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| # Last | # Surge Protection Anthology                      | # Su   |
|  updat |                                                   | rges H |
| ed Jun | **Part 6 -- Tutorials, Textbooks, and Reviews**   | appen! |
| e 2004 |                                                   |        |
|        | **Text of "System Protection Techniques" files**  |        |
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FOREWORD

This file contains the text part from eight papers on the following
subjects:

-   Surge protection techniques in low-voltage AC power systems (1979)

-   The coordination of transient protection for solid-state power
    conversion equipment (1982)

-   Lightning protection of roof-mounted solar cells (1983)

-   The protection of industrial electronics and equipment against power
    and data line disturbances (1984)

-   The protection of computer and electronic systems against power
    supply and data lines disturbances (1985)

-   Lightning and surge protection of photovoltaic installations (1989)

-   Protecting computer systems against power transients (1990)

-   Update on a consumer-oriented guide for surge protection (1999)

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="0.9576388888888889in"
height="0.4666666666666667in"}

Surge Protection Techniques

**in Low-Voltage AC Power Systems**

F. D. Martzloff

Corporate Research and Development

General Electric Company

Abstract

Designers involved in the ac power side of telecommunications equipment
have been justifiably concerned with surge protection because field
experience is rich in case histories of failures attributable to
transient overvoltages. Insufficient knowledge of the exact nature of
these overvoltages, however, has made their task difficult in the past.

After several years of data collection by a number of organizations, a
more definitive understanding of the surge environment is emerging. The
next few years\' publications from the IEEE, the IEC, NEMA, and other
interested groups will document that understanding. This paper presents
an overview of the results of data collection and environment
descriptions from the point of view of telecommunications power supply
problems, as well as a review of applicable techniques and devices.

From the early days of the introduction of semiconductors, voltage
surges have been blamed for device failures and system malfunctions.
Silicon semiconductors are, indeed, sensitive to overvoltages, more so
than their predecessors, such as the obsolete copper oxide or selenium
rectifiers. From an early period of frustration and poor knowledge of
the actual environment, progress has been made both in the area of
defining the environment and of providing new surge protective devices
and techniques to deal effectively with the problem.

Recent progress in the technology of transient voltage suppressors has
opened new opportunities to improve the level of protection of
semiconductors exposed to power system transients. In the past, direct
exposure to outdoor system surges required surge arresters with high
energy capability to survive the discharge currents associated with
direct or indirect lightning effects, at the cost of voltage-clamping
levels that were too high to protect sensitive semiconductors. The
approach at that time was a coordinated combination of arresters and
low-voltage suppressors, an approach that is still valid in many cases.
It is now possible, however, to apply a single suppressor, with
sufficient capability to withstand outdoor surges while clamping at a
level low enough to protect power semiconductors such .as power supply
rectifiers. Examples of coordinated protection as well as the
application of high power surge suppression devices, with experimental
verification of performance, will be given in the paper.

# THE ORIGIN OF SURGE VOLTAGES

Two major causes of surge voltages have long 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 tan
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 over
voltages (more than two or three per unit), but they may be very
difficult to suppress because the energies are high. 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.

# Lightning-Induced Surges

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.

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 (1), showing the number of storm days per year, is still the most
widely used description of the occurrence distribution (2).

# Switching Surges

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 (3) include circuit closing transients
produced when the two circuits on either side of the switch being opened
oscillate at different frequencies. In circuits having inductance and
capacitance (all physical circuits have at least some in the form of
stray capacitance and inductance) with little damping, these simple
switching transients are inherently limited to twice the peak amplitude
of the steady-state sinusoidal voltage. Another limit to remember in
analyzing transients associated with current interruption (circuit
opening) is that the circuit inductance tends to maintain the current
constant. At most, then, a surge protective device provided to divert
the current will be exposed to that initial current.

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

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

In response to these concerns, various committees and working groups
have attempted to describe ranges of transient occurrences or maximum
values occurring in power circuits. These transients include both surge
voltages and surge currents, although the primary emphasis is generally
given to surge voltages.

# EXISTING AND PROPOSED STANDARDS 

# ON TRANSIENT OVER VOLTAGES

Several Standards or Guides have been issued or proposed - in Europe by
VDE, IEC, CECC, Pro-Electron, and CCITT; in the USA by IEEE, NEMA, UL,
REA, FCC, and the Military - specifying a surge withstand capability for
specific equipment or devices and specific conditions of transients in
power or communication systems. Some of these specifications represent
early attempts to recognize and deal with the problem in spite of
insufficient 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 (4). The document is now
being reviewed by the IEEE Standards Board for eventual publication as a
standard. For some time now, a document prepared by a Relaying Committee
of IEEE under the title "Surge Withstand Capability" has been available
(5). The FCC has also published regulations concerning equipment
interfacing the communications and power systems (6).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 (7). These documents
will be reviewed in the pages that follow. Greatest emphasis, however,
will be placed on the IEEE document because it describes the transient
environment; the others 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 significant innovations in surge 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.

## Federal Communications Commission Requirements

The Federal Communications Commission (FCC) has issued regulations
describing tests to be applied to equipment interfacing the power
distribution system and the communication system. The intent of these
tests is protection of the equipment itself as well as protection of the
communications plant from surges originating on the ac power side of the
equipment. This concern is especially motivated by the recent
proliferation of terminal equipment being installed by telephone service
subscribers.

The most exacting test specified by these regulations is the application
on the ac side of equipment to be connected to the telephone system of a
1 x 10 μs impulse superimposed to the 60 Hz line voltage. The crest of
this voltage impulse is 2.5 kV, and the short-circuit capability of the
impulse source must be no less than 1 kA. This requirement of a
substantial short-circuit capability reflects the perceptions of
contributors to the regulation-making process that such surge currents
may occur in the real world, or it may express a wish to produce in the
laboratory a detectable burn-in of the fault following sparkover during
the application of the surge. Records on the background of this
regulation available to the author are not specific on which of the two
concerns was primary in the specification of such a high short-circuit
capability.

# The IEC SC/28A Report on Clearances

The Insulation Coordination Committee of the International
Electrotechnical Commission, following a comprehensive study of
breakdown characteristics in air gaps, included in its report a table
indicating the voltages that equipment must be capable of withstanding
in various system voltages and installation categories (Table 1).

The table specifies that it is applicable to a "controlled voltage
situation," which phrase implies that some surge-limiting device will
have been provided---presumably a typical surge arrester with
characteristics matching the system voltage in each case. The waveshape
specified for these voltages is the 1.2 x 50 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
farther removed from the outdoor environment. Thus, this document
addresses primarily the concerns of insulation coordination, and the
specification it implies for the environment is more the result of
efforts toward coordinating levels than efforts to describe the
environment and the occurrence of transients. The latter approach has
been that of the IEEE Working Group on Surge Voltages in Low-Voltage ac
Power Circuits, which we shall now review in some detail.

**The IEEE Working Group Proposal**

# *Voltages and Rates of Occurrence*

Data collected from a number of sources let 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 clearances.

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 is should not be applied indiscriminately to all
systems. Such application would penalize the vast majority of
installations, where the exposure is lower.

# *Waveshape of the Surges*

Many independent observations (8, 9, 10) have established that the most
frequent type of surge voltages 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. 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 2).

# *Energy and Source Impedance*

The energy involved in the interaction of a power system with a surge
source and a surge protective device will divide between the source and
the protective device in accordance with the characteristics of the two
impedances.

Unfortunately, not enough data have been collected on what value should
be assumed for the source impedance of the surge. Standards and
recommendations, .such as MIL STD-1399 or the IEC SC/28A Report, either
ignore the issue or indicate values applicable to limited cases, such as
the SWC test for high-voltage substation equipment. The IEEE 587.1
document attempts to relate impedance to categories of locations but
unavoidably remains vague on their definitions (Table II).

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 3),
will outline the method for assessing the need for surge protection and
selecting the appropriate device or system. Parallel work in other IEEE
working groups preparing test specification standards (11) 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.

# SURGE PROTECTIVE DEVICES

Various devices have been developed for protecting electrical and
electronic equipment against surge voltages. 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 surge voltages to
acceptable levels or make them harmless by diverting the surge current
to ground.

There are two categories of surge protective devices: those that block
the surge voltages, preventing their propagation toward sensitive
circuits, and those that divert surge currents, limiting residual
voltages. Since some of the surges originate from a current source, the
blocking of a surge voltage may not always be possible; the diverting of
the surge current is more likely to find general application. A
combination of diverting and blocking can be a very effective approach:
a first device diverts the surge current toward ground, a second
device - impedance or resistance - offers a restricted path to the surge
propagation but an acceptable path to the signal or power, and a third
device clamps the residual transient overvoltage. 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.

# 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 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 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.

The most significant limitation to crowbar applications in power systems
is the inability of the device to clear the circuit from the
power-follow current supplied by the power system after the sparkover of
the gap or turn-on of the device. Without some additional device to
limit the power-follow current, a crowbar is generally not acceptable in
a power system. On the other hand, a crowbar combined with a power
frequency current-limiting varistor - the conventional surge arrester -
has long been the most widely used protective device in power system.

# Voltage-Clamping Devices

Voltage-clamping devices have variable impedance, depending on the
current flowing through the device or the voltage across its terminal.
These components show a nonlinear characteristic - that is, Ohm\'s law
can be applied, but the equation has a variable R. Impedance variation
is monotonic and does not contain discontinuities, in contrast to the
crowbar device, which shows a turn-on action.

