© 1982 IEEE

Reprinted, with permission, from Conference Record, 

1982 IEEE/IAS International Semiconductor Power Converter Conference, May 1982.

Significance

Part 6 – Tutorials

Part 8 – Coordination of Cascaded SPDs

This paper was presented as a tutorial aimed at a semiconductor-oriented audience, giving an overview of the origin

of transient overvoltages and of IEEE and IEC documents under consideration in the early eighties, identifying and

categorizing transients. A brief review of available techniques and devices follows, with a description of the

principles of coordinated protection, specific experimental examples, and results reconciling the unknown with the

realities of equipment design.

The themes emphasized that effective protection of sensitive electronic equipment is possible through a systematic

approach where the capability of the equipment is compared to the characteristics of the environment, a basic tenet

of the electromagnetic compatibility documents. As more field experience is gained in applying these documents to

equipment design, the feedback loop can be closed to ultimately increase the reliability of new equipment at

acceptable costs, while present problems may also be alleviated based on these new findings in the area of

transient overvoltages.

Filename: Coordination 1982
THE COORDINATION OF TRANSIENT PROTECTION 

FOR SOLID-STATE POWER CONVERSION EQUIPMENT 

F.D. Martzloff 

r ,,---- +- 4 ..-A n '----- -A 

LulpvlaLG RGJG~ILI auu uevelupnlenL 

General Electric Company 

Schenectady, New York, USA 

ARSTRACT 

Transient avervo!!zges are nc !cnge: a: unknown :h;eai 

to the successful application of power conversion equipment, 

thanks to the availability of protective techniques and .d--.:--- vL:- WVIL~;J. LIII~ paper presents an overview of the origin of 

transient overvoltages and of recent IEEE and IEC documents identifying and categorizing transients. A brief review 

of available techniques and devices follows, with a description of. the principles of coordinated protection, specific 

experimental examples, and results reconciling the unknown 

with the realities of equipment design. 

INTRODUCTION 

Si~p !hektrnductian cf semicmd.;c:ors, tiaii~ieiii oxivoltages have been blamed for device failures and system 

malfunctions. Semiconductors are, indeed, sensitive to overvoltages. However, data have been collected for several 

years on the occurrence of overvoltages, to the point where 

the problem is now mostly a matter of economics and no 

longer one of lack of knowledge on what the environment of 

pwe: sj.s:ems can iiifici io poorlji proiecied semiconducior 

circuits. This statement may represent a slight oversimplification of the general problem because the environment is stiii defined in statistical terms, with unavoidable 

uncertainty as to what a specific power system can impress on 

a specific piece of power conversion equipment. 

The IEEE has published a Guide (1) describing the 

nature of transient overvoltages (surges) in low-voltage ac 

power circuits. T'nis Guide provldes miormation on the rate 

of occurrcnre, on the waveshape, ad 0:: :he eneigj.. 

associated with the surges, as a function of the location 

within the power system. In addition, the IEC has issued a 

report concerning insuiaiion coordination i2j, identifying 

four categories of installations, with a matrix of power system voltages and overvoltages specified for controlled siruarions. Other groups have also proposed test specifications, 

some of which are now enshrined in standards that may be 

applied where they are really not applicable, but have been 

applied because no other information was available at the 

time. 

At this time, the environment seems to be defined with 

~uffirie.~! de!ai!. Egwever, there is sti!! a !ack of giiibance on 

how to proceed for specific instances, and circuit designers 

may feel that they are left without adequate information to 

make informed decisions on the selection of component 

characteristics in the field of overvoltage withstand or protection. This situation has been recognized, and various groups 

concerned with the problem are attempting to close the 

by preparing application guides which will provide m

specific guidance !ha:: a mere de~ciipiisn of the envir

ment, although that description in itself is already a consid

able step forward. 

One of the difficulties in designing a protection schem

the industrial world of power conversion equipment is 

absence of an overall system coordinator, in contrast to 

world of electric utilities, for instance, which are gener

under the single responsibility of a centralized engineer

orgaiiization. The user or" power conversion equipmen

likely to purchase the materia! from a supp!ier ir.dependen

of other users of the same power system, and coordinat

of overvoltage protection is generally not feasible un

thqn 

. rnnA;+;c...n Ul-rn- ..-. . TV ULJG JGL, an iincoordinaied appiicaiion 

surge suppressors can lead to wasteful or ineffective resou

allocation, since independent users would each attempt 

provide protection in adjacent systems or independ

designers would provide protective devices in adjacent s

systems. 

To shed more light on this situation. this paper will bri

review some of the origins of transient overvoltages, w

reference to recently published IEEE and IEC docume

which provide g~idance on !he er?virar?men:. Tec!xiq.

and protective devices will then be discussed, and exam

of coordinated approaches presented. 

