Amansoor@epri-peac.comf.martzloff@ieee.orgKphipps@epri-peac.com

Reprinted from Proceedings. PQA ’99 Conference

Significance:

Part 2 Development of standards

Part 5 Monitoring instruments

Recent projects of monitoring Power Quality in AC systems have focused on voltage surges rather than current

surges. The predictable results of such monitoring – low apparent surge voltages – which in fact only monitors

whatever limiting voltage is allowed by the proliferating SPDs and PCs, do not reflect the surge activity, now confined

to surge currents flowing into the “attractive” paths of the SPDs and the capacitors included in the switch-mode power

supplies of PCs (and other electronic appliances).

At standard-writing times, questions have emerged as to why mention “large” surges when monitoring shows only low

voltages. W ith proper perspective, it becomes apparent that the proliferation of these voltage-limiting, surgeabsorbing SPD s and PCs are the explanation. W hile voltage surges might now

no longer be a threat, the possibility of substantial current surges is indeed a threat to equipm ent.
The Fallacy of Monitoring Surge Voltages: 

SPDs and PCs Galore! 

Arshad Mansoor and Kermit Phipps Fran~ois Martzloff 

EPRI-PEAC National Institute of Standards 

1052 1 Research Dr. and Technology ' 

Knoxville, TN 37932 Gaithersburg MD 20899 8 1 13 

Abstract 

To support the recommendation of shifting transienl rnonilori~ig liwi vvltage surges lo current 

surges, the paper presents experimental results as well as numerical modeling results demonstrating 

two mechanisms causing an apparent decrease of surge activity in low-voltage ac power circuits. 

The first mechanism is the proliferation of surge-protective devices, a situation which is by now well 

recognized. The second, which should also have been recognized, apparently escaped scrutiny 

so far: the proliferation of electronic appliances containing a switch-mode powcr supply that 

effectively places large surge-absorbing capacitors across the ac power systems. 

1. Introduction 

This paper is unabashedly tutorial, and some of the themes presented here might be quite familiar 

to some of our readers. However, we have observed that these concepts, which could almost be 

characterized as obvious when given the benefit of hindsight, are still not widely recognized. 

Therefore, we will present them to th~s forum, and illustrate their validity by experimental 

measurements and numerical modeling. The three major themes of this presentation are: 

1. There is a logical explanation for the apparent decrease in the level of voltage surges reported 

in recent power quality surveys: the proliferation of surge-mitigating devices. 

2. The present practice of recording voltage surges can lead to erroneous concepts on surge 

protection, which in turn can lead LU equiprr1~111 fililu1t;s. 

3. There is an unfulfilled need to develop and deploy power quality monitors that can 

characterize the energy-delivery capability of a surge event. 

The first theme will be introduced by a historical perspective showing how voltage surges 

became the focal point of monitoring surges, even before the term "power quality" was coined. 

Experimental measurements and numerical modeling will be described to illustrate the effect of 

the proliferation of surge-protective devices (SPDs) and of new electronic appliances that serve 

rcspcctivcly as intcndcd and unintcndcd surge-mitigating devices. The next two theme:; will be 

only briefly discussed because, once the first is accepted, these two follow quite logically. 

I Electricity Division, Electronics and Electrical Engineering Laboratory, Technology Adminrsrrution, 

U.S. Department of Comti~crce. 

Conrriburions from rhe Nariut~al Insrirure of Srunciurd.~ and Tet-hnolngj ore nor ~ubjecr ru U.S. Copyri~lzr. 
2. Historical Perspective 

The proliferation of SPDs in low-voltage ac power circuits has been recognized as nnc: nf the root 

causes of the apparent decrease of the surge levels recorded in recent power quality surveys. The 

change in the occurrence of surges began to be recognized [Dorr, 19951 ' and explanations were 

offered attributing the phenomenon to the emerging proliferation of SPDs in low-voltage ac power 

circuits [Martzloff, 19961. It is noteworthy that for many people, the term "surge" is equivalent 

to "transient overvoltage" to the point that the phenomenon has generally been recorded by 

instruments acting as voltmctcrs, and thc tcrm was without much scrutiny accepted in gcncl-a1 as 

meaning a voltage surge. 

