Voltage surge suppression power circuits

The utilization of clusters of two or more metal oxide varistors (MOVs) connected in parallel for suppressing surges and transients. Particular embodiments are disclosed for both grounded and ungrounded power systems. Also disclosed are two-stage and multi-stage uni-directional and bi-directional suppressors. Uni-directional means that the suppressor is intended to be placed between a source and a load for the purpose-of preventing voltage surges and transients from being propagated from the source into the load. Bi-directional suppressors are intended to be placed between a source and a load and provide suppression in both directions simultaneously.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of voltage surge protection and more 
particularly to the protection of equipment from large voltage surges and 
voltage transients such as result from lightning discharges and inductive 
load switching. 
2. Background Information 
Voltage surge and voltage transient suppressors are commonly used between a 
power source and its electrical load. Such suppressors protect the 
equipment from surges and transients or spikes as may occur on the power 
line due to switching of inductive loads on the power line or lightning 
strikes on the power line. In addition, surge suppressors prevent 
switching transients generated within a load from being reflected back 
into the power source and to other equipment. 
For certain applications, it is necessary that the surge suppressor meet 
the following characteristics. Spike voltages of amplitudes up to and 
including a 2500 volt peak as specified in MIL-STD-1399(NAVY), Section 
300A, and the standard 1.2.times.50 .mu.s, 6000 V and 8.times.20 .mu.s, 3 
kA voltage and current impulse waves respectively, specified in IEEE 
Standard 587-1980, must be attenuated to a level less than two times the 
peak voltage of the nominal system voltage. The suppressor must be 
effective in the frequency range of 2 kHz to 500 kHz. A series type 
suppressor must not cause more than 0.25% voltage drop at rated load and 
nominal operating line frequency. If more than one suppressor is used in a 
series arrangement, the total voltage drop of all units in series must be 
limited to 0.25% of the line voltage. A series type suppressor must be 
able to withstand repetitive inrush currents which, for example, in motor 
circuits can be six times the rated full load current. A shunt type 
suppressor must be capable of operating at rms voltage levels up to 121% 
of the nominal line voltage of the system being protected. The suppressor 
must be able to dissipate the energy contained in the spike as limited by 
the impedance of the source. The leakage or standby current drawn by the 
suppressor should be limited to 1% of the rated line current. The 
requirements of attenuating the spike voltage to a level less than two 
times the peak voltage of the nominal system voltage, and limiting the 
voltage drop across the suppressor to 0.25% of the line voltage, are 
particularly difficult to meet simultaneously. 
Several types of devices useful as surge suppressors are known in the prior 
art. These include gas tubes, silicon avalanche suppressors, capacitors, 
and metal oxide varistors (MOVs). 
A gas tube is basically a spark gap with the electrodes hermetically sealed 
in a gas-filled ceramic enclosure to lower the breakover (or breakdown) 
voltage. This type of device is small and inexpensive and has the 
capability of withstanding pulse currents up to 20000 amperes. When the 
device breaks over, the typical arc voltage ranges from 10 to 30 volts. 
However, the breakdown voltage of a spark gap device varies, for at a 
fixed set of conditions, the breakdown voltage is dependent on the rise 
time of the applied surge. For example, the typical sparkover voltage for 
a presently available gas tube rated for a 460 V.sub.ac application ranges 
from 1100 volts for a 100 volt per microsecond surge rise time to 1500 
volts for a 10 kilovolt per microsecond surge rise time. Note that these 
are typical breakover voltages which are subject to additional variations 
at distinct surge rise times. As a result, depending on the applied 
transient, several microseconds may elapse before a typical gas tube arcs 
over, leaving the leading portion of the surge intact to be passed on to 
the equipment operating on the power line. Although the gas tube diverts 
the majority of the surge current when it breaks over, the leading portion 
of the surge, frequently called a surge remnant, can contain a 
considerable amount of energy and have a high voltage amplitude. To clip 
the surge remnant, a common practice is to insert an L-section suppression 
circuit in the line following the gas tube. This circuit consists of a 
series impedance and a voltage clamping device, such as a MOV or a silicon 
avalanche suppressor, connected across the power line. The series 
impedance is connected between the gas tube and the clamp and can simply 
be a resistor or an inductor, or both; a resistor being suitable only for 
low voltage, low current applications. The impedance must be high enough 
in value to guarantee gas tube breakover so that the clamp only clips and 
diverts the energy in the remnant, not the energy in the entire surge. A 
major problem associated with gas tubes is "follow-on" current, the 
current from the power source which continues flowing through the gas tube 
after the surge current terminates. In ac circuits, the follow-on current 
clears when the line current goes through zero but the gas tube could be 
re-ignited on the next cycle. A typical gas tube is rated to handle a 60 
Hz, one-half cycle peak current of only 20 amperes, hence, if the power 
source can deliver much higher currents, i.e., a 460 V.sub.ac power line, 
the gas tube could be destroyed, particularly if it breaks over at the 
beginning of a cycle. In dc applications, a separate means for 
extinguishing the arc must be included in the circuit. Frequently, the 
follow-on current is limited to a safe value by connecting a low value 
resistor or a clamp such as a MOV in series with the gas tube. This 
technique, however, can significantly raise the clamping voltage if the 
surge current level is high. 
