+-------+------------------------------------------------------+-------+
| # Las | # Surge Protection Anthology                         | #     |
| t upd |                                                      |  Surg |
| ated  | **Part 6 \-- Tutorials, Textbooks, and Reviews**     | es Ha |
| April |                                                      | ppen! |
|  2004 | **Text of "Lecture Handouts" files**                 |       |
+-------+------------------------------------------------------+-------+

 

FOREWORD

This file contains the text part of the handouts from four lectures on
the following subjects:

-   Transient Overvoltage Protection Seminar (1979)

-   Transient Overvoltage Protection in the Undefined Real-World
    Environment (1979)

-   Electrical Protection of Cellular Radio Sites (1983)

-   Line Conducted Disturbances -- Origins and Control (1984)

This file is formatted as MS Word, allowing you to do a search for
keywords, but it does not support graphics as it was derived from an OCR
scan of hard-copy archives. However, should you wish to examine the
complete original format, each page in this file has an identifying
header and footer that contain a hyperlink to the pdf file for the
document being displayed on that page, regardless of the font size that
you select for optimum viewing. These headers and footers and hyperlinks
become accessible when you select the "Print Layout" in the "View" mode.

Best wishes and good browsing !

![](media/image1.png){width="1.0972222222222223in"
height="0.5347222222222222in"}

TRANSIENT OVERVOLTAGE PROTECTION SEMINAR

**Francois D. Martzloff**

**Corporate Research and Development**

**General Electric Company**

##### FOREWORD

Designers and users of electronic equipment have been justifiably
concerned with surge protection because field experience is rich in case
histories of failures attributable to transient overvoltages.
Insufficient knowledge of the exact nature of these overvoltages,
however, made their task difficult in the past. Also a wide choice of
protective techniques and devices is available, but direct comparisons
among them are often difficult because techniques vary from one
organization to another, and device specifications published by
suppliers are not always consistent. However, much progress has been
made in recent years to solve this problem. The seminar topics,
summarized in the five following written sections, present a brief
overview of the current knowledge.

The section on origins of surge voltages covers the two physical
phenomena that produce overvoltages: lightning discharges (not
necessarily directly to a power system) and switching surges. The second
considers fundamental protection techniques that are standard preventive
measures: (1) allowing the harmless conduction of surge current to
ground, (2) blocking the path between the sources of transients and
sensitive devices, and (3)protection against direct effects. The third
presents test methods and includes the rationale for testing, safety
considerations, and a demonstration of equipment. The fourth addresses
the wide choice of available surge protective devices, and the final
section gives examples of single device applications as well as
coordinated protection schemes.

# THE ORIGIN OF TRANSIENT OVERVOLTAGES

## INTRODUCTION

Transient overvoltages no longer pose an unknown threat to the
successful application of electronic equipment, for protective
techniques and devices are available. The appropriate selection of
these, however, remains a critical task because the exact nature of
transients in the real world is at best only statistically defined, so
that the choice involves technical and economic decisions based on
calculated risks rather than certainties. Nevertheless, information has
been collected and reviewed by various groups concerned with the
problems of transient protection. This work will result eventually in
guideline descriptions of the transient environment and proposed
standard tests. Such guides and standard tests will simplify the task of
ensuring protection.

Two major causes of transient overvoltates have long been recognized:
system switching transients and transients triggered or excited by
lightning discharges (in contrast to direct lightning discharges to the
power systems, which are generally destructive and for which economical
protection may be difficult to obtain). System switching transients can
involve a substantial part of the power system, as in the case of
power-factor-correction capacitor switching operations, disturbances
that follow the restoration of power after an outage, or load shedding.
However, these disturbances do not generally involve substantial
overvoltages (more than two or three per unit), but they may be very
difficult to suppress because the energies are considerable. Local load
switching, especially if it involves restrikes in the switchgear
devices, will produce higher voltages than the power system switching,
but generally at lower energy levels. Considering the higher impedances
of the local systems, the threat to sensitive electronics is quite real:
the few conspicuous case histories of failures blemish the record of a
large number of successful applications.

We will first review the basics of lightning phenom­ena, then briefly
discuss the mechanisms involved in switching transients, and conclude
with an overview of standards and guides on the characteristics of
representative transients.

### LIGHTNING PHENOMENA

The phenomenon of lightning has been the subject of intensive study by
many workers. The behavior of lightning is now fairly predictable in
general terms, but the exact knowledge of specific incidents is not
predictable. Protection against lightning effects includes two
categories: (1) direct effects concerned with the energy, heating,
flash, and ignition of the lightning current, and (2) indirect effects
concerned with induced overvoltages in nearby electrical and electronic
systems. Although we are examining the direct effects to appreciate the
phenomena involved, we are ultimately concerned in this discussion with
the indirect effects.

Because there is no conclusive evidence that lightning can be prevented,
one must design a facility taking appropriate measures to make lightning
strikes harmless. Designers should be familiar with two lightning
phenomena: the \"cone of protection\" and \"striking distance,\" as well
as with current theories on the formation of lightning. The descriptions
that follow are based on the work of Anderson and Hagenguth (1), Cianos
and Pierce (2), and Golde (3).

#### Generation of the Lightning Flash

The energy that produces lightning is provided by warm air rising into a
developing cloud. Several theories vary in detail, but all are based on
the observed evidence that the cloud, except for the top, is negatively
charged, with a small body of positive charge near the front base of the
cloud. Figure 1 shows a typical cloud distribution of charges. The solid
lines represent the direction of the air movement in the cloud, with the
cloud moving from right to left. As the cloud passes over a point on the
ground, an electrical charge is attracted under the cloud on the ground.

However, the first significant event in the formation of a lightning
flash occurs at the cloud. A column of ionized air, moving slowly by
steps of 30 to 50 meters forms at the cloud. This column is called the
pilot streamer. Second, the discontinuous or haphazard ionization and
filling of the column with charged particles results in a discharge
called a step-leader. The step-leader does not move in a straight line
toward the ground but, rather, seeks out the path with least electrical
resistance, producing the familiar zig-zag pattern of the final stroke.
Branching shows the several paths that may be formed in this process of
searching a weak path. Near the ground the last step is completed either
by continuation of the process or by meeting a leader of positive
charges originating from the ground.

With the path completed, a positive charge flows upward from the ground
into the negative channel left in the wake of the step-leader,
neutralizing the charge in this channel. This flow, carrying a current
at peaks of 1000 to 100,000 A with rise times in the order of one
microsecond, is what constitutes the lightning stroke.

When a very tall object is present, a step-leader can actually originate
from it and travel upward to meet the downward moving leader. Subsequent
charges will move in the ionized channel left by the first discharge.

One of the major factors to consider in determining the probability of
lightning damage, and thus the need for strong protection, is the number
of lightning flashes to earth in a given area for a given time. Such
statistics are not generally available; instead the number of
\"thunderstorm days\" is quoted. However, the term \"thunderstorm days\"
includes cloud-to-cloud discharges and does not include the duration and
intensity of each storm. Thus it does not represent an accurate
parameter. Progress is being made to improve statistics, but new
statistics are not yet available; therefore, the \"isokeraunic level"
map (4), showing number of storm days per year, is still the most widely
used description of the occurrence distribution. Contrary to some
popular beliefs, the density of lightning flashes on the average is
independent of terrain. However, ground objects (trees, buildings, and
hills) will produce a bias in the local distribution of this average.

This distribution at the ground level is determined by the final stages
of the step-leader coming from the cloud and depends to some extent on
what it finds on the ground. Thus, the actual point of termination can
be somewhat controlled, while the probability of a given area\'s
receiving a lightning stroke cannot. This is where the concept of the
striking distance, as explained by Golde (5), becomes useful.

[Striking Distance]{.underline}

As the step-leader approaches the ground in the haphazard path described
above, the point is reached where one more strike in the discontinuous
process will close the path. The distance between the tip of the leader
and the object about to be struck (or about to emit the meeting leader)
is called striking distance. The length of this distance is affected by
the field established by the leader, which in turn is determined by the
amount of charge existing in the ionized channel coming from the cloud.
With a large charge in the channel (eventually producing a high
discharge current), the field is more intense, so that breakdown can
occur for longer distances, while a shorter distance is necessary to
produce breakdown for the weaker fields established by smaller charges.

For instance, an average lightning current of 25 kA would correspond to
a striking distance of 40 m. Thus, for an average 25 kA stroke, details
of the terrain do not affect the point of termination of the stroke
beyond this distance, but within this distance will there be a race, or
a competition, as to which point will receive the flash, or invite it by
sending a meeting streamer. Conversely, very low amplitude flashes have
an even shorter striking distance, which means that they will seem to
ignore \"attractive\" points of termination. Amplitudes, together with
striking distance, explain some of the puzzling exceptions to the
generally assumed effect of tall structures, rods, etc.

[Cone of Protection]{.underline}

From the days of Benjamin Franklin (6), the concept of a cone of
protection has been used to provide an effective protection of objects
within the cone. Briefly, this concept states that objects contained
within a cone of 1:1 or 1:2 ratio of height to radius will not receive
the lightning stroke but that the object at the apex of the cone will.
In the elementary concept, only one object projecting above a ground
plane is being considered; in most practical situations involving
buildings, multiple cones should be considered.

A classical \"cone of protection\" rule for the building shown in Figure
2 would assure that the lightning mast shown provides dependable
protection for the building against an approaching step-leader. However,
if we consider the short striking distance shown by the circles of low
discharge in Figure 2(a), we can see that the advancing leader will have
ignored the lightning mast and traveled the shortest distance to Point
D, to the corner of the building within the \"cone of protection\"
rather than to ground. The path drawn here also exhibits the tell-tale
sharp inflection of the last step mentioned above and often
photographed.

Although both strokes pass through Point a, the step­leader drawn in
Figure 2 (b) for the case of a stroke of higher intensity, and thus a
longer striking distance, will terminate at the lightning mast. This
course represents the classical cone-of-protection situation; it implies
that the step-leader will find within its striking distance an intended
point of termination and strike that point, rather than an object that
chances to be close enough and shaped in a manner to initiate a streamer
to meet the step-leader.

Thus, the designer of a building or outdoor facility can provide
predictable and innocuous lightning termination points rather than
permit the destructive injection of lightning currents into the power or
communication system. (Such measures will be briefly addressed in
another section of the seminar.) Nevertheless, indirect effects of this
flow of lightning current will inescapably occur: induced voltages
caused by the rapid changes of magnetic flux in circuit loops, changes
in ground potential as a function of time and space, and/or excitation
of oscillations in the power or communication systems. From the point of
view of protecting electronic circuits, the indirect lightning effects
are therefore more important because they are less avoid­able than the
direct effects and likely to involve a greater part of a given system,
as compared to the massive but localized damage which may be caused by a
direct lightning strike to a circuit. In later discussions of
transient-producing mechanisms, the indirect effects will generally be
the only effects considered.

### SWITCHING TRANSIENTS

A transient is created whenever a sudden change occurs in a power
circuit, especially during power switch­ing - either closing or opening a
circuit. It is important to recognize the difference between the
intended switch­ing - that is, the mechanical action of the switch - and
the actual happening in the circuit. During the closing sequence of a
switch the contacts may bounce, producing openings of the circuit with
reclosing by restrikes and reopening by clearing at the high-frequency
current zero. Likewise, during an opening sequence of a switch,
restrikes can cause electrical closing(s) of the circuit.

[Simple Switching Transients]{.underline}

Simple switching transients (7) include circuit closing transients,
transients initiated by clearing a short-circuit, and transients
produced when the two circuits on either side of the switch being opened
oscillate at different fre­quencies. In circuits having inductance and
capacitance (all physical circuits have at least some in the form of
stray capacitance and inductance) with little damping, these simple
switching transients are inherently limited to twice the peak amplitude
of the steady-state sinusoidal voltage. Another limit to remember when
analyzing transients associated with current interruption (circuit
opening) is that the circuit inductance tends to maintain the current
constant. At most, then, a surge protective device provided to divert
the current will be exposed to that initial current. Without a surge
protective device the current is available to charge the circuit
capacitances at whatever voltage is required to store the inductive
energy from the current into capacitive energy.

[Abnormal Switching Transients]{.underline}

Several mechanisms are encountered in practical power circuits. These
mechanisms can produce transient overvoltages far in excess of the
theoretical twice-normal limit mentioned above. Two such mechanisms
occur frequently: current chopping and restrikes, the latter being
especially troublesome when capacitor switching is involved.

Current chopping is the name given to the rapid current reduction, prior
to the natural current zero of the power system, which fuses or circuit
breakers can force when clearing a circuit. When there is inductance in
the circuit, this rapid current change can produce high over­voltages -
some 10 times the normal circuit voltage. A classical example is an
unloaded transformer where the magnetizing inductance is high and the
energy stored in the magnetic core can charge only the winding
capacitance.

Capacitor switching can be troublesome if the switch restrikes after
current interruption: the capacitor voltage remains nearly constant at
maximum system voltage, since the interruption occurred at zero current,
which is 90o apart from the voltage zero, while the system voltage
follows the normal sine wave (Figure 3). At 180 degrees after
interruption, the switch has to support twice the system voltage, a
stress it might not be able to support with its contacts incompletely
separated. A restrike can occur under these circumstances. In such a
case the capacitance of the circuit will tend to drive the voltage not
toward the system voltage but beyond it, theoretically up to twice the
difference. Such an overshoot means a possible voltage of three times
the system voltage. While this high frequency oscillation takes place,
the switch may clear at a high-frequency current zero, only to restrike
again later with an even greater difference of voltage and escalating to
an even higher overshoot. The outcome will be either a breakdown in one
of the components or, if the switch eventually recovers enough
dielectric withstand in its opening gap, no further restrikes. But
severe overvoltages will have been impressed on the system.

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.
There is still some uncertainty as to where and when they occur because
the worst offenders result from some abnormal behaviour of a circuit
element. Lightning-induced transients are even less predictable because
there is a wide range of coupling possibilities. Moreover, one user,
assuming that his system will not be the target of a direct hit, may
take a casual view of protection while another, fearing his system will
experience a \"worst case,\" may demand the utmost protection.

In response to these concerns, various committees and working groups
have attempted to describe ranges of transient occurrences or maximum
values occurring in power circuits. Three such attempts will be
discussed in some detail at the end of this section.

### TRANSIENT OVERVOLTAGES IN COMMUNICATION CIRCUITS

Communication circuits share with power circuits their exposure to
lightning effects, but differ from power circuits in that they do not
have the power switching effects just discussed. On the other hand, they
are exposed to another source of overvoltages - the injection of
power-frequency current into the circuits. This injection can be the
result of accidental contact between fallen power wires on overhead
communication wires or cables, or even a malicious injection of
power-frequency voltage into terminals of communication equipment. This
injection can last until a power circuit breaker interrupts the fault it
senses on the power system, or it can be enduring, in which case the
communication circuit protective devices must provide an acceptable
failure mode to maintain the safety and integrity of the communication
plant - with interruption of the service until the fault condition is
removed.

### EXISTING AND PROPOSED STANDARDS ON TRANSIENT OVERVOLTAGES

Several Standards or Guides have been issued or proposed, in Europe by
VDE, IEC, CECC, Pro-Electron, CCITT, in the USA by IEEE, NEMA, UL, REA,
and the Military- specifying a surge withstand capability for spe­cific
equipment or devices and specific conditions of transients in power or
communication systems. Some of these specifications represent early
attempts to recognize and deal with the problem in spite of insufficient
data. As a growing number of organizations address the problem and as
exchanges of information take place, improvements are being made in the
approach. A Working Group of the Surge Protective Device Committee of
IEEE has completed a document describing the environment in low-voltage
ac power circuits (8). The document is now being reviewed by the IEEE
Standards Board for eventual publication as a standard. The Low Voltage
Insulation Coordination Subcom­mittee SC/28A of IEC has also completed a
Report, to be published in 1979, listing the maximum values of transient
overvoltages to be expected in power systems, under controlled
conditions and for specified system characteristics (9). For some time
now, a document prepared by a Relaying Committee of IEEE under the title
\"Surge With­stand Capability\" (10) has been available. These three
documents are now to be reviewed in the pages that follow. Greater
emphasis will be placed on the last because it describes the transient
environment; the other two assume an environment for the purpose of
specifying tests.

[The IEEE Surge Withstand Capability Test]{.underline}

One of the earliest published documents to address 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. This
document, which describes a transient generated by the arcing that takes
place when air-break disconnect switches are opened or closed in the
power system, presents significant innovations in transient protection.
The voltage waveshape specified is an oscillatory waveshape, not the
historical unidirectional waveshape; a source impedance, a
characteristic undefined in many other documents, is defined; and the
concept that all lines to the device under test must be subject to the
test is spelled out

Because this useful document was released at a time when little other
guidance was available, users attempted to apply the document\'s
recommendations to situations where the environment of a high-voltage
substation did not exist. Thus, an important consideration in the
writing and publishing of documents dealing with transients is a clear
definition of the scope and limitations of application.

[The IEC SC/28A Report on Clearances]{.underline}

The Insulation Coordination Committee of IEC, following a comprehensive
study of breakdown characteristics in air gaps, included in its report a
table indicating the voltages that equipment must be capable of
withstanding in various system voltages and installation categories
(Table I).

The table specifies that it is applicable to a "controlled voltage
situation," which phrase implies that some surge-limiting device will
have been provided\--presumably a typical surge arrester with
characteristics matching the system voltage in each case. The waveshape
specified for these voltages is the 1,2/50 μs wave, a specification
consistent with the insulation background of the equipment. No source
impedance is indicated, but four \"installation categories\" are
specified, each with decreasing voltage magnitude as the installation is
further removed from the outdoor environment. Thus, this document
addresses primarily the concerns of insulation coordination, and the
specification it implies for the environment is more the result of
efforts toward coordinating levels than efforts to describe the
environment and the occurrence of transients. The latter approach has
been that of the IEEE Working Group on Surge Voltages in Low-Voltage ac
Power Circuits, which we shall now review in some detail.

[The IEEE Working Group Proposal]{.underline}

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 4). These exposure levels are defined in
general terms as follows:

· Low Exposure - Systems in geographical areas known for low lightning
activity, with little load switching activity.

· High Exposure - Systems in geographical areas known for high lightning
activity, with frequent and severe switching transients.

· Extreme Exposure - Rare but real systems supplied by long overhead
lines and subject to reflections at line ends, where the charac­teristics
of the installation produce high sparkover levels of the clearances.

The two lower lines of Figure 4 have been drawn at the same slope, since
the data base shows reasonable agreement among several sources on that
slope. Both the low-exposure and high-exposure lines are truncated at
about 6 kV because that level is the typical wiring device sparkover.
The extreme-exposure line, by definition, is not limited by this
sparkover. Because it represents an extreme case, the extreme-exposure
line needs to be recognized, but it should not be applied
indiscriminately to all systems. Such application would penalize the
vast majority of installations, where the exposure is lower.

It is essential to recognize that a surge voltage observed in a power
system can be either the driving voltage or the voltage limited by the
sparkover of some clearance in the system. Hence, the term unprotected
circuit must be understood to be a circuit in which no low-voltage
protective device has been installed but in which clearance sparkover
will eventually limit the maximum voltage. The distribution of surge
levels, therefore, is influenced by the surge-producing mechanisms as
well as by the sparkover level of clearances in the system. This
distinction between actual driving voltage and voltage limited by
sparkover is particularly important at the interface between outdoor
equipment and indoor equipment. Outdoor equipment has generally higher
clearances, hence higher sparkover levels: 10 kV may be typical, but 20
kV is possible. In contrast, most indoor wiring devices used in 120-240
V systems have sparkover levels of about 6 kV; this 6 kV level,
therefore, can be selected as a typical cutoff for the occurrence of
surges in indoor power systems.

The voltage and current amplitudes presented in th Guide attempt to
provide for the vast majority of lightning strikes but none should be
considered \"worst case,\" since this concept cannot be determined
realistically. One should think in terms of the statistical distribution
of strikes, accepting a reasonable upper limit for most cases. Where the
consequences of a failure are not catastrophic but merely represent an
annoying economic loss, it is appro­priate to make a tradeoff of the cost
of protection against the likelihood of a failure caused by a high but
rare surge.

Waveshape of The Surges

Many independent observations (11, 12, 13) have established that the
most frequent type of transient overvoltage in ac power systems is a
decaying oscillation, with frequencies between 5 and 500 kHz. This
finding is in contrast to earlier attempts to apply the unidirectional
double exponential voltage wave, generally described as 1.2 x 50 or
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 Working
Group proposal specifies an oscillatory waveshape inside buildings and a
unidirectional waveshape outside buildings, and both at the interface
(Figure 5).

The oscillatory waveshape simulates those transients affecting devices
that are sensitive to dv/dt and to voltage reversals during conduction,
while the unidirectional voltage and current waveshapes, based on
long-established ANSI standards for secondary valve arresters, simulate
the transients where energy content is the significant parameter.

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. In a gap-type 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-follow current. In an energy-absorber protective
device, by its very nature, a substantial share of the surge energy is
dissipated in the suppressor, but its clamping action does not involve
the power-follow energy resulting from the 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 MIL STD-1399 or the IEC SC/28A
Report, either ignore the issue or indicate values applicable to limited
cases, such as the SWC test for high-voltage substation equipment. The
IEEE 587.1 document attempts to relate impedance to categories of
locations but unavoidably remains vague on their definitions (Table 11).

The 6 kV open-circuit voltage derives from two facts: the limiting
action of wiring device sparkover and the unattenuated propagation of
voltages in unloaded systems. The 3 kA discharge current in Category B
derives from experimental results: field experience in surge protective
device performance and simulated lightning tests. The two levels of
discharge currents for the 0.5 μs - 100 kHz wave derive from the
increasing impedance expected in moving from Category B to Category A.

Location Category C is likely to be exposed to substantially higher
voltages than Location Category B because the limiting effect of
sparkover is not available. The extreme exposure rates of Figure 4 could
apply, with voltage in excess of 10 kV and discharge currents of 10 kA,
or more. Installing unprotected load equipment in Location Category C is
not recommended; the installation of secondary arresters, however, can
provide the necessary protection.

Having defined the environment for low-voltage ac power circuits, the
Working Group is now preparing an Application Guide, where a
step-by-step approach, perhaps in the form of a flow chart (Figure 6),
will outline the method for assessing the need for transient protection
and selecting the appropriate device or system. Parallel work in other
IEEE working groups preparing test specification standards (14) for
surge protective devices will be helpful in this selection process.
Other groups in the U.S., as well as the international bodies of IEC and
CCITT, are now working toward further refinements and the reconciliation
of different approaches.

### CONCLUSIONS

The two major causes of transient overvoltages, lightning surges and
switching surges, have been identified with greater precision in
low-voltage ac circuits as well as in communication systems.

While standardizing the definition of the environment will not change
the environment itself, the emergence of realistic standards will enable
designers to increase the reliability of their products. Likewise, users
will be able to protect their equipment more effectively.

## REFERENCES

1\. J.H. Hagenguth and J.G. Anderson, \"Lightning To The Empire State
Building,\" Part Ill. AIEE [Transactions]{.underline}, Vol. 71, PAS,
August 1952.

2\. N. Cianos and E.T. Pierce, [A Ground-Lightning]{.underline}
[Environment for Engineering Usage]{.underline}, Stanford Re­search
Institute, Technical Report, August 1972.

3\. R.H. Golde, \"The Frequency of Occurrence and The Distribution of
Lightning Flashes to Transmission Lines,\" AIEE Transactions, Vol. 64,
1945.

4\. World Distribution of Thunderstorm Days, WMO-OM #21 TP6, World
Meteorological Organization, Genev 1953.

5\. R.H. Golde, \"The Lightning Conductor,\" [Lightning]{.underline},
Vol. II, R.H. Golde, Ed., Academic Press, New York, 1977.

