SURGE PROTECTION TECHNIQUES IN LOW-VOLTAGE AC POWER SYSTEMS 

ABSTRACT 

F.D. Martzloff 

Corporate Research and Development 

General Electric Company 

Designers involved in the ac power side of telecommunications equipment have been justifiably concerned with surge protection because field experience is 

rich in case histories of failures attributable to transient 

overvoltages. Insufficient knowledge of the exact nature 

of these overvoltages, however, has made the~r task 

difficult in the past. After several years of data 

collection by a number of organizations, a more definitive 

understand~ng of the surge environment is emerg~ng. The 

next few years' publications from the IEEE, the IEC, 

NEMA, and other interested groups will document that 

understanding. This paper presents an overview of the 

results of data collection and environment descriptions 

from the point of view of telecommunications power 

supply problems, as well as a review of applicable 

techniques and devices. 

INTRODUCTION 

From the early days of the introduction of semiconductors, voltage surges have been blamed for device 

failures and system malfunctions. Silicon semiconductors 

are, indeed, sensitive to overvoltages, more so than their 

predecessors, such as the obsolete copper oxide or 

selenium rectifiers. From an early period of frustration 

and poor knowledge of the actual environment, progress 

has been made both in the area of defining the environment and of providing new surge protective devices and 

techniques to deal effectively with the problem. 

Recent progress in the technology of transient 

voltage suppressors has opened new opportunities to 

improve the level of protection of semiconductors exposed 

to power system transients. In the past, direct exposure 

to outdoor system surges required surge arresters with 

high energy capability to survive the discharge currents 

associated with direct or indirect lightning effects, at the 

cost of voltage-clamping levels that were too high to 

protect sensitive semiconductors. The approach at that 

time was a coordinated combination of arresters and lowvoltage suppressors, an approach that is still valid in many 

cases. It is now possible, however, to apply a single 

suppressor, with sufficient capability to withstand outdoor 

surges while clamping at a level low enough to protect 

power semiconductors such as power supply rectifiers. 

Examples of coordinated protection as well as the 

application of high power surge suppression devices, with 

experimental verification of performance, will be given in 

the paper. 

THE ORIGIN OF SURGE VOLTAGES 

Two major causes of surge voltages have long bee

recognized: system switching transients and transient

triggered or excited by lightning discharges (in contrast t

direct lightning discharges to the power systems, whic

are generally destructive and for which economical pro

tection may be difficult to obtain). System switchin

transients can involve a substantial part of the powe

system, as in the case of power-factor-correction capac

itor switching operations, disturbances that follow th

restoration ~f power after an outage, or load shedding

However, these disturbances do not generally involv

substantial overvoltages (more than two or three per unit

but they may be very difficult to suppress because th

energies are high. Local load switching, especially if i

involves restrikes in the switchgear devices, will produc

higher voltages than the power system switching, bu

generally at lower energy levels. Considering the highe

impedances of the local systems, the threat to sensitiv

electron~cs is quite real: the few conspicuous cas

histories of failures blemish the record of a large numbe

of successful applications. 

Lightning-Induced Surges 

The phenomenon of lightning has been the subjec

of intensive study by many workers. The behavior o

lightning is now fairly predictable in general terms, bu

the exact knowledge of specific incidents is not predict

able. Protection against lightning effects includes tw

categories: I. direct effects concerned with the energy

heating, flash, and ignition of the lightning current, an

2. indirect effects concerned with induced overvoltage

in nearby electrical and electronic systems. 

One of the major factors to consider in determinin

the probability of lightning damage, and thus the need fo

strong protection, is the number of lightning flashes t

earth in a given area for a given time. Such statistics ar

not generally available; instead the number of "thunder

storm days" is quoted. However, the term "thunderstorm 

days" includes cloud-to-cloud discharges and does not in

clude the duration and intensity of each storm. Thus i

does not represent an accurate parameter. Progress i

being made to improve statistics, but new statistics ar

not yet available; therefore, the "isokeraunic level

map (I), showing the number of storm days per year, i

still the most widely used description of the occurrenc

distribution (2). 

CH1.502-417910000-0086$00.75 @ 1979 IEEE 
A transient is created whenever a sudden change 

occurs in a power circuit, especially during power switching - either closing or opening a circuit. It is important to 

recognize the difference between the intended switching - 

that is, the mechanical action of the switch - and the 

actual happening in the circuit. During the closing 

sequence of a switch the contacts may bounce, pf-oducing 

openings of the circuit with reclosing by restr~kes and 

reopening by clearing at the high-frequency current zero. 