Silicon avalanche suppressors are essentially large junction zener diodes 
specifically designed for transient protection, functioning as a clamp and 
providing suppression in just a few nanoseconds. Presently, the major 
limitation of this device is its low energy dissipation capability as 
compared with gas tubes and MOV's. 
A capacitor placed across the power line is a simple form of surge filter. 
The impedance of the capacitor forms a voltage divider with the surge 
source impedance resulting in the attenuation of transients at high 
frequencies, the higher the capacitance value, the greater the 
attenuation. Frequently, an inductor is placed in series in the line 
before the capacitor to form an L-section low pass filter which is an 
effective transient suppressor. A bi-directional transient suppressor is 
formed by a T-section low-pass filter which has an inductor in line on 
either side of a shunt capacitor. This simple approach may have 
undesirable side effects such as: unwanted resonances with the inductive 
components located in the circuit; high in-rush currents during turn-on 
and switching; excessive reactive load on the power system; high leakage 
current, especially in 400 Hz applications when the capacitance value is 
high; and, high voltage drop across the inductors. To limit the standby 
current, an inductor is sometimes connected in parallel with the filter 
capacitor to form a tank circuit tuned to the power line frequency. 
Although this allows high values of 'shunt capacitors to be used which 
provides greater attenuation of transients, a circulating current flows in 
the tank circuit continuously. Depending on the component values selected, 
this circulating current may be quite high, resulting in substantial 
heating of these circuit elements. Usually, such tank circuit components 
are large and heavy. Also, the problems applicable to low pass filters 
described above are equally applicable to tank circuits. 
Metal Oxide Varistors (MOVs) are devices which clamp and are usually 
connected directly across a power line. The device does not clamp until a 
voltage transient (spike) occurs which exceeds the line voltage by a 
sufficient amount. As the voltage transient rises, the MOV nonlinear 
impedance results in a spike current through the device which rises faster 
than the voltage across it. This produces the clamping action of the 
device. The clamping voltage depends on the line impedance and the 
impedance of the voltage spike source. If the spike source and line 
impedance are low, the spike current through the MOV is high, and hence 
the clamping voltage is high. If the spike source and line impedances are 
high, both the spike current through the MOV and the clamping voltage are 
low. When the spike source impedance is very low, several thousand amperes 
can flow through the MOV. Although MOVs can handle currents of this 
magnitude, they can do so only for a limited number of times before the 
device fails. To reduce the surge current through the MOV and thereby the 
clamping voltage, an inductor is connected in series to form an L-section 
as with a capacitor. By adding an inductor in the line on either side of 
the MOV, a bi-directional T-section transient suppressor is formed. As the 
series inductance value is increased, the surge current through the MOV is 
decreased. This results in a lower clamping voltage, but at the cost of 
high line voltage drop across the inductors. Hence, with this approach, 
the ability to limit a spike voltage amplitude of up to 6000 V peak to a 
level of less than two times the peak voltage of the nominal system 
voltage, and the ability to limit the voltage drop to less than 0.25% at 
rated load, and nominal operating line frequency cannot be met 
simultaneously. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide voltage surge and transient 
suppressors using clusters of metal oxide varistors. 
Yet another object of this invention is to provide a bi-directional voltage 
surge and transient suppressor using clusters of metal oxide varistors in 
a manner that the suppressors may be easily paralleled to provide 
suppressors of higher current carrying capability with a very low line 
voltage drop. 
An additional object of this invention is to provide a surge and transient 
suppressor using clusters of metal oxide varistors to attenuate surges and 
transients to a level less than two times the peak voltage of the nominal 
system voltage. 