When a voltage-clamping device is installed, the circuit remains
unaffected by the device before and after the transient for any
steady-state voltage below clamping level. Increased current drawn
through the device as the voltage attempts to rise results in
voltage-clamping action. Nonlinear impedance is the result if this
current rise is faster than the voltage increase. The increased voltage
drop (IR) in the source impedance due to higher current results in the
apparent clamping of the voltage. It should be emphasized that the
device depends on the source impedance to produce the clamping. A
voltage divider action is at work, where one sees the ratio of the
divider as not constant but changing (Figure 4).

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

**Avalanche Diodes**

> Avalanche diodes, the Zener diodes, were initially applied as voltage
> clamps, a natural outgrowth of their application as voltage
> regulators. Improved construction, specifically aimed at surge
> absorption, has made these diodes very effective suppressors.
> Large-diameter junctions and low thermal impedance connections are
> used to deal with the inherent problem of dissipating the heat of the
> surge in a very thin single-layer junction.
>
> The advantage of the avalanche diode, generally a PN silicon junction,
> is the possibility of achieving low clamping voltage and a nearly flat
> volt-ampere characteristic over its useful power range. Therefore,
> these diodes are widely used in low-voltage electronic circuits for
> the protection of 5 or 15 V logic circuits, for instance. For higher
> voltages, the heat generation problem associated with single junctions
> can be overcome by stacking a number of lower voltage junctions,
> admittedly at some extra cost.
>
> In the same category, we find silicon diodes used in the forward
> direction rather than in the reverse avalanche. A stack of such diodes
> is required to produce the necessary clamping voltage (0.75 V per
> diode), but the result is a protective system with large current
> capability.

# Varistors

The term varistor is derived from its function as a
[vari]{.underline}able resi[stor]{.underline}. The European usage is the
term voltage-dependent resistor, but the term seems to imply that the
voltage is the independent parameter in surge protection, a concept
which is misleading. Two very different devices have been successfully
developed as varistors: silicon carbide discs have been used for years
in the surge arrester industry; more recently, metal oxide varistors
(MOV) have come of age, with the result that these new varistors are
sometimes referred to as \"movistors\" (12).

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 (13, 14, 15, 16, 17).

Metal oxide varistors were initially developed as electronic circuit
protection devices. Later large metal oxide varistors were developed and
applied to large station surge arrestors (18). No device, however, was
available in a rating suitable for power distribution systems. The surge
currents occurring in these systems are excessive for electronic-type
varistors, a fact demonstrated by field failures resulting from
improperly applied varistors. Such failures could have been anticipated
had the data included in the proposed IEEE Guide reviewed above been
available at the time. In this context, it is worthwhile to examine the
implication of failure modes for the surge protective devices.

# Failure Modes

> 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 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 over voltage, 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 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
failed device is not cleared by a series overcurrent protective device
(fuse or breaker).

**Protection Coordination**

By protection coordination we mean a deliberate selection of two or more
protective devices used with the goal of reliable protection at minimum
cost. With the present situation of the unregulated and uncoordinated
application of protective devices, this may seem an unattainable goal
for complete systems. In specific cases, it is fully attainable, as the
example that follows will show. One can hope that success will
eventually spread the concepts and increase the drive to 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 low-power systems, a series
impedance is sometimes added in the circuit, but only after a
low-impedance diverting path has first been established; for high-power
systems, that option is generally not available.

# Coordination Between an Arrester and a Varistor

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

A circuit was set up in the laboratory (19), with 8 m of typical
two-wire cable between the arrester and the varistor. The current,
approximately 8 x 20 impulse, was raised until the arrester would spark
over about half of the time in successive tests at the same level, thus
establishing the transfer of conduction from the varistor to the
arrester. Figure 5A shows the discharge 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 sparked over, a voltage that would
propagate inside all of the building if there were no suppressor added.
However, when a varistor is added at 8 m, the voltage of Figure 5C is
attenuated to that shown in figure 5D, at the terminals of the varistor.

# Comparison between a High-Energy Varistor and an Arrester/Low-Power Varistor

In the case of power circuits where no regulation or centralized
engineering authority can mandate a coordinated approach, individual
protection of each piece of equipment may remain the only safe approach
to the manufacturer of equipment installed under this uncontrolled
situation. The choice of protection is then a question of economics and
calculated risks: provide equipment with low-cost, low-capability
protection, or with high-capability protection at a higher cost.

The first choice, low-cost, low-capability protection, will be effective
in the vast majority of indoors locations, such as Category A, or even
B, described in the proposed IEEE guide (4); there is, however, a finite
probability of failure if the equipment is installed close to the
outdoor environment in a high-exposure location. An arrester installed
at the service entrance would relieve the low-capability protection from
absorbing excess energy. In that case, the situation explored in the
experiment reported above would prevail: suitable coordination of the
respective protective levels of the two devices and the impedance
existing between them.

The second choice, provision of a high-capability protection in each
piece of Equipment, would obviously provide adequate protection and
might be justified where the cost of equipment failure in terms of
money, dead-time, or embarrassment outweighs the initial outlay of
component investment. It is doubtful, however, that even mass-production
could lower the cost of the high-capability device.

In the case of power circuits where a centralized engineering authority
is in a position to enforce coordinated protection and practices, the
appropriate procedure is evident and much more economical: provide a
single high-capability protective device at the service entrance to deal
with incoming lightning-induced surges and power system switching
surges; if necessary, complement the protection with coordinated
low-capability protective devices at individual pieces of equipment, to
deal with any internal switching transients that may occur. Indeed,
coordination of the two protective devices is imperative to prevent the
low-capability (and low clamping voltage) device from clamping the
incoming surge and thus absorbing all the energy.

Such coordination is now possible, since varistors with surge ratings to
25 kA for single surges and appropriate derating for multiple surges
(11) have become available. This rating is higher than the requirements
of ANSI Standards for secondary arresters (20, 21) and thus would be
suitable for Category C (10 kA) of the proposed IEEE Guide. These
high-capability varistors will clamp the voltage at a level sufficiently
low - typically 600 V in a 120 V system under a surge of 10 kA, or 1100
V in a 240 V system for the same 10 kA surge (by comparison,
conventional arresters have a protective level of 2 to 3 kV). The
availability of low clamping voltage devices for both the high-energy
service entrance duty and the low-cost individual equipment protection
makes an effective coordination easy to achieve.

# Conclusions

After many years of data collection and evaluation, a consensus is now
emerging on the definition of the ac power system transient over voltage
environment, including both voltage and current levels.

This definition will enable protection engineers in centralized
organizations, such as those existing in the operating communications
companies, to design coordinated schemes of protection, and will enable
equipment designers and manufacturers to assess the risks involved in
providing various levels of protection for their equipment on the basis
of economic and functional criteria.

**References**

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

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

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

4\. \"Guide on Surge Voltages in AC Power Circuits Rated Up to 600 V,\"
Final Draft, May 1979. Document prepared by Working Group 3.4.4 of the
Surge Protective Devices Committee of the Power Engineering Society,
Institute of Electrical and Electronics Engineers.

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

6\. Longitudinal Voltage Surge Test #3 - Section 68.302(e), Title 47,
\"Telecommunications,\" Code of Federal Regulations, U.S. Gov\'t.
Printing Office, Washington, DC, 1977.

7\. \"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).

8\. J.E. Lenz, \"Basic Impulse Insulation Levels of Mercury Lamp Ballast
for Outdoor Applications,\" Illuminating Engrg., Feb. 1964, pp. 133-140.

9\. 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.

10\. 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.

11\. \"Test Specifications for Varistor Surge-Protective Devices,\"
P465.3. Draft prepared by Task Force 3.3.6.3 of the IEEE Surge
Protective Devices for eventual publication as an IEEE Standard.

12\. Harnden, J.D., Martzloff, F.D., Morris, W.G., and Golden, F.B.,
\"The GE-MOV Varistor - The Super Alpha Varistor,\" Electronics 45, 21,
1972, p. 91.

13\. Matsuoka, M., Masa Yama, To, and Lida, Y., \"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.

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

15\. Richman, P., \"Diagnostic Surge Testing, Part I,\" Solid State
Power Conversion, Sept./Oct. 1979.

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

17\. Gauthier, N., \"Technologie et Applications des Varistances,\"
Toute L\'Electronique, January 1978

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

19\. 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.

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

21\. Guide for Application of Valve-Type Lightning Arresters for
Alternating Current Systems, ANSI Standard C62.2. (Rev. 1979 to be
published.)

**The Coordination of Transient Protection**

**for Solid-State Power Conversion Equipment**

F.D. Martzloff

Corporate Research and Development

General Electric Company

Schenectady, New York, USA

**ABSTRACT**

Transient overvoltages are no longer an unknown threat to the successful
application of power conversion equipment, thanks to the availability of
protective techniques and devices. This paper presents an overview of
the origin of transient overvoltages and of recent IEEE and IEC
docu­ments identifying and categorizing transients. A brief review of
available techniques and devices follows, with a descrip­tion of the
principles of coordinated protection, specific experimental examples,
and results reconciling the unknown with the realities of equipment
design.

**INTRODUCTION**

Since the introduction of semiconductors, transient over-voltages have
been blamed for device failures and system malfunctions. Semiconductors
are, indeed, sensitive to over-voltages. 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
may represent a slight over­simplification of the general problem because
the environ­ment is still defined in statistical terms, with unavoidable
uncertainty as to what a specific power system can impress on a specific
piece of power conversion equipment.

