(First presented as PE-114-PWRD-0-12-1997) 

Significance 

Part 8 – Coordination of cascaded SPDs 

The goal of implementing a well-coordinated cascade of SPDs with simple MOVs at both the service entrance of a 

building and point-of-use (the latter typically by an add-on plug-in SPDs typical of what consumers purchase from 

electronic stores – the so-called “TVSS”) presents a dilemma because the service entrance arresters tend to be 

designed with conservative MCOV ratings (hence relatively high limiting voltages) while the TVSSs tend to be 

designed with the lowest possible limiting voltage. Such relationship in the limiting voltages is the contrary of what 

is necessary to achieve coordination between the rugged service entrance arrester and the limited energy-handling 

capability of the TVSS. 

The situation has been created by the decision, early in the introduction of TVSSs and possibly motivated by the UL 

requirement to show the limiting voltage (with a misguided notion that a lower limiting voltage ensures better 

protection). By now, this de facto presence of millions of low limiting voltage for the TVSS makes it practically 

impossible to achieve coordination if the twp SPDs consist of simple MOVs. 

Ironically, upon introduction of MOVs in the mid-seventies, residential-type service entrance arresters that consisted 

of a series combination of a gap and a silicon carbide varistor were replaced by simple MOV discs, viewed at the 

time as a significant improvement of the protective level provided by a service entrance arrester – hence the 

“revisited” aspect of this paper. 

A solution to this dilemma might be to design the service entrance as a gapped arrester that can relieve the TVSS 

from the major part of the energy-dissipation stress, while the de facto TVSS can still provide point-of-use surge 

protection for the connected loads. 

This paper was designated “High Interest Paper” by the Power Engineering Society 

Filename: Gapped arresters
1174 IEEE Transactions on Power Delivery, Vol. 13, No. 4, October 1998 

Gapped Arresters Revisited : A Solution to Cascade Coordination 

Arshad Mansoor, Member, IEEE Fran~ois D. Martzloff, Fellow, IEEE Kermit 0. Phipps Member, IEEE 

Power Electronics National Institute of Power Electronics 

Applications Center Standards and Technology * Applications Center 

Knoxville TN Gaithersburg MD Knoxville TN 

Abstract - This paper provides a brief perspective on how the 

coordination of cascaded surge-protective devices (SPDs) has become an issue. We propose an approach where the 'ancient' technology of 

gapped arresters may well be the answer to the dilemma of the 

incompatibility of a service-entrance SPD having relatively high 

limiting voltage with the proliferation of built-in or plug-in SPDs 

having relatively low limiting voltage inside the buildings. The 

solution involves providing a gapped arrester at the service entrance 

and gapless SPDs inside the building. An example is given of such a 

combination, with experimental verification of the proposed solution 

and computer modeling that allows a parametric evaluation of the 

significant factors in any candidate combination of SPDs. 

I. INTRODUCTION 

A quarter of a century ago, metal-oxide varistors 

("MOVs"), initially developed as electronic components [I], 

121, were introduced to power-system applications and were 

promptly hailed as the revolutionary technology that would 

make possible the elimination of gaps in surge arresters and 

surge-protective devices (SPDs) in general [3]. The conventional arresters at that time combined a gap with a silicon 

carbide (Sic) varistor disc because the I-V characteristic of 

silicon carbide, for the desired protection level under surge 

conditions, resulted in excessive standby current under the 

normal power system conditions. 

For the high-voltage surge arresters, this Sic varistor-gap 

combination had reached great sophistication in the development of gap structures and construction with modular elements. 

For low-voltage applications, one Sic varistor disc and one gap 

were sufficient for the arrester function, but only a few of that 

type were used in residential applications. The gap sparkover 

characteristics made the device adequate enough for insulation 

protection but not effective for the protection of the emerging 

solid-state appliances [4]. Thus, a market was opened for allMOV arresters to replace Sic-based gapped arresters and, as 

the cliche goes, the rest is history. 

" Electricity Division, Electronics and Electrical Engineering Laboratory, 

Technology Administrution, U.S. Department of Commerce. 

Contributions,from the National Institute and Technology are not subject 

to U.S. Copyright. 

PE-114-PWRD-0-12-1997 A paper recommended and approved by the 

IEEE Surge Protective Devices Committee of the IEEE Power 

Engineering Society for publication in the IEEE Transactions on Power 

Delivery. Manuscript submitted June 9, 1997; made available for 

printing December 12, 1997. 

