GENERAL @ ELECTRIC 

General Electric Company 

Corporate Research and Development 

Schenectady, New York 12345 

Automation Systems Laboratory 

TECHNICAL INFORMATION 

SERIES 

lUlWll 

Martzloff, FD 

COUPOWTE 

RESEARCH AM0 IVELOPYEYT 

SCII€MEClADV, Y.V. 

The irresistible trend toward distributed computing systems may be a 

source of difficulties which the earlier, centralized arrangement had overcome: surge voltages and surge currents injected or induced into power 

supply lines and communication (data) lines. These surges may be produced by lightning, by power system switching, or by differences in the 

potential of points expected to be grounded. Uninformed users of terminal 

equipment and personal computers may also connect their equipment in a 

manner inviting difficulties. 

This report presents a summary of present knowledge concerning the 

occurrence of surges together with mitigating measures and recent case histories illustrating the pitfalls of inappropriate applications. 

SlmJUI 

computer protection - upset 

and damage 

REV WDIDS 

surges, transients, interference, computers, upset 

YO. 85CRD084 

DAn 

July 1985 

WE 

The Protection of Computer and Electronic Systems 

Against Power Supply and Data Lines Disturbances 

1NFORMATlON PREPARED FOR 

GE CLASS 1 

wo.~Es 38 

Additional Hard or Microfiche Copies Technical Information Exchange 

Available from Bldg. 5 Room 321, Schenectady, N.Y. 12345 
THE PROTECTION 

POWER 

OF COMPUTER AND ELECTRONIC SYSTEMS AGAINST 

SUPPLY AND DATA LINES DISTURBANCES 

Frangois D. Martzloff 

1. INTRODUCTION 

The irresistible trend toward greater numbers 

of distributed computing systems may be a source 

of difficulties which the earlier, centralized systems 

had overcome: surge voltages and surge currents 

injected or induced into power supply lines and 

communication (data) lines. These surges may be 

produced by lightning, by power system switching, 

or by differences in the actual potential of points 

expected to be at ground potential but which are, 

in fact, driven apart by a surge current. Uninformed users of terminal equipment or personal 

computers might also connect their equipment in a 

manner inviting difficulties. Another source of 

transients, not covered in this report, is static 

discharge. 

A different source of disturbances for computers is, of course, undervoltage transients, sometimes called "sags." These are largely due to 

power system switching or faults, some being the 

consequence of lightning. Unfortunately, it is 

more difficult to fill a void in the power delivery 

than to divert and block an excess energy, so that 

the techniques needed to protect computer systems 

against these sags require approaches and equipment different from those discussed in this report, 

whose scope is limited to overvoltage or overcurrent surges. A number of approaches for mitigation of externally caused sags have been successfully implemented by computer manufacturers 

and users. These approaches range from power 

supplies with sufficient storage capacity or motor 

generator sets with sufficient mechanical inertia to 

ride through short sags, to uninterruptible power 

supplies with storage batteries and solid-state 

inverters being instantaneously switched to take 

over a failing utility power supply. These are now 

classical remedies and, when properly applied, 

eliminate the problems of sags, while the problems 

associated with surges still seem to be with us. 

This report presents a summary of present 

knowledge on the occurrence of surges and on 

mitigating measures, with recent case histories 

illustrating the pitfalls of inappropriate applications. 

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 into the system. Furthermore, all systems involving several connections to 

external equipment face the risk of transient overvoltages associated with ground potential rise 

during the flow of surge currents. As stated previously, static discharge problems are not treated 

in this report. 

Lightning discharges may not necessarily mean 

direct termination of a lightning stroke onto the 

system. A lightning stroke terminating on some 

object near a power or data line will create a very 

fast-changing magnetic field that can induce voltages - and inject energy - into the loops 

formed by the conductors of the system. Lightning can also inject overvoltages in a system by 

raising the ground potential on the surface of the 

earth where the stroke terminates, while more distant "ground" points remain at a lower voltage, 

closer to the potential of "true earth." The literature provides information on the characteristics of 

lightning di~charges."-~) 

Surges from power system switching create 

overvoltages as a result of trapped energy in loads 

being switched off, or of restrikes in the 

switchgear. These transients will be examined in 

greater detail in the following paragraphs. 

2.1 Transients in power systems 

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 circuit. During the closing sequence of a switch, the 

contacts may bounce, producing openings of the 

Manuscript received June 17, 1985 
circuit with reclosing by restrikes and reopening by 

clearing at the high-frequency current zero. 

Prestrikes can also occur just before the contacts 

close, with a succession of clearings at the highfrequency current zero, followed by restrikes. 

Similarly, during an opening sequence of a switch, 

restrikes can cause electrical closing(s) of the 

circuit. 

Simple switching transientsf7' include circuit 

closing transients, transients initiated by the clearing of a short circuit, and transients produced 

when the two circuits on either side of the switch, 

being opened, oscillate at different frequencies. 

On the load side of the switch, for 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. On the load side of the switch 

and without a surge protective device, the current 

flowing just before switching is available to charge 

the circuit capacitances at whatever voltage is 

required to store the inductive energy of the 

current by converting it into capacitive energy; 

voltages can reach high levels, such as ten times 

the normal level. 

Several mechanisms are encountered in practical power circuits that can produce large transient 

overvoltages. Two such mechanisms occur frequently: current chopping and restrikes, the latter 

being especially troublesome when capacitor 

switching is involved. 

A similar scenario can unfold when an 

ungrounded power system experiences an arcing 

ground fault. The switching action is then not the 

result of a deliberate parting of contacts but the 

intermittent connection produced by the arc. 

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.'8' There is still some uncertainty as to 

when and where they occur because the worst 

offenders result from some abnormal, and thus 

rare,* behavior of a circuit element. Lightninginduced transients are much less predictable 

because there is a wide range of possibilities for 

the coupling mechanism. 

This rarity can take two different aspects: 1. In the vast 

majority of circuits, these abnormal transients can sometimes occur, but rarely ("when"). 2. Among all circuits, 

a few rare ones are frequently and consistently afflicted 

with such abnormal behavior ("where"). 

In response to these concerns, various commit

tees and working groups have attempted t

describe ranges of transient occurrences or max

imum values occurring in power circuits. Thre

such attempts are described in the next section

Figures 1, 2, and 3 show typical examples of tran

sients recorded in power systems. 

Figure 1. Lightning surge recorded on a 120 V 

overhead distribution system 

Figure 2. Switching surge recorded on a 120 V 

residential wiring system 

Figure 3. Capacitor switching transient 

recorded on a 460 V system 
2.2 Transients in data lines 

Data lines are different from power lines in the 

occurrence of surges: power systems can generate 

their own transients, as seen above, in addition to 

receiving injected surges. Data lines are only subjected to what the environment will inject. However, the operating voltages and the overvoltage 

tolerance of signal-processing components in these 

data systems are generally much lower than those 

of power system components. Thus, damage (not 

to discount problems of misoperation) is more 

likely to occur to data lines than to power system 

components from the same exposure to injected 

transients. 

The systematic effort at characterizing surges in Figure 4. Transients induced by lightning 

power circuits, exemplified by the IEEE standard in overhead data cable 

Vertical: 4 Vldiv described in the next section, has not been exten- Sweep: 1 msldiv 

sively reported by users of data lines, with the Cable: 1000 m long, 5 m high exception of the telephone ind~str~.'~-'~) In the (courtesy Digital Equipment 

telephone environment, much emphasis is placed Corporation) 

on lightning effects on long overhead or buried 

cables and lines, as well as on induced noise from 

adjacent power systems. The same IEEE group -. 

that produced the Guide on surge voltages is now 

addressing the lack of knowledge on data lines, 

such as interbuilding computer links and process 

control lines in chemical plants, which will be 

more relevant to computer users than are telephone environment data."4' Figures 4 and 5 

show transients recorded on typical data lines 

exposed to lightning effects. 

3. STANDARDS ON TRANSIENT 

OVERVOLTACES IN POWER LINES 

Several standards or guides have been issued 

or proposed in Europe and in the United Stgtes 

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 data. As a growing number 

of organizations address the problem and as 

exchanges of information take place, improvements are being made in the approach. Three of 

these are briefly discussed here. 

3.1 The IEEE Surge Withstand Capability Test 

(SWC) 'lS' 

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 high-voltage substations. 

Figure 5. Surge and noise induced by lightning 

in overhead data cable 

Peak: 48 V 

Duration: 22 ms 

Cable: 1000 m long, 15 m high 

(courtesy Digital Equipment 

Corporation) 

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 definition of the scope and limitations of the 

application. 
Table 1 

IEC Report 664 

PREFERRED SERIES OF VALUES OF IMPULSE 

WITHSTAND VOLTAGES FOR RATED VOLTAGES 

BASED ON A CONTROLLED VOLTAGE SITUATION 

(V rms and dc) I I I1 111 IV 

- - - 

Voltages Line-to-Earth 

Derived from Rated 

System Voltages, Up to: 

3.2 The IEC 664 ~e~ort''~) 

The Insulation Coordination Committee of the 

International Electrotechnical Commission (IEC) 

included in its report a table indicating the volPreferred Series of Impulse 

Withstand Voltages in 

Installation Categories 

tages that coordinated equipment must be capable 

of withstanding in various system voltages and 

installation categories (Table 1). The, table 

specifies its applicability to a conrrolled voltage 

situation, which implies that some surge-limiting 

device has been provided - presumably a typical 

surge arrester with characteristics matching the 

system voltage in each case. The waveshape 

specified for these voltages is the 1.2/50 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 farther away from the outdoor 

environment. Thus, this document primarily 

addresses the concerns of insulation coordination; 

the specification it implies for the environment is 

more the result of efforts toward coordinating the 

voltage 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 Characterization in Low-Voltage 

Circuits, which will be reviewed in detail. 

The designation, 1.2/50 ps, widely used in high-voltage 

dielectric tests, means a unidirectional impulse. double 

exponential, with 1.2 ps rise time and 50 ~s to half-value 

on the decaying tail. 

t The term "overvoltage categories" will replace "installation categories" in subsequent IEC recommendations. 

3.3 The IEEE Guide on Surge Volta es 

(ANSI/IEEE Std C62.41-1980) (lJ) 

3.3.1 Voltages and rate 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 6). 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. 

Medium Exposure - Systems in geographical 

areas known for high lightning activity, or with 

frequent and severe switching transients. 

0 High Exposure - Rare, but real, systems supplied by long overhead lines and subject to 

reflections at line ends, where the characteristics of the installation result in high sparkover 

levels of the clearances. 

The two lower lines of Figure 6 have been 

drawn at the same slope because 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 i

should not be indiscriminately applied to all sys

tems. Such application would penalize the vas

majority of installations where the exposure i

lower. 

The voltage and current amplitudes presented 

in the Guide attempt to provide for the vas
0.3 0.5 1 2 5 10 20 

SURGE CREST - kV 

*In some locations, sparkover of clearances may limit 

the overvoltages 

Figure 6. Frequency of occurrence vs level, 

from ANSI/IEEE C62.41-1980 

majority of lightning strikes, but none should be 

considered "worst case," because 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. 

Opon-Clrcult Vdtaga, 

Cumnt Doflnad by Tabla II 

hdoor Environment 

0.9 V,, 

0 3 v, 

3.3.2 Waveshape of the surges 

Many independent ~bservations"~-~" 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 specify the unidirectional doubleexponential 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 proposes two representative 

waveshapes: an oscillatory waveshape inside buildings, a unidirectional waveshape outside buildings, 

and both at the interface (Figure 7). 

The oscillatory waveshape simulates those transients affecting devices that are sensitive to dv/dt 

and to voltage reversals during cond~ction,'

while the unidirectional voltage and current 

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

equivalent of the transients where energy content 

is the significant parameter.* 

* Recent concerns on the occurrence of longer duration 

surges, or lower frequencies, such as the 5 kHz lower 

limit cited, will probably be reflected in the updating of 

this standard over the next several years. 

From a pragmatic point of view, the realization that 

oscillatin waves are unavoidably produced by practical test 

systems " will also be a driving force toward specification 

of oscillatory waveforms rather than unidirectional 

impulses in future standards. 