The IEEE has published a Guide (1) describing the nature of transient
overvoltages (surges) in low-voltage ac power circuits. 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, the IEC has issued a report
concerning insulation coordination (2), identifying 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 have been
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 various groups
concerned with the problem are attempting to close the 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 same power system, and
coordination of overvoltage protection is generally not feasible under
these conditions. Worse yet, an uncoordinated application of surge
suppressors can lead to wasteful or ineffective resource allocation,
since 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 on this situation, this paper will briefly review
some of the origins of transient overvoltages, with reference to
recently published IEEE and IEC documents, which provide guidance on the
environment. Techniques and protective devices will then be discussed,
and examples of coordinated approaches presented.

**THE ORIGIN OF TRANSIENT OVER VOLTAGES**

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, which are generally quite destructive, and against which
total protection 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 large
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 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 an adverse shadow over a large number of successful applications.

# VOLTAGE LEVELS

Two different approaches have been proposed to define voltage levels in
ac power systems. At this time, the diver­gences have not yet been
reconciled, as each proposal has its merits and justification. The IEEE
approach involves reciting a rate of occurrence as a function of voltage
levels, as well as of exposure in systems that do not necessarily use
protective devices. The IEC approach indicates only a maximum level for
each location category, but no higher values are expected because this
approach implies the application of protective devices. These two
proposals will be quoted in the following paragraphs.

**The IEEE Guide (IEEE Std 587-1980)**

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 in unprotected circuits (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.
>
> • Medium Exposure --- Systems in geographical areas known for high
> lightning activity, with frequent and severe switching transients.
>
> • High Exposure --- Rare but real systems supplied by long overhead
> lines and subject to reflections at line ends, where the
> characteristics of the installation correspond to high sparkover
> levels of the clearances.

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.

## Figure 1. Rate of surge occurrence versus voltage level in unprotected circuits (IEEE 587)

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 Approach (IEC Report 664, 1980)**

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 maximum
occurrences at different points of a power system, and at levels
dependent upon the system voltage (Table I). The report is not primarily
con­cerned with a description of the environment, but more with
insulation coordination of devices installed in these systems. This
approach rests entirely on the establishment of con­trolled levels in a
descending staircase, as the wiring systems progress within the building
away from the service entrance.

The fundamental assumption made in establishing the levels of Table I is
that a decreasing staircase of overvoltages will evolve from the outside
to the deep inside of a building (system), either as the result of
attenuation caused by the impedance network, or by the installation of
overvoltage lim­iters at the interfaces.

If the descending staircase of voltages is provided by a surge
protective device at each interface, it must be recog­nized that the
successive devices will interact; the situation is not one of one-way
propagation of the surges. Indeed, a protective device installed, say,
at the III/II interface might be so close (electrically) to the device
at interface IV/III that it could prevent the latter from operating; in
other words, the III/II device might face the surge duty normally
expected to be handled by the IV/III device. Thus, a vital aspect in the
selection of interface devices is that of ensuring proper coordination.

**Table I**

**Preferred Series Of Values Of Impulse Withstand Voltages**

**For Rated Voltages Based On A Controlled Voltage Situation**

In both the IEEE standard and the IEC report, the assumption has been
made that the surge is impinging the power system through the service
entrance and is occurring between phase and earth. Experience has shown
that a fre­quent cause of distress is the voltage differences existing
between conductors reputed to be at ground potential; in fact, one of
them is elevated above the other by the flow of surge current. This
situation, not addressed in either docu­ment, needs to be recognized and
dealt with on an individ­ual, case-by-case basis, lest a false sense of
security be created by restricting the protection to the power service
entrance.

# WAVESHAPE OF THE TRANSIENT OVERVOLTAGES

Observations in different locations (3-6) 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 1.2/50, although 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 transmission systems exposed to lightning. The
IEEE Guide recommends 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). 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 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, e.g., in the power
system series impedance or 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 Report 664 in its present published form \[Footnote:
Continuing studies by the IEC SC28A Working Group are now addressing
this issue, and additional publications are anticipated.\] they
sometimes indicate values applicable to limited cases, such as the SWC
test for electronic equipment operating in high-voltage substa­tions (8).
The IEEE Guide attempts to relate impedance with three categories of
locations, A, B, and C. For most industrial environments, Categories A
or B will apply; Category C is intended for outdoor situations (Table
II).

# MATCHING THE ENVIRONMENT WITH THE EQUIPMENT

On the basis of the various documents mentioned in the preceding
paragraphs, an equipment designer or user can take a systematic approach
to matching the transient over­voltage capability of the equipment with
the environment in which this equipment is to be installed. This design
may involve tests to determine the withstand levels (9), 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 discussed in the following paragraphs.

**Figure 2. Transient overvoltages and discharge currents in IEEE Std.
587-1980**

# 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). In this paper, a majority of the
discussion will center on the latter type, since voltage clamping
devices or crowbar devices are the most frequently used (10).

Voltage-clamping devices have a variable impedance, depending on the
current flowing through the device or the voltage across its terminals.
These components show a non­linear characteristic, i.e., Ohm's law E=RI,
can be applied but the equation has a variable R. Impedance variation is
monotonic and does not contain discontinuities, in contrast to the
crowbar device which shows a turn-on action. As far as volt-ampere
characteristics of these components are 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. Increased voltage drop (IZ) 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, Z,
to produce the clamping. A voltage divider action is at work where one
sees the ratio of the divider not constant, but changing (Figure 3). The
ratio is low, how­ever, if the source impedance is very low. The
suppressor cannot work at all with a limit zero source impedance. In
contrast, a crowbar-type device effectively short-circuits the transient
to ground. Once established, however, this short circuit will continue
until the current (the surge current as well as any power-follow current
supplied by the power sys­tem) is brought to a low level.

**Figure 3. Voltage clamping action of a suppressor**

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 crowbar 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, power-follow, and noise
generation 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 spark-over voltage achieved in
successive operations, since the pro­cess is statistical in nature. For
some devices, this sparkover voltage can also 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.
This difficulty can be alleviated by filling the tube with a gas having
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 power circuits, clearing is
even more uncertain. Additional means must, therefore, be provided to
open the power circuit if the crowbar device is not designed to provide
self-clearing action within specified limits of surge energy, system
voltage, and power-follow current.

A third limitation is associated with the sharpness of the sparkover,
which produces fast current rises in the circuits and, thus,
objectionable noise. A classic example of this kind of disturbance is
found in oscillograms recording the sparkover of a gap where the trace
exhibits an anomaly before the sparkover (Figure 4). 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 event delayed
by a few nanoseconds, so that in real time, the gap sparkover 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 5 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. 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 effective protec­tion at optimum cost. However, this combined
protection must be properly coordinated to obtain the full advantage of
the scheme. The following paragraphs will discuss some of the basic
principles of coordination and provide some examples of applications.

**Figure 4. Interference to oscilloscope circuits caused by gap
sparkover**

**Figure 5. Hybrid protector with gap**

# 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. Perhaps a long history of
testing insulation with voltage impulses 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.

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 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 (11). 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, 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
failed device is not cleared by a series overcurrent protective device
(fuse or breaker). Note that in this discussion, the term "fail-safe"
has 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. 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 II). 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.

**Table II**

#### Recommended Values From IEEE Std 587

Since 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 6).
Labora­tory proof-test of the retrofit demonstrated the capability of the
combined scheme to withstand 6 kA crest current surges (Figure 7A) and a
200^%^ margin from the proposed Category B requirement, as well as
reproduction of the field failure pattern (Figure 7B). 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 7C illustrates the effect of improper
installation of the suppressor, with eight inches of leads instead of a
direct connection across the input terminals of the circuit.

## Figure 6. Retrofit protection of control 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 and C62.2 standards
for secondary arresters. The only arresters available at the time which
could with­stand a 10 kA crest 8/20 μs impulse had a protective
(clamp­ing) level of approximately 2200 V (12). 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 the current
level at which 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 (13), with 8 m (24 ft) of #12
(2.05 mm) two-wire cable between the arrester and the varistor. The
current, approximately 8/20 μs 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 8A shows the discharge current level required from
the generator at which this transfer occurs. Figure 8B shows the voltage
at the varistor when the arrester does not spark over. Figure 8C shows
the voltage at the arrester when it sparks over; this voltage would
propagate inside all of the building if there were no suppressor added.
However, if a varistor is added at eight meters, the voltage of Figure
8C is attenuated to that shown in Figure 8D, at the terminals of the
varistor.

## Figure 7. Laboratory demonstration of retrofit effectiveness

## Figure 8. Transfer of conduction in a coordinated scheme of protection

# Matching Suppressor Capability to the Environment

It is a recognized fact that varistors exhibit, as do many other
components, an aging characteristic, so that a finite life can be
predicted. Most manufacturers provide information on this aspect of
application, and IEEE standards identify this parameter as one of the
significant evaluation tests (14). Carroll has shown (15) how
statistical information presented in IEEE Std 587 can be combined with
Pulse Lifetime Rat­ings published by manufacturers (16) to arrive at a
rational selection of device ratings, with a specific life goal, in a
cost-effective manner.

However, these ratings are generally expressed as a number of pulses of
constant value, e.g., the rated life of a given varistor may be 1 pulse
of 6 kA at 8/20, 10 pulses at 2 kA, 1000 pulses at 500 A, and so forth.
But since the surges encountered in real life have a range of values at
a slope of probability versus magnitude described by Figure 1, one must
consider the effect of this array of pulses with different values rather
than the constant pulses implied by the manufacturer's pulse lifetime
rating.

