TECHNICAL PAPER 

PRESENTED AT 

IN 

MUNICH, WEST GERMANY 

SEPTEMBER 17 - 21,1979 

PROTECTIVE DEVICES, SYSTEMS & METHODS 

Transient Overvoltage Protection 

Coordination in the Undefined 

Real World Environment 

by 

F. D. Martzloff 

General Electric Company 

1979 Solid-state PowerConversion (All rights reserved) 

P.O. Box 2889 

Oxnard. California. USA 
TRANSIENT OVI:.RVOI.TA<~E PILOTEC'flON COO11DlNA'flON 

IN TIIE 1INI)EFINIII) 11EAI. WORI.1) I~NVIItONMliN'I' 

P.D. Martzloff 

Corporate Ilcset~rch and I)evelopmcml 

General IClcctrir Company 

Schenectady. New York, IJSA 

Trn~rsient overvoltnges nrr no longer nn urtknowr~ tlrrent to the successful 

npplirntion of power conversion equipment, and protective techniqt~c~s ond devices nre 

nvuilnhle. The epproprinte selection of these, however, rcn~nins n difficult tusk 

becnuse the exnct nature of trnnsients in the real world is ot best only stntistienlly 

defined. Therefore, the choice involves teclmicnl and cconon~ic Occisions based on 

cnleulated risks rnther thttn drtern~inistic optimizution. In oddition to this, protection in n conrplcx system is required at more thnn one point of the circuits. Indiscrinlinnte or unroordi~cnted epplicntion of devices nt these sevrrnl points may be 

wnstcful or ineffective. 

This prrper presents an overview of the origin of trrrrrsielct overvoltnges and of 

current Il.;JiE rind IJiC activities to idrntify nt~d celegorizr transients. A brief review 

of avnilnhle techniques and devices follows, nr~d the major pnrt of the pnper tlescril~es 

the prinriplcs of coordinated protection with specific rxperirucntnl exnrnples nnd 

results reconciling tlle unknown with the realities of cquipnrcnt design. 

INTRODUCTION 

Sinoc the i~rlrotlurtion of senlieontluetors, trn~rsirnt overvoltnf:rs hnve been 

blnnred for devire failures and system rnalPunctions. Semiconductors arc, indeed, 

sensitive to overvoltccges. Ilowever, clntn hnve been collected for sever01 years on the 

ocleurrenee of ovcrvoltoges, to thr point where the problem is now nwstly n matter of 

economics nnd no longer one of luck of knowledge on wlrat the envirorrrnerrt of power 

systems con inflict to poorly protected semiconductor circuits. 'I'his stntement mny 

represent a slight oversimplifiention of the genernl problem, bernuse the environment 

is still defined in stntisticnl terms, with the unavoiclnhle uncertninty ns to whnt a 

specific power system cnn impress on a specific piece of power cotlversion equipmel~t. 

A Working (;roup of IliEl: hns prrpnred n Guide tlesrribing the nuture of tronsicnt overvolttlges(.i~rrges) itr ne power circuits rated up to fit10 V. 1 'This (hide 

provides infornlntion on the mte of orctclrrence, on the wrrvesh~~pe, old on the encrgy 

nssoeinted with the surges, us n ftnwtion of the locntion within the powrr system. In 

nclclition, a s~hcommitter of the It:(: lrns developed a report conrrrning Insulntion 

Coordinntion ? nnd Irns reroncmentled the use of four etrtegorics of installntions, with 

n matrix of power system voltnges und overvoltnges specified for corclr-olletl siltrations. 

Other groups hnvc nlso proposed test sprcifirntions, some of which are now enshrined 

in stnnd~trds tlrnt muy be npplirtl where tlwy are renlly not npplicnble, but were 

npplied bccSnuse no other inforrnntion wns nvnilnble at the time. 

At this time, the environment seems to t)c defined with sufficient detail. 

Ilowever, there is still n lnck of gcricinnce on trow to proceed for slwcifie instances, 

nnd circuit designers mny feel thnt they are left without ndcqurcte information to 

nrnke ir~fornrcd decisions on the selection of component chnrneteristies in the field of 

ovcrvoltnge withstand or protection. This situation has been recognized, and the 

various groups concerned with the problem are ettenlpting to close the gap by 

preparing Applicnlion Guides which will provide more specific guidnnce than a mere 

description of the environment, nlthouglr thnt description in itselP is alreedy n considernble step forward. 