Increased recognition of the upset aspect of transient 

overvoltages is also likely to result in the inclusion of a 

"fast transient" with rise time and duration in the 

nanosecond range. 

Opan Clrcult Vollaga Dlscharga Currant 

Outdoor and Near-Outdoor Environment 

Figure 7. Surges defined for power lines by ANSI/IEEE C62.41-1980 
Table 2 

(Reprinted by permission of the Institute of Electrical and Electronic Engineers, Inc.) 

Energy (joules) 

Type Deposited in a Suppressor* 

Comparable to Impulse of Specimen with Clamping Voltage of 

Location IEC No 664 Medium Exposure or Load 500V lOOOV 

Category Categon Wave form Amplitude Circuit (1 20 V System) (240 V System) 

A Long branch 

Circuits and 

outlets 

6 kV High impedance* - I1 0.6 ~100 kHz 200 A Low impedance%, 5 0.8 

B Major feedera. 1.2 X SO 6 kV High impedance* - - 

short branch 8X 2014s 3 kA Low impedance % 40 80 

circuits, and I11 6 kV High impedance - - 0.6 ps-100 kHz load center 500 A Low impedance+, 5 2 4 

*Other suppresson having different clamping voltages would receive different energy levels. 

t~or highh~edance tat specimen. or load circuit.. the voltage shown represents the surge voltage. In making simulation tests. use 

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

T~or low-impedance tat specimens or load circuits, the current shown represents the discharge current of the surge (not the shortdrcuit current of the power system). In making hulation testa. use that current for the short-circuit current of the test generator. 

S~he maximum amplitude (200 or 600 A) is specified. but the exact waveform will be influenced by the load characteristics. 

3.3.3 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. With a gaptype protective device, the low impedance of the 

arc after sparkover forces most of the energy to 

be dissipated elsewhere: for instance, in a resistor 

added in series with the gap for limiting the 

power-followt current, or in the impedance of the 

circuit upstream of the protective device. With an 

energy-absorber gapless protective device, 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 

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. Standards and 

recommendations, such as DOD 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. 

ANSI/IEEE C62.41 attempts to relate impedance 

to categories of locations but unavoidably remains 

vague on their definitions (Table 2). 

t Power-follow is defined in the IEEE Dictionary as "the 

current from the connected power source that flows 

through an arrester during and following the passage of 

discharge current." 

Having defined the environment for low

voltage ac power circuits, the Working Group i

now preparing an Application Guide, where a step

by-step approach will outline the method fo

assessing the need for transient protection an

selecting the appropriate device or system. Paral

lel work in other IEEE working groups preparing 

test specification for surge protec

tive devices will be helpful in this selection pro

cess. Other groups in the USA, as well as th

international bodies of the IEC and the ComitC 

Consultatif International TClCgraphique et T61b

phonique (CCITT), are now working towar

further refinements and the reconciliation o

different approaches. 

3.4 Previous and future surge recordings 

The supporting data cited in Appendix A of 

ANSI/IEEE C62.41 are based on voltage surge 

recordings made in the 1962-1975 period. In tha

period, digital instrumentation for surge monitor

ing was not as readily available as it is now, and

most significantly, the proliferation of surge protective devices, such as metal oxide varistors, had 

not reached the present level. 

Measurements, limited to voltage, were con

ducted with oscilloscope/camera systems or with 

peak-recording instruments. Voltages were gen

erally recorded between the line(s) and the neutra

of a single-phase or polyphase power system. No 

measurements had been reported as neutral-toground; some may have been between line and 

ground. Of course, that distinction is moot fo

measurements made at the service entrance where 

neutral and ground are bonded. 
Prior to the proliferation of varistors, a limitation had been recognized for peak voltages: the 

flashover of clearances, occurring typically between 

2 and 8 kV for low-voltage wiring devices. For 

that reason, the curves of Figure 1 in the Guide 

include the indication of a possible truncation of 

the distribution around 6 kV. Recent studies, still 

in progress, have indicated that benign flashover 

of clearances, without power follow and therefore 

not readily detectable, may be more prevalent than 

was previously believed. 

An estimate of the number of low-voltage 

surge protective devices such as varistors used in 

the United States since 1972 on ac power circuits 

is in the order of 500 million. An undefined but 

substantial portion of that number is installed in 

permanently connected equipment. Therefore, it 

is now very likely that a new limitation exists in 

the recording of voltage surges. A surge recording instrument installed indiscriminately at a random location may have a varistor connected across 

the line near the point being re~orded.'~"* This 

situation will have several implications for the 

recordings obtained in present and future measurements, as contrasted to those of previous measurement campaigns. 

1. Locations where voltage surges were previously identified - assuming no change in the 

source of surges - are now likely to experience lower voltage surges, while current surges 

will occur in the newly installed protective 

devices. 

2. Not only will the peaks of the observed voltages be changed, but also their waveforms 

will be affected by the presence of nearby 

varistors as follows: 

a. If a varistor is located between the source 

of the surge and the recording instrument, the instrument will record the 

clamping voltage of the varistor. This 

voltage will have lower peaks but longer 

time to half-peak than the original surge. 

b. If the instrument is located between the 

source of the surge and a varistor, or if a 

varistor is installed in a parallel branch 

circuit, the instrument will record the 

clamping voltage of the varistor, preceded 

by a spike corresponding to the inductive 

drop in the line supplying surge current 

into the varistor. 

c. If a varistor is connected between line 

and neutral with a surge impinging 

See Case History No. 7 in Section 7 of this report 

between line and neutral at the service 

entrance, a new situation is created: the 

line-to-neutral voltage is indeed clamped 

as intended, but the inductive drop in the 

neutral conductor between the point of 

connection of the varistor and the service 

entrance creates a spike voltage between 

the neutral and the grounding connector 

at the point of connection of the varistor 

and downstream points supplied by the 

same neutral. Because this spike will 

have a short duration, it will be enhanced 

by the open-end transmission line effect 

between the neutral and grounding conductor~.'~~)~ 

3. The surge voltage limitation function performed by flashover of clearances is more 

likely to be assumed by new surge protective 

devices that are constantly being added to the 

systems. 

4. The considerations discussed in paragraphs 1, 

2, and 3 above will produce a significant 

reduction in the mean of recorded voltage 

surges in a population of different locations. 

This reduction will continue as more and 

more varistors are installed. The upper limit, 

however, will remain the same for locations 

where no varistor has yet been installed. A 

sense of false security and an incorrect 

description of the environment might be 

created by attention given only to the average 

of voltage surges presently recorded in power 

systems. Furthermore, the need for adequate 

surge current handling capability of a new 

candidate surge suppressor might be underestimated if partial surge diversion is already 

being performed by a nearby varistor. This 

risk will be exacerbated if an attempt is made 

to clamp at lower voltages by the installation 

of a new protective device with a clamping 

voltage lower than that of the device already 

installed.'*' 

4. SURGE PROPAGATION 

4.1 The limitations of arbitrary division into 

categories 

The standards cited in the preceding paragraphs 

describe surges which may be expected at specific 

points of a wiring system; the implication is that 

the surges will proceed downstream, at the same 

amplitude and waveform, until some interface 

somehow produces a staircase-like decrease in 

t See an example of this situation in Section 6.5 of this report. 
amplitude. IEC 664 proposes a voltage staircase 

with its overvoltage categories (Table I), while 

ANSVIEEE C62.41 proposes a current staircase 

with its location categories (Figure 8). 

This staircase representation is useful to simplify the real world into a manageable set of 

assumptions, but it is a simplification that can 

mask the 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 9) on one hand and the shunt 

impedance (Z2) on the other hand, form a voltage 

divider. If the series impedance is low and the 

shunt impedance is high (light loading of the system), the voltage divider does not produce high 

attenuation of the voltage surges. In addition to 

an attenuation of the amplitude, a waveform 

change takes place which is most apparent for fast 

front and short duration pulses.(29' Conversely, 

current surges, if they are the result of a current 

source such as a lightning strike, will produce high 

voltages unless a low-impedance diverting path is 

offered to the flow of current. If the current surge 

in a system is the result of a combined current 

source and multipath to ground (Figure lo), there 

is then a division of the current among the paths 

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 

the system to another. The solution lies in surge 

diversion, offering to the surge a path where the 

current flow can occur harmlessly. 

Zz "u V~we 

Figure 9 Voltage divider effect 

Lightning Stroke 

I I 

or Lightning- I 

Induced Current I 

+i I = Ground 

User Has No Control 

Over This Path User's World 

' or V = llZl - I2Z' if 2' is Significant 

Figure 10. Multipath current division 

4.2 The limitations of transmission line 

analysis 

In qualitative discussions of surge propagation

the classical behavior of a transmission line i

often called upon to provide explanations of th

situation. In particular, the reflections occurring a

the end of a line are cited in accordance with th

theory that the impulse is doubled if the line i

open-ended, and that an inverse impulse i

returned if the line is shorted. However, thes

discussions sometimes lose sight of the fact tha

the concept is applicable only if the line length i

sufficient to contain all of the surge front. If th

surge has a rise time longer than the propagatio

time along the line, the point is moot and, by th

time the surge reaches its peak, the voltage at th

receiving end of the line does not differ from th

voltage applied at the sending end. Figure 1

Figure 11. Voltages at sending end and receiving 

end of a line 

B: Black or phase conductor 

W: White or neutral conductor 

G: Green or grounding conductor 
1. THE ANSYIEEE STD C62.41-1980 CONCEPT OF LOCATION CATEGORIES IN UNPROTECTED CIRCUITS 

LOCATION CATEOORY LOCATION CATEGORY LOCATION CATEOORV C 0 A 

10 kV or more VOLTAGES a kV lmpu~w or Rtng 6 kV Rlng 

10 kA DT more 

CURRENTS 

2. TYPICAL EXAMPLES OF INDUSTRIAL OR RESIDENTIAL CIRCUITS 

AItnne1. 

Undemround -hA 

LEGEND- FA: Fixed spplknc; 

ID Induslrlml drlvs s-mm M: Driven molor 

M6: Meln bm.lrar 

P T~nmlent pml.Elor 

SE: Swwlca entrance (may lam many lorn% 

dawndlng on sputftc case of sysleml 

WRT: Wmll ncmplulw wlthoul ellenuation 

CA Codann8cI.d appllurca 

COUP Capvter wth buffed lnpul 

ICS Industrial conlml rralem LC Lfns wwsr condll~onar 

P3 surge wrlctot WRZ WUI recep~~ca~ wnn 

aIt.nuallon prw8d.d by Z-8edas Impsd.nce 

C-shun1 1rnp.d-e 

. Cable Input 

See Note 3 

3. THE IEC REPORT 684-1880 CONCEPT OF CONTROLLED VOLTAGES 

Uncontrol1.d 

VOLTAGES 

111 f4Y.S kV 

INSTALLATION CATEOORV 

IV INSTALLATION CATEGOAV II (2 sws kv 

Rmwy supply 1- 0verPm.d line. 111 

mld cable SVS~N~S ln~luding dl8trlbu. 1 1NSTALLA110N CATEGORY I (1SY)IkV 

Itom bus end 11s as.osie1.d onrcvrrml Fmed mslellatmn follormg 11 

protect,on wulpnmnt 1 ms1.11.110. C.lagOl" IV 

Aoolmnc.~ 00rlebl~ .auiom.nt 1 INSTALL*TKW CATEGORY 

Independently from the location of a devlct or equipment in the above flgurt, It should remain safe (no fires, no personnel hazard) over the full ranse of available swges at any point wlthln the installation. It may also be desirable, under particular clrcumstances and for specilk devices, to proscribe damage as a result of testlng at hlgher levels than miaht be suggested by its 

typical location. 

Notes: 

(1)The Controlled Voltage Situation of IEC Report 664 requires the presence of interfaces; these can be surge protective devices 

such as PI. P2. P3 or P4, or the existence of well-defined impedance networks such as Z and C shown in the circuit diagram 

upstream of WR2. 

