contact with base 
portion 358. Crown 356 of MOV 354 supports MOV 352, thereby creating 
series spark gap 374 between upper face 360 of MOV 354 and lower face 362 
of MOV 352. Similarly, crown 356 of MOV 352 supports upper plate electrode 
340 such that series spark gap 372 is formed between electrode 340 and 
upper face 360 of MOV 352. Crowns 356 of MOV's 352 and 354 include an 
upper surface 364 which is preferably left unmetalized to increase the 
series resistance to current flow through crown 356 along the steady-state 
current path denoted by arrows P.sub.1. The surge current path is denoted 
by arrows P.sub.2. In this embodiment, crown portions 356 formed on MOV's 
352, 354 perform the same function as combination of conductive rings 326 
and unmetalized portions 319 previously described with reference to FIG. 
16. More specifically, crown 356 provides an additional resistance to 
current flow in path P.sub.1, physically defines the dimensions of gaps 
372 and 374, and resistively grades or controls the spark-over of gaps 372 
and 374. As shown in FIG. 17, the steady-state current path is thus formed 
along the outer periphery of MOV's 352, 354, while surge-induced current 
is conducted through the central portion of the MOV's 352, 354 and across 
gaps 372, 374 along the path denoted by arrows P2. 
FIG. 18 shows another alternative embodiment of a multiple current path 
arrester of the present invention. Referring to FIG. 18, arrester assembly 
380 comprises a column of MOV elements 382, 384 stacked in series 
relationship along with plate electrodes 340, 342 and conductive rings 
326, electrodes 340, 342 and rings 326 being previously described with 
reference to FIG. 16. MOV 382 and 384 include a cylindrical base portion 
386 having upper face 390 and lower face 392. Formed atop upper face 390 
of MOV's 382, 384 is dome 388 preferably made from the same metal oxide 
formulation as used for base portion 386. Dome 388 includes metalized 
contact surface 394, leaving an unmetalized peripheral portion 391 on 
upper face 390. Lower face 392 of MOV 382 and 384 includes a central 
metalized contact surface 396 and an unmetalized peripheral portion 393. 
Metalized contact surfaces 394 and 396 are substantially identical to 
contact surface 28 previously described with reference to FIGS. 1 and 3. 
With the exception of the addition of dome 388 on its upper face, MOV's 
382, 384 are substantially identical to MOV 20, likewise described 
previously with reference to FIGS. 1 and 3. 
As shown in FIG. 18, lower electrode 342 supports and is in electrical 
contact with lower face 392 of MOV 384. Conductive ring 326 is disposed 
between MOV 384 and 382 along the opposing unmetalized peripheral portions 
391 and 393. An identical conductive ring 326 is disposed between MOV 382 
and upper electrode 340. In this arrangement, a series gap 398 is formed 
between electrode 340 and dome 388 of MOV 382. Likewise, spark gap 400 is 
formed between lower central contact surface 396 of MOV 382 and contact 
surface 394 of MOV 384. By varying the height of dome 388 and the 
thickness or height of conductive ring 326, the width of gaps 398 and 400 
may be controlled. 
Because there is no metalized contact surface between insulative ring 326 
and base portions 386 of MOV's 382, 384, a high resistance path is created 
for the steady-state current which flows along the path denoted by arrows 
P.sub.1. When a transient condition exists having a magnitude great enough 
to cause the arrester to operate, series gaps 398 and 400 will begin to 
conduct, permitting the resulting surge current to flow through arrester 
assembly 380 across spark gaps 398, 400 and through MOV's 382, 384 along 
the path denoted by arrows P.sub.2. 
FIG. 19 shows another alternative embodiment of an arrester of the present 
invention having coaxial current paths. Referring to FIG. 19, there is 
shown arrester assembly 410 which generally includes MOV's 412, 414, 416 
stacked in columnar fashion with electrodes 430, 340 and 342. Each MOV 
412, 414, 416 comprises a generally cylindrical base portion 418 and a 
crown portion 420. Base 418 includes upper face 422 and lower face 424. 
Crown 420 is formed on the periphery of upper face 422 of MOV's 412, 414, 
416. Upper face 422 of each MOV includes a metalized central contact 
surface 426. Similarly, the upper surface of crown 420 of each MOV is 
metalized at 428 and lower face 424 of each MOV includes a metalized 
contact surface 429. 
Electrode 430 is preferably deep drawn of brass but may be manufactured by 
other means and may comprise copper, aluminum or other conductive 
material. Electrode 430 generally comprises rim 432 and a set off central 
portion 434. 
