THE PROTECTION OF INDUSTRIAL ELECTRONICS AND POWER CONVERSION 

EQUIPMENT AGAINST POWER LINE AND DATA LINE DISTURBANCES 

Fran~ois D. Martzloff, Fellow IEEE 

Corporate Research and Development 

General Electric Company 

Schenectady, New York 12345 

Abstract 

The irresistible trend toward distributed computing systems for the 

control of industrial power electronic equipment is adding new dimensions to the old problem of surge protection. Harmful surges may be 

produced by lightning, power switching, or by differences in ground 

potentials. The objective of the paper is to provide increased awareness and better understanding of the fundamentals of protection as 

well as the characteristics of protective devices. 

1. Introduction 

The continuing development of power conversion equipment, 

further promoted by the success of microelectronics in controlling this 

equipment, raises the question of vulnerability to surge voltages or 

surge currenls appearing at the power or control input terminals of the 

equipment. The very origin of these surges (lightning, switching, electrostatic discharges) makes it somewhat difficult to predict with accuracy thcir characteristics, so that surge protection has sometimes been 

presented as "art rather than science." There is, however, enough 

practical experience and data available to apply good engineering judgment in developing an understanding of the practical aspects of surge 

protection based on fundamental concepts and on protective device 

characteristics. The present paper is offered as a contribution toward 

better surge protection, neither as an art nor as a science, but as a nononsense, sound, and cost-effective engineering practice. 

The paper presents First a discussion of the various and complex 

origins of lightning and switching surges (electrostatic discharges are 

acknowledged but not addressed in a limited space discussion). The 

paper also shows how standards can help simplify the application of 

protection. A brief discussion is given on aspects of the propagation 

of these surges, followed by a presentation of some fundamental 

approaches to protection. Basic characteristics of protective devices are 

outlined. Three examples are given on how this knowledge and 

engineering judgment can be applied to answer specific questions and 

to avoid pitfalls. 

2. The Origins of Transient Overvoltages 

Transient overvoltages in power systems originate from one cause. 

energy being injected into the power system, but from two sources: 

lightning discharges. or switching within the power system In communication or data systems, there is another source of transients the 

coupling of power system transients lnto the system Furthermore, all 

systems involving several connections to the environment have the 

potential risk of transient overvoltages associated with ground potential 

rise during the flow of surge currents As stated previously, stauc 

discharpe problems are not treated in this paper. 

Lightning discharges may not necessarily mean direct termination of 

a lightning stroke onto [he system A lightning stroke terminating 

near a power line. either by hitting a tall object or the bare earth, will 

create a very fast-changing magnetic field that can induce voltages - and inject energy - into the loop formed by the conductors of the 

system. Lightning can also inject overvoltages in a system by raising 

the ground potent!al on the surface of the earth where the stroke terminates. while more distant "ground" points remain at a lower volrage. closer to the potential of "true earth." The literature provides 

information on the characteristics of lightning discharges [I-61. 

Surges from power systeni switching create overvoltages as a result 

of trapped energy in loads being switched off. or of restrikes in the 

switchgear. These uill be examined in greater detail in the following 

paragraphs. 

2.1 Transients in Power Sy.tems 

A transient is created whenever a sudden change occurs in a power 

circuit. especially during power switching - either the closing or opening. It is important to recognize the difference between the intended 

switching (the mechanical action of the switch) and the actual happening in the circuti During the closing sequence of a switch, the conCH2040-4/84/0000-0018$01 .oO(C 1984 IEEE 

tacts may bounce, producing openings of the circuit wlth reclosing by 

restrikes and reopening by clearing at the high-frequency current zero 

Prestrikes can a150 occur just before the contacts close, with a succes

sion of clearings at the h~gh-frequency current zero, followed by re

strikes. Similarly~ during an opening sequence of o switch. restrike

can cause electrical closing(s) oC the circuit. 

Simple switching transients 171 include clrcuit closing transients

transients initiated by the clearing of a short circuit, and transients pro

duced when the two circuits on either side or the switch being opene

oscillate at different frequencies. In circuits having induclance an

capacitance (all physical circuits have at least some, in the form o

stray capacitance and inductance) with little damping, these simpl

switching transients are ~nherently limited to twice the peak amphtud

of the steady-state sinusoidal voltage Without a surge protectiv

device, the current flowing just before switching is available lo charg

the circuit capacitances at whatever voltage is required lo store th

inductive energy of the currenl by converting it into capacitive energy. 

Several nlechdnisms are encountered In practical power circuits tha

can produce transient overvoltages Par in excess of the theoretica

twice-normal limit rnent~oned above. Two such mechanisms occur fre

quently- currenl chopping and restrikes, the latter being especial

troublesome when capacitor swilching is involved 

A similar scenarlo can unfold when an ungrounded power system 

experiences an arclng ground fault. The switching action is then no

the result of a dellherate parting of contacts but the intermittent con

neclion produced by the arc 

These switching overvoltdges. high d5 they may be, are somewha

predictable and can be estimated with reasonable accuracy from the ci

cuit paramerers, once the mechanism lnvolved has been identified

There is still some uncertainty as to where and when they occu

because the worst offenders result from some abnormal behavior of 

circuit element Lightning-induced transients dre much less predictab

because there is a wde range of coupling possib~lities. 

In response lo these concerns. various committees and workin

groups have attempted to describe ranges of transient occurrences o

maximum values occurring in power circuits Three such auempts a

described in the next section Figures I. 2. and 3 show typical exam

ples of transients recorded In power systems 

3. Standards on Transient Overvoltages in Power Lines 

Several standards or guides have been issued or proposed in Europ

and in the United States, specifying a surge withstand capability fo

specific equipment or devices and specific conditions of transients 

power or communication systems. Some of these specification

represent early attempts to recognize and deal with the problem 

splte of insufficient data. As a growing number of organization

address the problem and as exchanges of information take place

improvements are being made in the approach. Three of these a

brlefly discussed here 

0 5 10 20 30 40 75 2 

WICROSECONDS 

Figure 1. Lightning surge recorded on a 

120 V overhead distribution systep 
Table 1 

IEC Report 664 

0 5 10 20 30 40 IS 100 

MICROSECONDS 

Figure 2. Switching surge recorded on a 

120 V residential wiring system 

1000 

500 

VOLTS 0 

500 

1000 

0 I 2 3 4 

MILLISECONDS 

Figure 3. Capacitor switching transient recorded on a 460 V system 

3.1 The lEEE Surge Withstand Capability Test (SWC) 181 

One of the earliest published documents which addressed new problems facing electronic equipment exposed to power system transients 

was prepared by an IEEE committee dealing with the exposure of 

power system relaying equipment to the harsh environment of highvoltage substations. 

Because this useful document was released at a time when little 

other guidance was available, users attempted to apply the recommendations of this document to situations where the environment of a 

high-voltage substation did not exist. The revised version of this standard, soon to be issued, recognizes the problem and attempts to be 

more specific (and restrictive) in its scope. Thus, an Important consideration in the writing and publishing of documents dealing with 

transients is a clear defin~tion of the scope and limitations of the 

application. 

