f.martzloff@ieee.org

Reprinted from Proceedings, Open Forum on Surge Protection Application, NISTIR4654, August 1991

Significance:

Part 2 – Development of standard – Reality checks

Demonstration ad absurdum: 

Accepting the premise of prevalent 100/1300 high-energy surges and subjecting typical metal-oxide varistors to the

stress from a test performed with a prototype generator leads to the conclusion that most of the billions of varistors

in service should fail at alarming rates – but we know they do not. Ergo, the premise is not valid. 

This paper was part of a successful effort to discourage the IEC TC77 from including that 100/1300 test in the

regimen of across-the board EMC testing.

(See also paper “Validating Surge Tests ..." in this Part 2 for a demonstration by numerical modeling.)

Filename: MOV - VDE
gy 

Abstract - High-energy surge tests have been pe$ormed on metal-oxide varistors of a type in common use, a

a proposed ZEC standard derived from German Standard WE 01 60. l%e surge generator used for the test was 

commercial device developed especially to deliver the 1000300 ps waveform speczjied by WE Standard 0160. 

on the position of the varistor within its manufacturing tolerance band, failure or degradation can occur, va

concern that this test requirement may be too severe for universal application. 

INTRODUCTION 

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1983 jijj have id ~e German organization to specify a iiigii-energy siirge test to 

to electronic equipment installed in industrial environments (VDE 0160, 1988 [2]). Essentially

requires discharging into the ac line interface of the equipment under test (EUT) a capacitor of suc

that the specified waveform is generated, initially charged at a voltage suitable for producing a peak of 

the power-system sine-wave peak (Figure 1). Technical Committee 77 of the IEC has included this 

menu of surge immunity tests (TC77BlWG3, 1990 [3]), without limiting the scope of application to 

environments intended by the Meissen paper. Thus, this test is likely to become a general requiremen

on commercial and consumer equipment, unless its implications are recognized. In the absence of 

available surge generator, computer modeling of the test had previously been performed (Fenimore & 

1990 141, 1991 [5]). The findings of these simulations have shown that typical varistors, of wh

millions have been installed and continue to operate satisfactorily, cannot survive the proposed IEC

because excessive energy would be deposited in these varistors during the surge. The recent availa

prototype surge generator made it possible to subject typical varistors to the VDEIIEC surge, as repor

Schaffner*, a manufacturer of surge generators, has now developed a prototype that can produce the 

surge; in response to an invitation to try out this prototype, an informal work session was condu

Schaffner facility to subject typical varistors to the VDE 0160 surge. The generator includes the 

capacitor, up to 6000 pF, the necessary dc supply to charge the capacitor, a 220-V ac supply (for 

environments), and suitable means to decouple the test specimen circuit from the laboratory ac system

of the circuits are still proprietary, and only the output of the generator is described in this 

chronological recitation of the work session would require first a discussion of the various consider

conditions of the test. Recognizing the natural curiosity of the readers, let it be stated here that on

was destroyed during the test, and the other (barely) survived, consistent with the predictions of the 

modeling. Having thus given away the outcome, let us now proceed with the detailed recitatio

ronsid_eration; and conditions; 

Y As a pojicy, the idaiiond irisii'iciie of Standards arid Teciiriologii disci'ai~ms aiiy i~xp~'ieii eidoiseiiieiit of a coiiiiiieicia! p

identifying such products for the sole purpose of adequately describing the equipment used in the experiment. In this pa

the prototype generator used in the tests was the only one known to be available. Furthermore, there is no certainty that S

offer a commercial product based on this prototype. 
after the tests. This computation yields results of sufficient magnitude (that is, large overstress of the varistor) 

to make precise computing unnecessary in evaluating the outcome of the test. 