Likewise, during an opening sequence of a switch, restrikes can cause electrical closing(s) of the circuit. 

Simple switching transients (3) include circuit 

closing transients, transients initiated by clearing a shortcircuit, and transients produced when the two circuits on 

either side of the switch being opened oscillate at 

different frequencies. In circuits having inductance and 

capacitance (all physical circuits have at least some in the 

form of stray capacitance and inductance) with little 

damping, these simple switching transients are inherently 

limited to twice the peak amplitude of the steady-state 

sinusoidal voltage. Another limit to remember in analyzing transients associated with current interruption (circuit 

opening) is that the circuit inductance tends to maintain 

the current constant. At most, then, a surge protective 

device provided to divert the current will be exposed to 

that initial current. 

Several mechanisms generating abnormal switching 

transients are encountered in practical power circuits. 

These mechanisms can produce overvoltages far in excess 

of the theoretical twice-normal limit mentioned above. 

Two such mechanisms occur frequently: current chopping 

and restrikes, the latter being especially troublesome 

when capacitor switching is involved. 

These switching overvoltages, high as they may be, 

are somewhat predictable and can be estimated with 

reasonable accuracy from the circuit parameters, once 

the mechanism involved has been identified. There is still 

some uncertainty as to where and when they occur 

because the worst offenders result from some abnormal 

behavior of a circuit element. Lightning-induced overvoltages are even less predictable because there is a wide 

range of coupling possibilities. Moreover, one user, 

assuming that his system will not be the target of a direct 

hit, may take a casual view of protection while another, 

fearing his system will experience a "worst case," may 

demand the utmost protection. 

In response to these concerns, various committees 

and working groups have attempted to describe ranges of 

transient occurrences or maximum values occurring in 

power circuits. These transients include both surge 

voltages and surge currents, although the primary emphasis is generally given to surge voltages. 

EXISTING AND PROPOSED STANDARDS ON 

TRANSIENT OVERVOLTAGES 

Several Standards or Guides have been issued or 

proposed - in Europe by VDE, IEC, CECC, Pro-Electron, 

and CCITT; in the USA by IEEE, NEMA, UL, REA, FCC, 

and the Military - specifying a surge withstand capability 

for specific equipment or devices and specific conditions 

of transients in power or communication systems. Some 

of these specifications represent early attempts to recognize and deal with the problem in spite of insufficient 

problem and as exchanges of information take pla

improvements are being made in the approach. A Work

Group of the Surge Protective Device Committee of IE

has completed a document describing the environment 

low-voltage ac power circuits (4). The document is n

being reviewed by the IEEE Standards Board for event

publication as a standard. For some time now, 

document prepared by a Relaying Committee of IE

under the title "Surge Withstand Capability"' has be

available (5). The FCC has also published regulatio

concerning equipment interfacing the communications a

power systems (6). The Low Voltage Insulation Coordi

tion Subcommittee, SC/28A, of IEC has also complete

report, to be published in 1979, listing the maxim

values of transient overvoltages to be expected in pow

systems, under controlled conditions and for specif

system characteristics (7). These documents will 

reviewed in the pages that follow. Greatest empha

however, will be placed on the lEEE document because 

describes the transient environment; the others assume 

environment for the purpose of specifying tests. 

The IEEE Surge Withstand Capability Test 

One of the earliest published documents to add

new problems facing electronic equipment exposed 

power system transients was prepared by an IEEE comm

tee dealing with the exposure of power system relay

equipment to the harsh environment of high-volt

substations. This document, which describes a trans

generated by the arcing that takes place when air-br

disconnect switches are opened or closed in the po

system, presents significant innovations in surge pro

tion. The voltage waveshape specified is an oscilla

waveshape, not the historical unidirectional waveshap

source impedance, a characteristic undefined in m

other documents, is defined; and the concept that all l

to the device under test must be subject to the tes

spelled out. 

Because this useful document was released a

time when little other guidance was available, u

attempted to apply the document's recommendation

situations where the environment of a high-voltage 

station did not exist. Thus, an important consideratio

the writing and publishing of documents dealing w

transients is a clear definition of the scope and limitat

of application. 