A further object of the present invention is to provide a surge and 
transient suppressor that is light in weight and compact in size. 
Briefly, this invention contemplates the provision of clusters of two or 
more MOV's in parallel for suppressing surges and transients. Particular 
embodiments are disclosed for both grounded and ungrounded power systems. 
Also disclosed are uni-directional and bi-directional suppressors. 
Uni-directional means that the suppressor is intended to be placed between 
a source and a load for the purpose of preventing voltage surges and 
transients from being propagated from the source into the load. 
Bi-directional suppressors are intended to be placed between a source and 
a load to provide suppression in both directions simultaneously. 
Suppressors in accordance with the present invention are characterized in 
that they are easily paralleled formulating suppressors of higher current 
carrying capacity and lower line voltage drop than one such circuit 
operating individually. Suppression is provided by appropriately placed 
clusters of metal oxide varistors. Connecting several MOVs in parallel 
makes the clamping voltage lower than that achieved with one MOV operating 
alone. Each MOV in a cluster is matched with each other MOV in the cluster 
by means of high current pulse tests. Paralleling MOVs of the same disc 
size and rms voltage rating, in a cluster, also increases the amount of 
surge energy that can be dissipated. Moreover, the number of high current 
surges that a cluster of MOVs can withstand is significantly greater than 
that of a single MOV. In turn, paralleling circuits of the present 
invention allows an increase in the amount of steady state current that 
can be applied to a load while limiting the line voltage drop to a low 
level.

DETAILED DESCRIPTION OF THE INVENTION 
This invention encompasses both uni-directional and bi-directional surge 
and transient suppressors. As used within this specification, 
uni-directional surge and transient suppressors protect a load from 
voltage surges and transients originating at the power source. A 
bi-directional surge suppressor of the present invention not only protects 
the load from surges, and transients from the supply, but also protects 
the supply from surges and transients that may be generated within the 
load and reflected back toward the source. 
Referring now to FIG. 1, a first embodiment of a hi-directional surge 
suppressor of the present invention is illustrated generally as 10. A 
bi-directional surge suppressor will protect the load from voltage surges 
generated by the power supply, but will also prevent any voltage surges 
generated in the load such as by switching of inductors, from being 
reflected back into the power source for transmission to other loads or 
especially in the case of an electronic power source, from damaging the 
power source. Surge suppressor 10 has input nodes or terminals 11 and 12 
and output nodes 13 and 14. A power source is represented generally as 15 
and is an ungrounded power source. When the power source is grounded, such 
that node 12 is grounded, alterations may be made in the circuitry as are 
explained below. Surge suppressor 10 is considered to be a multi-stage 
surge suppressor, with the input nodes of the surge suppressor 10 being 
nodes 11 and 12, and the output nodes of the surge suppressor 10 being 
nodes 13 and 14. Although a single stage will provide significant 
attenuation, a multi-stage configuration is preferred since it has been 
found that in most cases, multi-stages are required to attenuate the 
voltage surge to a level of less than two times the peak voltage of the 
nominal system voltage. 
Within surge suppressor 10, a cluster of MOVs, 32 and 42, are connected 
across the line at each end of the suppression circuit for reasons which 
will become apparent below. An MOV cluster comprises two or more discrete 
MOVs connected in parallel, as will be explained in more detail 
subsequently. The discrete parallel connected MOVs are illustrated 
schematically in connection with cluster 42 of FIG. 1. In the remaining 
Figures, a single MOV or a cluster of MOVs will be designated by the 
numerals 22 and 32 respectively, when connected between intermediate nodes 
16 and 17 and between output nodes 13 and 14. 
MOV clusters 32 and 42 preferably comprise at least three, preferably four, 
or possibly five individual MOVs connected in parallel to share the surge 
current. Connecting three or more smaller disc size MOVs of the same disc 
size and rms voltage rating in parallel, acts to reduce the clamping 
voltage of the cluster when the cluster is subjected to the voltage surge, 
as shall be explained in greater detail at the end of this description of 
operation. A parallel combination of inductor 23, damping resistor 26, and 
MOV 21 are connected between input node 11 and intermediate node 16, and a 
parallel combination of inductor 24, damping resistor 27 and MOV 20 are 
connected between input node 12 and intermediate node 17. A parallel 
combination of inductor 33, damping resistor 35, and MOV 31 are connected 
between intermediate node 16 and output node 13, and a parallel 
combination of inductor 34, damping resistor 36 and MOV 30 are connected 
between intermediate node 17 and output node 14. For the shunt position 
between intermediate nodes 16 and 17, a single MOV 22 is used so that only 
two line-to-line varistor clusters are ever required regardless of how 
many surge suppression circuits are paralleled. Series R-C filters 19 and 
19a are connected across the line at each end of the surge suppression 
circuit as shown in FIG. 1. 