The test specimens (EUT) were 20-mm diameter varistors, consisting of two 130-V rms rated devices connected 

in series, a good approximation of the practice of applying 250-V rated varistors in the 220-V equipment used 

in Europe (Martzloff and Leedy, 1989 [6]). The nominal voltage, V,,,, of each varistor (voltage measured 

with 0.5 rnA or 1 mA dc injected in the varistor) was determined before the test for each device. One varistor 

pair (referred to as EUT #1) had a nominal voltage of 392.6 V, the other pair (EUT #2), 399.5 V. The 

nominal voltage for a 250-V rms varistor is 390 V, the minimum 354 V, and the maximum 429 V (Harris 

iVianuai, 1996 171). nus, ET~T is siiuaid at 1 % i?"oove the nominal of a 250-xv7 

EUT u2 is at 2.5% above the nominai vaiue. 

To test the varistors under the worst case condition (that is, the varistor at 10% beiow nominai, thus drawing 

energy from the generator for a longer portion of the surge waveform), the test voltage should be raised above 

the voltage specified for nominal test conditions. To place the varistor under conditions equivalent to those 

prevailing for a -10% specimen, a varistor at some tolerance level must be subjected to the same current as that 

occurring for a -10% varistor at the nominal test voltage. With the nominal VDE 0160 test voltage of 2.3 times 

the 220-V peak (714 V), the available EUT varistor specimen can be tested in a manner equivalent to a -10% 

tolerance varistor by raising the test voltage. 

For EUT #I which is 1 % above the V,,, of a 250-V rated varistor, the test voltage should be 10% higher than 

the nominal 714-V peak, plus I%, that is, 792 V. For EUT #2, 2.5% above the V,,,,, the test voltage should 

be 12.5% higher, 803 V. This increased test voltage will place the varistor at the correct value of current on 

its I-V characteristic, but raises the power dissipated in the varistor by the same percentage. Thus, the energy 

deposition in varistors other than -10% tested under the artificially raised test voltage received 11 % or 12.5% 

more energy than what a varistor at -iO% wouid have received. However, considering tine energy ieveis 

observed In the tests reported beiow (about 200% of rated ieveis, this ii-i2.5% excess does not affect t'he 

conclusions. The significant parameter to be observed is the current level, and that correct level was ~ndeed 

achieved by raising the test voltage. 

The VDE 0160 document states that the specified surge test voltage should be maintained across the terminals 

of the EUT, rather than the usual method of having a preset open-circuit voltage, and then connect the EUT 

without changing the generator setting (the so-called 'let-it-rip' mode [5], and (ANSUIEEE C62.41-1987, 

[8]). Meissen confirmed this interpretation of the document [9], so that the charging voltage of the 

generator capacitor was increased toward obtaining the specified voltage with the EUT connected, using an 

expendable EUT varistor during preliminary tests. However, the prototype generator output voltage, with 

maximum charging voltage and with varistor connected, could only be raised to 774 volts (Figure 2) instead 

of the 792 V or 803 V necessary to place the #1 and #2 varistors in the -10% tolerance situation. Thus, EUT 

#1 was actually tested in a condition corresponding to 7741792 = 98% of the worst case level, and EUT #2 at 

7741803 = 96% of the worst case level. In other words, EUT #1 was tested as if it were at a -8% tolerance 

level, and EUT #2 at a -6% tolerance level with respect to a 0% tolerance on their V,,,. 

The manufacturer's specifications 171 show a 70-5 single-pulse energy rating for the 130-V varistor, or 140 J 

for two in series. Figure 3, from Ref 151, shows the predicted energy deposition as a function of the varistor 

position in its tolerance band, for the test condition where the voltage is maintained across the EUT by 

readjusting the surge generator charging voltage. 
set up by Schaffner included approximately 5 m of leads "suitable for a 16 A load" between the v

the output of the generator. Thus, the impedance presented by the test specimen to the applied VDE 

includes a resistance that will reduce the stress of the varistor; however, this reduction is not readily 

by the simple mention in the figure of a 5-m maximum lead length, and the cross-section of the co

not specified. Operators can interpret the test procedure in a way producing maximum stress (a s

large cross section) or a minimum stress (maximum of 5 m of leads with small cross section). 