(2)Voltage levels following the designation of Installation Category (IV, Ill, 11 or I) are shown in parentheses for a system with 

300 V phase-to-ground voltage, and next for 150 V phase-to-ground voltage. The voltages shown are implied as 1.2/50 ps 

impulsas. 

(3)This diagram makes no allowance for the possibility of surges associated with ground potential differences that may occur, for 

instance, with a sensor connection to the ICS control system, a cable TV connection to the line-isolated TV set. etc.. or the flow 

of ground current in the impedance of the grounding conductors. 

Figure 8. Similarities and differences between the location categories concept of ANSI/IEEE 

Std C62.41-1980 and the installation categories concept of IEC Report 664-1980, 

applied to a typical example 
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. 

There is a minor difference during the rise, where 

the initial front is doubled at the receiving end, 

but the final crest values are the same. 

The effects of reflections and attenuations 

along a line are further illustrated in Figures 12, 

13, and 14, excerpted from Reference 29. A 

three-wire, conduit-enclosed line, 225 m long was 

subjected to short pulses applied through an 

impedance-matching network to provide minimum 

interaction between the surge generator and the 

line. Access points at 75 and 150 m, in addition 

to the far end at 225 m, were provided within 

reach of the measuring oscilloscope probes by 

folding the line in a zig-zag configuration. 

Figure 12 shows the propagation of a 200 nswide pulse when a matching impedance is connected at the receiving end. The attenuation of 

this pulse is quite apparent, with an average ratio 

of 0.7 between the voltages at points 75 m apart 

in the line. The travel time can also be seen as 

1.1 ps from sending to receiving and, corresponding to 225 m/l.l ps = 204 m/p, or two-thirds 

the speed of light in vacuum. 

Figure 13 shows the same pulse propagating in 

the same line, but with a doubling of the voltage 

at the receiving end, which was left open. In 

spite of this doubling affect, there has been 

Figure 13. Propagation of the same pulse as 

Figure 12, with open-ended line 

Figure 14. Propagation of a 2 ps-wide pulse 

in the line of Figure 12 

Figure 12. Propagation of a 200 ns-wide pulse in a line terminated by its matching impedance 
enough attenuation of the short spike over the 

225 m that the pulse at the receiving end is lower 

than the pulse at the sending end. However, for 

shorter lines, the attenuation would not have 

taken its toll before doubling at the end: doubling 

the pulse which appears at the intermediate 75 m 

of the 225 m line implies that, at the end of an 

open-ended 75 m, one could have a pulse 20% 

greater than at the sending end. Note also that, 

while the amplitudes are attenuated, the rise time 

tends to be increased; therefore, the time integral 

of the pulse (and therefore its potential for 

damaging energy-sensitive components) is not 

attenuated as quickly as is the amplitude along the 

line length. 

Figure 14 shows a 2 ps-wide pulse propagating 

along the same line, with matched termination. 

The attenuation is lower than for the 200 ns 

pulse, with an average ratio of 0.9 between the 

voltages of points 75 rn apart in the line. This 

lower attenuation at longer pulse duration (lower 

frequencies) results from the decreased effect of 

the shunt capacitance of the line. 

These propagation characteristics will be discussed again in conjunction with the performance 

of protective devices, in Section 6 of this report. 

Thus, for many installations contained in a building, the line lengths are short compared to the 

length required to contain, say, a 1 ps front traveling at the speed of 200 m/ps. A more detailed 

report describing some of the aspects of the propagation of surges and their implications is given 

in References 29 and 30. 

4.3 Common Mode or Normal Mode? 

Another aspect of the propagation of surges 

concerns the dichotomy normal mode/common 

mode. In power lines, the issue is whether 

significant surges occur line-to-line (black-to-white, 

or phase-to-neutral), the situation described by 

"normal mode," or whether they occur between 

any - or all - of the lines and ground (black-togreen, white-to-green, or [black-and-whitel-togreen), the situation described by "common 

mode." These terms were first defined in the context of signal lines, where the concern for balanced circuits reflects the fact that apparently 

innocuous common mode noise can be converted 

into objectionable normal mode when circuit 

impedances along the two signal-carrying wires are 

not symmetrical with respect to the ground (cornmon) conductor (Figure 15). 

Conversely, Figure 16 shows how an attempt 

to protect against a normal mode surge can pro- 

*. 

COMMON MODE It-' 

A. Common mode voltage on primary of 

transformer couples capacitively. 

Unequal impedances Z create normal mode 

voltage across secondary 

B. Current iflowing in the two lines of a data li

because of common mode voltage K produce 

normal mode voltage e, even if currents are 

equal, when ZI and 22 are unequal 

Figure 15. Conversion of a common mode surg

into a normal mode surge 

Neutral 

Groundlng 

Conductor 

Voltage added as a 

result of surge 

current flow 

through protective devlce 

Figure 16. Current flowing in line-to-neutral 

protective device causes voltage to 

appear between neutral and groundi

duce a harmful common mode-like surge: at the conductors 
end of a branch circuit, the user installs a surgeprotective device connected line-to-neutral, in 

order to protect his load equipment against a normal mode surge impinging the origin of the 

branch circuit, where the user in this assumed 

scenario has no access to provide a surge protective device. Common knowledge that the neutral 

and grounding conductors are bonded at this origin might lead the user to believe that no harmful 

voltage can occur at his end between neutral and 

grounding conductors. Yet, the very installation 

of the protective device between line and neutral, 

results in a significant L diM voltage developed 

along both line and neutral conductors when the 

surge current flows into the protective device; half 

of the total voltage appears at the user's end 

between the neutral and the grounding conductors. 

Because there is generally no admittance between 

the neutral and grounding conductors at the user's 

end, the situation is then that of an open-ended 

line, which produces a doubling of the impinging 

neutral-to-ground voltage. An example of this 

situation is discussed in Section 6.5. 

Thus, the answer to the normal/common mode 

dichotomy is that, in most cases, both modes 

must be considered, because one can convert 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. Case History 

No. 5, later in this report, shows quantitative 

measurement results of the effects of various 

methods of connecting a surge protective device at 

the end of a line. 

5. FUNDAMENTAL PROTECTION 

TECHNIQUES 

The protection of a power system, a computer 

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 effective 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 that 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 tech

niques that is limited in space and scope has th

risk of becoming an inventory of a bag of tricks

yet, there are a few fundamental principles an

fundamental techniques that can be useful i

obtaining transient immunity, especially at th

design stages of a computer system or circuit. A

too often, the need for protection become

apparent at a late stage, when it is much mor

difficult to apply those fundamental technique

which are most effective and economical whe

implemented at the outset. 

5.1 Basic techniques 

Protection techniques can be classified int

several categories according to the purpose and th

system level at which the engineer is working

For the system as a whole, protection is primaril

a preventive effort. One must consider the physi

cal exposure to transients - in particular, th

indirect effects of lightning and power system 

faults resulting from building design, location

physical spread, and coupling to other disturbanc

sources - as well as such inherent susceptibilit

characteristics as frequency response and nomina

voltage. A data processing system using low

voltage signals, high-impedance circuits, an

installed over a wide area such as a chemical plan

spread over several kilometers, would presen

much more serious problems than the same sys

tem confined to a single building. As discussed i

Case History No. 1, the installation of remote ter

minals in separate buildings is a prime candidat

for trouble unless some basic precautions ar

observed. 

For the system components or electronic blac

boxes, the environment is often beyond the con

trol of the designer or user, and protectio

becomes a curative effort - learning to live an

survive in an environment which is imposed

Quite often this effort is motivated by fiel

failures, and retrofit is needed. The technique

involved here tend to be the application of protec

tive devices to circuits or a search for inheren

immunity rather than the elimination or diversio

of surges at their origin. 

Another distinction can be made in classifyin

protective techniques. While surges are unavoid

able, one can attempt to block them, divert them

or strive to withstand them; the latter, however, 

generally difficult to achieve alone. 

5.2 Shielding, Bonding, and Grounding 

Shielding, bonding, and grounding are thre

interrelated methods for protecting a circuit from 

external transients. Shielding is the practice o
enclosing the circuit components in a conductive 

enclosure, which theoretically cancels out any electromagnetic field inside the enclosure; actually, it 

is more an attenuation than a cancellation because 

the enclosure is rarely complete and perfect. 

Bonding is the practice of providing low-impedance 

connections between adjacent metal parts, such as 

the panels of a shield, cabinets in an electronic 

rack, or rebars in a concrete structure. Grounding 

is the practice of providing a low impedance to 

earth or a well-defined reference ultimately connected to earth,* through various methods of driving conductors into the soil. Each of these techniques has its limitations, and each can sometimes 

be overemphasized. 

5.3 Shielding 

Shielding conductors by wrapping them in a 

grounded sheath or 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 electromagnetic environment. 

Difficulties arise, however, when the concept of 

"grounded" is examined in detail. Difficulties 

also arise when the goals of shielding for noise 

immunity conflict with the goals of shielding for 

surge immunity. 

A shield can be the size of a matchbox or an 

airplane fuselage; it can cover a few centimeters 

of wire or kilometers of buried or overhead 

cables. Effective grounding of these diverse 

shields is not always an easy thing to do because 

the impedance to earth of the grounding connection must be acknowledged. The situation is 

made even more controversial because of the 

conflict between the often-proclaimed design rule 

"ground cable shields at one end only" - a rule 

justified by noise immunity performance, in particular common mode noise reduction - and the 

harsh reality of current flow and Ohm's law when 

lightning strikes or when power systems faults 

occur. 

The difficulty may be caused by a perception 

on the part of the noise prevention designers that 

the shield serves as an electrostatic shield in which 

longitudinal currents associated with common 

mode noise coupling should not flow. This concept is exemplified in the terminology of shielde

cable users, when they describe the shield con

struction of some cable design as having a foi

plus "drain wire," as if there were electrostati

charges that needed to be removed (drained)

Indeed, electrostatic charges can be drained b

connecting only one end of the shield. Further

more, if the two ends of the shield of a cabl

spanning some distance are connected to the loca

ground at each end, there is a definite possibilit

that some power frequency current may flow i

the shield. For low-level signals, this curren

could produce noise (hum) in the signals. Fo

that reason, many system designers will insist o

the one-end-only grounding rule, and they ar

correct from that point of view. Sometimes th

shield is used as a return path for the signal cir

cuit, in which case shield currents will cause vol

tage drops added to the signal. But the fact i

that, when surge currents flow near the circuits

they will unavoidably inject magnetic flux varia

tions into the circuits; hence induced voltages

Worse yet, in the case of a lightning stroke or o

a power system fault injecting current into th

earth in the area spanned by the one-end-onl

grounded shield, the potential of one end of th

cable defined as "ground" is not the same as th

"ground" at the other end of the cable. Ver

high voltages can be developed (Figure 17

between the floating end of the shield and th

local ground. No practical insulation can with

stand these levels, and breakdown will occur

allowing surge currents to flow in spite of th

designers' intent to prevent them. The path o

these currents will be determined by the com

ponents most likely to fail when the voltag

rises - the low-level logic circuits, of course. I

contrast, by deliberately allowing a small part o

these surge currents to flow in the shields, on

obtains a cancellation of the voltages tha

Connection of the reference to earth is a general safety 

requirement involving low frequency fault currents for 

which adequate low-impedance connection is a matter of 

conductor resistance. At the high frequencies involved 

with transient disturbances, the inductance of the connection becomes significant. See detailed discussion in paragraph 5.5. 

Figure 17. Voltage developed by a lightning 

stroke at the ungrounded end 

of a shield 
otherwise would be induced in the circuits, and 

the currents will follow a well-defined path that 

can be designed to produce harmless effects. 

This conflict is actually very simple to resolve 

if recognized in time: provide an outer shield, 

grounded at both ends (and at any possible intermediate points); inside 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 continuity 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 the plastic 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. Case History 

No. 1, given later in this report, illustrates the 

penalty inflicted by nature when the one-and-onlyone-ground rule was misapplied by the designer. 