THE ORIGIN OF TRANSIENT OVERVOLTAGE

Two majnr causes of transient. nvervn!!ages have 

been recognized: system switching transients, and transi

triggered or excited by lightning discharges (in contras

direct lightning discharges to the power systems, which 

generally quite destructive, and against which total pro!ec!i

may not be economical in the average application). Sys

switching transients can involve a substantial part of 

power system, as in the case of power factor correc

capacitor switching operations, disturbances following rest

tion of power after an outage, and load shedding. Howe

these do not generally involve large overvoltages (more t

two or three per unit), but may be very difficult to supp

since the energies are considerable. Local load switch

especially if it involves restrikes in switchgear devices, 

piodiice higher voiiages ihan ihe power system switch

but generally at lower energy levels. Considering, howe

the higher impedances of the local systems, the threat to 

sitive electronics is quite real, and only a few conspicu

case histories of failures can cast an adverse shadow ov

large number of successful applications. 
VOLTAGE LEVELS 

Two different approaches have been proposed to define 

voltage levels in ac power systems. At this time, the divergences have not yet been reconciled, as each proposal has its 

merits and justification. The IEEE approach involves reciting 

a rate of occurrence as a function of voltage levels, as well as 

of exposure in systems that do not necessarily use protective 

devices. The IEC approach indicates only a maximum level 

for each location category, but no higher values are expected 

because this approach implies the application of protective 

devices. These two proposals will be quoted in the following 

paragraphs. 

The IEEE Guide (IEEE Std 587-1980) 

Data collected from a number of sources led to plotting a 

set of lines representing a rate of occurrence as a function of 

voirage for three iypes of exposiires iii iiiipiokcted ciifiits 

(Fig~re !!. These exposure levels are defined in general 

terms as follows: 

0 Lnw Exposure - Systems in geographical areas known 

for low lightning activity, with little load switching 

activity. 

0 Medium Exposure - Systems in geographical areas 

known for high lightning activity, with frequent and 

severe switching transients. 

High Exposure - Rare but real systems supplied by long 

overhead lines and subject to reflections at line ends, 

where the characteristics of the installation correspond 

to high sparkover levels of the clearances. 

It is essential to recognize that a surge voltage observed 

in a power system can be either the driving voltage or the 

voltage limited by the sparkover of some clearance in the 

SURGE CREST. kV 

*In some locations, sparkover of clearances may limit 

the overvoltages 

Figure 1. Rate of surge occurrence versus voltage level in 

unprotected circuits from IEEE Std 587 

system. Hence, the term unprotected circuit must be u

stood to be a circuit in which no low-voltage prote

device has been installed, but in which clearance spark

will eventually limit the maximum voltage. The distrib

of surge levels, therefore, is influenced by the s

producing mechanisms as well as by the sparkover lev

clearances in the system. 

The voltage and current amplitudes presented in 

Guide attempt to provide for the vast majority of ligh

strikes but should not be considered as "worst case," 

this concept cannot be determined realistically. One s

think in terms of the statistical distribution of strikes, ac

ing a reasonable upper limit for most cases. Wher

consequences of a failure are not catastrophic but m

represent an annoying economic loss, it is appropria

make a tradeoff of the cost of protection against the 

lihood of a failure caused by a high but rare surge. 

The IEC Approach (IEC Report 664, 1980) 

In a report dealing with clearance requirements for in

A: - - ----A:--*:- -..-----" LIUII CuululllilLlUii YUI~VJ~~, th~ !EC S~bc~i~~~i

SC

recommends a set of impulse voltages to be consider

representative of the maximum occurrences at dif

points of a power system, and at levels dependent upo

system voltage (Table I). The report is not primarily 

cerned with a description of the environment, but more 

insulation coordination of devices installed in these sys

This approach rests entirely on the establishment of 

trolled levels in a descending staircase, as the wiring sy

progress within the building away from the service entra

The fundamental assumption made in establishin

levels of Table I is that a decreasing staircase of overvo

will evolve from the outside to the deep inside of a bu

(system), either as the result of attenuation caused b

impedance network, or by the installation of overvoltag

iters a: the interf~ces. 

surge pratective device a! each interface, it must be r

nized that the successive devices will interact; the situat

not one of one-way propagation of the surges. Inde

protective device installed, say, at the III/II interface 

be so close (electrically) to the device at interface IVIII

it could prevent the latter from operating; in other w

the 111111 device might face the surge duty normally exp

to be handled by the IVIIII device. Thus, a vital asp

the selection of interface devices is that of ensuring p

coordination. 

Table I 

PREFERRED SERIES OF VALUES OF IMPUL

WITHSTAND VOLTAGES FOR RATED VOLTA

BASED ON A CONTROLLED VOLTAGE SITUA

Voltages line-to-earth Preferred series of impulse withst

derived frx? ra!d , vn!t.ges in ins!a!!a!ion ca!eenries 

system voltages, up to: 1- 

500 BOO 

1500 2500 

2500 4000 

1000 4000 6000 
In both the IEEE standard and the IEC report, the 

assumption has been made that the surge is impinging the 

power system through the service entrance and is occurring 

between phase and earth. Experience has shown that a frequent cause of distress is the voltage differences existing 

between conductors reputed to be at ground potential; in 

fact, one of them is elevated above the other by the flow of 

surge current. This situation, not addressed in either document, needs to be recognized and dealt with on an individual, case-by-case basis, lest a false sense of security be 

created by restricting the protection to the power service 

entrance. 