The method described by Carroll in the referenced paper provides a
computation that can be applied in general terms, but repeating it here
would be too lengthy. Rather, we will take two examples of application
and develop a table showing how the Pulse Lifetime Ratings can be
combined with the data from IEEE Std 587 to make a reasonable estimation
of the rated life consumption. The computations shown in the tables have
been made with four digits for the sake of allow­ing a check of the
arithmetic, but the base data are far from four significant digits in
their accuracy, and the numbers are read from curves with rather coarse
logarithmic scales. How­ever, these examples do illustrate the method and
the results that can be expected.

The first task is to convert the voltage surge density prob­ability of
Figure 1 into a histogram of surge currents. A family of surge voltage
cells can be defined from the Figure 1 line, with the density read at
the center of the cell. The number of occurrences for any cell is then
the value of the ordinate of the line, minus the number of total
occurrences of all cells to the right of the cell of interest. In the
compu­tations of Table III, this conversion is shown in the first three
columns, indicating the voltage level at the cell center, the number per
year, and the number of occurrences per year.

From the description of the Category B in IEEE Std 587, one can deduce
an implied source impedance of 6 kV/3 kA for a surge or 8/20 μs, or 2 Ω
as the most severe in Category B. The current that will flow in a
varistor con­nected at this Category B location is then the surge
voltage, minus the varistor clamping voltage, divided by the 2 Ω source
impedance of the surge. The varistor clamping volt­age can be determined
if the current is known, so an itera­tion would be required to obtain the
clamping voltage. How­ever, one can assume a clamping voltage, and later
check the validity of the assumption against the resulting current
obtained. The fourth column of Table III shows this assumed and
subsequently checked value of the clamping voltage, hence the value of
the available driving voltage in the next column, and the resulting
surge current value, assumed to be an 8/20 μs waveshape.

**Table III**

**Life Consumption --- 14 mm, 130 V RMS Varistor, Category B, Low
Exposure**

Turning then to the published Pulse Lifetime Ratings, one can read the
rated number of pulses corresponding to the surge current for each cell.
Table III is computed with the ratings for a 14 mm varistor (Figure 9a);
Table IV is computed for a 32 mm varistor (Figure 9b). Note that this
"rated life" is defined as the condition reached when the varistor
nominal voltage has changed by 10%; this is not the end of life for the
varistor, but only an indication of some permanent change beginning to
take place. The varistor has still retained its voltage clamping
capability at this point.

For each level of surge current, the number of pulses is read on the
family of curves of Figures 9a or 9b, along the vertical axis, since
these are 8/20 μs impulses. The number of pulses with constant amplitude
is shown in the next-to-last column of Table III. We can now define, for
each level, the percentage of life consumed for one year of exposure at
that level. For instance, at the 2500 V level of Table III, there will
be 0.01 surges of 1010 A per year, with 10 allowed by the ratings.
Therefore, in percent, the life consumption is (0.01/yr x 100)/10, or
0.10%. Likewise, taking the 900 V level, the consumption is (0.8/yr x
100)/2000 = 0.04%. The total of these life consumptions at all cell
levels is then 0.43% of the rated life in one year, yielding an
estimated 232 years for this 14 mm varistor to reach its rated life in
the Low-Exposure Category B environment.

# Figure 9. Pulse lifetime ratings

Similar computations for a 32 mm varistor in a Category B, Medium
Exposure, are shown in Table IV. In the case of this "Medium Exposure,"
we note the high fre­quency of occurrences below 3000 V. reflecting the
"fre­quent and severe switching transients" cited in the IEEE definition
of Medium Exposure. Thus, a still very conserva­tive estimate would be
that as many as half of the occurrences would be due to lightning, with
the attendant 8/20 μs high energy surges, while the other half would be
switching transients, having a lower energy content than the 8/20 μs
surges accounted in this computation, being oscilla­tory as typified by
the 0.5 μs --- 100 kHz wave. This translates to 13 surges of 760 A, 35
surges of 525 A, and 250 surges of 285 A, still a high number of
lightning surges and therefore certainly conservative. Using this
conservative estimate of half of the low-magnitude surges and all of the
high-magnitude surges being 8/20 μs lightning-related surges, the
computation of Table IV yields 21 years to reach rated life for the 32
mm varistor. In this case, where the rated life is reached earlier, it
should be pointed out that the results are strongly influenced by the
assumption made for the source impedance. Using the IEEE 587 implied
value of 2 Ω leads to these conservative results. For example, the FCC
test for communication equipment interfacing with power lines (17)
implies a 2.5 Ω source impedance. Current studies for complementary data
to the IEC Report 664 make the assumption of a surge originating on the
primary of a dis­tribution transformer, with a 63 Ω source impedance,
yield­ing currents of less than 1 kA available at the service entrance
interface. Thus, there is still room for more precise definitions of the
source impedance, but we should recognize that any attempt to make broad
generalizations will always encounter the contradiction of some special
cases.

**Table IV**

**Life Consumption --- 32 mm, 150 V RMS Varistor, Category B, Medium
Exposure**

**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. With the publication of the IEEE Guide, and of
application guides in the near future, we can expect better knowledge 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 costs, while present 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 Characterization in Low-Voltage AC Power Circuits. Robert
Mierendorf emphasized the significance of clearance sparkover; and Peter
Richman's critique of the concepts relating to source impedance proved
very effective; and Eric Carroll's cross examination of the IEEE Std 587
led to the concept of relating varistor pulse lifetime data to the
distribution of expected surge currents.

**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\. J.E. Lenz, "Basic Impulse Insulation Levels of Mercury Lamp
> Ballast for Outdoor Applications," Illuminating Engrg., pp. 133-140
> (February 1964).
>
> 4\. F.D. Martzloff and G.J. Hahn, "Surge Voltage in Resi­dential and
> Industrial Power Circuits," IEEE PAS 89 (6), pp. 1049-1056
> (July/August 1970).
>
> 5\. R. Hasler and R. Lagadec, "Digital Measurement of Fast Transients
> on Power Supply Lines," Proc. 3rd Symposium and Technical Exhibition
> on Electromagnetic Compatibility, Rotterdam, Holland, pp. 445-448 (May
> 1979).
>
> 6\. F.D. Martzloff "Transient Overvoltages in Secondary Systems,"
> General Electric Company Report 81CRD 121, Schenectady, NY (1981).
>
> 7\. P. Chowdhuri, "Transient-Voltage Characteristics of Sili­con Power
> Rectifiers," IEEE IA‑9 (5), p. 582 (September/October 1973).
>
> 8\. "Guide for Surge Withstand Capability," (SWC) ANSI/IEEE
> C37.90a-1974.
>
> 9\. F.A. Fisher and F.D. Martzloff, "Transient Control Lev­els, A
> Proposal for Insulation Coordination in Low-Voltage Systems," IEEE
> PAS-95 (1), pp. 120-129 (January/February 1976).
>
> 10\. Transient Voltage Suppression Manual, Chapter 2, General Electric
> Company, Auburn, NY, pp. 13-17 (1978).
>
> 11\. Transient Voltage Suppression Manual, Chapter 2, General Electric
> Company, Auburn, NY, p. 15 (1978).
>
> 12\. F.D. Martzloff, "Transient Overvoltage Protection: The
> Implications of New Techniques," Proc. 4th Symposium and Technical
> Exhibition on Electromagnetic Compatibility, Zurich, Switzerland, pp.
> 505-510 (March 1981).
>
> 13\. F.D. Martzloff, "Coordination of Surge Protectors in Low-Voltage
> AC Power Circuits," IEEE PAS-99 (1), pp. 129-133 (January/February
> 1980).
>
> 14\. "Test Specifications for Varistor Surge-Protective Devices," IEEE
> STd C62. 33, 1982.
>
> 15\. E. Carroll, "Transient Attenuation with Metal Oxide Vans­tors for
> AC Mains Powered Equipments," presented at "Comel," University of
> Twente, Holland (No­vember 1980).
>
> 16\. Transient Voltage Suppression Manual, Chapter 4, General Electric
> Company, Auburn, NY, pp. 39-41 (1978).
>
> 17\. Longitudinal Voltage Surge Test #3, Code of Federal Regulation,
> Section ^6^8.302(e), Title 37, Telecommunica­tions, Washington, D.C.,
> U.S. Government Printing Office. 197T

Lightning Protection of Roof-Mounted Solar Cells

F. D. Martzloff

General Electric Company

Schenectady, New York

# 

# SUMMARY

Lightning interception by an unprotected building 30 feet high x 45 feet
long on flat land, in the Cleveland area, can be estimated at one strike
per 200 years on the average. However, peculiar configuration can easily
affect this estimate upward or downward. Means are available to install
air terminals so that interception will be done by these, without harm
to the roof-mounted photovoltaic array. Economic trade-off on the
decision to invest in a lightning protection system during the life of
the development project must also include intangible factors such as
schedule delays and postmortem apologies for insufficient protection.

**1.0 INTRODUCTION**

This discussion is presented as an introductory guideline to alert
system designers and provide a basic understanding of the phenomena
governing the techniques of lightning protection. No definitive
numerical prediction is given at this time, pending availability of
architectural details; an example of estimated interception rate is
given for an assumed building in the Cleveland area.

A brief description is given of the lightning phenomena, as well as of
the basic concepts which need to be understood for effective application
of protective techniques. Prevention of direct effects, that is, the
effects associated with current flow in the system, to be avoided, is
first discussed. Indirect effects are also mentioned, and the document
is completed by a bibliography for further reading by interested
designers, and an Appendix discussing in more detail the history and
rationale of the cone of protection concept.