Federal Communications Commission Requirements 

The Federal Communications Commission (F

has issued regulations describing tests to be applied 

equipment interfacing the power distribution system 

the communication system. The intent of these test

protection of the equipment itself as well as protectio

the communications plant from surges originating on 

ac power side of the equipment. This concern 

especially motivated by the recent proliferation of 

minal equipment being installed by telephone ser

subscribers. 

The most exacting test specified by these reg

tions is the application on the ac side of equipment to 

connected to the telephone system of a 1 x 10 us imp

superimposed to the 60 Hz line voltage. The crest of 

voltage impulse is 2.5 kV, and the short-circuit capabi

of the impulse source must be no less than I kA. 
requirement of a substantial short-circuit capability reflects the perceptions of contributors to the regulationmaking process that such surge currents may occur in the 

real world, or it may express a wish to produce in the 

laboratory a detectable burn-in of the fault following 

sparkover during the application of the surge. Records on 

the background of this regulation available to the author 

are not specific on which of the two concerns was primary 

in the specification of such a h~gh short-circuit capability. 

The IEC SC/28A Report on Clearances 

The Insulation Coordination Committee of the 

International Electrotechnical Commission, following a 

comprehensive study of breakdown characteristics in air 

gaps, included in its report a table indicating the voltages 

that equipment must be capable of withstanding in various 

system voltages and installation categories (Table I). 

Table I 

PREFERRED SERIES OF VALUES OF IMPULSE 

WITHSTAND VOLTAGES FOR RATED VOLTAGES 

BASED ON A CONTROLLED VOLTAGE SITUATION 

Voltages line-to-~aith 

Derived from Rated Preferred Sertes of Impulse W~thstand 

System Voltages, Up to: Voltages in Installation Categories 

(V rms and dc) 

The table specifies that it is applicable to a "controlled voltage situation," which phrase implies that some 

surge-limiting device will have been provided - presumably a typical surge arrester with characteristics matching the system voltage in each case. The waveshape 

specified for these voltages is the 1.2 x 50 ~s wave, a 

specification consistent with the insulation background of 

the equipment. No source impedance is indicated, but 

four "installation categories" are specified, each with 

decreasing voltage magnitude as the installation is farther 

removed from the outdoor environment. Thus, this 

document addresses primarily the concerns of insulation 

coordination, and 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 we shall now 

review in some detail. 

The IEEE Working Crwp Proposal 

Voltages at~d Rates of Occurrence 

Data collected from a number of sources led to 

plotting a set of lines representing a rate of occurrence as 

a function of voltage for three types of exposures 

(Figure 1). These exposure levels are defined in general 

terms as follows: 

0 Low Exposure - Systems in geographical areas 

known for low lightning activity, with little load 

switching activity. 

High Exposure - Systems in geographical are

known for high lightning activity, with freque

and severe switching transients. 

0 Extreme Exposure - Rare but real systems su

plied by long overhead lines and subject 

reflections at line ends, where the characte

istics of the installation produce high sparkov

-. levels of the clearances. 

SPARKOVER ---\ 

LOW - \ EXPOSURE \ 

\ 

\ 

I 

SURGE CREST - kV 

Figure 1. Rate of Surge Occurrence Versus Voltage Lev

Both the low-exposure and high-exposure lines a

truncated at about 6 kV because that level is the typic

wiring device sparkover. The extreme-exposure line, 

definition, is not limited by this sparkover. Because 

represents an extreme case, the extreme-exposure li

needs to be recognized, but it should not be appli

indiscriminately to all systems. Such application wou

penalize the vast majority of installations, where t

exposure is lower. 

Waveshape of the Surges 

Many independent observations (8, 9, 10) ha

established that the most frequent type of surge voltag

in ac power systems is a decaying oscillation, w

frequencies between 5 and 500 kHz. This finding is 

contrast to earlier attempts to apply the unidirection

double exponential voltage wave, generally described 

1.2 x 50. Indeed, the unidirectional voltage wave has 

long history of successful application in the field 

dielectric withstand tests and is representative of t

surges propagating in power transmission systems expos

to lightning. In order to combine the merits of bo

waveshape definitions and to specify them where they a

applicable, the Working Group proposal specifies 

oscillatory waveshape inside buildings and a unidirection

waveshape outside buildings, and both at the interfa

(Figure 2). 
Open-Circult Voltage, 

Current Defined by 

Table II 

Indoor Environment 

Open Circu~t Voltage Discharge Current 

Outdoor and Near-Outdoor Environment 

Figure 2. Proposed lEEE 587.1 Transient O v e r v o l t a g e s a n d Discharge Currents 