5.4 Bonding 

We have already mentioned one aspect of 

bonding in describing the continuity of the outer 

shield. Another instance of bonding occurs where 

the shield of an incoming cable is connected to an 

equipment cabinet in order to allow shield current 

flow, The shield current flows in the connecting 

pigtail and creates electromagnetic radiation at the 

point of cable entry. 

Adjacent cabinets in a lineup must be bonded 

together for safety as well as transient and noise 

immunity. In principle, a flat strap has a lower 

inductance than a round wire of the same area. 

This concept may be somewhat overused; actually 

several strategically located smaller wires provide a 

much more effective bond than one massive strap, 

either round or flat. The difficulty lies in implementing this alternate view, and overcoming the 

comforting sight of a large grounding strap at the 

bottom of the cabinet lineup. Such a strap does 

no harm and is a good safety practice, but it may 

not do as much good as expected from the point 

of view of surge protection, compared to multiple 

point bonds. 

A significant subset of the general subject of 

bonding is the termination of cable shields by 

connectors at the junction to an equipment 

cabinet. The search for low-cost construction 

often results in the shield being connected 

through one pin of the multiple-pin connector. 

Add to this construction the misguided concept 

that all grounding connections in a cabinet should 

be made to a single point, and the result will be 

the worst possible practice, as shown in 

Figure 18A: the shield current is injected inside 

the cabinet along a tortuous path, creating 

interference in all circuits. Figure 18B shows a 

tolerable practice, where the connection is still 

made through the connector, for convenience, but 

a very short lead makes the connection to the 

cabinet frame inside the cabinet. Figure 18C 

shows an improvement, with the connection made 

outside the cabinet. A variation of this arrangement is encountered in RS232 connectors with a 

metal shell where the two securing screws provide 

a double (and symmetrical) bond to the equipment chassis. The ultimate and best bond, of 

course, is obtained by using a connector with a 

continuous bond provided by a cable connector 

with metallic shell and ring screwed to the chassis 

connector all around the cable (Figure 18D). 

5.5 Grounding 

Grounding, or earthing, has different meanings 

as well as different roles. The primary definition 

is the connection of the circuit, shield, or reference to earth. But what is earth? System 

designers, construction crews, inspectors, and 

technical conference authors are concerned with 

establishing, measuring, and maintaining a low 

ground resistance, often determined by dc meas

urements on rods driven into the ground. Driving 

many rods into the ground at great expense doe

not ensure a low impedance under the transien

conditions of a high rate of current change associ

ated with lightning discharges. 

When one deals with a reasonably compact sys

tem, be it cabinet-size, room-size, or building-size

it is more effective to view the grounding as a 

well-bonded connection to the outer shield (i

any), building frame, or cabinet enclosure, acting 

as a zero-reference. The resistance (impedance

from that reference to earth is not very significan

as long as other wires at ground potential are no

brought to the system. Since there is little chanc

of dealing with an absolutely isolated system 

(short of a flying aircraft, which does quite well

thank you, without an earrh connection), the ques

tion is: What should be done with incoming 

wires? These wires can be isolated from the loca

ground during normal operation, but one mus

recognize that, during transient conditions o
A The ultlmate atroclty 

The currents flow~ng In the shleld (or a smgle 

parallel grounding contactor) enter the 

cab~net and couple ~nterference radlated 

lnslde the cablnet 

B. A close second (or perhaps the prize) 

The shield is not connected (sometimes th~s 

is done by deliberate choice in an attempt to 

rnmim~ze noise In poorly designed circuits) 

The clrculattng currents cannot flow, but 

capacitance between the wires and the shleld 

transfers the current onto the wlres, addlng 

spurlous slgnals 

C. An acceptable, but not very good approach Is 

to bond the shield (grounding conductor) 

IMMEDIATELY upon entering the cabinet. In 

the absence of a connector providing the "E" 

conflgural~on, thls is often done by connectIng a shleld pigtail by means of one pln of 

the connector 

of aN conductors entering a system, as shown i

Figure 19. The system can be a single cabine

room-size equipment, or a complete floor in 

building - the principle remains the same. Th

ground window must be specified from the begin

ning, as retrofits are generally difficult to make. 

Powel 

GNL 

MULTIPLE GROUND 

CONFIGURATION 

Ctrculating 

ground currents 

Involve the 

system clrcults 

Power - 

Lt.23 = 

I 

0. A much better way IS to connect a short 

ptgtail (or the grounding conductor) on the 

outside of the cabinet, next to the point of 

entry of the other wires. This requlres a 

screwed connection in addition to a plug. -- and for that reason is not welcome, but 

it IS better then C 

E. The ultlmate good practice. A coaxlal shell 

connector bonds the shield to the cabmet 

wall, no coupling occurs onto the Internal 

wires 

Figure 18. Bonding of cable shields to 

equipment cabinets 

lightning surges or power system faults, high voltages will appear across these isolated wires and 

local ground - voltages which, in some cases, are 

far beyond the withstand capability of insulation. 

That insulation, then, must be protected by suitable devices which, in fact, do connect the wires 

to the local ground, but only for the duration of 

the transient. This type of momentary grounding 

is one function of transient suppressors. 

An effective approach to limiting the adverse 

effects of ground potential differences is the 

enforcement of a "ground window"' arrangement 

The concept and the term "ground window" were 

developed by workers in the Bell System; the author was 

introduced to th,e concept by Paul Speranza of Bell Communications Research during discussions at IEEE working 

group meetings: this sharing of information is gratefully 

acknowledged. 

GROUND WINDOW 

CONFIGURATION 

Circulating 

ground currents 

exchange takes 

place through 

jumper at window. 

outside circuits 

1 + power DATA1 3 

3, power 1- "Earth ground 

DATA 

Figure 19. Multiple cable entry vs single 

ground window 

5.6 Isolation of subsystems 

In the case of systems involving separate build

ings, remote sensors, or the interconnection of 

power system with a communication system, oth

requirements may dictate the isolation of the sub

systems, creating the illusion that protectio

against overvoltages has also been accomplishe

And yet we have seen that during transient cond

tions high voltages can occur between the subsy

tems. 

Where moderately high voltages only ca

occur, effective isolation can be accomplished b

the insertion of an isolating transformer or a

opto-isolator; if metallic isolation is not required, 

filter can also be used if it does not degrade da

pulse shapes or system frequency response. 
Where the voltages will reach levels exceeding 

the withstand capability of economically or technically feasible insulation, two possible solutions 

exist. The first, already mentioned, and applicable 

to power as well as data systems, is to bond the 

grounds or references of the two systems during 

the transient by means of a surge protective device, which returns to a high level of insulation 

after the transient has subsided. The second, 

applicable only to low-power data transmission, is 

to use other methods, such as insertion of audio 

couplers or a fiber optics link. Complete decoupling of electrical transients and noise resulting 

from ground potential differences can be achieved 

in this manner; however, these techniques will not 

guard against noise collected by the circuits themselves and faithfully transmitted by the link to the 

other end. 

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 around the sensitive equipment. 

There are two categories of transient suppressors: those that block transients, preventing their 

propagation toward sensitive circuits, and those 

that divert transients, limiting voltages to an 

acceptable residual level. Because 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 20). 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. Depending on the type 

of device, this voltage nonlinearity is the result of 

two different mechanisms - a continuous increase 

in the device conductivity as the current increases

or an abrupt switching as the voltage increases. 

Because the technical and trade literature con

tains many articles on these devices, a discussion 

of the details will be limited and review of the 

references is suggested. Some comparisons will be 

made, however, to point out the significan

differences in performance; clarification of some 

issues resulting from unwarranted concern will also 

be given. We will first examine the basic princi

ples of single-component suppressors, then the 

application of these devices to protect circuits, a

single- or multiple-component packaged devices. 

Figure 20. Hybrid approach for surge 

suppression 

RESTRICT 

DIVERT CLAMP 

- - 

6.1 Crowbar devices 

PROTECTED 

CIRCUIT 

The principle of crowbar devices is simple

upon occurrence of an overvoltage, the devic

changes from its normal high-impedance state to 

low-impedance state, offering a low-impedanc

path to divert the surge to ground. This switchin

can be inherent to the device, as in the case o

spark gaps involving the breakdown of a gas o

the recently introduced two-terminal multijunction 

semiconductors. Some applications have also been 

made of externally triggered devices, such as trig

gered vacuum gaps in high-voltage technology o

thyristors in low-voltage circuits, where a contro

circuit senses the rising voltage and turns on th

surge-rated device to divert the surge. 

The major advantage of the crowbar device i

that its low impedance allows the flow of substan

tial surge currents without dissipation of high 

energy within the device itself; the energy has t

be spent elsewhere in the circuit. This so-calle

"reflection" of the impinging surge can also be 

disadvantage in some circuits when the transien

disturbance associated with the gap firing is being 

considered. Where there is no problem of power

follow (discussed below), such as in communica

tion circuits, the spark gap has the advantage o

very simple construction with potentially low cost. 

The crowbar device, however, has three majo

limitations. The first limitation concerns the volt

time sensitivity of the breakdown process. As th
voltage increases across a spark gap, a significant 

conduction of current - and hence the voltage 

limitation of a surge - cannot take place until the 

transition occurs to the arc mode of conduction, 

by avalanche breakdown of the gas between the 

electrodes. The load is left unprotected during 

the initial rise because of this delay time (typically 

in microseconds). Considerable variation exists in 

the sparkover voltage achieved in successive 

operations, because the process is statistical in 

nature. In addition, this sparkover voltage can be 

substantially higher after a long period of rest than 

after successive discharges. From the physical 

nature of the process, it is difficult to produce 

consistent sparkover voltage for low-voltage ratings. This difficulty is increased by the effect of 

manufacturing tolerances on very small gap distances, but it can be alleviated by filling the tube 

with a gas having a lower breakdown voltage than 

air. However, if the enclosure seal is lost and the 

gas is replaced by air, this substitution creates a 

reliability problem due to the substantially higher 

sparkover of the air gap. 

In communication circuits using a pair of conductors, protection is often provided by connecting 

a gas tube between each conductor and ground. 

Upon occurence of a common mode surge, which 

would leave the input circuits unaffected, an 

nature of the process, it is difficult to produce 

consistent sparkover voltage for low-voltage ratings. This difficulty is increased by the effect of 

manufacturing tolerances on very small gap distances, but it can be alleviated by filling the tube 

with a gas having a lower breakdown voltage than 

air. However, if the enclosure seal is lost and the 

gas is replaced by air, this substitution creates a 

reliability problem due to the substantially higher 

sparkover of the air gap. 

In communication circuits using a pair of conductors, protection is often provided by connecting 

a gas tube between each conductor and ground. 

Upon occurence of a common mode surge, which 

would leave the input circuits unaffected, an 

undesirable condition results from the volt-time 

variation between the two devices: unavoidable 

manufacturing tolerances between the two devices, 

plus the statistical variation for each tube breakdown cause one tube to fire before the other. 

During the time separating the two firings, a substantial normal surge is applied to the input circuitry, with possible destructive effects (Figure 21). 

This problem can be avoided by using threeelectrode tubes where firing of the first gap causes 

firing of the second gap with no delay 

(Figure 22). 

COMMON MODE 

IMPINGING 

SURGE ------, G2 

"PROTECTED" 

LOAD 

I G2 FIRES / 

-- 

ZERO HIGH 

-w ZERO 

NORMAL NORMAL NORMAL 

VOLTAGE VOLTAGE VOLTAGE 

Figure 21. Normal mode surge resulting from 

difference of sparkover time of two 

separate gas tubes 

COMMON MODE 

IMPINGING 

SURGE- 4) PROTECTED 

LOAD 

Figure 22. Threeelectrode gas tube avoiding the 

problem of Figure 21 

The second limitation is associated with t

sharpness of the sparkover, which produces fa

current rises in the circuits and, thus, objectio

able noise. A classic illustration of this proble

is found in oscillograms recording the sparkover 

a gap where the trace exhibits an anomaly befo

the sparkover (Figure 23). This anomaly is d

to the delay introduced into the oscilloscope c

cuits to provide an advanced trigger of the swee

Figure 23. Anomaly in recording showing 

gap interference 
What the trace actually shows is the event delayed 

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

gap sparkover occurs and noise enters the oscilloscope by stray coupling, while the electron beam 

is still writing the pre-sparkover rise. Another, 

more objectionable, effect of this fast current 

change can be found in some hybrid protective 

systems. The circuit of one such commercial device is shown in Figure 24. The gap does a very 

nice job of discharging the impinging high-energy 

surges, but the magnetic field associated with the 

high di/dt induces a voltage in the loop adjacent to 

the second suppressor, adding what can be a substantial spike to the expected clamping voltage 

provided by the second device. An illustration of 

this effect is discussed in Case History No. 1. 

