Reprinted with permission from Proceedings, 23rd International Conference on Lightning Protection, Florence, 1996

Significance:

Part 2 Development of Standards – Reality checks

Three examples of reality checks are given that shed some light on issues raised during standards development

! The apparent reduction in surge voltage activity is explained by the proliferation of surge mitigating devices.

! A proposed high-energy surge would cause failure of ubiquitous SPDs, but they do not fail in the field.

! Allegedly frequent high-level surges would cause frequent failure of light bulbs, but they do not in the field
A DDT VllTP 

RP A T TTV OvOVC 

nAAu~~~~ux\lrnblI I ~11 ~110 

TO STANDARDS ON THE SURGE ENVIRONMENT 

Arshad Mansoor and Doni Nastasi 

Power Eiectronics ~ppiications Center 

Knoxville TN USA 

Abstract - The paper identifies several realities of surge 

environment and equipment survival that are sometimes ignored 

in surge-protection practices. It questions the quest for what 

could be overly conservative requirements for surge immunity or 

surge mitigation by presenting "reality checks" based on field 

experience or laboratory data. A first check focuses on the fact 

that some recent field recordings of surges may be misleadingly 

low in today's surge environment. Additional checks, aimed at 

moderating the overly conservative requirements, include the case 

history of a proposed high-swess 10011300 ps surge test, data on 

failure levels of clock motors and light bulbs that can serve as 

benchmarks for severity levels, and measurements, validated by 

parametric modeling, showing that large currents cannot 

propagate into long cables without causing a flashover of the 

wiring devices at the beginning of the cable, effectively limiting 

the energy-delivery capability of a surge at the end of the cable. 

1. Introduction 

Some proposals for standards on equipment surge 

immunity or on performance of surge-protective devices 

are driven by the commendable quest for conservative 

ratings. However, when this quest produces compounded 

safety factors, the result may not be cost-effective. 

The purpose of our paper is to present facts and 

rationalizations in support of the development of realistic 

standards on the surge environment in low-voltage ac 

power systems. In addition to contributors to the ICLP 

Conferences, two major players in the development of 

:-*--...+:--- -*--A...*A- -.. +he 1. I.*..:..- " ..--- ...-.,.------ 4 uwxlrauu~Ld auu~ucuuo UIL ULG ~iguu~u~g DUI~G GLIY~UIUI~LII 

are IEC Technical Committee 77 on Electromagnetic 

Compatibility and Technical Committee 81 on Lightning 

Protection. Other contributors addressing switching surges 

as well as lightning surges include IEC Subcommittee 28A 

on Insulation Coordination, Subcommittee 37A on SurgePmtecfve Devices, and r?c!e FEE Cnmminee nn SllrgeProtective Devices. The standards developed by these 

bodies reflect the collective experience of individual 

contributors as well as the "corporate memory" of these 

groups. In this paper, four items are described that may 

serve as foundations, or reality checks, to be added to this 

corporate memory. The first check can be seen as a 

reminder note to resist the mistake of dismissing surge 

threats because contemporary recordings show benign 

occurrences of surge voitages. Tine three other checks can 

serve as a tempering note, to avoid economically unjustified over-specification of surge protection. 

* Electriciry Division, Electronics and Electrical Engineering Laboratory, 

Technology Administration, U.S. Deparrment of Commerce. 

Fran~ois D. Martzloff 

Nationai Institute of Standards and Technology * 

Gaithersburg MD USA 

2. The ninety-five percent rule 

Debates and definitions concerning compatibil

levels, immunity limits, or emissions limits genera

acknowledge that requirements drawn from these consid

ations cannot apply simultaneously to 100% of the ca

and 100% of the time. The term "high probability" appe

in many definitions, rather than a hard number. T

avoidance of hard numbers is the result of the difficulty 

secure a consensus from individuals who represe

different communities of end-users with different perce

tions of what is an acceptable level of probability. 

The military and aerospace communities often spe

of zero tolerance or zero defects where economics 

downplayed. In contrast the commercial, mass-product

communities speak of realistic requirements where co

effectiveness is a very prominent factor. 

An intermediate position can be found in indus

where a rational balance is sought between the costs 

exhaustive mitigation and the consequential costs of limit

mitigation. 

Our reality checks are offered as a guide to striki

such a balance, recognizing the diversity of circumstan

and expectations from a diversity of end-users. 