Tr;msicnt Overvoltage Protection Coordination in the llntletined I<wI World I~nvironmcnt I 

PC-79-3.4 
One of tho difficulties in designing a protection scheme in the industrial world 

of powcr conversiotl eq~~ipment is the absence of an overall system coordinator, in 

contrnst to the world of electric utilities, for instance, which are generally undcr the 

single rcsponsibility of a cenlralized engineering organization. The user of power 

c*onversion eqc~ipment is likely to purchase the ~nnterial from a supplier independently 

of ott~cr users of the snlne power system, and coordinntion of overvoltage protection 

is gcnernlly not feasible under tl~ese conditions. Worse yet, an uncoordinn tcd applicatio~r of surge suppressors co111d lead to wasteful or ineffective resource ~llocation as 

indcperident users would each attempt to provide protection in adjacent systems, or 

independent designers would provide protective devices in adjacent subsystems. 

To shed more light onto this situation, this paper will briefly review some of 

the origins of transient overvoltnges, with reference to IEEE and IEC documents 

recently completed but not yet published thnt will provide guidance on the environment. Techniques and protective devices will then be discussed, and examples of 

coordina ted nppro~chcs presented. 

'RI R ORIGIN OF TJt ANSIENT OVRRVOI*TAGB 

Two major cuuses of transieri t overvoitages have long heen recognized: system 

switctkirrg transients and transielrts triggered or excited by lightning discharges (in 

contrast to direct lightning dischat-gcs to the powcr systems, as these are generally 

quite destructive, and protection against these mny not be econornicol in the average 

application). System switching transients con involve n substantifll part of the power 

system, as in the case of power factor correction capacitor switching operations, 

disturbunccs following restoration of power nfter an outage, and load shedding. 

flowever, these do not generally involve substnntinl overvoltnges (more th~n two or 

three pcr unit) hut may be very dirficult to suppress since the energies nre considerablc. Locnl load switching, espccidly if it i~ivolves restrikes in the switchgear 

devices, will produce higher voltages than the power system switching, but generally 

nt lower energy levels. Considering, however, the higher impednnces of the local 

systems, the thrent to sensitive electronics is quite real, and only a few conspicuo~~

case histories of failures can cast a very adverse shadow over a large number of 

strccessf ul applicat ions. 

VO J,TAGl? 1.EVEI.S 

Two different approaches have been proposed to def he the voltnge levels in ac 

power systems. At this time, the divergences llnve not yet been reconciled, as each 

proposal has its merits and justification. As proposed by the Working Group already 

rnt?ntioned, the IEEK approach involves reciting a rate of' occurrence as a frlnction of 

voltngc levels as well ns of exposure. The IEC approncl'r indicates only a rnnximum 

level Tor each locntion cntegory. I'lrese two proposals wifl be quoted in the following 

pnrngraplis. 

The iEl?E Working Group Proposal 

Data collected from a number of sources lcd to plotting a set of lines representi~~g e rate of occurrence as a function of voltnge for three types of exposures 

(Figwe 1). These exposure levels are defined in gcncral terms as follows: 

0 Low Expoatre - Systenw in geogl-oplrictll nrens kr~own for low 

lighlning activity, with little load swilclring activity. 

0 High L.:xposru.e - Systems in geograpt~ical areas known for 

high lightning activity, with frequent arid severe switching 

transier~ts. 

0 Extrertle Exposrrre - Rere but real systems s~~pplied by long 

overlie~d lirrcs and subject to reflcctio~rs at line ends, where 

the ct~aracteristics of tile installation produce high spnrkover 

levels of the clcnrances. 

2 'Trnnsioni Ovcrvolt:~go l'rotcction Coordination in the Undcfincd Real World Environme

I'C-79-3.4 
10" 

VI 

z 

0 

t- 9 lo' 

!3 

B 

I- F 

H Z lo1 

3 

t- 4 

a 

9 > 

a 

UI 

a I 

U) 

W c3 a 

'n 3 

4L 

0 

m w 10 m 

I, 

z 

10 

03 05 1 2 5 10 

SIJRGE CREST kV 

Figure 1. Rate of Surge Occlrrrence 

Versus Voltage 1,evel in 

Unprotected Circuits 

Ttre two lower lines of Figure 1 hnve heen drawn ~t the same slope, since the 

data buse shows reasonnble agreenlent among several sources on thnt slopc. Both the 

low-cxposure and high-exposure lines are trl~ncated at about 6 kV becnusc that level 

is the typical wiring device sparkover. The extreme-exposure line, by definition, is 

not limited by this sparkover. Becatrse it represents an extreme cnse, the extremeexposure line needs to be recognized, but it should not be applied indiscrimin~tely to 

all systen~s. Such application would pennlize the vast rnajority of installntions, where 

the exposure is lower. 