Table 11 

SURGE VOLTAGES AND CURRENTS DEEMED TO REPRESENT THE INDOOR ENVIRONMENT 

AND RECOMMENDED FOR USE IN DESIGNING PROTECTIVE SYSTEMS 

T~pe Enrrgy (joules) 

Comparable to Impulse of Sprr me11 Drpouted in a 5uppre5wr 

Locdtlon IEC SC28A H~glr Expowre or Lodd iiith Cldmpmg Voltdge of 

Category Category U abeiorrr Arnpli tudc ( ir( ult 500V l OOOV 

A. Long Rrdnc ti 

C~rr u~ts and 

Outlet5 

R. Mqor Ferdrr5, 1.2 x 50 ps 6 kV High Impeddrm ril) .. .. 

hhort Branch 111 8 x 20bi 3 kA Lou' lrrlpeddncc (') 40 80 

C1rrwt5, and 

Load Center 0.5 115 - 100 hH7 H~gh Irr~prdance'~' .. .- 

Low 1") peddnc~ (*' 2 4 

- Notrs: (1) For hlgh ~rnpedanre te5t sperlrneni or load clrcult5, the voltdgc 5hown represents thc >urge voltdge. In rrldklng s~irrirIdt~on test>, use thdt vdlu? for the open clrcult voltage of the tot generator. 

(2) For IOU-lrnpeddnce teit specimens or load clrruiti, the current shown repre\ent\ the diri hdrge r urrent of thr ivrjic 

(not the \hart-r~rcult current of the power svsterr~). In rnak~ng s~rnuldtion test\. me thdt currrnt for the ihort r Irr u~t 

current of thc test generator. 

Energy and Source Impedance 

The energy involved in the interaction of a power 

system with a surge source and a surge protective device 

will divide between the source and the protective device 

in accordance with the characteristics of the two impedances. 

Unfortunately, not enough data have been collected 

on what value should be assumed f o r t h e source impedance 

of the surge. Standards and recommendations, such as 

MIL STD-1399 or the IEC SC/28A Report, either ignore 

the issue or indicate values applicable to limited cases, 

such as the SWC test for high-voltage substation equip 

ment. The lEEE 587.1 document attempts to relate 

impedance to categories of locations but unavoidably 

remains vague on their definitions (Table 11). 

Having defined the environment for low-voltage ac 

power circuits, the Working Group is now preparing an 

Application Guide, where a stepby-step approach, per

haps in the form of a flow chart (Figure 31, will outlln

the method for assessing the need for surge protection an

selecting the appropriate device or system. Parallel wor

in other IEEE working groups preparing test specificatio

standards (11) for surge protective devices will be helpfu

in this selection process. Other groups in the U.S., as we

as the international bodies of IEC and CCITT, are no

working toward further refinements and the reconciliatio

of different approaches. 

SURGE PROTECTIVE DEVICES 

Various devices have been developed for protectin

electrical and electronic equipment against surge volt

ages. They are often called "transient suppressors

although, for accuracy, they should be called "transien

limiters," "clamps," or "diverters" because they canno

really suppress transients; rather they limit surge voltage

to acceptable levels or make them harmless by divertin

the surge current to ground. 

Voltage-Clamping Devices 

Voltage-clamping devices have variable impedance, 

depending on the current flowing through the device or 

the voltage across its terminal. 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. 

When a voltage-clamping device is installed, the 

circuit remains unaffected by the device before and after 

the transient for any steady-state voltage below clamping 

level. Increased current drawn through the device as the 

voltage attempts to rise results in voltage-clamping 

action. Nonlinear impedance is the result if this current 

rise is faster 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 the clamping. A voltage 

divider actlon is at work, where one sees the ratio of the 

divider as not constant but changing (Figure 4). 

Figure 4. Voltage-Clamping Action of a Suppressor 

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: the polycrystalline varistors 

and the single-junction avalanche diodes. Another 

technology, the selenium rectifier, has been practically 

eliminated from the field because of the improved 

characteristics of modern varistors. 