WAVESHAPE OF THE 

TRANSIENT OVERVOLTAGES 

Observations in different locations (3-6) have established +,.a *L- -,.,.* r ---..,.- ..-- .r &----:--A 

LllaL LIIG IIWJL IIG~UGIIL typ~ UI LI~IIJIGIIL oveivoiiage in ac 

power systems is a decaying oscillationj with frequencies 

between 5 and 500 kHz. This finding is in contrast to earlier 

attempts to apply the unidirectional double exponential 

voiiage wave, generaiiy described as i.2i50, aitnough .the 

unidirectional voltage wave has a long history of successful 

application in the field of dielectric withstand tests and is representative of the surges propagating in transmission systems 

exposed to lightning. The IEEE Guide recommends two 

waveshapes, one for the indoor environment, and one for 

the outdoor and near-outdoor environment (Figure 2). Not 

only is a voltage impulse defined, but the discharge current, 

or short-circuit current of a test generator used to simulate 

these transients, is also defined in the IEEE document. 

The oscillatory waveshape simulates those transients 

affecting devices that are sensitive to dvldt and to voltage 

reversals during conduction (7). The unidirectional voltage 

and current waveshapes, based on long-established ANSI 

standards for secondary valve arresters, simulate the transients wiieie eneigji conieni is ihe significant parameier. 

ENERGY AND SOURCE IMPEDANCE 

The energy involved in the interaction of a power system 

with a surge source and a surge suppressor will divide 

between the source and the suppressor in accordance with 

the characteristics of the two impedances. In a gap-type 

suppressor, the low impedance of the arc after sparkover 

I \ T = lops (1 = 100 kHz) 

OpenCircuit Voltage, 

Curreni Deiined by Table ii 

Indoor Environment 

forces most of the energy to be dissipated elsewhere, e.g

the power system series impedance or in a resistor adde

series with the gap for limiting the power-follow current. 

an energy-absorber suppressor, by its very nature, a subs

tial share of the surge energy is dissipated in the suppres

but its clamping action does not involve the power-fo

energy resulting from the short-circuit action of a gap. I

therefore, essential to the effective use of suppres

devices that a realistic assumption be made about the sou

impedance of the surge whose effects are to be duplicated

Unfortunately, not enough data have been collected 

what this assumption should be for the source impedanc

the transient. Standards or recommendations either ign

the issue, such as MIL STD-1399 or the IEC Report 66

its present published form,' ot they sometimes indi

values applicable to limited cases, such as the SWC test 

eieciionic eqiiipmerii operating in high-voirage sub

tions (8). The !FEE Guide z!!emp!s to re!a!e impeda

with three categories of locations, A, B, and C. For m

industrial environments, Categories A or B will ap

Category C is intended for outdoor sttuations (Table IIj. 

MATCHING THE ENVIRONMENT 

WITH THE EQUIPMENT 

On the basis of the various documents mentioned in 

preceding paragraphs, an equipment designer or user 

take a systematic approach to matching the transient o

voltage capability of the equipment with the environmen

which this equipment is to be installed. This design 

involve tests to determine the withstand levels (91, s

measurements andlor analysts (r, determine the degre

hostility of the environment, and a review of available 

tective devices. The latter will be discussed in the follow

paragraphs. 

Transient Suppressors 

Two methods and types of devices are available 

suppress transients: blocking the transient through s

fl;iei, oi diveriiiig it to gro"iid ibroiig'il some 

linear device. This nonlinearity may he ei!her a frequ

nonlinearity (high-pass filter) or a voltage nonlinea

Continuing studies by the IEC SC128A Working Group are 

addressing this issue, and additional publications are anticipated. 

Open Circuit Voltage Discharge Current 

Outdoor and Near-Outdoor Environment 

Figure 2. Transient overvoltages and discharge currentsin IEEE Std. 587-1980 
(clamping action or crowbar action). In this paper, a majority 

of the discussion will center on the latter type, since voltage 

clamping devices or crowbar devices are the most frequently 

used (10). 

Voltage-clamping devices have a variable impedance, 

depending on the current flowing through the device or the 

voltage across its terminals. These components show a nonlinear characteristic, i.e., Ohm's law E=RI, can be applied 

but the equation has a variable R. Impedance variation is 

monotonic and does not contain discontinuities, in contrast 

to the crowbar device which shows a turn-on action. As far 

as volt-ampere characteristics of these components are concerned, they are time-dependent to a certain degree. However, unlike sparkover of a gap or triggering of a thyristor, 

time delay is not involved here. 

When a voltage-clamping device is installedj the circuit 

remains unaffected by the device before and after the transient for any steady-state voltage below clamping level. 

increased current drawn through the device as the voitage ottomnto tn rieo rocralta in lrnltooo rlomninn artinn Tnrroorprl U.."L..YLL L" ..Y" LVYULLI 1.1 ."..Ub" ".Ul..yl.lb YILIVLI. .I'"I"U.,"U 

voltage drop (IZ) in the source impedance due to higher 

current results in the apparent clamping of the voltage. It 

should be emphasized that the device depends on the source 

impedance, Z, to produce the clamping. X voltage divider 

action is at work where one sees the ratio of the divider not 

constant, but changing (Figure 3). The ratio is low, however, if the source impedance is very low. The suppressor 

cannot work at all with a limit zero source impedance. In 

contrast, a crowbar-type device effectively short-circuits the 

transient to ground. Once established, however, this short 

circuit will continue until the current (the surge current as 

well as any power-follow current supplied by the power system) is brought to a low level. 

