This subject was addressed in several other papers (See in Part 5 “Keeping up” 1995; “No joules” 1996; “Make 

sense” 1996; Joules Yes-No” 1997; and “Galore,” 1999 in Part 2) in persistent but unsuccessful attempts to 

persuade manufacturers and users of power quality monitors to address the fallacy of continuing to monitor surge 

voltages in post-1980 power distribution systems. As it turned out, the response was polite interest but no action. 
A NOVEL TRANSDUCER 

FOR MONITORING SURGE CURRENTS IN AC MAINS: 

Expectations and Reality 

Franqois D. Martzloff 

National Institute of Standards 

and Technology t 

Gaithersburg MD 20899-8 1 13 USA 

Abstract - The proliferation of surge-protective devices 

and switch-mode power supplies has radically changed 

the surge environment in the low-voltage AC mains. 

Monitoring surge voltages has lost significance and can 

be misleading. Instead, surge currents should be 

monitored. The paper describes the performance of a 

silicon-based surge-protective device acting as a "magnet" 

for impinging surges in the presence of multiple metaloxide varistors. This "magnet" is the first step toward the 

development of a transducer producing a voltage signal 

reflecting the surge current that can be delivered by the 

power system. That signal can then be applied to the 

voltage input of existing power quality monitors. 

I. BACKGROUND 

The need to shift focus from voltage surge monitoring to 

current surge monitoring was presented at the EMC1996 

Roma Symposium [I]. The need for this change of focus 

i,s the proliferation of surge-protective devices (SPDs) to 

the point that monitoring voltage surges only shows what 

these SPDs allow, not the real surge activity [2]. The 

proposed approach was to use a transducer to deliver a 

voltage signal suitable for the high-frequency response 

channels of commercial power quality monitors. Most 

existing monitors equipped with current input channels 

are designed for monitoring low-frequency harmonics; 

these inputs are not suitable for recording surge currents. 

This voltage signal would represent the maximum surge 

current that the power system can deliver at any point of 

the system, by offering to any impinging surge the most 

attractive path to earth - that is, the lowest impedance - 

thanks to a highly nonlinear characteristic. Hence, the 

terms "magnet" or "low biddern used in this paper to 

characterize the function of the transducer. 

The initial 1996 proposal placed emphasis on developing 

a simple, low-cost adapter, even though the result would 

only be an approximation of the true current waveform 

(the peak amplitude would be calibrated). In the state of 

affairs prevailing at that time, and still at present, it was 

felt that an imperfect approximation would still be an 

improvement over the complete lack of information on 

possible surge currents. 

Kermit 0. Phipps 

EPRI PEAC 

942 Corridor Park Blvd 

Knoxville TN 37932 USA 

Figure I, reproduced from the 1996 paper [I], shows the 

difference between the response of the low-cost system 

versus a laboratory-grade current-viewing transducer. For 

the measurements reported in the present paper, the issue 

of low cost vs. laboratory precision was set aside to 

concentrate on assessment of the "magnet" performance. 

Fi 3 - Voltage ad the SAD string and 

corresponding ctvrrnt signnlr deliver& by 

the laboratory-grade transformer and the 

J 

Figure 1 - Performance of the 1996 breadboard 

One of the limitations acknowledged in the 1996 proposal 

was the risk of failure for the low-cost "magnet" in case of 

very large surges or temporary overvoltages. To address 

that concern, a new set of measurements was undertaken 

with a silicon-avalanche diode (SAD) device that has been 

successfully deployed in the field as a commercial SPD, 

demonstrating its survival capability. 

A new urgency for this shift of focus has emerged as the 

International Electrotechnical Commission (IEC) has 

recently undertaken the development of a new standard 

for measurements of power quality parameters [3]. If the 

feasibility of recording current surges rather than, or in 

addition to, voltage surges can be demonstrated in time, 

the IEC standard might include such a shift of focus in its 

recommendations. 

t Electriciry Division, Electronics and Electrical Engineering Laboratory, Technology Administration, U.S. Department of Commerce 

Contributions from the National Institute of Standards and Technology are not subject to U.S. Copyright. 