3. Shrinking surges vs. expanding mitigation 

A reality check can explain an apparently puzzli

deveiopment which has been observed since the beginni

of studies concerning the characterization of the sur

environment: starting in the sixties, results published 

researchers monitoring the occurrence of surges ha

reported a gradual decrease in the relative severity 

surges (Martzloff & Hahn, 1970 [I]; Allen & Segall, 19

:2]; D,d! & Nethercott, 1375 3;; Gddsiein aid Speran

1982 [4]; Goedbloed, 1987 [5]; Dorr, 1995 [6]). 

This pmz!lng trend can he exp!&ned by referen

to the reality: the spectacular expansion of surge-protect

devices (SPDs) in low-voltage ac power circuits n

makes it almost impossible to make measurements at a s

where there is not some hidden SPD. Monitors will n

report the residual voltage of these SPDs, not the me su

activity in the environment (Aspnes et al. and discussi

1985 [7]). So, it is not that surges have shrunk, but rath

that mitigation has greatly expanded, masking the persist

occurrence of transients which remain a potentiai he

This threat has shifted from observed surge voltages 

now mitigated -- to the possibility of excessive su

currents into candidate mitigation means with low limit

voltage which would be particularly attractive to the surg
Future surveys of the occurrence of surges in lowvoltage ac power systems should shift from the recording 

of surge voltages to the recording of surge currents [8]. 

Unless this shift is implemented, the false sense of security 

will be further promoted that potentially damaging surges 

are less frequent than once believed. The 1980 title of a 

seminal IEEE document on the occurrence of surges 

(Guide on Surge Voltages in Low-Voltage AC Power 

Circuits [9]) was appropriate at the time, but should now be 

amended to reflect the shift. 

4. The saga of the proposed 100/1300 ps surge 

A proposal was made in the early eighties to require 

a high-stress surge test for industrial equipment. A reality 

check would have revealed the contradiction between the 

failure rates that would result from such a test and the 

observed field failures. 

Starring with observations of the occurrence of long 

duration surges, typically associated wth iuse biowing, the 

proposal was to require an additional test with a surge 

longer than the classic 1 ,Z5O - 8120 ps impulse (Meissen, 

1983 [lo]; VDE 0160, 1989, [Ill; IEC 1000-4-1, 1990 

[12]). The proposed surge would be characterized by a 

10011300 ps waveform, with peaks as high as 1,3 per unit, 

added to the ac sine wave peak. In the original VDE 01 60 

implementation, the test circuit involved the discharge of a 

24000 pF energy-storage capacitor. An amendment to the 

VDE 0160 standard [13] scaled the capacitance value 

down to 6000 pF, still a large stress for the equipment 

under test. 

For instance, such surges would impose large 

stresses to the millions (or perhaps billions) of small varistors now installed in low-voltage ac power systems. One 

would observe a conspicuous failure rate in the field as 

typical 20-mm diameter varistors cannot survive such 

surges, but one does not observe this fahie rate. An 

objection to this surge specification was first proposed on 

the basis of computer simulation of the event (Fenimore & 

Martzloff, 1990 [14]) because no generator was commercially available to produce that surge. Table 1 shows 

an excerpt from [14] where the current that would be 

caused by the proposed 10011300 ps surge in a 20-mrn 

varistor rated 250 V rms was computed and compared to 

the published varistor pulse rating for current-handling 

capability. Only a varistor at the high end of its 210% 

tolerance band could survive one application of that surge 

(th.e c~xespcnding COEI~~~! ctme~! pea! is nn!y 269 A, 

compared to a rating of 300 A). In all the other cases, the 

varistor rating is exceeded and likelihood of failure 

increases for lower tolerances and for more than one pulse. 

Later on, when a prototype generator capable of 

delivering this surge was developed by a surge generator ----.t--+..-a- +,m ..At-anrnA h.r nna nf thm nlathfirr rrnA hv IllalulabLuGl, LGJ~ wrulrsaru vj vur v~ YAW UYY~V-Y U..Y YJ 

W.Meissen (the originator of the 10011300 ps surge 

proposal), did confirm the fact that 20-mm varistors are 

destroyed by that surge (Martzloff, 1991 1151). We now 

understand that the IEC proposals and the VDE 0160 

requirements for the 10011300 ps surge have been pulled 

back or are being reconsidered. 