Figure 3. Voltage clamping action of a suppressor 

The crowbar device will often reduce the line voltage 

below its steady-state value, but a voltage clamping device 

will not. Substantial currents can be carried by the crowbar 

suppressor without dissipating a considerable amount of 

energy within the suppressor, since the voltage (arc or 

forward-drop) during the discharge is held very low. This 

characteristic constitutes the major advantage of these 

suppressors. However, limitations in volt-time response, 

power-follow, and noise generation are the price paid for this -A 

au vauiagc. AS -.-I.^-- vulrage IIIU cases au uss a spark-gap, 

significant conduction cannot take place until transition to the 

arc mode has taken place by avalanche breakdown of the gas 

between the electrodes. The load is left unprotected during 

the iniiiai rise due to this deiay time itypicaiiy in 

microseconds). Considerable variation exists in the sparkover voltage achieved in successive operations, since the process is statistical in nature. For some devices, this sparkover 

voltage can also be substantially higher after a long period of 

rest than after successive discharges. From the phys~

nature of the process, it is difficult to produce consiste

sparkover voltage for low voltage ratings. This difficulty 

increased by the effect of manufacturing tolerances on ve

small gap distances. This difficulty can be alleviated by filli

the tube with a gas having lower breakdown voltage than a

However, if the enclosure seal is lost and the gas is replac

by air, this substitution creates a reliability problem becau

the sparkover of the gap is then substantially higher. 

Another limitation occurs when a power current from t

steady-state voltage source follows the surge dischar

(follow-current or power-follow). In ac circuits, this powe

follow current may or may not be cleared at a natural curre

zero. In dc power circuits, clearing is even more uncertai

Additional means must, therefore, be provided to open t

power circuit if the crowbar device is not designed to provi

seif-ciearing action within specified iimits of surge energ

'..rrtnm .rnltona ",.A nn..mr~f~llP...r 0.. -ma..* ojorul~~ vvrrtlgr, ~LLV pvwrn -IVIIVW ~ULL~I~L. 

A third iimitation is associated with the sharpness of t

spxkover, which produces fgs! current rises in !he circ~

and, thus, objectionable noise. A classic example of th

kind of disturbance is found in oscillograms recording t

sparkover of a gap where the trace exhibits an anomaly befo

the sparkover (Figure 4). This anomaly is due to the del

introduced in the oscilloscope circuits to provide an advanc

trigger of the sweep. What the trace shows is the eve

delayed by a few nanoseconds, so that in real time, the g

sparkover occurs while the trace is still writing the pr

sparkover rise. Another, more objectionable effect of th

fast current change can be found in some hybrid protecti

systems. Figure 5 shows the circuit of such a device. 

(a) Open clrcult lOkV 1 rs (b) SWrkover of 120 V Arrester 

at 1600 V In 550 ns 

Figure 4. Interference to oscilloscope circuits caused by 

gap sparkover 

Figure 5. Hybrid protector with gap 
found In the commerce. The gap does a very nice job of 

discharging the impinging high-energy surges, but the magnetic field associated with the high di/dt induces a voltage in 

the loop adjacent to the secondary suppressor, adding what 

can be a substantial spike to the expected secondary clamping 

voltage. Consequently, most electronic circuits are better 

protected with voltage clamping suppressors than with 

crowbars, but sometimes the energy deposited in a voltage 

clamping device by a high current surge can be excessive; a 

combination of the two devices can provide effective protection at optimum cost. However, this combined protection 

must be properly coordinated to obtain the full advantage of 

the scheme. The following paragraphs will discuss some of 

the basic principles of coordination and provide some 

examples of applications. 

PROTECTION COORDINATION 

One of the first concepts to be adopted when considering 

a coordinated scheme is that current, not voltage, is the 

independent vanable involved. The physics of overvoltage 

generatian invoke &hei ;ig:l;iiing or load swiiciiing. Both 

are current sources, and it is only the voltage drop associated 

with the surge current flow in the system impedance which 

appears as a transient overvoltage. Perhaps a long history of 

testing insulation with voltage impulses has reinforced the 

erroneous concept that voltage is the given parameter. Thus, 

overvoltage protection is really the art of offering low 

impedance to the flow of surge currents rather than attempting 

to block this flow through a high series impedance. In combined approaches, a series impedance is sometimes added in 

the circuit, but only after a low impedance diverting path has 

first been established. 