It is essential to recognize that a surge volt~ge observed in a power system can 

be either the driving voltage or the voltage limited by the sparkover of sotrle 

elenrance in the system. Ilerrcc, the term rrnprotec ted circuit must be understood to 

be a circuit in which m low-voltage protective device has been installed but in which 

clearance sparkover will eventu~lly limit the maximum voltage. The distribution of 

surge levels, therefore, is influenced by the surge-producing mechanisn~s as well as by 

the sparkover level of clearances in the system. This distinction between actual 

'driving voltage and voltage limited by sparkover is parlicularly important nt the 

interface between outdoor equipment and indoor equipment. Outdoor equipment has 

generally higher cleernnces, hence higher sparkover levels: 10 kV mny be typical, but 

20 kV is possible. In contrast, most indoor wiring devices used in 120-240 V systems 

have sparkover levels of about 6 kV; this 6 kV level, therefore, can be selected as a 

typical cutoff for the occurrence of surges in indoor power systems. 

The voltage and current amplitudes presented in the Guide nt tempt to provide 

for the vast majority of lightning strikes but should not be considered as "worst cnse," 

since this concept cannot be determined realistically. One should think in terms of 

the statistical distribution of strikes, accepting a reasonable upper limit for most 

cases. Where the consequences of a failure are not catastrophic but merely represent 

an annoying economic loss, it is appropriate to make a tratleoff of the cost of 

protection against the likelihood of a failure c~used by a high but rare surge. 

Transient Ovcrvoltage Protoction Coordination in the Undefined Kcal World Ihviror~nient 3 PC-79-3.4 
'Ilre IEC-SC28A ~O~OSII~ 

In a report dealing wit!\ clearance requirements for insulotiori coordination 

purposes, the IEC Subcommittee SC/28A recommends e set of impulse voltagcs to be 

considered as representative of the occurrences at different points of a power 

system, and at Icvcls dependent upon the systcni voltage (Table 1). The report is not 

primarily concerned with a description of the environment, but more with inst~lation 

coordination of devices installed in these systems. 

Table I 

PREFERREI) SERIES 01: VALUES OF IMPULSE WI'MISTAND VOLTAGES FOR 

R A'I'ED VOL'I'AQ ES RASE11 ON A CONTROLLED VOLTAGE SITUATION 

. WAVESlfAPE OF TIIB TRANSlIiNT OVERVOLTAGES 

Many independent observations 3.4.5 ti~ve established that the most frequent 

type of transient overvoltnge in ac power systems is a decoying oscillration, with 

lreqtrcricies between 5 und 500 kflz. This finding is in contrast to earlier attrmpts to 

apply the 1111idirectio11~1 double exponelltial voltage wave, generatly described ns 

1.2 x 50 or 1,2/50. Indeed, the unidirectional voltage wnye has a long history of 

successflrl npplicntion in the field of dielectric withstand tests and is representative 

of the surges propngating in tr~nsmission systenis exposed to lightning. Still, this is 

tllc wnveshnpe selected by the 1EC-SC28/A for low-voltttge systems. Other grotips 

have promoted nn oscillation, such 8s that specified in the SWC tests. 6 l'he IKEE 

Working Cirorrp is recornmending two waveshapcs, one for the indoor environment, and 

onc for llie outdoor and near-outdoor environment (Figure 2). Not only is a voltnge 

impulse defined, but the discharge current, or short-circuit current of a test 

generntor used to simulate these transients, is also defined in the'lEEE document. 

Volt~ges line-to-earth 

dcrived Iron, ruled 

system voltagcs, 11p to: 

(V rrns nntl dc) 

5 0 

100 

150 

300 

ti00 

1000 

'I'he oscillnlory wavcslmpe simulates thosc transients offectiog devices that 

we sensitive to rlvfdt nnd to voltnge seversuls during conduction, ' while the unidirectior~al voltnge and c18rre11t wavesliapes, based on long-established ANSl standnrds 

for secondt~ry valve nrreslers, simulnte the transients where energy content is the 

signifirwnt pururnctcr. 