However, this apparently happy state of affairs with the 

new, improved, MOV-based gapless SPDs is not the end of the 

story. Arresters developed with electric utility applications in 

mind were designed by specialists with strong motivation to 

ensure a reliable, long-life and ultimately cost-effective 

application of their products. This philosophy included due 

consideration of the maximum continuous operating voltage 

(MCOV), where the drive for low protection levels was 

tempered by the need to survive the variations and extremes of 

the power system environment. This criterion was well 

understood by utilities and manufacturers. 

In this paper, we propose to show the opportunity to 

revive - revisit - the approach of a gapped arrester that was 

all but abandoned, as a possible solution to the dilemma of 

coordination between an arrester designed with a prudent and 

conservative MCOV at the service entrance, and the many 

SPDs proliferating inside the building and having a de facto 

low limiting voltage. This paper is not a product announcement 

but is an invitation to both manufacturers and users to recognize 

the opportunity and develop a viable product based on this 

revisited approach. We only suggest that an appropriate 

coordination is possible between an arrester capable of 

withstanding high temporary overvoltages, according to utility 

practice, and the small, de facto SPDs inside the building. We 

leave the actual product design to the ingenuity and skill of SPD 

manufacturers responding to the need of the utilities. 

11. THE RACE FOR LOWEST PROTECTION LEVEL 

Those designs are now found throughout utility systems, 

down to the service entrance of the end-user customers. 

Meanwhile, the designers of appliances, driven by the economic 

pressures of mass production, had selected solid-state components with relatively low surge immunity. This fateful design 

and marketing decision led to the need for adding surgeprotective devices at the equipment level (incorporated at the 

power port of the appliance), or as an interface plug-in device 

separately purchased and installed by the end-user. There, the 

motivation became one of offering the lowest conceivable 

protection level, for instance 330 V for 120-V applications [5]. 

However, some of the implications of this race for the lowest 

protection level were not fully recognized [6]. 

Now, an additional concern is emerging as the idea of the 

so-called "wholehouse surge protection" is gaining popularity. 

In that scheme, a relatively large SPD is installed at the service 

entrance and additional, smaller SPDs are installed inside the 

building to complement the first line of protection provided at 

the service entrance. The service-entrance arrester would be a 

simple (gapless) varistor SPD, based on the conservative 

0885-8977/98/$10.00 O 1997 IEEE 
approach of the utilities (sufficient MCOV, hence medium level 

limiting voltage for the SPD). However the de facto situation 

inside the building is the uncontrolled proliferation of small 

SPDs with low limiting voltage. Note that given the uncoordinated status of cascaded SPDs, it would be pointless to try and 

pin down precisely the qualifiers of 'high', 'medium' and 'low' 

limiting voltage. The point is only to indicate a relative level. 

This situation is uncontrolled because the design and 

surge immunity of appliances has not benefitted from generic 

standards on surge immunity. The result is that the small SPDs 

can in fact 'protect' the service entrance arrester and invite the 

largest part of an impinging surge to pass by the entrance 

arrester - intended to divert the large surges but by-passed - to be dissipated into the small devices - that might not be 

suitable for the large surge. 

At this point of our discussion, we deliberately use the 

vague qualifier "large" to refer to the size and energy-handling 

capability of an SPD and to the stress threat of the impinging 

surge [7]. An additional concern is that inviting the flow of 

large surge currents inside the building has adverse side effects 

from the electromagnetic compatibility (EMC) point of view by 

shifting the potential of signal reference points associated with 

the equipment grounding conductors [8]. 

111. EMERGENCE OF COORDINATION ISSUES 

These emerging issues led to the recognition of "Cascade 

Coordination" as an important objective for the application of 

SPDs. A coordinated cascade is the parallel connection of two 

or more SPDs across the line, one upstream and one or more 

downstream, each with voltage limiting characteristics that 

ensure sharing of the surge energy in a ratio commensurate with 

the energy-handling capability of each SPD. 

The stage was set nearly two decades ago, with the 

publication of IEC Report 664 on insulation coordination [9] 

proposing "Installation Categories" with a descending staircase 

of voltages from the service entrance to the end of the branch 

circuits in a building. That concept was valid at the time, based 

on the availability of conventional arresters using a siliconcarbide varistor in series with a gap. Consequently, equipment 

manufacturers, including manufacturers of SPDs, became 

biased toward a philosophy that advocated higher limiting 

voltage at the service entrance and progressively lower limiting 

voltages inside the building. 