The metalized surface 428 of each MOV crown 420 is in electrical contact 
with rim 432 of electrode 430. Rim 432 of the uppermost electrode 430 also 
supports upper plate electrode 340 creating a series current path 
therethrough. Lower plate electrode 342 is in electrical contact with 
contact surface 429 of lower face 424 of MOV 416. In this configuration, a 
spark gap 440 is formed between central portion 434 of each electrode 430 
and the contact surface 426 of the adjacent upper face 422 of MOV's 412, 
414, 416. 
In this embodiment of FIG. 19, the steady-state current path is denoted by 
arrows P.sub.1, the steady-state current generally flowing through 
assembly 410 along the outer periphery of MOV's 412, 414, 416 through rims 
432 of electrodes 430. The height of crowns 420 provides for a longer and 
thus more resistive path for the steady-state current P.sub.1 as compared 
to the path for the surge-induced current, denoted by arrows P.sub.2, and 
also provides the grading resistance for controlling the sparkover of gaps 
440. When a transient of sufficient magnitude occurs, spark gaps 440 will 
begin to conduct such that impulse current is conducted across gaps 440 
and through MOV's 412, 414, 416 along the path denoted by arrows P.sub.2. 
When the impulse current reaches the lower face 424 of MOV's 412, 414, 
416, it is conducted to the peripheral portion of face 424 by metalized 
contact surface 429 which provides for good electrical contact with 
electrode 430. In this manner, electrode 430 becomes energized and the 
spark gap 440 that is formed between the now-energized electrode 430 and 
the next adjacent MOV will begin to conduct. 
Another alternative embodiment of the present invention is shown in FIG. 
20. Referring to FIG. 20, there is shown an arrester subassembly 450 which 
generally comprises MOV 452 and electrode 462. Electrode 462 is identical 
to electrode 430 described above with reference to FIG. 19. MOV 452 
comprises cylindrical base portion 454 and a crown portion 456. Base 454 
includes upper face 458 and lower face 460. Crown 456 is formed on the 
periphery of upper face 458 of base portion 454. In this embodiment, crown 
456 is formed of a metal oxide that is different from the metal oxide used 
to form base portion 454. More specifically, crown portion 456 is 
preferably formed of a modified formulation of zinc oxide material having 
an exponent n within the range of approximately 5 to 10, and preferably 
10, at the steady state system voltage. By contrast, base portion 454 
comprises a metal oxide formulation having an exponent n within the range 
of about 10-25, and preferably about 20, at the steady state voltage. The 
resistance of crown 456 is used to control the voltage at which gap 470 
becomes conductive. 
Crown 456 includes an upper metalized face 457 which supports and 
electrically engages rim 464 of electrode 462. Lower face 460 includes a 
metalized contact surface 459 for engaging an adjacent electrode 462 which 
would be provided when assembly 450 was stacked in series with addition 
such subassemblies 450 in an arrester housing. Upper face 458 includes a 
central contact surface 461. As shown in FIG. 20, spark gap 470 is formed 
between central portion 466 of electrode 462 and contact surface 461 of 
MOV 452. Steady-state current is conducted along the path denoted by 
arrows P.sub.1. During surge conditions, gap 470 will conduct such that 
surge current is conducted through MOV 452 along the path denoted by 
arrows P.sub.2. 
FIG. 21 shows another alternative embodiment of the present invention 
including an MOV arrester subassembly 480 having coaxial, multiple current 
paths. Referring to FIG. 21, arrester subassembly 480 generally comprises 
MOV 482, electrode 430 (previously described with reference to FIG. 19) 
and semiconductor coating 494. As shown, MOV 482 includes a generally 
cylindrical base portion 484. Base portion 484 includes upper face 488 and 
lower face 490. Upper face 488 includes a central metalized contact 
surface 500. Lower face 490 of MOV 482 is metalized about its entire 
cross-sectional area forming contact surface 501. MOV 482 further includes 
a crown portion 486 formed on the periphery of upper face 488 of base 484. 
In this embodiment, crown portion 486 and base 484 are formed of the same 
metal oxides having n equal to approximately 20 at the steady-state system 
voltage. A semiconductive coating 494 made of zinc oxide or similar 
material is disposed about the outer surface 493 of MOV 482 and on top 
face 492 of crown 486. Rim 432 of electrode 430 is supported by and in 
electrical contact with semiconductive coating 494 atop crown 486. As 
shown in FIG. 21, series spark gap 502 is formed between central portion 
434 of electrode 430 and upper face 488 of MOV base 484. 
In this embodiment, the semiconductive coating 494 disposed between crown 
486 and electrode 430 provides added resistance in the path for 
steady-state current denoted by arrows P.sub.1. The added resistance 
provided by semiconductor 494 and the resistance of crown portion 486 
combine to provide resistive grading of gap 502. Surge current will be 
conducted across gap 502 and through MOV disk 482 along the path denoted 
by arrows P.sub.2 when gap 502 is caused to conduct by a transient 
overvoltage of magnitude equal to or greater than a predetermined 
magnitude. 