Avalanche Diodes 

Avalanche diodes, the 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 of 

the surge in a very thin single-layer junction. 

The advantage of the avalanche diode, generally a 

PN 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, admittedly at some 

extra cost. 

In the same category, we find silicon diodes used 

the forward direction rather than in the reverse av

lanche. A stack of such diodes is required to produce t

necessary clamping voltage (0.75 V per diode), but t

result is a protective system with large current capabili

Varistors 

The term varistor is derived from its function a

- variable resistor. The European usage is the te

voltage-dependent resistor, but the term seems to im

that the voltage is the independent parameter in su

protection, a concept which is misleading. Two v

different devices have been successfully developed 

varistors: silicon carbide discs have been used for year

the surge arrester mdustry; more recently, metal ox

varistors (MOV) have come of age, with the result t

these new varistors are sometimes referred to 

"movistors" (12). 

Metal oxide varistors depend on the conduct

process occurring at the boundaries between the la

grains of oxide (typically zinc oxide) grown in a carefu

controlled sintering process. Detailed descriptions of 

process can be found in many publications (13, 14, 15, 

17). 

Metal oxide varistors were initially developed 

electronic circuit protection devices. Later large me

oxide varistors were developed and applied to large s

tion surge arrestors (18). No device, however, was av

able in a rating suitable for power distribution system

The surge currents occurring in these systems are exc

sive for electronic-type varistors, a fact demonstrated 

field failures resulting from improperly applied varisto

Such failures could have been anticipated had the d

included in the proposed IEEE Guide reviewed above b

available at the time. In this context, it is worthwhile 

examine the implication of failure modes for the su

protective devices. 

Failure Modes 

An electrical component is subject to failure eit

because its capability was exceeded by the applied str

or because some latent defect in the component w

unnoticed in the quality control processes. While t

situation is well recognized for ordinary components

surge protective device, which is no exception to th

limitations, tends to be expected to perform miracles

at least to fail graciously in a "fail-safe" mode. The te

"fail-safe," however, may mean different failure modes 

different users and, therefore, should not be used. 

some users, fail-safe means that the protected hardw

must never be exposed to an overvoltage, so that fail

of the protective device must be in the fail-short mo

even if it puts the system out of operation. To ot

users, fail-safe means that the function must be m

tained, even if the hardware is left temporarily unp

tected, so that failure of the protective device must b

the open-circuit mode. It is more accurate and 

misleading to describe failure modes as "fail-short" 

"fail-open," as the case may be. 

When the diverting path is a crowbar-type dev

little energy is dissipated in the crowbar, as noted earl

In a voltage-clamping device, more energy is deposite

the device, so that the energy-handling capability o

candidate protective device is an important paramete
consider in the designing 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 protectice device and thus applied to 

the protected circuit, 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, either because of an error made in the assumption, 

or because nature tends to support Murphy's law, or 

because of human error in the use of the device, 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 powerfrequency currents can be supplied by the power system, 

the fail-short protective device generally terminates as 

fail-open when the power system fault in the failed device 

is not cleared by a series overcurrent protective device 

(fuse or breaker). 

PROTECTION COORDINATION 

By protection coordination we mean a deliberate 

selection of two or more protective devices used with the 

goal of reliable protection at minimum cost. With the 

present situation of the unregulated and uncoordinated 

application of protective devices, this may seem an 

unattainable goal for complete systems. In specific cases, 

it is fully attainable, as the example that follows will 

show. One can hope that success will eventually spread 

the concepts and increase the drive to generahe the 

approach. 

One of the first concepts to be adopted when a 

coordinated scheme is considered is that current, not 

voltage, is the independent variable involved. The physics 

of overvoltage generation involves either lightning or load 

switching. Both are current sources, and it is only the 

voltage drop associated with the surge current flow in the 

system impedance which appears as a transient overvoltage. Furthermore, there is a long history of testing 

insulation with voltage impulses which has reinforced the 

erroneous concept that voltage is the given parameter. 

Thus, overvoltage protection is really the art of offering 

low impedance to the flow of surge currents rather than 

attempting to block this flow through a high series 

impedance. In low-power systems, a series impedance is 

sometimes added in the circuit, but only after a lowimpedance diverting path has first been established; for 

high-power systems, that option is generally not available. 