It took some time and several contributions from 

independent researchers to recognize that this downward 

staircase cannot be implemented by a cascade of parallelconnected, varistor-type SPDs, even if separated by some 

distance along the wiring from the service entrance to the end 

of the branch circuits. This reality was first discussed in several 

unpublished committee working papers before a rush of 

published papers brought the realization into the open [lo], 

[ll], [12], [13], [14]. It turns out that SPDs included in 

equipment or added by users have lower lirmting voltages than 

all-varistor SPDs installed at the service entrance and thus 

unintentionally "protect" the service entrance SPD by attracting 

the surge current to the device with the lowest limiting voltage. 

IV. A POSSIBLE SOLUTION: RETURN 

TO A GAPPED ARRESTER 

This gapped arrester will use a varistor with a limiting 

voltage lower than that of the downstream SPDs (in all the 

following text, "varistor" is to be understood as short-hand for 

metal-oxide varistor). The gap in series prevents steady-state 

application of the line voltage which the varistor cannot sustain 

for more than one half-cycle. An impinging surge will cause 

the gap to spark over, inserting the low-limiting varistor ahead 

of the downstream varistors. We have postulated that by 

appropriate selection and design of the gap, the powerfrequency current which will flow in the varistor after the surge 

will be cleared by the gap at the first natural current zero. 

4.1 Criteria for coordination 

The basic principle of coordination for a cascade is that 

the two SPDs - for instance one upstream at the service 

entrance, and one downstream at the end of a branch circuit or 

incorporated in the connected equipment - are decoupled from 

each other by some impedance. With a gapped arrester at the 

service entrance with a varistor with limiting voltage lower than 

that of the downstream SPlDs can serve as the most attractive 

SPDs in the cascade and thus divert the surge current away 

from internal branch circuits after the gap has sparked over. 

The gap can also serve to provide a higher MCOV and allow 

the arrester to survive the loss of neutral in a 1201240-V system. 

4.2 Experimental verification 

To demonstrate that it. is possible to obtain a satisfactory 

coordination, we used our replica of a residential wiring system 

[8], connecting two of its branch circuits, one 4.5 m long, the 

other 36 m long (Figure 1). We then installed a gap-varistor 

combination at the service entrance of the replica and a 

downstream varistor either at the end of the 4.5-m branch circuit 

or at the end of the 36-m branch circuit. Figure 1 shows the 

configuration of the circuit and defines the various current and 

voltages that will be cited in reporting the results. 

I, : Current delivered by the generator V, : Voltage at arrester 

I, : Current flowing in gapped arrester V, :Voltage of SPD when at O 

I, : Current flowing in SPD when at O V, :Voltage of SPD when at O 

I, . Gurrent flowlng in WD when at W YGi ~wltage acirom gap 

Figure 1 - Test circuit for experimental verification of coordination 

between a gapped arrester installed at the service entrance (Position@) 

and an SPD installed at the end of branch circuits (Positions @ or @) 
In our replica, the power wiring uses the conventional 

non-metallic jacket, 2-conductor plus equipment grounding 

conductor (2 mm dia., AWG #12). The gapped arrester, suitable 

for a 1201240-V system voltage, consisted of a varistor in series 

with a gas tube. The downstream SPD was a typical varistor 

used in plug-in SPDs, rated 130 V rms [15], [16]. 

The surge, applied at the service entrance of the replica, 

was produced by a generator capable of delivering a 6 kV, 

1.2150 ps open-circuit voltage or a 5 kA, 8/20 ys short-circuit 

current, as described in IEEE C62.41-1991 [17]. Suitable t 

differential voltage probes and current-viewing transformers 

were used to monitor voltages and currents during a surge 

event. Tests were conducted in accordance with procedures 

described in IEEE C62.45-1987 [18]. Instruments used for 

measurements are listed in the appendix, which also includes, 

as a contribution toward the updating of C62.45, examples of 

pitfalls in interpretation of digital oscilloscope recordings. 

Aware of the fact that the critical point for coordination is 

not the maximum surge current that may be encountered in the 

application, but some intermediate current for which the 

transition occurs as the gap first sparks over, we sought that 

transition point for each of the line lengths considered in the 

experiment. We would expect that in the case of the short 

decoupling line, it would be more difficult to produce coordination for a given combination of downstream limiting voltage 

and gap sparkover, as the inductive drop would be smaller than 

in the case of the longer line. Nevertheless, we made both 

experiments because the long line, for which coordination is 

easier, creates other problems, as we will see later. 