539 
This paper reports tests performed to assess the viability 

of the "low bidder" function, and the implications for a 

successful deployment of the transducer in the presence 

of competing metal-oxide varistors (MOVs) such as those 

used in the many plug-in packaged surge suppressors 

found in today's installations. Hopefully, these test 

results will help arouse interest among would-be users 

and manufacturers for developing a complete package, 

an activity which is not in the scope of the authors' 

charter. These tests included: 

1. Evaluating the effects of separation distance between 

competing MOVs and the SAD in a simple multibranch circuit. 

2. Identifying the relationship between the SAD and 

MOVs likely to be found in a facility, when connected 

directly in parallel. 

3. Investigating the behavior of the SAD in the face of 

likely sustained power-frequency ovewoltages. 

The terms "magnet" and "low biddern are used in this 

discussion to emphasize the present situation in end-user 

installations where a proliferation of SPDs has occurred. 

These SPDs are essentially highly nonlinear devices and 

offer a low-impedance path toward earth for the flow of 

surge currents. Effectively connected in parallel, they act 

as "bidders" to the surge source, and the one with the 

lowest impedance becomes the winner of the competition 

- so goes it in theory. 

The reality is not quite so simple. Two major, different 

SPD applications have developed in parallel: SADs and 

MOVs. Ever since their competing introduction in the 

seventies, the perception has prevailed that SADs exhibit 

a sharper knee in their V-I characteristic, compared to that 

of MOVs. Thus, if placed directly in parallel, the SAD 

would be expected to act more aggressively than the many 

MOVs in capturing the largest part, if not all, of an 

impinging surge. 

The literature reports many tests and simulations of what 

is called "cascade coordination" [4], [5], [6], [7], [8], a 

situation where the two "competingn SPDs are separated 

by some inductance, in contrast with the direct parallel 

connection, the subject of the second test cited in (2) 

above. The effect of this inductance is that an SPD with 

a dynamic impedance slightly higher than that of its 

competitor can still take on most of the current if the 

competitor is located downstream along the surge path, 

because the voltage drop caused by the inductance is 

added to the clamping voltage of the SPD. 

However, competition for offering the lowest dynamic 

impedance has the undesirable side effect of lowering the 

voltage beyond which a temporary ovewoltage could 

severely overstress the SPDs [9]. 

Manufacturers publish MOV specifications that include a 

min-max range of the nominal voltage at the 1 mA DC 

level [lo], [I 11. For instance, 20 mm discs rated 130 V 

rms - a popular choice for many packaged SPDs in 

120 V AC applications - have a nominal voltage of 

200 V, with a minimum of 184 V and maximum of 228 V 

or 220 V. Therefore, the strongest competition would 

come from those devices on the low side of the tolerance 

range for the nominal voltage. 

To obtain some feel of the actual range - not a universal 

statement - a lot of one hundred 130-V MOVs was 

purchased directly from a distributor, presumably coming 

from the same manufacturing lot. The nominal voltage at 

1 mA DC was measured for all 100 devices. Only three 

devices were at or below 190 V, only four at or above 

207 V, and the median was 198 V. Therefore, a typical 

competition for the SAD would be with a 198-V varistor. 

An unfavorable handicap for the SAD would be to 

compete with a varistor with nominal voltage of 190 V or 

below, and an easy win would be for the upper half, say 

at 205 V. Devices with these three levels of nominal 

voltage were selected from the one-hundred lot to perform 

the coordination tests described in the following section. 

The test circuit shown schematically in Figure 2 was 

configured with conventional non-metallic jacket cable of 

three conductors, #12 AWG (2.05 mm diameter). Three 

lengths of cable were provided: 3 m, 10 m, and 30 m, all 

connected in parallel at a common point (as they would be 

in the service panel of an actual installation). A surge 

generator capable of delivering a 6 kA 8/20 p current 

was used to inject the surge at this common point. A 

fixed setting of 3 kA available short circuit current was 

maintained for all tests. 