3.2 The IEC 664 Report 191 

The Insulation Coordination Committee of IEC included in its 

report a table indicating the voltages that equipment must be capable 

of withstanding in various system voltages and installation categories* 

(Table 1). The table spec~fies its appl~cabihty to a controlled voltage 

situation, which implies that some surge-limiting device has been provided - presumably a typical surge arrester with characterist~cs matching the system voltage in each case. The waveshape specified for 

these voltages is the 1.2/50 ps wave, a specification consistent with the 

insulation withstand concerns of the group that prepared the document. 

No source impedance is indicated, but four "installation categories" 

are specified, each with decreasing voltage magnitude as the installation 

is further removed from the outdoor environment. Thus, this document primarily addresses the concerns of insulation coordinat~on; the 

specification it implies for the environment is more the result of 

efforts toward coordinating levels than efforts to describe the environment and the occurrence of transients. The latter approach has been 

that of the IEEE Working Group on Surge Voltages in Low-Voltage 

AC Power Circuits, which will be reviewed in detail. 

3.3 The lEEE Guide on Surge Voltages 

(ANSVIEEE Std C62.41-1980) [I01 

3.3.1 Voltages and Rate of Occurrence. Data collected from a 

number of sources Icd to plotting a set of lines representing a rate of 

PREFERRED SERIES OF VALUES OF IMPULSE 

WITHSTAND VOLTAGES FOR RATED VOLTAGES 

BASED ON A CONTROLLED VOLTAGE SITUATION 

Derived from Rated Withstand Voltages in 

(V rms and dc) 

occurrence as a function of voltage for three types of exposures (Figure 4). These exposure levels are defined in general terms as follows: 

Low Exposure - Systems in geographical areas known for low 

lightning activity, with l~ttle load switching activity. 

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 produce high sparkover levels of the 

clearances. 

The two lower lines of Figure 4 have been drawn at the same slope 

since the data base shows reasonable agreement among several sources 

on that slope. All lines may be truncated by sparkover of the clearances. at levels depending on the withstand voltage of these clearances. 

The high exposure line needs to be recognized, but it should not be 

indiscriminately applied to all systems. Such application would penalize 

the vast majority of installations where the exposure is lower. 

The voltage and current amplitudes presented in the Guide attempt 

to provide for the vast majority of lightning strikes but none should be 

considered "worst case," as this concept cannot be determined realistically. It is necessary to think In terms of the statistical distribution of 

strikes, and to accept 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 tradeoff of the 

cost of protection against the likelihood of a failure caused by a high 

but rare surge. 

01 05 i 2 5 (0 

SURGE CREST LV 

.In some locataons, sparkover of clearances may lam01 

(he overvoltages 

Figure 4. Frequency of occurrence versus level from 

ANSI/IEEE Std C62.41-1980 
0pen.Clrcult Voltage, 

Current Defined by Teble II 

Indoor Environment 

Open Clrcult Vdtrge Dlschrrga Current 

Outdwr and Near-atdoor Environment 

Figure 5. Surges defined for power lines by ANSWEEE Std C62.41-1980 

3.3.2 Waveshape of the Surges. Many independent observations Ill13) have established that the most frequent type of transient overvoltage in ac power systems is a decaying oscillation, with frequencies 

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

to apply the unidirectional double-exponential voltage wave that is generally described as 1.2/50. Indeed, 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 power 

transmission systems exposed to lightning. In order to combine the 

merits of both waveshape definitions and to specify them where they 

are applicable, the Guide specifies an oscillatory waveshape inside 

buildings, a unidirectional waveshape outside buildings, and both at the 

interface (Figure 5). 

The oscillatory waveshape simulates those transients affecting devices that are sensitive to dvldt and to voltage reversals during conduction [141, while the unidirectional voltage and current waveshapes, 

based on long-established ANSI standards for secondary valve arresters, 

simulate the transients where energy content is the significant 

parameter. 

3.3.3 Energy and Source Impedance.. The energy involved in the 

interaction of a power system with a surge source and a surge prolctive 

device will divide between the source and the protective device in 

accordance with the characteristics of the two impedances. In a gaptype protective device, the low impedance of the arc after sparkover 

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

added in series with the gap for limiting the power-follow current, or 

on the impedance of the circuit upstream of the protective device. In 

an energy-absorber protective device, by its very nature, a substantial 

share of the surge energy is dissipated in the suppressor, but its clamping action does not involve the power-follow energy resulting from the -- 

' Note the difference belween ssr.?e Ct~e~Io~rce or the hne. also called characler~sltc 

impedance. and the mipedanc~ ro rlre surge. Z, 41 F~gure 8 d~h,rnl br anra "/ R. L. C 

or he frequenry corresponding to rhe ssslye. 

short-circuit action of a gap. It is therefore essential to the effective 

use of surge protective devices that a realistic assumption be made 

about the source impedance of the surge whose effects are to be 

duplicated. 

Unfortunately, not enough data have been collected on what this 

assumption should be for the source impedance of the transient. Stan

dards and recommendations, such as MIL STD-1399 or the IEC 664 

report. either ignore the issue or indicate values applicable to limited 

cases, such as the SWC test for high-voltage substation equipment

ANSIIIEEE Std C62.41-1980 attempts to relate impedance to categories 

of locations but unavoidably remains vague on their definitions 

(Table 2). 

3.3.4 New Reports on Transient Occurrence. The importance of 

defining the environment has motivated a number of workers to instal

recording devices on their power systems. and thus created a marke

for commercial transient recorders in the last several years. Unfor

tunately. at least one such recorder contains a surge suppressor in ir

own power supply (Figure 6). 

1-3 kV open-circuit \ , 3 kV open-circuit 

. . 'with analyzer plugged in 

4 1 , 4 lV- -d 

(a) 200 A short-circuit current (b) 500 A short-circuit current 

Figure 6. Effect of connecting a "Disturbance Analyzer" on a power 

system subjected to the ANWIEEE C62.41 ring wave 

Table 2 

Description of Surge Environment in ANSI/IEEE Std C62.41-1980 

Location 

Category 

A Long branch 

Circuits and 

outlets 

B Major feeders. 

short branch 

circuits, and 

load center 

Comparable to Impulse 

IEC No 664 Medium Exposure 

Category Waveform Amplitude 

I1 0.5 us-100 kHz 6 kV 

200 A 

1.2 X 50 ps 6 kV 

111 8 X 20 ps 3 kA 

0.5 ps-100 kHz 6 kV 

500 A 

Type of Specimen 

or Load 

circuit 

High impedancet 

Low impedance?. 3 

High impedance* 

Low impedance? 

High impedance* 

Low impedance+. 8 

Energy (joules) 

Deposited in a Suppressor* 

with Clamping Voltage of 

500v lO0OV 

(1 20 V System) (240 V System) 

*Other suppressors hanng different clamping voltages would receive different energy levels. 

*For high-impedance test specimens or load circuits. the voltage &own represents the surge voltage. In making simulation tests- that value for be open-cimuit voltaee of the test generator. ?For low-impedance test suecimens or load circuits. the current shown represents the discharge current of the suqe (not the shortcircuit current of the Power system). In making simulation tests. use that current for the short-circuit current of the test generator. t~he maximum amplitude (200 or 500 A) is specified, but the exact waveform will be influenced by the load characteristics. 
Consequently. if the line being monilored is that in which the 4. Surge Propagation 

analyzer has its power cord plugged. the transients that occurred on 

the line before the analyzer was tnstalled disappear. This irony makes 4.1 The Limitations of Arbitrary D i v i s i o n i n t o Categories 

my facetious remark or 1969. ". . .[he best surge suppressor is a surge The standards cited in the preceding paragraphs descr~he surges 

n~onitor." (121 the truth. Therefore. some of the recenlly published which may be expected at specific points of a wiring system; the implisurveys on the occurrence of surges may show a lower number of cation is that the surges will proceed downstream, at the same arnpllsurges thm itctui~lly occur. and therefore data should be examined with tude, until some interface somehow produces a decrease in amplitude 

caution until the ambiguity caused by this cord-connected suppressor is down to the next lower level of voltage or lo the next lower level of 

resolved. current specified by the next downstream category (Figure 7). 