Figures 2,3, and 4 show respectively, for the case of the 

long branch circuit, the transition from no gap sparkover to gap 

sparkover, occurring first on the tail of the wave, then on the 

front of the wave as the impinging surge current is raised. 

In Figure 2, the 700-V voltage developed across the 

arrester is insufficient to sparkover the gap, and all the applied 

current (140 A peak) goes to the downstream varistor. In the 

experiment where the current I, reflects the interaction of the 

circuit with the generator, the current is reduced by the 

impedance of the long branch circuit; compared with the larger 

1, (440 A) of Figure 3 after gap sparkover. In the real world 

where the impinging surge is a current source, there would not 

be that reduction of the surge current and all of the impinging 

current, unimpeded, would be forced into the downstream 

varistor and flow in the branch circuit, an EMC problem [8]. 

t The measurements reported in this paper have been made with 

&~r,-,t,r;~, f wJ&-1% Ae --b;,,,d -,,ta;mty should mot ,eed 

k5% to 6%. Given the process of applying the measurement results 

to the response of surge-protective devices exposed to environments 

with characteristics that are at best known within an order of 

magnitude, this level of uncertainty does not affect the practical 

conclusions. 

Certain commercial instruments are identified in the appendix list of 

instrumentation in order to adequately describe the test procedure. 

Such ident$cation does not imply recommendation or endorsement by 

the National Institute of Standards and Technology, nor does it imply 

that these instruments are necessarily the best for the purpose. 

From top to bottom traces (5 psldiv sweep): 

V, - 200 Vidiv: 700 V peak 

I, - 100 Ndiv: 140 A peak 

I, - 100 Ndiv: No current in arrester 

l3 - 100 Ndiv: 140 A peak (= lo) 

Figure 2 - Voltage and currents for a surge producing 

a voltage lower than gap sparkover (long branch circuit) 

In Figure 3, the 750-V level developed across the arrester 

is sufficient to cause sparkover of the gap, but still in the tail of 

the wave, 4 ps into the surge. This sparkover transfers the 

impinging current to the upstream arrester, limiting the rise of 

current into the downstream varistor at 65 A instead of 140 A. 

The only stress left on the downstream varistor is to 

slowly discharge the energy stored in the 36-m branch circuit by 

the initial rise of current. Note the sudden increase in I, at 4 ys 

as the load impedance presented to the generator changes from 

36 m of cable to the short path between generator and upstream 

arrester. 

From top to bottom traces (5 psldiv sweep): 

V, - 200 Vldiv: 750 V peak 

I, - 200 Ndiv: 440 A peak 

I, - 200 Ndiv: 380 A peak 

I, - I00 Ndiv: 65 A peak 

Figure 3 - Voltage and currents for a surge producing a 

voltage causing gap sparkover on the tail (long branch circuit) 
With the current rise shut off in the downstream varistor 

as the upstream arrester starts conducting, the current in the 

downstream varistor is then limited to 65 A: a greater surge 

current results in less current in the downstream varistor after 

the transition of current levels from no gap sparkover to gap 

sparkover: "more begets less!" [19]. 

In Figure 4, the larger applied surge (1450 A) results in 

the gap sparking over on the front of the wave, with very little 

delay to allow only the beginning of current build-up in the 

downstream varistor. However, the higher voltage after sparkover (400 V, compared to 350 V in Figure 3) produces further 

increase in the current I, an increase that does not stop until the 

voltage V, falls below 350 V, 15 ps into the surge. This figure 

was recorded to show the complete event, including the end of 

the current pulse, and provide a comparison with Figure 2 and 

Figure 3 at the same sweep rate. As discussed in the Appendix, 

the sharp spike at the front of the voltage trace must arouse 

suspicions that the digital oscilloscope might have missed the 

peak because the need of displaying a SO ps window means that 

the resulting sampling rate, reflecting the memory size, is not 

sufficient to resolve the peak. The value of this figure is then 

limited to indicating current values and the timing of events, but 

not the peak of the voltage spike. 