When the diverting path is a crowbar-type device, little 

energy is dissipated in the crowbar, as noted earlier. In a 

voltage clamping device, more energy is deposited in the 

device, so !hat the energy h.nd!ing c~psbi!i!y ef I candldI!e 

suppressor is an important parameter to consider when 

designing a protection scheme. With nonlinear devices, an 

error made in the assumed value of the current surge pro- duces little +he LLL~ .7-l+o"o v vrragb dv o.,"--aA vb!urbu "---"" aw UJJ +I.- 

suppressor and thus applied to the protected circuit (1 11, b

the error is directly reflected in the amount of energy whi

the suppressor has to absorb. At worst, when surge curren

in excess of the suppressor capability are imposed by th

environment, because of an error made in the assumption 

because nature tends to support Murphy's law or because 

human error in the use of the device, the circuit in need 

protection can generally be protected at the price of failure 

the short-circuit mode of the protective device. However, 

substantial power-frequency currents can be supplied by th

power system, the fail-short protective device generally te

minates as fail-open when the power system fault in th

failed device is not cleared by a series overcurrent protectiv

device (fuse or breaker). Note that in this discussion, th

term "fail-safe" has carefully been avoided since it can mea

opposite failure modes to different users. To some, fail-sa

means that the protected hardware must never be exposed 

an overvoltage, so that failure of the protective device mu

be in the faii-short mode, even ~f ~t puts the system out 

=peratla. TG ether users, fail-safe means that the Yiiiiii

must be maintained. even if the hardware is left temporari

unprotected, so that failure of the protective device must 

in the open-circuit mode. 

EXAMPLES OF COORDINATED 

SURGE PROTECTION 

Retrofit of a Control Circuit Protection 

In this case history, a field failure problem was caused 

lack of awareness (on the part of the circuit designer) of t

degree of hostility in the environment where the circuit w

to be installed. A varistor had been provided to protect t

control circuit components on the printed circuit board, b

its capability was exceeded by the surge currents occurring 

a Category B location (Table 11). To the defense of the c

cuit designer, however, it must be stated that the data 

Table I1 were not available to him at the time. 

Since a nurnher of devices were in service, co~pl

redesign was not possible, and a retrofit - at an acceptab

cost - had to be developed. Fortunately, the power co

sumption of this controi circuit was iirilited so that it w

Table I1 

RECOMMENDED VALUES FROM IEEE STD 587 

Surge Voltages and Currents Deemed to Represent the Indoor Environment 

and Recommended for Use in Designing Protective Systems 

Energy (joules) 

Type Deposited in a Suppressor* 

Comparable to Impulse of Specimen with Clamping Voltage of 

Location IEC No 664 Medium Exposure or Load 500V lO0OV 

Category Category Waveform Amplitude Circuit (120 V System) (240 V System) 

A Long branch 

Circuits and 

outlets 

6 kV High impedancet - I1 0.5 ps-100 kHz 200 A Low impedance+, 8 0.8 

3 Mn:cr feeders. A>& .-, v A =n ov pa ..- 6;(;. rug11 in~pedance* - - ..>-L ~ 

short branch III 8 X 20 ps 3 kA Low impedance? 40 80 

circuits, and 6 kV High impedancer - - load center 0.5 p-100 kHz 500 A Low impedance*, 8 2 4 ~ - - ~ -- - - - - - - - - 

*nthor -.---. r..nnrnrlnrr hs.ri-,. rliffnrr.-.4 ,.1^--.-:.."-.^1*.." - 12 -.:re ---- ---. . ,-,,.,wv.r --.r~r..* r~a~uplrqj ru~m&s wuuru ~ssslvr uu~rrrui ellergy levels. 

t~or high-impedance test specimens or load circuits, the voltage shown represents the surge voltage. In making simulation tests. use 

that value for the open-circuit voltage of the test generator. 

 or low-impedance test specimens or load circuits, the current shown represents the discharge current of the surge (not the shortcircuit current of the power system). In making simulation tests, use that current for the short-circuit current of the test generator. 

 he maximum amplitude (200 or 500 A) is specified, but the exact waveform will be influenced by the load characteristics. 
possible to insert some series impedance in the line, ahead of 

the low-capacity varistor, while a higher capacity varistor was 

added at the line entrance to the circuit (Figure 6). Laboratory proof-test of the retrofit demonstrated the capability of 

the combined scheme to withstand 6 kA crest current surges 

(Figure 7A) and a 200% margin from the proposed 

Category B requirement, as well as reproduction of the field 

failure pattern (Figure 7B). The latter is an important aspect 

of any field problem retrofit. By simulating in the laboratory 

the assumed surges occurring in the field (Table II), 

verification of the failure mechanism is the first step toward 

an effective cure. Figure 7C illustrates the effect of imprope 

installation of the suppressor, with eight inches of leads 

instead of a direct connection across the input terminals of 

the circuit. 

TERMINAL RETRO-FIT MULTl~PlN 1 - BOARO PROTECTIVE CONNECTOR ------, DEVICE P.C. BOARD PACKAGE 

Figure 6. Retrofit protection of control circuit 

Coordination Between a Secondary Surge Arrester and a 

Varistor 

In this example, the objective was to provide overvoltage 

protection with a maximum of 1000 V applied to the protected circuit, but to withstand current surges on the service 

entrance of magnitudes associated with lightning, as defined 

in ANSI C62.1 and C62.2 standards for secondary arresters. 

The only arresters available at the time which could withstand a iO kA crest 320 ps impuise naci a protective kiamping! !eve! cf ~pproximate!y 220s V (!2!. Some dls!axce was 

available between the service entrance and the location of the 

protected circuit, so that impedance was in fact inserted in 

series between the arrester and the protected circuit where a 

varistor with lower clamping voltage would be installed. The 

object was to determine the current level at which the 

arrester would spark over for a given length of wire between 

the two protective devices, relieving the varistor from the 

excessive energy that it would absorb if the arrester would 

not spark over. 