A third limitation occurs when a power current 

from the steady-state voltage source follows the 

surge discharge (follow-current or power-follow). 

In ac circuits, this power-follow current may or 

may not be cleared at a natural current zero. In 

dc circuits, clearing is even more uncertain. Additional 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-follow current. This combination of a 

gap with a current-limiting, nonlinear varistor has 

been very successful in the utility industry as a 

surge arrester, often referred to as a "valve-type 

arrester." 

6.2 Voltage-clamping devices 

Voltage-clamping devices 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 discontinuity by turn-on action. As far as 

volt-ampere characteristics are concerned, these 

components are time-dependent to a certain 

Figure 24. Typical hybrid circuit subject 

to induced voltage 

degree. However, unlike the sparkover of a gap 

or the triggering of a thyristor, time delay is no

involved. 

When a voltage-clamping device is installed in 

a circuit, the circuit remains essentially unaffected 

by the device before and after the transient fo

any voltage below clamping level. Increased 

current drawn through the device as a surge vol

tage attempts to rise results in voltage-clamping 

action. Nonlinear impedance means that thi

current increases more than the voltage. Th

increased voltage drop (IR) in the sourc

impedance due to higher current results in th

apparent clamping of the voltage. It should b

emphasized that the device depends on the sourc

impedance to produce clamping. A voltage divide

action is at work where the ratio of the divider i

not constant, but changing. If the sourc

impedance were very low, the ratio would be low

and eventually the suppressor could not work a

all with a zero source impedance (Figure 25). I

contrast, a crowbar type of device effectively shor

circuits the transient to ground; once established

however, this short circuit will continue until th

current (the surge current as well as any power

follow current supplied by the power system) i

brought to a low level. 

The principle of voltage clamping can b

achieved with any device exhibiting this nonlinea

impedance. Two categories of devices, having th

same effect but operating on very different phys

cal processes, have found acceptance in the indus

try: polycrystalline varistors and single-junctio

avalanche diodes. Another technology, selenium 

rectifiers, has been practically eliminated from th

field because of the improved characteristics o

modern varistors. 

Figure 25. Voltage divider effect of shuntconnected suppressor 

6.3 Avalanche diodes 

Avalanche diodes, or Zener diodes, were in

tially applied as voltage clamps, a natural ou

growth of their application as voltage regulator

Improved construction, specifically aimed at surg

absorption, has made these diodes very effectiv

suppressors. Large-diameter junctions and lo
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 V 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. 

Silicon avalanche diodes are available with 

characteristics tailored to transient suppression. 

These should not be confused with regulator-type 

Zener diodes although many engineers tend to use 

the generic term "Zener diode." May Zeus help 

them if they misapply a regulator-type Zener, 

expecting to achieve good protection! 

Since the junction is very thin, the capacitance 

of an avalanche diode is appreciable. This can be 

a concern. The effect of capacitance can he 

minimized by using series combinations with lowcapacitance diodes (Figure 26). 

Slllcon Transient 

Suppressor 

Slllcon 

Translent 

Suppressor 

Capacltance 

Low Capac~tance 

D~odes 

Figure 26. Reduction of effective capacitance 

of a suppressor 

6.4 Varistors 

The term varistor is derived from the function 

of the device as variable resistor. This device has 

also been called a voltage-dependent resistor, but 

that description tends to imply that voltage is the 

independent parameter in surge protection, while 

in fact surge current is the given parameter. Two 

very different devices have been successfully 

developed as varistors: silicon carbide blocks have 

been used for years in the surge arrester industry, 

and more recently, metal oxide varistors have 

become widely used. 

Metal oxide varistors depend on the conduction 

process occurring at the boundaries between 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 literat~re.'~'-~~' 

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 behavior of 

a varistor can be understood by examination o

the equivalent circuit of Figure 27. The major 

element is the varistor proper, R,,, whose V-

characteristic is assumed to be the perfect power 

law, I = kVU. In parallel with this varistor, there 

is a capacitor, C, and a leakage resistance, R,. In 

series with this three-component group, there i

the bulk resistance of the zinc oxide grains, R,, 

and the inductance of the leads, L. 

Figure 27. 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, all but the leakage resistance are significant: the varistor provides low 

impedance to the flow of current, but eventually 

the series resistance will produce an upturn in the 

V-1 characteristic; the lead inductance can give 

rise to spurious overshoot problems if not dealt 

with properly; and the capacitance can offer either 

a welcome additional path for fast transients or an 

objectionable loading at high frequency, depending 

on the application. 

When the V- I characteristic is plotted on a 

log-log graph, the curve of Figure 28 is obtained. 

Three regions result from the dominance of R,, 

then R,,, and finally R, as the current in the device increases from nanoamperes to kiloamperes. 

The V-1 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 factors, discussed in 
Figure 28. V-I characteristic of a varistor 

detail in the information available from manufacturers, must also be taken into consideration. 

Some of these factors are: 

Selection of the appropriate nominal voltage 

for the line voltage of the application. 

0 Selection of the current-handling capability 

(including consideration of the source 

impedance of the transient, the waveshape, 

and the number of occurrences). Reference 31 provides general information on the 

selection process for these two factors, and 

Reference 8 gives an example of the problems which can occur when one attempts to 

provide a relatively low clamping voltage by 

using a device found by hindsight to be 

insufficient for the environment. 

Proper installation in the circuit (lead 

length). Because experience has shown that 

the lead effects are sometimes misunderstood, two aspects of their importance are 

presented as case histories in Section 7.6. 

6.5 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 now been settled in a study 

published by EPRI.(~~) 

Component surge protective devices such as 

gaps, varistors, or avalanche diodes are used by 

manufacturers for incorporation into the circuitry of 

their products. In contrast, packaged suppressors 

are applied by prudent users as preventive and 

complementary protection, or by aggrieved users as 

retrofit protection. These packaged suppresso

may contain only a single protective device or 

combination of devices; they are available f

power-line protection, for data-line protection, a

also in combined power/data lines protection. 

The combined power/data lines protecti

packages offer not only convenience for protecti

both lines of peripheral or remote equipment b

also, and very important, permit the implement

tion of a common reference (ground) between t

power and the data line. This common referen

can be located right at the point of installatio

and thus realize the ground window approach d

cussed in the preceding section. 

A new type of suppressor has also appeared 

the market, the so-called "tracking protectors

This type of device provides a voltage-limitin

action over a narrow band of deviation from t

power-frequency sine wave, rather than the fixe

absolute voltage limit of clamping or crowbar de

ices. Typical circuits involve the switching on 

a shunt capacitor when the instantaneous volta

deviation exceeds a preset limit. 

Packaging of the suppressors accomplishes tw

desirable goals: convenience of insertion by t

user and coordination of the design for multipl

component protective schemes. Unfortunatel

this packaging sometimes intentionally obscur

the principles of protection being offered, maki

an evaluation of performance claims difficult. T

competitive nature of these products is an un

voidable reality, which does not justify obscuri

performance characteristics, even if some users a

only interested in simple assurances that t

devices packaged will provide them with adequa

protection. 

One reason for the frequent lack of inform

tion on the performance of the packages bei

offered is a lack of standards that would provi

manufacturers and users with realistic and unifor

application requirements. Component protecti

devices have the benefit of presently available te

specification standards, (24-27' but standards-writi

groups have not yet completed their projects 

packaged suppressors. In particular, the IEEE h

an ongoing project that will take several years 

work before publication; the Underwriters' Labor

tories (UL) are approaching release of a doc

ment'36' that will provide not only safety guidan

but also a more uniform basis of comparison 

the packages being offered in the trade. 

As an example of power line packaged suppre

sors, the recently introduced General Electric VS

device offers plug-in protection with both line-t
neutral and neutral-to-ground protective devices 

(Figure 29). Further protection against low-level, 

high-frequency disturbances is obtained with the 

VNS device, which has an L-C filter added to the 

package (Figure 30). 

The presence of neutral-to-ground protection in 

these two packages is a definite improvement over 

the simpler packages, which provide only line-toneutral or neutral-to-line protective devices. Each 

of these two simpler devices may leave parts of 

the "protected" load unprotected during the 

occurrence of certain types of surges, as mentioned in the common mode/normal mode discussion of the preceding section. Figure 31 illustrates 

the complete protection provided by the dual devices, and Figure 32 shows the incomplete protection of the simple devices, where a normal mode 

Figure 29. General Electric VSS suppressor 

(a) Voltage between Hne and neutral 

Figure 30. General Electric VNS suppressor 

surge is converted into a voltage transient between 

the neutral and the grounding conductor. 

6.6 Line conditioners as surge suppressors 

The need to provide "clean power" to sensitive electronic equipment has promoted the 

development of a wide variety of devices generally 

described as "line conditioners." Depending upon 

the design and principle involved, these devices 

can perform several of the following functions: 

line isolation, voltage regulation (medium- and 

long-term), noise suppression, surge suppression, 

common mode suppression, and back-up power 

supply. Close inspection of the specifications is 

(b) Voltage between neutral and ground 

Figure 31. Clamping of the ANSVIEEE C62.41 ringwave vs the VSS suppressor 

Vertical: 500 V/div 

Sweep: 2 ps/div 
A. Line-to-neutral voltage B. Neutral-to-ground voltage 

Figure 32. Incomplete protection by a single suppressor at the end of a branch circuit 

with normal mode ringwave applied at the service entrance 

Vertical: 500 V/div 

Sweep: 2 &div 

required to distinguish the details of the performance of these devices. Their principle of operation can be used to categorize the types: 

r Ferroresonant transformers (primarily regulation) 

r Isolation transformers with filters (primarily 

common-mode decoupling) 

r Motor-generator sets (primarily decoupling and 

short-time ride-through) 

r Magnetic synthesizers (primarily regulation) 

0 Electronic synthesizers (primarily regulation) 

r Electronic rectifiers/battery/inverters (primarily 

uninterruptible power supplies) 

When properly understood and applied, these devices can provide not only their desired primary 

functions, but surge suppression as well - a welcome side effect. A notable exception, resulting 

from misunderstanding of the application, is discussed in Case History No. 4. 

6.7 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 is 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 temporarily out of 

operation. To others, fail-safe means that the 

function 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 fail-short 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 deposited in the device, 

the current-handling 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 does not affect the protective function 

(Figure 33). However, 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 (for 

example, an error made in the assumption, a 

human error in the use of the device, or nature's 

tendency to support Murphy's law), the circuit in 

need of protection can generally be protected at 

the price of failure of the protective device in the 

short-circuit mode. 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 quickly cleared by 

a series overcurrent protective device (fuse or 

breaker). 
Assume an open-circuit voltage of 3000V I 

If the source impedance is 2, = 500 

with a suppressor impedance of 2, = 80 I 

the expected current is: 

I = 3000 = 51.7AandV. = 8~51.7 =414V 50 + 8 

The maximum voltage appearing across the terminals of a typical nonlinear V130LA20A 

varistor at 5 1.7A is 330V. 

Note that: 

2, x I = 50 X 51.7 = 2586V 

V, x I = 8 x 51.7 = 414V 

= 3ooov 

dthe source impedance is only 50 (a 10: 1 error in the assumption), the voltage across the same 

Iimar 80 suppressor is: 

However, the nonlinear varistor has a much lower impedance; again, by iteration from the 

chatacteristic curve, try 400V at 500A. which is correct for the V130LA20A; to prove the 

correctness of our "educated guess" we calculate I. 

which justifies the "educated guess" of 500A in the circuit.* 

3000V "OPEN-CIRCUIT" TRANSIENT VOLTAOE 

Protective Level Assumed Source Impedance 

Achieved 50Q Sn 

Linear 8n 4 14V 18SOV 

Nonlinear Varistor 330V 400V 

Similar calculations can be made, with similar conclusions, for an assumed error in 

open-circuit voltage at a fixed source impedance. In that case, the linear device is even more 

acnsitive to an error in the assumption. The calculations are left for the interested reader to 

work out. 