In accordance with the interpretation of the Figure 4 diagram, the voltage measured and shown in 

the total of the voltage developed across the varistor and the lead drop. To evaluate the implicat

interpretation, the next test was performed, without changing the generator setting (at its maximu

voltage), with the voltage measurement made at the varistor terminals (Figure 5). Note the 700-V 

test, ~r a 74-V difference (!O%Or-) from th,e value recorded ir, Fig~re 2. 1: the m~de!ing of Referen

[5], tk,e effect ~f is 5-112 test !ead had not been inchibed, so that the c~iiclti~ioiis of the modeliii

pessimistic than the conseqcieiices of a test coiiditioii wi;4 a lead leiigih included. Ti~us, the varisto

under 10% less voltage siress ('~eep iin mind the noniinear relationship between voitage and curre

model prediction, and possibly could survive. 

THE DEATH OF A VARISTOR 

According to a subsequent amendment to the VDE 0160 test specification, the maximum* capacito

the duration of the surges may be reduced to 300 ps for equipment installed in circuits protected 

less than 35 A continuous rating. This reduction will provide significant relief to varistors inclu

industrial environments. However, the IEC document [3] does not include that reduction. The te

for EUT #1 included two surges with this reduced stress (Figure 6), followed by surges with the fu

capacitance and full 1300 ps duration, at the maximum available generator voltage, as shown in 

Before and after each surge, the varistor Vnom was recorded to track any shift in characteristics, c

to the maximum shift of 10% allowed in the manufacturer's specifications. 

The test sequence and results for EUT #1 (a specimen in the -8% tolerance position), starting wi

surges applied: were the following: 

Shot 1: 718 V crest, 400 ps duration, Vnom shift of 1 % (Figure 6) 

Shot 2: 768 V crest, 1100 ps duration, V,, shift of 1 % 

Shot 3: 774 V crest, 1400 ps duration, Vno, shift of 1 % (Figure 2) 

Shot 4: Repeat, same settings as shot 3 (voltage measured at varistor, Figure 5), 

Varistor (a) of pair punctured 

Varistor (b) of pair externally intact, but VnOm = 0 (short circuit) 

Energy deposited in the varistor; approximately 300 J (215 % of rating) 

The same test sequence was then applied to EUT #2, that is, first two shots at reduced stress, an

stress for shot 3 and four additional shots. The V,,, shift grew from 1 % after the first shot to 6% a

shot, as measured after cooling down following the test. By the time the author had returned to 

States (20 days iater), the shifi in Vnom, determined by more systematic measurement at NIST, was 

4%. 'The difference between the 6% immediately after the test and the 4% after 20 days may be t

a slow recovery of the material, or a difference in the precision of the measurements, or both. 

* The surge duration is the specified parameter in the VDE 01 60 document, therefore the required value of the capacito

dependent upon the impedance of the EUT. 
the (late) realization of the stress reduction provided by the lead length does somewhat change the situation. 

In other words, the 10% loss of voltage caused by the leads places the varistors used in these tests at 

respectively +2% and +4% in the tolerance band, a condition that the prediction describes as marginal 

survival. The joule rating specified by the manufacturer tends to be conservative, so that it may take more than 

140 J to destroy a varistor. Furthermore, a larger population of test specimens may produce a distribution of 

more failure as well as more survivals as only two test points can only provide an indication, not a certainty. 

However, the conclusion is clear, that varistors of common use in commercial and consumer equipment would 

be in severe jeopardy if the full 10011300 us surge were applied, even with the mitigating effect of the 5-m lead 

--.. length. Discussing the test results with Meissen, we agreed on the following conclusions: 

1. There is no disagreement that the basic phenomenon of fuse blowing can lead to the high-energy surges 

described by Meissen in the heavy industrial environment (circuits with fuses above 35 A). 