ENERGY AND SOURCE IMPEDANCE 

'We energy involved in the interaction of a power system with o surge source 

ur~d a surge strppressor will divide bclweeri the source nnd the suppressor in 

nccnrtlrrrrcrc with the clinrtlckristics or the two impednnccs. In a gap-type suppressor, 

tile low irnpcdnucc of the arc cifler sparkover forces most of the energy to be 

dissipr~lcd clsewhcre, for i~rstnt~ce in a resistor odrted in scries with the gap for 

li rn it ing t Ire power-follow current. 111 RII energy-absorber suppressor, by its very 

Preferred series of impulse withstnnd 

volt~ges in ir~slallation cntegories 

4 *'l'ransicnt OvcrvrAtngc Protaction Coordination in the Undcfincd Real World t<nviromnent 

PC!-79-3.4 

I 

. 330 

500 

800 

1500 

2500 

4000 

IV 

1500 

2500 

4000 

6000 

8000 

12000 

I I 

550 

80R 

1500 

2500 

4000 

GO00 

f ll 

BOO 

1500 

2500 

4000 

6000 

8000 
Open-Circuil Vollage, Current Derined by 

Table It 

Indoor Environment 

Open Clrcult Vollage Discharge Current 

Outdoor and Near-Outdoor Environment 

1:igure 2. Proposed IEEE 587.1 Transient Overvoltnges and Discharge Currents 

nature, a substantial share of the surge energy is dissipated in the suppressor, but its 

clamping act ion does not involve the power-follow energy resulting from the shortcircuit action of a gnp. It is, therefore, essential to the effective use of slppression 

devices that a realistic assumption be made about the source impedance of the surge 

whose effects are to be duplicated. 

Unfortunately, not enough data hnve been collected on what this assumption 

should be for the sotlrce impedance of the transient. Strlndards or recommendations 

either ignore the issue, such as h'tlt STD-I399 or the IEC SC/28A Report, or 

sometimes indicate values applic~ble to limited cases, such as the SWC test for highvoltage substation equipment. The IEEE 587.1 document is at tempting to relate 

impedance with categories of locations, but unavoidably has to remain somewhat 

vngue on their definitions (Table 11). 

The 6 kV open-circuit voltage derives from two facts: the limiting action of 

wiring device spnrkover and the unatterwated propagation of voltages in unloaded 

systems. The 3 kA discharge current in Category B derives from experimental 

results: field experience in suppressor performance and simulated lightning tests. 

The two levels of discharge currents for the 0.5~ s - 100 kHz wave derive from the 

increasing i~rrpedunce expected in moving from Category B to Category A. 

1,ocation Cntegory C is likely to be exposed to substantially higher voltages 

thm Location Cutegory B because the limiting effect of spr~rkovcr is not available. 

Transient Ovcrvoltago Protection Coordination in the Undefined Real World Endronri~znt 5 

PC-79-3.4 
I 

The extreme exposure rates of Figure 1 could apply, with voltage in excess of 10 kV 

and discharge currents of 10 kA, or more. Installing unprotected lmd equipment in 

Location Category C is not recommended; the installation of secondary arresters, 

however, cnn provide the necessnry protection. 

l'sblc I1 

SURGE VOLTAGES AND CURRENTS DEEMED TO REPRESENT 

1'11E INDOOR ENVIRONMENT AND RECOMMENDED FOR USE 

IN DESIGNING PROTECTIVE SYSTEMS 

MA'IY:IIING TlIE ENVIRONMENT WITH TllE I:@UIPMENT 

On the h~sis of the various docitments described in the preceding pnrngraphs, 

nn cquipinent ciesigner or a user can toke a systematic approach at matching the 

cnpnbility or req~tircment for withstand of the equipment with the environment in 

wl~ich this equipment is to be instnlled. Figure 3 shows a flow chart concept of this 

systerriatic approcwh. This may involve tests to determine the withstand levels, 8 

some ~rie~surclnents and/or ~nnlysis to determine the degree of hostility of the environlnent, and n review of available protective devices. The latter will be briefly 

surveyed in the following paragrnphs. 