For instance, in the bi-lingual publications of the International Elecrrotechnical Commission (IEC), 

the French text which parallels the English text uses the term "surtension" (meaning overvoltage), 

because there is a mind set, reinforced by the lack of a neutral term in French that could be 

applied to either or both voltage surges and current surges. In the United States, the Underwriters 

Laboratories perpetuates this narrow perspective by calling SPDs "Transient Voltage Surge 

Suppressors" (italics ours) [UL Std 1449, 19961, although both recent IEC and IEEE definitions 

of SPDs introduce and emphasize the concept of current surges as well as voltage surges. 

Now less recognized but significant, a similar cause for the apparent decrease of voltage surge levels 

is becoming important as more and more electronic appliances depend on a switch-mode power 

supply with a rectifier-capacitor DC link. Through the rectifies, the DC link capacitor which is 

typically in the order of 200 pF to 500 pF, offers a low-impedance path to current surges 

impinging on the power port of these appliances. With many such appliances connec~ed in an enduser installation, the effect is that of quite a large capacitor being connected across the ac mains. 

Small wonder then that even largc surge currents (for instance, with the capability of delivering 

currents of 3 kA, 8/20 ps) [ANSVIEEE C62.41- 19951 can no longer raise the voltage across the 

mains to the high values sometimes reported in earlier surveys of surge voltage occurrences. 

Another historical mind-set has been to recognize the origin of surges only as a voltage event 

while in fact it can be either a true induced-voltage event, or the end-result of the injection of a 

surge current somewhere in the power system. Typical induced-voltage surges are associated 

with the electromagnetic coupling into the power circuits of the field created by a nearby (but 

not direct) lightning flash. Such voltage surges, which can develop substantial voltages in highimpedance circuit loops [Martzloff et al., 19951 can easily be mitigated with relatively small 

SPDs because their energy-delivery capability is relatively small [IEC document 64/1034/CD, 

19981. In contrast. current surges are produced either by the dispersion of the current associated 

with a lightning flash when a direct strike injects current at some point of the power system, in 

close vicinity or at a more remote point of the power distribution system [Mansoor et al., 19981. 

Another source of current surges is switching surges involving the injection of residual energy 

into parts of the power system. The energy-delivery capability of these current surges can be 

substantial, and be a threat to the survival of improperly sized SPDs. Note in passing the use of 

thc tcrm "cncrgy-dclivcry capability" and not "cncrgy in thc surgc." Somc of our rcndcrs arc by 

now familiar with that theme [Lindes et al., 19971 - perhaps even tired of seeing it repeated - 

but the sad truth is that usage of the term "surge energy" is still rampant. 

I Citations appearing in the text as [Author, date] are listed in alphabetical order in Section 7, Bihlio~raphy 
The significance of making the distinction between recording current surges versus recording 

voltage surges is very important for equipment designers. A decision to provide only modest 

surge withstand capability for an SPD incorporated at the powcr port of thc cquipment might be 

made because the contemporary surveys reveal few and moderate (voltage) surges. When combined 

with the misconception that "the lower the clamping voltage, the better" [Martzloff et al., 19891, 

the result can be disastrous. We have in our laboratory 'morgue' two examples of such massproduced devices incorporating an inappropriate SPD that led to early mortality of the product. 

Without identifying the culprit - perhaps a harsh word for a designer who was lulled into this 

position by referring to misleading reports on surge actlvlty - but to illustrate the situation, we 

can name the two products: a compact fluorescent bulb, and a remotely-controlled ceiling fan. 

To recite a recurrent theme in our tutorial presentations ("Transients Are Here to Stay"), voltage 

surges might appear to have faded away, but current surges are still here, ready to destroy a small 

SPD incorporated in a design based on the misperception of fading voltage surges, fostered by 

recording only voltage surges. 