Four current-viewing transducers were installed in the 

circuit, to record the currents identified as lo, 13, 110, and 

I30 in Figure 2. The output signals were fed to a digital 

signal analyzer capable of displaying the waveforms and 

performing the computations for direct reading of the 

peak values of the currents. 

Figure 3 is a typical oscillogram, with all the complex 

numerical data of the display edited out. The scale factors 

of the oscillograms were adjusted to obtain visible 

deflections, with the peak values read from the display 

transcribed on the right of the oscillogram The complete 

set of results of the coordination measurementstare 

summarized in Table 1 and Table 2. 

t The measurements reported in this paper have been made with 

instruments and transducers with cumulative relative uncertainty 

that should not exceed 5% to 6%. Given the goal of applying these 

results to monitoring highly scattered values of current surges, as 

they occur in thefield, the conclusions are not affected by this level 

of uncertainty. 
Current Surge Generator 

Figure 2 - Test circuit for cascade coordination 

III-a - Tests with SAD at the panel bus 

Table 1 shows the results of the first test series where the 

SAD was connected at the panel bus, the point identified 

as 0 m in Figure 2. 

Table 1 

Current sharing between SAD connected at point 0 m 

and one or more MOVs connected at points 3 m, 10 m, 

and 30 m with a 3 kA setting for the available current 

Varistor 

nominal 

voltage 

(V) 

Point(s) of 

connection 

of MOVs 

-- 

Percent of 

impinging 

current 

captured 

by MOVs t 

( 3 rn only 1 11.1 1 

1 3 rn only I 10.1 I 

190 

198 

10 rn only 

30 rn only 

All3rn, lorn, 30rn 

3 rn only 

10 rn only 

30 rn only 

All3rn, 10rn,30rn 

205 

lo 

2600 A peak 

3.9 

2.2 

15.2 

10.2 

3.6 

2.2 

14.5 

All3rn, 10rn,30m 

1- :6 A peak 

10 rn only 

30 rn onlv 

13.6 

k 110 - none 

3.6 

2.0 

t The three single-MOV values do not add to the value for 

all three MOV being connected, because of the interactions 

in sharing the total current. 

\SO - none . . . . . . . . . . 

Horizontal sweet) (time): 10 u%/division ,. , . Vertical: variable, with computed values shown at right 

Figure 3 

Current sharing between SAD located at 0 m and single 

198-V MOV located at 3 m, 3 kA 8/20 available current 

Three sets of measurements were performed with the three 

levels of MOV nominal voltage, as discussed above. In 

each set, first only one varistor was connected at the end 

of a branch circuit, then all three branch circuits were 

equipped with a varistor. The impinging surge was 

obtained from the generator at a constant setting of 3 kA 

available short-circuit current. With the SAD taking most 

of the current, the actual impinging surge was not much 

affected by the small variations in the currents drawn 

through the MOVs, and was in the range of 2500 A to 

2600 A. 

Inspection of the table shows that over the range of MOV 

nominal voltages, there are relatively small changes in the 

currents being diverted away from the "magnet" SAD. 

With three MOVs, the error in accepting the SAD current 

as representing the system delivery capability would be 

about 15%. Given the present lack of knowledge on the 

possible current surges, and the certain wide scattering of 

their amplitudes, a 15% error in reporting values is a small 

price to pay for new information on SPD applications. 

111-b - SAD at end of a long branch 

With the SAD connected at the end of a long branch 

circuit - as it might be if the goal were to determine the 

current surge delivery at a point of use, a legitimate 

concern for a user - the sharing can be expected to be 

quite different. 