6\. H. Cavendish, W. Watson, B. Franklin, and J. Robertson, \"Report of
The Committee Appointed by The Royal Society to Consider a Method of
Securing the Powder Magazines at Purfleet,\" [Philosophical]{.underline}
[Transactions of The Royal Society]{.underline}, London, Vol. 63, 1773,
and Vol. 68, 1777.

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

8\. [Guide on Surge Voltages in AC Power Circuits Rated Up
to]{.underline} 600 V, Final Draft, May 1979. Document prepared by
Working Group 3.4.4 of the Surge Protective Device Committee of the
Power Engineering Society, Institute of Electrical and Electronics
Engineers.

9\. \"Insulation Coordination Within Low-Voltage Systems Including
Clearances and Creepage Distances for Equipment,\" International
Electrotechnical Commission Report SC28A (Central Office) 5 (to be
published in 1979).

10\. Guide for Surge Withstand Capability (SWC) [Tests]{.underline},
ANSI Standard C37.90a, 1974; IEEE Standard 472-1974.

II\. J.E. Lenz, \"Basic Impulse Insulation Levels of Mercury Lamp
Ballast for Outdoor Applications,\" [Illuminating Engrg.]{.underline},
133-140 (Feb. 1964).

12\. F.D. Martzloff and G.J. Hahn, \"Surge Voltage in Residential and
Industrial Power Circuits,\" [IEEE PAS]{.underline} 89, [6]{.underline},
1049-1056 (July/August 1970).

13\. R. Hasler and R. Lagadec, \"Digital Measurement of Fast Transients
on Power Supply Lines,\" Proc. [3rd]{.underline} [Symposium and
Technical Exhibition on Electro]{.underline} [Magnetic]{.underline}
Compatibility, Rotterdam, Holland, 445-448 (May 1979).

14\. Test for Gas Tube Surge Protective Devices[,]{.underline} IEEE
Standard 465.1-1977.

##### FUNDAMENTAL PROTECTION TECHNIQUES

## INTRODUCTION

Protection of a power system, of a communication system, or of 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
insuring immunity and success, but, rather, there are a number of valid
approaches that can be combined as necessary to achieve the goal. The
competent protection engineer can contribute his knowledge and
perception to the choice of approaches against a threat which is
imprecise and unpredictable, keeping in mind the balance between the
technical goal of maximum protection and the economic goal of realistic
protection at an acceptable cost. However, just as in the case of
accident insurance, the cost of the premium appears high before the
accident, not after.

A discussion of fundamental protection techniques that is limited in
time and scope has the risk of becoming an inventory of a bag of tricks;
yet, there are some fundamental principles and fundamental techniques
that can be useful in obtaining transient immunity, especially at the
design stages of an electronic system or circuit. All too often, the
need for protection becomes apparent at a late stage, when it is much
more difficult to apply fundamental techniques.

Our primary goals in this section, then, will be to alert the system and
circuit engineer to various aspects of protec­tion and to present some
basic concepts of approach.

### BASIC TECHNIQUES

Protection techniques can be classified into several categories
according to the purpose and the system level at which the engineer is
working. For the system as a whole protection is primarily a preventive
effort. One must consider the physical exposure to transients - in
particular, the indirect effects of lightning resulting from building
design, location, physical spread, and coupling to other disturbance
sources - as well as such inherent susceptibility characteristics as
fre­quency response and nominal voltage. A data processing system built
with low-voltage signals, high-impedance circuits, and installed over a
wide geographical area, such as a railroad switching yard or a chemical
plant spread over thousands of meters, would, of course, present serious
problems.

For the system components, the electronic black boxes, the environment
is often beyond the control of the designer or user, and protection
becomes a curative effort - learning to live and survive in an
environment which is imposed. Quite often this effort is motivated by
field failures and retrofit is needed. The techniques involved here tend
to be the application of protective devices to circuits rather than the
elimination of surges at their origin.

Another distinction can be made in classifying protective techniques.
Granted that surges will be unavoidable, one can attempt to block them,
divert them, or strive to withstand them; the latter, however, is
generally difficult to achieve alone.

### SHIELDING, BONDING, AND GROUNDING

Shielding, bonding, and grounding are three interrelated methods for
protecting a circuit from external transients. Shielding consists in
enclosing the circuit wiring in a conductive enclosure, which in theory
cancels out any electro­magnetic field inside the enclosure; actually, it
is more an attenuation than a cancellation. 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,\" through various methods of driving conductors
into the soil. Each of these techniques has its limitations, and each
can sometimes be overemphasized. One of Dr. Golde\'s favorite remarks is
that he could retire on a small percentage of the cost of the useless
copper buried in the ground to provide \"better grounding.\" We shall
now examine each of these techniques, pointing out some of their
limitations and some of the controversies concerning them.

[Shielding]{.underline}

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 and when the goals of
shielding for noise immunity conflict with the goals of shielding for
lightning surge immunity.

A shield can be the size of a matchbox or an airplane fuselage; it can
cover a few inches of wire or kilometers of buried or overhead cables.
\"Grounding\" these diverse shields is not an easy thing to do because
the impedance to earth of the grounding connection must be 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, and the
harsh reality of current flow and Ohm\'s law when lightning strikes.

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 should not flow. Sometimes the shield is
used as a return path for the circuit, in which case shield currents can
cause voltage drops added to the signal. But the fact is that, when
surge currents flow near the circuits, they will unavoidably inject
magnetic flux variations into the circuits, hence induced voltages. By
deliberately allowing part of these surge currents to flow in the
shields, one obtains a cancellation of the magnetic flux.

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.\" However, in
many installations there is a metallic conduit through which the cables
are pulled; with simple 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. That
additional cost, then, is the insurance premium, which is well worth
accepting.

[Bonding]{.underline}

We 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 the box of the circuit or to the
building ground. The principle is simple: the shield can be viewed as an
extension of the box, and thus bonding of the shield to the box should
be continuous over 360 degrees. In practice, unless special connectors
are used, this is difficult to achieve, for often a shielded cable is
terminated at a connection board with the shield peeled back and turned
into a pigtail, which in turn is connected to the \"ground\" terminal of
the connection board. One can imagine the many possible variations of
current flow, with the shield current now flowing in the pigtail and the
creation of the corresponding 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 advertised.

[Grounding]{.underline}

Grounding, which is also referred to as \"earthing,\" has different
meanings as well as different roles. The primary definition is the
connection of the circuit, shield, or reference to [earth]{.underline}.
But what is \"earth\"? System designers, construction crews, inspectors,
and technical conference authors are concerned with establishing,
measuring, and maintaining a low ground [resistance]{.underline}, often
determined by dc measurements on rods driven into the ground. Driving
rods into the ground does not ensure a low impedance under the transient
conditions of high rate of current change associated with lightning
discharges. This remark is not intended as a criticism of the efforts
going into achieving a low resistance but, rather, to alert the system
designer that there is more to it than just low resistance, and that one
can overdo the act of burying copper in the ground.

When one deals with a reasonably compact system, 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 (if any), building
frame, or cabinet enclosure. The resistance (impedance) from that
reference to \"earth\" is not very significant as long as other wires at
\"ground\" potential are not brought to the system. Since there is
little chance of dealing with an absolutely isolated system, the
question is What should be done with incoming wires? These wires can be
isolated from the local ground during normal operation, but one must
recognize that, during transient conditions of lightning surge or power
system faults, high voltages will appear across these isolated wires and
local ground, which in some cases are totally beyond the withstand
capability of insulation. That insulation, then, must be protected by
suitable devices which in fact do ground the wires for the duration of
the transient. This type of grounding is one function of transient
suppressors.

Power System [Grounding]{.underline}

In the context of grounding it is appropriate to mention the questions
raised by proponents of ungrounded and grounded power systems. Indeed, a
discussion of grounding practices would not be complete without
reference to these questions.

It has been a long-established practice to operate some three-phase
power systems without an intentional ground connection of the neutral.
The intent is to increase service continuity, in principle allowing the
system to continue operating with one ground fault, as opposed to
grounded systems, where an outage will be the result of the first ground
fault to occur. Examples of such concerns are found in military power
systems (\"ride through the first shell\") and industrial low-voltage
systems.

Closer analysis of the consequences of this choice indicates, however,
that the overall performance of the ungrounded systems may not
necessarily be improved over the safer, more predictable grounded
system. An excellent discussion of the advantages and disadvantages can
be found in IEEE Standard 142-1972. A detailed description is presented
of multiple faults to ground, arcing faults to ground (which can be
caused by sparkover after a transient overvoltage and result in massive
equipment damage), location of faults, personnel safety, performance
with overvoltages, and system costs.

The effects of these concepts on the roles performed by shielding,
bonding, and grounding can be summarized in a set of relatively simple
design guidelines, which are stated in the Appendix.

[Isolation of Subsystems]{.underline}

In the case of systems involving separate buildings, remote sensors, or
the interconnection of a power system with a communication system, other
requirements may dictate the isolation of the subsystems, creating the
illusion that protection against overvoltages has also been
accomplished. \'And yet we have seen that during transient conditions
high voltages can occur.

Where moderately high voltages only can occur, effective isolation can
be accomplished by the insertion of isolating transformers, or when
metallic isolation is not required, by insertion of a filter.

Where the voltages will reach levels exceeding the withstand capability
of economically or technically feasible insulation, two possible
solutions exist. The first, already mentioned, is to bond the two
systems during the transient by means of a nonlinear surge protective
device, which returns to a high level of insulation after the transient
has subsided. Another method, and one which is becoming increasingly
attractive, is the insertion of a fiber optics link into a control or
data system. Complete decoupling of electrical transients and noise can
be achieved in this manner.

### CONCLUSIONS

A number of fundamental protection techniques are available to limit the
penetration of lightning surges into a system. These are best
implemented at the early stages of design. They include the provision of
points where a lightning stroke can attach and be diverted without harm
to earth, shielding of circuits, bonding of enclosures, and grounding of
the shields and reference points.

It is a dangerous illusion to believe that lightning effects can be
eliminated by the isolation of conductors or subsystems. It is much
safer, and quite acceptable if included in the design, to provide
bonding during transient conditions by suitable protective devices. The
important point to remember is that lightning is a fairly well-defined
phenomenon, with known characteristics and effects in general, but its
probability of occurrence at a particular location is unknown. For the
successful operation of a system, foresight is needed in applying
fundamental protection techniques at the beginning.

## BIBLIOGRAPHY

Boehne, E.W., and Golde, R.H., eds., [Research on the]{.underline}
[Lightning]{.underline} Phenomenon, Journal of the [Franklin
Institute]{.underline}, Vol. 283, No. 6, June 19 7, pp. 451-478.

Fisher, F.A., and Plumer, J.A., [Lightning Protection of]{.underline}
Aircraft, NASA RP 1008, National Aeronautics and Space Administration,
U.S. Gov\'t Printing Office, Wash­ington, DC 20402, 1977.

Golde, R.H., edLightning, 2 vols., Academic Press, London and New York,
1977.

Greenwood, A., Electrical Transients in [Power Systems]{.underline},
Wiley Interscience, New York, London, 1971.

[IEEE Recommended Practice for Grounding of Industrial]{.underline} [and
Commercial Power]{.underline} Systems, IEEE Standard 142-1972, 1972.

Shelkunoff, S.A., \"Electromagnetic Theory of Coaxial Transmission Lines
and Cylindrical Shields,\" [Bell System]{.underline}
[Technical]{.underline} Journal, 13, 1934, pp. 532-579.

[The Protection of Telecommunication]{.underline} Lines and Equip[ment
Against Lightning]{.underline} Discharges, Comite Consultatif
International de Telegraphe et Telephone, International
Telecommunication Union, Geneva

Uman, M.A., Understanding Lighnting , Bek Technical Publications,
Carnegie, PA 15106, 1971.

# APPENDIX

###### BASIC CONSIDERATIONS ON SURGE CONTROL FOR ELECTRONIC SYSTEMS

Frank A. Fisher, General Electric Company,

**PROTECTION FROM THE DIRECT EFFECTS OF LIGHTNING**

1\. Try to divert the stroke away from the system, the farther the
better.

2\. If the stroke cannot be diverted away completely, it must be carried
to ground along a path where it does the least damage. This basically
means providing a system of lightning rods and lightning conductors to a
low-resistance ground.

3\. Reduce the resistance along the current-carrying path as much as
feasible. This is particularly true of the ground resistance where the
lightning conductor is grounded.

4\. In buildings housing electronic equipment, establish a uniform
potential ground plane over as much of the building as possible. In new
buildings all reinforcing steel in concrete and structural steel members
should be bonded together and connected to ground rods. These should be
located around the periphery of the building. Bonding should be done at
many points. All water pipes and utility conduits should be bonded to
this ground system where they enter and at frequent intervals within the
building.

5\. Insofar as feasible, the building should be arranged to form a
grounded metal enclosure (Faraday cage). Exter­nal magnetic and electric
fields will not penetrate into a perfect Faraday cage.

6\. Power systems should be protected with commercially available surge
arresters.

7\. Avoid the use of ground systems for electronic equipment that are
isolated from building or power system grounds. This means using a
multiple ground system rather than a single-\'Point ground system.

8\. Connect the cases of all electronic equipment to the nearest
building ground point. This insures that as a minimum the cases will not
assume a high potential, under lightning flash conditions, relative to
the surrounding structure and so will not present an electrical hazard
to people operating the equipment. Ground leads should be as short and
direct as possible, possessing a minimum of resistance and (especially)
inductance.

9\. All wiring between different locations in the \[system\] should be
carried in shielded cables, with shields grounded at both ends.

10\. Electronic equipment should be designed to withstand surge
voltages. Surges can be carried into such equipment on input and output
leads and on power supply leads, and consideration should be given to
the use of protective devices on these circuits.

### PROTECTION COMPATIBLE WITH NOISE ATTENUATION DESIGN

An approach which has been used successfully to provide lightning
protection of communication systems without compromising the systems\'
steady-state noise performance is shown on \[the figure that follows\] .
The important elements of the system are as follows:

1\. Electrical cables carrying signals between sensitive or critical
apparatus in separated locations must have an overall shield. This
shield must be continuous and must be grounded at each end (A and B) to
the building ground systems.

2\. The individual cables within this bundle should be shielded, but
this is not an absolute requirement.

3\. The shields on these individual cables should be grounded at each
end (C and D), but may, at the circuit designer\'s discretion, be
grounded at only one end (E and F) if such a practice is preferable for
the control of steady-state noise. As long as the designer leaves the
overall shield alone, he can do whatever he wishes with any internal
shields.

4\. The electronic equipment to which the cables connect should have the
housings (G and H) connected to the building ground system (1).

5\. A ground bus within the individual pieces of electronic equipment,
but isolated from the equipment case, is often desirable. If such ground
busses are provided (J and K), it is preferable from the viewpoint of
lightning protection that they be connected to the building ground
system at the point of entry (L) of the building ground system.

6\. If direct connection of the ground systems cannot be tolerated, a
separate electronic ground (M) can be provided. A spark gap or other
voltage-limiting device (N) should then be provided to limit the surge
voltages that can be developed between, the two ground systems.

7\. If the electronic ground system is very extensive, it may be
desirable to provide other voltage-limiting elements (0) between the
electronic ground system and the building ground system.

8\. Electronic cables are generally carried between locations in cable
trays. These trays, though not shown on \[the\] figure, should be
connected to the building ground system at each end. The cable trays
should be electrically continuous over their entire length. Cable trays,
if used, do not eliminate the need for an overall shield on the cables
within the tray.

9\. If cables are carried between two locations in electrically
continuous metallic conduits, such conduits being con­nected to the
building ground system at each end, the overall shield on the cables may
be eliminated, since the conduits take the place of the shield. Cables
carried in nonmentallic conduits, however, must have the overall shield.

**SURGE TESTING**

## INTRODUCTION

Surge testing is very important in the successful application of surge
protection devices for two reasons: (1) subjecting equipment to expected
environmental surges makes possible an assessment of protection needs
and (2) subjecting protective components to expected environmental
surges makes possible an evaluation of component performance. Another
aspect of surge testing also needs recognition, although it may be
somewhat painful. A large portion of the surge protection engineer\'s
work involves postmortems, because even a conservative engineer will
often push to the limit the optimization of low cost with performance
and because a field failure is one way to calibrate the design limits.
Confronted with the remains of a field failure (often obscured by the
resulting effects of a transient-induced power-follow), the engineer
must duplicate in the laboratory the effects of a field overvoltage
before he can evaluate the effectiveness of a proposed retrofit or
redesign.

Thus, for surge testing the engineer needs transient=­generating
equipment as well as transient-measuring equipment. The literature is
fairly abundant on high-voltage techniques but scant on the transients
and the circuits involving low-voltage electronics.

### TRANSIENT MEASUREMENTS

Transient measurements in the laboratory are easier to obtain than are
transient measurements for monitoring purposes, but precautions are
nevertheless necessary - technical precautions to ensure the accuracy of
the experiment and safety precautions to ensure the survival of the
experimenter.

The fundamental tool for measurements is still the cathode\--ray
oscilloscope, although digital instruments are being improved in
performance and convenience. Great advances over the cumbersome taking
of photographs with a non-persistent oscilloscope display have been made
in fast-storage oscilloscopes that speed the examination of test data.
Major oscilloscope manufacturers offer a wide choice of instruments, and
their catalogues and application information are valuable for selecting
the correct instrument. Where fast transients are involved, it is useful
to remember that minor secondary oscillations, which may not be of any
consequence to the circuit performance, tend to produce a very fast
sweep of the electron spot, often making the trace invisible except at
the peaks of the secondary oscillation and producing a dotted line trace
rather than the clean record expected in principle. It is the clean
oscillogram record that is valuable in under­standing phenomena and that
makes the oscilloscope the best friend (if trustworthy) of the engineer.

This trustworthiness of the oscilloscope can and should be tested by the
engineer before he draws hasty conclusions from transient measurements.
Here again the literature will provide guidance; in addition there are
some simple checks: freedom from background noise and radiation into the
oscilloscope preamplifiers, alertness to the pitfalls of the built-in
delay line, proper compensation of attenuator probes, and avoidance of
ground loops.

When repetitive transient tests are involved, digital instruments are
valuable and require less operator training. For large evaluation
programs, incoming or outgoing quality control, automated equipment is
now available, either as separate monitors or in combination with the
surge generators.

## TRANSIENT (SURGE) GENERATORS

The basic surge generator is generally a charged capacitor, discharged
through a wave-shaping circuit into the test piece. A variant can be a
pulse-forming network providing a quasi-constant pulse form, generally
of the flat-top variety. In some cases involving a current source,
special circuits have been developed to provide constant-current,
switchable power supplies.

Since most surge tests are based on some agreed upon standard, there are
generally only a limited number of test waveshapes to be produced for a
given application. Ideally, the surge generator should be so powerful
that undesirable loading by the test piece could not occur. In some
cases, however, loading of the circuit by the interaction of the
generator source impedance and the test piece impedance is precisely the
point of the experiment. Practical limitations also exist on the cost of
a large-capacity machine, the difficulty in maintaining fast fronts for
high-power impulses, and the safety hazards of large energy storage.

Some test standards include in their specifications the test circuit to
be used; in that case the engineer has little choice except to build the
circuit or to find a commercial equipment supplier of the gear. Other
standards specify only the electrical characteristics of the impulse,
leaving the exact test circuit to be determined by the laboratory
engineer.

Traditionally, manufacturers of utility equipment have maintained
competent staffs of laboratory engineers and technicians to perform the
required tests. Such finely honed competence is not always found in the
laboratories of electronics manufacturers, in part because the need has
not been as obvious as it is in the high-voltage equipment. While it may
be highly educational for the electronic engineer to build his own surge
generator, the process can be frustrating and dangerous if safety
precautions are not rigidly observed because many capacitors are not
simply capacitors but have internal inductance, and inductors have stray
capacitance. Instead of obtaining test results on his equipment, the
busy engineer observes overshooting, ringing, and exploding carbon
resistors. Thus, for purely economic reasons, the typical electronics
engineer would be better advised to purchase a commercial surge
generator, even if it is custom made. These are the reasons that an
equipment demonstration has been included in the program.

### BIBLIOGRAPHY

Hawley, W.G., [Impulse Voltage Testing]{.underline}, Chapman and Hall,
London, 1959.

[High Voltage Impulse Testers]{.underline}, Haefely, Basel, Switzerland,
Ref . E II 1.

[IEEE Sandard Techniques]{.underline} for [High Voltage
Testing]{.underline}, IEEE Standard 4-1978.

\"Instrumenting Surge Tests,\" KeyTek Application Note #103, KeyTek
Instrument Corporation, Burlington, MA 01803.

Lutz, M., [Surge Test in the Voltage Range of 0.5 - 35kV]{.underline},
Haefely, Basel, Switzerland, Ref. E 1-45.

Martzloff, F.D., [Transient Control Level Test
Genera]{.underline}­[tors]{.underline}, Report 77CRD241, Corporate
Research and Development, General Electric Company, Schenectady, NY,
1977.

Marx, E., \"Versuche uber die Prufung von Isoltaren mit
Spannungsstossen,\" Electro. Tech. Zeit., Vol. 45, 1924.

Modrusan, M., Long Duration Impulse Current Generator for Arrester Test
to IEC Recommendations, Haefely, Basel, - E I- -

Modrusan, M., Normgerechte Pruefung von Ableitern mit
[Exponentiellen]{.underline} Sirss roe-en, Haefely, Basel, Switzerland,
Ref. D 1-36.

Phillpott, J., \"Simulation of Lightning Currents in Relation to
Measured Parameters of Natural Lightning,\" Proceedinge on Lightning and
Static tor 2. 14-1 April I Aspects an Test Criteria Society of London,
December 1975, pp.

Richman, P., \"Diagnostic Surge Testing, Part II,\" Solid [State
Power]{.underline} Conversion, to be published Nov.\[De-c. 1979.

Richman, P., \"Instrumenting Surge Tests for Circuits and Systems,\"
Interference [Technology Engineers]{.underline} Master, R & B
Enterprises, Plymouth Meeting, PA 19462, 1979.

[Test Specifications]{.underline} for Gas Tube Surge Protective Devices,
IEEE Standard 46 1-1977.

Timperley, J.E., \"Construction and Application of a SWC Generator,\"
IEEE Power Engineering Society Winter Meeting, New York, NY, Conference
Paper C72-040-9, January 1972.

**TRANSIENT SUPPRESSORS**

## INTRODUCTION

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.

There are two categories of transient suppressors: those that block
transients, preventing their propagation toward sensitive circuits, and
those that divert transients, limiting residual voltages. Since some of
the transients originate from a current source, the blocking of a
transient may not always be possible; the diverting of the transient is
more likely to find general application. As we shall see in the section
on coordination, 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 - 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. Thus, we are
primarily interested in the diverting devices. These diverting devices
can be of two kinds: voltage-clamping devices and short-circuiting
devices (crowbar). Both involve some nonlinearity, either frequency
nonlinearity (as in filters) or, more usually, voltage nonlinearity.
This voltage nonlinearity is the result of two different mechanisms - a
con­tinuous change in the device conductivity as current increases or an
abrupt switching as voltage increases.

Because technical and trade literature contains many articles on these
devices, we shall limit the discussion of the details and refer the
reader to the bibliography at the end of this section. We shall,
however, make some comparisons to point out the significant differences
in performance.

### CROWBAR DEVICES

The principle of crowbar devices is quite simple: upon occurrence of an
overvoltage, the device changes from a high-impedance state to a
low-impedance state, offering a low-impedance path to divert the surge
to ground. This switching can be inherent to the device, as in the case
of spark gaps involving the breakdown of a gas or in a new breed of
solid state devices involving a switching action. Some applications have
also been made of triggered devices, such as triggered vacuum gaps in
high voltage technology or thyristors in low-voltage circuits where
control circuits sense the rising voltage and turn on the power-rated
devices to divert the surge.