A circuit was set up in the laboratory (13), with 8 m 

(24 ft) of #12 (2.05 mm) two-wire cable between the 

arrester and the varistor. The current, approximately 

8/20 ps impulse, was raised until the arrester would sparkover about half of the time in successive tests at the same 

level, thus establishing the transfer of conduction from the 

varistor to the arrester. Figure 8A shows the discharge 

current level required from the generator at which this 

transfer occurs. Figure 8B shows the voitage at the varistor whan 

7.'. WL' +ha LUW UL 

nrr~~t~~ '"L". Anor 

Y"V.7 nnt ""L a,,'&. ~n,-k =ye:, Figure 8C &=.,y~ the 

voltage at the arrester when it sparks over: this voltage 

would propagate inside all of the building if there were no 

suppressor added. However, if a varistor is added at eight 

meters, the voltage of Figure 8C is attenuated to that shown 

Upper trace: Voltage across 

V15OLAl varistor on PC board, 

200 Vldiv. 

Lower trace: Applied surge current, 

2000 P.!diu. 

Sweep speed: 10 &div. 

Additional surge protection removed: 

Vl5OL.Al varistor on PC board is 

the only protection. 

Upper trace: Voltage across -.. --- V I >ULA i varistor 

Lawr trace: Vxis!gr curren! 

200 Aldiv. Sparkover occurs at 

about 700 A: 60 Hz power-follow 

de~tioy~ ;he PC board. 

Sweep speed: 10 &div. 

Same as A, but with varistor 

mounted on e~ght-~nch leads irom 

!ermim! bgerd. 

Figure 7. Laboratory demonstration of retrofit 

effectiveness 

in Figure 8D, at the terminals of the varistor. 
Figure 8. Transfer of conduction in a coordinated 

scheme of protection 

rnrn-.-L.-- '7 C.--L4. 2- ~vlia~cning auppressur ~n~no~riry to the Environmeni 

It is a recognized fact that varistors exhibit, as do many 

other components, an aging characteristic, so that a finite life 

can be predicted. Most manufacturers provide information 

on this aspect of application, and IEEE standards identify this 

parameter as one of the significant evaluation tests (14). 

Carroll has shown (15) how statistical information presented 

in IEEE Std 587 can be combined with Pulse Lifetime Ratings published by manufacturers (16) to arrive at a rational 

selection of device ratings, with a specific life goal, in a costeffective manner. 

However, these ratings are generally expressed as a 

number of pulses of constant value, e.g., the rated life of a 

given varistor may be 1 pulse of 6 kA at 8/20, 10 pulses at 

2 kA, 1000 pulses at 500 A, and so forth. But since the 

si;iges ei;coiiii;eieb in rea: :ifs have a range of a 

slope of probability versus magnitude described hy Figure I, 

one must consider the effect of this array of pulses with 

,,

UUG~W~ VL~~UG~ I~UIF.~ ~11.11 the coiisiaiii piiises impiied 

the manufacturer's pulse lifetime rating. 

The method described by Carroll in the referenced pa

provides a computation that can be applied in general term

but repeating it here would be too lengthy. Rather, we w

take two examples of application and develop a table show

how the Pulse Lifetime Ratings can be combined with 

data from IEEE Std 587 to make a reasonable estimation 

the rated life consumption. The computations shown in 

tables have been made with four digits for the sake of allo

ing a check of the arithmetic, but the base data are far fr

four significant digits in their accuracy, and the numbers 

read from curves with rather coarse logarithmic scales. Ho

ever, these examples do illustrate the method and the resu

that can be expected. 

The first task is to convert the voltage surge density pr

abiiiiy of Figure i inio a hisiogram of surge currents. 

farr?i!y of surge vo!!age ce!!s car? be defir?ed frnm the Figur

line, with the density read at the center of the cell. T

number of occurrences for any cell is then the value of 

ordinate of the line, minus the number of total occurren

of all cells to the right of the cell of interest. In the comp

tations of Table 111, this conversion is shown in the f

three columns, indicating the voltage level at the cell cen

the number per year, and the number of occurrences 

year. 

From the description of the Category B in IEEE Std 5

one can deduce an implied source impedance of 6 kV/3 

for a surge or 8/20 ps, or 2 0 as the most severe 

Category B. The current that will flow in a varistor c

nected at this Category B location is then the surge volta

minus the varistor clamping voltage, divided by the 2 

source impedance of the surge. The varistor clamping v

age can be determined if the current is known, so an ite

tion would be required to obtain the clamping voltage. Ho

ever, one can assume a ciamping voiiage, and iaier checic 

va!ic!i!y of !he assumption agains! !he reru!!ing curr

obtained. The fourth column of Table111 shows 

Table 111 

LIFE CONSUMPTION - 14 mm, 130 V RMS VARISTOR, 

CATEGORY B, LOW EXPOSURE 

Assumed Voltage Number clamping Available Surge current Rated number Percent life 

surge per occurrences voltage of driving @ 2 0 pulses consumed level year per year at varistor voltage A for this 

v per year above level level v surge current 

3000 0.01 0.01 500 2500 1250 

2500 0.02 0.01 480 2020 1010 

1700 0.10 0.08 450 1250 625 

1300 0.20 0.10 420 880 440 

900 1 0.80 400 500 250 

700 2 1 380 320 160 

500 10 8 370 230 115 

Cumulative life consumption per year 

Time to reach rated life. years 
assumed and subsequently checked value of the clamping 

voltage, hence the value of the available driving voltage in 

the next column, and the resulting surge current value, 

assumed to be an 8/20 ps waveshape. 