1. THE ANSIIIEE,!? STD C62.41-1980 CONCEPT OF LOCATION CATEGORIES IN UNPROTECTED CIRCUITS 

LOCATION CATEGORY LOCATION CATEGORY LOCATION CATEGORY 

C B A 

-0~tstde and Serwce Entrance" : "Major Feeders and Short Branch Circuits" : "Outlets and Long Branch Circuits" 

. VOLTAGES 6 kv Impulse or Ring 6 kV Rmg 

CURRENTS 

2, TYPICAL EXAMPLES OF INDUSTRIAL OR R E S I D E N T I A L C I R C U I T S 

Altsmats~ 

Underground 

cable service 

I 

Overhead I service 

Note 1 

Data line 

See Note 3 

Cable input 

See Note 3 

I PI. Surge arrester ID lndustrml drwe system LEGEND: P3 Surge protector (secondary ratlngl M Owen motor ICS lndustral control system * P4 Surge protector 

M Wall.hour meter FA Flxed applmnce WRZ Wall receptacle wth PC Personal computer 

P2 Surge arrester WRI. Wail receplacle w~thoul attenuat~on atlentuatm prowded HF Consumer electroncs 

fsecondarv ratln~l SE Serv~ce entrance (may take maw farms TV Transformer isolated 

MB Mam breiker 

AW. Arc welding supply P Transient p~oleclor 

CA Cord connected appliance 

COMP Computer wlth buflered nput LC Llne power cond!t?oner 

3. THE IEC REPORT 664-1980 CONCEPT OF CONTROLLED VOLTAGES 

uncontrolled 

VOLTAGES 

lli: f4\25 kV 

Figure 7. 

Similarities and differences between the location categories concept of ANSI/IEEE Std C62.41-1980 

and the installatipn categories concept of IEC Report 664-1980, applied to a typical example 

INSTALLATION CATEGORY 

IV 

supply level. Overhead lines 

and -ble systems including dlstrlbu- lion bus and 11s associated overcurrent 

equipment. 

. ,- - 

INSTALLATION CATEGORY 1 11 (2.5)i 5 kV 

111 I INSTALLATION CATEGORY 1 (1SP 8 kV 

Fwd ~nstallatmn lollow+ng installallon category IV. I1 

Appliances portable equipment etc INSTALLATION CATEGORY I f0lIW.n~ mslallal~on category Ill I SP ec...." N,. I:., ,.,.. "4, 

men1 following lnslallatlon category I. 

telecommunmtlon, etectronlc, etc 
(Notes for Figure 7) 

I I THE ANSlilEEE Sld C62.41-I980 CONCEPT OF LOCATION CATEGORIES IN UNPROTECTED CIRCUITS 

MThe voltage levels shown in the three IoCatiOn CategOrleS represenl h~gh tmpedance circult cond~tlons llghl load~ng and no 

surge protective dev~ces PI. P2. P3 or P4 The 10 kV voltage of Calegory C IS reduced lo a maxlmum of 6 kV In both Calegor~es 

Band A by the tlkely sparkover of clearances, should a 10 kV surge Impinge on the servtce entrance 

(b)The currenl levels shown In the three locatlon categor~es In a descendmg slatrcase lrom C to A represenl tow wnpedance ctr 

cull condthon for surges, such as the lnSlallatlOn of one or more surge prolecllve devlces PI P2 P3 or P4 Another low 

mpedance condlllon 1s the case of equtpment sparkover (installed equipment In an actual system or EUT durmg a test) 

(c) If mulliple surge protective devices are lnslalled on the system. the currenl waveform Imposed on the downstream protect~ve 

device is tnfluenced by the clampling characteristics cf the upstream device 

1 2. TYPICAL EXAMPLES AND THE IEC REPORT 664 CONCEPT NOTES: 

(1) The Controlled Voltage Sttuallon of IEC Report 664 requires Ihe Presence of mlerfaces lhese can be surge protecllve devlces 

such as PI. P2 P3 or P4 or the existence of well-defined Impedance networks such as Z and C shown In the clrcuil d~agram 

upstream of WR2 

I Surge arresters or protectors PI. P2. P3 and P4 may be any pr0leCl1ve dev~ce sullable lor Ihe surge current levels expected at 

that pomt of the system PI and P2 are shown connecled Ime-to ground P3 and P4 may be connected ltne to neulral or be a 

, 

combmallon of Ime-to-neutral wtth addlhonal neutral-to-ground 

Surge arrester P2 may also be connecled on the load slde of tne maln ctrcu~l breaker ME In lhal case MB would then be con 

1 s~dered to be m Inntallatton Category IV 

(2)Voltage levels totlow~ng the des~gnal~on ol Installallon Category IIV Ill 11 or I) are shown m Parentheses lor a system wlth 300 v 

phase-to-ground voltage. and next lor 150 V Phase-lo-ground voltage The voltages shown are ~m~lted as I 2/50 mpulses 

Example IV (6)4 means 6 kV 1 2/50 ps lor a 240 V System 4 kV 1 2/50 ps tor a 120 V system See IEC Report 664-1980 [g] lor 

the complete (able of levels correspondmg to system voltages from 50 lo 1000 V 

(3)Thls d~agram makes no allowance for the poss~b~lily of surges associated wllh ground potenllal d~fferences that may occur, for 

instance w~th asensor connectton to the ICS control system a cable TV connectton to the line- so la led TV set etc . or the flow 

of ground current In the lmpedance of the groundmg conductors 

(4)Ttansient prolector P in the hne feedtng the welder AW la lypocal example of transient generator mlernal to the system) 

Intended to protect the system lrom the welder rather than to prolect me welder lrom Ihe system 

(51 Power llne cond~l~oner LC while pertormlng the major task of Cond~lionmg Ihe power supply to the computer may perform a 

tunct~on stmllar to that of the protector Pat the welder In blockmg conducted mlerterence lrom the load loward the system 

(6)Many appltances or electronic devtces might be equtpped wllh Internal surge orotecllve devices and therefore be sullable lor 

mslalfatton In other categor~es than II 

This s~aircase represenlation is useful lo simplify the real world into 

a manageable set of assumptions. but it is a simplification that can 

mask ihe reality. Surges will propagate in the system starting at the 

point of entry: voltage surges will be attenuated to the extent that the 

series impedance between the point of interest and the source (Z,. 