The major advantage of the crowbar device is that its low impedance
allows the flow of substantial surge currents without the development of
high energy within the device itself; the energy has to be spent
elsewhere in the circuit. This \"reflection\" of the impinging surge can
also be a disadvantage in some circuits when the transient disturbance
associated with the gap firing is being considered. Where there is no
problem of power-follow (discussed below), such as in some communication
circuits, the spark gap has the advantage of very simple construction
with potentially low costs.

The crowbar device, however, has two major limitations. One is the
volt-time sensitivity of the breakdown process. As the voltage increases
across a spark gap, significant conduction of current-and hence the
voltage limitation of a surge - cannot take place until the transition
to the arc mode of conduction by avalanche breakdown of the gas between
the electrodes occurs. 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, since the process is statistical in nature. This
sparkover voltage, in addition, 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 because the sparkover of the gap is then substantially higher.

Another 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 or surge diverter. The principles
and applications of these devices will be examined in some detail in
later paragraphs.

### VOLTAGE-CLAMPING DEVICES

Voltage-clamping devices have variable impedance, depending on the
current flowing through the device or the voltage across its terminal.
These components show a nonlinear characteristic - that is, Ohm\'s law
can be applied, but the equation has a variable R. Impedance variation
is monotonic and does not contain discontinuities, in contrast to the
crowbar device, which shows a turn-on action. As far as their
volt-ampere characteristics are concerned, these com­ponents are
time-dependent to a certain degree. However, unlike the sparkover of a
gap or the triggering of a thyristor, time delay is not involved.

When a voltage-clamping device is installed, the circuit remains
unaffected by the device before and after the transient for any
steady-state voltage below damping level. Increased current drawn
through the device as the voltage attempts to rise results in
voltage-damping action. Nonlinear impedance is the result if this
current rise is faster than the voltage increase. The increased voltage
drop (IR) in the source impedance due to higher current results in the
apparent clamping of the voltage. It should be emphasized that the
device depends on the source impedance to produce the clamping. A
voltage divider action is at work where one sees the ratio of the
divider as not constant but changing. The ratio is low, however, if the
source impedance is very low. The suppressor cannot work at all with a
limit-zero source impedance (Figure 1). In contrast, a crowbar-type
device effectively short circuits the transient to ground, but, once
established, this short circuit will continue until the current (the
surge current as well as any power-follow current supplied by the power
system) is brought to a low level.

The principle of voltage clamping can be achieved with any device
exhibiting this nonlinear impedance. Two categories of devices, having
the same effect but operating on very different physical processes, have
found acceptance in the industry: the polycrystalline varistors and the
single-junction avalanche diodes. Another technology, the selenium
rectifier, has been practically eliminated from the field because of the
improved characteristics of modern varistors.

[Avalanche Diodes]{.underline}

Avalanche diodes, the Zener diodes, were initially applied as voltage
clamps, a natural outgrowth of their application as voltage regulators.
Improved construction, specifically aimed at surge absorption, has made
these diodes very effective suppressors. Large-diameter junctions and
low thermal impedance connections are used to deal with the inherent
problem of dissipating the heat of the surge in a very thin single-layer
junction.

The advantage of the avalanche diode, generally a PN silicon junction,
is the possibility of achieving low clamping voltage and a nearly flat
volt-ampere characteristic over its useful power range. Therefore, these
diodes are widely used in low-voltage electronic circuits for the
protection of 5 or 15 V logic circuits, for instance. For higher
voltages, the heat generation problem associated with single junctions
can be overcome by stacking a number of lower voltage junctions,
admittedly at some extra cost.

[Varistors]{.underline}

The term varistor is derived from its function as a [variable
resistor]{.underline}. It is also called a voltage-dependent resistor,
but that description tends to imply that the voltage is the independent
parameter in surge protection, a concept which we will contest
repeatedly in this Seminar. Two very different devices have been
successfully developed as varistors: silicon carbide disks have been
used for years in the surge arrester industry, and, more recently, metal
oxide varistor technology has come of age.

In silicon carbide varistors as well as in metal oxide varistors, the
relationship between the current flowing in the device and the voltage
appearing across its terminals can be represented approximately by a
power function I - kVa, where the higher the value of a, the more
effective the clamping. Hence there has been a race between
manufacturers and specification writers for higher and higher values of
a. We will see, however, that there are practical limits to this race
and that, in fact, better performance can be obtained at high current
densities by departing somewhat from the large values of the exponent a.

In silicon carbide varistors, the physical process of nonlinear
conduction is not completely understood, and the manufacturing of the
material has remained an art, successful as it is. It seems that the
process takes place at the tips of the grains of silicon carbide, held
together by a binder. The story goes that the device action was found
accidentally by having a grinding wheel on a disorderly work bench
connected to an experimental circuit; for many years silicon carbide
varistors indeed looked like grinding wheels, each complete with a hole
in the center.

Metal oxide varistors depend on the conduction process occurring at the
boundaries between the large grains of oxide (typically zinc oxide)
grown in a carefully controlled sintering process. Detailed descriptions
of the process can be found in many publications. For more effective
application of these devices, it is worthwhile for this purpose to
consider their electrical behavior rather than details of the
construction and solid-state physics involved.

### ELECTRICAL CHARACTERISTICS OF VARISTORS

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 of the equivalent circuit of Figure 2. The
major element is the varistor proper, R , whose I-V characteristic is
assumed to be the perfect power law I = k V^a^ . In parallel with this
varistor, there is a capacitor, C, and a leakage resistance, RD. In
series with this three-component group, there is the bulk resistance of
the zinc oxide grains, R., and the inductance of the leads, L.

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-I characteristic; the lead inductance
can give rise to spurious overshoot problems if it is not dealt with
properly; the capacitance can offer either a welcome additional path
with fast transients or an objection­able loading at high frequency,
depending on the application.

[V-I Characteristic]{.underline}

When this V-I characteristic is plotted on a log-log graph, the curve of
Figure 3 is obtained, with three regions as shown, resulting from the
dominance of R~p~, R~v~, R~s~ as the current in the device goes from
nanoamperes to kiloamperes.

This V-I 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, which are discussed in
detail in the information available from manufacturers, must also be
taken into con­sideration. Some of these factors are the selection of the
appropriate nominal voltage for the line voltage of the application, the
selection of energy-handling capability (including source impedance of
the transient, waveshape, and number of occurrences), heat dissipation,
and finally, proper installation in the circuit. In fact, enough
instances of poor installation practices have been observed, and enough
ques­tions have been raised on alleged \"overshoot\" that a brief
discussion of lead effects is in order.

[Overshoot: A Lead Effect]{.underline}

To illustrate the effect of lead length on the overshoot, two
measurement arrangements were used. As shown in Figures 4(a) and 4 (b),
respectively, 0.5 cm**^2^** and 22 cm**^2^** 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 4 (c) and 4 (d). With a slow current front of 8 μs, there is
little difference in the voltages occurring with a small or large loop
area, even with a peak current of 2.7 kA. With the steep front of 0.5
μs, the peak voltage recorded with the large loop is nearly twice the
voltage of the small loop. (Note on Figure 4 (d) that at the current
peak, L di/dt = 0, and the two voltage readings are equal; before the
peak, L di/dt is positive, and after, it is negative.)

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 μs.

[Failure Modes]{.underline}

An electrical component is subject to failure either because its
capability was exceeded by the applied stress or because some latent
defect in the component went by 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 to exposed to an overvoltage, so that
failure of the protective device must be in the fail-short mode, even if
it puts the system out of operation. To other users, 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, more energy is deposited in the device, so that the
energy-handling capability of a candidate protective device is an
important parameter to consider in the designing of a protection scheme.
With nonlinear devices, an error made in the assumed value of the
current surge produces little error on the voltage developed across the
protective device and thus applied to the protected circuit, but the
error is directly reflected. in the amount of energy which the
protective device has to absorb. At worst, when surge currents in excess
of the protective device capability are imposed by the environment,
either because of an error made in the assumption, or because nature
tends to support Murphy\'s law, or because of human error in the use of
the device, the circuit in need of protection can generally be protected
at the price of failure in the short-circuit mode of the protective
device. However, if substantial power-frequency 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 faded device
is not cleared by a series overcurrent protective device (fuse or
breaker).

### COMPARISONS OF PROTECTIVE DEVICES

[Linear Versus Nonlinear Devices]{.underline}

When a protection scheme is designed for an electronic system operating
in the environment which is not completely defined (as discussed in the
first part of this Seminar), it is often necessary to make an assumption
about the parameters of the transients expected to occur. In particular,
if an error is made in assuming the source impedance of the transient,
the consequences for a linear protective device and for a nonlinear
protective device are dramatically different, as demonstrated by the
following comparison.

> *\[See pdf file for equations\]*

Spark Gap Versus Varistor

The choice between these two devices will be influenced by the inherent
characteristics of the application. Where power-follow is a problem,
there is little opportunity to apply a simple gap. Where very steep
front transients occur, the gap alone may let an excessive voltage go by
the \"protected\" circuit until the voltage is limited by sparkover.
Where the capacitance of a varistor is objectionable, the low inherent
capacitance of a gap seems attractive. If very high energy levels can be
deposited in a varistor, compared to the lower levels inherent with the
crowbar action of a gap, a high­capacity surge arrester may be required,
with a combination of a lower clamping voltage varistor farther into the
circuit (see Applications of Transient Protection).

Avalanche Diode Versus Varistor

The basic performance characteristics of these two devices are similar,
and therefore the choice may be dictated by clamping voltage
requirements (the avalanche diode is available at lower damping
voltages), by energy-handling capabilities (the avalanche diode is
generally lower in capability per unit of cost), and by packaging
requirements (the varistor material is more flexible and does not
require hermetic packaging).

[Conventional Surge Arresters Versus Gapless Arresters]{.underline}

Surge arresters (diverters) have reached a high degree of sophistication
over the years by using precision gaps in series with silicon carbide
varistors. For high voltage applications the arrester is made of a stack
of modules, generally of 3 or 6 kV each. With the use of
current-limiting gaps, the clearing of power-follow current in dc
application was made possible. A series gap was required in all these
arresters using silicon carbide because, for a set discharge voltage and
discharge current, the standby current at the power frequency would
otherwise be excessive.

With the advent of metal oxide varistors, the high exponent of the V-I
characteristic reduced the standby current to a very low level, one that
can be tolerated by the varistor under steady-state condition. The
series gap can thus be eliminated, producing three considerable
improvements in performance (elimination of abrupt sparkover); in
reliability (elimination not only of the gap and all trigger circuitry
but also of the parallel voltage-grading varistors); and in
contam­ination withstand (elimination of effects of leakage current on
the outer shell).

In low-voltage circuits, where secondary arresters using gaps were
heretofore the only devices capable of meeting the ANSI requirement of a
10 kA, 8/20 discharge current, high-energy varistors are now capable of
meeting this requirement, without the problems associated with series
gaps.

### CONCLUSIONS

Surge protective devices are available for protecting low-voltage
electronics. Two basic types offer different characteristics: crowbar
devices have high-current capability but generally involve power-follow
when applied on a power system; voltage clamping devices, either silicon
avalanche or varistors, are free from the power-follow problem.

Avalanche diodes offer low clamping voltage, which makes them most
suitable for low-voltage, low-power electronics. Metal oxide varistors
are now available in a wide range of clamping voltages and
energy-handling capacities. Each of these devices has its own best field
of application, insuring greater reliability of the circuits in the
not-quite defined electromagnetic environment of power and communication
systems.

**BIBLIOGRAPHY**

Barksdale, C.B., \"New Gas Tube Protector Technique for Circuits,\"
Telephony, Vol. 171, 1970, pp. 21-28.

Bazarian, A., \"When Should You Use Gas-Discharge Protectors?,\"
[Electronic Design News]{.underline}, Dec. 20, 1973, pp. 58-52.

Clark, O.M., and Pizzicaroli, J.J., \"Effect of Lead Wire Lengths on
Protector Clamping Voltages,\" [Federal Aviation]{.underline}
[Administration-Florida Institute of Technology]{.underline} [Workshop
on Grounding and Lightning Technology]{.underline}, FAARD-79-6, March
6-8, 1979, Melbourne, Florida, U.S. Dept. of Transportation, Washington,
DC 20590.

Gauthier, N., \"Technologie et Applications des Varistances,\" Toute
[L\'Electronique]{.underline}, January 1978, pp. 39-42.

Harnden, J.D., Martzloff, F.D., Morris, W.G., and Golden, F.B., \"The
GE-MOV Varistor - The Super Alpha Varistor,\" [Electronics,]{.underline}
45, No. 21, 1972, p. 91.

Kotheimer, W.C., \"Control Circuit Transients,\" [Power]{.underline}
Engineering, Vol. 73, 1969.

Levinson, L.M., and Philipp, H.R., Low Temperature AC Properties of
Metal-Oxide Varistors, 78CRD102, General Electric Company, Schenectady,
NY 1978.

Mahan, G.D., Levinson, L.M., and Philipp, H.R., Theory of Conduction in
ZnO Varistors, 78CRD205, General Electric Company, Schenectady, NY,
1978.

Matsuoka, M., Masa Yama, T., and Lida, Y., \"Nonlinear Electrical
Properties of Zinc Oxide Ceramics,\" Proc. of First Conf. Solid State
Devices, Tokyo, 1969, [J. Japan Soc. Appl. Phys.,]{.underline} 39 1970,
Suppl. p. 94.

Matsuoka, M., \"Non-Ohmic Properties of Zinc Oxide Ceramics,\" [Jap. J.
Appl. Phys. 10, 1971, p. 736.]{.underline}

Morris, W.G., and Martzloff, F.D., VARPAR - A Program for Calculating
Parallel Varistor Characteristics, 74CRD278, General Electric Company,
Schenectady, NY, 1974.

Philipp, H.R., and Levinson, L.M., [High Temperature Electrical Studies
on Metal Oxide Varistors]{.underline}, 78CRD131, 1978. General Electric
Company, Schenectady, NY, 1978.

Philipp, H.R., and Levinson, L.M., [Long]{.underline} Time Polarization
Currents in Metal Oxide Varistors General Electric Company Schenectady,
NY, 76CRD026, 1976.

Richman, P., \"Diagnostic Surge Testing, Part I,\" [Solid State Power
Conversion]{.underline}, Sept./Oct. 1979.

Sakshaug, E.C., Kresge, J.S., and Miske, S.A., \"A New Concept in
Station Arrester Design,\" [IEEE PAS-96]{.underline}, No. 2, March-April
1977, pp. 647-656.

Smith, M.N., Practical Application and Effectiveness of Commercially
Available Pulse Voltage Transient Suppressors, Naval Civil Engineering
Laboratory, Port Hueneme, CA., 1973.

\"Surge Voltage Protectors - Application Note and Suggested
Applications,\" SERT, [The J. of the Soc. of Electronic and Radio
Technicians]{.underline}" Vol. 9, No. 4, April 1975, pp. 85-91.

Transient Voltage Suppression Manual, Second Edition, General Electric
Company, Auburn, NY 1978.

Vodicka, V.W., \"Voltage Transients Tamed by Spark-Gap Arresters,\"
[Electronics]{.underline}, Vol. 39, 1966, pp. 109-113.

**APPLICATIONS OF TRANSIENT PROTECTION**

**INTRODUCTION**

Applications of surge protective devices for transient protection
involve two situations: the retrofit of protection into an appliance or
system found vulnerable in the field (too often the common situation)
and the initial design of protection in a new product (hopefully the
result of seminars such as this). Before looking into some specific
examples of applications, it is worthwhile for us to examine a concept
which could solve the problem of transient protection: the Transient
Control Level system proposed to the electrical and electronics
community in the U.S.A. (1) and in Europe (2). We shall then examine
three fields where transient protection schemes have been applied with
different devices and different degrees of success, and conclude with an
appeal for the coordination of protective devices on a system-wide
basis.

### THE TRANSIENT LEVEL CONTROL CONCEPT

Until recently there did not appear to be a clear approach for achieving
compatibility between the transient withstand capability of devices and
the transients to which such devices are exposed. This situation was
somewhat like that which prevailed many years ago in the electric power
industry. Transients produced by lightning frequently caused failure of
such vital and expensive power equipment as transformers and generators.
Those transient problems were solved by engineering design guided by the
concept of insulation coordination and the establishment of a series of
Basic Insulation Levels (BILs). At present the Transient Control Level
concept of testing and coordination promises a solution to the problem
of compatibility.

According to the Transient Control Level concept, instead of
retrofitting protective devices into a finished product or design, the
first step is to determine at what level the transients occurring in the
system can be limited by suitable protective devices. It is then
sufficient to build the equipment to withstand only that level
established by the protective device. The result is a well-defined
situation, with adequate margins between the maximum level of transients
and the demonstrated withstand capability of the equipment. The specific
proposals made in that concept are the following.

> 1\. That there be defined for electronic equipment (and other
> low-voltage equipment) a standard transient voltage similar in concept
> to, but different in wave-shape from, the 1.2/50 μs wave used in the
> coordination of insulation in high-voltage power apparatus.
>
> 2\. That there be defined for electronic equipment (and other
> low-voltage equipment) a series of TCLs similar in concept to the
> BILs.
>
> 3\. That a start be made on assigning one of these standard levels to
> individual electronic components and electronic devices.
>
> 4\. That individual protective devices be rated in terms of their
> ability to control transients to levels no greater than, and
> preferably lower than, one of the above levels.
>
> 5\. That equipment and procedures be developed by which equipment may
> be tested by vendors to determine which TCL is appropriate to assign
> to individual components and equipment.
>
> 6\. That TCLs begin to be used in purchase specifications.
>
> 7\. That such equipment and procedures be used by purchasers to
> evaluate vendor-supplied equipment to determine its compliance with
> such purchase specifications.
>
> 8\. That such TCLs begin to appear in regulatory specifications for
> consumer apparatus in which the consumers cannot make the appropriate
> tests or prepare appropriate specifications.

The engineering community in the U.S.A. responded favorably to the
proposal, and some of the concepts found their way into the IEEE Working
Group Guide discussed in the first section of this Seminar (3). With the
writing of Application Guides by the IEEE as well as the IEC, there is
an opportunity to advance the proposal further.

### EXAMPLES OF APPLICATIONS

[Communications]{.underline}

The communications systems, primarily the telephone installations, were
successful in protecting the subscriber as well as the central station
equipment against overvoltages as long as mechanical switching equipment
was used. However, the increasing use of semiconductors creates a
potential problem which is recognized by most equipment manufacturers
and telephone operators but which has not been solved at this time.

The long success story of spark gaps, carbon blocks or gas tube
protectors, has created a situation where specifications of environment
and device capability are intermeshed to the point where one is not
quite sure which came first. Therefore any proposal to apply new
protective devices in these circuits is likely to confront a requirement
for meeting specifications to fit the capability of spark gaps but not
necessarily the capability (and limitations) of the candidate
replacement. Nevertheless, there is a great need to apply surge
protective devices capable of clamping surges at voltages well below the
capability of the old-time spark gaps used to protect old-time
electromechanical equipment.

Carbon block protectors are still widely used in tele­phone systems. The
reasons for this continued use are technical as well as economic.
Technically, the carbon block has provided satisfactory performance in
protecting the equipment, and the line voltage and short-circuit current
conditions after the crowbar action of the block allow clearing of the
circuit. This would not be the case with a power line. Economic reasons
include the cost of replacing on a large scale an existing device, still
obtainable at low cost, by a technically better device which may be more
expensive. Nevertheless, there is a trend to replace carbon blocks by
gas tubes, and some telephone companies make it a routine procedure to
substitute a gas tube protector for the old carbon block whenever a
serviceman is on the premises for some other reason.

The unsettled controversy over two-electrode versus three-electrode gaps
remains. The issue is that different sparkover voltages of two separate
two-electrode gaps used to protect a two-wire system can transform a
common-mode transient impinging on the circuit into a more damaging
line-to-line transient during the interval separating sparkover of the
first gap and sparkover of the second gap. Three-electrode gaps avoid
the problem because both lines are almost instantly connected to ground
whenever one of the two line-to-ground gaps contained in the same
envelope sparks over. The problem would disappear if varistors were used
for this protective function.

One of the requirements for protectors in the telephone systems is that
the power-frequency current associated with fallen wires or induced
voltages be conducted through the protector for a specified time, or a
specified I2t, corresponding to the melting of the wire connecting the
incoming cable to the main frame. This requirement is often met by
providing a heat-sensitive element in combination with the protective
device. The combination produces a direct short circuit and removes the
protective device from the current path, either in a resettable mode or
in a destructive mode making the protector expendable but protecting the
equipment while failing. Such a combination has the strength of
successful experience with carbon blocks, but it has yet to be tried
with realistic goals for varistors. Current requirements in the U.S.A.
vary from a few amperes for several seconds to a few hundred amperes for
a few cycles. In Europe the CCITT recommends 3 levels, 5, 20, and 50 A,
for repeated durations of 1 s at each current level (4).

Of course a significant part of the telephone plant exposure involves
the power supplies connected to the power system. These are not
essentially different from any electronic system supplied by the power
grid; the discussion for that subject is given under the heading
\"Mains.\"

Another exposure involves the growing number of customer-owned terminal
equipment (teletypes, computer terminals, recorders, and dialers)
connected on one side to the telephone line and on the other side to the
local mains. Insufficient withstand capability of this equipment on the
mains side could present some risk to the telephone system, so that, in
the U.S.A. for instance, strict withstand requirements have been
specified by the Federal Communications Commission: 2.5 kV, 1/10 μs,
with a short-circuit capability of 1 kA (5).

As solid-state electronics are applied in the switching equipment, well
inside the central station system, surge protective devices are also
applied to the inputs of these circuits. This additional protective
device, downstream from the protective device at the cable entrance, is
generally referred to as a secondary protection, the primary protection
being at the cable entrance. A potential problem exists if the secondary
protection has such a low voltage level that it will prevent the primary
protection from sparking over and thus become subjected to the full
surge current. Examples of coordination approaches will be given later
in this section to illustrate the concept.

[Automotive Circuits]{.underline}

Overvoltages occurring in automotive circuits (6, 7) have durations
ranging from microseconds to minutes. Overvoltage can be a steady state
condition in the case of a regulator failure or quasi-steady state with
24 V booster starts. Clearly, no voltage-clamping device can limit these
overvoltages, nor would a crowbar allow operation of the system while it
is being short circuited by the crowbar. From a list of overvoltages
published by the Society of Automotive Engineers (SAE) (8), the longest
duration which may be amenable to treatment as a transient overvoltage
is the so-called load dump, which is the accidental disconnection of the
battery while charging is proceeding at a high rate. Voltages in the
order of 125 V can occur, involving energy depositions of 10 J or more.
Such high depositions occur infrequently but often enough to mandate
that protection be provided. At the other end of the range, repetitive
transients occur in the ignition circuit, with only a few volts but with
high repetition rates.

Two examples of protection against these extreme cases are described in
detail in the Transient Voltage Suppression Manual (9) and thus will be
mentioned only briefly here. For the convenience of the reader, the
detailed calculations have been included in the Appendix. Figure 1 shows
the circuit of a solenoid driver, for which the most harmful overvoltage
would be the load dump condition. Varistors could be connected in
Positions A or B; analysis for each position involves simple
calculations of the current flowing in the varistor and the resulting
clamping voltage, with consideration of the impulse duration and number
of expected occurrences in the lifetime of the system.

After evaluating the two positions with several candidate varistors, the
designer reaches the conclusion that Position B is the preferred
location, with a recommendation of a V24ZA50 or V24ZA60 varistor.

Figure 2 shows the circuit for a typical electronic ignition circuit
where the switching transistor needs to be protected against
overvoltages associated with the energy stored in the ignition coil.
Detailed numerical analysis, starting with the coil current parameters,
yields a specific recommendation for a candidate varistor protection, a
V220MA2A or a V270MA4B varistor in this example.

[Protection Against Transients in the Mains]{.underline}

Protection against transients in the mains is the field in which
varistors have found their most effective role, and at this time a large
fraction of varistor production is applied to this function (10, 11).
Even in the high-voltage electric utility field the advent of metal
oxide varistors is expected to have considerable impact (12), but a
detailed discussion of high voltage surge arresters (diverters) is
beyond the scope of this Seminar.