Referring briefly to FIG. 22, there is shown a modification which may be 
made to the MOV's previously described with reference to FIGS. 19-21. For 
purposes of example only, the modification will be described with 
reference to MOV disk 412 shown in FIG. 19. Referring to FIG. 22, MOV 412 
is shown including base portion 418 and crown 420. In this embodiment, MOV 
412 further includes a knee 506 formed at the junction of crown 420 and 
upper face 422 of base 418. Knee 506 comprises an ion generator understood 
by those skilled in the art as useful for sparkover stabilization. 
FIG. 23 shows another embodiment of an arrester assembly including coaxial 
current paths which is a part of the present invention. Referring to FIG. 
23, an arrester subassembly 510 is shown comprising MOV's 512, 513 stacked 
in columnar fashion. Each MOV 512, 513 generally comprises a cylindrical 
central portion 516, crown 514 and lower extension 518. In this 
embodiment, it is contemplated that crown 514, central portion 516 and 
lower extension 518 would all be comprised of the same metal oxide 
material such as a zinc oxide having an exponent n equal to 20; however, 
for added resistance in the steady-state current path denoted by arrows 
P.sub.1, crown 514 may be comprised of a more resistive metal oxide than 
that used to form portions 516 and 518. Central portion 516 of MOV's 512, 
513 include an upper face 520 having a central contact surface 521. 
Similarly, lower extension 518 includes a central metalized contact 
surface 523. 
Semiconductor coating 532 is applied around the outer surface of MOV's 512, 
513 and on upper face 526 of crown 514. Additionally, semiconductive 
material 532 is also applied on lower face 524 of central portion 516. As 
shown in FIG. 23, a series gap 530 is formed between contact surface 523 
of MOV 512 and contact surface 521 of MOV 513. 
The adjacent and contacting surfaces of semiconductor 532 between MOV's 512 
and 513 provides additional resistance in the steady-state current path 
denoted by arrows P.sub.1 and grades gap 530. As shown, steady-state 
current generally flows along the periphery of MOV's 512 and 513. When 
assembly 512 experiences a transient sufficient to cause series gap 530 to 
conduct, surge current flows through the central portion of MOV's 512, 513 
and across gap 530 along the path denoted by arrows P.sub.2. 
FIG. 24 shows still another alternative embodiment of the present 
invention. Referring to FIG. 24, there is shown an arrester assembly 540 
generally comprising substrate 542, MOV material 544, upper electrode 546 
and lower electrode 548. Substrate 542 generally comprises a cylindrical 
insulator made of porcelain or other insulative material. MOV material 544 
is disposed about the outer surface 550 of substrate 542 and preferably 
has a thickness of approximately 0.015-0.05 inches. MOV material 544 is 
formulated to have an exponent n within the range of approximately 10-25, 
and preferably about 20, at the system's steady-state voltage. As shown, a 
channel 552 is formed in spiral fashion through MOV material 544 along the 
entire length of assembly 540. This channel 552 has a depth substantially 
equal to the thickness of MOV material 544. In the preferred embodiment, 
arrester assembly 540 is manufactured with MOV material 544 completely 
covering substrate 550 and with channel 552 thereafter being formed 
through the MOV material 544. Alternatively, MOV material 544 may be 
applied to substrate 550 in a ribbon-like fashion, the voids between the 
adjacent edges of MOV material 544 thereby defining channel 552. Using 
either method of manufacture, series gaps 560 are formed across channel 
552 between the adjacent segments of MOV material 544. Upper and lower 
electrodes 546 and 548, respectively, are disposed at the ends of 
substrate 550 in electrical contact with the edges of MOV material 544 as 
shown at edge 554. 
In the steady-state condition, the current flowing through arrester 
assembly 540 is conducted through electrode 546 into MOV material 544 at 
edge 554. The steady-state current thereafter is conducted through the MOV 
material 544 to lower electrode 548 in a long spiral path as denoted by 
arrows P.sub.1. When arrester assembly 540 experiences an overvoltage 
condition of sufficient magnitude, series spark gaps 560 will become 
conductive such that the surge current is conducted through arrester 
assembly 540 through MOV material 544 in a direction substantially 
parallel to the longitudinal axis of arrester assembly 540 along the path 
denoted by arrows P.sub.2. 
While the preferred embodiments of this invention have been shown and 
described, modifications thereof can be made by one skilled in the art 
without departing from the spirit of the invention. The embodiments 
described herein are exemplary 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 protection is not 
limited by the above description, but is only limited by the claims which 
follow, that scope including all equivalents of the subject matter of the 
claims.