3. Experimental Measurements 

To illustrate the effect of nonlinear SPDs as well as linear capacitors connected across the mains, 

the Power Electronics Applications Center (PEAC) "Upside-Down House" [Key et al., 19941 

was used to inject surges into the service entrance of the I Jpside Down House with various 

combinations of SPDs and/or personal computer (PC) power supplies connected at the end of' 

two branch clrcu~ts, one 9-m long, the other 3h-rn long. In Figure I, currents In the branches a~id 

voltages at the nodes are identified respectively as I, I, I, and Vo, V, V,, . The charging voltagc 

setting of the Combination Wave surge generator was kept constant to provide a 2 kV, 1.2150 ps 

open-circuit voltage (OCV). In a second series of experiments, the setting was increased to 4 kV. 

This second series, not reported here in detail because of limited space, confirmed the expected 

nonlinear response of varistors and the linear response for capacitors only. 

Figure 1 - Upside-Down House branch circuits 

3.1 Metal-oxide varistors only 

Given the known and predictable behavior of multiple SPDs - often reported in the literature 

to the point that bibliographic citations would take several lines to list all of them - this part of 

the experiment was performed only to provide a baseline. Metal-oxide varistors (MOVs) rated 

150 V, 20 mm diameter, were used for this experiment. To record all interesting currents and 

voltages, two shots are necessary when using the 4-channel digital signal analyzer; therefore each 

oscillogram in the figures contains the trace of Vo to serve as a common reference (Figure 2). 
Vo 1 kV/div 

1, 550 Ndiv 

1, 500 Aldiv 

I, 5OONdiv /- 

Sweep: 5 pddw 

2 

.- 

Sweep: 5 ps/dlv 

Vo 500 Vldiv 

V, 500 Vldiv 

V,, 50OVldiv 

Currents in branches Voltages at nodes 

790 1000 900 790 410 ' 330 

Figure 2 - Typical recordings1 made during the first experiment - SPDs only. 

In Figure 2, the current traces (left oscillograrn) show the unequal sharing between the two 

MOVs, reflecting the difference in the inductance of the two branch circuits. Because the peaks 

uT 11ic cwu ~I~~IICII-L~ILLI~L ULII~~;I~~S ale nut simultancuus (the cu~-~-cnt inhc longcr branch circuit 

takes longer to build up) their sum seems to exceed the peak of the injected current, I,. 

The voltage traces (right oscillogram) show how the voltage at the service entrance, Vo, is 

mitigated from the 2 kV open-circuit voltage supplied by the generator that would propagate 

without attenuation in the absence of a surge-mitigating device [Martzloff et al., 19861. This 

voltage Vo is the sum of the clamping performed by the varistor at the node V, and the inductive 

voltage drop in the 9-m long connection. Note that this inductive voltage is additive during the 

rise of the current I, and subtractive during the fall, hence the apparent "overshoot" in the trace 

of V, compared to the flat-top trace of the varistor at V,. The voltages at V, and V,, are the 

typical clamping voltages of the MOVs corresponding to the current they carry. 

In a subsequent experiment with a 4 kV open-circuit voltage setting of the generator (twice the 

value of Figure 2, but not reported here in detail because of limited space), the voltages across 

the varistors, predictably, were nor substantially increased. However, the substantially increased 

current in the 9-rn long branch circuit (from I000 A to 2800 A, resulting from the nonlinear 

response of the varistor) produced an increased inductive effect to the point that the voltage at 

the service entrance was raised to 1300 V from the 790 V recorded for the case of Figure 2. 

Table 1, at the end of this section, presents a summary of the peak values recorded in the various 

combinations of components, branch circuits, and amplitudes of the injected surge. 

The experimental values shown in Figures 2, 3, and 4, and in Table I were recorded with a digital signal analyzer. 

They have been rounded off to the nearest ten to convey a simpler set of numbers, uncluttered by a precise last digit. 

Uncertainty in these measurements is not an issue here as it does not affect the general conclusions. 
In a second set of experiments, less easily predictable would bc thc bchavior of the still all-linear 

circuit involving the capacitors of a PC power supply (440 pF each in this experiment) when 

receiving a surge originating from the complex RLC wave-shaping network of a Combination 

Wave surge generator (Figure 3). 