Figure 33. The importance of correct assumptions on some impedance 

when using nonlinear surge protective devices 
With the failure mode of a suppressor being of 

the fail-short type, the system protection with 

fuses can take two forms (Figure 34). For the 

user concerned with maintaining the protection of 

expensive equipment, even if failure of the protector means the loss of the function, Alternative A 

must be selected. Conversely, if the function is 

paramount, Alternative B must be selected. 

7. EXAMPLES AND CASE HISTORIES 

To illustrate the preceding considerations, some 

practical examples are given in this section as the 

basis for sound design approaches, or as horror 

tales where the namcs of the "guilty" have been 

withheld: it is always easy, with hindsight, to see 

what should have been done, but less easy at the 

outset for engineers unfamiliar with surge protection and more concerned with other system considerations. 

These case histories show how unintentional 

but clear violations of the principles described in 

this report resulted in the problems that follow: 

Case 1: 

Case 2: 

Case 3: 

Case 4: 

Case 5: 

Case 6: 

Case 7: 

Case 8: 

Ground potential differences on data 

lines caused by discontinuous shield 

Ground potential differences on 

power lines caused by ignorance of 

the ground window approach 

Insufficient and poor utilization of 

protection 

Misunderstanding of common versus 

normal mode 

Controversies on connection options 

Measurement problems 

Unwanted surge suppression by an 

instrument 

Undersized protective device 

The detailed explanations of the problems and 

cures in these case histories are offered as a help 

to avoid these types of pitfalls. 

7.1 Case History No. 1 - Computer graphics 

system: Ground potential difference on data 

lines 

A CAD/CAM graphics system had been 

installed by a computer graphics vendor linking a 

central processing unit to remote terminals located 

in separate buildings. In a span of 5 weeks during 

the first summer after the system was commissioned, three lightning storms occurred in the 

A - Protection mainlalned, 

functlon interrupted 

B - Functlon maintained, 

protection lost 

Figure 34. Fusing alternatives for suppressors 

area; no direct strikes were reported on the buildings, but extensive damage was done to the circuit 

boards on terminals and central processing unit 

(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 terminals at the onset of a lightning storm. This did 

not help. Next, isolating transformers were 

installed but, again, did not help. At this point, 

the author was called in for consultation, and the 

following proposed diagnosis 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 plastic conduit buried between 

the buildings and, tru to the controversial 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 35 shows how this 

arrangement can produce high voltages between a 

floating end of the shield and the local ground, an 

arrangement that is bound to produce a flashover 

and flow of surge currents along unwanted paths 

in the circuit components. Thus, the problem was 

not 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 
Buldmg A Building B 

I Shield Tied to 

r CPU Ground I I 

+Steel !In Eh~ild~ngs Condu~t /e 

Data Pair 

Figure 35. Shield connections of data cables 

grounding connection of 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 bundle, would indeed 

have eliminated the differential ground potential 

problems, but were considered too expensive or 

too long to install. 

Incidentally, part of the original bewilderment 

at the failures was the notion that opto-isolators 

provided in the data link route should have served 

to avoid 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 in principle, would have been the 

replacement of the plastic underground conduit by 

a continuous steel conduit linking the steel conduits used inside the buildings. This additional 

metal would have provided equalization of the 

ground potentials along the data cable, while 

allowing, within the conduit, the desired use of 

shields with one end only at ground. However, 

that solution was not acceptable to the plant facility organization. (One does not dig up the front 

lawn two times within a few months at an industrial park!) 

In this particular location, a spare conduit, 

buried next to the data line conduit, offered the 

possibility of pulling a heavy ground cable in the 

spare conduit,. close to the data cables. This cable 

could then bond the two corners of the building's 

steel frame 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 facilities: 

because there is a ground grid tying the 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:(38) 

inserting an array of diodes (Figure 36) at one 

end of the shields reconciled the need for noise 

prevention during 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 circulating current that would inject noise 

into the data cables. During a lightning strike, 

however, the diodes would allow flow of current 

to compensate and cancel the ground potential 

differences. 

These two cures were implemented during the 

summer of 1980, and no further problems 

occurred for the rest of the lightning season of 

that year. While these two solutions might have 

been sufficient, the concern over another possible 

failure of the system was sufficient 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 

Shield Grounded Twisted 

at Other End Data Pair 

Forward Drop of Diodes 1 - 

Isolate Shield End at 60 Hz. - 

Conduction of Diodes Durin~ Local Groun

a Surge Effectively rounds ] 

Shield. 

Figure 36. Diode array for grounding both ends 

of shield during a surge 
the computer vendor, so that it was not implemented until the next lightning season, the spring 

of 1981. Thus, the system survived all of the 

remainder of the 1980 lightning season with only 

the first two remedies. 

Voltage clamps were designed for insertion at 

each end of the data cable, at the point where the 

cables were terminated prior to connection to the 

CPU or to the terminal (Figure 37). The objective was to clamp any transient developed in the 

pair, with respect to ground, at less than 25 V, 

without introducing excessive degradation of the 

pulses' fronts in the signals transmitted by the 

cables. Vendors of hybrid protection packages 

were consulted and samples were obtained for 

evaluation. All these samples consisted of a gas 

tube protective device connected between the line 

and ground and followed by some resistance, and 

a silicon avalanche suppressor between line and 

ground (Figure 38). The criteria were the optimization of the RC parameters of the suppressor - 

R being the series resistance, and C the capacitance of the silicon diode - for maximum 

suppression with limited and acceptable decrease in 

the steepness of the data pulses. 

During evaluation testing of the various candidate suppressors submitted by prospective vendors, 

it became apparent that the physical layout of the 

components had an effect on the clamping voltage 

obtained: when the gas tube gap sparked over, 

the high dildt in the gap produced a high d~$/dt 

in adjacent loops, in particular the loop involving 

the second-stage avalanche diode and the load 

(Figure 39). While the clamping voltage of the 

avalanche diode was, in fact, 15 V, as much as 

45 V spikes were recorded across the output of 

the packaged hybrid suppressors, raising doubts 

about the effectiveness of the protection. These 

spikes were extremely short (nanoseconds), but 

the vendor of the integrated circuits that had 

failed in the initial problems would not agree to 

consider more than the specified maximum 25 V 

overvoltage, even for these very short spikes. 

Since the proposed connection between the 

interface box in the terminal or CPU rooms and 

the terminal equipment involved a flexible connection by shielded pairs, a simple solution was possible. The inherent capacitance of the pairs to their 

shield, combined with an additional resistance at 

the output of the hybrid suppressor could reduce 

this induced spike below 25 V (Figure 40). Thus, 

an acceptable package was designed, providing for 

the clamping of any surge to a level below the 

tolerance level of the integrated circuits of the line 

drivers or line receivers, but still not producing an 

Building A Building 8 

Local Clamps 

Figure 37. Insertion of voltage clamps 

at interfaces 

Figure 38. Voltage clamping retrofit on data cable 

objectionable degradation of pulse fronts. The 

complete protection scheme was installed before 

the 1981 lightning season, and no further problems have been reported. 

Experience has shown that conclusions on the 

effectiveness of lightning protection scheme

should wait perhaps as much as 10 years before 

being proclaimed, because of the large variations 

in lightning activity. However, after several years 

of trouble-free operation compared to three major 

failures in 5 weeks, the cure would seem effective

In retrospect, then, the following recommendations can be drawn from this horror tale,' for 

retrofits or new installations: 

1. Data cables linking separate buildings or span

ning 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 diktat of the system vendor, then these cables should be installed 

within a continuous metal shield. This con

tinuous shield can be either a double shield of 

the flexible cable or simply a metal condui

with both ends grounded and proper attention 

to maintenance of its continuity. 

* Which, in the last several years, was found repeated at 

several other facilities involving different systems but the 

same basic problem. Thus, this case history has achieved 

the status of "classic" or "textbook"importance. 
Vertical: 20 V/div 

Sweep: 50 ns/div 

Figure 39. Voltage induced by gap sparkover 

25 R 

t 

Cable Vertical: 10 V/div 

Capacitance Sweep: 50 nsldiv 

Figure 40. Reduction of induced voltage 

Substantial relief can be obtained in retrofits phase-control of power thyristors, limited to a sinby grounding both ends of existing shields gle room in a building, had suffered system 

through a low-voltage clamp, such as a diode crashes and memory component damage on 

array, that will block noise-inducing power fre- repeated occasions. Suspicions developed that 

quency currents but will allow the flow of there was some correlation between the crashes or 

ground-potential equalizing currents during damage and the operation of another developmensurges. tal power system in an adjacent laboratory. The 

The ultimate protection may be the insertion 

of surge-protective devices in each line. However, this solution requires careful design so 

that degradation of the signals does not occur 

and residual spikes are not allowed to pass 

through. 

While damage prevention can be accomplished 

by these approaches, data errors may still be 

produced. If data integrity is an absolute 

requirement, metallic connections should be 

avoided for data links spanning remote terminals. 

Case History No. 2 - Computer-aided 

industrial control: Ground potential 

differential on the power lines (absence of 

ground window) 

software engineers working on the control system 

were attempting to continue their work by staggering the schedule with their neighbors; they were 

also considering installation of an independent 

power feeder to their system. 

From another point of view, focused by the 

author, this case presented an opportunity to 

learn, while still in the laboratory, what the real 

world can inflict on unprotected computerized 

power systems. Rather than blame the power supply, a more fruitful approach for the long run 

would be to develop immunity to interference and 

damage. In defense of the system designers, it 

should be pointed out that their system had 

enough challenges to be tackled in the main objectives that it might have seemed reasonable to 

overlook the interference problems in the initial 

stages. Sooner or later, however, the ignored 

problems will crop up, and, sooner is better than 

In this situation, a novel adaptive control using later when fundamental design concepts are 

sophisticated microprocessor-based sensors and involved. 
A review of the total system revealed the 

existence of ground loops. On one side, the 

power supply for the computer and some signal 

processing circuits were obtained from the room 

outlets of the laboratory 120 V system, including 

the grounding conductor (green safety wire). On 

the other side, the power supply for the highpower circuit was obtained from a feeder coming 

directly from the building power center, including 

again a grounding conductor run alongside this 

power line, properly installed by electricians, and 

bonded to the frame of the machine being controlled. One of the signals used for controlling 

the process was derived from a voltage pickup 

referred to the frame of the machine, while the 

chassis of the computer and its zero reference 

were bonded to the grounding conductor of the 

120 V room supply. Thus, a double ground loop 

was formed: one between the grounding conductor 

of the 120 V room supply and the power-feed 

grounding conductor, and the other between the 

overall zero reference of the signal processing and 

the voltage pickup with its separate ground reference (Figure 41). 

Power Circuit 

Disconnect 1-1 

Switch 

I \ : 

120" I s":" 

Wall 

Outlet I w: 

B 

W 

v -I + i ::.. . . . . . . . . . . . . . . . . . . . . . Load' ----------- 

Center 

Ground 

Microprocessor 

Control 

Figure 41. Ground loops in power and 

control systems 

With hindsight, it was not difficult to conclude 

that during transient conditions involving the highpower feed to this system and the neighboring 

system, substantial current could flow between the 

two grounding wires linked by the computer reference wiring. An immediate cure was to open this 

path for surge ground currents by inserting an 

isolating transformer in the 120 V supply to the 

computer, and bonding the secondary side of this 

transformer to the single ground point derived from 

the high-power feed (a National Electrical Code 

requirement). This correct application of an isolating transformer, to open a ground loop, is in 

contrast to the misconception that isolating 

transformers can eliminate line-to-line spikes, as 

discussed in Case History No. 4. 