TR ANSIRNT su PPR ESORS 

Two methods tlnd types of devices are available to suppress transients: 

blocking the transient through some low-pnss filter, or diverting it to ground through 

some nonlinear device. This nonlinearity may be either a frequency nonlinearity 

(high-pass filter) or a voltage nonlinearity (clamping action or crowbar action). We 

will be moslly discussing the second type in this paper, since voltage clamping devices 

or crowbar devices are the most frequently used. 9 

Voltage-clamping devices have variable impedance, depending on the current 

flowing through the device or the voltage across its terminal. IKese components 

show a noillinear characteristic, Le., Ohm's law can be applied but the equation has a 

variable R. Impedance varintion is monotoriic and does not contain discontinuities, in 

contrast to the crowbar device which shows a turn-on action. AS far as volt-ampere 

charncteristies of these components is concerned, they fire time-dependent to n 

certain degree. IIowever, utrlike sparkover of a gap or triggering of a thyristor, time 

dcley is not involved Iwe. 

When a voltage-clamping device is installed, the circuit rem~ins unaffeclcd by 

the device before and after the trnnsient for any steady-state voltage below clamping 

level. Increased current drawn through the ckvice as the voltage attempts to rise 

rcsults in voltage clamping action. Nonlinear itnpcdnnce is the result if this current 

rise is faster than the voltage increase. lncrcnsed voltage drop (IR) in the source 

itn()ctlance due to higher ctrrrent restrlts in Ihe apparent clun~ping of the vollnge. It 

should be emphasized that the device depends on the source impedance to produce the 

6 Trntrsicnt Overvoltage Protection Chordination ill the IJndefined Keal 'World Environtt~cnt 

PC-7 9 -3.4 
clamping. A voltage divider action is at work where one sees the ratio of the divider 

not constant, but changing. The ratio is low, however, if the source impedance is 

very low. The suppressor cannot work at all with a limit zero source impedance 

(Figure 4). In contrast, a crowbar type device effectively short-circuits the transient 

to ground; but once established, this short circuit will continue until the current (the 

surge current as well as any power-lollow current supplied by the power system) is 

brought to a low level. 

The crowbar device will often reduce the line voltage below its steady-state 

value, but a voltage clamping device will not. Substan tiat currents can be carried by 

the sktppresor without dissipating a considerabie amount of energy within the 

suppressor. since the voltage (arc or forward-drop) during the discharge is held very 

low. This characteristic const itues the mejor advantage of these suppressors. 1 lowever, limitations in volt-time response and power-follow are the price paid for this 

advantage. As voltage increases across a spark-gap, significant ~ondll~ti~n 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 initial rise 

due to this delay time (typically in microseconds). Considerable vnrifttior~ exists in 

thc sp~rkover voltage achieved in succassive operations, since the prowss is statistical in nature. This sparkover voltage can, in addition, be substanlirllly higher 

Trntlsient Overvoltage Protection Coordination in the Undclined Real World Environrilcnt 

PC-79-3.4 
Figrrre 4. Volt~ge Clamping 

of a Suppressor 

Action 

after a long period of rest than after successive discharges, for some devices. Prom 

the physical nntrrre of the process, it is difficult to produce consistent sparkover 

voltnge for low voltage ratings. Illis difficulty is increased by the effect of rnanufacturing tolerances on very small gap distances. This difficulty can be ttllevieted by 

filling the tube with a gas having lower breakdown voltage than air. IIowever, if the 

enclosure seal is lost and the gns replaced by air, this substitution creates a reliability 

problem because the sparkover of the gap is then substnntially higher. 

Another lirnitetiorl occurs when a power current from the steady-sttlte voltnge 

source follows the surge disclrnrge ( follow-current, or power-follow). This powerfollow current mny or mny not be cleared at a natural current zero, in ac circuits. In 

dc circrrils, clearing is even more uncertain. Additional means must, therefore, be 

provided to open the power circuit, if the crowbar device is not designed to provide 

self-clearitig net ion within specified limits of surge energy, system voltage, and 

power-follow current. Consequently, most eleclronic circuits nre better protected 

with voltnge ct~imping suppressors than with crowbars, but sometimes the energy 

deposited in a voltage clamping device by a high current surge crm be excessive; a 

combinntion of the two devices cnn provide an cffcctive protection at optimhm cost. 