A major reason for the effectiveness of protection by varistors is the
absence of the power-follow problem, problem which makes the application
of gaps impossible on circuits having high power-frequency short-circuit
currents. The bilateral conduction of varistors, as opposed to the
polarized single direction of avalanche diodes, also makes them
cost-effective in ac power circuits.

At the present time, there is no mandatory protection specification on
low-voltage mains, in contrast to the BIL concept mentioned earlier for
the primary utility systems. Should the Transient Level Concept become
standard practice on all low-voltage distribution systems, the situation
would be quite different, but the problem is that each appliance or each
industrial load may require individual protection as long as there is no
central protective device installed at the service entrance.
Publications such as the Guide on Surge Voltages (3) can provide
guidance to manufacturers of electronic appliances or instruments that
will be connected to the mains, in making an economic tradeoff between
no system protection with some attendant expected failure rate - and
protection for each piece of equipment - at an appreciable cost but with
a substantially lower expected failure rate.

The result of the present uncontrolled situation may be a proliferation
of surge protective devices in a given household or industrial circuit,
for each equipment manufacturer may assume that he alone is to provide
protection - that is, that he cannot depend on the user to provide
central protection and therefore he must build protection into his
equipment. This proliferation is not only an obvious waste but also a
potential problem because the protective device installed in a piece of
equipment - similar to the secondary protection of the telephone - might
assume the role of primary protection if its clamping voltage is lower
than that of the device (if any) installed at the service entrance.
Thus, proper coordination of protective devices in low-voltage circuits
should be a high-priority goal in the efforts of standardizing bodies,
manufac­turers, and regulatory agencies.

The situation is made more confusing by the role played by the sparkover
of wiring devices in limiting the transient overvoltages in low-voltage
systems. There is evidence that the clearances in wiring devices in some
systems can spark over, providing protection without producing a
power-follow fault. Thus the field experience is influenced by this
unintentional protective function which occurs in some installations but
not in others. Relying on uncontrolled sparkover for protection is
hazardous because there are three possible outcomes to a clearance
sparkover:

> 1\. A power-follow current occurs with destructive effects on the
> components.
>
> 2\. A power-follow current occurs, but overcurrent protection (breaker
> or fuse) limits the damage. The system can be restored to operation
> after a mere nuisance interruption.
>
> 3\. No power-follow current takes place; the overvoltage protective
> function of the system can be considered as accomplished.

The concept of protecting solid insulation by allowing clearances to
spark over first is actively promoted by the Low Voltage Insulation
Coordination Subcommittee of the International Electrotechnical
Commission (13). The concept is worth attention because cost reductions
and system reliability can be obtained but only where it is properly
applied.

### PROTECTION COORDINATION

By protection coordination we mean a deliberate selection of two or more
protective devices used with the goal of reliable protection at minimum
cost. With the present situation of unregulated and uncoordinated
application of protective devices, this may seem an unattainable goal
for complete systems. In specific cases, however, it is fully
attainable, as the three examples that follow will show. One can hope
that success will eventually spread the concepts and increase the drive
to generalize the approach.

One of the first concepts to be adopted when a coordinated scheme is
considered is that current, not voltage, is the independent variable
involved. The physics of overvoltage generation involves either
lightning or load switching. Both are current sources, and it is only
the voltage drop associated with the surge current flow in the system
impedance which appears as a transient overvoltage. Furthermore, there
is a long history of testing insulation with voltage impulses which has
reinforced the erroneous concept that voltage is the given parameter.
Thus, overvoltage protection is really the art of offering low impedance
to the flow of surge currents rather than attempting to block this flow
through a high series impedance. In combined approaches, a series
impedance is sometimes added in the circuit, but only after a
low-impedance diverting path has first been established.

[Retrofit of a Control Circuit Protection]{.underline}

In one retrofit case history, a field failure problem was caused by the
lack of awareness, at the time the original circuit was designed, of the
hostility of the environment in which the circuit was to be installed. A
varistor had been provided on the printed circuit board of the device,
to protect the control circuit components, but its capability was
exceeded by the surge currents occurring in that location (Category B of
the Guide on Surge Voltages \[ 3\] ).

Because a number of devices were in service, complete redesign was not
possible, and a retrofit at an acceptable cost had to be developed.
Fortunately, the power consumption of this control circuit was limited
so that it was possible to insert some series impedance in the line,
ahead of the low-capacity varistor, while a higher capacity varistor was
added at the line entrance to the circuit (Figure 3). Laboratory
proof-test of the retrofit demonstrated the capability of the combined
scheme to withstand 6 kA crest current surges (Figure 4A), a 200% margin
from the proposed Category B requirement, as well as reproduction of the
field failure pattern (Figure 4B). The latter is an important aspect of
any field problem retrofit. Simulation in the laboratory of the assumed
surges occurring in the field and subsequent verification of the failure
mechanism are the first steps to an effective cure. Figure 4C
illustrates the effect of improper installation of the suppressor, with
20 cm of leads instead of a direct connection across the input terminals
of the circuit.

[Coordination Between an Arrester and a Varistor]{.underline}

The example described here involves a load circuit for which the maximum
transients had to be limited to 1000 V (on a 120 V ac line) although
lightning surges were expected on the incoming service. The only
arresters available at the time which could withstand a 10 kA crest,
8/20 impulse had a protective (clamping) level of approximately 2200 V.
Some distance was available between the service entrance and the
location of the protected circuit, so that impedance was in fact
inserted in series between the arrester and the protected circuit where
a varistor with lower clamping voltage would be installed. The testing
objective was to determine at what current level the arrester would
spark over for a given length of wire between the two protective
devices, relieving the varistor from the excessive energy that it would
absorb if the arrester did not spark over.

A circuit was set up in the laboratory, with 8 m of typical two-wire
cable between the arrester and the varistor. The current, approximately
8/20 impulse, was raised until the arrester would spark over about half
of the time in successive tests at the same level, thus establishing the
transfer of conduction from the varistor to the arrester. Figure 5A
shows the discharge current level required from the generator at which
this transfer occurs. Figure 5B shows the voltage at the varistor when
the arrester did not spark over. Figure 5C shows the voltage at the
arrester when it sparks over, a voltage that would propagate inside all
of the building if there were no suppressor added. However, when a
varistor is added at 8 m, the voltage of Figure 5C is attenuated to that
shown in Figure 5D, at the terminals of the varistor.

[Lightning Surge Injection into Ground]{.underline}

A lightning surge current flowing in the ground of a power system can
induce substantial overvoltages in the phase wires of the system without
having these phase wires directly involved in conducting the lightning
current. To illustrate this situation, a laboratory simulation circuit
was set up (14), under the following valid assumptions:

1\. A 100 kA lightning strike terminated on the over­head primary circuit
causing flashover to ground or sparkover of an arrester at the pole.
This 100 kA represents a severe occurrence, not exceeded in more than 5%
of all lightning strokes (15).

2\. Seventy percent of this current is assumed to flow into the ground
at the pole and into the next two pole grounds, leaving 30 kA to flow
into the service entrance in question (Figure 6).

3\. The service overhead entrance has the phase wires wrapped around the
ground wire.

Accordingly, a series of 8/20 current impulses was injected into a
representative wiring system, consisting of service entrance box with
circuit breakers and branch circuit terminating at wall receptacles.
Provision was made to connect a secondary arrester at the service box
or, alternatively, a high capacity varistor with a low-capacity varistor
at the outlet the latter typical of a consumer\'s electronic package.
Some of the interesting observations made during this test series
follow. First, the injection of a unidirectional 8/20 surge current into
the ground conductor of the service entrance caused oscillator voltage
transients in the phase-to-ground outlets within the building wiring
system. Second, the impedance of the equivalent source could be
estimated by comparing the open-circuit voltage at the outlet with the
lower voltage observed when a known load resistance was connected across
the outlet. Third, while applicable only in the simulated condition,
some numerical data can be quoted to illustrate the possible
consequences of injecting high current into the ground conductors, i.e.,
as if a direct lightning stroke occurred in the distribution system
outside the building. Table 1 shows some of the values recorded.

From the first and second observations, one can compute an approximate
equivalent source impedance for the 1.5 kA surge:

> Z = 130 Ω x 2200/1400
>
> = 75 Ω

The second test level, 10 kA, produced flashover (or sparkover) at
voltage levels which are predictable for the type of wiring devices
being used. Ironically, the better the wiring device, the higher the
overvoltage imposed on the system; the poorer the wiring device, the
lower the overvoltages, provided that this sparkover of the device
clearance will not produce a power-follow.

The third test level, representing a very severe occurrence with very
low probability at any one location, did not, however, threaten the
integrity of an arrester designed to ANSI standards for secondary
arresters.

### CONCLUSIONS

-   Applications of surge protective devices can be retrofit protection
    following field problems or preventive protection against possible
    problems.

-   Although the Transient Control Level concept has not yet gained full
    acceptance, it should be actively discussed and promoted in order to
    attain more cost-effective application of protective devices.

-   Coordination of protective devices will become a more important task
    as more protective devices are installed in \"Parallel\" across the
    mains. Effective coordination does require matching the
    characteristics of protective devices with the source impedance as
    well as the impedance separating the two protective devices.

## REFERENCES

1\. F.A. Fisher, and F.D. Martzloff, \"Transient Control Levels, a
Proposal for Insulation Coordination in Low-Voltage Systems[,\" IEEE
PAS-95,]{.underline} 1, January/February 1976, pp. 120-129.

2\. F.D. Martzloff, and F.A. Fisher, \"Transient Control Level
Philosophy and implementation: The Reasoning Behind the Philosophy,\"
[Proceedings Second Symposium on EMC]{.underline}, 77CH1224-5EMC,
Montreux, June 1977.

3\. [Guide on Surge Voltages in AC Power Circuits Rated Up to 600
V]{.underline}, Final Draft, May 1979. Document prepared by Working
Group 3.4.4 of the Surge Protective Device Committee of the Power
Engineering Society, Institute of Electrical and Electronics Engineers.

4\. [Proceedings Fifth Plenary Assembly, Geneva, 4-15 December 1972,
Vol. IX Protection, Geneva:]{.underline} The International
Telecommunications Union, 1972.

5\. Longitudinal Voltage Surge Test #3 - Section 68.302 (e), Title 47,
\"Telecommunications,\" [Code of Federal Regulations]{.underline},
Washington: U.S. Gov\'t. Printing Office, 1977.

6\. N. Rittmannsberger, \"Semiconductors for Automotive Electronics,\"
Proc. [Automotive Electronics: An International Conference on Its
Various Aspects]{.underline}, August 1976, pp-17-20.

7\. J.R. Morgan, \"Transients in the Automotive Electrical System,\"
[IEEE Vehicular Technology Conf]{.underline}. Record, 1973.

8\. [Preliminary Recommended Environmental Practice for Electronic
Equipment Design]{.underline}, New York, Society of Automotive
Engineers, 2 Pennsylvania Plaza, NY, NY 10001.

9\. [Transient Voltage Suppression Manual]{.underline}, Second Edition,
Auburn NY, General Electric Company, 1978, pp. 73-78.

10\. M.N. Smith, Practical Application and Effectiveness of Commercially
Available Pulse Voltage Transient Suppressors, Port Hueneme, CA., Naval
Civil Engineering Laboratory, 1973.

11\. J.B. Clarke, [Transient Suppression on 120 Volt Power
Circuits]{.underline}, Wright-Patterson AFB, Ohio, Air-Force Institute
of Technology, 1975.

12\. E.C. Sakshaug, J.S. Kresge, and S.A. Miske, \"A New Concept in
Station Arrester Design,\" [IEEE PAS-96]{.underline}, No. 2, March-April
1977, pp. 647-656.

13\. \"Insulation Coordination Within Low-Voltage Systems Including
Clearances and Creepage Distances for Equipment,\" International
Electrotechnical Commission Report SC28A (Central Office) 5 (to be
published in 1979).

14\. F.D. Martzloff, \"Coordination of Surge Protectors in Low-Voltage
AC Power Circuits,\" Preprint No. F29 6354 for IEEE PES Summer Meeting,
Vancouver, Canada (July 1979).

15\. N. Cianos, and E.T. Pierce, [A Ground-Lightning Environment for
Engineering Usage]{.underline}, Stanford Research, Institute, Technical
Report, August 1972.

**EPILOGUE**

This Seminar, starting with the origin of transient overvoltages,
presented basic concepts on protection techniques and protective
devices, and gave specific examples of the application of protective
schemes and surge generating equipment. Condensing the field in a brief
handout and a one-day presentation was a challenge. Surely the author
has failed to bring out all the relevant facts; for this he accepts full
responsibility, and he will sincerely welcome comments from the readers.

This epilogue presents an opportunity to acknowledge the support of
International General Electric in sponsoring this Seminar, the
participation of the Heafely Company and KeyTek Instrument Corporation
in demonstrating equipment, and also to thank Catharine Fisher for
editing these notes.

The author expresses his appreciation to the Seminar participants for
their attention. As one who has experienced the problems caused by
transients and the satisfaction of solving them, the author extends to
the protection engineer his best wishes for success through the
effective application of protective techniques and devices.

> Francois D. Martzloff
>
> Schenectady, N.Y., July 1979

TRANSIENT OVERVOLTAGE PROTECTION COORDINATION

**IN THE UNDEFINED REAL WORLD ENVIRONMENT**

F.D. Martzloff

Corporate Research and Development

General Electric Company

**ABSTRACT**

Transient overvoltages are no longer an unknown threat to the successful
application of power conversion equipment, and protective techniques and
devices are available. The appropriate selection of these, however,
remains a difficult task because the exact nature of transients in the
real world is at best only statistically defined. Therefore, the choice
involves technical and economic decisions based on calculated risks
rather than deterministic optimization. In addition to this, protection
in a complex system is required at more than one point of the circuits.
Indiscriminate or uncoordinated application of devices at these several
points may be wasteful or ineffective.

This paper presents an overview or the origin of transient overvoltages
and of current IEEE and IEC activities to identify and categorize
transients. A brief review of available techniques and devices follows,
and the major part of the paper describes the principles of coordinated
protection with specific experimental examples and results reconciling
the unknown with the realities of equipment design.

### INTRODUCTION

Since the introduction of semiconductors, transient overvoltages have
been blamed for device failures and system malfunctions. Semiconductors
are, indeed, sensitive to overvoltages. However, data have been
collected for several years on the occurrence of overvoltages, to the
point where the problem is now mostly a matter of economics and no
longer one of lack of knowledge on what the environment of power systems
can inflict to poorly protected semiconductor circuits. This statement
rally represent a slight oversimplification of the general problem,
because the environment is still defined in statistical terms, with the
unavoidable uncertainty as to what a specific power system can impress
on a specific piece of power conversion equipment.

A Working Group of IEEE has prepared a Guide describing the nature of
transient overvoltages (surges) in ac power circuits rated up to 600 V.
\[1\] This Guide provides information on the rate of occurrence, on the
waveshape, and on the energy associated with the surges, as a function
of the location within the power system. In addition, a subcommittee of
the IEC has developed a report concerning Insulation Coordination \[2\]
and has recommended the use of four categories of installations, with a
matrix of power system voltages and overvoltages specified for
controlled situations. Other groups have also proposed test
specifications, some of which are now enshrined in standards that may be
applied where they are really not applicable, but were applied because
no other information was available at the time.

At this time, the environment seems to be defined with sufficient
detail. However, there is still a lack of guidance on how to proceed for
specific instances, and circuit designers may feel that they are left
without adequate information to make informed decisions on the selection
of component characteristics in the field of overvoltage withstand or
protection. This situation has been recognized, and the various groups
concerned with the problem are attempting to close tire gap by preparing
Application Guides which will provide more specific guidance than a mere
description of the environment, although that description in itself is
already a considerable step forward.

One of the difficulties in designing a protection scheme in the
industrial world of power conversion equipment is the absence of an
overall system coordinator, in contrast to the world of electric
utilities, for instance, which are generally under the single
responsibility of a centralized engineering organization. The user of
power conversion equipment is likely to purchase the material from a
supplier independently of other users of the smile power system, and
coordination of overvoltage protection is generally not feasible under
these conditions. Worse yet, an uncoordinated application of urge
suppressors could lend to wasteful or ineffective resource allocation as
independent users would each attempt to provide protection in adjacent
systems, or independent designers would provide protective devices in
adjacent subsystems.

To shed more light onto this situation, this paper will briefly review
some of the origins of transient overvoltages, with reference to IEEE
and IEC documents recently completed but not yet published that will
provide guidance on the environment. Techniques and protective devices
will then be discussed, and examples of coordinated approaches
presented.

### THE ORIGIN OF TRANSIENT OVERVOLTAGES

Two major causes of transient overvoltages have long been recognized:
system switching transients and transients triggered or excited by
lightning discharges (in contrast to direct lightning discharges to the
power systems, as these are generally quite destructive, and protection
against these may not be economical in the average application). System
switching transients can involve a substantial part of the power system,
as in the case of power factor correction capacitor switching
operations, disturbances following restoration of power after an outage,
and load shedding. However, these do not generally involve substantial
overvoltages (more than two or three per unit) but may be very difficult
to suppress since the energies are con­siderable. Local load switching,
especially if it involves restrikes in the switchgear devices, will
produce higher voltages than the power system switching, but generally
at lower energy levels. Considering, however, the higher impedances of
the local systems, the threat to sensitive electronics is quite real,
and only a few conspicuous case histories of failures can cast a very
adverse shadow over a large number of successful applications.

### VOLTAGE LEVELS

Two different approaches have been proposed to define the voltage levels
in ac power systems. At this time, the divergences have not yet been
reconciled, as each proposal has its merits arid justification. As
proposed by the Working Group already mentioned, the IEEE approach
involves reciting a rate of occurrence as a function of voltage levels
as well as of exposure. The IEC approach indicates only a maximum level
for each location category. These two proposals will be quoted in the
following paragraphs.

# The IEEE Working Group Proposal

Data collected from a number of sources led to plotting a set of lines
representing a rate of occurrence as a function of voltage for three
types of exposures (Figure 1). \'These exposure levels are defined in
general terms as follows:

· Low Exposure - Systems in geographical areas known for low lightning
activity, with little load switching activity.

· High Exposure - Systems in geographical areas known for high lightning
activity, with frequent and severe switching transients.

· Extreme Exposure - Rare but real systems supplied by long overhead
lines and subject to reflections at line ends, where the characteristics
of the installation produce high sparkover levels of the reflections.

The two lower lines of Figure 1 have been drawn at the slime slope,
since the data base shows reasonable agreement among several sources on
that slope. Both the low-exposure and high-exposure lines are truncated
at about 6 kV because that level is the typical wiring device sparkover.
The extreme-exposure line, by definition, is not limited by this
sparkover. Because it represents an extreme case, the extreme exposure
fine needs to be recognized, but it should not be applied
indiscriminately to all systems. Such application would penalize the
vast majority of installations, where the exposure is lower.

It is essential to recognize that a Surge voltage observed in a power
system can be either the driving voltage or the voltage limited by the
sparkover of some clearance in the system. Hence, the term unprotected
circuit must be understood to be a circuit in which no low-voltage
protective device has been installed but in which clearance sparkover
will eventually limit the maximum voltage. The distribution of surge
levels, therefore, is influenced by the surge-producing mechanisms as
well as by the sparkover level of clearances in the system. This
distinction between actual driving voltage and voltage limited by
sparkover is particularly important at the interface between outdoor
equipment and indoor equipment. Outdoor equipment has generally higher
clearances, hence higher sparkover levels: 10 kV may be typical, but 20
kV is possible. In contrast, most indoor wiring devices used in 120-240
V systems have sparkover levels of about 6 kV; this 6 kV level,
therefore, can be selected as a typical cutoff for the occurrence of
surges in indoor power systems.

The voltage and current amplitudes presented in the Guide attempt to
provide for the vast majority of lightning strikes but should not be
considered as \"worst case,\" since this concept cannot be determined
realistically. One should think in terms of the statistical distribution
of strikes, accepting a reasonable upper limit for most cases. Where the
consequences of a failure are not catastrophic but merely represent an
annoying economic loss, it is appropriate to make a tradeoff of the cost
of protection against the likelihood of a failure caused by a high but
rare surge.

## The IEC-SC28A Proposal

In a report dealing with clearance requirements for insulation
coordination purposes, the IEC Subcommittee SC/28A recommends a set of
impulse voltages to be considered as representative of the occurrences
at different points of a power system, and at levels dependent upon the
system voltage (Table 1). The report is not primarily concerned with a
description of the environment, but more with insulation coordination of
devices installed in these systems.

## WAVESHAPE OF THE TRANSIENT OVERVOLTAGES

Many independent observations \[3, 4, 5\] have established that the most
frequent type of transient overvoltage in ac power systems is a decaying
oscillation, with frequencies between 5 and 500 kHz. This finding is in
contrast to earlier attempts to apply the unidirectional double
exponential voltage wave, generally described as 1.2 x 50 or 1,2/50.
Indeed, the unidirectional voltage wove has a long history of successful
application in the field of dielectric withstand tests and is
representative of (lie surges propagating in transmission systems
exposed to lightning. Still, this is the waveshape selected by the
IEC-SC28/A for low-voltage systems. Other groups have promoted an
oscillation, such as that specified in the SWC tests. \[6\] The IEC
Working Group is recommending two waveshapes, one for the indoor
environment, and one for the outdoor and near-outdoor environment
(Figure 2). Not only is a voltage impulse defined, but the discharge
current, or short-circuit current of a test generator used to simulate
these transients, is also defined in the IEEE document.

The oscillatory waveshape simulates those transients affecting devices
that are sensitive to dv/dt and to voltage reversals during conduction,
\[7\] while the unidirectional voltage and current waveshapes, based on
long-established ANSI standards for secondary valve arresters, simulate
(line transients where energy content is the significant parameter.

### ENERGY AND SOURCE IMPEDANCE

The energy involved in the interaction of a power system with a surge
source and a surge suppressor will divide between the source and the
suppressor in accordance with the characteristics of the two impedances.
In a gap-type suppressor, the low impedance of the arc after sparkover
forces most of the energy to be dissipated elsewhere, for instance in a
resistor added in series with the gap for limiting the power-follow
current. In an energy-absorber suppressor, by its very nature, a
substantial share of the surge energy is dissipated in the suppressor,
but its clamping action does not involve the power-follow energy
resulting from the short-circuit action of a gap. It is, therefore,
essential to the effective use of suppression 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 or recommendations either ignore the issue, such as MIL
STD-1399 or the IEC SC/28A Report, or sometimes indicate values
applicable to limited cases, such as the SWC test for high-voltage
substation equipment. The IEEE 587 \[1\] document is attempting to
relate impedance with categories of locations, but unavoidably has to
remain somewhat vague on their definitions (Table II).

The 6 kV open-circuit voltage derives from two facts: the limiting
action of wiring device sparkover and the unattenuated propagation of
voltages in unloaded systems. The 3 kA discharge current in Category B
derives from experimental results: field experience in suppressor
performance and simulated lightning tests. The two levels of discharge
currents for the 0.5 μs - 100 kHz wave derive from the increasing
impedance expected in moving from Category B to Category A.

Location Category C is likely to be exposed to substantially higher
voltages than Location Category B because the limiting effect of
sparkover is not available.

The extreme exposure rates of Figure 1 could apply, with voltage in
excess of 10 kV and discharge currents of 10 kA, or more. Installing
unprotected load equipment in Location Category C is not recommended;
the installation of secondary arresters, however, can provide the
necessary protection.

## MATCHING THE ENVIRONMENT WITH THE EQUIPMENT

On the basis of the various documents described in the preceding
paragraphs, an equipment designer or a user can take a systematic
approach at matching the capability or requirement for withstand of the
equipment with the environment in which this equipment is to be
installed. Figure 3 shows a flow chart concept of this systematic
approach. This may involve tests to determine the withstand levels,
\[8\] some measurements and/or analysis to determine the degree of
hostility of the environment, and a review of available protective
devices. The latter will be briefly surveyed in the following
paragraphs.