**2.0 LIGHTNING PHENOMENA**

The phenomenon of lightning has been the subject of intensive study by
many workers (see bibliography) and its behavior is fairly predictable
in general terms, although the exact description of specific incidents
is not predictable. Protection against lightning effects includes two
categories: direct effects concerned with the energy, heating, flash,
ignition of the lightning current, and indirect effects concerned with
induced overvoltages in nearly electrical and electronic systems.

Claims to the contrary notwithstanding, there is no conclusive evidence
that lightning can be prevented. Consequently, one has to design and
implement a facility to recognize the possibility of a lightning strike,
and take appropriate measures to make this strike harmless. Lightning
protection is then an approach where one can make the proper moves if
the characteristics of the enemy are anticipated.

Two concepts must be understood to apply effective lightning protection
devices: the \"cone of protection\" and the \"striking distance.\" These
will be discussed with some detail in the following pages. Other
fundamentals of electricity such as the impedance of a circuit to
fast-changing currents impulses are assumed familiar to the reader.

We will first review current theories on the formation of lightning,
then go on to the concept of the cone of protection and striking
distance. The following descriptions are based on the work and papers of
Fisher, Cianos & Pierce, and Golde.

**2.1 Generation of the lightning flash**

The energy that produces lightning is provided by warm air rising upward
into a developing cloud. In detail, several theories vary, but all are
based on the observed evidence that the cloud,, except for the top, is
negative, with a small body of positive charge near the front base of
the cloud. Figure 2.1 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. The average electric field at the surface of the ground
will change from its fair-weather value of about 300 volts/meter to
several thousand volts/meter. The gradient will be concentrated around
sharp protruding points on the ground, and can exceed the breakdown
strength of air, typically 30 kV/cm.

However, the first significant event toward formation of a lightning
flash occurs at the cloud. A slow-moving column of ionized air forms at
the cloud, called pilot streamer, moving by steps of 30 to 50 meters,
and followed by a more intense discharge called step-leader, because of
the discontinuous process of ionization and filling of the column with
charged particles.

The step-leader does not move in straight line towards the ground, but
seeks out the path with least electrical resistance, producing the
familiar zig-zag pattern of the final stroke, with branches as several
paths may be formed in the process of searching a weak path. The
interval between the successive steps is about 50 microseconds, allowing
progressive build-up of charges on the ground as the charged column
advances toward the ground. Finally, a stage is reached with the
step-leader being one step only away from the ground, when the last step
is completed, either by continuation of the process, or by meeting a
leader of positive charges originating from the ground. We will discuss
the implications of this in some detail in the section dealing with the
striking distance concept.

With the path now completed, a positive charge then flows upward from
the ground into the negative channel left in the wake of the
step-leader, neutralizing the charge in this channel and moving at
roughly one third of the speed of light. This is what constitutes the
lightning stroke, carrying the current at peaks of 1000 to 100,000
amperes, with a decrement to half-value in the order of 50 microseconds.
The first one of these which occurs in a flash is called the return
stroke.

The first return stroke neutralizes the ionized column as well as a
small pocket of changes in the cloud; a second or more return strokes,
sometimes called re-strikes or subsequent strokes, can take place, using
the same ionized channel, but moving much faster. Thus, a series of
strokes, such as shown in Figure 2.2, can occur in an interval in the
order of a second. Each of these strokes increases from a very low
quiescent value to a very high amplitude in a very short time, resulting
in rates of current change up to 1 x 10^11^ amperes/second.

When very tall objects are present, a step-leader can actually originate
from this object, and travel upward to the cloud,, rather than the more
general case of downward-moving leader. Subsequent charges, however,
will be similar, that is, move in the ionized channel left by the first
discharge.

In addition to the short (tens of microseconds) discharges just
described, a low amplitude current can exist between the individual
strokes. Although low in amplitude (a few hundred amperes), the long
duration of this continuing current (tens to hundreds of milliseconds)
is significant because of its total charge, resulting in most of the
burning and metal-melting effects of a lightning flash.

The wave form and amplitude of lightning stroke and continuing currents
vary over such a wide range that information has to be presented in
statistical form; Cianos & Pierce have published a comprehensive set of
statistics, from which the data of Figures 2-3 through 2-8 are derived,
giving the reader a feel for the orders of magnitude involved.

**2.2 Frequency of occurrence**

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. This is
not generally available, and instead the number of \"thunderstorm days\"
is quoted. However, this does also include the cloud-to-cloud
discharges, and does not represent an accurate parameter, since it does
not include the duration and intensity of each storm. Progress is being
made in improved statistics, but these are not yet available, and
therefore the \"isokeraunic level\" maps, showing the number of
storm-days per year, is still the most widely used description of the
occurrence distribution. An empirical equation has been derived,
relating the density of flashes to ground and the number of storms per
year, as follows:

> density in flashes per km^2^ per year: D
>
> thunderstorm-days per year: T
>
> D = 0.02 T^l,6^

This corresponds to approximately 1 flash per year per km^2^ at an
isokeraunic level of 10, and 10 flashes per year per km^2^ at a level of
40.

The significance of this situation is that, contrary to some popular
beliefs, the density of lightning flashes, [on the]{.underline}
[average]{.underline}, is independent of terrain. However, detail of the
ground objects (trees, buildings, hills) will produce a bias in the
local distribution of this average.

**2.3 Striking distance**

This distribution at the local scale is determined by the final stages
of the step-leader coming from the cloud, so to speak, without knowing
what it will find on the ground. Thus the actual point of termination
can be somewhat controlled,, while the probability of a given area to
receive a lightning stroke cannot. This is where the concept of the
striking dis­tance, as explained by Golde, becomes very useful.

As the step-leader has approached 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 top
of the leader and the object about to be struck (or about to emit the
meeting leader) is called [striking distance]{.underline}. The length of
this distance is affected by the field established by the leader, which
in turn is determined by the amount of charges existing in the ionized
channel coming from the cloud. With large charges in the channel, the
field is more intense, so that breakdown can for longer distances, while
a shorter distance is necessary to produce breakdown for the weaker
fields established by smaller charges. Figure 2-9 shows the relationship
between the stroke current (which reflects the charge existing in the
ionized channel) and the striking distance, as computed by Golde.

For instance, an average lightning current of 25 kA would correspond to
a striking distance of 40 meters. Thus, for an average stroke, details
of the terrain do not affect the point of termination of the stroke
beyond this distance, but only 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, meaning that they will
ignore \"attractive\" points of termination, explaining some of the more
puzzling exceptions to the generally assumed effect of tall-structures,
rods, etc. In other words, once a lightning step-leader has approached
the ground within just short of the striking distance, no amount of
devices beyond the striking distance will have any effect on the
occurrence of the stroke, just the details of the area within the
striking distance will determine the point of termination; the area is
committed to receive the stroke, and it is now up to the humans
controlling the shape of the objects on the ground to provide a least
harmful point of termination, and make it most attractive to the
approaching leader (\"take me to your leader\"\...)

A number of photographs have been collected, and reported in the
literature, showing a lightning flash approaching the ground in a
somewhat wandering but generally downward direction, but with a sharp
\"turn\" near the lower part of the path. This, according to the
striking distance just described, can be readily explained as being the
point where the downcoming step-leader met the upcoming leader from the
ground. A photograph of a particularly clear occurrence of this will be
found in the Appendix.

We are now ready to tackle the concept of the \"cone of protection,\"
having understood how the striking distance concept can explain some of
the otherwise unexplained exceptions to the generally accepted and
verified protection afforded by projecting objects, natural or
artificial.

4.  **Cone of protection**

From the days of Benjamin Franklin, the concept of a cone of protection
has been used to provide 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 (Figure 2-10) will not receive
the lightning stroke, but that the object at the apex of the cone will.
In the elementary concept, only one projecting object above a ground
plane is being considered; in most practical situations involving
buildings, multiple cones will be or should be considered.

Historically, a 1:2 cone was considered acceptable. However., some
exceptions to the \"rule\" of protection (as if Zeus should abide by
\"rules\") have recently led to a more conservative use of a 1:1 cone of
protection; the exceptions can be explained by the striking distance
concept, as we shall now see.

Classical \"cone of protection\" rule for the building shown in Figure
2-11 would assure that the lightning mast shown provides dependable
protection for the building against an approaching step-leader. However,
if we consider the striking distance shown by the circles, at each step
of the leader advance, we can see that the leader will have ignored the
lightning mast, and that at the fateful last decision point D, the
shortest distance is to the corner of the building within the \"cone of
protection,\" rather than to ground, even less to the lightning mast.
The path drawn here also exhibits the tell-tale sharp inflexion of the
last step mentioned above and often photographed.

By contrast, the step-leader drawn in Figure 2-12 for the case of a
stroke with higher prospective intensity, and thus longer striking
distance, will terminate at the lightning mast, starting from the same
point \"A\" of its path. This corresponds to the classical cone of
protection situation; it implies that the step-leader can find within
its striking distance a point of termination which is intentional,,
rather than an object which happens to be close enough and shaped in a
manner promoting the initiation of a streamer which will \"win\" the
race to meet the step-leader.

**3.0 DIRECT EFFECTS**

**3.1 Conduction of current to ground**

We have now reached the state where lightning has struck and have seen
that it may strike in some rather unexpected (and unprotected)
locations, and are faced next with an evaluation of the effects of the
stroke and continuing current flow. In the case of a roof-top array of
voltage-sensitive cells, the real story begins with the flow of current
below the first metallic termination point. Therefore, lightning
protection in this context will mean making sure that the lightning does
terminate where we want it to terminate, and that from there it is led
to earth in an acceptable manner, along a safe, controlled path.