Figure 8) and the shunt impedance (Z:) forni a voltage divider. If 

the series impedance is low and the shunt impedance is high (lighl 

loading of the system). the voltage divider does not produce high 

attenuation of the voltage surges. Conversely, current surges, if they 

are the resuh of a current source such as a lightning strike. will produce high voltages unless a low-impedance diverting path is offered lo 

the flow of current. If the current surge in a system is the result of a 

combined current source and multipath to ground (Figure 9), there is 

then a division of the current among the palhs that is governed by the 

inverse ratio of the impedances. If a user has control over only one 

of the paths, he can decrease the amplitude of the current surge in his 

path only by forcing a greater share of the total current to flow 

through the other paths: thus, surge blocking is likely to be an exercise 

in passing the problem from one point of Ihe system to another. The 

solution lies in surge drversion, offering to the surge a path where the 

current flow can occur harmlessly. 

Voltage 

~20 1 seen by 

User 

* 

Figure 8. Voltage divider effect 

4.2 The Limitations of Transmission Line Analysis 

In qualitative discussions of surge propagation, the classical beha

of a transmission line is often called upon to provide explanations 

the situation. In particular, the theoretical reflections occurring at 

end of a line are cited: doubling the impulse if the line is open-end

returning an inverse impulse if the line is shorted. ~oweverT

discussions sometimes lose sight of the fact that the concept. cpn 
Lightning Stroke or Lightning. 

Induced Current 

User Has No Control 

Over This Path 

User's 

Y =I1I1. 11 Equipment 1'2 

%I r Ground 

User's World 

orV = IIZ1 - I2Z' ~f 2' is Significant 

Figure 9. Multipsth current division 

applied only if the line length is sufficient to contain all of the surge 

front. If the surge has a rise time longer than the propagation time 

along the line, the point is moot and, by the time the surge reaches 

its peak, the voltage at the receiving end of the line does not differ 

from the voltage applied at the sending end. Figure 10 illustrates this 

situation showing the voltage at the sending end (SD) and the voltage 

at the receiving end (RC) of a conduit-enclosed three-wire line 

(B: black or phase conductor, W: white or neutral conductor, G: green 

or grounding conductor). There is a minor difference during the rise, 

where the initial front is doubled at the receiving end, but the crests 

are the same. Thus, for many installations in buildings, the line 

lengths are short compared to the length required to contain, say, a 

I rs front traveling at a typical speed of 200 ml~s. 

A more detailed report describing some of the aspects of the propagation of surges and their implications is given in Reference IS. 

4.3 Common Mode or Normal Mode? 

Another aspect of the propagation of surges concerns the dichotomy 

normal mode/common mode. In other words, the issue is whether 

significant surges occur line-to-line (black-to-white, or phase-toneutral), the situation described by "normal mode." or whether they 

occur between any - or both - of the lines and ground (black-togreen, white-to-green, or [black-and-whitel-to-green), the situation 

described by "common mode." 

The answer to that dichotomy is that, in most cases, both modes 

must be considered, since one often converts into the other, depending 

upon the coupling, the wiring practices, and the attempts made at 

suppressing the mode perceived as the greatest threat. Here again, the 

pervasive and pervert reality is too often that a solution aimed at 

suppressing one effect only displaces the problem. Examples 1 and 2, 

in the Appendix, show how controversies on the method of suppressing a surge assumed to occur between two wires can lead to partial or 

misleading solutions when the realities of the situation are not 

recognized. 

5. Fundamental Protection Techniques 

The protection of a power conversion system or an electronic black 

box against the threats of the surge environment can be accomplished 

in different ways. There is no single truth or magic cure ensuring 

immunity and success, but, rather, there are a number of valid 

approaches that can be combined as necessary to achieve the goal. 

The competent protection engineer can contribute his knowledge and 

perception to the choice of approaches against a threat which is imprecise and unpredictable, keeping in mind the balance between the 

technical goal of maximum protection and the economic goal of realistic protection at an acceptable cost. However, just as in the case of 

accident insurance, the cost of the premium appears high before the 

accident, not after. 

A discussion of fundamental protection techniques that is limited in 

space'and scope has the risk of becoming an inventory of a bag of 

tricks: yet. there are a few fundamental principles and fundamental 

techniques that can be useful in obtaining transient immunity, especially at the design stages of a computer system or circuit. All too 

often. the need for protection becomes apparent at a late stage, when 

Figure 10. Voltages at sending and receiving ends of a line 

it is much more difficult to apply the fundamental techniques which 

are most effective and economical when implemented at the outset. 

5.1 Basic Techniques 

Protection techniques can be classified into several categorie

according to the purpose and the system level at which the engineer is 

working. For the system as a whole, protection is primarily a preven

tive effort. One must consider the physical exposure to transients - in particular, the indirect effects of lightning resulting from building 

design, location, physical spread, and coupling to other disturbanc

sources - as well as such inherent susceptibility characteristics as fre

quency response and nominal voltage. A system depending on low

voltage signals, high-impedance circuits, and installed over a wide area 

would present much more serious problems than the same system 

confined to a single building or a single cubicle 

For the system components or electronic black boxes. the environ

ment is often beyond the control of the designer or user, and protec

tion becomes a curative effort - learning to live and survive in an 

environment which is imposed. Quite often this effort is motivated by 

field failures. and retrofit is needed. The techniques involved here 

tend to be the application of protective devices to circuits or a searc

for inherent immunity rather than the elimination of surges at thei

origin. 

Another distinction can be made in classiiying protective tech

niques. While surges are unavoidable, one can attempt to block them

divert them, or strive to withstand them; the latter, however, is gen

erally difficult to achieve alone. 

5.2 Shielding, Bonding, and Grounding . Shielding, bonding, and grounding are three interrelated method

for protecting a circuit from external transients. Shielding consists o

enclosing the circuit wiring in a conductive enclosure, which theoreti

cally cancels out any electromagnetic field inside the enclosure: actu

ally. it is more an attenuation than a cancellation. Bonding is th

practice of providing low-impedance connections between adjacen

metal parts, such as the panels of a shield. cabinets in an electroni

rack, or rebars in a concrete structure. Grounding is the practice o

providing a low impedance to earth. through various methods of driv

ing conductors into the soil. Each of these techniques has its limita

tions, and each can sometimes be overemphasized. Many texts an

papers discuss the subject at length, so that a detailed discussion is no

necessary here. with one exception, which follows. 

5.3 Ground One End or Both Ends of the Shield? 

Shielding conductors by wrapping them in a grounded sheath o

shielding an electronic circuit by enclosing it in a grounded conductive 

box is a defensive measure that occurs very naturally to the system 

designer or the laboratory experimenter anticipating a hostile elec

tromagnetic environment. Difficulties arise. however, when [he con

cept of "grounded" is examined in detail. Difficulties also arise when 

the goals of shielding for noise immunity conflict with the goals o

shielding for lightning surge immunity. 

A shield can be the size of a matchbox or an airplane fuselage: i

can cover a few inches of wire. or miles of buried or overhead cables

Grounding these diverse shields is not an easy thing to do because the 

impedance to earth of the grounding connection must be acknowl

edged. The situation is made even more controversial because of t

conflict between the often-proclaimed design rule "ground cable shield

at one end only" - a rule justified by noise immunity performance, i

particular common mode noise reduction - and the harsh reality o

current flow and Ohm's law when lightning strikes. 

23 
noi\e prevention designers that the shield serves as an electrostatic 

shield in which longitudinal currents associated with common mode 

noire coupling should not flow. This concept is exemplified in the terminology of shielded cable users. when they describe the shield construction of some cable design as having a foil plus drairr wire. as if 

there were elecrrosrarrc charges that needed to be removed (drairred. 