Coordination Between an Arrester and a Varistor 

This example involves a load circuit for which the 

maximum transient overvoltage had to be limited to 

1000 V (on a 120 V ac line) although lightning surges were 

expected on the incoming service. The only arresters 

available at the time which could withstand a 10 kA crest, 

8 x 20 us impulse had a protective (clamping) level of 

approximately 2200 V. Some distance was available 

between the service entrance and the location' of the 

protected circuit, so that impedance was in fact inserted 

in series between the arrester and the protected circuit 

where a varistor with lower clamping voltage would be 

installed. The testing objective was to determine at what 

current level the arrester would spark over for a given 

length of wire between the two protective devices, 

relieving the varistor from the excessive energy that it 

would absorb if the arrester did not spark over. 

A circuit was set up in the laboratory (19), with 

8 m of typical two-wire cable between the arrester and 

the .varistor. The current, approximately 8 x 20 impulse

was raised until the arrester would spark over about half 

of the time in successive tests at the same level, thus 

establishing the transfer of conduction from the varistor 

to the arrester. Figure 5A shows the discharge curren

level required from the generator at which this transfer 

occurs. Figure 58 shows the voltage at the varistor when 

the arrester did not spark over. Figure 5C shows the 

voltage at the arrester when it sparked over, a voltage 

that would propagate inside all of the building if there 

were no suppressor added. However, when a varistor is 

added at 8 rn, the voltage of Figure 5C is attenuated to 

that shown in Figure 5D, at the terminals of the varistor. 

Figure 5. Transfer of Conduction In a Coordinate Scheme 

Comparison Between a High-Energy Varistor 

and an ArresterILow-Power Varistor 

In the case of power circuits where no regulation o

centralized engineering authority can madate a coor

dinated approach, individual protection of each piece o

equipment may remain the only safe approach to th

manufacturer of equipment installed under this uncon

trolled situation. The choice of protection is then 

question of economics and calculated risks: provid

equipment with low-cost, low-capability protection, o

with high-capability protection at a higher cost. 

The first choice, low-cost, low-capability protec

tion, will be effective in the vast majority of indoor

locations, such as Category A, or even B, described in th

proposed IEEE Guide (4); there is, however, a finit

probability of failure if the equipment 1s installed close t

the outdoor environment in a high-exposure location. An 

arrester installed at the service entrance would reliev

the low-capability protection from absorbing exces

energy. In that case, the situation explored in the experi

ment reported above would prevail: suitable coordination 

of the respective protective levels of the two devices an

the impedance existing between them. 

The second choice, provision of a high-capability 

protection in each piece of equipment, would obviousl

provide adequate protection and might be justified wher

the cost of equipment failure in terms of money, dead

time, or embarrassment outweighs the initial outlay o

component investment. It is doubtful, however, that eve

mass-production could lower the cost of the high

capability device. 
In the case of power circuits where a centralized 

engineering authority is in a position to enforce coordinated protection and practices, the appropriate procedure 

is evident and much more economical: provide a single 

high-capability protective device at the service entrance 

to deal with incoming lightning-induced surges and power 

system switching surges; if necessary, complement the 

protection with coordinated low-capability protective 

devices at individual pieces of equipment, to deal with any 

internal switching transients that may occur. Indeed, 

coordination of the two protective devices is imperative 

to prevent the low-capability (and low clamping voltage) 

device from clamping the incoming surge and thus 

absorbing all the energy. 

Such coordination is now possible, since varistors 

with surge ratings to 25 kA for single surges and appropriate derating for multiple surges (11) have become 

available. This rating is higher than the requirements of 

ANSI Standards for secondary arresters (20, 21) and thus 

would be suitable for Category C (10 kA) of the proposed 

IEEE Guide. These high-capability varistors will clamp 

the voltage at a level sufficiently low - typically 600 V in 

a 120 V system under a surge of 10 kA, or 1100 V in a 

240 V system for the same 10 kA surge (by comparison, 

conventional arresters have a protective level of 2 to 

3 kV). The availability of low clamping voltage devices 

for both the high-energy service entrance duty and the 

low-cost individual equipment protection makes an effective coordination easy to achieve. 

CONCLUSIONS 

After many years of data collection and evaluation, 

a consensus is now emerging on the definition of the ac 

power system transient overvoltage environment, including both voltage and current levels. 