The most important consideration is then what happens to the potential
drop along the current path to ground. We have seen that the current
increases very rapidly on the front of the wave. Therefore the inductive
drop L di/dt over the current path will be extremely large, in other
words, during initial phase of the discharge, the \"grounded\" lightning
rods and downcomers of a building network will be elevated to
substantial potentials above earth ground, in the order of tens or
hundreds of kilovolts. Any circuitous path in the downcomer will
increase this voltage drop, with the attendant risk that a flashover may
occur to bridge and shorten this path, defeating the intent of carrying
the current directly to earth, away from the array.

Another consideration is the difference of potential established during
the initial phase, when the current increases at extremely fast rates,
between the lightning system and the cell array. The upper portions of
the downcomers are elevated at very high potential because of the
inductive drop, while the upper metallic parts of the array remain
uninvolved in the current path, and therefore remain at \"ground\"
potential. We are writing \"ground\" in quotes as this is an arbitrary
definition in the context of potential distribution during a lightning
stroke.

With reference to Figure 3-1, the \"ground\" point can be represented as
the common point \"G\" at which the electric power system of the house
is tied to the downcomer conductors; any further impedance below point
\"G\" and \"true\" earth is not relevant to the difference of potential
V established across the wall of the building at point A. If the length
(inductance) between points B and G is large, and the distance between A
and B small, the voltage V will exceed the dielectric strength of the AB
gap, and a \"side flash\" will occur between A and B, with possible
disastrous effect.

**3.2 Design of the air terminals**

Protection of an experimental roof-top array is different from the
protection to be provided, if any, for future mass-produced arrays. In
the case of the experimental system, the high cost of a lightning strike
should prevail over aesthetic considerations, so that the objective will
be to maximize protection. Once the cost of the roof array is
established as a commercial product, the trade off between economics and
aesthetics will be quite different. In this discussion., however, we are
addressing the case of the experimental house.

There are two basic approaches to providing sufficient protection:
lightning masts, at some distance from the house, with sufficient height
to provide an effective cone of protection, and lightning conductors
above the roof. Neither can provide absolute protection against all
possible strikes; however, the likelihood of a strike attaching to the
roof-mounted array will be decreased by several orders of magnitude if a
properly designed system is installed.

**3.2.1 *Lightning masts***

The discussion of paragraph 2.4 above, and the possible exception to the
protection expected for a strict interpretation of the cone of
protection formula, give some guidance on the design of a mast system.
For the experimental model, two masts at minimum are recommended, with
four being even more effective. (In the case of a project involving a
group of closely spaced houses, it may be possible to provide a common
mast at each lot boundary. Figure 3-2 shows how such a mast system can
be implemented. One of the advantages would be complete independence of
the power system and lightning system at all but one point.

**3.2.2** ***Lightning conductors***

A possibly more effective protection can be expected from a lattice of
lightning conductors strung some distance above the roof. The number of
conductors, and the distance from the roof are in inverse proportions.
Therefore, there is a possible trade-off between a few conductors high
above the roof, and many conductors close to the roof. The boundaries of
the trade-off correspond to the situation where the many conductors
would be so close to the roof that side-flash becomes a problem, or
where only a few, very distant conductors would allow a low-amplitude
stroke, with attendant short striking distance, travel around the
theoretical cone of protection, as discussed in paragraph 2.4.

In any case, the down conductors for the cable lattice have to be routed
far enough away from the roof to prevent any side flash. Figure 3.3
shows a possible arrangement for a cable lattice system.

**4.0 INDIRECT EFFECTS**

**4.1 Indirect voltages**

The major indirect effect of lightning strokes for this project is the
voltage induced on the power system by the rapidly changing magnetic
flux associated with the high di/dt of the lightning current. A less
important but still significant effect would be the voltage produced by
electrostatic coupling between the roof array and the charges associated
with atmospheric electricity.

Typical lightning strokes involve currents of 50 kA reaching crest in 1
microsecond. Thus, the do//dt near the lightning conductors will be
quite high and capable of inducing destructive voltages in any loop
which would link a substantial flux from the lightning current.
Therefore, the power system, cell array and control circuits must be
designed to minimize intrinsic coupling, or be suitably shielded.

**4.2 Effect on incoming power system**

There is a certain amount of information available from various sources
on the magnitude of transient overvoltage entering a house wiring system
by the route of the utility service. While not exhaustive, the data
collected so far have indicated that transients up to 6 kV can occur,
infrequently, but still often enough to cause concern for sensitive
electronics. This problem of course is not unique to photo-voltaic
systems, but is faced by every appliance manufacturer.

In the present case, recognition of the problem is all that is necessary
since there is a wide variety of commercial devices available to
suppress these surges.

**5.0 FLASH INTERCEPTION RATE**

The relationship between annual thunderstorm days and flash density has
been defined by empirical formulae and plotted by Cianos & Pierce. Table
5-1 shows the computed ground flash density, obtained by multiplying the
flash density by a factor relating the [ground]{.underline} flashes to
the total flashes. This factor depends on the latitude, as suggested by
Cianos & Pierce.

Using the cone of protection concept in reverse, we can estimate that
the effective area of an object protruding over the ground surface, in
attracting a strike, increases as its height increases. For a height h,
a circle of radius 2h can be considered as candidate for flash
interception. Thus, assuming a building of 45 feet (15 m) x 30 ft (10m),
with a height of 30 ft (10 m), the effective area is indicated by the
sketch of Figure 5-1. The area corresponding to an elongated double cone
originating from a roof antenna on one side and the peak of the roof on
the other side, at a 2:1 angle, is approximately

50 m x 50 m = 2500 m^2^ = 0. 0025 km^2.^

Applying the ground flash density of Table 5-1 for an isokeraunic level
of 40 (see map of Figure 5-2), we obtain an estimated interception rate
of 0.0025 x 2.1 = 0.005 flashes per year on the area covered by the
house and its associated cones.

In other words, the house in this example can be expected to intercept a
lightning strike once every two hundred years. This prediction would be
based on a house projecting above flat terrain, which we understand to
be the case here. One must bear in mind that in the absence of adjacent,
overlapping cones of protection from other buildings, trees or
structures, the house and its cones can be treated as completely
independent from the surroundings. On the other hand, hill tops and
elevated details of the terrain can affect the local isokeraunic level
upward, while depressed locations will affect it downward.

A simple trade-off based on economics would then consist in stating that
for an estimated repair cost of say, \$ 200 000, and a project life of
two years, the break-even value of a lightning protection system is
20000 x 0.005 x 2 = \$ 2000. However, this trade-off undoubtedly would
be modified to take into consideration the \"utility\" concept in a game
plan. This concept introduces subjective factors which tend to magnify
the perceived cost of a failure, but decrease the perceived cost of a
desired event. Stated in other terms, a trade-off in this project must
also take into consideration intangible factors such as delays in the
study and the embarrassing situation of a major failure caused by a
single lightning strike.

Therefore we submit that a well designed lightning protection system, at
this time divorced from aesthetic considerations, would be a sound
investment. This situation may change when the cost of the arrays will
be lower, giving greater weight to aesthetics.

**6.0 BIBLIOGRAPHY**

Cavendish, H. , Watson, W. , Franklin, B. , and Robertson, J. \"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]{.underline} (London, England), vol. 63, 1773, and
vol. 68, 1777.

Walter, B., \"Uber die Schutzwirkung von Fernblitzableitern und die
Blitzchlaghaufigbeit in einigen Grosstadten, \" [Deutsche]{.underline}
[offentlichrechtliche Versicherung]{.underline}, no. 18, 1935.

Mc Cann, G. D., \"The Measurement of Lightning Currents in Direct
Strokes, \" [AIEE Transactions]{.underline}, vol. 63, 1944.

Golde, R. H. , \"The Frequency of Occurrence and the Distribution of
Lightning Flashes to Transmission Lines, \" [AIEE
Transactions]{.underline}, vol. 647 1945.

Lewis, W. W. , [The Protection of Transmission Lines Against
Lightning]{.underline}, John Wiley, New York, 1950.

Westinghouse Electric Corp. , [Electrical Transmission and
Distribution]{.underline} [Reference]{.underline} Book, Chapter 16, East
Pittsburgh, Pa. , 1950.

Hagenguth, J. H. and Anderson, J. G. , \"Lightning to the Empire State
Building - Part III, [\"AIEE Transactions]{.underline}, vol. 71, Part
III (Power Apparatus and Systems), August 1952.

British Standard Code of Practice CP 326:1965, \"The Protection of
Structures Against Lightning, \" British Standards Institution, British
Standards House, 2 Park Street, London, W. I.

Golde, R. H. , \"The Lightning Conductor, \" Journal of the Franklin
Institute, vol. 283, No. 6, June 1967.

Berger, K., \"Novel Observations of Lightning Discharges: Results of
Research on Mount San Salvatore, \" Journal of the Franklin Institute,
vol. 283, No. 6, June 1967.

[Blitzschutz]{.underline}, Eighth Edition, VDE-Verlag Gmbh., 1 Berlin
12, Bismarckstrasse 33.

Cianos, N. and Pierce, E. T. I \"A Ground-Lightning Environment for
Engineering Usage, \" Stanford Research Institute Technical Report 1,
August 1972.