Indeed, electrostatic charges can be drained by connecting only one 

end of the shield. Furthermore, if the two ends of the shield of a 

cable spanning some distance are connected to the local ground at each 

end, there is a definite possibility that some power frequency current 

may flow in the shield. For low-level signals. this current would produce nolse (hum) in the signals. For that reason. many system 

designers will insist on the one-end-only grounding rule, and they are 

correct from that point of view. Sometimes the shield is used as a 

return path for the c~rcuir, in which case shield currents can cause voltage drops added to the signal. But the fact is that, when surge 

currents flow near the circuits, they will unavoidably inject magnetic 

flux variations into the circuits, hence mduced voltages. Worse yet, in 

the case of a lightning stroke injecting current into the earth in the 

area spanned by the one-end-only grounded shield, the potential of 

one end cf the cable defined as "ground" is not the same as the 

"ground" at the other end of the cable. Very high voltages can be 

developed (Figure 11) between the floating end of the shield and the 

local ground. No practical insulation can withstand these levels, and 

breakdown yill occur, allowing surge currents to flow in spite of the 

designers' intent to prevent them; the path of these currents will be 

determined by the components most likely to fail when the voltage 

rises - the low-level logic circuits, of course. In contrast, by deliberately allowing part of these surge currents to flow in the shields, -~ 

one obtains a cancellation of the voltages that otherwise would be 

induced in the circuits, and the currents will follow a welldefined path 

that can be designed to provide harmless effects. 

This conflict is actually very simple to resolve if recognized in time: 

~rovide an outer shield, grounded at both ends (and at any possible 

intermediate points), mside this shield the electronic designer is then 

free to enforce his single-point grounding rules. The only drawback to 

this approach is the hardware cost of double shields. In many installations. however. there is a metallic conduit through which the cables 

are pulled; with simple but close attention to maintaining the continu~ty of this conduit path, through all the joints and junction boxes, 

a very effective outer shield is obtained at negligible additional cost. 

In the case of underground conduit runs, the most frequent practice is 

to use plastic conduit, which unfortunately breaks the continuity. System designers would be well advised to require metal conduits where 

the circuits are sensitive or, at a minimum, to pull a shielded cable in 

rhe plasuc conduit where the shield is used to maintain continuity 

between the above-ground metal conduits. That additional cost, then, 

is the insurance premium, which is well worth accepting. Example 3 

in the Appendix shows the consequence of the mtsapplied "ground 

one end only" rule 

Figure 11. Voltage developed by a lightning stroke 

at the ungrounded end of a shield 

6. Transient Suppressors 

Various devices have been developed for protecting electrical and 

electronic equipment against transients. They are often called "transient suppressors" although, for accuracy, they should be called "transient limiters," "clamps," or "diverters" because they cannot really 

suppress transients; rather, they limit transients to acceptable levels or 

make them harmless by diverting them to ground. 

g

transients, preventing their propagation toward sensitive circuits, and 

those that divert transients. limiting residual voltages. Since many of 

the transients originate from a current source, the blocking of a transient may not always be possible: thus. diverting the transient is more 

likely to find general application. A combination of diverting and 

blocking can be a very effective approach. This approach generally 

takes the form of a multistage circuit, where a first device diverts the 

transient toward ground, a second device - an impedance or resistance - offers a restricted path to the transient propagation but an 

acceptable path to the signal or power. and a third device clamps the 

residual transient (Figure 12). Thus, we are primarily interested in the 

diverting devices. These diverting devices can be of two kinds

voltage-clamping devices or short-circuiting devices (crowbar). Both 

involve some nonlinearity, either frequency nonlinearity (as in filters) 

or, more usually, voltage nonlinearity. This voltage nonlinearity is the 

result of two different mechanisms - a continuous change in the dev

ice conductivity as the current increases, or an abrupt switching as the 

voltage increases. 

Because the technical and trade literature contains many articles on 

these devices, a discussion of the details will be limited. Some com

parisons will be made, however, to point out the significant difference

in performance. 

RESTRICT 

DIVERT CLAMP PROTECTEO 

CIRCUIT 

Figure 12. Three-step approach for surge suppression 

u 

6.1 Crowbar Devices 

The principle of crowbar devices is quite simple: upon occurrenc

of an overvoltage, the device changes from a high-impedance state to 

low-impedance state, offering a low-impedance path to divert the surg

to ground. This switching can be inherent to the device, as in th

case of spark gaps involving the breakdown of a gas. Some applica

tions have also been made of triggered devices, such as triggere

vacuum gaps in high-voltage technology or thyristors in low-voltage cir

cuits when a control circuit senses the rising voltage and turns on th

power-rated device to divert the surge. 

The major advantage of the crowbar device is that its low 

impedance allows the flow of substantial surge currents without th

developmenc of high energy within the device itselr; the energy has t

be spent elsewhere in the circuit. This "reflection" of the impingin

surge can also be a disadvantage in some circuits when the transiec

disturbance associated with the gap firing is being considered. Wher

there is no problem of power-follow (discussed below), sueh as i

some communication circuits, the spark gap has the advantage of ver

simple construction with potentially low cost. 

The crowbar device, however, has three major limitations. The fir

limitation concerns the volt-time sensitivity of the breakdown proces

As the voltage increases across a spark gap, a significant conduction o

current - and hence the voltage limitation of a surge - cannot tak

place until the transition occurs to the arc mode of conduction, b

avalanche breakdown of the gas between the electrodes. The load 

lefl unprotected during the initial rise because of this delay time (typ

cally in microseconds). Considerable variation exists in the sparkov

voltage achieved in successive operations, since the process is statistic

in nature. In addition, this sparkover voltage can be substantiall

higher after a long period of rest than after successive discharge

From the physical nature of the process, it is difficult to produce con

sistent sparkover voltage for low voltage ratings. 

The second limitation is associated with the sharpness of the spark

over, which produces fast current rises in the circuits and. thus, obje

tionable noise. A classic example is found in oscillograms recordin

the sparkover of a gap where the trace exhibits an anomaly before th

sparkover (Figure 13). This anomaly is due to the delay introduced 

the oscilloscope circuits to provide an advanced trigger of -weep. 

What the trace shows is the event delayed by a few nanoseconds, 

that in real time, the gap sparkover occurs and noise enters the oscill

scope by stray coupling while the electron beam is still writing the pr

sparkover rise. 

2
Figure 13. Anomaly in recording showing gap interference 

A third limitation occurs when a power current from the steadystate voltage source follows the surge discharge (follow-current, or 

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

dot be cleared at a natural current zero. In dc circuits, clearing is 

even more uncertain. Addittonal means, therefore, must be provided to 

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

self-clearing action within specified limits of surge energy, system voltage, and power-follgw current. This combination of a gap with a nonlinear varistor that limits the power-follow current has been very successful in the utility industry as a surge arrester or surge diverter. 

6.2 Voltage-Clamping Devices 

Voltage-clamping devices will exhibit a variable impedance, depending on the current flowing through the device or the voltage across its 

terminals. These components show a nonlinear characteristic - that 

is, Ohm's law 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 are concerned, these components are 

time-dependent to a certain degree. However, unlike the sparkover of 

a gap or the triggering of a thyristor, time delay is not involved. 