From top to bottom traces (5 pldiv sweep): 

V1 - 200 Vldiv: Not resolved - See Appendix 

I, - 500 Ndiv: 1450 A peak 

I, - 500 Ndiv: 1400 A peak 

I, - 100 Ndiv: 50 A peak 

Figure 4 - Timing of sparkover and currents for a surge producing 

sparkover of the gap on the front of the wave (long branch circuit) 

Turning now to the case of the SPD connected at the end 

of the short (4.5 m) branch circuit, Figure 5 shows the transition 

from no sparkover to sparkover. In this example, the sparkover 

occurs early in the tail of the wave. Instead of the spike shown 

in Figure 4, the occurrence of the sparkover in the tail provides 

suffi~ienf dats p~ints to obtain a v&d display of the voltage. 

In this more difficult coordination scenario (smaller 

decoupling impedance afforded by the short branch circuit), the 

build-up of the current I, in the downstream varistor is greater 

than for the case of the long branch circuit. 

, ............................................ 

... 

From top to bottom traces (5 psldiv sweep): 

V, - 200 Vldiv: 840 V peak 

I, - 500 Ndiv: 1010 A peak 

I, - 500 Aldiv: 780 A peak 

I, - I00 Ndiv: 230 A peak 

Figure 5 - Voltage and currents for a surge causing 

gap sparkover into the tail (short branch circuit) 

In Figure 5, the current I, reaches 200 A before the 

arrester shuts off the fast increase, about 2 ps into the event, 

leaving the current with only a modest increase to 230 A before 

it slowly decreases, half-way into the surge event. Thus, the 

stress caused by the energy deposition into the downstream 

varistor is greater than for the case of the long branch circuit. 

Even so, it is still acceptable for the 20-mrn diameter varistor 

typically used for plug-in SPDs [Ill. Note also the ringing 

visible as the voltage V, reaches its maximum (840 V), resulting 

from the oscillation of the open-ended 36-m branch circuit. 

The appearance of ringing noted in Figure 5 serves as a 

warning that the propagation of surges is not a simple matter 

1201. To give an example of such complexity, and to give an 

answer to the frequently asked question "do we need an SPD on 

each branch circuit, or is one sufficient ?" Figure 6 shows the 

voltage V, at the end of the 36-m branch circuit (Position O, 

Figure 1) during a surge scenario similar to that shown in 

Figure 5 (one only downstream SPD located at Position O, 

none in Position @). 

In the scenario of Figure 6, the long branch circuit was 

left open at Position @, producing a ringing caused by 

reflections and undamped oscillations at that end. In this test, 

the driving voltage V, developed at the upstream gapped arrester 

(Position a) is only 730 V, but the voltage at the end of the 

long branch circuit (Position @) exceeds 1100 V during the 

ringing. Note that for an actual installation, a load connected at 

Position O, where an SPD would be present in this scenario, 

would not be subjected to this relatively high voltage ringing. 

At Position @, a load that would be connected at the end of the 

long branch circuit assumed to be without SPD, where the 

ringing occurs, is likely to damp out the ringing. 

To validate Ihis expectation, we connected a rcsistivc. load 

at the end of the 36-m branch circuit (Position@), showing that 

the ringing can be considerably reduced, if not completely 

eliminated. An unloaded branch circuit, by its very definition, 

raises no concern for equipment since none is present. 
A light load, such as a solid-state control circuit during the 

off-state of the controlled load, would be the worst case by 

being at the same time a light load and potentially the most 

vulnerable type of load. 

This situation provides an incentive for the so-called 

"whole-house protection" where, as mentioned in Section 11, a 

service-entrance arrester as well as plug-in SPDs are provided 

as a complete package. It is this package approach that will 

make possible the specification, and actual implementation, of 

a coordinated gapped arrester and simple varistor plug-in SPDs. 

Table 1 shows, for a range of load resistances, how the 

oscillations (recorded during our tests with a narrow window as 

discussed in the appendix but not shown here, to limit the 

length of the paper) are reduced as the load resistance is 

decreased. The large decrease from 500 Q to 100 Q occurs 

because above 125 8, the characteristic impedance of the line 

[21], a voltage enhancement occurs while below, a voltage 

reduction occurs 

From top to bottom traces: 

V, - 200 Vldiv: 730 V peak 

V, - 200 Vldiv: Peaks not resolved - See Table 1 

I, - 500 Aldiv: 750 A peak 

(5 psldiv sweep) 

Figure 6 - Voltages at the service entrance and at the end of a long 

open-ended branch circuit for a sparkover occurring in the service 

entrance arrester 

TABLE 1 

PEAK OF THE RINGING VOLTAGE AT THE END OF THE 36-m 

BRANCH CIRCUIT AS A EONCTION OF THE CONNECTED LOAD. 