### TRANSIENT SUPPRESSORS

Two methods and types of devices are available to suppress transients:
blocking the transient through some low-pass filter, or diverting it to
ground through some nonlinear device. This nonlinearity may be either a
frequency nonlinearity (high-pass filter) or a voltage nonlinearity
(clamping action or crowbar action). We will be mostly discussing the
second type in this paper, since voltage clamping devices or crowbar
devices are the most frequently used. \[9\]

Voltage-clamping devices have variable impedance, depending on the
current flowing through the device or the voltage across its terminal.
These components show a nonlinear characteristic, i.e., Ohm\'s law can
be applied but the equation has a variable R. Impedance variation is
monotonic and does not contain discontinuities, in contrast to the
crowbar device which shows a turn-on action. As for as volt-ampere
characteristics of these components is concerned, they are
time-dependent to a certain degree. However, unlike sparkover of a gap
or triggering of a thyristor, time delay is not involved here.

When a voltage-clamping device is installed, the circuit remains
unaffected by the device before and after the transient for any
steady-state voltage below clamping level. Increased current drawn
through the device as the voltage attempts to rise .results in voltage
clamping action. Nonlinear impedance is the result if this current rise
is faster than the voltage increase. Increased voltage drop (IR) in the
source impedance due to higher current results in the apparent clamping
of the voltage. It should be emphasized that the device depends on the
source impedance to produce the clamping. A voltage divider action is at
work where one sees the ratio of the divider not constant, but changing.
The ratio is low, however, if the source impedance is very low. The
suppressor cannot work at all with a limit zero source impedance (Figure
4). In contrast, a crowbar type device effectively short-circuits the
transient to ground; but once established, this short circuit will
continue until the current (the surge current as well as any
power-follow current supplied by the power system) is brought to a low
level.

The crowbar device will often reduce the line voltage below its
steady-state value, but a voltage clamping device will not. Substantial
currents can be carried by the suppressor without dissipating a
considerable amount of energy within the suppressor, since the voltage
(arc or forward-drop) during the discharge is held very low. This
characteristic constitutes the major advantage of these suppressors.
However, limitations in volt-time response and power-follow are the
price paid for this advantage. As voltage increases across a spark-gap,
significant conduction cannot take place until transition to the arc
mode has taken place by avalanche breakdown of the gas between the
electrodes. The load is left unprotected during the initial rise due to
this delay time (typically in microseconds). Considerable variation
exists in the sparkover voltage achieved in successive operations, since
the process is statistical in nature. This sparkover voltage can, in
addition, be substantially higher after a long period of rest than after
successive discharges, for some devices. 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. This difficulty
can he alleviated by filling the tube with a gas having lower breakdown
voltage than air. However, if the enclosure seal is lost and the gas
replaced by air, this substitution creates a reliability problem because
the sparkover of the gap is then substantially higher.

Another limitation occurs when a power current from the steady-state
voltage source follows the surge discharge ( follow-current, or
power-follow). This power-follow current may or may not be cleared at a
natural current zero, in ac circuits. fit dc circuits, clearing is even
more uncertain. Additional means must, therefore, be provided to open
(lie 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. Consequently, most electronic
circuits are better protected with voltage clamping suppressors than
with crowbars, but sometimes the energy deposited in a voltage clamping
device by a high current surge can be excessive; a combination of the
two devices can provide an effective protection at optimum cost.
However, this combined protection must be properly coordinated to obtain
the fall advantage of the scheme. The following paragraphs will discuss
some of the basic principles of coordination and provide some examples
of applications.

### PROTECTION COORDINATION

One of the first concepts to be adopted when considering a coordinated
scheme is that current, not voltage, is the independent variable
involved. The physics of overvoltage generation involve either lightning
or load switching. Both are current sources, and it is only the voltage
drop associated\] with the surge current flow in the system impedance
which appears as a transient overvoltage. Furthermore, there is a long
history of testing insulation with voltage impulses which has reinforced
the erroneous concept that voltage is the given parameters Thus,
overvoltage protection is really the art of offering low impedance to
the flow of surge currents rather than attempting to block this flow
through a high series impedance. In combined approaches, a series
impedance is sometimes added in the circuit, but only after a low
impedance diverting path tins first been established.

When the diverting path is a crowbar type device, little energy is
dissipated in the crowbar, as noted earlier. In a voltage clamping
device, more energy is deposited in the device, so that energy handling
capability of a candidate suppressor is an important parameter to
consider when designing a protection scheme. With nonlinear devices, an
error made in the assumed value of the current surge produces little
error on the voltage developed across the suppressor and thus applied to
the protected circuit, \[10\] but the error is directly reflected in the
amount of energy which the suppressor has to absorb. At worst, when
surge currents in excess of the suppressor capability are imposed by the
environment, either because of an error made in the assumption, or
because nature tends to support Murphy\'s law, or because of human error
in the use of the device, the circuit in need of\' protection can
generally be protected at the price of failure in the short-circuit mode
of the protective device. However, if substantial power-frequeney
currents can be supplied by file power system, the fail-short protective
device generally terminates as fail-open when the power system fault in
the failed device is not cleared by a series overcurrent protective
device (fuse or breaker). Note that in this discussion, the term
\'fail-safe\' hits carefully been avoided since it can mean opposite
failure modes to different users. To some, fail-safe means that the
protected hardware must never be exposed to an overvoltage, so that
failure of the protective device must be in the fail-short mode, even if
it puts the system out of operation. To other users, 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.

**EXAMPLES OF COORDINATED SURGE PROTECTION**

### Retrofit of a Control Circuit Protection

In this case history, a field failure problem was caused by 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 (the
first question asked in the flow chart of Figure 3). A varistor had been
provided to protect the control circuit components on the printed
circuit board, but its capability was exceeded by the surge currents
occurring in a Category B location (Table 11). To the defense of the
circuit designer, however, it must be stated that the data of Table II
were not available to him at the time.

Because a number of devices were in service, complete redesign was not
possible, but a retrofit - at an acceptable cost - had to be developed.
Fortunately, the power consumption of this control circuit was limited
so that it was possible to insert some series impedance in the line,
ahead of the low-capacity varistor, while a higher capacity varistor was
added at the line entrance to the circuit (Figure 5). Laboratory
proof-test of the retrofit demonstrated the capability of the combined
scheme to withstand 6 kA crest current surges (Figure 6A), a 200 %
margin from the proposed Category B requirement, as well as reproduction
of the field failure pattern (Figure 6B). The latter is an important
aspect of any field problem retrofit. By simulating in the laboratory
the assumed surges occurring in the field (Table II), verification of
the failure mechanism is the first step toward an effective cure. Figure
6C illustrates the effect of improper installation of the suppressor,
with 8 inches of leads instead of a direct connection across the input
terminals of the circuit.

**Coordination Between a Secondary Surge Arrester and a Varistor**

In this example, the objective was to provide overvoltage protection
with a maximum of 1000 V applied to the protected circuit, but to
withstand current surges on the service entrance of magnitudes
associated with lightning, as defined in ANSI C62.1/2 Standards for
secondary arresters. The only arresters available fit the time which
could withstand a 10 kA crest 8x20 impulse bad a protective (clamping)
level of approximately 2200 V. Some distance was available between the
service entrance and the location of the protected circuit, so that
impedance was in fact inserted in series between the arrester and the
protected circuit where a varistor with lower clamping voltage would be
installed. The object was to determine at what current level the
arrester would spark over for a given length of wire between the two
protective devices, relieving the varistor from the excessive energy
that it would absorb if the arrester would not spark over.

A circuit was set up in the laboratory, with 8 meters (24 ft) of #12
two-wire cable between the arrester and the varistor. The current,
approximately 8x20 impulse, was raised until the arrester would
sparkover about half of the time in successive tests at the same level,
thus establishing the transfer of conduction from the varistor to the
arrester. Figure 7A shows the discharge current level required from the
generator at which this transfer occurs. Figure 7B shows the voltage at
the varistor when the arrester (does not spark over. Figure 7C shows the
voltage at the arrester when it sparks over; this voltage would
propagate inside all of the building if there was no suppressor added.
However, if a varistor is added at 8 meters, the voltage of Figure 7C is
attenuated to that shown in Figure 7D, at the terminals of the varistor.

### Surge Injected into Ground System

Lightning surge currents flowing in the ground conductors of a power
system can induce substantial overvoltages in the phase conductors,
without having lightning current directly injected into these phase
conductors. A laboratory simulation of this situation was conducted,
\[11\] from which interesting observations were made. First, the
injection of a unidirectional 8x20 surge current in the ground conductor
of the service entrance to a building caused oscillatory voltage
transients in the phase-to-ground outlets within the simulated building
wiring system. Second, the impedance of the equivalent source could be
estimated by comparing the open-circuit voltage at the outlet with the
lower voltage observed when a known load resistance was connected across
the outlet. Third, while applicable only in the simulated condition,
some numerical data can be quoted to illustrate the possible
consequences of injecting high current into the ground conductors, i.e.,
if a direct lightning stroke were to occur in the distribution system
outside the building. Table III shows some of the values recorded.

From the first observation, one can compute an equivalent source
impedance of about 80 SI for the discharge current of 1.5 kA source,
consistent with the orders of magnitude shown in Table II. The clearance
sparkover of the second observation is predictable and confirms the
comments made earlier in discussing the rate of occurrence of Figure 1.
The 30 kA example, which may seem a very high current, corresponds to a
lightning discharge of 100 kA, or the 5 % level of crest discharges 12
where 70 kA is assumed to flow directly into the pole ground and
overhead ground conductors to adjacent poles of the outdoor distribution
system. Thus, for this extreme assumption, an acceptable current level
is imposed at the service entrance arrester (for which the ANSI
requirement is 10 kA).

## CONCLUSION

Effective protection of sensitive electronic equipment is possible
through a systematic approach where the capability of the equipment is
compared to the characteristics of the environment. The combined efforts
of several organizations have produced a set of data which provide the
circuit designer with reasonable information, albeit not fine
specifications, on the assumptions to be made in assessing the hostility
of the environment. A Guide and Application Guides will be available in
the near future to better define the characteristics of the power system
environment. As more field experience is gained in applying these
documents to equipment design, the feedback loop can be closed to
ultimately increase the reliability of new equipment at acceptable cost,
while current problems may also be alleviated based on these new
findings in the area of transient overvoltages.

### ACKNOWLEDGMENTS

The data base for the Guide quoted in the paper as well as the writing
of that Guide was provided by the members of the IEEE Working Group on
Surge Voltages in AC Power Circuits Rated 600 V or Less. Robert
Mierendorf made available the data of Table I and emphasized the
significance of clearance sparkover; Peter Richman\'s critique of the
concepts relating to source impedance proved very effective; Paul
Speranza suggested the flow chart approach developed in Figure 5.

**REFERENCES**

1\. [Guide on Surge Voltages in AC Power Circuits Rated Up to 600
V]{.underline}, Final Draft, May 1979. Document prepared by Working
Group 3.4.4 of the Surge Protective Device Committee of the Power
Engineering Society, Institute of Electrical and Electronics Engineers.

2\. "Insulation Coordination Within Low-Voltage Systems Including
Clearances and Creepage Distances for Equipment," International
Electrotechnical Commission Report SC28A (Central Office) 5 (to be
published in 1979).

3\. J.E. Lenz, \"Basic Impulse Insulation Levels of Mercury Lamp Ballast
for Outdoor Applications,\" [Illuminating Engrg.]{.underline}, 133-140
(Feb. 1964).

4\. F.D. Martzloff and G.J. Hahn, "Surge Voltage in Residential and
Industrial Power Circuits," [IEEE PAS]{.underline} 89, [6,]{.underline}
1049-1056 (July/August 1970).

5\. R. Hasler and R. Lagadec, "Digital Measurement of Fast Transients on
Power Supply Lines," [Proc. 3^rd^ Symposium and Technical Exhibition on
Electromagnetic Compatibility]{.underline}, Rotterdam, Holland, 445-448
(May 1979).

6\. Surge Arrestors for Alternating-Current Power Circuits, IEEE
Standard 28­1974; ANSI Standard C62.1-1975; IEC Standard 99-2.

7\. P. Chowdhuri, "Transient-Voltage Characteristics of Silicon Power
Rectifiers." [IEEE IA-9]{.underline}, [5]{.underline}, 582 (Sept./Oct.
1973).

8\. F.A. Fisher, and F.D. Martzloff, "Transient Control Levels, a
Proposal for Insulation Coordination in Low-Voltage Systems," [IEEE
PAS-95]{.underline}, [1]{.underline}, 120-129 (Jan./Feb. 1976).

9\. [Transient Voltage Suppression Manual,]{.underline} Chapter 2, pp.
13-17, General Electric Company, Auburn, NY (1978).

10\. [Transient Voltage Suppression Manual]{.underline}, Chapter 2, p.
15, General Electric Company, Auburn, NY (1978).

11\. F.D. Martzloff, "Coordination of Surge Protectors in Low-Voltage AC
Power Circuits," Preprint No. F29 635-4 for IEEE PES Summer Meeting,
Vancouver, Canada (July 1979).

12\. N. Cianos and E.T. Pierce, A [Ground-Lightning Environment for
Engineering Usage]{.underline}, Stanford Research Institute, Menlo Park,
CA 94205 (August 1972).

ELECTRICAL PROTECTION OF CELLULAR RADIO SITES

François D. Martzloff

General Electric Company

Schenectady NY

# INTRODUCTION

Hardware installed at a cellular radio site will be exposed to transient
overvoltages which can upset the data processing or, worse, cause
permanent damage of sensitive electronic components.

The hazards associated with lightning or power system switching
transient overvoltages (surges) have existed ever since electronic
equipment began to displace the older electro-mechanical devices.
Therefore, these hazards are not a new threat to the successful
operation of cellular radio systems; rather, the combination of three
ports connected to the hardware presents some new problems, but they can
be solved by proper application of known techniques and devices. The
connection to the power supply, the connection to the telephone cable
plant, and the exposure of the antenna to lightning strikes are three
ports through which upsetting or damaging transient overvoltages can
impinge on the hardware. While a click in a telephone conversation would
not be objectionable, an upset of digital commands might be. Permanent
damage, of course, should be avoided as much as possible.

This presentation will give an overview of the nature of the surges
likely to enter by each of the three ports. A brief review will then be
given of the techniques and devices available for diverting or
attenuating these surges. The effectiveness of these techniques and
devices has been demonstrated in other existing electronic systems;
therefore, reliable protection of cellular radio hardware can also be
expected under most operating conditions.

**IMPINGING SURGES**

**Power Supply System**

The power supply system for a site will be whatever the local electric
utility supplies to its customers. Studies and standards have been
published, describing the nature of the surges that can be expected to
occur on such a power supply.

Fortunately for the reliability of the cellular radio equipment, the
actual power supply to the electronics is a dc source, needed to assure
continuity of service in case of failure of the utility system. This
supply takes the form of a dc battery with a charger powered by the
utility grid. Provided that the charger can withstand the impinging
surges from the utility supply, the battery will act as a very effective
buffer for the disturbances, including surges, occurring in the utility
grid.

The existence of power supply surges has been recognized in the
telephone industry, and FCC regulations (Part 68) do provide a standard
for withstand capability of the ac input of electronics interfacing the
power supply and the cable plant. Therefore, cellular radio hardware
will be required to meet the same requirements as all other peripherals
now being used at subscribers facilities. The FCC standard is a voltage
impulse of 2500 V peak with a duration of 10 μs and a short-circuit
current of 1000 A. It is a severe test, but equipment designed to
withstand that impulse has been found to have satisfactory survival
capability in service, which is the ultimate proof that the standard is
appropriate.

# Cable Plant

The wire connection of the cell hardware to the telephone system will
present the same characteristics as a conventional connection to a
multi-line subscriber. The telephone industry has a long and successful
experience in dealing with surges that can appear on the wire pairs. Of
course, there has been a downward trend in the withstand capability of
terminal equipment as old electromechanical equipment has been
superseded by modern solid-state equipment. Fortunately, concurrent
advances in protective devices have enabled system operators to maintain
reliability of these more sensitive components. A wide range of
protective devices is available, meeting requirements designed
specifically for this type of service.

# Antenna Feed

The very nature of an antenna, especially a top-mounted omni-directional
antenna, makes it a very good lightning rod. Thus, there is a definite
likelihood that a lightning bolt will, from time to time, terminate on
an antenna. It is possible to provide some protection---mostly by
diverting the lightning current to earth---but some damage to the
antenna by the current burning effect, or to the electronics by the
overvoltage, is still likely to occur. The probability of a tower being
hit by a lightning bolt can be estimated with reasonable accuracy for
the general case, but not with good precision for any particular site.
Equations, which are based on actual experience, relate the number of
strikes terminating on a tall object to its height and geographical
location. A recent study has been published, assessing the lightning
damage threat in the United States, which provides a better picture of
the situation than the traditional "isokeraunic map" based on the number
of days where thunder is heard.

# Ground Potential Differential

The three sources of surges discussed above have all been taken one at a
time, with the implication that they might carry a surge voltage within
themselves. This type of surge is the so-called "normal mode," or
"transverse mode." In site hardware, the power supply and the antenna
feed conductors include a connection to ground, while the telephone
connection will be a balanced pair. There is another way for surges to
appear, the so-called "common mode" where, for instance, two pairs of
telephone cable are simultaneously elevated in potential above ground.
However, as the telephone terminal equipment is connected across the
pair, the amplifiers inputs are not stressed between terminals, but the
insulation with respect to ground will be stressed.

Each of the three ports at the cell can bring a surge with respect to
the ground, and therefore produce different potentials with respect to
ground, unless adequate common grounding is applied. It is a dangerous
fallacy to proclaim that electrical noise can be avoided by using
separate grounds.

# FUNDAMENTAL PROTECTION TECHNIQUES

Protection of a power system, of a communication system, or of an
electronic black box against the threats of the surge environment can be
accomplished in different ways. There is no single truth or magic cure
ensuring immunity and success, but, rather, there are a number of valid
approaches that can be combined as necessary to achieve the goal. The
competent protection engineer can contribute his knowledge and
perception to the choice of approaches against a threat which is
imprecise and unpredictable, keeping in mind the need to balance the
technical goal of maximum protection and the economic goal of realistic
protection at an acceptable cost. However, just as in the case of
accident insurance, the cost of the premium appears high before the
accident, not after.

A discussion of fundamental protection techniques that is limited in
time and scope has the risk of becoming an inventory of a bag of tricks;
yet, there are some fundamental principles and fundamental techniques
that can be useful in obtaining transient immunity, especially at the
design stages of an electronic system or circuit. All too often, the
need for protection becomes apparent at a late stage, when it is much
more difficult to apply the fundamental techniques which are most
effective and economical when implemented at the outset.

Protection techniques can be classified into several categories
according to the purpose and the system level at which the engineer is
working. For the system as a whole, protection is primarily a preventive
effort. One must consider the physical exposure to transients---in
particular, the indirect effects of lightning resulting from building
design, location, physical spread, and coupling to other disturbance
sources---as well as such inherent susceptibility characteristics as
frequency response and nominal voltage.

For many system components or electronic black boxes, the environment is
often beyond the control of the designer or user, and protection becomes
a curative effort---learning to live and survive in an environment which
is imposed. Quite often this effort is motivated by field failures, so
that retrofit is needed. The techniques involved here tend to be the
application of protective devices to circuits or a search for inherent
immunity rather than the elimination of surges at their origin. However,
in the case of cellular radio hardware, the opportunity exists to
integrate all protective measures at the outset of the design and thus
optimize the protection.

Another distinction can be made in classifying protective techniques.
Granted that surges will be unavoidable, one can attempt to block them,
divert them, or strive to withstand them; the latter, however, is
generally difficult to achieve alone.

**SHIELDING, BONDING, AND GROUNDING**

Shielding, bonding, and grounding are three interrelated methods for
protecting a circuit from external transients. Shielding consists in
enclosing the circuit wiring in a conductive enclosure, which in theory
cancels out any electromagnetic field inside the enclosure; actually, it
is more an attenuation than a cancellation. 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," through various methods of driving conductors into
the soil. Each of these techniques has its limitations, and each can
sometimes be overemphasized.

# 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 laboratory experimenter anticipating a hostile
electromagnetic environment. Difficulties do arise, however, when the
concept of "grounded" is examined in detail and when the objectives of
shielding for noise immunity conflict with the objectives of shielding
for lightning surge immunity.

A shield can be the size of a matchbox or a building; it can cover a few
inches of wire, or kilometers of buried overhead cables. "Grounding"
these diverse shields is not 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.

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. In this approach, the
only draw-back is the hardware cost of "double shields." However, in
many installations there is a metallic conduit through which the cables
are pulled; with simple but close attention to maintaining the
continuity of this conduit pat, 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.

**Bonding**

We have already mentioned one aspect of bonding in describing the need
for continuity of the outer shield. Another instance of bonding occurs
where the shield of an incoming cable is connected to the box of the
circuit of to the building ground. The principle is simple: the shield
can be viewed as an extension of the box, and thus bonding of the shield
to the box should be continuous over 360°In practice, unless special
connectors are used, this is difficult to achieve, for often a shielded
cable is terminated at a connection board with the shield peeled back
and turned into a pigtail, which in turn is connected to the "ground"
terminal of the connection board. One can imagine the many possible
variations of current flow, with the shield current now flowing in the
pigtail and the creation of the corresponding 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. 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.

# Grounding

Grounding, which is also referred to as "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
measurements on rods driven into the ground. Driving rods into the
ground does not ensure a low impedance under the transient conditions of
high rate of current change associated with lightning discharges. This
remark is not intended as a criticism of the efforts going into
achieving a low resistance but, rather, to alert the system designer
that there is more to it than just low resistance, and that one can
overdo the act of burying copper in the ground. Driving rods into the
ground does not ensure a low impedance under the transient conditions of
high rate of current change associated with lightning discharges. This
remark is not intended as a criticism of the efforts going into
achieving a low resistance but, rather, to alert the system designer
that there is more to it than just low resistance, and that one can
overdo the act of burying copper in the ground.

When one deals with a reasonably compact system, be it cabinet-size,
room-size, or building-size, it is more effective to view the grounding
as well bonded connection to the outer shield (if any), building frame,
or cabinet enclosure. The resistance (impedance) from that reference to
"earth" is not very significant as long as other wires at "ground"
potential are not brought to the system. This desirable situation is
best obtained with single-point grounding approaches, as opposed to
multiple grounding. There is a controversy over the respective
advantages and disadvantages of the two approaches for general cases,
but examination of the characteristics of specific cases will resolve
the controversy. Since there is little chance of dealing with an
absolutely isolated system, the question is: What should be done with
incoming wires? These wires can be isolated from the local ground during
normal operation, but one must recognize that, during transient
conditions of lightning surge or power system faults, high voltages will
appear across these isolated wires and local ground, voltages which, in
some cases, are totally 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 for the duration of the
transient suppressors.

It is a dangerous illusion to believe that lightning effects can be
eliminated by the isolation of conductors or sub-systems. It is much
safer and quite acceptable, if included in the design, to provide
bonding during transient conditions by suitable protective devices. The
important point to remember is that while lightning is a fairly
well-defined phenomenon, with known characteristics and effects in
general, its probability of occurrence at a particular location is
unknown. For the successful operation of a system, foresight is needed
in applying fundamental protection techniques at the beginning.

# PROTECTIVE DEVICES

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.

There are two categories of transient suppressors: those that block
transients, preventing their propagation toward sensitive circuits, and
those that divert transients, limiting residual voltages. Since many of
the transients originate from a current source, the blocking of a
transient may not always be possible; the diverting of 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 -- 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. Thus, we are primarily interested in the diverting devices.
These diverting devices can be of two kinds: short-circuiting devices
(crow-bar) or voltage-clamping devices.