Golde, R. H. , \"Lightning Protection, \" Edward Arnold Pub. , London,
1973.

[Lightning Protection]{.underline} Code, 1975, NFPA-78, National Fire
Protection Association, 60 Batterymarch St. , Boston, Mass. 02110.

The Protection of Industrial Electronics

and Power Conversion Equipment

**Against Power Line and Data Line Disturbances**

François D. Martzloff, Fellow IEEE

Corporate Research and Development

General Electric Company

Schenectady, New York

# Abstract

The irresistible trend toward distributed computing systems for the
control of industrial power electronic equipment is adding new
dimensions to the old problem of surge protection. Harmful surges may be
produced by lightning, power switching, or by differences in ground
potentials. The objective of the paper is to provide increased awareness
and better understanding of the fundamentals of protection as well as
the characteristics of protective devices.

**1. Introduction**

The continuing development of power conversion equipment, further
promoted by the success of microelectronics in controlling this
equipment, raises the question of vulnerability to surge voltages or
surge currents appearing at the power or control input terminals of the
equipment. The very origin of these surges (lightning, switching,
electrostatic discharges) makes it somewhat difficult to predict with
accuracy their characteristics, so that surge protection has sometimes
been presented as \"art rather than science.\" There is, however, enough
practical experience and data available to apply good engineering
judgment in developing an understanding of the practical aspects of
surge protection based on fundamental concepts and on protective device
characteristics. The present paper is offered as a contribution toward
better surge protection, neither as an art nor as a science, but as a
no-nonsense, sound, and cost-effective engineering practice.

The paper presents first a discussion of the various and complex origins
of lightning and switching surges electrostatic discharges are
acknowledged but not addressed in a limited space discussion). The paper
also shows how standards can help simplify the application of
protection. A brief discussion is given on aspects of the propagation of
these surges, followed by a presentation of some fundamental approaches
to protection. Basic characteristics of protective devices are outlined.
Three examples are given on how this knowledge and engineering judgment
can be applied to answer specific questions and to avoid pitfalls.

**2. The Origins of Transient Overvoltages**

Transient overvoltages in power systems originate from one cause, energy
being injected into the power system, but from two sources: lightning
discharges, or switching within the \'power system. In communication or
data systems, there is another source of transients: the coupling of
power system transients into the system. Furthermore, all systems
involving several connections to the environment have the potential risk
of transient overvoltages associated with ground potential rise during
the flow of surge currents. As stated previously, static discharge
problems are not treated in this paper.

Lightning discharges may not necessarily mean direct termination of a
lightning stroke onto the system. A lightning stroke terminating near a
power line, either by hitting a tall object or the bare earth, will
create a very fast-changing magnetic field that can induce voltages -
and inject energy - into the loop formed by the conductors of the
system. Lightning can also inject overvoltages in a system by raising
the ground potential on the surface of the .earth where the stroke
terminates, while more distant \"ground\" points remain at a lower
voltage, closer to the potential of \"true earth.\" The literature
provides information on the characteristics of lightning discharges
\[1-6\].

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

**2.1 Transients in Power Systems**

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

Simple switching transients \[7\] include circuit closing transients,
transients initiated by the clearing of a short circuit, and transients
produced when the two circuits on either side of the switch being opened
oscillate at different. frequencies. 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. Without a surge protective
device, the current flowing just before switching is available to charge
the circuit capacitances at whatever voltage is required to store the
inductive energy of the current by converting it into capacitive energy.

> Several mechanisms are encountered in practical power circuits that
> 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.
>
> 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 behavior of a
> circuit element. Lightning-induced transients are much less
> predictable because there is a wide range of coupling possibilities.
>
> 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 are described
> in the next section. Figures 1, 2, and 3 show typical examples of
> transients recorded in power systems.

**3. Standards on Transient Overvoltages in Power Lines**

Several standards or guides have been issued or proposed in Europe and
in the United States, 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 a\$ exchanges of information take place, improvements are
being made in the approach. Three of these are briefly discussed here.

> **3.1 The IEEE Surge Withstand Capability Test (SWC) \[8\]**

One of the earliest published documents which addressed new problems
facing electronic equipment exposed to power system transients was
prepared by an IEEE committee dealing with the exposure of power system
relaying equipment to the harsh environment of high-voltage substations.

Because this useful document was released at a time when little other
guidance was available, users attempted to apply the recommendations of
this document to situations where the environment of a high-voltage
substation did not exist. The revised version of this standard, soon to
be issued, recognizes the problem and attempts to be more specific (and
restrictive) in its scope. Thus, an important consideration in the
writing and publishing of documents dealing with transients is a clear
definition of the scope and limitations of the application.

**3.2 The IEC 664 Report \[9\]**

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

**3.3 The IEEE Guide on Surge Voltages (ANSI/IEEE Std 62.41:1980)
\[10\]**

***3.3.1 Voltages and Rate of Occurrence***. Data collected from a
number of sources led to plotting a set of lines representing a rate of
occurrence as a function of voltage for three types of exposures (Figure
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.

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

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

The two lower lines of Figure 4 have been drawn at the same slope since
the data base shows reasonable agreement among several sources on that
slope. All lines may be truncated by sparkover of the clearances, at
levels depending on the withstand voltage of these clearances. The high
exposure line needs to be recognized, but it should not be
indiscriminately applied to all systems. Such application would penalize
the vast majority of installations where the exposure is lower.

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

***3.3.2 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 5).

The oscillatory waveshape simulates those transients affecting devices
that are sensitive to dv/dt and to voltage reversals during conduction
\[14), while the unidirectional voltage and current waveshapes, based on
long-established ANSI standards for secondary valve arresters, simulate
the transients where energy content is the significant parameter.

> ***3.3.3 Energy and Source Impedance.*** The energy involved in the
> interaction of a power system with a surge source and a surge
> 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, e.g.,
> in a resistor added in series with the gap for limiting the
> power-follow current, or on the impedance of the circuit upstream of
> the protective device. In an energy-absorber protective device, by its
> very nature, a substantial share of the surge energy is dissipated in
> the suppressor, but its clamping action does not involve the
> power-follow energy resulting from the 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. ANSI/IEEE Std C62.41-1980 attempts to relate impedance to
> categories of locations but unavoidably remains vague on their
> definitions (Table 2).

***3.3.4 New Reports on Transient Occurrence.*** The importance of
defining the environment has motivated a number of workers to install
recording devices on their power systems, and thus created a market for
commercial transient recorders in the last several years. Unfortunately,
at least one such recorder contains a surge suppressor in its own power
supply (Figure 6).

Consequently, if the line being monitored is that in which the analyzer
has its power cord plugged, the transients that occurred on the line
before the analyzer was installed disappear. This irony makes my
facetious remark of 1969, ". . .*the best surge suppressor is a surge
monitor*." \[12\] the truth. Therefore, some of the recently published
surveys on the occurrence of surges may show a lower number of surges
than actually occur, and therefore data should be examined with caution
until the ambiguity caused by this cord-connected suppressor is
resolved.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

\* Note the difference between surge impedance of the line. also called
characteristic impedance, and the impedance to the surge. Z1 of Figure
8. defined in terms of R. L. C at the frequency corresponding to the
surge.

**4. Surge Propagation**

> **4.1 The Limitations of Arbitrary Division into Categories**

The standards cited in the preceding paragraphs describe surges which
may be expected at specific points of a wiring system; the implication
is that the surges will proceed downstream, at the same amplitude, until
some interface somehow produces a decrease in amplitude down to the next
lower level of voltage or to the next lower level of current specified
by the next downstream category (Figure 7).

This staircase representation is useful to simplify the real world into
a manageable set of assumptions. but it is a simplification that can
mask the reality. Surges will propagate in the system starting at the
point of entry: voltage surges will be attenuated to the extent that the
series impedance between the point of interest and the source (Z1,
Figure 8) and the shunt impedance (Z2). form a voltage. divider. If the
series impedance is low and the shunt impedance is high (light loading
of the system), the voltage divider does not produce high attenuation of
the voltage surge\'s. Conversely, current surges, if they are the result
of a current source such as a lightning strike, will produce high
voltages unless a low-impedance diverting path is offered to the flow of
current. If the current surge in a system is the result of a combined
current source and multi path to ground (Figure 9), there is then a
division of the current among the paths that is governed by the inverse
ratio of the impedances. If a user has control over only one of the
paths, he can decrease the amplitude of the current surge in his path
only by forcing ,a greater share of the total current to flow through
the other paths: thus, surge blocking is likely to be an exercise in
passing the problem from one point of the system to another. The
solution lies in surge diversion, offering to the surge a path where the
current flow can occur harmlessly.

**4.2 The Limitations of Transmission Line Analysis**

In qualitative discussions of surge propagation. the classical behavior
of a transmission line is often called upon to provide explanations of
the situation. In particular, the theoretical reflections occurring at
the end of a line are cited: doubling the impulse if the line is
open-ended, returning an inverse impulse if the line is shorted.
However, these discussions sometimes lose sight of the fact that the
concept can be applied only if the line length is sufficient to contain
all of the surge front. If the surge has a rise time longer than the
propagation time along the line, the point is moot and, by the time the
surge reaches its peak, the voltage at the receiving end of the line
does not differ from the voltage applied at the sending end. Figure 10
illustrates this situation showing the voltage at the sending end (SD)
and the voltage at the receiving end (RC) of a conduit-enclosed
three-wire line (B: black or phase conductor, W: white or neutral
conductor, G: green or grounding conductor). There is a minor difference
during the rise, where the initial front is doubled at the receiving
end, but the crests are the same. Thus, for many installations in
buildings, the line lengths are short compared to the length required to
contain, say, a 1 μs front traveling at a typical speed of 200 m/μs.