Turning then to the published Pulse Lifetime Ratings, 

one can read the rated number of pulses corresponding to 

the surge current for each cell. Table 111 is computed with 

the ratings for a 14 mm varistor (Figure 9a); Table IV is 

computed for a 32 mm varistor (Figure 9b). Note that this 

"rated life" is defined as the condition reached when the 

varistor nominal voltage has changed by 10%; this is not the 

end of life for the varistor, but only an indication of some 

permanent change beginning to take place. The varistor has 

still retained its voltage clamping capability at this point. 

For each level of surge current, the number of pulses is 

read on ihe famiiy of curves of Figures 9a or 9b, aiong the 

vertica! ~xis, since these 2re 8/20 ps impc!ses. The number 

of pulses with constant amplitude is shown in the next-to-last 

column of Table 111. We can now define, for each level, the 

percentage of life consumed for one year of exposure at that 

level. For instance, at the 2500 V level of Table 111, there 

will be 0.01 surges of 1010 A per year, with 10 allowed by 

the ratings. Therefore, in percent, the life consumption is 

(O.Ol/yr x 100)/10, or 0.10%. Likewise, taking the 900 V 

level, the consumption is (O.l/yr x 100)/2000 = 0.04%. 

The total of these life consumptions at all cell levels is then 

0.43% of the rated life in one year, yielding an estimated 

232 years for this 14 mm varistor to reach its rated life in the 

Low-Exposure Category B environment. 

Similar computations for a 32 mm varistor in a 

Category B, Medium Exposure, are shown in Table IV. In 

the case of this "Medium Exposure," we note the high frequency of occurrences below 3000 V, reflecting the "frequent and severe switching transients" cited in the IEEE 

definition of Medium Exposure. Thus, a still very conserva- 

!ive es:ima:e :v~u!d be !ha: as many as ha![ of :he 

occurrences would be due to lightning. with the attendent 

8/20 ps high energy surges, while the other half would be 

switching transients, having a lower energy content than the 

8/20 ps surges accounted in this computation, being oscillatory as typified by the 0.5 ps - 100 kHz wave. This 

End of l8letmw IS defsned as d deqradatlon faalute which occurs when the 

devkce exh~txts a sh~ft In the varlstor voltage a1 one Ill m~ll~ampere In excess 

of + looo of the ln~tm value Thts type 01 fatlure Is normally a result of a 

decredslnq V1 v~lue but does not prevent the device from conllnulng to funct8o

However the VdrlStOr will no longer meet the origlnal Speclficartons 

Figure 9. Pulse lifetime ratings 

translates to 13 surges of 760 A, 35 surges of 525 A, and 2

surges of 285 A, still a high number of lightning surges a

therefore certainly conservative. Using this conservati

estlmate of half of the low-magn~tude surges and all of t

I.:"l. ^..--^" LA:-- 8/?(? ps I:-L.^:^- -^l^.^A ~tte~t-~~~a~ut~uub 3~1563 ~61115 II~$IIUIIII~-I GI~

surges. the computation of Table IV yields 21 years to rea

rated life for the 32 mm varistor. In thls case, where t

rated life is reached earlier, ~t should be pointed out that t

results are strongly influenced by the assumption made f

the source impedance. Using the IEEE 587 implied value 

Table IV 

LIFE CONSUMPTION - 32 mm, 150 V RMS VARISTOR, 

CATEGORY B, MEDIUM EXPOSURE 

Voltage Number Total Occurrences Clamping Available Surge Rated number 

surge per occurrences due to voltage driving current of pulses for life 

level year per year at lightning of varistor voltage @ 2 0 this surge consumed 

V above level level V V A current 

Cumulative life consumption per yea1 

Time to reach rated life, years 
2 R ieads to these conservative resuits. For example, the 

FCC test for communication equipment interfacing with 

power lines (17) implies a 2.5 il source impedance. Current 

studies for complementary data to the IEC Report 664 make 

the assumption of a surge originating on the primary of a distribution transformer, with a 63 il source impedance, yielding currents of less than I kA available at the service 

entrance interface. Thus, there is still room for more precise 

definitions of the source impedance, but we should recognize 

that any attempt to make broad generalizations will always 

encounter the contradiction of some special cases. 

CONCLUSION 

Effective protection of sensitive electronic equipment is 

possible through a systematic approach where the capability 

of the equipment is compared to the characteristics of the 

environment. The combined efforts of severa! orga~izations 

have produced a set of data which provide the circuit 

designer with reasonable information, albeit not fine 

specificaiions, on the assumpiions to be made in assessing 

the hos!i!ity of the e~~virmme~~!. With the pub!ica!im of the 

IEEE Guide, and of application guides in the near future, we 

can expect better knowledge of the power system environment. As more field experience is gained in applying these 

documents to equipment design, the feedback loop can be 

closed to ultimately increase the reliability of new equipment 

at acceptable costs, while present problems may also be 

alleviated based on these new findings in the area of transient overvoltages. 