Because the technical and trade literature contains many articles on
these devices, we shall limit the discussion of the details and refer
the reader to References 16-29. We shall, however, make some comparisons
to point out the significant differences in performance.

Cr**owbar Devices**

The principle of crowbar devices is quite simple: upon occurrence of an
overvoltage. the device changes from a high-impedance state to a
low-impedance state. offering a low-impedance path to divert the surge
to ground. Spark gaps, including gas tubes and carbon blocks extensively
used in the telephone industry, are examples of crowbar devices.

The major advantage of the crowbar device is that its low impedance
allows the flow of substantial surge currents without the development of
high energy within the device itself; the energy has to be spent
elsewhere in the circuit. This \"reflex ion\" of the impinging surge can
also be a disadvantage in some circuits when the transient disturbance
associated with the gap firing is being considered. Where there is no
problem of power-follow (discussed below), such as in some communication
circuits, the spark gap has the advantage of very simple construction
with potentially low cost.

The crowbar device, however, has three major limitations. One is the
volt-time sensitivity of the breakdown process. As the voltage increases
across a spark gap, 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, since the process is statistical in nature. This
sparkover voltage, in addition, can be substantially higher after a long
period of rest than after successive discharges.

The second limitation is associated with the sharpness of the sparkover,
which produces fast current rises in the circuits and, thus,
objectionable noise. An effect of this fast current change can be found
in some hybrid protective systems. The gap does a very nice job of
discharging the impinging high-energy surges, but the magnetic field
associated with the high rate of current change (di/dt) induces a
voltage in the loop adjacent to the secondary suppressor, adding what
can be substantial spike to the expected secondary clamping voltage.

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. Additional means, therefore, must
be provided to open the power circuit if the crowbar device is not
designed to provide self-clearing action within the 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 or surge
diverter.

# Voltage-Clamping Devices

Voltage-clamping devices have variable impedance, depending on the
current flowing through the device or the voltage across its terminal.
These components show a nonlinear characteristic - that is, Ohm\'s law
can be applied, but the equation has a variable. Impedance variation
does not contain discontinuities, in contrast to the crowbar device,
which shows a turn-on action. As far as their volt-ampere
characteristics are concerned, these components are time-dependent to a
certain degree. However, unlike the sparkover of a gap or the triggering
of a thyristor, time delay is not involved.

When a voltage-clamping device is installed, the circuit remains
essentially unaffected by the device before and after the transient for
any steady-state voltage below clamping level. Increased current drawn
through the device as the surge voltage attempts to rise results in
voltage-clamping action.

The principle of voltage clamping can be achieved with any device
exhibiting this nonlinear impedance. Two categories of devices, having
the same effect but operating on very different physical processes, have
found acceptance in the industry: the polycrystalline varistors and the
single-junction avalanche diodes. Another technology, the old selenium
rectifier, has been practically eliminated from the field because of the
improved characteristics of modern varistors.

Avalanche diodes, the Zener diodes, were initially applied as voltage
clamps, a natural outgrowth of their application as voltage regulators.
Improved construction, specifically aimed at surge absorption, has made
these diodes very effective suppressors. Large-diameter junctions and
low thermal impedance connections are used to deal with the inherent
problem of dissipating the heat of the surge in a very thin single-layer
junction. (26,27)

The advantage of the avalanche diode, generally a P-N silicon junction,
is the possibility of achieving low clamping voltage and a nearly flat
volt-ampere characteristic over its useful power range. Therefore, these
diodes are widely used in low-voltage electronic circuits for the
protection of 5 or 15 V logic circuits, for instance. For higher
voltages, the heat generation problem associated with single junctions
can be overcome by stacking a number of lower voltage junctions,
admittedly at some extra cost.

The term varistor is derived from its role as a variable resistor. It is
also called a voltage-dependent resistor, but that description tends to
imply that the voltage is the independent parameter in surge protection,
which is not correct. Two very different devices have been successfully
developed as varistors: silicon carbide disks have been used for years
in the surge arrester industry, and, more recently, metal oxide varistor
technology has come of age.(28)

In silicon carbide varistors, the physical process of nonlinear
conduction is not completely understood, and the manufacturing of the
material, successful as it is, has remained an art. It appears that the
process takes place at the tips of the grains of silicon carbide which
are held together by a binder. The story goes that the device action was
found accidentally by having a grinding wheel, on a disorderly work
bench, accidentally connected to an experimental circuit; for many years
silicon carbide varistors indeed looked like grinding wheels, each
complete with a hole in the center.

Metal oxide varistors depend on the conduction process occurring at the
boundaries between the large grains of oxide (typically zinc oxide)
grown in a carefully controlled sintering process. The physics of the
nonlinear conduction mechanism have been described in the
literature.(22-24)

# COMPARISONS OF PROTECTIVE DEVICES

**Spark Gap Versus Varistor**

The choice between these two devices will be influenced by the inherent
characteristics of the application. Where power-follow is a problem,
there is little opportunity to apply a simple gap. The operation of a
gap on a power system can cause sags in the voltage when the crowbar
action persists until the next current zero; varistors do not cause
these sags. Where very steep front transients occur, the gap alone may
let an excessive voltage go toward the \"protected\" circuit until the
voltage is limited by sparkover. Where the capacitance of a varistor is
objectionable, the low inherent capacitance of a gap seems attractive.
If very high energy levels (compared to the lower levels inherent with
the crowbar action of a gap) are likely to be deposited in a varistor by
an impinging surge, then a high capacity surge arrester near the service
entrance may be combined with a lower clamping voltage varistor
installed farther into the circuit. This combined protection, however,
requires adequate coordination between the two suppressors.(30)

# Avalanche Diode Versus Varistor

The basic performance characteristics of these two devices are similar,
and therefore the choice may be dictated by clamping voltage
requirements (the avalanche diode is available at lower clamping
voltages), by energy-handling capabilities (the varistor is generally
higher in capability per unit of cost), and by packaging requirements
(the varistor material is more flexible and does not require hermetic
packaging).

# 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 by 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 out of operation. To other
users, 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.

**SPECIFIC APPLICATIONS**

Trial systems of cellular radar (AMPS and ARTS) as well as conventional
mobile radio base stations provide a basis for field experience in the
techniques of protection. The practices or recommendations described in
this section reflect the combined experience of three organizations
involved in this technology: Bell Laboratories, Mobile Radio Department
of General Electric, and Motorola Communications and Electronics.

Actual damage experience has been limited to one outage after a major
lightning strike, but a single module re-placement by field personnel
restored the normal operation. Automatic monitoring of the antenna
performance has been sufficient to detect minor antenna damage due to
lightning; this type of degradation can then be corrected by routine
maintenance.

This success story is the result of thorough application of the
principles discussed earlier. Inspection of a cell site reveals a
grounding scheme that includes extensive grounding rods and well-bonded
rings (called \"halo\" when installed at the top of the inside wall of
the building, in contrast to floor-level rings). Systematic bonding of
conduits, cabinets, and enclosures is mandatory.

At a typical site, the power line protection starts at the utility
distribution transformer with a primary surge arrester. The meter socket
and service antenna panel are grounded to the local unit. Surge
arresters are provided on the load side of the service panel, protecting
the rectifiers for the dc supply to the 24 V storage batteries. The
limited 120 V ac loads (test instruments and fans) are supplied by an
inverter fed from the battery.

The cable input from the wireline plant is first protected at the point
of entry by conventional protectors. While in some installations this
has been the only protection, further protection is recommended by the
use of three-electrode gas tubes, as a prudent and ultimately
cost-effective backup protection.

The major threat associated with lightning has been successfully
overcome by extensive grounding and shielding. The tubular post
construction for the antenna, rather than the lattice tower, offers a
very effective method of shielding the antenna feed line. Lightning
current flows on the outer shell of the post, leaving the coax cables
unharmed inside. The extent of the outside grounding network (rods vs.
mats or radials) can be determined by prudent practice, guided by
cost-weighing factors involving the past history and practices of the
general site area, and the water table condition of the site on a
year-round basis. Both power companies and local telephone companies
generally have well-developed experience in this area.

Field experience has shown that problems arose when the antenna elements
were not bonded to the overall ground (\"dc ground\" as opposed to \"RF
ground\") system. This bonding is done at the tower top, at intermediate
tie points, and. most important, at the base of the tower and hatch
plate entrance to the building. Some of the relatively small details in
grounding practices, which can be described as 2% increments in
protection costs, have been found to yield a 50% improvement in
lightning withstand capability.

When the precautions have been applied, operators and manufacturers
express confidence in the reliability. While none would be foolhardy to
claim immunity, all have found that an effective protection can be
achieved.

Systematic application of protective techniques, including shielding,
bonding, and grounding will allow diversion of lightning currents
directly to earth, with a minimum amount of residual stress for the
electronics. The residual stress can then be dealt with by suitable
surge protective devices.

# CONCLUSIONS

Electrical protection of cellular radio sites can be obtained by
appropriate application of protective techniques and protective devices.
The effectiveness of these has been demonstrated in trial systems of
cellular radio as well as in their long-successful application in power
systems, telephone wireline systems, and conventional mobile radio
systems.

Surge protective devices are available for protecting low-voltage
electronics. Two basic types offer different characteristics: crowbar
devices have high-current capability but generally involve power-follow
when applied on a power system; voltage clamping devices, either silicon
avalanche or varistors, are free from the power-follow problem.

Avalanche diodes offer low clamping voltage, which makes them most
suitable for low-voltage, low-power electronics. Metal oxide varistors
are now available in a wide range of clamping voltages and
energy-handling capacities. Spark gaps, while having some limitations,
are widely used on telephone circuits, mostly in the form of gas tubes.
Each of these devices has its own best field of application, insuring
greater reliability of the circuits in the electromagnetic environment
of power and communication systems.

# ACKNOWLEDGEMENTS

The contribution of data and comments made by the following individuals
and organizations are gratefully acknowledged: W. Bryson. Phelps Dodge
Communications Company; E.J. Cohen, Rural Electrification
Administration; F.A. Fisher and E.K. Guthrie, General Electric Company;
G. Fell, Motorola National Service; D. Horn, Motorola Communications and
Electronics; K.G. Nelson, AMPS of Illinois Bell; J. Nicholas. P.D.
Speranza and T.K. Wingard. Bell Telephone Laboratories.

**REFERENCES**

> 1\. IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits,
> IEEE Std 587-1980.
>
> 2\. "Insulation Coordination Within Low-Voltage Systems Including
> Clearances and Creepage Distances for Equipment," International
> Electrotechnical Commission Report 664, 1980.
>
> 3\. M. Goldstein and P.D. Speranza, "The Quality of U.S. Commercial
> Power." INTELEC Conference Proceedings, IEEE, 1982.
>
> 4\. Regulation, Section 68.302(e) Title 37, Telecommunications,
> Washington, D.C., U.S. Government Printing Office, 1977.
>
> 5\. Test Specifications for Gas Tube Surge Protective De vices, IEEE
> Std C62.31-1982.
>
> 6\. Test Specifications for Low-Voltage Air Gap Surge Protective
> Devices, IEEE Std C62.32, 1981.
>
> 7\. Test Specifications for Varistor Surge Protective De vices, IEEE
> Std C62.33, 1982.
>
> 8\. Test Specifications for Avalanche Junction Protective Devices,
> IEEE Std C62.34, to be published in 1983.
>
> 9\. World Distribution of Thunderstorm Days, WMO-OMM #21 TP6, World
> Meteorological Organization, Geneva, 1953.
>
> 10\. R.S. Valazquez, V. Gerez, D. Mukhedar, and Y. Gervais,
> "Probabilistic Calculations of Lightning Protection for Tall
> Buildings," IEEE Transactions on Industry Applications 1A-18 (3),
> May/June 1982, pp. 252-259.
>
> 11\. F.A. Fisher and D.W. Caverly. "Lightning Protection Systems for
> Walt Disney World." General Electric Company, Pittsfield, MA, 1969.
>
> 12\. J.F. Henz and E.W. Paul, "A Lightning Damage Assessment of the
> United States," U.S. Nuclear Regulatory Commission, NUREG/CR-2013,
> 1981.
>
> 13\. Recommended Guide for Measuring Ground Resistance and Potential
> Gradients in the Earth, No. 81, American Institute of Electrical
> Engineers. New York, New York, May 1962.
>
> 14\. IEEE Recommended Practice for Electric Power Systems in
> Commercial Buildings, IEEE Std 241-1974.
>
> 15\. "Electrical Protection Grounding Fundamentals," REA Manual,
> Section 802, June 1975.
>
> 16\. M.N. Smith, Practical Application and Effectiveness of
> Commercially Available Pulse Voltage Transient Suppressors, Naval
> Civil Engineering Laboratory, Port Hueneme, CA, 1973.
>
> 17\. "Surge Voltage Protectors \-- Application Note and Suggested
> Applications," SERT, The. Journal of the Society of Electronic and
> Radio Technicians 9 (4), pp. 85-91, April 1975.
>
> 18\. C.B. Barksdale, "New Gas Tube Protector Technique for Circuits,\"
> Telephony 171, pp. 21-28, 1970.
>
> 19\. A. Bazarian, "When Should You Use Gas-Discharge Protectors?\"
> Electronic Design News, December 20, pp. 58-62, 1973.
>
> 20\. V.W. Vodicka, "Voltage Transients Tamed by Spark-Gap Arresters,"
> Electronics 39, 1966, pp. 109-113.
>
> 21\. J.D. Harden, F.D. Martzloff, W.G. Morris, and F.B. Golden, "The
> GE-MOV Varistor \-- The Super Alpha Varistor," Electronics 45 (21),
> 1972, p. 91.
>
> 22\. G.D. Mahan, L.M. Levinson, and H.R. Philipp, "Theory of
> Conduction in ZnO Varistors," 78CRD205, General Electric Company,
> Schenectady, NY, 1978.
>
> 23\. M. Matsuoka, T. Masa. Yama, and Y. Lida, "Nonlinear Electrical
> Properties of Zinc Oxide Ceramics," Proc. of First Conf. Solid State
> Devices, Tokyo, 1969, J. Japan Soc. Appl. Phys. 39, 1970, Suppl., p.
> 94.
>
> 24\. M. Matsuoka, "Non-Ohmic Properties of Zinc Oxide Ceramics,\" Jap.
> J. Appl. Phys. 10, 1971, p. 736.
>
> 25\. S. Korn, "Transients and Transient Suppressors For Power
> Conversion Equipment,\" INTELEC Conference, Washington, D.C., 1982.
>
> 26\. O.M. Clark, and J.J. Pizzicaroli, "Effect of Lead Wire Lengths on
> Protector Clamping Voltages," Federal Aviation Administration-Florida
> Institute of Technology Workshop on Grounding and Lightning
> Technology, FAA-RD-79-6, March 6-8, 1979, Melbourne, Florida, U.S.
> Dept. of Transportation, Washington, DC 20590.
>
> 27\. "Data Book," General Semiconductor Industries, Tempe, AZ.
>
> 28\. Transient Voltage Suppression Manual, Third Edition, General
> Electric Company, Auburn, NY, 1982.
>
> 29\. D. Bodle, "Electrical Protection Guide for Land-Based Radio
> Facilities," Joslyn Electronic Systems, 1971.
>
> 30\. F.D. Martzloff, "Coordination of Surge Protectors in Low-Voltage
> AC Power Circuits, "IEEE PAS-99(1), Jan-Feb 1980, pp. 129-133.

## For Further Reading

####### Lightning

H. Cavendish, W. Watson, B. Franklin, and J. Robertson, "Report of the
Committee Appointed by the Royal Society to Consider a Method of
Securing the Powder Magazines at Purfleet," Philosophical Transactions
of the Royal Society, London, 63, 1773, and 68, 1777.

Golde, R.H., ed., Lightning, 2 vols., Academic Press, London and New
York, 1977.

N. Cianos and E.T. Pierce, A Ground Lightning Environment for
Engineering Usage, Stanford Research Institute, Technical Report, August
1972.

M.A. Uman, Understanding Lightning, Bek Technical Publications,
Carnegia, PA 15106, 1971.

E.W. Boehne, and R.H. Golde, eds., "Research on the Lightning
Phenomenon,\" Journal of the Franklin ***Institute 283 (6), June 1967,
pp. 451-478.***

***Surge Occurrence and Propagation***

F.D. Martzloff and G.J. Hahn, "Surge Voltage in Residential and
Industrial Power Circuits," IEEE PAS 89 (6),July/August 1970, pp.
1049-1056.

R. Hasler and R. Lagadec, "Digital Measurement of Fast Transients on
Power Supply Lines," Proc. 3rd Symposium and Technical Exhibition on
Electro-Magnetic Compatibility, Rotterdam, Holland, May 1979, pp.
445-448.

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

F.D. Martzloff, "The Propagation and Attenuation of Surge Voltages and
Surge Currents in Low-Voltage AC Circits," IEEE PAS-102(5), May 1983,
pp. 1163-1170.

####### Surge Suppression

"The Protection of Telecommunication Lines and Equipment Against
Lightning Discharges," Comite Consultatif International de Telegraphe et
Telephone, International Telecommunication Union, Geneva, Switzerland,
1974.

####### Cellular Radio

"Advanced Mobile Phone Service," The Bell System Technical Journal 58
(1), January 1979.

F.H. Blecher, "Advanced Mobile Phone Service," IEEE Transactions on
Vehicular Technology VT-29 (2), May 1980, pp. 238-244.

# LINE CONDUCTED DISTURBANCES --- ORIGINS AND CONTROL

François D. Martzloff

Corporate Research and Development

General Electric Company

Schenectady, New York 12345

## 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. However, in
communication and data systems, there is another source of transients:
the coupling of power system transients into the data system.

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. However, in
communication and data systems, there is another source of transients:
the coupling of power system transients into the data system.

Lightning discharges may not necessarily mean direct termination of a
lightning stroke onto the power system. A lightning stroke terminating
near a power line, either by hitting a tree or the bare earth, will
create a very fast-changing magnetic field that can induce voltages --
and inject energy -- into the loop formed by the conductors of the power
system. Lightning can also inject overvoltages in a power 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
and other presentations in this conference provide information on the
characteristics of lightning discharges^.(1-6)^

Surges from power system switching create overvoltages as a result of
either trapped energy in loads being switched off. or restrikes in the
switchgear. These will be examined in greater detail in the following
paragraphs.

### SWITCHING TRANSIENTS

A transient is created whenever a sudden change occurs in a power
circuit, especially during power switching -- either the closing or
opening of a circuit. 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 circuit with
reclosing by restrikes and reopening by clearing at the high-frequency
current zero. In medium-voltage switchgear, pre-strikes can occur just
before the contacts close, with a succession of clearings at the
high-frequency current zero, followed by restrike. Similarly, during an
opening sequence of a switch. restrikes can cause electrical closing(s)
of the circuit.

**Simple Switching Transients**

Simple switching transients^(7)^ include circuit closing transients.
transients initiated by clearing a short circuit. and transients
produced when the two circuits on either side of the switch being opened
oscillate at different frequencies. In circuits having inductance and
capacitance (all physical circuits have at least some in the form of
stray capacitance and inductance) with little damping, these simple
switching transients are inherently limited to twice the peak amplitude
of the steady-state sinusoidal voltage. Another limit to remember when
analyzing transients associated with current interruption (circuit
opening) is that the circuit inductance tends to maintain the current
constant. At most, then, a surge protective device provided to divert
the current will be exposed to that initial current. Without a surge
protective device the current is available to charge the circuit
capacitances at whatever voltage is required to store the inductive
energy from the current by converting it into capacitive energy.

### Abnormal Switching Transients

Several mechanisms are encountered in practical power circuits. These
mechanisms can produce transient overvoltages far in excess of the
theoretical twice-normal limit mentioned above. Two such mechanisms
occur frequently: current chopping and restrikes, the latter being
especially troublesome when capacitor switching is involved.

Current chopping is the name given to the rapid current reduction which
fuses or which circuit breakers can force when clearing a circuit prior
to the natural current zero of the power system. When there is
inductance in the circuit, this rapid current change can produce high
overvoltages -- some 10 times the normal circuit voltage. A classical
example is an unloaded transformer where the magnetizing inductance is
high and the energy stored in the magnetic core can charge only the
winding capacitance.

Capacitor switching can be troublesome if the switch restrikes after
current interruption. Restrike occurs following the initial
interruption: the capacitor voltage remains nearly constant at maximum
system voltage, since the interruption occurred at zero current. which
is 90\' apart from the voltage zero. while the system voltage follows
the normal sine wave (Figure 1). At 180° after interruption, the switch
has to support twice the amount of system voltage, a stress it might not
be able to withstand with its contacts not completely separated. A
restrike can occur under these circumstances. In such a case, the
capacitance of the circuit will tend to drive the voltage not toward the
system voltage but beyond it -- theoretically up to twice the
difference. Such an overshoot means a possible voltage of three times
the system voltage. While this high-frequency oscillation takes place.
the switch may clear at a high-frequency current zero. only to restrike
again later with an even greater difference of voltage and then
escalating to an even higher overshoot The outcome will be either a
breakdown in one of the components. or. it the switch eventually
recovers enough dielectric withstand in its opening gap, no further
restrikes. But severe over-voltages will have been impressed on the
system.

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 some-what
predictable and can be estimated with reasonable accuracy from the
circuit parameters, once the mechanism involved has been identified.
There is still some uncertainty as to where and when they occur because
the worst offenders result from some abnormal behavior of a circuit
element Lightning-induced transients are much less predictable because
there is a wide range of coupling possibilities. Moreover, one user,
assuming that his system will not be the target of a direct hit may take
a casual view of protection. while another. fearing his system will
experience a ^\"^Worst case.\" may demand the utmost protection.

In response to these concerns. various committees and working groups
have attempted to describe ranges of transient occurrences or maximum
values occurring in power circuits. Three such attempts will be
discussed now.

### STANDARDS ON TRANSIENT OVERVOLTAGES

> Several standards or guides have been issued or proposed in Europe by
> VDE. IEC. CECC. Pro-Electron. CCITT. and in the USA by IEEE, NEMA, UL.
> REA, and the military, specifying a surge withstand capability for
> specific equipment or devices and specific conditions of transients in
> power or communication systems. Some of these specifications represent
> early attempts to recognize and deal with the problem in spite of
> insufficient data. As a growing number of organizations address the
> problem and as exchanges of information take place. improvements are
> being made in the approach. The IEEE has recently published a new
> standard describing the environ­ment in low-voltage ac power
> circuits.^(8)^ The Low-Voltage Insulation Coordination Subcommittee
> SC/28A of IEC has also completed a report IEC 664. listing the maximum
> values of transient overvoltages to be expected in power systems under
> controlled conditions and for specified system characteristics.^(9)^
> For some time now, a document prepared by a Relaying Committee of IEEE
> under the title Surge Withstand Capability^(10)^ has been available.
> These three documents are reviewed in the pages that follow. Greater
> emphasis will be placed on the first cited above because it describes
> the transient environment: the other two assume an environment for the
> purpose of specifying tests.

**The IEEE Surge Withstand Capability Test**

One of the earliest published documents to address 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. This
document, which describes a transient generated by the arcing that takes
place when air-break . disconnect switches are opened or closed in the
power system. presents innovations in transient protection:

-   The specified waveshape of the voltage is oscillatory wave-shape,
    not the historical unidirectional waveshape

```{=html}
<!-- -->
```
-   A source impedance, a characteristic undefined in many other
    documents. is defined

-   The concept that all lines to the device under test must be
    subjected to the test is spelled out

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 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.