Load, i2 open 10 k 5 k I k 500 100 50 

Peak. V 1170 1170 1150 1020 920 680 650 

4.3 Modeling the experiment 

A numerical model of the wiring was developed with the 

EMTP code [22] for the equivalent parameters of the circuit, as 

measured in our replica of residential wiring [8]. The "Line 

Constants" subroutine of EMTP was used to generate various 

models which were subsequently used in the main data file to 

compute the response of the circuit to various surge waveforms. 

A time step of 0.01 ps was used for the EMTP simulation [23]. 

Experimentally recorded waveforms of surge current were 

digitized. Using the least-squares fitting technique, parameters 

for the current source were determined. Using the "Freeform 

FORTRAN" expression capability of the EMTP code, any 

surge current waveform that can be expressed as a closed form 

equation can be modeled. 

This capability provides a powerful tool for analyzing 

circuit response to various other surge waveforms now under 

consideration by standards-writing organizations. 

The characteristics of the varistors are represented by a set 

of I-V points derived from published characteristics [15] and 

verified by measurements at several current values. In our first 

approximation, the gap is represented by a switch that closes 

when the voltage across it reaches 1100 V. In the future, we 

plan to increase the sophistication of the model by adding an arc 

voltage to the gap characteristic and the presence of fuses to be 

provided as the disconnector device required by the SPD 

standards now being developed. 

The equation used for the impinging current is a damped 

sine wave that allows a close approximation of the current 

delivered by typical Combination Wave generators into 

inductive loads [13]. It is known that actual generators tend to 

produce an "undershoot" when connected to an inductive load, 

and this case was no exception. However, computational 

artifacts occur when using a simple damped sine wave because 

its di/dt derivative (a cosine) is not zero at time zero. Furthermore, we know that nature does not allow an instantaneous 

jump of current from zero to a steep rise. By adding a 

multiplier term [1 - e (-t)], these artifacts are eliminated and the 

waveform has a "gentle toe" [19] which is a better model of 

reality. This improved equation is then: 

I = 2121 * sin(0.126t) * e(t'26.1) * [I- e(-t)] (1) 

with I in amperes and t in microseconds. 

Figures 7 and 8 show plots obtained from modeling the 

same case as that of Figure 4, that is, the application of a surge 

current such that sparkover of the gap will occur on the front of 

the wave. Figure 7 shows the voltage V,, similar to the timestretched trace of Figure A.2 in the Appendix. 

I ' Time (ns) 

Figure 7 - Model plot of the voltage across arrester, for conditions 

similar to those of~i~ure 4. (See also Figure A.2 in the Appendix) 
I, - 500 Ndiv; - 1400 A 

I, - 500 Ndiv : 1350 A 

I, - 100 Ndiv : 50 A (Change of scale by plotting software) 

Figure 8 - Model plot of currents, for conditions similar to Figure 4 

Figure 8 shows the three current traces, similar to the 

current traces of Figure 4. The top trace is the applied surge, 

1400 A, postulated according to Eq. (1) to match the current 

involved in the measurement of Figure 4. Practically the same 

peak values are obtained for the resulting currents, respectively 

1300 A for the current in the arrester, I,, and 50 A for the 

current in the downstream SPD, 13. (Note that to present the 

three traces on the same software-driven plot, the l3 trace is 

scaled by a factor of five, to fit the 500 Aldiv versus 100 Ndiv 

of the respective scales of Figure 4). 

4.4 Other important factors 

The objective of this paper, as stated in the introduction, is 

only to show how the dilemma of cascade coordination might be 

resolved by recourse to a gapped arrester at the service entrance. 

We have shown that effective coordination becomes possible by 

appropriate selection of the limiting voltages of the varistors and 

of the gap sparkover characteristics. However, there are other 

factors that will need to be addressed by designers before this 

approach can be transitioned to viable hardware. We have not 

attempted at this stage to study in detail all of these factors, but 

suggest the following list of topics for consideration. 