V, 1 kV1div 

1, 550 Ndiv 

1, 500 Ndiv 

I, 5OONdiv 

Sweep: 5 psldiv 

Currents in branches 

V, 500 V/dw 

V9 500 Vld~ 

V,, 500VId1v 

Sweep: 5 psldiv 

Voltages at nodes 

Figure 3 - Typical recordings made during the second experiment - Capacitors only 

In Figure 3, the voltage at the service entrance, V,, is mitigated from the 2 kV open-circuit 

voltage supplied by the generator. However, because of the interaction between the RLC 

components in the wave-shaping network of the Combination Wave generator on the one hand, 

and the capacitances of the PC power supplies and inductances of the branch circuits on the other 

hand, this voltage rings around an average voltage level of 1300 V, reaching a peak of 1700 V. 

To reflect this situation, the tabulation of the voltages in the figure shows two lines, peak and 

average. Thus, the mitigation effect. is degraded by the ringing. Nevertheless, one can expect 

that a< more PC% wni~ld he added, the ringing freqi~ency would hecome lower and the voltage 

peaks lower. 

3.3 Capacitor and MO V 

In a third set of experiments, a capacitor was connected at the end of the Y-m branch circult 

and an MOV was connected at the end of the 36-m branch circuit. Interest in this particular 

configuration was motivated by the desire to show how an MOV would mitigate the ringing 

that was observed in the preceding experiment at the end of the 36-m line (V,, in Figure 3). 
V, 1 kVldiv 

1, 550 Ndiv 

l9 500 Aldiv 

I,, 500Ndiv 

I-/ 

Sweep: 5 psldlv 

Currents in branches 

/ '0 1 Is 1 '9 I36 1 

(peak V) (peak A) (peak A) (peak A) 

Sweep: 5 pldiv 

Voltages at nodes I I I 1 

Figure 4 - Typical recordings made during the third experiment - capacitor and MOV. 

Experiments 2 and 3 (Figures 3 and 4) do show decreasing levels of voltage surges at various 

points of the Upside Down House, compared to the open-circuit conditions. However, the 

interactions between the capacitors of the generalor RLC wave-shaping network on the one hand, 

and the PC capacitors on the other hand, make detailed interpretation of the waveforms tedious 

and beyond the scope of our illustrative examples. Numerical modeling with a current source, 

as reported in the following section, avoids this interaction and provides further evidence on the 

"PCs Galut~" effc~t. FUI 1t;adel~s interested in thc dctails, Tablc 1 documents thc rcsults 

concerning peak voltages noted from the oscillograms kept on file but not reproduced here. 

Readers interested primarily in the big picture may skip a close examination of this table. 

Table 1 - Summary of node voltages for component combinations 

Dctailcd cxpcrimcnt 

description number 

3.1 - MOVs only 

3.3 - Capacitor and MOV 

1 3.2 - Capacitors only 

Device 

Combination 

Capacitor 

9 m 

Open 

MOV 

Open 12000 11600 12100' 1 Not done(linear) 

MOV 1 1600 1 1500 1 370 1 Ca~acitor failed. end test 

Ringing at the open-ended line is the cause of this voltage being greater than Vo. 

1 Capacitor 1 1700 1 1550 

Voltages for 2 kV OCV 

(peak V) 

36 m 

Open 

MOV 

Capacitor 

Open 

MOV 

Capacitor 

"0 

2500 

1700 

1800 

860 

790 

720 

Voltages for I kV OCV 

(peak V) 

"0 / "9 / v36 

Not done (linear) 

1100 

1300 

Not done (linear) 

v9 

2600* 

330 

1900 ' 

400 

41 0 

420 

400 370 

v36 

3300' 

1900 ' 

1300 

1300 ' 

330 

700 

Not done (linear) 

1400 

1300 

1300 

400 

390 

390 

2000 ' 

400 

990 
Our previous experience with modeling cascaded SPDs [Lai et al., 19931 and the behavior of the 

Upside Down House with installed SPDs [Martzloff et al., 19951 can be readily applied to the 

prediction of the behavior of the three circuits subjected to the experimental measurements, with 

an imposed current source, free from the unavoidable interactions that occurred in the preceding 

experimental measurements. This approach yields the best of the two methods: a computation 

that has been well demonstrated as suitable for modeling nonlinear SPDs, and the freedom to 

Impose any fixed waveform - what reality Imposes on a res~dence is not the surge from an 

impedance-limited surge generator - combined with the possibility to model many branch 

circuits and many combinations of SPDs andlor capacitors. 