## The IEC 664 Report

The Insulation Coordination Committee of IEC. following a comprehensive
study of breakdown characteristics in air gaps, included in its report a
table indicating the voltages that equipment must be capable of
withstanding in various system voltages and installation categories
(Table 1). The table specifies that it is applicable to a controlled
voltage situation, which implies that some surge-limiting device will
have been provided -- presumably a typical surge arrester with
characteristics matching the system voltage in each case. The waveshape
specified for these voltages is the 1.2/50 uus wave. a specification
consistent with the insulation withstand con­cerns of the group that
prepared the document No source impedance is indicated. but four
\"installation categories\" are specified, each with decreasing voltage
magnitude as the installation is further removed from the outdoor
environment Thus. this document addresses primarily the concerns of
insulation coordination, and the specification it implies for the
environment is more the result of efforts toward coordinating levels
than efforts to describe the environment and the occurrence of
transients. The latter approach has been that of the IEEE Working Group
on Surge Voltages in Low-Voltage AC Power Circuits. which we shall now
review in some detail.

**The IEEE Guide on Surge Voltages (IEEE Std 587-1980)**

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 2). These exposure levels are defined in
general terms as follows:

-   Low Exposure - Systems in geographical areas known for low lightning
    activity, with little load switching activity.

```{=html}
<!-- -->
```
-   Medium Exposure - Systems in geographical areas known for high
    lightning activity, with frequent and severe switching transients.

```{=html}
<!-- -->
```
-   High Exposure - Rare but real systems supplied by long overhead
    lines and subject to reflections at line ends, where the
    characteristics of the installation produce high sparkover levels of
    the clearances.

The two lower lines of Figure 2 have been drawn at the same slope since
the data base shows reasonable agreement among several sources on that
slope. All lines may be truncated by spark-over of the clearances, at
levels depending on the withstand voltage of these clearances. The high
exposure line needs to be recognized, but it should not be
indiscriminately applied to all systems. Such application would penalize
the vast majority of installations where the exposure is lower.

It is essential\' to recognize that a surge voltage observed in a power
system can be either the driving voltage or the voltage limited by the
sparkover of some clearance in the system. Hence, the term unprotected
circuit must be understood to be a circuit in which no low-voltage
protective device has been installed but one in which clearance
sparkover will eventually limit the maximum volt-age. The distribution
of surge levels, therefore, is influenced by the surge-producing
mechanisms as well as by the sparkover level of clearances in the
system. This distinction between actual driving voltage and voltage
limited by sparkover is particularly important at the interface between
outdoor equipment and indoor equipment. Outdoor equipment has generally
higher clearances, hence higher sparkover levels: 10 kV may be typical,
but 20 kV is possible. In contrast, most indoor wiring devices used in
120 to 240 V systems have sparkover levels of about 6 kV; this 6 kV
level, therefore, can be selected as a typical cutoff for the occurrence
of surges in indoor power systems.

The voltage and current amplitudes presented in the Guide attempt to
provide for the vast majority of lightning strikes but none should be
considered \"worst case,\" as this concept cannot be determined
realistically. It is necessary to think in terms of the sta­tistical
distribution of strikes, and to accept a reasonable upper limit for most
cases. Where the consequences of a failure are not catas­trophic but
merely represent an annoying economic loss, it is appropriate to make a
tradeoff of the cost of protection against the likelihood of a failure
caused by a high but rare surge.

Waveshape of the Surges

Many independent observations^(11-13)^ have established that the most
frequent type of transient overvoltage in ac power systems is a decaying
oscillation, with frequencies between 5 and 500 kHz. This finding is in
contrast to earlier attempts to apply the unidirectional double
exponential voltage wave that is generally described as 1.2/50. Indeed,
the unidirectional voltage wave has a long history of successful
application in the field of dielectric withstand tests and is
representative of the surges propagating in power transmission systems
exposed to lightning. In order to combine the merits of both waveshape
definitions and to specify them where they are applicable, the Guide
specifies an oscillatory waveshape inside buildings, a unidirectional
waveshape outside buildings. and both at the interface (Figure 3).

The oscillatory waveshape simulates those transients affecting devices
that are sensitive to dv/dt and to voltage reversals during conduction.
while the unidirectional voltage and current waveshapes. based on
long-established ANSI standards for secondary valve arresters, simulate
the transients where energy content is the significant parameter.

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. In a gap-type 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-follow current. In an energy-absorber protective
device, by its very nature, a substantial share of the surge energy is
dissipated in the suppressor, but its clamping action does not involve
the power-follow energy resulting from the 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 MIL STD-1399 or the IEC 664 report, either
ignore the issue or indicate values applicable to limited cases, such as
the SWC test for high-voltage substation equipment. The IEEE 587
standard attempts to relate impedance to categories of locations but
unavoidably remains vague on their defi­nitions (Table 2).

The 6 kV open-circuit voltage derives from two facts: the limiting
action of wiring device sparkover, and the unattenuated propagation of
voltages in unloaded systems. The 3 kA discharge current in Location
Category B derives from experimental results: field experience in surge
protective device performance and simulated lightning tests. The two
levels of discharge currents for the 0.5 μs--100 kHz wave derive from
the increasing impedance expected in moving from Category B to Category
A.

Category C is likely to be exposed to substantially higher volt-ages
than Category B because the limiting effect of sparkover might not be
available. The high exposure rates of Figure 2 could apply, with voltage
in excess of 10 kV and discharge currents of 10 kA or more. Installing
unprotected load equipment in Category C is not recommended; the
installation of secondary arresters, how-ever, can provide the necessary
protection.

Having defined the environment for low-voltage ac power circuits, the
Working Group is now preparing an Application Guide. where a
step-by-step approach will outline the method for assessing the need for
transient protection and selecting the appropriate device or system.
Parallel work in other IEEE working groups preparing test specification
standards ^(14-16)^ for surge protective de-vices will be helpful in
this selection process. Other groups in the U.S. as well as the
international bodies of IEC and CCITT. are now working toward further
refinements and the reconciliation of different approaches.

## PROPAGATION OF SURGES

Voltage surges propagate in wiring at speeds in the order of 2/3 the
speed of light. i.e., about 200 m/uus. Thus, for short lengths of
wiring, the classical transmission line behavior of reflections is not
applicable: with surges having a rise time of 1 uus. many round-trips
between the sending end and the far end can occur during the rise. so
that the final voltage is not affected by the reflections; only during
the rise can one notice a small difference between the two ends (Figure
4).

Figure 4 also illustrates the fact that a line with an unloaded end does
not attenuate the voltage. This fact prevails over the miscon­ception
that the wiring system in a building, for instance, inherently produces
a progressive attenuation of surges from the service en-trance to the
locations inside the building. ^(9)^

Likewise, suggestions are sometimes made that an \"isolating
transformer\" can attenuate surges. This is true only if the
trans-former is fully loaded. It is not difficult to imagine a scenario
where the transformer will operate at very light load, for instance,
with only an electronic control energized during the off-cycle of the
power cir­cuit Figure 5 shows how an impinging 0.5 μs-100 kHz transient
of 8 kV (IEEE 587) produces a 7 kV output at the unloaded secondary
terminals. The second reprint in these notes, \'The Propagation and
Attenuation of Surge Voltage and Surge Currents in Low-voltage AC
Circuits,\" presents more detailed discussions of these concerns.

## THE TRANSIENT CONTROL LEVELS (TCL) CONCEPT

Until recently, there did not appear to be a clear approach for
achieving compatibility between the transient withstand capability of
devices and the transients to which such devices are exposed. This
situation was somewhat like that which prevailed many years ago in the
electric power industry. Transients produced by lightning frequently
caused failure to such vital and expensive power equip­ment as
transformers and generators. Those transient problems were solved by
engineering design guided by the concept of insulation coordination and
the establishment of a series of Basic Insula­tion Levels (BILs). At
present the transient control levels concept of testing and coordination
promises a solution to the problem of compatibility.

According to the transient control levels concept, instead of
retrofitting protective devices into a finished product or design. the
first step is to determine at what level the transients occurring in the
system can be limited by suitable protective devices. It is then
sufficient to build the equipment to withstand only that level
established by the protective device. The result is a well-defined
situation, with adequate margins between the maximum level of transients
and the demonstrated withstand capability of the equipment. The specific
proposals made in that concept are the following:

1.  That there be defined for electronic equipment (and other
    low-voltage equipment) a standard transient voltage similar in
    concept to, but different in waveshape from, the 1.2/50 μs wave used
    in the coordination of insulation in high-voltage power apparatus.

2.  That there be defined for electronic equipment (and other
    low-voltage equipment) a series of TCLs similar in concept to the
    BILs.

3.  That a start be made on assigning one of these standard levels to
    individual electronic components and electronic devices.

4.  That individual protective devices be rated in terms of their
    ability to control transients to levels no greater than, and
    preferably lower than, one of the above levels.

5.  That equipment and procedures be developed by which equipment may be
    tested by vendors to determine which TCL is appropriate to assign to
    individual components and equipment

6.  That TCLs begin to be used in purchase specifications.

7.  That such equipment and procedures be used by purchasers to evaluate
    vendor-supplied equipment to determine its compliance with such
    purchase specifications.

8.  That such TCLs begin to appear in regulatory specifications for
    consumer apparatus in which the consumers cannot make the
    appropriate tests or prepare appropriate specifications.

The engineering community in the United States responded favorably to
the proposal, and some of the concepts found their way into the IEEE
Guide discussed in the first section of these notes. With the writing of
Application Guides by the IEEE as well as by the IEC, there is an
opportunity to advance the proposal further. A reprint of the original
paper presenting this proposal is included in the notes.

## CONCLUSIONS

The two major causes of transient overvoltages. lightning surges and
switching surges, have been identified with greater precision in
low-voltage ac circuits as well as in communication systems.

While standardizing the definition of the environment will not change
the environment itself, the emergence of realistic standards will enable
designers to increase the reliability of their products. Likewise, users
will be able to protect their equipment more effectively.

## REFERENCES

1.  J.H. Hagenguth and J.G. Anderson. \"Lightning to the Empire State
    Building,\" Part Ill. AIEE Transactions 71 PAS, August 1952.

2.  N. Cianos and ET. Pierce. A Ground Lightning Environment for
    Engineering Usage. Stanford Research Institute. Technical Report.
    August 1972.

3.  R.H. Golde. \"The Frequency of Occurrence and the Distribution of
    Lightning Flashes to Transmission Lines.\" AIEE Transactions 64,
    1945.

4.  World Distribution of Thunderstorm Days. WMO-OMM #21 TP6, World
    Meteorological Organization, Geneva. 1953.

5.  R.H. Golde. The Lightning Conductor,\" Lightning II, R.H. Golde.
    ed., Academic Press. New York. 1977.

6.  H. Cavendish. W. Watson. B. Franklin. and J. Robertson, \"Report of
    the Committee Appointed by the Royal Society to Consider a Method of
    Securing the Powder Magazines at Purfteet,\" Philosophical
    Transactions of the Royal Society. London. 63, 1773. and 68. 1777.

7.  A. Greenwood. \"Electrical Transients in Power Systems.\" Wiley
    Interscience. New York. 1971.

8.  IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits. IEEE
    Std 587-1980.

9.  \"Insulation Coordination Within Low-Voltage Systems Including
    Clearances and Creepage Distances for Equipment.\" International
    Electrotechnical Commission Report 664. 1980.

10. Guide for Surge Withstand Capability (SWC) Tests. ANSI Standard
    C37.90a. 1974; IEEE Std. 472-1974.

11. J.E Lenz, \"Basic Impulse Insulation Levels of Mercury Lamp Ballast
    for Outdoor Applications.\" Illuminating Engrg.. 133-140 (February
    19641.

12. F.D. Martzloff and G.J. Hahn. \"Surge Voltage in Residential and
    Industrial Power Circuits.\" IEEE PAS 89 6. 1049-1056 (July/August
    1970).

13. R. Hasler and R. Lagadec. \"Digital Measurement of Fast Transients
    on Power Supply Lines.\" Proc. 3rd Symposium and Technical
    Exhibition on Electro-Magnetic Compatibility, Rotterdam. Holland.
    445-448 (May 1979).

14. Test Specifications for Gas Tube Surge Protective Devices. IEEE Std.
    C62.31-1982.

15. Test Specifications for Low-Voltage Air Gap Surge Protective
    Devices. IEEE Std C62.32. 1981.

16. Test Specifications for Varistor Surge Protective Devices. IEEE Std
    C62.33. 1982

#### FUNDAMENTALS OF PROTECTION

### INTRODUCTION

Protection of a power system, of a communication system, or of an
electronic black box against the threats of the surge environment can be
accomplished in different ways. There is no single truth or magic cure
ensuring immunity and success, but, rather, there are a number of valid
approaches that can be combined as necessary to achieve the goal. The
competent protection engineer can contribute his knowledge and
perception to the choice of approaches against a threat which is
imprecise and unpredictable, keeping in mind the balance between the
technical goal of maximum protection and the economic goal of realistic
protection at an acceptable cost. However, just as in the case of
accident insurance, the cost of the premium appears high before the
accident, not after.

A discussion of fundamental protection techniques that is limited in
time and scope has the risk of becoming an inventory of a bag of tricks;
yet, there are some fundamental principles and fundamental techniques
that can be useful in obtaining transient immunity, especially at the
design stages of an electronic system or circuit. All too often, the
need for protection becomes apparent at a late stage. when it is much
more difficult to apply the fundamental techniques which are most
effective and economical when implemented at the outset.

Our primary goals in this section, then, will be to alert the system and
circuit engineer to various aspects of protection and to present some
basic concepts of approach.

### BASIC TECHNIQUES

Protection techniques can be classified into several categories
according to the purpose and the system level at which the engineer is
working. For the system as a whole, protection is primarily a preventive
effort. One must consider the physical exposure to transients -- in
particular, the indirect effects of lightning resulting from building
design, location, physical spread, and coupling to other disturbance
sources -- as well as such inherent susceptibility characteristics as
frequency response and nominal voltage. A data processing system built
with low-voltage signals, high-impedance circuits, and installed over a
wide geographical area, such as a rail-road switching yard or a chemical
plant spread over thousands of meters. would present much more serious
problems than the same system confined in a single building.

For the system components or electronic black boxes, the environment is
often beyond the control of the designer or user, and protection becomes
a curative effort -- learning to live and survive in an environment
which is imposed. Quite often this effort is motivated by field
failures, and retrofit is needed. The techniques involved here tend to
be the application of protective devices to circuits or a search for
inherent immunity rather than the elimination of surges at their origin.

Another distinction can be made in classifying protective techniques.
Granted that surges will be unavoidable, one can attempt to block them,
divert them, or strive to withstand them: the latter, how-ever. is
generally difficult to achieve alone.

## SHIELDING. BONDING, AND GROUNDING

Shielding, bonding, and grounding are three interrelated methods for
protecting a circuit from external transients. Shielding consists in
enclosing the circuit wiring in a conductive enclosure, which in theory
cancels out any electromagnetic field inside the enclosure; actually, it
is more an attenuation than a cancellation. 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\" through various methods of driving conductors
into the soil. Each of these techniques has its limitations. and each
can sometimes be overem­phasized. One of Dr. Golde\'s favorite remarks
was that he could retire on a small percentage of the cost of the
useless copper buried in the ground to provide \"better grounding.\" We
shall now examine each of these techniques, pointing out some of their
limitations and some of the controversies concerning them.

### 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 and when the goals of
shielding for noise immunity conflict with the goals of shielding for
lightning surge immunity.

A shield can be the size of a matchbox or an airplane fuselage; it can
cover a few inches of wire, or kilometers of buried or over-head cables.
\"Grounding\" these diverse shields is not 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.

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. Sometimes the shield is used as a return path
for the circuit, in which case shield currents can cause voltage drops
added to the signal. But the fact is that, when surge currents flow near
the circuits, they will unavoidably inject magnetic flux variations into
the circuits, hence induced voltages. By deliberately allowing part of
these surge currents to flow in the shields, one obtains a cancellation
of the voltages that otherwise would be induced in the circuits.

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.\" However, in
many installations 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.

### 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 the current flow, with
the shield current now flowing in the pigtail and the creation of the
corresponding 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.

## Grounding

Grounding, which is also referred to as \"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 deter-mined by dc
measurements on rods driven into the ground. Driving rods into the
ground does not ensure a low impedance under the transient conditions of
high rate of current change associated with lightning discharges. This
remark is not intended as a criticism of the efforts going into
achieving a low resistance but rather, to alert the system designer that
there is more to it than just low resistance. and that one can overdo
the act of burying copper in the ground.

When one deals with a reasonably compact system, be it cabinet-sae.
room-size. or building-size. it is more effective to view the grounding
as a well-bonded connection to the outer shield (if any), building
frame, or cabinet enclosure. The resistance (imped­ance) from that
reference to \"earth\" is not very significant as long as other wires at
\"ground\" potential are not brought to the system. Since there is
little chance of dealing with an absolutely isolated system. the
question is: What should be done with incoming wires? These wires can be
isolated from the local ground during normal operation, but one must
recognize that during transient con­ditions of lightning surge or power
system faults, high voltages will appear across these isolated wires and
local ground. voltages which, in some cases. are totally 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 for the duration of the transient. This type of grounding
is one function of transient suppressors.

### Power System Grounding

In the context of grounding it is appropriate to mention the questions
raised by proponents of ungrounded and grounded power systems. Indeed, a
discussion of grounding practices would not be complete without
reference to these questions.

It has been a long-established practice to operate some three-phase
power systems without an intentional ground connection of box of the
circuit or to the building ground. The principle is simple: the shield
can be viewed as an extension of the box, and thus bonding ^.^of the
shield to the box should be continuous over 360 - degrees. In practice,
unless special connectors are used, this is difficult to achieve, for
often a shielded cable is terminated at a con­nection board with the
shield peeled back and turned into a pigtail. which in turn is connected
to the \"ground\" terminal of the connection board. One can imagine the
many possible variations of the neutral. The intent is to increase the
reliability of service continuity, in principle allowing the system to
continue operating with one ground fault as opposed to grounded systems,
where an outage will be the result of the first ground fault to occur.
Examples of such concerns are found in military power systems (\"ride
through the first shell\") and industrial low-voltage systems.

Closer analysis of the consequences of this choice indicates. however.
that the overall performance of the ungrounded systems may not
necessarily be improved over the safer, more predictable grounded
system. An excellent discussion of the advantages and disadvantages can
be found in IEEE Standard 142-1972. A detailed description is presented
of multiple faults to ground. arcing faults to ground (which can be
caused by sparkover after a transient overvoltage and result in massive
equipment damage), location of faults. personnel safety, performance
with overvoltages, and system costs.

The effects of these concepts on the roles performed by shielding,
bonding. and grounding can be summarized in a set of relatively simple
design guidelines, which are stated in the appendix.

## Isolation of Subsystems

In the case of systems involving separate buildings. remote sensors, or
the interconnection of a power system with a communication system, other
requirements may dictate the isolation of the sub-systems. creating the
illusion that protection against overvoltages has also been
accomplished. And yet we have seen that during transient conditions,
high voltages can occur.

Where moderately high voltages only can occur, effective isolation can
be accomplished by the insertion of isolating transformers or. when
metallic isolation is not required. by insertion of a filter.

Where the voltages will reach levels exceeding the withstand capability
of economically or technically feasible insulation, two possible
solutions exist. The first already mentioned, is to bond the two systems
during the transient by means of a nonlinear surge protective device,
which returns to a high level of insulation after the transient has
subsided. Another method and one which is becoming increasingly
attractive, is the insertion of a fiber optics link into a control or
data system. Complete decoupling of~.~ electrical transients and noise
can be achieved in this manner.

**CONCLUSIONS**

A number of fundamental protection techniques are available to limit the
penetration of lightning surges into a system. These are best
implemented at the early stages of design. They include the provision of
points where a lightning stroke can attach and be diverted without harm
to earth, shielding of circuits, bonding of enclosures, and grounding of
the shields and reference points.

It is a dangerous illusion to believe that lightning effects can be
eliminated by the isolation of conductors or subsystems. It is much
safer and quite acceptable, if included in the design, to provide
bonding during transient conditions by suitable protective devices. The
important point to remember is that lightning is a fairly well-defined
phenomenon. with known characteristics and effects in general, but its
probability of occurrence at a particular location is unknown. For the
successful operation of a system, foresight is needed in applying
fundamental protection techniques at the beginning.

## FOR FURTHER READING

Boehne, E.W., and R.H. Golde, eds., \"Research on the Lightning
Phenomenon,\" Journal of the Franklin Institute 283 (6). June 1967, pp.
451-478.

Fisher, FA.. and JA. Plumer, Lightning Protection of Aircraft NASA RP
1008, National Aeronautics and Space Administration, U.S. Government
Printing Office. Washington. DC 20402. 1977.

Gary. C.. \"Les Principes Physiques Guidant la Protection des Batiments
Contre la Foudre,\" Revue Generale de L\'Electricite 89, May 1980. pp.
351-367.

Golde. R.H., ed.. Lightning. 2 vols.. Academic Press. London and New
York, 1977.

Greenwood, A., Electrical Transients in Power Systems, Wiley
Inter-science, New York, London, 1971.

IEEE Recommended Practice for Grounding of Industrial and Commercial
Power Systems. IEEE Standard 142-1972, 1971.

Lee. R.H., \"Lightning Protection of Buildings.\" IEEE Transactions on
Industry Applications 1A-15, May/June 1979, pp. 236-240.

Shelkunoff, SA~\"~ \"Electromagnetic Theory of Coaxial Transmission
Lines and Cylindrical Shields.\" Bell System Technical Journal 13. 1934,
pp. 532-579.

\"The Protection of Telecommunication Lines and Equipment Against
Lightning Discharges,\" Comite Consultatif International de Telegraphe
at Telephone, International Telecommunication Union. Geneva.
Switzerland, 1974.

Uman, M.A.. Understanding Lightning. Bek Technical Publications,
Carnegie. PA 15106, 1971.

Velazquez. R.S. V. Gerez. D. Mukhedar, and Y. Gervais. \"Probabilistic
Calculations of Lightning Protection for Tall Buildings.\" IEEE
Transactions on industry Applications 1A-18 (3), May/June 1982. pp.
252-259.

**Appendix**

**BASIC CONSIDERATIONS ON SURGE CONTROL FOR ELECTRONIC SYSTEMS**\
(From Surge Protection of Electronics, University of Wisconsin. March
1979, Conference Notes)\
Frank A. Fisher. General Electric Company

### PROTECTION FROM THE DIRECT EFFECTS OF LIGHTNING

1.  Try to divert the stroke away from the system. the farther the
    better.

2.  If the stroke cannot be diverted away completely, it must be carried
    to ground along a path where it does the least damage. This
    basically means providing a system of lightning rods and lightning
    conductors to a low-resistance ground.

3.  Reduce the resistance along the current-carrying path as much as
    feasible. This is particularly true of the ground resistance where
    the lightning conductor is grounded.

4.  In buildings housing electronic equipment, establish a uniform
    potential ground plane over as much of the building as possible. In
    new buildings, all reinforcing steel in concrete and structural
    steel members should be bonded together and con­nected to ground
    rods. These should be located around the periphery of the building.
    Bonding should be done at many points. All water pipes and utility
    conduits should be bonded to this ground system where they enter and
    at frequent intervals within the building.

5.  Insofar as feasible, the building should be arranged to form a
    grounded metal enclosure (Faraday cage). External magnetic and
    electric fields will not penetrate into a perfect Faraday cage.

6.  Power systems should be protected with commercially available surge
    arresters.

7.  Avoid the use of ground systems for electronic equipment that are
    isolated from building or power system grounds. This means using a
    multiple ground system rather than a single-point ground system.

8.  Connect the cases of all electronic equipment to the nearest
    building ground point. This ensures that, at a minimum, the cases
    will not assume a high potential under lightning flash conditions,
    relative to the surrounding structure, and so will not present an
    electrical hazard to people operating the equipment. Ground leads
    should be as short and direct as possible, possessing a minimum of
    resistance and (especially) inductance.

9.  All wiring between different locations in the system should be
    carried in shielded cables, with shields grounded at both ends.

10. Electronic equipment should be designed to withstand surge voltages.
    Surges can be carried into such equipment on input and output leads
    and on power supply leads, and consideration should be given to the
    use of protective devices on these cir­cuits.