These are familiar to arrester manufacturers and this list is 

not intended to tutor them, but simply to place the idea in 

perspective so that no false expectations are raised that an 

immediate and easy solution is already at hand. We will have 

accomplished our purpose if the old idea is just given new 

consideration. Among the topics to be studied, the following are 

most important: 

- Ability of the varistor to reduce the follow current to a level 

that will allow the gap to clear at the first current zero - as 

postulated. 

- Ability of the varistor to conduct the follow current that the 

power system can deliver at the point of installation. 

- Ability of the gap to withstand the unavoidable powerfrequency overvoltages of the power system without going 

into conduction and yet to have an acceptable sparkover 

voltage. 

V. THE NEW OPPORTUNITY 

The results of our experimental measurements, which can 

be expanded by parametric modeling, show how a happy state 

of affairs - an effective coordination of cascaded SPDs - 

could be obtained by gapped arresters at the service entrance. 

These arresters would combine the best of the two technologies, 

gas tubes and metal-oxide varistors. This will not happen, 

however, if the decision is not made to apply such a gapped 

arrester. That decision must be made by utilities and installers. 

In contrast, the de facto situation inside the building, imposed 

by millions of installed appliances, is now hopelessly 

immovable. Typically, when these appliances include a built-in 

SPD or, when the end-user purchases and installs an add-on, 

plug-in SPD, these SPDs are of the type with low limiting 

voltage [5], resulting in difficult if not impossible coordination. 

This very difficult coordination, however, should not be 

construed as a recipe for disaster. The reality of the present 

situation is that these low limiting voltage SPDs manage in 

general to survive even in the absence of a service entrance 

arrester. As discussed earlier, this is not a desirable situation, 

hence the proposals for whole-house surge protection. But if 

the proposed service entrance arrester were designed to use a 

simple varistor with ratings commensurate with utility practices, 

it is most likely that the internal SPDs will "protect" the service 

entrance arrester, which then serves no useful purpose and is a 

waste of resources. Furthermore, as more electronics and 

equipment with low logic voltages are installed, the existing 

practices may lose effectiveness. 

Standards or regulations cannot prescribe the particular type 

of service entrance arrester (furthermore, the provision of a 

service entrance arrester is required in only a few countries), so 

the decision is left to the community of utilities, SPD 

manufacturers and end-users. The manufacturers would 

probably respond to the need for gapped arresters if informed 

system designers were to call back from retirement the 'ancient' 

gapped device and, with appropriate technology update, give the 

old idea a new lease on life. 

VI. CONCLUSIONS 

1. The dilemma of coordjnating a cascade of surge-protective 

devices can be solved by providing a gapped arrester at the 

service entrance, that will coordinate with the de facto 

situation inside the building. 

2. The need for a service-entrance arrester to withstand the 

scenario of lost neutral can be satisfied by a gapped arrester 

having sufficient maximum continuous operating voltage 

capability. 

3. Experimental verification of this coordination has been 

demonstrated for typical branch circuit lengths and limiting 

voltages applicable to the 1201240-V systems used in residential applications in North America. The same principles 

can be applied to other power systems with appropriate 

adaptation of voltage ratings and careful consideration of the 

local grounding practices. 
4. The behavior of a complex system such as the interactions 

between circuit impedances and the nonlinear characteristics 

of surge-protective devices can be successfully modeled to 

allow parametric studies. 

5. Other factors need attention, for which good engineering 

practice applied by surge-protective device manufacturers can 

provide adequate design. 

6. While the idea appears sound, it cannot be implemented by 

individual end-users. It will take an initiative by a centralized 

organization, such as the utility serving the district, to 

persuade manufacturers that a market opportunity exists to 

which they can contribute. 

VII. ACKNOWLEDGMENTS 

Support for the development of this revisited approach was 

provided by the Power Quality Research groups of Delmarva 

Power & Light and of Pacific Gas & Electric. Support for the 

testing and modeling was provided by the Electric Power 

Research Institute. 

VIII. REFERENCES 

1. Matsuoka, Masyama & Ida, Supplementary Journal of Japanese Society 

ofApplied Physics, Vol 39, pp 94-101, 1970 

2. Harnden, Martzloff, Morris, and Golden, "Metal-oxide Varistor: a New 

Way to Suppress Transients," Electronics, October 1972. 

3 Sakshaug, Kresge, and Miske, "A New Concept in Station Arrester 

Design," IEEE Transactions-PES-96, MarchlApril 1977. 

4. Martzloff, "Transient Overvoltage Protection: The Implications of New 

Techniques," Proceedings, EMC Zurich 1981. 