Table 1 

Computed peak currents for a postulated 100/1300 ps surg

and varistor rated peak current 

Tolerance 

on varistor 

nominal 

voltage 

rating 

(%) 

- 10 

0 

+10 

'FWHM: Full width at half maximum 

5. Failure levels of clock motors 

and incandescent light bulbs 

Computed results: 

Current peak and 

duration of surge 

in the varistor 

Tuln simp!e devices have been in service ir? --!!inn

of households and can be seen as surge threshold detect

motor-driven clocks (before the advent of digital clo

and still today, the ubiquitous incandescent light bulb. 

have some knowledge on the failure rate of some of th

devices under surge conditions, from which we can de

some inferences on approximate limits on the leve

frequent surges. As we will see in the following 

subsections, the data are not precise and are subjec

undetermined statisticai variations. However, the ia

number of observations does give some value to 

inferences, compared to the limited number of observat

obtained by more precise recording instruments. 

658 625 45 

269 

Peak 

(A) 

Varistor rating: Allowa

peak A for number o

pulses of computed 

FWHM 

5.1 Failures of motor-driven clocks 

p"A",Hf"y 

(ps) 

4 IVV nn 

pulses 

In the sixties, a report was published citing a 10

reduction in the failure rate of clock motors when 

withstand ievei of the motor was raised from 2 icV to 6 

[9], thus providing a verification of the "slope" of su

nr-c~rrence rate vs. p.ak reported hy my researchers [

Figure 1 shows a plot of the frequency of oc

rence of surge voltages versus their peak amplitud

reported by many researchers, and the two-point line of 

100:l relationship between 2 kV and 6 kV occurren

The parallel position of all these lines is remarkable. W

p!ss Tep&d by tuhl~ rer~&r&err &re g,~er,l!y !i-..

to a few thousand hours of observation, at only a 

locations, the clock data represent the integration of m

than 10000 clocks over a period of two to three years

4n I V 

pulses 

Because the data reflect only the number of clo

returned by dissatisfied customers, the exact numbe

clocks involved in "monitoring" the surge voltages, and 

duration of the observation are not known. Theref

rigorous statistical analysis would be meaningless. 

point of our reality check, however, is still that a very l

number of observation points were involved, for a pe

of severai years, something that no survey couid do -- 

for this particular case history, it was all done before 

environment began to be tamed by the proliferation of 

new generation of low-voltage SPDs. 

pu
Peak Surge 

8-l - Bull & lethercot (~o~posite) U - Martzloff Can - Cannova 

6-5 - fioldsteln 4 Speranza UBB - Yernstrom et *I (upper llmlt) 

60e - Poedbloed Clock - 2 kV L 6 kV points only 

Figure 1 - Plots of frequency of occurrence versus peak amplitude 

reported in 1970-1987 surveys by independent researchers. The 

line marked "clock" only shows the slope from 2 kV to 6 kV, not 

the actual frequency of occurrence. Note how the lines are 

essentially parallel. (Plot reproduced from 1161.) 

5.2 Failure of incandescent.light bulbs under surges 

TesG recently perfo-a by fie authors 120-V 

incandescent light bulbs show that few bulbs will survive 

surges in excess of 1200 to 1500 V 1., and that a surge of as 

little as 800 V, when it occurs under the most sensitive 

condition, can trigger the failure . Limited tests performed 

on 240 V bulbs yield similar results. Comparison of tests 

performed by a researcher in Austria and by the authors on 

both 120 V and 240 V bulbs might be available at the time 

of presentation of this paper. 

Since we do not hear reports of endemic failure of 

light bulbs beyond what can be expected from their known 

service life, we must conclude that the reality is that there 

is not a high rate of occurrence of surges at levels in excess 

of 1500 V. 

t The measurements reported in this paper have been made with 

instnunentation for which the combined uncertainty should not "...-""A LC r. +OI^ rx.,",, +ha "r,mrr nt-""nl.,;nn ths ...~"r.r..a - 

crcccu u 6" -v I". u'rcr' rrrc yrvs.c.ee vj uyy.,v.rrg ra- rr-wurrment results to the failure levels of light bulbs exposed to 

environments with characteristics that are at best hown within 

an order of magnitude, this level of uncertainry does nor affect 

the practical conclusions. 

This observation merits a brief summary of 

findings about the mechanism of bulb failure triggered 

a surge. We emphasize the triggered aspect because sur

of relatively low amplitude do not cause directly the fail

of the bulb, but cause a secondary flashover at the pow

frequency that burns out the filament. Surges nf hig

amplitudes can deposit enough energy into the filamen

melt it, but our point for the reality check is that even lo

amplitude surges can result in failure of the bulb. 