Opening the second ground loop involving the 

voltage pickup could not be as readily implemented because it required a differential voltage 

pickup circuitry. Plans for further refinements of 

the system included this change, but even with 

only the first ground loop opened, the major 

crashes and damage stopped; only occasional 

interference occurred, probably not associated with 

the second group loop but, rather, with a more 

subtle software problem associated with sensing 

the power flow in the system. Clearly, the firs

ground loop was one of the major sources of the 

problem, a problem that would have been avoided 

if the system had been arranged with a single 

ground window. 

7.3 Case History No. 3 - Outdoor recorder 

retrofit: Insufficient surge protection agains

line-to-ground surges 

In this case history, a field failure problem wa

caused by a lack of awareness (on the part of the 

circuit designer) of the degree of hostility in the 

environment where the circuit was to be installed

A varistor had been provided to protect contro

circuit components on the printed circuit board

but its capability was exceeded by the surg

currents occurring in the particular location. In 

defense of the circuit designer, however, it mus

be stated that he was unaware of the data pub

lished in IEEE Std 587 (now ANWIEEE C62.41)

Since a number of devices were in service

complete redesign was not possible, and 

retrofit - at an acceptable cost - had to b

developed. Fortunately, the power consumptio

of this control circuit was limited, so that it wa

possible to insert some series impedance in th

line, ahead of the low-capacity varistor, while 

higher capacity varistor was added at the lin
entrance to the circuit (Figure 42). Laboratory 

proof-testing of the retrofit demonstrated the capability of the combined scheme to withstand 6 kA 

crest current surges (Figure 43), which is a 200°h 

margin from the suggested IEEE/ANSI (262.41 

Category B level. Furthermore, it demonstrated 

reproduction of the field failure pattern (Figure 43). The latter is an important aspect of any 

field problem retrofit. By simulating in the laboratory the assumed surges occurring in the field, 

verification of the failure mechanism is the first 

step toward an effective cure. Figure 43 illustrates 

the effect of improper installation of the suppressor in a first retrofit attempt with 8 inches of 

leads instead of a direct connection across the 

input terminals of the circuit. The author has 

observed far too many applications of varistors 

with excessive lead lengths, to the point that the 

protection is substantially reduced for fast rising 

surges, the present case being typical. 

- 

BOARD 

CONTROL 

CIRCUIT 

Figure 42. Retrofit of a protection package 

7.4 Case History No. 4 - Does an isolating 

transformer 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 44 through 

46 are offered to support the position that these 

quotations are misconceptions. When properly 

applied, isolating transformers are useful to break 

ground loops, but they do not by themselves 

attenuate spikes that occur line-to-line, the socalled "normal mode." 

Figure 44 shows the propagation - or worse, 

the enhancement - of a voltage impulse in a 1:l 

isolating transformer. The 6 kV, 0.5 ps-lOO kHz 

impinging wave of ANSMEEE C62.41 is applied 

to the primary of the transformer, HIH3 to ff2H4. 

The output voltage, measured at X1X3 to X2X4, 

appears as a 7 kV crest on the secondary side of 

this "isolating" transformer. 

PC BOARD VARISTOR 

Upper trace: Voltage across VlSOLAl 

varistor on PC board, 200 Vldiv 

Lower trace: Applied surge current, 

2000 A/div 

Sweep speed: 10 ps/div 

SPARKOVER OF RUNS UITHOUT PROTECTION 

Additional surge protection removed: 

VlSOLAl varistor on PC board is the 

only protection 

Upper trace: Voltage across V1 SOLA 1 

varistor 

Lower trace: Varistor current 

200 A/div. Sparkover occurs at about 

700 A: 60 Hz power-follow destroys the 

PC board 

Sweep speed: 10 ps/div 

LONG LEADS 

Same as A, but with varistor mounted 

on 8 in. leads from terminal board 

Figure 43. Performance of retrofit package 
- 

1 kVA GENERAL PURPOSE 1201120 TRANSFORMER 

Figure 44. Surge propagation through isolating 

transformer 

1 kVA LOW CAPACITANCE 

"LINE ISOLATOR" TRANSFORMER 

Figure 45. Surge propagation through 

"line isolator" transformer 

Figure 46. Effect of loading on the secondary side 

Figure 45 shows the similar behavior of a 

transformer offered as a line isolator. This transformer is intended to provide low effective capacitance and ground loop isolation between primary 

and secondary windings, but here again, the 

author has observed that users of this device 

expect arrenuation of spikes. The response of this 

isolator, due tc its internal construction, is 

different from that of the simple two-winding 

transformer of Figure 44, but we also note that a 

crest of 8 kV occurs on the secondary side during 

the second half-cycle. Hardly an improvement. 

Figures 44 and 45 were recorded with no load 

on the transformer secondary, which represents 

the extreme case of a low-power electronic control 

in standby mode. Figure 46 shows the primary 

and secondary voltages of the transformer with a 

10 W (1500 n> and a 100 W (150 a) load on 

the secondary side, at the same surge generator 

setting as Figure 44. 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 46, still 

slightly higher than the input. 

With the 100 W load shown in Figure 46, the 

attenuation is now apparent, but is only 2:l. 

Capacitive loads would, of course, produce a 

greater attenuation than resistive loads for the 

inductive series impedance of the transformer, at 

the frequency spectrum of this fast, 2 ~s, wide 

spike. For surges of longer duration, the attenuation would be even smaller. 

These examples show that, unless a welldefined load is connected to the transformer, 

expecting attenuation from the transformer may 

prove to be hazardous to the health of low-power 

electronics connected on the secondary side of a 

transformer. 

By contrast, decoupling is possible with a ferroresonant line conditioner which is primarily 

intended for line voltage regulation but which also 

provides a high degree of surge suppression. Figure 47 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 

Figure 47. Surge decoupling by ferroresonant 

line conditioner 
attenuation to less than 10 V was observed. The 

nature of the ferroresonant line conditioner is 

such that the decoupling improves with loading, 

while the simple transformers of Figures 44, 45, 

and 46 can only act as linear dividers with load 

changes. Conversely, the decoupling between primary and secondary sides of the line conditioner 

is further seen on the oscillogram recorded on the 

input side of the line conditioner. This oscillogram is, in fact, a photograph of two successive 

measurements, one with no load on the line conditioner and one with a 100 W load. The input 

waves are exactly superimposed. 

This decoupling reflects the nonlinear behavior 

of the ferroresonant line conditioner, which is 

significant in this case, compared to the linear 

behavior of transformers: For surge sources of 

lower impedance than the generator used in these 

tests, or for frequencies lower than the frequency 

contained in the 0.5 ps - 100 kHz spike, the 

transformer attenuation would become lower, in 

direct proportion to the corresponding impedance 

change, while the ferroresonant line conditioner 

would keep the decoupling unchanged. See also 

Case History No. 7 for an application of a ferroresonant line conditioner to decouple a surge 

protective device from the power supply, the 

inverse situation of what is described here. 

For worst-case demonstration, the two oscillograms of the output were recorded with the spike 

timed to occur at the peak of the 60 Hz line voltage demonstration. The peak-to-peak amplitude 

of the line voltage is indicated by the gray band 

recorded on the oscillograms by photographically 

superimposing repetitive traces of the line voltage. 

For timings other than at peak, the small voltage 

oscillation on the output voltage would be completely contained within the normal peak-to-peak 

band of the 60 Hz line voltage. 

7.5 Case History No. 5 - Connections options 

for suppressors and effects on residual 

voltages 

The author has witnessed lively controversies 

over the most effective transient suppression 

configuration to be applied. Taking, as an example, the task of specifying the protection of an 

appliance or equipment connected at the end of a 

line with no opportunity to divert the transient 

closer to the source (for instance, at the service 

entrance), the options would be to connect one, 

two, or three varistors between the three wires 

(black, white, and green) at the end of the line. 

However, additional information needs to be 

known: Will the impinging surge be in the normal mode (black-to-white) or in the common 

mode ([black-and-whitel-to-green)? Where in the 

equipment is the most sensitive component: lineto-line (most likely) or line (black or white)-togreen? Clearly, the situation is confusing, and 

there will not be a single, simple answer applicable 

indiscriminately to all cases. The National Electrical specifically allows the connection of 

surge arresters between neutral and grounding conductors (Article 280-22) if the interconnection 

occurs only by operation of the surge arrester during the surge. Since the standby current of varistors is very low, this requirement can be met; 

furthermore, there will not be any interference 

with the operation of Ground Fault Circuit Interrupters if there are only a small number of 

protectors. 

The set of measurements recorded in Figure 48 

shows an example of these many options with 

increasing protection, albeit at increasing cost, 

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- 

[white-and-green], since white and green are tied 

together at the service entrance. The line is a 

75 m line and the surge is that available from the 

generator set for a 2000 A 8/20 ps short-circuit 

impulse. Rather than attempt to modify the setting of the generator for each case in order to 

maintain a constant current crest for the various 

configurations (an impossible task if waveform is 

also to be maintained), the generator was left 

unchanged, to discharge a constant total energy 

into the system - not a bad hypothesis for the 

real world. The current crests are all in the range 

of 300 to 380 A, which is not a significant variation for comparing varistor clamping voltages. 

If only one varistor is allocated to protect the 

equipment, the black-to-white varistor connection 

(first row) affords maximum protection for the 

electronics, which are also likely to be connected 

black-to-white. However, the voltages between 

either black or white and green are large; that voltage is the stress that will be applied to the clearances of the equipment. This situation is a good 

example of the conversion of a normal mode transient into a common mode, as dicussed in Section 6.5. 

The configuration with varistor black-to-green 

(second row) does not afford very good protection 

for components connected black-to-white; therefore, it should be used only if there is a special 
I 3WA I 360 v max I 780 v mar 

I-SQ , + *v v* .A 4 zw-v WB 1 La 

Iv NWWN,tre lob6 KXllv nHY#+Ki lOuS roolV WyWO 10y) 

, 

mv mw~cco IW 

I 320 A 360V max 630 V max I 360 V man I 

V - WHITE.TO.GREEN 

I-XW , , + ?-V V.PC/sU J +Wv-rM# J rlOrWY.16AP ,A 

1v mu lOlJ *v SHW lOuS mv nV)w l0vs mv 7SyI.a l0vs 

300 A 1 360 V ma^ 380 V max 100 V max 

I IV mwmm IW I mv mww iovs 1 ,mv wwm JW j mv mnm IW 

V- BLACK.TO-GREEN 

Figure 48. Connections options and effect on common mode or normal mode overvoltages 

CIRCUIT V- BLACK.TO-WHITE 

need to clamp black-to-green at a low voltage with 

only one varistor available or allowed. 

An improved protection is obtained with a 

varistor connected black-to-white, complemented 

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. 

TOTAL CURRENT 

7.6 Case History No. 6 - Measurement 

problems 

Considerable controversy has been raised on an 

"overshoot" associated with the performance of 

varistors under fast pulses. Actually, this 

overshoot is primarily a measurement problem 

associated with lead effects. To illustrate the 

effect of lead length on the overshoot, two 

measurment arrangements were used. As shown 

in Figures 49(a) and 49(b), respectively, 0.5 cm2 

and 22 cm2 of area were enclosed by the leads of 

the varistor and of the voltage probe. 

The corresponding voltage measurements are 

shown in the oscillograms of Figures 49(c) and 

49(d). With a slow current front of 8 us. there is 

little difference in the voltages occur& with a 

small or large loop area, even with a peak current 

of 2.7 kA. With the steep front of 0.5 ps, the 

peak voltage recorded with the large loop is nearly 

twice the voltage of the small loop. Note in 

Figure 49(d), that at the current peak L dudt = 0 

and the two voltage readings are equal; before the 

peak L di/dt is positive and after, it is negative. 

Other measurement errors can be introduced 

by the connection of the voltage probes, as illustrated by the following experiment. When making 

voltage measurements across a clamping device for 

evaluating its performance, one must recognize 

possible difficulties requiring special precautions. 