For purposes of explanation, assume that a voltage surge, represented 
schematically as 25, originates from voltage source 15. Initially, 
inductors 23 and 24 do not pass any surge current. Four-MOV cluster 42 
consisting of, for example, V150LA20B MOVs is connected across the line at 
the voltage source end of the surge suppressor with no inductance between 
the surge source and the cluster, except for the power lines. Without 
inductor isolation, the surges are applied directly to the MOV cluster 42, 
thus preventing any large increases in impulse current width through this 
cluster. This four-MOV cluster 42 passes the bulk of the impulse current 
and sharply drops the surge voltage 25 before the surge reaches the 
parallel combination of the inductor 23 and 24, MOV 21 and 20, and the 
damping resistor 26 and 27. The remaining (or residual) impulse current is 
both reduced in amplitude and increased in width; first, by inductor pair 
23 and 24 and then by inductor pair 33 and 34. As a result, the clamping 
voltages across MOV 22 and MOV cluster 32 are successively lowered and the 
surge voltage at the load 37 is easily reduced to two times the peak 
voltage of the nominal system voltage, as required. A four-MOV cluster 
pair 28 and a four-MOV cluster pair 29 are connected from line-to-ground 
at both ends of the suppression circuit in order to suppress 
line-to-ground surges. The four-MOV cluster pairs 28 and 29 are comprised 
of V150LA20B varistors in case a power line is permanently or 
intermittently grounded. If this should happen, full line voltage will be 
present across the MOV cluster connected from the still ungrounded power 
line to ground. Series R-C filters 19 and 19a are across the line at each 
end of the surge suppression circuit in order to prevent a clamping 
voltage overshoot at the load and the power source, respectively. Air core 
inductors and noninductive resistors are preferred. The primary functions 
of damping resistors 26, 27, 35, and 36 are to reduce oscillations and 
ringing, and to limit the inductive voltage kick that occurs across 
inductors 23, 24, 33 and 34 when a rated load current is interrupted by 
opening switch 51. These damping resistors, however, will not limit the 
voltage across the aforementioned inductors to a sufficiently low value 
under fault conditions. Referring to FIG. 1, should either load L.sub.1 
or L.sub.2 fail shorted, the corresponding fuse 52 or 53 will blow after 
drawing very high fault current. If slow-blow fuses are used, the problem 
is exacerbated because the fault current will be even higher. If only 
circuit breaker protection 60 is present, which is the case in many 
instances, heavy fault current will flow before the circuit breaker trips. 
Returning to the fuse scenario: after a fuse opens, a voltage spike is 
developed across each line inductor which increases in amplitude in 
accordance with the equation V.sub.L =-di.sub.fc /dt, where i.sub.fc is 
the fault current that was interrupted. Even if the damping resistors are 
limited in value to several ohms, the sum of the voltages developed across 
each resistor-inductor pair will cause line-to-line varistor 22 and 
varistor clusters 32 and 42 to conduct and clamp these voltage spikes. As 
a result, the voltage across any other load operating on the power line 
will exceed the specified limit. 
To solve this problem, a V82ZA12 varistor 20, 21, 30 and 31 is connected in 
parallel with each inductor-resistor pair 24 and 27, 23 and 26, 34 and 36, 
and 33 and 35 respectively, as shown in FIG. 1. This MOV has an rms 
voltage rating of 50 V.sub.ac and a maximum specified clamping voltage of 
145 Volts at 50 amperes. At 150 amperes, the maximum specified clamping 
voltage is about 160 volts, hence, the total voltage across the load 37 
cannot exceed 320 volts and will be less due to the clamping action of MOV 
cluster 32 which protects the load. Moreover, depending on the magnitude 
of the load current, MOVs 20, 21, 30 and 31 provide better protection of 
the load than the damping resistors when a heavy load current is 
interrupted by switch 51. For 440 V.sub.ac applications, a V150ZA8 MOV 
should be used. Another advantage to this approach is that MOVs 20, 21, 30 
and 31 will remain inactive when a surge or transient originates from 
either the power source 15 or the load 37. When this occurs, MOV clusters 
42 or 32 clamp the transient or surge to such a low level that the voltage 
differential across each parallel inductor-resistor-MOV combination--which 
is one-half of the difference in clamping voltage developed between an 
external MOV cluster 42 or 32 and the centrally located MOV 22--is too low 
to cause MOVs 20, 21, 30 and 31 to conduct. 