Ilowevcr, this combined protection must be properly coordinated to obtain the full 

~dvnntege of the schcme. The following pnrogrnphs will discuss some of the basic 

principles of coordination and provide some exarnples of npplicafions. 

Orle of the first concepts to be adopted when considering a coordinated scheme 

is thnt ctirrent, n3t voltnge, is the irldcpcndr~nt varieble involved. The physics of 

overvolttige gerieration involve either lightning or load switching. Both are current 

sources, ar~d it is only the voltnge drop associntetl with the surge current flow in the 

system impedance which appears as a trunsient overvoltage. Further~nore, there is R 

long history of testing insulation with voltage impulses which has reinforced the 

erroneoils concept that voltage is the given parfimeter. 'I'llus, overvoltage protection 

is redly the art of offering low impedance to the flow of surge currents rather than 

attcrnptirig to block this flow through a high series impedance. In combined 

npproac*hcs, a series impcdnnce is sometimes added in the circuit, but only after a low 

impedance diverlir~g pnth tins first been established. 

Wlren tlie diverting pth is a crowbar type device, little energy is dissipated in 

the crowbnr, as noled earlier. 111 R voltnge clamping device, more energy is deposited 

in lhc device, so tlwt ertcrgy hnndlirlg capability of a candidate suppressor is an 

importnnt paranictcr to consider when designing a protection schernc. With nonlinear 

devices, an error nraclc in tlie nss~rrnetf vdue of the current surge produces little error 

on the voltage developed across the suppressor and thus applied to the protected 

circuit, 10 but tl~e error is directly reflected in the amotlrit of energy which the 

suppressor 110s to ohsorb. At worst, when surge cr~rrents in excess of the suppressor 

ctipnbility nre i~~iposed bythe environment, either because of Rn error rnctde in tlie 

nssuriiption, or becflr~se nature ter~cls to support Murphy's jaw, or bkcnuse of hun~nn 

8 Trilnsiont Overvollnge Prntection Coordination in the Undefined Heal World I<nvironmcnt 
error in the use of the device, the circuit in need of protection cnn gei~ernlly be 

protected at the price of failure in the strort-circuit rnotfc of the protective device. 

Ilowever, if substuntin1 power-frequency currents coil he supplied by the powe

system, the fail-short protective device generally terminates ns fail-open when the 

power system feult in the f~iled device is not cleared by a series overcurrent 

protective device (fuse or breaker). Note that in this discussion, the term 'fail-safe' 

has cnrcfully been avoided since it can mean opposite failure nlodes to different 

users. To some, fail-safe means that the protected hardware must never be exposed 

to nn ovcrvoltage, so that failure of the protective device must be in the fail-short 

mode, even if it puts the system out of operation. To other users, f~il-safe means 

that the function must be m~intaincd, even if the hardware is left temporarily 

unprotected, so that failure of the protective device must be in the open-circuit 

mode. 

EXAMPLES OF COORDINATED SURGE PHOTECTION 

Retrofit of a Control Circuit Protection 

In this case history, a field failure problem wns caused by lnck of awareness 

(on the pnrt of the circuit designer) of the degree of hostility in the environment 

where the circuit was to be installed (the first question asked in the flow chart of 

1:igctr-e 3). A varistor had been prcrvided to protect the control circuit conlp l1erlt.s on 

the printed circuit board, but its capability was exceetled by the surge currents 

occurring in a Category B location (Table 11). To the defense of the circuit designer, 

however, it must be stated that the data of Teble 11 were not ~vailnble to him at the 

time. 

Becnuse a number of devices were in service, complete redesign was not 

possible, but a retrofit - at an acceptable cost - had to be developed. Fortunately, 

the power consumption of this control circuit was limited so that it was possible to 

insert some series impedance in the line, ahead of the low-cnpacity varistor, while a 

higher capacity varistor was added at the line entrance to the circuit (Figure 5). 