4.1 Modeling combinations of loads and branch circuit lengths 

In a series of modeling runs similar to the combinations of the experimental measurements, the 

model used the circuit of Figure 5. A current source feeds a fixed current surgc via a common 

service drop to the panel bus of the service entrance, where three branch circuits made of 2-mm 

diameter conductors (#I 2 AWG) take off, with length of respectively 5 m, 10 m, and 20 rn. The 

choice of these lengths was based on curiosity about the effect of the long distance connection 

(and thus an effect that might bc delayed 01 degraded) to the irnmediate occurrencc of an 

overvoltage at the end oS shorter branch circuits. The currents in the circuit are identified In 

Figu~e 5 respectively as I,, I, I,, I,,, and I,,, wth the node voltages at the servicc bu\ and hanch 

circuit ends respectively as V, V, V,, and V,,. 

FlgUre 5 - Service drop and branch circuits for modeling combinations 

As in our previous modeling applications, the EMTP program [EPRI, 19891 was used. To avoid 

computational artifacts, a finite 10 kQ resistance was always postulated at the ends of all branch 

circuits, whether these were left in "open" or "loaded" condition. The loads that were modeled 

included combinations of SPDs, capacitors, and 100 Q resistors. The SPDs were all 20-mm 

diameter, 130-V rated MOVs. The capacitors included a capacitance of 440 pF and a series 

resistance ("ESR") of 0.25 0 to represent an electrolytic capacitor. Initial conditions for the 

models stipulated an initial charge on the capacitor to represent the normal condition of the DC 

link. The 100 Q resistor was selected as a typical value for a 150-W power-consuming appliance 

connected at the end of the branch circuit. 
g

Compared with the 100 0 load at the end of the branch circuits, the series impedance has a vcry 

small effect and the impinging surge current I, divides almost equally (3000 A I4 = 750 A) in the 

nearly identical four branches I,, I,, I,,, and I,, (I, being the current in the service entrance 

'branch', with a length of 0). Thus, the voltage developed by this 750 A current across 100 In 

would simply be expected to be 75 000 V (seventy five thousand volts). We say "would be 

expected" because, of course, the insulation level of a real-world low-voltage insulation cannot 

withstand such a voltage. The result of this theoretical case is another illustration of the theme 

"More Begets Less," according to which a high-amplitude, steep-front surge cannot propagate 

in branch circuits because a flashover will occur at the origin [Mansoor et al., 19981. 

am not am OUJ . . 

Currents in branches: Time in milliseconds, peaks in amperes Voltages at nodes: Tlme in milliseconds, peaks ~n volts 

Figure 6 - Currents and voltages for the baseline case of 100 R loads without SPDs 

Figure 7 shows the example of two PCs, onc at thc cnd of thc 5-m branch circuit, the othcr at 

the end of the 10-m branch circuit: the surge current is shared (unequally) between the two 

capacitors, with a corresponding decrease of the voltage level at each DC link, and also a 

reduction of the voltage at the open end (unprotected) of the 20-m branch circuit. This example 

shows the beginning of the "PCs Galore !" effect. Several intermediate and further steps were 

modeled by adding combinations of surge-mitigating devices to build our case file and confirm 

the expected effects, but we will spare the reader from a tedious recitation. 

The results shown in Figure 6 call for several comments: 

The unequal division of the currents I, and I,, reflects the effect of the larger impedance of the 

10-m branch circuit, compared to the 5-m branch circuit. 

Substantial mitigation is obtained at the nodes V, and V,,, as a result of the filter-like action 

of the line inductance and the capacitance of the PC power supply. 

Thc voltngc surgc dcvclopcd at thc nodc Vo propagates, unabatcd, to the end of thc opcncircuited 20-m branch circuit, a reminder that the notion of voltage surges being attenuated 

as they propagate in building wiring [IEC Report 664, 19801 was incorrect. 
Currents in branches: Time in milliseconds, peaks in amperes Voltages at nodes: Time in milliseconds, peaks in volts 

Figure 7 - Currents and voltages for the case of two PCs without SPDs 

The voltage results are especially worth noting, in the context of what a power quality monitor 

would report in such an installation, depending upon its point of connection. 