As stated above, the voltage surge suppression circuit of FIG. 1 is 
intended for use on ungrounded power supply systems such as are found 
aboard U.S. Navy ships. In power supply systems where one line is grounded 
at input node 12, hereafter designated as common node 18, inductors 24 and 
34, resistors 27 and 36, four MOV clusters 29, and MOVs 20 and 30, as 
shown in FIG. 1, may be eliminated, resulting in a circuit configuration 
as shown in surge suppression circuit 10a in FIG. 2. In this instance, 
preferably, inductors 23a and 33a have twice the inductance value as 
inductors 23 and 33 of FIG. 1 when each of the circuits is designed for a 
similar steady state current carrying capacity. 
The circuits of FIG. 1 and FIG. 2 are readily adaptable for controlling 
voltage surges where the load current is in the 15 ampere range and load 
37 is supplied from a 50 to 60 Hz power source. At much higher loads, and 
when the power source frequency is 400 Hz such as that used on Navy ships 
and aircraft, the voltage drop across suppressor 10 will exceed the 0.25 
percent limit. In higher current and frequency applications, these 
limitations may be overcome by paralleling two or more substantially 
identical surge suppressors in a manner illustrated in FIG. 12 for 
suppressors 10 and 10a. As a more detailed example, additional suppressors 
such as illustrated by 10c in FIG. 13 can be added in parallel, as shown, 
to the bi-directional multi-stage surge suppressor for an ungrounded 
system of FIG. 1 to provide additional current carrying capacity while 
limiting the voltage drop to the specified level. Paralleling the 
suppressors effectively reduces the series inductance. If two 
substantially identical surge suppression circuits 10 and 10a are 
paralleled, the effective series inductance is cut in half and an equal 
amount of surge current and line current is drawn by each circuit 10 or 
10a. Therefore, since each series inductor is now carrying one-half the 
load, the cross sectional area of the conductor in inductors 23, 24, 33, 
and 34 in FIG. 1 and in inductors 23a and 33a in FIG. 2 can be made 
smaller, reducing the physical size of each inductor. Moreover, 
paralleling surge suppressors 10 or 10a engenders a reduced clamping 
voltage across each centrally located line-to-line MOV 22. Consequently, 
each individual inductor in the circuit may be reduced in value as the 
number of paralleled suppressors 10 or 10a is increased in order to 
provide a still further reduction in total series inductance and keep the 
line voltage drop across the surge suppressor within the required limit. 
For any selected number of paralleled suppressors 10 or 10a, the lowest 
permissible inductance value for each individual inductor in suppressors 
10 or 10a can be established by subjecting the paralleled circuits to a 
standard test wave, and verifying that the test surge is clamped to two 
times the peak voltage of the nominal system voltage, or less, by MOV 
cluster 32, as required. An important advantage derived from paralleling 
any number of suppressors 10 or 10a is that the centrally located MOVs 22 
in the paralleled sections form a nearly perfectly matched MOV cluster. 
Whichever direction the surge or transient emanates from, an outermost 
cluster 42 or 32 absorbs nearly all of the surge energy, as explained 
above. The remaining surge current is divided equally by the in-line 
inductor-resistor-varistor combinations in each of the paralleled units, 
an equal portion of the remaining surge current flowing through each 
centrally located MOV 22 with the small leftover surge currents remnants 
being summed in the MOV cluster farthest from the surge source. The surge 
current sharing by each centermost varistor 22 will not be perfect 
however, since each varistor 22 will have a slightly different clamping 
voltage. 
A modification of the circuit shown in FIG. 1 results in a uni-directional 
voltage surge suppression circuit 70 for an ungrounded power source, FIG. 