When a voltage-clamping device is installed, the circuit remains 

essentially 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 surge voltage attempts to rise results in 

voltage-clamping action. Nonlinear impedance is the result if this 

current rise is greater than the voltage increase. The increased voltage 

drop (IR) 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 to produce clamping. A voltage divider action is at work where the ratio of the divider as not constant, but changing. If the source impedance is very low, the ratio is 

low, and eventually the suppressor could not work at all with a zero 

source impedance (Figure 14). 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. 

The principle of voltage clamping can be achieved with any device 

exhibiting this nonlinear impedance. Two categories of devices, having 

the same effect but operating on very different physical processes, have 

found acceptance In the industry: polycrystalline varistors and singlejunction avalanche diodes. Another technology, selenium rectifiers. has 

been practically eliminated from the field because of the improved 

of modern varistors. 

Figure 14. Voltage divider effect of shunt-connected suppressor 

6.3 Avalanche Diodes 

Avalanche diodes, or Zener diodes, were initially applied as voltage 

clamps, a natural outgrowth of their application as voltage regulators. 

Improved construction, specifically aimed at surge absorption, has made 

these diodes very effective suppressors. Large diameter junctions and 

low thermal impedance connections are used to deal with the inherent 

problem of dissipating the heat deposited by the surge in the small 

volume of a very thin single-layer junction. 

The advantage of the avalanche diode, generally a P-N silicon junction, is the possibility of achieving low clamping voltage and a nearly 

flat volt-ampere characteristic over its useful power range. Therefore, 

these diodes are widely used in low-voltage electronic circuits for the 

protection of 5 or 15 V logic circuits, for instance. For higher voltages, the heat generation problem associated with single junctions can 

be overcome by stacking a number of lower voltage junctions. 

6.4 Varistors 

The term varislor is derived from its function as a variable resistor. 

It is also called a voltage-dependent resisior, but that description tends to 

imply that the voltage is the independent parameter in surge protection. Two very different devices have been successfully developed as 

varistors: silicon carbide disks have been used for years in the surge 

arrester industry, and more recently, metal oxide varistor technology 

has come of age. 

Metal oxide varistors depend on the conduction process occurring at 

the boundaries between the large grains of oxide (typically zinc oxide) 

grown in a carefully controlled sintering process. The physics of the 

nonlinear conduction mechanism have been described in the literature 

(16-201. 

Because the prime function of a varistor is to provide the nonlinear 

effect, other parameters are generally the result of tradeoffs in design 

and inherent characteristics. The electrical behav~or of a varistor can 

be understood by examination of the equivalent circuit of Figure 15. 

The major element is the varistor proper, R,,, whose V- I characteristic is assumed to be the perfect power law I = kV". In parallel w~t

this varistor, there is a capacitor, C, and a leakage resistance, R,. In 

series with this three-component group, there is the bulk resistance of 

the zinc oxide grains, R,, and the inductance of the leads, L. 

Figure 15. Equivalent circuit of a varistor 

Under dc conditions, at low-current densities because obviously no 

varistor could stand the high energy deposited by dc currents of high 

density, only the varistor element and the parallel leakage resistance 

are significant. Under pulse conditions at high-current densities, al

but the leakage resistance are significant: the varistor provides low 

impedance to the flow of current, but eventually the series reststance 

will produce an upturn in the V-I characteristic; the lead inductance 

can give rise to spurious overshoot problems if it is not dealt with 

properly; the capacitance can offer either a welcome additional path 

with fast transients or an objectionable loading at high frequency, 

depending on the application. 

6.4.1 V-I Characteristic. When the Y- I characteristic is plotted on a 

log-log graph, the curve of Figure 16 IS obtained. Three regions result 

from the dominance of R,, R,., and R, as the current in the device 

goes from nanoamperes to kiloamperes. 

The V-l characteristic is then the basic application design tool for 

selecting a device in order to perform a protective function. For a

successful application, however, other tactora, discussed in detail in the 

information available from manufacturers, must also be taken into consideration. Some of these factors are: 
Figure 16. V-I characteristic of a varistor 

Selection of the appropriate nominal voltage for the line voltage of the application 

Selection of the energy-handling capability (including consideration of the source impedance of the transient, the waveshape, and the number of occurrences 1211) 

Heat dissipation 

Proper installation in the circuit (lead length) [221 

6,s Packaged Suppressors 

The need for protection and the opportunity to provide packaged 

protective devices to concerned computer users has prompted the 

marketing of many packaged suppressors, ranging from the very simple 

and inexpensive to the complicated (not necessarily much better) and 

expensive. The field has also seen a number of devices claiming 

energy savings in conjunction with transient suppression; there is no 

foundation for such a claim, and the issue hopefully has been settled 

in a study published by EPRl 1231. 

Line conditioners using a ferro-resonant transformer are offered prlmarily as line-voltage regulators, but also provide high attenuation of 

transient overvoltages, a performance that isolating transformers do not 

provide. Example No. 2, in the Appendix, gives examples of the poor 

performance of isolating transformers and of the effective performance 

of line conditioners in dealing with normal mode transients 

6.6 Failure Modes 

Failure of an electrical component can occur because its capability 

was exceeded by the applied stress or because some latent defect in 

the component went unnoticed in the quality control processes. While 

this situation 1s well recognized for ordinary components, a surge protective device, which is no exception to these limitations, tends to be 

expected to perform miracles, or at least to fail graciously in a fail-safe 

mode. The term "fail-safe," however, may mean different failure 

modes to different users and, therefore, should not be used. To some 

users. fail-safe means that the protected hardware must never be 

exposed to an overvoltage, so that failure of the protective device 

must be in the fail-short mode, even if it puts the system out of 

operation. To others, fail-safe means that the funcrion must be maintained, even if the hardware is left temporarily unprotected, so that 

failure of the protective device must be in the open-circuit mode. It 

is more accurate and less misleading to describe failure modes as failshort or fail-open, as the case may be. 

When the diverting path is a crowbar-type device. little energy is 

dissipated in the crowbar. as noted earlier. In a voltage-clamping device. because more energy is depos~ted in the device. the energyhandling capability of a candidate protective device is an important 

parameter to consider in the design of a protection scheme. With 

nonlinear devices. an error made in the assumed value of the current 

surge produces little error on the voltage developed across the protective device and thus applied to the protected c~rcuit, but the error is 

directly reflected in the amount of energy which the protective device 

has to absorb. At worst, when surge currents in excess of the 

protective device capability are imposed by the environment, e.g., an 

error made in the assumption, a human error in the use of the device, 

or because nature tends to support Murphy's law, the circuit In need 

of protection can generally be protected at the price of failure in the 

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

pwer-frequency currents can be supplied by the power system, the 

fail-short protective device generally terminates as fail-open when the 

mwer system fault in the failed device is not quickly cleared by a 

series overcurrent protective device (fuse or breaker) 

With the failure mode of a suppressor being of the fail-short type. 

the system protection with fuses can take two forms (Figure 17). For 

the user concerned with maintaining the protection of expensive equipment, even if failure of the protector means the loss of the function, 

AlternativeA must be selected. Conversely. IT the function is 

paramount, Alternative B must be selected 

a) Protect~on maintained 

function interrupted 

m 

b) Funcl~on mainta~ned 

protechon lost 

Figure 17. Fusing alternatives for suppressors 

7. Conclusions 

Power system disturbances can inject damaging overvoltages i

power lines as well as data lines. Lightn~ng surges can be equall

damaging. by direct termination of a stroke, or by induction or eve

differences in ground potential caused by the flow of the curren

Beware of differential ground potential rise! 