The MOVs, now upstream of the SAD, will take on a 

greater portion of the impinging current. When testing 

with a current generator, rather than the real-world 

scenario of a constant current source of the surges, the 

current delivered by the generator will be affected by the 

variable load resulting from different MOV locations. 
To assess this situation, a second test series was 

performed, with the SAD now connected at the end of the 

30-m branch. Only two values of MOV nominal voltage 

were investigated, at the extremes of the nominal voltage 

range, that is, 190 V and 205 V. The results of this test 

are summarized in Table 2. 

Table 2 

Current sharing between SAD connected at point 30 m 

and one or more MOVs connected at points 0 m, 3 m, 

and 10 m with 3 kA setting for available current 

Percent of 

by SAD t 

0 rn only 

10 rn only 

All three 2460 5.5 

0 m only 

205 

10 m only 

All three 2460 6.5 - 

t Percentage computed for each case where the impinging 

current varies from 1400 A to 2460 A, depending on the 

MOV configuration. 

Note that in this table, the percentages shown are the values 

captured by the SAD, not the MOVs. 

The percentage values shown in Table 2 reveal the 

importance of the branch circuit inductances in limiting 

the propagation of the current surges. With an MOV at 

the service panel, the mechanisms of cascade coordination 

result in only a very small percentage of the impinging 

surge being captured by the distant SAD. Even if the 

MOV is connected on a different branch circuit, but a 

branch circuit shorter than that of the SAD, the percentage 

captured by the SAD remains low. 

An even more important observation concerning this 

situation is the dependence of the captured percentage on 

the configuration and location of competing MOVs. In an 

actual installation, one can expect an ever-changing 

arrangement of user-installed, plug-in surge suppressors. 

Therefore, any measurement conducted at an intended 

point of use can only be a snapshot in time of a constantly 

changing situation. Making such a measurement, as 

stated previously, might be a legitimate attempt by the 

user to evaluate the surge threat at that location, but the 

results could be completely misleading. 

Consequently, we must conclude that the only valid 

connection point of the transducer will be at the service 

entrance. This observation should be contrasted with 

measurements of voltage surges, for we know that their 

propagation suffers very little attenuation in a building 

wiring system [12]. 

IV. DIRECT PARALLEL CONNECTION 

To assess the intrinsic "magnet" characteristic of the SAD, 

a test was performed to eliminate most of the inductive 

effects associated with the relative location of the MOVs 

and the SAD. In its commercial package, the SAD was 

provided with 30-cm long leads, probably the minimum 

practical length for installation in or near a service panel. 

The MOVs used in the tests were simple components, not 

packaged, with their usual 2-cm long leads. To make a 

fair comparison with the SAD package, the MOVs were 

also fitted with a pair of 30-cm long leads, and then 

connected together at the point of injection of the surge. 

The same current-viewing transducers and digital signal 

analyzer used in the coordination tests were used to record 

the currents in the two parallel-connected SPDs. 

In addition to the selected MOVs used in the fust series of 

tests, other readily available MOV discs on hand in the 

laboratory included some lower ratings, with a known 

nominal voltage. These would have been unsuitable for 

connection to a powered 120 V system, but provided an 

opportunity to compare the "magnet" further down in the 

dynamic characteristic. Table 3 shows how the impinging 

current was divided between the two SPDs, and one 

example of recording is shown in Figure 4. The table can 

be interpreted as a comparison between the ability of the 

SAD and the ability of the MOVs to divert impinging 

surges. 

Table 3 

Current sharing between parallel-connected SAD and 

MOVs with equal lead length, 3 k.4 8/20 ps available 

short-circuit from the surge generator 

in SAD 

t The two percentages add to more than 100% because the 

peaks are not simultaneous. 
The 50% cross-over where the two devices have an 

identical performance is between the 95 V and the 60 V 

rating of the MOVs. In other words, one could say that 

the SAD, in the current range of 1000 A, behaves as an 

80-V MOV would. However, Figure 4 also shows that 

the MOV reaches its peak earlier than the packaged SAD. 

This is probably caused by the additional internal 

inductance made necessary by the SAD design. In the 

case of the moderate rise time of the 8/20 surge, this 

slower response is not a problem, but might raise some 

questions in the case of surges with very short rise times. 