3, that is used primarily for protecting the load 37 from voltage surges 
and transients 25 originating from the source 15. The uni-directional 
circuit configuration is basically the same as the bi-directional version, 
except for a few changes. The line-to-line R-C filter at the power source 
has been eliminated, the four-MOV cluster protecting the load has been 
replaced with a single V150LA20B varistor, and the four-MOV clusters 
connected from each line to ground at the load have been replaced with 
single V150LA20B varistors. For FIG. 3 and the remaining FIGS. 5, 8 and 
10, single MOVs connected from each line to ground at the load are 
identified by the symbol 39. Although this circuit configuration has been 
classified as a uni-directional voltage surge suppressor in this patent 
specification, it also functions to protect the source 15 from surges and 
transients generated by the load 37. However, since only a single MOV 
instead of an MOV cluster is now connected line-to-line at the load end of 
surge suppressor 70, MOV cluster 42, in FIG. 3, clamps at a higher voltage 
level than MOV cluster 42, in FIG. 1, under identical load generated surge 
voltage conditions. 
FIG. 4 shows a uni-directional multi-stage surge suppressor circuit for use 
in grounded power systems. Inductors 24 and 34, resistors 27 and 36, MOVs 
39 and MOVs 20 and 30, as shown in FIG. 3, may be eliminated and replaced 
with short circuits resulting in a circuit configuration as shown in FIG. 
4. In this instance, preferably, inductors 23a and 33a have twice the 
inductance value as inductors 23 and 33 of FIG. 1 when each of the 
circuits is designed for a similar steady state current carrying capacity. 
FIG. 5 shows an alternative uni-directional multi-stage voltage and 
transient surge suppressor. In this embodiment, MOV cluster 42 has been 
eliminated and an MOV cluster 32 has been connected between intermediate 
nodes 16 and 17 in place of single MOV 22. With MOV cluster 42 removed, 
line-to-ground clusters 28 and 29, which are effectively connected in 
series across the power line, act as a substitute for the eliminated 
cluster 42 and clamp impulse voltage 25 to a voltage level which is twice 
that of cluster 42 in FIG. 3. Since the clamping voltage at this first 
stage in the suppression circuit is double that of the preferred 
uni-directional embodiment, the remaining impulse current passed on to the 
succeeding suppression stages is higher than in the preferred embodiment. 
To equal the performance of the preferred uni-direction embodiment, the 
following circuit modifications are made. First, an MOV cluster 32 is 
connected between intermediate nodes 16 and 17 in place of a single MOV 22 
to absorb the additional surge energy and to effect a reduction in 
clamping voltage. Recall that a cluster of matched MOVs has a lower 
clamping voltage than a single MOV, as stated above. Secondly, the 
inductance value of inductors 23b and 24b is increased to reduce the 
remaining impulse current amplitude, which results in an additional 
decrease in the clamping voltage of MOV cluster 32. The inductance value 
of inductors 33b and 34b in the final suppression stage is proportionally 
reduced so that the sum of the inductances of inductors 23b, 24b, 33b and 
34b in FIG. 5 is equal to that of the inductors 23, 24, 33 and 34 in the 
preferred embodiment. With these changes, the desired performance 
characteristics are achieved by this alterative uni-directional voltage 
surge suppression circuit. The alternative suppression circuit does, 
however, have two drawbacks which are not present in the preferred 
embodiment. First, when a number of alternative uni-directional units are 
paralleled like the configurations in FIGS. 1 through 4 to limit the line 
voltage drop, the number of line-to-line MOV clusters 32 required equals 
the number of paralleled units, whereas in the preferred uni-directional 
embodiment only one line-to-line MOV cluster is ever needed, regardless of 
the number of paralleled units. Secondly, under surge voltage conditions 
in 440 V.sub.ac applications, the voltage differential across the 
paralleled inductor-resistor-MOV combinations, 23b, 26 and 21, and 24b, 27 
and 20, preceding the line-to-line MOV cluster 32 will be higher than that 
across the corresponding combinations in the preferred embodiment, so 
damping resistors with an adequate working voltage rating are needed here. 
As in the case of the voltage surge suppression circuit configurations 
shown in FIGS. 1 through 4, when several of the alternative units are 
paralleled, the surge current is divided equally among the parallel paths, 
whether the surge originates from the source 15 or the load 37. 
Consequently, the matched MOV clusters 32 connected between intermediate 
nodes 16 and 17 in each of the paralleled alternate units are also closely 
matched to each other with no selection process required. Each centrally 
located line-to-line cluster 32 will have a slightly different clamping 
voltage, so the current sharing between clusters will not be identical. 