Laboratory proof-test of the retrofit demonstrated the capability of the 

r-------I- l I RETROFIT 

-4 

CIRCUIT BOARD 

V1 : V15flLA1 varistor 

V2 : V150LA20A varistor 

R : 10 R 1 W carbon resistor 

Figure 5. Retrofit Protection of Control Circuit 

Transient Overv~ltaga Prutcction Cuordin:~tion in the Undefined Heal World Environment 

PC-79-3.4 
sc-heme to witl~stand 6 kA crest current surges (Figure 6A), a 200 % margin from the 

proposed Category I3 requirement, as well ns reprod~~ction of the field failure pattern 

(Figure 6B). The intter is nn important aspect of any field problem retrofit. Dy 

siln~tir~tirrg in the laboratory the assumed surges occurring in the field (Table Il), 

vcrificnlion of the failure mechnnisn~ is the first step townrd on effective cure. 

Figure 6C illirstrntes the effect ol' improper instellation of the suppressor, with 

8 inches of teads instend of a direct connection ncross the input terminah of the 

circuit. 

Figure 6. Isborntory Demonstration of Retrofit Effectiveness 

Coordination netween a Secondary Surge Arrester end a Varistor 

In this example, the objective was lo provide overvol tage protection with a 

mnxirnum 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 

C62.1/2 Standards for secondary arresters. The only arresters available nt the time 

which could withstand a 10 kA crest 8x20 impulse hrrd a protective (clamping) level of 

~pproximately 2200 V. Some distetlce was avnilnble between the service entr~nce nnd 

the location of the protected circuit, so that impedance was in fact inserted in series 

between the errester nnd the protected circuit where a varistor with lower clamping 

voltage would be installed. 'me object was to determine at what current level the 

arrester would spark over for a given length of wire bctween the two protective 

devices, relieving the varistor from the excessive energy that it would absorb if the 

arrester would not spnrk over. 

A circuit was set up in the laboratory, with 8 meters (24 it) of #12 two-wire 

cnhle between the nrrcster and the varistor. The c~trrent,-approximntely 8x20 impulse, 

was raised until the errester would sparkover ebout half of the time in successive tests 

nt the same level, thus establishing the transfer of conduction from the varistor to the 

arrester. Figure 7A shows the discharge current level required from the generator at 

which this transfer occurs. Figure 7B shows the voltage at the varistor when the 

arrester does not spnrk over. Figt~re 7C shows the voltnge nt the arrester wt~en it 

sparks over; this voltnge would propngate inside nll of the building if there was no 

suppressor added. Ifowever, if a varistor is added at 8 meters, the voltage of 

Figure 7C is nttenuntcd to that shown in Figure 7D, at the terminals of the varistor. 

Surge Injected into Ground System 

Lightning surge currents flowing in the ground conductors of a power system 

cnn induce sl~t)stantial overvoltnges in the phase conductors, without t!eving lightning 

10 Transient 0vcrvnlt;tgc Protection Coordination in the IJndcfincil Real World Ewironn~e

PC-79-3.4 
Figure 7. Transfer of Conduct ion 

current directly injected into these phase conductors. A laboratory simulation of this 

situation was conducted, l1 from which interesting observations were made. First, 

the injection of a unidirectional 8x20 surge current in the ground conductor of the 

service entrance to a building caused oscillatory voltage transients in the phase-toground outlets within the simttlnted- building wiring system. Second, the impedance of 

the equivalent source could be estimated by comparing the open-circuit voltage at 

the outlet with the lower voltage observed when a known load resistance was 

connected across the outlet. Third, while applicable only in the simulated condition, 

some numerical data can be quoted to illustrate the possible consequences of 

injecting high current into the ground conductors, Le., if a direct lightning stroke 

were to occur in the distribution system outside the building. Tabte 111 shows soine of 

the values recorded. 

Table 111 

RESULTS OF SURGE INJECTION TESTS 

Injected current 

into ground of 

1.5 kh 

Observations Inside the 'Building' 

open-circuit 2200 V crest, 500 kIlz at 6 rn (20 ft) from entrance 

with 130 load, 1400 V crest, fast damping at snme poilit 

1 . 8 kV 0ben eimuit voltages in wiring produces spnrkover of the 

1 clearances of the wiring devices 

an arrester connected at the service entrance will discharge 

about 3-5 kA between the phase conductors and ground 

From the first observation, one can compute an equivalent source impedance 

of about 8012 for the discharge current of 1.5 kA source, corlsistcnt with the orders of 

magnitude shown in Table 11. The clearance sparkover of the second observation is 

predictable and confirms the comments made earlier in discrtssing the rate of 

occurrence of Figure 1. The 30 kA example, which may seem a very high current, 

corresponds to s lightning discharge of 100 kA, or the 5 % level of crest discharges 12 

where 70 kA is ~ssumed to flow directly into the pole ground and overhead ground 

conductors to adjacent poles of the outdoor distribution system. Thus, for this 

extreme nssumption, an acceptable current level is imposed at llw service entrance 

arrester (for which the ANSI requirement is 10 kA). 