5. UL Standard 1449, Transient Voltage Surge Suppressors (1980). 

6. Martzloff and Leedy, "Selecting Varistor Clamping Voltage: Lower Is 

Not Better!" Proceedings, EMC Zurich 1989. 

7 Key, Mansoor, and Martzloff "No Joules for Surges: Relevant and 

Reahtic Assessment of Surge Stress Threats," Proceedings, 7th 

International Conference on Harmonics and Quality of Power, Las 

Vegas, September 1996. 

8. Key and Martzloff, "Surging the Upside-Down House: Looking into 

Upsetting Reference Voltages," Proceedings, PQ.4'94, Amsterdam, 

October 1994. 

9. IEC Publication 664. Insulation coordination within low-voltape 

systems, including clearances and creepage distances,for equipment, 

1980. 

10. Standler, "Coordination of Surge Arresters For Use on Low-voltage 

Mains," Proceedings, EMC Zurich 1991. 

11. Lai and Martzloff, "Coordinating Cascaded Surge-protective Devices: 

High-Low vs. Low-High," Conference Record. IEEE-IAS Meeting, 

October 1991. Also in IEEE Transactions, IAS 29-4 1993. 

12. Hostfet and Huse, "Coordination of Surge Protective Devices in Power 

Supply Systems: Needs for Protection," Proceedings, ICLP 1992. 

13. Hasse, Wiesinger, Zahlmann and Zischank, "Principle for an Advanced 

Coordination of Surge Protective Devices in Low Voltage Systems," 

Proceedings, ICLP 1994. 

14. Rousseau and Perche, "Coordination of Surge Arresters in the LowVoltage Field," Proceedings, 17th International Telecommunications 

Conference (INTELEC 93, 1995. 

15. Transient Voltage Suppression Devices, Harris Corp., 1990. 

16. IEEE C62.33-1982 (Reaffirmed 1989). IEEE Standard Test Specifications,for Varistor Surge-Protective Devices. 

17. IEEE C62.41-1991, IEEE Recommended Practice on Surge Voltages 

in Low-Voltage AC Power Circuits. 

18. IEEE C62.45-1992, IEEE Guide on Surge Testing for Equipment 

Connected to Low-Voltage AC Power Circuits. 

19. Mansoor and Martzloff, "Driving High Surge Currents into Long 

Cables: More Begets Less," IEEE Transactions PWRD 12, No.3, July 

1997. 

20. Goedde, Dugan and Rowe, "Full Scale Lightning Surge Tests of 

Distribution Transformers and Secondary Systems," IEEE - PES 

Transmission and Distribution Conference Record, Dallas TX, 

September 1991. 

21. Martzloff and Gauper, "Surge and High-Frequency Propagation in 

Industrial Power Lines," IEEE Transactions IA-22, July 1986. 

22. EPRI, "Electromagnetic Transients Program (EMTP), Version 2.0; 

Volume 1; Main Program; Volume 2: Auxiliary Routines" EPRI 

Report EL-6421-L, July 1989. 

23. Martzloff, Mansoor, Phipps and Grady, "Surging the Upside-Down 

House: Measurements and Modeling Results," Proceedings, PQA'95 

Conference, EPRI, 1995. 

Arshad Mansoor (M' 1995) is an 

Electrical Systems Engineer at the EPRI 

Power Electronics Applications Center 

(PEAC). He received his MS and Ph.D. in 

electrical engineering from the University of 

Texas, Austin in 1992 and 1994 

respectively. His areas of interest include 

Power Quality, power systems transients 

analysis, harmonics, surge propagation and 

protection, and EMTP model development. 

Franqois Martzloff (M'1956, F' 1983) 

Born and educated in France, with 

additional MS degrees from Georgia Tech 

and Union College, worked at General 

Electric for 29 years and now 12 years at 

the National Institute of Standards and 

Technology. He is contributing to several 

committees for the development of 

standards on EMC, surge protection and 

Power Quality in the IEEE and the IEC. 

Kermit Phipps is in charge of testing and 

evaluating equipment performance in 

accordance with standards of ANSI, IEEE, 

IEC, Military, UL, and the EPRI System 

Compatibility Test Protocols. At PEAC, he 

has conducted laboratory investigations of 

lamp ballasts, surge-protective devices, 

unintermptible power supplies, and personal 

computers. He is the author of test protocols 

and research papers on surge propagation 

and power quality.