ACKNOWLEDGMENTS 

The data base for the Guide quoted in the paper as well 

as the writing of that Guide was provided by the members of 

the IEEE Working Group on Surge Characterization in LowVoltage AC Power Circuits. Robert Mierendorf emphasized thn .,,, ,;,,,;fi,,,,,, o.nn. rn..na of c!eaiance sparkovei; aiid Peter Richiiiaii's 

critique of the concepts relating to source impedance proved 

very effective; and, Eric Carroll's cross examination of the 

IEEE Std 587 led to the concept of relating varistor pulse I:''-*.-- 

IHGL~I~IG data io the d~siiibuiion of expected surge currents. 

REFERENCES 

"IEEE Guide for Surge Voltages in Low-Voltage AC 

Power Circuits," IEEE Std 587-1980. 

"Insulation Coordination Within Low-Voltage Systems 

Including Clearances and Creepage Distances for Equipment," International Electrotechnical Commission 

Report 664 (1980). 

J.E. Lenz, "Basic Impulse Insulation Levelg of Mercury 

Lamp Ballast for Outdoor Applications," Illuminating 

Engrg., pp. 133-140 (February 1964). 

F.D. Martzloff and G.J. Hahn, "Surge Voltage in Residential and Industrial Power Circuits," IEEE PAS 89 

(6), pp. 1049-1056 (JulyIAugust 1970). 

R. Hasier and R. iagadec, "Digital Measurement of Fast Tr.lno;nrrtn - D-..7ar C..-nl.r T '7 DWnn ?FA P.--.rno;r,- l lallolrllro "fi I vnbl OupplJ LLIIbD, 1 I VL. JI U U.YlllpVI)IIIIII 

and Technical Exhibition on Electromagnetic C~m~uatibili~. 

Rotterdam, Holland, pp. 445-448 (May 1979). 

6. F.D. Martzloff "Transient Overvoltages in Secon

Systems," General Electric Company Re

81CRD121, Schenectady, NY (1981). 

7. P. Chowdhuri, "Transient-Voltage Characteristics of 

coti Power Rectifiers," IEEE IA-9 (5), p. 

(SeptemberIOctober 1973). 

8. "Guide for Surge Withstand Capability," (S

ANSIIIEEE C37.90a-1974. 

9. F.A. Fisher and F.D. Martzloff, "Transient Control 

els, A Proposal for Insulation Coordination in L

Voltage Systems," IEEE PAS-95 (11, pp. 120

(JanuaryIFebruary 1976). 

10. Transient Voltage Suppression Manual, Chapter 2, Gen

Electric Company, Auburn, NY, pp. 13-17 (1978). 

11. Transient Voltage Suppression Manual, Chapter 2, Gen

Electric Company, Auburn, NY, p. 15 (1978). 

12. F.D. Mzrtz!e!?', "Transien: Oveivoltage Proieciion: 

Implications of New Techniques," Pror, 4!!! Sy.mp

and Technical Exhibition on Electromagnetic Compatib

Zurich, Switzerland, pp. 505-510 (March 1981). 

13. F.E. ?vfaiiziuiT, "Coordination of Surge Protector

Low-Voltage AC Power Circuits," IEEE PAS-99 

pp. 129-133 (JanuaryIFebruary 1980). 

14. "Test Specifications for Varistor Surge-Protec

Devices," IEEE Sfd C62.33, 1982. 

15. E. Carroll, " Transient Attenuation with Metal Oxide V

tors for AC Mains Powered Equipments," presented 

"Comel," University of Twente, Holland (

vember 1980). 

16. Transient Voltage Suppression Manual, Chapter 4, Gen

Electric Company, Auburn, NY, pp. 39-41 (1978). 

17. Longitudinal Voltage Surge Test #3, Code of Fed

Regulation, Section 68.302(e), Title 37, Telecommu

tions, Washington, D.C., U.S. Government Prin

Office. 1977. 

Frangnis I). !Vlar!z!e!T @4'56! 

born in France, and received 

undergraduate degree at the E

- Spkciaie ve ~kcanique 

E'E!ectricit& ia 1951 ; he recei

the MSEE degree from Geo

Tech in 1952 and the MSIA deg

from Union College in 1971. 

Since 1956 he has been with 

General Electric Company, wh

he gained experience in the Transformer and Switch

Divisions. Upon joining General Electric Corporate Rese

and Development in 1961, he became involved in po

semiconductor circuits and overvoltage protection. He 

participated in the introduction and application of metal o

varistors since 1971. 

In IEEE, Mr. Martzloff is active on the Surge Protec

Devices Committee. He is chairman of the Working Gr

on Surge Characterization in Low-Voltage Circuits. H

also a member of the Ad Hoc Advisory Subcommittee of 

USA Advisory Committee on IEC SIC 28A and ANSI 

Su&-omiiii;;ee oii Loii;-Vakage Suige Pio;ec;ii;e Eevices, 

has been awarded 10 U.S. patents, primarily in the fiel

varistors and transient protection.