### PROTECTION COMPATIBLE WITH NOISE ATTENUATION DESIGN

An approach which has been used successfully to provide lightning
protection of communication systems without compromising the systems\'
steady-state noise performance is shown in Figure A-1. The important
elements of the system are as follows:

1.  Electrical cables carrying signals between sensitive or critical
    apparatus in separated locations must have an overall shield. This
    shield must be continuous and must be grounded at each end (A and B)
    to the building ground systems.

2.  The individual cables within this bundle should be shielded, but
    this is not an absolute requirement.

3.  The shields on these individual cables should be grounded at each
    end (C and D). but may. at the circuit designees discretion. be
    grounded at only one end (E and F) if such a practice is preferable
    for the control of steady-state noise. As long as the designer
    leaves the overall shield alone, he can do whatever he wishes with
    any internal shields.

4.  The electronic equipment to which the cables connect should have the
    housings (G and H) connected to the building ground system (I).

5.  A ground bus within the individual pieces of electronic equipment,
    but isolated from the equipment case, is often desirable. If such
    ground busses are provided (J and K). it is preferable from the
    viewpoint of lightning protection that they be connected to the
    building ground system at the point of entry (L) of the building
    ground system.

6.  If direct connection of the ground systems cannot be tolerated, a
    separate electronic ground (M) can be provided. A spark gap or other
    voltage-limiting device (N) should then be provided to limit the
    surge voltages that can be developed between the two ground systems.

7.  If the electronic ground system is very extensive, it may be
    desirable to provide other voltage-limiting elements (0) between the
    electronic ground system and the building ground system.

8.  Electronic cables are generally carried between locations in cable
    trays. These trays, although not shown on the figure. should be
    connected to the building ground system at each end. The cable trays
    should be electrically continuous over their entire length. Cable
    trays, if used, do not eliminate the need for an overall shield on
    the cables within the tray.

9.  If cables are carried between two locations in electrically
    continuous metallic conduits, such conduits being connected to the
    building ground system at each end, the overall shield on the cables
    may be eliminated, since the conduits take the place of the shield.
    Cables carried in nonmetallic conduits, however, must have the
    overall shield.

#### TRANSIENT SUPPRESSORS

**INTRODUCTION**

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
can-not really suppress transients; rather, they limit transients to
accept-able levels or make ethem harmless by diverting them to ground.

There are two categories of transient suppressors: those that block
transients, preventing their propagation toward sensitive circuits. and
those that divert transients, limiting residual voltages. Since many of
the transients originate from a current source, the blocking of a
transient may not always be possible; the diverting of the transient is
more likely to find general application. As we shall see in the section
on coordination, 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 -- 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 1). 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 non-linearity (as in filters) or, more usually, voltage
nonlinearity. This voltage nonlinearity is the result of two different
mechanisms -- a continuous change in the device conductivity as current
increases. or an abrupt switching as voltage increases.

Because the technical and trade literature contains many articles on
these devices, we shall limit the discussion of the details and refer
the reader to the bibliography at the end of this section. We shall,
however, make some comparisons to point out the significant differences
in performance.

### CROWBAR DEVICES

The principle of crowbar devices is quite simple: upon occurrence of an
overvoltage. the device changes from a high-impedance state to a
low-impedance state. offering a low-impedance path to divert the surge
to ground. This switching can be inherent to the device, as in the case
of spark gaps involving the breakdown of a gas. Some applications have
also been made of triggered devices. such as triggered vacuum gaps in
high voltage technology or thyristors in low-voltage circuits where a
control circuit senses the rising voltage and turns on the power-rated
device to divert the surge.

The major advantage of the crowbar device is that its low impedance
allows the flow of substantial surge currents without the development of
high energy within the device itself; the energy has to be spent
elsewhere in the circuit. This \"reflection\" of the impinging surge can
also be a disadvantage in some circuits when the transient disturbance
associated with the gap firing is being con­sidered. Where there is no
problem of power-follow (discussed below), such as in some communication
circuits. the spark gap has the advantage of very simple construction
with potentially low cost.

The crowbar device, however, has three major limitations. One is the
volt-time sensitivity of the breakdown process. As the voltage increases
across a spark gap, 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. since the process is statistical in nature. This
sparkover voltage. in addition. 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. How-ever, if the enclosure seal is lost and
the gas is replaced by air, this substitution creates a reliability
problem because the sparkover of the gap is then substantially higher.

The second limitation is associated with the sharpness of the sparkover,
which produces fast current rises in the circuits and. thus,
objectionable noise. A classic example is found in oscillograms
recording the sparkover of a gap where the trace exhibits an anomaly
before the sparkover (Figure 2). This anomaly is due to the delay
introduced in the oscilloscope circuits to provide an advanced trigger
of the sweep. What the trace shows is the events delayed by a few
nanoseconds, so that in real time, the gap spark-over occurs while the
trace is still writing the pre-sparkover rise.

Another, more objectionable effect of this fast current change can be
found in some hybrid protective systems. Figure 3 shows the circuit of
such a device, as found in the commerce. The gap does a very nice job of
discharging the impinging high energy surges, but the magnetic field
associated with the high di/dt induces a voltage in the loop adjacent to
the secondary suppressor, adding what can be a substantial spike to the
expected secondary clamping voltage.

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 veristor has been very successful in the
utility industry as a surge arrester or surge diverter. The principles
and applications of these devices will be examined in some detail in
later paragraphs.

## VOLTAGE-CLAMPING DEVICES

Voltage-clamping devices have variable R. impedance, depending on the
current flowing through the device or the voltage across its terminal.
These components show a nonlinear characteristic --- that is, Ohm\'s law
can be applied, but the equation has a variable. Impedance variation is
monotonic and does not contain discontinui­ties, in contrast to the
crowbar device. which shows a turn-on action. As far as their
volt-ampere characteristics are concerned these components are
time-dependent to a certain degree. However, unlike the sparkover of a
gap or the triggering of a thyristor. time delay is not involved.

When a voltage-clamping device is installed, the circuit remains
essentially unaffected by the device before and after the transient for
any steady-state voltage below clamping level. Increased current drawn
through the device as the surge voltage attempts to rise results in
voltage-clamping action. Nonlinear impedance is the result if this
current rise is greater than the voltage increase. The increased voltage
drop (IR) in the source impedance due to higher current results in the
apparent clamping of the voltage. It should be emphasized that the
device depends on the source impedance to produce the clamping. A
voltage divider action is at work where one sees the ratio of the
divider as not constant but changing. If the source impedance is very
low, the ratio is low, and eventually the suppressor could not work at
ail with a zero source impedance (Figure 4). In contrast a crowbar-type
device effectively short circuits the transient to ground, but once
established, this short circuit will continue until the current (the
surge current as well as any power-follow current supplied by the power
system) is brought to a low level.

The principle of voltage clamping can be achieved with any device
exhibiting this nonlinear impedance. Two categories of devices, having
the same effect but operating on very different physical processes, have
found acceptance in the industry: the polycrystalline varistors and the
single-junction avalanche diodes. Another technology, the selenium
rectifier, has been practically eliminated from the field because of the
improved characteristics of modern varistors.

**AVALANCHE DIODES**

Avalanche diodes, the Zener diodes, were initially applied as voltage
clamps, a natural outgrowth of their application as voltage regulators.
Improved construction. specifically aimed at surge absorption, has made
these diodes very effective suppressors. Large-diameter junctions and
low thermal impedance connections are used to deal with the inherent
problem of dissipating the heat of the surge in a very thin single-layer
junction.

The advantage of the avalanche diode, generally a P-N silicon junction.
Is the possibility of achieving low clamping voltage and a nearly flat
volt-ampere characteristic over its useful power range. Therefore, these
diodes are widely used in low-voltage electronic circuits for the
protection of 5 or 15 V logic circuits. for instance. For higher
voltages, the heat generation problem associated with single junctions
can be overcome by stacking a number of lower voltage junctions,
admittedly at some extra cost.

### Characteristics of Avalanche Diodes

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!

Figure 5 shows a typical family of V-1 characteristics for one product
The parameter / is the peak of a specified current waveform, generally
the 10/1000 μs double exponential preferred by communication engineers
in contrast to the 8/20 μs wave preferred by power engineers. These
curves are remarkably flat from 1 to 20 amperes, with clamping voltages
low enough to protect sensitive electronics.

Manufacturers of avalanche diodes generally rate their devices in terms
of maximum peek power for a specified pulse duration. Although the power
decreases with increasing pulse duration. this does not occur at
constant energy (Figure W. A two-decade increase in pulse duration
reduces the power by only about one decade. primarily because of
increasing heat transfer for longer pulses.

Since the junction is very thin, the capacitance of an avalanche diode
is appreciable. This can be a concern. It is possible to minimize this
effect by using series combinations with low capacitance diodes (Figure
7).

Properly packaged and wired avalanche diodes exhibit a quick response to
steep, front pulses, and have been widely used for NEMP protection of
electronic equipment However, this quick response can be completely
obliterated by improper wiring (lead length). The effect of lead length,
discussed in detail in the next section. is applicable to any transient
suppressor.

## VARISTORS

The term varistor is derived from its function as a variable resistor.
It is also called a voltage-dependent resistor, but that description
tends to imply that the voltage is the independent parameter in surge
protection. This concept will be contested repeatedly during the
conference. Two very different devices have been successfully developed
as varistors: silicon carbide disks have been used for years in the
surge arrester industry, and, more recently, metal oxide varistor
technology has come of age.

In silicon carbide varistors. as well as in metal oxide varistors. the
relationship between the current flowing in the device and the voltage
appearing across its terminals can be represented approximately by a
power function I= kV^a^, where the higher the value of a. the more
effective the clamping. Hence, there has been a race between
manufacturers and specification writers for higher and higher values of
a. We will see, however, that there are practical limits to this race
and that in fact better performance can be obtained at higher current
densities by departing somewhat from the large values of the exponent a.

In silicon carbide varistors, the physical process of nonlinear
conduction is not completely understood, and the manufacturing of the
material, successful as it is. has remained an art. It appears that the
process takes place at the tips of the grains of silicon car-bide which
are held together by a binder. The story goes that the device action was
found accidentally by having a grinding wheel, on a disorderly work
bench, accidentally connected to an experimental circuit: for many years
silicon carbide varistors indeed looked like grinding wheels. each
complete with a hole in the center.

Metal oxide varistors depend on the conduction process occur-ring at the
boundaries between the large grains of oxide (typically zinc oxide)
grown in a carefully controlled sintering process. The physics of the
nonlinear conduction mechanism have been described in the
Iiterature;^(1-10)^ in these application notes, we will be more
concerned with the behavior of the varistors as a two-terminal
electrical component.

### Electrical Characteristics of Varistors

Because the prime function of a varistor is to provide the non-linear
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 of the equivalent circuit of Figure 8. The
major element is the varistor proper. R~v~,. whose V-I characteristic is
assumed to be the perfect power law I=kV^a^. In parallel with this
varistor, there is a capacitor. C. and a leakage resistance. R~p~. In
series with this three-component group, there is the bulk resistance of
the zinc oxide grains. R~s~. and the inductance of the leads. L.

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 law
impedance to the flow of current. but eventually the series resistance
will produce an upturn in the V -I characteristic; the lead inductance
can give rise to spurious overshoot problems if it is not dealt with
properly; the capacitance can offer either a welcome additional path
with fast transients or an objectionable loading at high frequency,
depending on the application.

V-I Characteristic. When the V-1 characteristic is plotted on a log-log
graph. the curve of Figure 9 is obtained, with three regions as shown,
resulting from the dominance of R~0~, Rv, Rs as the current in the
device goes from nanoamperes to kiloamperes.

The V -I 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, which are discussed in
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

```{=html}
<!-- -->
```
-   Selection of energy-handling capability (including consideration of
    the source impedance of the transient. the waveshape. and the number
    of occurrences)

```{=html}
<!-- -->
```
-   Heat dissipation

```{=html}
<!-- -->
```
-   Proper installation in the circuit (lead length).

In fact enough instances of poor installation practices have been
observed and enough questions have been raised on alleged \"overshoot\"
that a brief discussion of lead effects is in order.

Overshoot: A Lead Effect. To illustrate the effect of lead length on the
overshoot two measurement arrangements were used. As shown in Figures
10(a) and 10(b). respectively. 0.5 cm^2^ and 22 cm^2^ of area were
enclosed by the leads of the varistor and of the volt-age probe.

The corresponding voltage measurements are shown in the oscillograms of
Figures 10(c) and 10(d). With a slow current front of 8uus. there is
little difference in the voltages occurring with a small or large loop
area, even with a peak current of 2.7 kA. With the steep front of 0.5
As. the peak voltage recorded with the large loop is nearly twice the
voltage of the small loop. Note in Figure 10(d). that at the current
peak. L di/dt = 0, and the two volt-age readings are equal: before the
peak. L di/dt is positive, and after. it is negative.

## PROBE CONNECTIONS

When making voltage measurements across a clamping device. e.g.. for
evaluating its performance, one must recognize possible difficulties
requiring special precautions. Two precautions must be taken:

1.  Use two probes in a differential mode to make a measurement directly
    at device terminals.

2.  Avoid contaminating the true device voltage by the additional
    voltage caused by magnetic coupling.

Commercial oscilloscope preamplifiers offer a wide choice of
differential mode operation, either through an \[add + invert one\] mode
of two-channel preamplifiers. or through a differential amplifier built
specifically for high common-mode rejection, sometimes at the expense of
bandwidth. Thus, careful attention must be given to this aspect of
measurements.

The voltage measured by the two probes is the sum of the actual clamping
voltage existing across the device and a spurious voltage caused by
magnetic coupling. This spurious voltage is induced into the loop formed
by the clamping device length and the two probes by the changing
magnetic field of the current flowing in the device.

To further illustrate this situation, the measurement circuit shown in
Figure 11 was set up in the output circuit of a generator producing a
8/20 uus 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 also soldered. brought to the mid-point of the hollow
conductor and in close contact with the conductor; from the midpoint
outward. they were twisted in the same manner as the inside pair. A
third set of wires were 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 13
shows the measured voltage versus radial distance of the opposite side
of the rectangle, plotted from the oscillograms of Figure 12.

This example shows that not only one must 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 this 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 example 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 1uus.

**COMPARISONS OF PROTECTIVE DEVICES**

**Linear Versus Nonlinear Devices**

When a protection scheme is designed for an electronic system operating
in an environment which is not completely defined, it is often necessary
to make an assumption about the parameters of the transients expected to
occur. In particular, if an error is made in assuming the source
impedance of the transient, the consequences are dramatically different
between a linear protective device and a nonlinear protective device, as
illustrated in a simplified comparison of the two suppressor devices
(see box).

### Spark Gap Versus Varistor

The choice between these two devices will be influenced by the inherent
characteristics of the application. Where power-follow is a problem,
there is little opportunity to apply a simple gap. Where very steep
front transients occur, the gap alone may let an excessive voltage go by
the \"protected\" circuit until the voltage is limited by sparkover.
Where the capacitance of a varistor is objectionable, the low inherent
capacitance of a gap seems attractive. If very high energy levels,
compared to the lower levels inherent with the crowbar action of a gap,
can be deposited in a varistor, then a high capacity surge arrester near
the service entrance may be combined with a lower clamping voltage
varistor installed farther into the circuit. This combined protection,
however, requires adequate coordination between the two suppressors,

Avalanche Diode Versus Varistor

The basic performance characteristics of these two devices are similar,
and therefore the choice may be dictated by clamping volt-age
requirements (the avalanche diode is available at lower clamping
voltages), by energy-handling capabilities (the avalanche diode is
generally lower in capability per unit of cost), and by packaging
requirements (the varistor material is more flexible and does not
require hermetic packaging).

### Conventional Surge Arresters Versus Gapless Arresters

Surge arresters (diverters) have reached a high degree of sophistication
over the years by using precision gaps in series with silicon carbide
varistors. For high-voltage applications, the arrester is made of a
stack of modules, generally of 3 or 6 kV each. With the use of
current-limiting gaps, the clearing of power-follow current in dc
application was made possible. A series gap was required in all these
arresters using silicon carbide because, for the specified discharge
voltage and discharge current, the standby current at the normal voltage
would be excessive.

With the advent of metal oxide varistors, the high exponent of the V-I
characteristic reduced the standby current to a very low level, one that
can be tolerated by the. varistor under steady-state conditions. The
series gap can thus be eliminated, thereby producing three considerable
improvements:

-   Performance (elimination of abrupt sparkover)

-   Reliability (elimination not only of the gap and all trigger
    circuitry but also of the parallel voltage-grading varistors)

-   Contamination withstand (elimination of effects of leakage current
    on the outer shell).

Previously, where low-voltage secondary arresters using gaps were the
only devices capable of meeting the ANSI requirement of a 10 kA. 8/20 μs
discharge current. high-energy varistors are now capable of meeting this
requirement, without the problems associated with series gaps.

**MATCHING VARISTOR CAPABILITY TO THE ENVIRONMENT**

Manufacturers of metal oxide varistors publish ratings which include a
family of curves. generally described as \"Pulse Lifetime,\" that show
the number of surges which a given varistor can absorb before some
arbitrary limit of characteristic shift is reached. These curves show
numbers of surges of equal amplitudes for a set of waveshapes. The
actual occurrence of surges, however, is a full range of values, from
the lowest to the highest, and also of different waveshapes.

Attempts to simplify the description of surge occurrences have resulted
in the statistical information of IEEE Std 587-1980, where a family of
curves has been obtained from actual surge recordings; these curves show
the frequency of occurrence of surges as a function of the voltage surge
level, for various exposures (Figure 14). Carroll first developed a
method^(12)^ whereby the statistical information of IEEE 587 can be
combined to predict the effect of the range of surge, as opposed to
constant amplitude surges, on the varistor Pulse Lifetime. Korn has
shown^(13)^ how a simple set of assumptions and computations based on
Figure 14 and Pulse Lifetime curves can provide an estimate of the time
required to reach the varistor rated Pulse Lifetime. The last reprint.
\"Matching Surge Protective Devices to Their Environment,\" provides a
detailed explanation of the method, with two specific examples.

## Failure Modes

Failure of an electrical component can occur either because its
capability was exceeded by the applied stress or because some latent
defect in the component went by 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 out of operation. To other users, 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
energy-handling capability of a candidate protective device is an
important parameter to consider in the designing of a protection scheme.
With nonlinear devices, an error made in the assumed value of the
current surge produces little error on the voltage developed across the
protective device and thus applied to the protected circuit, but the
error is directly reflected in the amount of energy which the protective
device has to absorb. At worst, when surge currents in excess of the
protective device capability are imposed by the environment. such as an
error made in the assumption, human error in the use of the device, or
because nature tends to support Murphy\'s law, the circuit in need of
protection can generally be protected at the price of failure in the
short-circuit mode of the protective device. However, if substantial
power-frequency 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).

When there is a need to provide elimination of a failed protector at the
specific equipment level, insertion of the fuse in the line provides
protection of the equipment (Figure 15a). while insertion of the fuse in
series with the shunt-connected protector provides protection of the
function. Albeit with loss of overvoltage protection (Figure 15b).

## CONCLUSIONS

Surge protective devices are available for protecting low-voltage
electronics. Two basic types offer different characteristics: crowbar
devices have high-current capability but generally involve power-follow
when applied on a power system; voltage clamping devices, either silicon
avalanche or varistors. are free from the power-follow problem.

Avalanche diodes offer low clamping voltage, which makes them most
suitable for low-voltage. low-power electronics. Metal oxide varistors
are now available in a wide range of clamping voltages and
energy-handling capacities. Each of these devices has its own best field
of application, insuring greater reliability of the circuits in the
not-quite-defined electromagnetic environment of power and communication
systems.

**FOR FURTHER READING**

Barksdale, C.B., \"New Gas Tube Protector Technique for Circuits,\"
Telephony 171, 1970. pp. 21-28.

Bazarian. A., When Should You Use Gas-Discharge Protectors?\" Electronic
Design News. December 20. 1973, pp. 58-62.

Clark. O.M., and J.J. Pizzicaroli. \"Effect of Lead Wire Lengths on
Protector Clamping Voltages.^\"^ Federal Aviation Administration-Florida
Institute of Technology Workshop on Grounding and Lightning Technology.
FAA-RD-79-6, March 6-8. 1979. Melbourne. Florida, U.S. Dept. of
Transportation. Washington, DC 20590.

Kotheimer. W.C.. \"Control Circuit Transients.\" Power Engineering 73.
1969.

Richman. P., \"Diagnostic Surge Testing. Part I.\" Solid State Power
Conversion. September/October 1979.

Smith. M.N.. Practical Application and Effectiveness of Commercially
Available Pulse Volage Transient Suppressors, Naval Civil Engineering
Laboratory. Port Hueneme. CA. 1973.

\"Surge Voltage Protectors --- Application Note and Suggested
Applications,\" SEPT. The Journal of the Society of Electronic and Radio
Technicians 9. (4). April 1975. pp. 85-91.

Vodicka. V.W., \"Voltage Transients Tamed by Spark-Gap Arresters.\"
Electronics 39, 1966. Pp. 109-113.

### 

### REFERENCES

1.  Harden. J.D.. F.D. Martzloff, W.G. Morris, and F.B. Golden. \"The
    GE-MOV Varistor --- The Super Alpha Varistor.\" Electronics 45
    (21), 1972. ^p^. 91.

2.  Levinson. LM, and H.R. Philipp. Low Temperature AC Properties of
    Metal-Oxide Varistors. 78CRD102. General Electric Company.
    Schenectady, NY, 1978.

3.  Mahan. G.D.. LM. Levinson. and H.R. Philipp. \"Theory of Conduction
    in ZnO Varistors,\" 78CR0205, General Electric Company, Schenectady.
    NY. 1978.

4.  Matsuoka. M. T. Mesa Yama, and Y. Lida, \"Nonlinear Electrical
    Properties of Zinc Oxide Ceramics.\" Proc. of First Conf. Solid
    State Devices, Tokyo. 1969, J. Japan Soc. App!. Phys. 39, 1970.
    Suppl. p. 94.

5.  Matsuoka. M., \"Non-Ohmic Properties of Zinc Oxide Ceramics.\"
    Jap. J. Appl. Phys. 10. 1971. p. 736.

6.  Morris. W.G. and F.D. Martzloff, \"VARPAR --- A Program for
    Calculating Parallel Varistor Characteristics.\" 74CR0278, General
    Electric Company. Schenectady, NY. 1974.

7\. Philipp. H.R., and L.M. Levinson,. \"High-Temperature Electrical
Studies on Metal Oxide Varistors.\" 78CR0131, General Electric Company.
Schenectady. NY. 1978.

8.  Philipp, H.R. and L.M. Levinson. \"Long Time Polarization Currents
    in Metal Oxide Varistors.\" 76CRD026. General Electric Company.
    Schenectady. NY. 1976.

9.  Sakshaug, EC. J.S. Kresge. and S.A. Miske. \"A New Concept in
    Station Arrester Design,\" \"IEEE PAS-96, (2). March/April 1977, pp.
    647-656.

10. Transient Voltage Suppression Manual. Third Edition. General
    Electric Company, Auburn, NY. 1982.

11. \"Test Specifications for Varistor Surge-Protective Devices.\" IEEE
    Std C62.33. 1982.

12. Carroll, E. \"Transient Attenuation with Metal Oxide Varistors for
    AC Mains Powered Equipments.\" presented at \"Comet.\" University of
    Twente. Holland. November 1980.

13. Korn, S. \"Transients and Transient Suppressors For Power Conversion
    Equipment.\" INTELEC Conference. Washington. D.C., 1982.

+-----------------------------------------------------------------------+
| ## François Martzloff                                                 |
+-----------------------------------------------------------------------+
| ######## END OF FILE "Text Lectures"                                  |
+-----------------------------------------------------------------------+
| ## April 2004                                                         |
+-----------------------------------------------------------------------+