Fundamental precautions, best applied in the design and construc

tion stages. can provide effective protection a1 a small cost compare

to the alternative of failures and later relrofils. The cost of insuranc

premiums always seems high before the accident. 

Shielding. bonding, and ground~ng are the classical preventiv

methods at the system and component level. Conflicts between trad

tional grounding practices for noise reduction can be reconciled w~

the requirements of surge protection. Grounding the shields at on

end only invites trouble. 

A combined approach of fundamental precautions and protectiv

devices can provide effective protection over the range of natural an

man-made disturbances. However, these devices must be applied a

part of a concerted effort. Coordination of protective devices is th

key to functional and cost-effective protection. 

Acknowledgments 

Motivation for presenting this paper was provided by the reported cas

histor~es and the penetrating questions raised by students at th

University of Wisconsin annual conferences on surge protection, 

well as by d~scussions with members of the IEEE Surge Protectiv

Dev~ces Committee. Catharine Fisher and Elizabeth Zivanov of CR

contributed valuable revhews and comments toward development of th

final text. 

References 

1. J.H. Hagenguth and J.G. Anderson, "Lightning to kEmpir

State Building," Part 111, AIEE Transactions 71 PAS, August 1952

2. N. Cianos and E.T. Pierce, A Ground Llghlning Envrronment fo

Engineerrng Usage, Stanford Research Institute Technical Repor

August 1972. 

2

To illustrate the preceding discussion, some practical examples are 

given in this appendix as the basis for sound design approaches. 

Example No. 1 - Does an isolating tr~sformer help? The author 

has witnessed and engaged In many discussions on the merits of isolating transformers, sparked by the misconception that "spikes are attenuated by transformers" or "spikes do not pass through transformers." 

Figures 18 and 19 are offered to support the position that these quotations are misconceptions. When properly applied, isolating transformers are useful to break ground loops and reduce common mode effect. 

but they do not by themselves attenuate spikes that occur line-to-line 

in the normal mode. 

Figure 18 shows the propagation - or worse, the enhancement - of a voltage impulse in a 1.1 isolating transformer. The 6 kV, 0.5- 

100 kHz impinging wave of ANSIIIEEE Std C62.41-1980 is applied to 

the primary of the transformer, H1H3 to H2H4. The output voltage. 

measured at XIX, to X2X4, appears as a 7 kV crest on the secondary 

side of this "isolating*' transformer. 

0.5-lrn 

B 

W 

G 

1 WA GENERAL PURPOSE 120112~ - TRANSFORMER II1JIIIIFigure 18. Surge propagation through isolating transformer 

Figure 18 was recorded with no load on the transformer secondary, 

which represents the extreme case of the low-power electronic control 

in the standby mode. Figure 19 shows the primary and secondary 

voltages of the transformer with a 10 W (1500 fl) and a 100 W 

(150 R) load on the secondary side, at the same generator setting as 

Figure 18. With the 10 W load that might be typical of an electronic 

control in standby mode, the combined series reactance of the 

transformer and shunt resistance of the load produce the output shown 

in Figure 19A, still slightly higher than the input. 

With the 100 W load shown in Figure 19B, the attenuation is now 

apparent, but is only 2'1. Capacitive loads would, of course, produce a 

greater attenuation than resistive loads for the inductive series 

impedance of the transformer, a1 the frequency spectrum of this fast 

2 ps-wide spike. For surges of longer duration, the attenuation would 

be even smaller. 

These examples show that, unless a well-defined load is connected 

to the transformer, expecting attenuation from the transformer may 

prove to be hazardous to the health of low-power electronics connectkd 

on the secondary side of a transformer. 

Figure 19. Effect of loading on the secondary side 

In contrast, decoupling is possible with a ferro-resonant line conditioner which is primarily intended for line voltage regulation, but 

which also provides a high degree of surge suppression. Figure 20 

shows the 6 kV incoming wave being attenuated to 60 V (100:l) on 

the secondary side of the unloaded line conditioner, and to 40 V 

(150:l) with a load of only 10%; at full load, an attenuation to less 

than LO V was observed. The nature of the ferro-resonant line conditioner is such that the decoupling improved with loading, while the 

Figure 20. Surge decoupling by ferro-resonant line conditioner 

simple transformer of Figure 18 can only act as linear dividers w

load changes. Conversely, the decoupling between primary a

secondary sides of the line conditioner is further seen on the oscil

gram recorded on the input side of the line conditioner. This oscil

gram is, in fact, a photograph of two successwe measurements, o

with no load on the line conditioner and one with a 100 W load. T

input waves are exactly superimposed. 

This decoupling reflects the nonlinear behavior of the ferro-resona

line conditioner, which is significant in this case, compared to t

linear behavior of transformers: For surge sources of lower impedan

than the generator used in these tests, or for frequencies lower th

that contained in the 0.5 ps - 100 kHz spike, the transform

attenuation would become lower, in direct proportion to t

corresponding impedance change, while the ferro-resonant line con

tioner would keep the dec?upling unchanged. 

FOJ worst-case demonstration, the two oscillograms of the outp

were recorded with the spike timed to occur at the peak of the 60 H

line voltage. demonstration. The peak-to-peak amplitude of the l

voltage js indicated by the gray band recorded on the oscillograms 

photographically superimposing repetitive traces of the line voltag

For timings other than the peak, the small voltage oscillaiion on t

output voltage would be completely contained within the normal pea

40-peak band of the 60 Hz line voltage. 

There is at present a trend to an upward spiral in specifying dB's 

attenuation in line conditioners. In the author's opinion. the point 

citing a 120 dB attenuation is moot because typical installation pract

will degrade that level of decoupling. 

Example No. 2 - Connections options for suppressors an! effects 

residual voltages. The author has w~tnessed l~vely cpntroversl

among varlous appl~cat~on lnforrnat~on sources on the most effect

translent suppression configurat~on to be apphed Taklng, as 

smphfied example, the task of spec~fy~ng the protection of a s~ng

phase equlpment connected at the end of a he w~th no opportunity 

d~vert the translent closer to the source (for instance, at the servl

entrance), the optlons wo&d be to connect one. two or three varisto

between the three wlres (black, wh~te. and green) at the end of t

line However, addmonal mformat~on needs to be known WtII t

Impinging surge be In the normal mode (black to wh~te) or In t

common mode ([black-and-wh~tel-to-green)? Where In th; equlpme

is the most sensitwe component he-to-he (most I~kely) or h

(black OR whtte)-to-greej17 Clearly, the sltuatlon ts confus~ltg;-

there wtll not be a smgle, s~mple answer apphcable md~scr~m~nat

all cases The Natlonal Electr~cal Code 1241 spec~fically allows the co

nectlon of surge arresters (Artlcle 280-22) ~f the tnterconnectlon occu

only by operation of the surge arrester dur~ng the surge Sme t

2 
s(andby current of varistors is very low. this requirement can be met; 

furthermore. there will not be any interference with the operation of 

~~ound Fault Circuit Interrupters if there IS only a small number of 

protecton 

The set of measurements recorded in Figure 21 shows an example 

of these many options with increasing protection, albeit at increasing 

wt. from a single varistor to three varistors. The selection would 

depend on the vulnerability level and location of the equipment to be 

protected. The impinging surge is assumed to be black-to-jwhite-and- 

#reen], since white and Breen are tied together at the service entrance. 