Recall that this characteristic is also present in the centrally located 
MOVs 22 in the surge suppression circuits of FIGS. 1 through 4 when these 
units are similarly paralleled, as explained above. 
For rated load current levels below ten amperes, the uni- and 
bi-directional surge suppression circuits in FIGS. 6 through 10 can meet 
the required line voltage drop and clamping voltage limits using only two 
surge suppression stages, as shown. The suppressors in FIGS. 6 through 10 
are basically two-stage versions of FIGS. 1 through 5 respectively, as a 
comparison between the corresponding drawings readily shows. In each of 
these suppressors, the total series inductance is made higher than the 
total inductance of the corresponding multi-stage configuration. The 
increased series inductance effects a greater reduction in the magnitude 
of the residual surge current passed on to the second suppression stage 
than occurs in the multi-stage units, therefore, only one additional 
suppression stage is needed in these suppressor configurations to limit 
the surge voltage at the load to two times the peak amplitude of the 
nominal system voltage, as required. Although the increased series 
inductance causes an increase in the voltage drop across the two-stage 
suppression circuits presented in FIGS. 6 through 10, this increase is 
effectively negated by the reduced voltage drop across these suppressors 
due to the lower load current. 
FIG. 6 is an ungrounded bi-directional two-stage surge suppressor. This 
embodiment is similar to FIG. 1. Inductors 33 and 34, damping resistors 35 
and 36, MOVs 30 and 31, and MOV 22, components which essentially comprise 
a surge suppression stage, have been eliminated. Inductors 23 and 24 have 
been replaced by inductors 23c and 24c respectively, each of which has an 
inductance value greater than two times the inductance value of inductors 
23 and 24, as explained above. 
FIG. 7 is a grounded hi-directional two-stage surge suppressor. This 
embodiment is similar to FIG. 2. Here, inductor 33a, damping resistor 35, 
and MOVs 22 and 31 have been eliminated. Inductor 23a has been replaced by 
inductor 23d which has an inductance value greater than twice that of 
inductor 23a. 
FIG. 8 is an ungrounded uni-directional two-stage surge suppressor. This 
embodiment is similar to FIG. 3. Inductors 33 and 34, damping resistors 35 
and 36, and MOVs 22, 30 and 31 have been eliminated. As in FIG. 6, 
inductors 23 and 24 have been replaced by inductors 23c and 24c 
respectively, each of which has an inductance value greater than two times 
the inductance value of the replaced inductors. At the load, either an MOV 
22 or an MOV cluster 32 can be connected across the line, and either a 
pair of MOV clusters 29 or a pair of individual MOVs 39 can be connected 
from each line to ground, as shown. 
FIG. 9 is a grounded uni-directional two-stage suppressor. This embodiment 
is similar to FIG. 4. Inductor 33a, damping resistor 35, and MOVs 22 and 
31 have been eliminated. As in FIG. 7, inductor 23a has been replaced by 
inductor 23d which has an inductance value greater than twice that of 
inductor 23a. At the load, either an MOV 22 or an MOV cluster 32 can be 
connected across the line. 
FIG. 10 shows an alternative uni-directional two-stage surge suppressor. 
This embodiment is similar to and functions like the alternative 
uni-directional multi-stage surge suppressor shown in FIG. 5. The 
electrical circuit configuration is the same as that of FIG. 8, except 
that MOV cluster 42 has been eliminated. 
The most simple surge suppressor is to just use MOV clusters with four, or 
perhaps five, paralleled metal oxide varistors as shown in FIG. 11. This 
totally eliminates the problems with: line voltage drop, paralleling 
multi-stage units, damping resistors, circuit complexity, power line 
frequency, size and weight, ad infinitum. Although this embodiment cannot 
limit the voltage surge 25 at a load 37 to two times the peak amplitude of 
the nominal system voltage, a cluster of four closely matched 20 mm 
diameter V150LA20B MOVs connected in parallel, will clamp voltage surge 25 
to a maximum peak voltage of 430 volts or less as shown in FIG. 14; a 
sufficiently low voltage level for the majority of other applications with 
less stringent requirements. 