Two precautions must be taken: 

1. Use two probes in a dflerential mode to make a 

measurement direct& at device terminals. Commercial oscilloscope preamplifiers offer a wide 
AREA - 22 cm2 

AREA = 0.6 cm2 A7 OUTPUT LEAD FROM 

TRANSIENT GENERATOR 

PATH 

CURRENT VAPISTOR 

PATH 

la) Mln~rnal Loop Area I b) Exceaswe Loop Area 

Icl Current R~se of 8 us (dl Current Rlse of 0 5 us 

Figure 49. Effect of lead length on overshoot 

Figure 50. Circuit configuration 

same manner as the inside pair. A third set of 

wires was soldered at the end points of the hollow 

conductor, and arranged to form a rectangle, the 

hollow conductor being one side of that rectangle. 

Several widths could be set up for the rectangle, 

and each time the measured voltage was recorded. 

Figure 52 shows the measured voltage versus 

radial distance of the opposite side of the rectangle, plotted from the oscillograms of Figure 51. 

current in Conductor. 80A/~s Voltaae bv coaxial orobes 

the current flowing in the device. Voltage by side probes Voltage 1x10 cm loop 

To illustrate the second point, the measurement circuit shown in Figure 50 was set up in the 

output circuit of a generator producing a 8/20 1s 

impulse. The "device" was a hollow conductor, 

with a hole at the center through which a twisted 

pair was fed, one wire of the pair branching out 

to each end of the conductor, separated by 10 cm. 

At the same 10 cm separation, but outside of the 

hollow conductor, two thin wires were attached " and brought .to the midpoint of the hollow conductor, in close contact with the conductor; from Figure 51. Voltages recorded for various 

the midpoint outward, they were twisted in the probe connections 
Figure 52. Voltage versus area 

This experiment shows that not only must one 

connect the probes as close as possible to the terminals of a clamping device but still strive to 

minimize the area established by the probes close 

to the device. 

In the case of a low-voltage suppressor, it 

would be better to solder short leads to the device 

terminals, bring them together while tightly hugging the device, then twist them in a pair and 

connect the oscilloscope probes some distance 

away from the device. 

This experiment also shows the importance of 

wire layout in making the connections of a protective device in an actual circuit. As discussed in 

Case History No. 1, creating a loop near the protective device is an invitation to induce additional 

voltages in the output of the protective device, 

thus losing some of its effectiveness. 

Hence, when one is making measurements as 

well as when one is designing a circuit for a protection scheme, it is essential to be alert to the 

effects of lead length (or more accurately of loop 

area) for connecting the varistors. This warning is 

especially important when the currents are in 

excess of a few amperes with rise times of less 

than 1 ps. 

7.7 Case History No. 7 - The best surge 

suppressor is a surge monitor! 

Increasing recognition of the existence of transients on power lines has encouraged extensive 

use of commercial disturbance analyzers as a first 

step in identifying a potential transient problem. 

Surveys have also been made on the quality of 

the available ac power, based on recordings 

obtained from such analyzers. However, the 

results of such measurements may be ambiguous 

as a result of the design of at least one of these 

instruments, as made by Dranetz Technologies, 

Inc., until recently. 

The problem arises from a characteristic of the 

Dranetz equipment, which exists for both Models 

606 and 626, and which was not recognized at the 

time some measurements were made but is now 

pointed out in more recent Dranetz instruction 

manuals. 

In order to protect the electronics of the Disturbance Analyzer from damage by overvoltages in 

the power supply to these internal electronics, a 

surge suppressor has been provided in the input to 

the power supply - not the monitoring input of 

course. However, if the ac power system being 

monitored is the same as the power system in 

which the instrument power supply cord is plugged 

- a likely possibility in the general case and precisely the situation of some reported measurements - then the observations of surge 

occurrences on that power system are those of a 

system whose transients have been suppressed! 

To support this claim, Figure 53 shows an 

oscillogram recorded at the output of a surge generator which provides both 120 V ac power and 

the IEEE/ANSI C62.41 Ring Wave, Category B. 

The oscillogram shows the surge without the 

Dranetz analyzer plugged into the test system output, and, superimposed, the effect of plugging the 

power cord only of the Dranetz analyzer into the 

test system output. Without the analyzer, the 

open-circuit voltage is 3 kV; with the analyzer 

plugged in, the output voltage is reduced to 

1.1 kV. 

Figure 53. Open-circuit voltage output of KeyTek 

surge generator Model 711 with PI 

plug-in and attenuated voltage with 

Dranetz 606 power cord connected at 

output of generator 

Vertical: 1 kV/div 

Sweep: 2 psldiv 

The ANSI/IEEE C62.41 Category B characteristics are 6 kV open-circuit voltage, 500 A shortcircuit current, therefore a source impedance of 

6000 : 500 = 12 R. From the circuit values of 
Figure 54, the unknown effective impedance of 

the analyzer, Z, can be computed to be only 7 fl. 

That low impedance, when connected in parallel 

with the voltage measurement leads, will load the 

source of the transient and yield lower voltage 

recordings than the actual occurrence would have 

been without the analyzer connected. This situation makes the facetious remark in the 1970 paper 

come true ("the best surge suppressor is a surge 

monitor!") 

While it is too late to correct data already 

recorded, there is a very simple solution to the 

problem. Ferroresonant line conditioners not only 

provide surge isolation at their output but also 

decoupling of the input from the output.(30' Figure 55 shows the open-circuit output of the surge 

generator at 6 kV (upper trace) and the output 

with the line conditioner feeding the analyzer 

plugged in (lower trace). There is no detectable 

effect on the impinging surge. Thus, by merely 

inserting the line conditioner in the power cord of 

the analyzer, the issue disappears, and measurements can be obtained without the ambiguity 

which can cause a sense of false security in the 

relatively low levels of impulse cited in some of 

the published reports. 

7.8 Case History No. 8 - Varistor versus 

environment: Winning the rematch 

During the initial startup of a solid-state motor 

drive in a chemical processing plant, difficulties 

arose with the varistor and its protective fuse at 

the input of the thyristor circuits. Frequent 

blowing of the fuse was observed, with occasional 

failure of the varistor. The plant substation, fed 

at 23 kV from the local utility, included a large 

capacitor bank with one-third of the bank switched 

on and off to provide power factor and system 

voltage regulation. These frequent switching 

operations were suspected of generating highenergy transients that might be the cause of the 

failure of the fuses and varistors, because literally 

thousands of similar drive systems have been 

installed in other locations without this difficulty. 

On-site measurements performed after repeated 

blowing of fuses and occasional failure of varistors 

connected at the input to the thyristor drive indicated that indeed the devices were not matched to 

their environment. From this point on, specifying 

larger sizes, sizes appropriate to the environment, 

solved the problem. Immediate relief was secured 

by the installation of a larger varistor at the same 

point of the circuit; long-term protection was 

obtained by the addition of a gapless metal-oxide 

Reference 19, p. 1055, conclusion of Case History No. 1. 

3kv9 TI DRANETZ 

Figure 54. Computation of effective input 

impedance of the disturbance analyzer 

power supply 

Figure 55. Output voltage of KeyTek 711/P1 

surge generator 

Top trace: No connected load 

Lower trace: 

Line conditioner 

GE Cat 9T91L130G3 

plugged in output 

Vertical: 2 kV/div 

Sweep: 2 ps/div 

varistor arrester on the primary side of the stepdown transformer feeding the drive. The situation 

has been changed from failures occurring every 

few days to no further problems in the 3 years 

since the larger varistor was installed. A complete 

description of this case history is given in Reference 8; a summary is given in this report. 

This case history illustrates how surge protective devices that are successfully applied for the 

majority of cases can occasionally suffer failure 

when exposed to exceptionally severe surge 

environments. It also shows how little attenuation 

occurs, at the frequencies produced by switching 

surges, between the distribution level (23 kV) and 
the utilization level (460 V), even though a long 

line and two step-down transformers exist between 

the source of the transient and the point of measurement. 

A typical total event recorded on one of the 

phases during a capacitor bank closing is shown in 

Figure 56A. A low-frequency oscillation with a 

period of 3 ms (330 Hz) and initial peak-to-peak 

amplitude of 450 V decayed in about 10 ms. The 

high-frequency oscillations are resolved in the 

recording of Figure 56B (recorded during a similar 

switching sequence). This high frequency has an Figure 56. Capacitor switching transient initial peak-to-peak amplitude of 2000 V, decaying Vertical: 500 V/div 

in about 5 ms. The period is 180 ps (5.5 kHz). Sweep: 0.5 ps/div A similar, third event is shown in Figure 56C. 

For scaling the amplitudes, the steady-state voltage 

is shown in ~igure-56~. - 

Figure 57 shows recordings of transient 

currents in all of the three varistors. Figure 57A 

shows a train of current pulses in the range of 10 

to 40 A. In the burst of Figure 57B, the 

recorded current pulses range from 5 A to 200 A. 

Conclusive evidence, therefore, was obtained 

that substantial current pulses were absorbed by 

the varistors during capacitor switching. The magnitude and duration of these pulses were excessive 

for the capability of a 20 mm disc used originally; 

many similar drives installed elsewhere do not Figure 57, Current surge bursts during capacitor experience the failures encountered at that particu- switching lar location. Another significant finding from 

these measurements is the fact that the switching further changes for the long term. Therefore, the transients, generated at the 23 kV level, propagate change to a 32 mm size, connected at the same 

to the point of utilization at the 460 point of the circuit, was immediately implemented level. for that particular environment. 

An obvious remedy would be to use a varistor 

with greater current-handling capability. The 

32mm size offers such a possibility. The 

improvement in the number of pulses is 50 times 

more pulses until pulse rating is reached. The 

improvement in the number of pulses until varistor failure occurs, however, is not necessarily 

50 times more pulses. Because of the imprecision 

in the margin between end of pulse rating and 

ultimate failure, that margin is not necessarily the 

same for the two sizes, 20 mm and 32 mm, but it 

In addition to the proposed upgrading of protection at the 460 V level, three other remedies 

could be considered: installation of surge arresters 

at the 2300 V level, installation of surge arresters 

at the 23 kV level, or a change in the circuits 

involved in the capacitor switching, designed to 

reduce the severity of the transients at their origin. In a second phase of the retrofit described 

here, 2300 V arresters were installed at the 

transformer primary. 

is reasonable to expect the same order of magni- CONCLUSIONS tude improvement in the ultimate failure as in the 

pulse rating. This expectation of a 50 times Power system disturbances can inject damaging 

improvement would change the time between overvoltages into power lines as weH as data lines. 

failures from the few days observed with the Lightning surges can be equally damaging, by 

20 mm size to perhaps 1 year with the 32 mm direct termination of a stroke, by induction, or 

size, providing immediate relief and time to make especially, by differences in ground potential 
caused by the flow of the current into earth. 

Beware of differential ground potential rise! 

Fundamental precautions, best applied in the 

design and construction stages, can provide 

effective protection at a small cost compared to 

the alternative of failures and retrofits. The cost 

of insurance premiums always seems high before 

the accident. 

Shielding, bonding, and grounding are the classical preventive methods at the system and component level. Conflicts between traditional 

grounding practices for noise reduction can be 

reconciled with the requirements of surge protection. Grounding the shields at only one end 

invites trouble. 

A combined approach of fundamental precautions and protective devices can provide effective 

protection over the range of natural and manmade disturbances. However, these devices must 

be applied as part of a concerted effort. The 

coordination of protective devices is the key to 

functional and cost-effective protection. 

9. ACKNOWLEDGMENTS 

Motivation for preparing this report was provided by the reported case histories and the 

penetrating questions raised by students at the 

University of Wisconsin annual conferences on 

surge protection of computers and electronic systems, as well as by discussions with members of 

the IEEE Surge Protective Devices Committee. 

Maurice Tetreault of Digital Equipment Corporation graciously made available the recordings of 

transients on data cable. Virginia Barnum, Catharine Fisher, and Elizabeth Zivanov of Corporate 

Research and Development contributed valuable 

review and comments toward development of the 

final text. 

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R.H. Golde, "The Frequency of Occurrence 

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20. R. Hasler and R. Lagadec, "Digital Measurement of Fast Transients on Power Supply 

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Testing Powered Equipment, Proc. of the 6th 

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