2. The prediction of varistor failure through modeling is consistent with the tests; the mitigating effect of the 

allowable EUT lead reduces the forecast of widespread failures, but varistors in the lower tolerance bands are 

still at risk. 

3. The amendments to VDE 0160 providing for reduced maximum capacitance values (see the footnote on page 

3) and reduced duration make the test more realistic. Further evaluation of these reduced stress levels would 

show appropriate limits of application. 

4. However, this stress reduction has not yet been acknowledged by the IEC proposals (Figure 1; showing only 

one value of 1.7 ms is excerpted from the IEC documenti not the amended VDE 0160 where the alternate 

dmitinn nf 0.3 ms is shnwn). This paper is therefnre s~lhmitted tc! the engineering cnmm~nity at !arge as a 

rec~rnrnefidati~fi ~f !kiting the fi~!! duratim ~f a I300 ps surge and its high energy tc the industria! 

environment for which it was first proposed. 

5. The concept of readjusting the surge generator charging voltage to maintain a specified test voltage across 

the specimen is different from the usual practice of maintaining a fixed open-circuit voltage for the generator. 

However, it may be compared to the practice of readjusting the surge generator used for surge arrester tests at 

a specified test current level. As long as the implications of the procedure are recognized, either method may 

be suitable, if uniformly interpreted. 

6. In its present form, the VDE 0160 document leaves open the possibility of different interpretations by 

different operators. Should the principle of a high-energy test be adopted by the IEC, more detailed 

specifications need to be developed and agreed upon by interested parties. 
REFERENCES 

Meissen, W., uberspannungen in Niederspannungsnetzen (Overvoltages in low-voltage 

Elektrotechnische Zeitschrift, Vol. 104, 1983. 

German Standard VDE 0160, Ausriistung von Starkstromanlagen mit electronischen Betreibsmitteln (

with electronic operating controls for use on power systems), May 1988, amended April 1989. 

IEC Draft Standard DIS 77B(CO)lO(c), Electromagnetic Compatibility - Part 4: Testing and 

techniques - Overview on electromagnetic compatibility tests, 1991. 

Fenimore, C. and Martzloff, F.D., Validating Surge Test Standards by Field Experience: High-Energ

Varistor Performance, Conference Record, IEEE/IAS Annual Meeting, October 1990. 

Fenimore, C. and Martzloff, F.D., Incompatibility Between the 10011300 ps Surge Test and Varis

Rates, Proceedings, 1991 Ziirich EMC Symposium. 

Martzloff, F.D. and Leedy, T.F., Selecting Varistor Clamping Voltage: Lower is no

Proceedings, 1989 Ziirich EMC Symposium. 

Transient Voltage Suppression Devices, Harris Corporation, 1990. 

IEEE Guide on Surge Testing-for Equipment Connected to Low-Voltage AC Power Circuits, AN

C62.45-1987. 

Private communications, March 15 and May 15, 1991. 
Figure 1. High-energy waveform specification (From Ref. [3]) 

Volts 

EGG 

400 

0 

Amperes 

800 

400 

0 

2 ms/div 

Figure 2. Voltage across and current through EUT #1 
loo0 i 

Energy into EUT #1 

I / /' for shots #3 2% ii4 

Figure 3. 

Energy deposited in varistor as a function of tolerance of device compared to nominal value 

(From Ref. [q) 

Test Measuring instrument 

equipment e.g. oscilloscope 

Sample 

Supply system 

XL 

. , - 

Supply 

4 

cable 

(max. 5 m) 

Figure 4. Elementary test circuit diagram (From Ref. 121) 
Amperes 

UVV onn

400 

0 

Volts I 

Figure 5. Voltage across and current through varistor only 

Volts 

800 

400 

0 

1 I 

Amperes 

800 

400 

0 

Figure 6. Voltage across and current through varistor with reduced stress 

I