Figures 2, 3, 4, and 5 present oscillograms of 

voltage across an energized 120-V bulb and of the curr

flowing in the bulb. Figure 2 shows a narrow wind

commensurate with the duration of the surge delivered 

a 1,2150 ps - 8120 ps surge generator. We observe 

chopping of the voltage wave, typical of a gap sparkov

and the rise of the surge current after the sparkover. T

scale of the current trace selected to record the su

(hundreds of amperes) does not show the normal curr

(1 A) in the bulb. Observations during this test inclu

hearing a pinging noise and seeing a bright flash of lig

followed by darkness as the filament can then be se

broken at its points of attachment to the stems. 

.. i .. . i. i.... i. .. i. ~-- i .. - ~p i. i .... 

Top trace: Voltage across bulb terminals, 200 Vld

Bottom trace: Current in bulb. 100 Ndiv 

Sweep: 10 psldiv 

Figure 2 - Voltage and current in light bulb during applicati

of a 1,2450 ps - 8/20 ps surge, resulting in surge sparkover 

Figure 3 was recorded (for a new bulb) with 

ionger window to dispiay two fuii cycies of the pow

frequency. At that sweep rate, the surge is no long

resolved, and its apparent peak on the trace may be low

than the actual peak because not enough data samples a

collected around the peak. However, the timing of t

surge, and the events following the surge are what 

:.-&.-* :.. *l.:- c -.-- n- --.-- ------- lllllL I I LC. LLIC UI~C CVCIIL appc~ as

voltage spike and a current spike, followed by return 

practically normal voltage and no visible large curre

Then, suddenly, a pulse of power-frequency curre

appears, with a large amplitude -- the source of t

observed flash. We believe that it is this current that cau

the burn-out of the filament, not the "trigger" surge. 
.................... .... I....,....,.... 

- - 

Top trace: Voltage across bulb terminals, 200 Vldiv 

Bottom trace: Current in bulb, 100 Ndiv 

Sweep: 5 msldiv 

Figure 3 - Voltage and current in light bulb during application of 

a 1,USO ps - 8/20 ps surge at 30°, resulting in surge sparkover, 

followed by power-frequency flashover 

This belief is supported by the recordings shown in 

Figures 4 and 5. Figure 4 is actually the recording of a 

surge application to the bulb of Figure 3, in a test that 

preceded the fatal surge recorded in Figure 3. The applied 

surge was the same (900 V) for both Figure 3 and Figure 4. 

The randomness of the process of igniting the power arc is 

such that in the case of Figure 4, the power arc was not 

ignited, while in the subsequent surge appiication on the 

same bulb (Figure 3) and in the same conditions, the power 

arc was ignited, resulting in bum-out of the filament. 

.. i.... i ... i... ... i .... It: ......... ....... 

:i: -, 

: .t J 

...................... ....................... 

. , . . , , , , .... ,..,.I....I....l.. .. 

Top trace: Voltage across bulb terminals, 200 Vldiv 

Bottom trace: Current in bulb, 100 Aldiv 

Sweep: 5 msidiv 

Figure 4 - Voltage and current in light bulb during application 

of a 1,USO ps - 8/20 ps surge at 30°, with surge sparkover but 

no power-frequency flashover 

As f~rht er evidence, Fi-me 5 shows the process (in 

a new bulb) when the surge was applied at zero degrees, 

the time at which there is the least power-frequency voltage 

to ignite a power arc. in that test sequence, the iirsi appiied 

surge had an amplitude of 1000 V. Sparkover under the 

impulse did occur (the pinging noise was heard), but the 

power-frequency arc was not ignited and the bulb survived. 

.I : 

..; ...... :......... + ...... !.. :.. ....

I i f'f ..L. .... 1 ....I.... ,....1....I....,....I....!....I... 

Top trace: Voltage across bulb terminals, 200 Vl

Bottom trace: Current in bulb, 100 AJdiv 

Sweep: 5 msldiv 

Figure 5 - Voltage and current in light bulb during applicati

of a 1 ,Z50 ps - 8/20 ps surge at 0°, resulting in surge sparkov

and delayed power-frequency flashover 

The applied surge was then raised in 100 V step

still with pinging heard but no fatal power-frequency ar

Figure 5 was recorded when the applied surge voltage w

increased to 1300 V. At that level of energy deposited 

the surge, enough plasma was generated in the path of t

surge current to eventually ignite the power-frequency a

but it had to wait until the power,frequency voltage h

----+A :*- ..a-l, I cau GU I w par.. 

To conclude this summary, Table 2 shows t

relationship between the timing of the surge with respect 

the sine wave and the amplitude of the surge sufficient 

trigger ignition of the power-frequency arc. When t

surge is applied at 90 degrees (the peak of the sine wav

making immediate ignition of the power arc easiest)

surge of 800 V is sufficient to trigger the power arc. 

zero degrees, the surge must be raised to 1500 V 

produce sufficient plasma to result in a subsequent pow

frequency arc. 