At this point, our readers might wonder why the MOVs 

were characterized by their nominal voltage (voltage at 

1 mA DC) but the SAD had not been so characterized. 

The answer is that the SAD, being a commercial package, 

is provided with condition-indicating lights that draw 

more than 1 mA at the rated line voltage, so that it is not 

possible to make the measurement of nominal voltage 

without going into the package to disconnect these 

auxiliary circuits. This arrangement is found in most 

commercial SPD packages, a regrettable situation that 

makes it difficult to assess any change in the condition of 

an SPD package after a long service or exposure to 

repetitive stresses. 

Impinging 

current 

2650 A peak 

I 1 1040 A peak 

Current in SAD 

1720 A peak 

Horizontal sweep (time): 10 psldivision 

Vertical: variable, with computed values shown at right 

Figure 4 

Current sharing between SAD and 95 V rms MOV 

connected in parallel, 3 kA 8120 available current 

V. TEMPORARY OVERVOLTAGE IMMUNITY 

As undesirable as they are, temporary overvoltages do 

occur in AC power systems, raising the issue of immunity 

to these events, especially in the case of a "low bidder." 

One condition that has been frequently reported is the loss 

of the neutral conductor in single-phase, four-wire 

1201240 V systems. 

Theoretically, the voltage across the side that has the 

lightest load can reach almost the full 240 V of the 

system. The more general case of some loads, albeit 

unbalanced, on both sides of the system can produce 

temporary overvoltages that equipment might sustain, 

undetected, for some time. Highly nonlinear SPDs are not 

so fortunate, to the point that the common wisdom is that 

most of in-service failures of SPDs are attributable to 

temporary overvoltages rather than to "large" surges. 

Therefore, it would be desirable to assess the behavior of 

the SAD in case of moderate temporary overvoltages. In 

the case of severe temporary overvoltages, much of the 

equipment in an installation is likely to fail or cause 

tripping of overcurrent protective devices, so that a failure 

- in an acceptable mode - of the SAD would be taken in 

that perspective. 

The SAD was exposed to two levels of temporary 

overvoltages: a sustained 12596, and a brief 150%. The 

waveform of the current drawn by the SAD under those 

conditions was monitored to detect any sign of incipient 

thermal runaway. 

At 125% of rated line voltage, a noticeable peaking of the 

current was evident, and the housing became warm, but 

not hot to the touch. Due to limitations in the available 

laboratory test time, the test was not pursued beyond two 

hours, but it appears that the condition could have been 

sustained longer. 

At 150%, peaking of the current was again quite 

noticeable, but without any sign of thermal runaway after 

two minutes of exposure to this severe overvoltage. The 

housing was hot, but without any sign of duress or 

hazardous condition. However, when the housing was 

opened for examination, it was found that several of the 

soldered diode connections had melted. Some diodes 

were loose in the housing. This condition would impact 

the ability of the SAD to continue its satisfactory 

performance as the "magnet", but nevertheless the 

reassuring absence of an objectionable failure mode was 

a positive result. 

VI. DISCUSSION 

The cascade coordination tests indicate that the initial 

concept of a simple, universal transducer that could be 

connected at any point of the power system was optimistic 

even as desirable as it was. Nevertheless, an SAD with 

field-demonstrated reliability can be used to serve as the 

magnet for attracting and characterizing current surges. 

The key to success -credible and useful measurements - 

is to make these measurements as close as possible to the 

service entrance. 

Two configurations might be possible, depending on the 

physical arrangement of the conductors at the service 

entrance, as described in the following alternatives. 
A package consisting of the SAD and a suitable 

current-viewing transducer inserted on one of the 

leads can be connected across the two active 

conductors. These connections would be similar to 

those routinely made for monitoring voltage surges 

with existing power quality monitors. Users of these 

monitors are quite familiar with all arrangements and 

precautions for such measurements. 