From the (impossible) high voltages of the 100 Q baseline (no SPDs), the voltage that would 

be recorded by a monitor installed at the service entrance is now reduced to "only" 1960 V. 

A voltage-only monitor installed at the point of use of the power (typical selection of point of 

~nstallat~on In many surveys), namely the two reccptacles at 5 m anti 1 U m fcedlng thc I'Cs, 

would report respectively 5 10 V and 290 V, creating the illusion Cfallacy) that there is 110 

significant surge activity at these points. 

In reality a current surge I, of nearly 2000 A is carried by the PC rectifier into the capacitor, 

via a line fuse. Such a high current - undetected by a voltage-only monitor - could very 

wcll bc fatal for thc rcctificr or thc input fusc, or for thc capacitor of thc PC powcr port. Such 

failures were found in post-mortems of equipment recently performed at PEAC. That 

scenario is also what occurred in the experimental test of Section 3 for a 4 kV open-circuit 

voltage and capacitor-MOV combination (shown in next-to-last row of Table 1). 

4.2 Modeling PCs Galore 

As a grand finale for illustrating our major theme, the case of an increasing number of PCs was 

modeled, with each of the PCs connected at the end of a dedicated branch circuit, with all branch 

circuits having the same length of 20 m, still with the 10-m long service drop driving a 3 kA 

current surge into the installation. 

Table 2 shows the resulting voltages at the service entrance and at the point of connection of the 

PC>, as well as 111e i~npusecl irr~yir~gi~~g culreIil ad Ll~e resullii~g cu1-rerrls in the individual power 

ports of the PCs. Indeed, the effect is linear with increasing numbers of PCs, and the resulting 

decrease in the voltages expected to be reported by a power quality monitor installed at the point 

of use is quite apparent. 
Table 2 

 Effect of an increasing number of connected PCs 

Inspection of thc tablc shows that thc cffcct is practically proportional to thc numbcr of PCs in 

use in the installation. While we have used the short acronym of PC in the title and preceding 

discussions, the proliferation that we observe is not limited to PCs, but includes many electronic 

Number 

of 

appliances, such as home entertainment, heating and air conditioning with adjustable speed drive, 

that use a DC link with large energy-storage capacitor. 

5. Action Items 

Impinging current 

(A) 

The customary closure of a paper is to list conclusions. However, in this case we suggest action 

items for industry, rather than academic conclusions: 

Thc cxamplcs givcn hcrc clcarly show that thc fallacy of littlc surge activity can bc crcatcd 

by limiting power quality measurements to voltage surges. With undetected current surge 

activity, users of electronic appliances will be puzzled by unexplained failures in the face of 

reporls of little voltage surge activity. 

We offer the explanation that these unexplained failures are likely to be associated with 

the (heretofore not characterized) surge current delivery capability of the environment. 

Such failures could have been avoided, had designers been better informed. 

With our leading theme now solidly established, questions arise about what to do about it. 

This matter requires the dedicated attention of both manufacturers and users of power quality 

monitoring instruments. 

Unfortunately, more than three years after beginning the crusade to overcome the fallacy of 

limiting surge measurements to voltage surges [Martzloff, 19951, there is no commercially 

available power quality monitor capable of characterizing the energy-delivery capability of 

a surge event. 

Worse yet, among the international delegates to an IEC working group chartered to develop a 

standard on measurement of power quality parameters (which is likely to dominate the design 

of future monitoring instruments), there are some claims being made that "a current surge is 

not a power quality parameter" and therefore it should not be included in the emerging 

standard prescribing measurements methods. This misconception needs to be corrected. 

Therefore, the crusade must be pursued with perseverance, and it is the goal and hope of the 

authors that sufficient recognition of the fallacy will eventually create a market demand for 

appropriate instruments, which forward-looking manufacturers may have developed or may 

be in the process of developing in anticipation of such recognition. 

Current at PC port 

(A) 

Voltage at service entrance 

('4 

Voltage at PC point of use 

(V) 
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