The phenomena are of course subject to t

statistical variations of sparkover. The values shown 

Table 2 are the averages of tests performed on a total of 

bulbs of the same manufacturer, rated 100 W, replicati

the test at several timing angles. This paper is not t

medium for reporting in detail our series of experime

with other manufacturers and other watt ratings whi

produced similar results; the point is, however, that o

inferences are not hased on just the 20 bulbs of Table 2

Table 2 

Relationship between timing angle of the surge and amplitu

necessary to produce a fatal power-frequency flashover 

' One specimen produced 'pinging" starting at 1000 V, but 

power arc. Eventually. the bulb failed at 1600 V without power a

as a direct result of the energy dumped into the filament by 

surge alone. 

I~ngle(')I o 1 15 1 30 1 45 1 90 1 135 / 150 (16

Peak (V) 1500 

- - 

1200 900 800 800 850 850 11
6. Limits to pushing surges into branch circuits 

Scme propesds have beea mde tc require SPDs 

intended for installation on indoor circuits to withstand 

surges with relatively high peaks and short rise time. Such 

a requirement would mean that a substantial voltage drop 

would be developed (L. di/dt) along the wiring. Added to 

the !ifif r?g w!tage cf the SPD at the end cf th,e !in=, tk 

voltage necessary to drive such a surge at the origin of the 

line would very likely cause flashover of wiring devices at 

the origin of the line. 

This flashover, occurring during the rising part of 

the surge, would effectively shut off further propagation of 

the surge toward the SPD (except for the energy stored in 

the line during the current rise, which is easily dissipated 

by the SPD). Thus, the requirement of a large surge 

capability for SPDs installed with even a modicum of line 

length would be unrealistic. In support of this statement, 

we present here a summary of measurements on actual 

wiring ZK! EMTP [I?] tempter simu!ztior! ef a mqge cf 

parameters. 

Figure 6 shows the experimental circuit with a 

varistor connected at the downstream end of a "branch 

circuit" consisting of two copper conductors of 2-mm2 

cross-section (#I2 AWG), typical of residential wiring. The 

first current transformer monitors the total current 

impinging at the upstream end. The second current 

transformer monitors the current flowing toward the 

downstream end, which will be imposed on the varistor. 

The clearances at the upstream end, such as clearances in 

a service-entrance panel, are represented by a discrete gap 

that can be set to produce sparkover at some given voltage 

during the test as well as in the model. 

To determine the response of the circuit without the 

clearance limitation and verify that the model produces the 

same result, the gap setting was adjusted for this particular A rL 1.- ~cst SO umi no sparnwver occurred at the upsueam voirage 

developed for the current delivered by the generator. Once 

the model was validated, parametric variations could be 

performed for any combination of circuit length, applied 

surge, and clearance (gap) sparkover. 

9 meters 

Surge 

generator 

i Dig 1 

signal 

analyzer printer 

Figure 6 - Test setup for driving surges into a varistor 

installed at the end of a 9 m branch circuit. 

Top trace: Gap voltage, 500 Vldiv 

Center trace: Total current, 500 Ndiv 

Bottom trace: MOV current, 500 Ndiv 

Sweep: 10 psldiv 

Figure 7 - Voltage and currents 

measured in the circuit of Figure 6 

Figure 8 - Plot from model of voltage across the gap 

(compare with voltage trace of Figure 7) and impinging 

current (compare with the two current traces of Figure 7) 

for the circuit of Figure 6 

Inspection of Figures 7 and 8 clearly shows th

agreement between real-world measurements and mode

The voltage traces are quite comparable. In Figure 7, th

two current traces are identical since no current is divert

in the arrester. In Figure 7, the current trace is the on

ponu!atec! in the mnde!ing. %is conespmdence a!!ow

us to make parametric variations in the model wi

postulated impinging surges of higher values, such as tho

being proposed for consideration in some standards-writin

groups. Another advantage of the model is that it allow

postulating a current source -- the consensus choice amo

lightning researchers - rather than using a surge generat

which is only a charged capacitor with a wave-shapi

network. Consequently, the surge generator interacts wi

the circuit into which the surge is injected, changing t

waveform and losing the postulated constant wavefor

This changing of the waveform makes it more difficu!t 

perform parametric tests, compared to the ease 

modeling. Table 3 below shows the results of su

computations for the current waveform of Figures 7 and