This definition will enable protection engineeers in 

centralized organizations, such as those existing in the 

operating communications companies, to design coordinated schemes of protection, and will enable equipment 

designers and manufacturers to assess the risks involved in 

providing various levels of protection for their equipment 

on the basis of economic and functional criteria. 

REFERENCES 

1. C.F. Wagner and G.D. McCann, "Lightning Phenomena," Electrical Transmission and Distribution Reference Book, Westinghouse Electric Corp., East Pittsburgh, PA, 1950, pp. 556-559. 

2. N. Cianos and E.T. Pierce, A Ground-Lightning 

Environment for Engineering Usage, Stanford Research Institute, Technical Report, August 1972. 

3. A. Greenwood, "Electrical Transients in Power Systems," Wiley Interscience, New York, 1971. 

4. "Guide on Surge Voltages in AC Power Circuits Rated 

Up to 600 V," Final Draft, May 1979. Document prepared by Working Group 3.4.4 of the Surge Protective 

Devices Committee of the Power Engineering 

Society, Institute of Electrical and Electronics Engineers. 

5. Guide for Surge Withstand Capability ISWCI Tests, 

ANSI Standard C37.90a, 1974; IEEE Standard 472- 

1974. 

6. Longitudinal Voltage Surge Test #3 - Sect

68.302(e), Title 47, "Telecommunications," Code 

Federal Regulations, U.S. Gov't. Printing Offi

Washington, DC, 1977. 

7. "lnsulation Coordination Within Low-Voltage Syste

Including Clearances and Creepage Distances 

Equipment," International Electrotechnical Comm

sion Report SC28A (Central Office) 5 (to be publish

in 1979). 

8. J.E. Lenz, "Basic Impulse Insulation Levels of M

cury Lamp Ballast for Outdoor Applications," nlum

nating Engrg., Feb. 1964, pp. 133-140. 

9. F.D. Martzloff and G.3. Hahn, "Surge Voltage in Re

dential and Industrial Power Circuits," IEEE PAS-8

6, July/August 1970, pp. 1049-1056. 

0. R. Hasler and R. Lagadec, "Digital Measurement 

Fast Transients on Power Supply Lines," Proc. 

Symposium and Technical Exhibition on Elect

Magnetic Compatibility, Rotterdam, Holland, M

1979, pp. 445-448. 

1. "Test Specifications for Varistor Surge-Protect

Devices," P465.3. Draft prepared by Task Fo

3.3.6.3 of the IEEE Surge Protective Devices 

eventual publication as an IEEE Standard. 

12. Harnden, J.D., Martzloff, F.D., Morris, W.G., a

Golden, F.B., "The GE-MOV Varistor - The Su

Alpha Varistor," Electronics 45, 21, 1972, p. 91. 

13. Matsuoka, M., Masa Yama, T., and Lida, Y., "No

linear Electrical Properties of Zinc Oxide Ceramic

Proc. of First Conf. Solid State Devices, Tokyo, 19

J. Japan Soc. Appl. Phys., 39, 1970, Suppl. p. 94. 

14. Mahan, G.D., Levinson, L.M., and Phillip, H.R

Theory of Conduction in ZnO Varistors, 78CRD2

General Electric Company, Schenectady, NY, 1978

15. Richman, P., "Diagnostic Surge Testing, Part I," So

State Power Conversion, Sept./Oct. 1979. 

16. Transient Voltage Suppression Manual, Second E

tion, General Electric Company, Auburn, NY, 1978

17. Gauthier, N., "Technologie et Applications des Var

tances," Toute LfElectronique, January 19

pp. 39-42. 

18. Sakshaug, E.C., Kresge, J.S., and Miske, S.A., "A N

Concept in Station Arrester Design," IEEE PAS-

No. 2, March-April 1977, pp. 647-656. 

19. F.D. Martzloff, "Coordination of Surge Protectors 

Low-Voltage AC Power Circuits," Preprint N

F29 6354 for IEEE PES Summer Meeting, Vancouv

Canada, July 1979. 

20. Surge Arresters for Alternating-Current Power C

cuits, IEEE Standard 28-1974; ANSI Standard C62

1975; IEC Standard 99-2. 

21. Guide for Application of Valve-Type Lightning Arr

ters for Alternating Current Systems, ANSI Stand

C62.2. (Rev. 1979 to be published.)