The voltage surge and transient suppression circuits presented in FIGS. 1 
through 10, 12 and 13 cannot limit the line voltage drop across them to 
0.25%, while simultaneously limiting spike and surge voltages at the load 
to a level of two times the peak voltage of the nominal system voltage or 
less, without utilizing appropriately placed clusters of paralleled 
smaller diameter MOVs, as shown. FIG. 14 lists the maximum specified 
clamping voltage, V.sub.c, and other characteristics of selected 
individual and paralleled 150 V.sub.ac MOVs with disc sizes ranging from 
20 mm to 60 mm in diameter when these devices are subjected to a standard 
3000 ampere 8.times.20 impulse current wave as recommended by IEEE 
standard 587-1980 (1980), "IEEE Guide for Surge Voltages in Low-Voltage AC 
Power Circuits". The listed clamping voltages were obtained from the MOV 
characteristics published by the manufacturer of these devices. In 
general, the larger the disc size, the lower the clamping voltage for a 
specific impulse current magnitude. Also, for any MOV, the clamping 
voltage decreases as the magnitude of the pulse current through the device 
decreases. This property means that for a specific impulse current 
magnitude, connecting several of the same model number MOVs in parallel to 
form a cluster will result in a significantly lower clamping voltage than 
that of a single MOV of the same type if the devices in the cluster are 
matched to ensure reasonably good current sharing. This is clearly shown 
in FIG. 14, wherein a single V150LA20B MOV has a maximum clamping voltage 
of 540 volts when subjected to a 3 kA peak current surge whereas a 
perfectly matched cluster of four paralleled V150LA20B MOVs has a maximum 
clamping voltage of 430 volts under this same test condition, a 
considerable decrease in clamping voltage of 110 volts. An additional 
advantage gained from using clusters of four matched 20 mm MOVs is that 
the lifetime number of 8.times.20, 3 kA current surges such a cluster can 
withstand is considerably greater than that of a single 20 mm MOV; 
approximately 150 vs. 4 respectively, as shown in FIG. 14. 
From a comparison of the current, energy, and lifetime current pulse 
ratings given in FIG. 14, it is readily apparent that for all practical 
purposes, a cluster comprised of four matched 20 mm disc size V150LA20B 
MOVs connected in parallel is the equivalent of a single 40 mm disc size 
V151DA40 or a V151DB40 MOV. Similarly, a cluster of three matched 
V150LA20B MOVs is equivalent to a 32 mm disc size V150HE150 MOV. Moreover, 
the maximum clamping voltages of the four- and three-MOV clusters 
comprised of 20 mm devices are 430 volts and 450 volts respectively, 
compared to 530 volts for both the 40 mm and 32 mmMOVs at a peak impulse 
current of 3 kA. In fact, the maximum clamping voltage of a cluster of 
four matched model number V150LA20B MOVs is 50 volts lower than that of a 
single V151BA60 or V151BB60 MOV, the largest available MOVs which have a 
disc diameter of 60 mm. An individual 60 mm MOV does, however, have a 
significantly higher maximum peak current and energy rating than a 
four-MOV cluster comprised of 20 mm devices and can also withstand a 
greater number of 3 kA, 8.times.20 current impulses; approximately 500 vs. 
150 for a 20 mm four-MOV cluster as estimated from the manufacturer's data 
sheets. 
Incrementally increasing the number, n, of MOVs in a cluster will result in 
an ever lower clamping voltage, but the reduction in V.sub.c slows rapidly 
as the number of MOVs grows large. This occurs because the reduction in 
peak current, I.sub.p, through each MOV in a cluster diminishes in 
accordance with the formula .DELTA.I =I.sub.p /n+1-I.sub.p /n whenever 
another device is added to the cluster. With a 3 kA impulse current for 
example (refer to FIG. 14), when the number of MOVs is increased from two 
to three, the shared impulse current through each device drops from 1500 
to 1000 amperes, a significant 500 ampere reduction. This yields a drop in 
maximum clamping voltage of 30 volts, from 480 to 450 volts, for the 
cluster containing V150LA20B MOVs. Should the number of MOVs in this 
cluster be increased, say, from ten to eleven, the impulse current through 
each device in the cluster drops from 300 to 272 amperes, a 28 ampere 
decline. According to the published characteristics of these particular 
devices, this will yield a minuscule clamping voltage reduction of 5 volts 
or less. A cluster with a large number of MOVs will also be both difficult 
and costly to match; cluster cost, size and complexity would also 
increase. Therefore, an optimum MOV cluster consists of a minimum of 
three, preferably four, or a maximum of five 20 mm V150LA20B type MOVs 
connected in parallel. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modification within the spirit and scope of the appended 
claims.