This arrangement makes it possible to wind more than 

one turn in the window of a high-frequency current 

transformer, thus providing sufficient voltage to the 

input of the power quality monitor. 

The disadvantage of this approach is that in a system 

where many MOVs might be connected on branch 

circuits, the "magnet" would capture only about 80 % 

of the impinging surge. Still, compared to the present 

total lack of information, 80% is not bad. 

2. If it is possible to insert a current transformer on the 

incoming line - by a clamp-on type so as not to 

interrupt the loads -then the SAD can be connected, 

separately from the transducer, across the active 

conductors in the panel, downstream from the 

transducer. However, this arrangement limits the 

primary of the current transformer to one turn. 

Readily available high-frequency current transformers 

might not have a sufficiently high output voltage to 

ensure good resolution of the signal by presently 

available power quality monitors. 

While this approach has the advantage of the SAD 

capturing all the impinging surge, the simplicity of 

connecting an integral package simply across active 

conductors is lost. 

VII. CONCLUSIONS 

The need for shifting focus from voltage surges to 

current surges has not decreased, if anything it is 

greater. 

The initial concept of providing a "magnet" to attract 

and measure current surges is applicable if the 

"magnet" is connected close to or at the service 

entrance. 

VIII. REFERENCES 

[I] Martzloff, F.D., "Surge Recordings that Make 

Sense: Shifting focus from voltage to current 

measurements," Proceedings, EMC1996 Roma, 

1996. 

[2] Mansoor, A,, Pliipps, K., and Martzloff, F.D., 

"The Fallacy of Monitoring Surge Voltages: 

SPDs and PCs Galore!" Proceedings, PQA '99 

Conference, May 1999. 

[3] IEC 77AJ3011CD - Electromagnetic Compatibility 

-Part 4-30: Testing and measurement techniques 

-Power Quality Measurement Methods. 

[4] Martzloff, F.D., "Coordination of Surge 

Protectors in Low-Voltage AC Power circuits," 

IEEE Transactions PAS-99, JanuaryIFebruary 

1980. 

[5] Standler, R.B., "coordination of surge arresters 

and suppressors for use in low-voltage AC 

power circuits," Proceedings, 1991 EMC Zurich 

Symposium, 1989. 

[6] Stringfellow, M.F. and Stonely, B.T., 

"coordination of Surge Suppressors in LowVoltage AC Power Circuits," Proceedings, 

Open Forum on Surge Protection Applications, 

NISTIR-4657, 1991. 

[7] Hostfet, O.T., Hervland, T., Nansen, B., and 

Huse, J., "Coordination of surge-protective 

devices in power supply systems: Needs for 

secondary protection," Proceedings, 

Internutiowl Conference on Lightning 

Protection, Berlin, September 1992. 

[8] Lai, J.S. and Martzloff, F.D., "Coordinating 

Cascaded Surge Protection Devices: High-Low 

versus Low-High," IEEE Transactions IA-29, 

JulyIAugust 1993. 

[9] Martzloff, F.D. and Leedy, T.F., "Selecting 

varistor clamping voltage: Lower is not better !" 

Proceedings, 1989 EMC Zurich Symposium, 

1989. 

For an easy across-the line connection, the presence of 

many MOVs in the system might reduce the measured [lo] IEEE Std C62.33-1981, IEEE Standard Test 

value to about 80% of the truly available surge SpeciJications for Varistor Surge-Protective 

current. While regrettable, this is not unacceptable. Devices, Reaffirmed 1995. 

If a current-viewing transducer can easily be inserted [Ill Harris Semiconductor Corp.9 "Transient Voltage 

upstream of the entrance service panel, then all of the Suppression Devices, " 199 1. 

available current surge will be recorded. [12] Martzloff, F.D., "Coupling, Propagation, and 

Thus, the concept is still attractive and should be Side Effects of Surges in an Industrial Building," 

implemented in forthcoming power quality surveys. IEEE Transactions IA-26, March-April 1990.