Transient Ovcrvdtage Protection Coordination in the llndcfinod Real World Environmet~t 1

PC-79-3.4 
CONC I*USION 

Effective protection of sensitive electronic equipment is possible through a 

systematic approach where the capability of tlw eqtliptnent is compared to the 

cllnrr\cteristics of the environment. The combined efforts of several organizations 

tbave produced a set of data which provide the circuit designer with rcasonnble information, albeit not fine specifications, on the assumptions to be made in nssessing 

the Imstility of the environrncr~t. A Guide and Application Gt~idcs will be ~vnilnblc in 

the new future to better define the characteristics of the power system environment. 

As more field experience is gained in applying these documents to equipment design, 

the feedbnck loop can be closed to ultimately increase the reliability of new 

equipment at acceptable cost, while current problems may also be alleviated based on 

these ncw firtdirtgs itr the area of transient overvoltages. 

ACKNOWLEDGMEN'l'S 

The data base for the Guide quoted in the ptlper as well as the writing of that 

Guide was provided by the members of the IEEE Working Group on Surge Voltages in 

AC Power Circuits Rated 600 V or Less. Robert Mierendorf made available t h e d a t a 

of Table I and ernphnsizcd the significance of clearance sparkover; Peter Richman's 

critique of the concepts relating to source impedance proved very effective; Paul 

Sperrtnza suggested the flow chart approach developed in Figure 5. 

Guide on Srwge Voltages in AC Power Circuits Rated Up to 600 V, Final Draft, 

May 1979. Doct~rnent wepared bv Working Grou~ 3.4.4 of the Suree Protective .. . . 

~eiice Committee of- the power ~n~ine&n &ciety, Institute of Electrical 

and Electronics Engineers. 

"Insulation Coordination Within Low-VoJ tage Systems Including Clearances and 

Creepnge Distances for Equipment," International Electrotechnical Commission Report SCZBA (Central Office) 5 (to be published in 1979). 

J.E. Lenz, Wrtsic Impulse Insulation Levels of Mercury Lamp Rellast for 

Outdoor Applications," Illuminatjng Engrg., 133-140 (Feb. 1964). 

F.D. Martzloff and O.J. tiahn, "Surge Voltage in Residential and Industrial 

Power Circuits," IEEE PAS 89, 6, 1049-1056 (July/August 1970). 

R. IIasler and R. Lugadec, "Digital Measurement of Fast Transients on Power 

Supply Lines," Proc. 3rd Symposium and Technical Exhibition on Electromagne tic Cornpatabilit y, Rot tcrdam, Holland, 445-448 (May 1979). 

Surge Arrestors ior Alternating-Current Power Circuits, IEEE Standard 28- 

1974; ANSI Standard C62.1-1975; IEC Stnndard 99-2. 

P . Chowdhurl," Transient-Voltage Cherac teristics of Silicon Power Rectifiers,: 

lEEE IA-9, - 5, 582 (SeptJOct. 1973). 

F.A. Fisher, and F.D. Martzloff, "Transient Control Levels, a Proposal for 

Insulation Coordination in Low-Voltage Systems," IEEE PAS-95, - 1, 120-129 

(Jen./Feb. 1976). 

Transient Voltage Suppression Manual, Chapter 2, pp: 13-17, General Electric 

Company, Auburn, NY (1978). 

Transient-Voltage Suppression Manual, Chapter 2, p. 15, General Electric 

Company, Auburn, NY (1978). 

F.D. Martzloff, "Coordination of Surge Protectors in Low-Voltage AC Power 

Circuits," Preprint No. F29 635-4 for iEEE PES Summer Meeting, Vancouver, 

Canada (July 1979). 

N. Cianos and I?.T. Pierce, A Ground-Lightning Environment for Engineering 

Usage, Stanford Research Institute, Menlo I'nrk, CA 94205 (August 1972). 

Transient Overvoltage Protection Coordination in the Undefined Rcnl World 

Ihvironnient PC-79-3.4