The line is a 75 m line and the surge is that available from the genorator set for a ZOO0 A 8/20 ps short-circuit impulse. Rather than 

attempt to modify the setting of the generator for each case in order 

to maintain constant current crest for the various configurations (an 

impossible task if wave form is also to be maintained). the generator 

was left unchanged. to discharge a constant total energy in the system - not a bad hypothesis for the real world. The current crests are 

a]] in the range of 300 to 380 A. which is not a significant change for 

comparing varistor clamping 'rollages. 

If only one varistor is allocated to protect the equipment. the blackto-white varistor connection (first row) affords maximum protection for 

the cltetronics which are also likely to be connected black-to-white. 

However. the voltages between either black or white and green are 

large: this is the stress that will be applied to the clearances of the 

equipment. This is a good example of conversion of a normal mode 

transient into a common mode. 

The configuration with varistor black-to-green (second row) does 

not afford very good protection for components connected black-towhite: therefore. it should be used only if there is a special need to 

damp black-to-green at a low voltage with only one varistor available 

An improved protection is obtalned with a varistor connected blackto-white. complem~nted by a second varistor connected white-to-green 

(third row). The ultimate protection is, of course. one varistor in 

every position (fourth row). but this should be required only for 

exceptionally sensitive loads 

Example NO. 3 - Ground potential rise on data lines. A distributed computer system had been installed with a central processing unit 

and remote terminals located in separate buildings. In a span of 

5 weeks during the first summer after commissioning the system, three 

lightning storms occurred in the area; no direct strikes were reported 

on the buildings, but extensive damage was done to the circuit boards 

on terminals and CPU inputs. 

After the first occurrence, power line surges were suspected and 

some precautions were applied, when access to the hardware was possible. by pulling out the ac power plugs from the CPU or terminal at 

the onset of a lightning storm. This did not help. Next, isolating 

transformers were considered but, again, did not help. At this point. 

the author was called in for consultation, and the following proposed 

diagnostic was established: the surges were not coming from the ac lines, but rather were due to differences in the ground potential existing between the separate buildings during flow of lightning currents. 

The data cables had been run in PVC conduit buried between the 

buildings and, true to one tenet of steady-state noise prevention, only 

one end of the shield of the wire pairs had been grounded, with the 

other left floating. Figure 22 shows how this arrangement can produce 

high voltage between a floating end of the shield and the local ground, 

a practice which is bound to produce a flashover and flow of surge 

currents along unwanted paths in the circuit components. Thus, the 

problem was no1 power line surges, but differential ground potential. 

Worse. by pulling out the ac line plugs but leaving the incoming data 

cables connected, the operators had unwittingly removed the local 

grounding connection from the hardware frame, leaving only the data 

cables coming in, with a possibility of raising the complete hardware 

several thousand volts above local grounds in the room - a dangerous 

condition. 

A solution to the problem could take several approaches. Radical 

solutions, such as a microwave link or a fiber optics cable, would 

indeed have eliminated the differential ground potential problems, but 

were considered too expensive or too long to install. 

Figure 21. Conneetlons options and effect on eommon mode or normal mode overvoltages 

29 
Building I3 

?=-I m 

~F&~~gt/~ Data Pair 

Figure 22. 2004 correction, caption should have read: 

Problem of single-ended shield 

Incidentally. part of the original puzzlement at the failures was the 

notion that opto-isolators provided in the data link route should have 

avoided problems. Close scrutiny of the circuits. however, disclosed 

that the opto-isolators had been provided for some other purpose: in 

fact, the ground potential loop was closed by the power supply to the 

opto-isolator feeding the amplifiers from a local source rather than the 

remote source, negating the isolation function. 

Another solution, really the most simple and effective. would have 

been the replacement of the plastic underground conduit by a continuous steel conduit linking the steel conduits used inside the buildings. 

This would have provided equalization of the ground potentials along 

the data cable, while allowing the desired use of shields with one end 

only at ground. However, that solution was not acceptable to the plant 

facility organization. 

In this particular location. a spare conduit had been buried next to 

the data line conduit. This offered the possibility of pulling a heavy 

ground cable in this conduit, close to the data cables. which could then 

tie the two corners of the buildings at the point of entry of the data 

cables, as a first step toward reducing differential ground potentials. 

At first. this concept was somewhat difficult to sell to plant facil~ties: 

because there is a ground grid tying lo two buildings for 60 Hz faults. 

further ties between the two buildings did not seem necessary. However. our thesis was that this grid would have too high an impedance 

to serve the purpose. and furthermore that the data cable run. located 

away from the ground grid. would form a flux-collecting loop with the 

ground grid. After lengthy discussions. the thesis was accepted and 

the cable was installed. 

Simultaneously, the concept of tying the two ends of the cable 

shields to ground was proposed. with the provision of a barrier that 

would avoid the circulation of power-frequency currents 1251: inserting 

an array of diodes (Figure 23) at one end of the shields reconciled the 

needs of noise prevention under normal operation and the requirement 

of grounding at both ends during lightning events. The forward drop 

of the two diodes in series (1.5 V) was enough to block any 60 Hz 

Shield Grounded Twisted 

at Other End Data Paw 

Forward drop of diodes 

isolate shield end at M Hz. - - !.ocal Ground 

Conduction of diodes during a sur e effectively grounds 

shiel!. 

Figure 23.2004 correction, caption should have read: 

Single-ended shield corrected by second bond with diodes 

circulating current that would inject noise in the data cables. During a 

lightning strike. however. the diodes would allow flow of current to 

compensate and cancel the ground potenlial direrences. 

These two cures were implemented during the first summer, and no 

further problems occurred for the rcst of the lightning season. While 

these two solutions might have been sufficient. the concern over 

another possible failure of the system was suficient to motivate the 

design of further protection: the insertion of a voltage clamp in each 

data pair. This solution required some design and acceptance testing 

from the system manufacturer. so that it was not implemented until 

the next lightning season. Thus. the system survived the remainder of 

the first lightning season with only the first two remedies. 

Experience has shown that conclusions on the effectiveness of 

lightning protection schemes should wait perhaps as much as 10 years 

before being proclaimed. because of the large variations in lightning 

activity. However, after three summers of trouble-lree operation compared to three major failures in 5 weeks, the cure would seem 

effective. Hopefully. these words will not have to be eaten by the 

author in a few years. 

In retrospect, then. the following recommendations can be drawn 

from this horror tale. for retrofits or new installations: 

I Data cables linking separate buildings or spanning beyond a single 

room within one building should have a shield tied to local ground 

at both ends of the cable If the first shield provided with the 

cables must be left with one end floating by dikht of the system 

vendor. then these cables should be installed within a conrintrous 

metal shield. This continuous shield can be either a double shield 

of the flexible cable. or simply a metal conduit. with both ends 

irounded 

2. Substantial relief can be obtained in retrofits by grounding both 

ends of existing shields through a low-voltage clamp. such as a 

diode array, that will block noise-inducing power frequency 

currents, but will allow the flow of ground-potential equalizing 

currents during surges. 

3. The ultimate protection may be the insertion of surge-protective 

devices in each line. However, this solution requires carerul 

design so that degradation of the signals does not occur, and residual spikes are not allowed to uass thrmvh