A more detailed report describing some of the aspects of the propagation
of surges and their implications is given in Reference 15.

**4.3 Common Mode or Normal Mode?**

Another aspect of the propagation of surges concerns the dichotomy
normal mode/common mode. In other words, the issue is whether
significant surges occur line-to-line (black-to-white, or phase-to.
neutral), the situation described by \"normal mode,\" or whether they
occur between any - or both - of the lines and ground (black-to-green,
white-to-green, or \[black-and-white\]-to-green) , the situation
described by \"common mode.\"

The answer to that dichotomy is that, in most cases, both modes must be
considered, since one often converts into the other, depending upon the
coupling, the wiring practices, and the attempts made at suppressing the
mode perceived as the greatest threat. Here again, the pervasive and
pervert reality is too often that a solution aimed at suppressing one
effect only displaces the problem. Examples 1 and 2, in the Appendix,
show how controversies on the method of suppressing a surge assumed to
occur between two wires can lead to partial or misleading solutions when
the realities of the situation are not recognized.

**5. Fundamental Protection Techniques**

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

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

**5.1 Basic Techniques**

Protection techniques can be classified into several 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 system depending on
low-voltage signals, high-impedance circuits, and installed over a wide
area would present much more serious problems than the same system
confined to a single building or a single cubicle.

For the system components or electronic black boxes, the 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.
While surges are unavoidable, one can attempt to block them, divert
them, or strive to withstand them; the latter, however, is generally
difficult to achieve alone.

**5.2 Shielding, Bonding, and Grounding**

Shielding, bonding, and grounding are three interrelated methods for
protecting a circuit from external transients. Shielding consists of
enclosing the circuit wiring in a conductive enclosure, which
theoretically 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. Many texts and
papers discuss the subject at length, so that a detailed discussion is
not necessary here, with one exception. which follows.

**5.3 Ground One End or Both Ends of the Shield?**

Shielding conductors by wrapping them in a grounded sheath or shielding
an electronic circuit by enclosing it in a grounded conductive box is a
defensive measure that occurs very naturally to the system designer or
the laboratory experimenter anticipating a hostile electromagnetic
environment. Difficulties arise, however, when the concept of
\"grounded\" is examined in detail. Difficulties also arise when the
goals of shielding for noise immunity conflict with the goals of
shielding for 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 miles of buried or overhead cables.
Grounding these diverse shields is not an easy thing to do because the
impedance to earth of the grounding connection must be 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. This concept is exemplified in the terminology
of shielded cable users, when they describe the shield construction of
some cable design as having a foil plus drain wire. as if there were
electrostatic charges that needed to be removed (drained). Indeed,
electrostatic charges can be drained by connecting only one end of the
shield. Furthermore, if the two ends of the shield of a cable spanning
some distance are connected to the local ground at each end, there is a
definite possibility that some power frequency current may flow in the
shield. For low-level signals, this current would produce noise (hum) in
the signals. For that reason, many system designers will insist on the
one-end-only grounding rule, and they are correct from that point of
view. Sometimes the shield is used as a return path for the 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. Worse yet, in the case of a lightning stroke
injecting current into the earth in the area spanned by the one-end-only
grounded shield, the potential of one end of the cable defined as
\"ground\" is not the same as the \"ground\" at the other end of the
cable. Very high voltages can be developed (Figure 11) between the
floating end of the shield and the local ground. No practical insulation
can withstand these levels, and breakdown will occur, allowing surge
currents to flow in spite of the designers\' intent to prevent them; the
path of these currents will be determined by the components most likely
to fail when the voltage rises - the low-level logic circuits, of
course. In contrast, by deliberately allowing part of these surge
currents to flow in the shields, one obtains a cancellation of the
voltages that otherwise would be induced in the circuits, and the
currents will follow a well-defined path that can be designed to provide
harmless effects.

> This conflict is actually very simple to resolve if recognized in
> time: provide an outer shield, grounded at both ends (and at any
> possible intermediate points); inside this shield the electronic
> designer is then free to enforce his single-point grounding rules. The
> only drawback to this approach is the hardware cost of double shields.
> In many installations, however, there is a metallic conduit through
> which the cables are pulled; with simple but close attention to
> maintaining the continuity of this conduit path, through all the
> joints and junction boxes, a very effective outer shield is obtained
> at negligible additional cost. In the case of underground conduit
> runs, the most frequent practice is to use plastic conduit, which
> unfortunately breaks the continuity. System designers would be well
> advised to require metal conduits where the circuits are sensitive or,
> at a minimum, to pull a shielded cable in the plastic conduit where
> the shield is used to maintain continuity between the above-ground
> metal conduits. That additional cost, then, is the insurance premium,
> which is well worth accepting. Example 3 in the Appendix shows the
> consequence of the misapplied \"ground one end only\" rule.

**6. Transient Suppressors**

Various devices have been developed for protecting electrical and
electronic equipment against transients. They are often called
\"transient suppressors\" although, for accuracy, they should be called
\"transient limiters,\" \"clamps,\" or \"diverters\" because they cannot
really suppress transients; rather, they limit transients to acceptable
levels or make them harmless by diverting them to ground.

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; thus, diverting the transient is
more likely to find general application. A combination of diverting and
blocking can be a very effective approach. This approach generally takes
the form of a multistage circuit, where a first device diverts the
transient toward ground, a second device - an impedance or resistance -
offers a restricted path to the transient propagation but an acceptable
path to the signal or power, and a third device clamps the residual
transient (Figure 12). Thus, we are primarily interested in the
diverting devices. These diverting devices can be of two kinds:
voltage-clamping devices ,or short-circuiting devices (crowbar). Both
involve some nonlinearity, either frequency nonlinearity (as in filters)
or, more usually, voltage nonlinearity. This voltage nonlinearity is the
result of two different mechanisms - a continuous change in the device
conductivity as the current increases, or an abrupt switching as the
voltage increases.

Because the technical and trade literature contains many articles on
these devices, a discussion of the details will be limited. Some
comparisons will be made, however, to point out the significant
differences in performance.

**6.1 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 (he 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. The first
limitation concerns the volt-time sensitivity of the breakdown process.
As the voltage increases across a spark gap, a significant conduction of
current - and hence the voltage limitation of a surge - cannot take
place until the transition occurs to the arc mode pf 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. In addition, this sparkover voltage can be
substantially higher after a long period of rest than after successive
discharges. From the physical nature of the process, it is difficult to
produce consistent sparkover voltage for low voltage ratings,

> The second limitation is associated with the sharpness of the
> spark-over, 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 13). 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 event delayed by a few
> nanoseconds, so that in real time, (he gap sparkover occurs and noise
> enters the oscilloscope by stray coupling while the electron beam is
> still writing the pre-sparkover rise.

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 mayor 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
nonlinear varistor that limits the power-follow current has been very
successful in the utility industry as a surge arrester or surge
diverter.

2.  **Voltage-Clamping Devices**

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

When a voltage-clamping device is installed, the circuit remains
essentially unaffected by the device before and after the transient for
any steady-state voltage below clamping level. Increased current drawn
through the device as the surge voltage attempts to rise results in
voltage-clamping action. Nonlinear impedance is the result if this
current rise is greater than the voltage increase. The increased voltage
drop (IR) in the source impedance due to higher current results in the
apparent clamping of the voltage. It should be emphasized that the
device depends on the source impedance to produce clamping. A voltage
divider action is at work where the ratio of the divider as not
constant, but changing. If the source impedance is very low, the ratio
is low, and eventually the suppressor could not work at all with a zero
source impedance (Figure 14). In contrast, a crowbar-type device
effectively short circuits the transient to ground; once established,
however, this short circuit will continue until the current (the surge
current as well as any power-follow current supplied by the power
system) is brought to a low level.

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

> **6.3 Avalanche Diodes**

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

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

**6.4 Varistors**

The term 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. Two very different devices have been successfully developed
as varistors: silicon carbide disks have been used for years in the
surge arrester industry, and more recently, metal oxide varistor
technology has come of age.

Metal oxide varistors depend on the conduction process occurring at the
boundaries between the large grains of oxide (typically zinc oxide)
grown in a carefully controlled sintering process. The physics of the
nonlinear conduction mechanism have been described in the literature
\[16-20\].

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 15. The
major element is the varistor proper, Rv, whose V -I characteristic is
assumed to be the perfect power law I = kV\". In parallel with this
varistor, there is a capacitor, C, and a leakage resistance, Rp. In
series with this three-component group, there is the bulk resistance of
the zinc oxide grains, Rs, and the inductance of the leads, L.

Under de conditions, at low-current densities because obviously no
varistor could stand the high energy deposited by de 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.

***6.4.1 V-I Characteristic.*** When the V -I characteristic is plotted
on a log-log graph, the curve of Figure 16 is obtained. Three regions
result from the dominance of Rp, Rv, and 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, 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 volt age
    of the application

-   Selection of the energy-handling capability (including consideration
    of the source impedance of the transient, the wave-shape, and the
    number of occurrences \[21\])

-   Heat dissipation

-   Proper installation in the circuit (lead length) \[22\]

**6.5 Packaged Suppressors**

The need for protection and the opportunity to provide packaged
protective devices to concerned computer users has prompted the
marketing of many packaged suppressors, ranging from the very simple and
inexpensive to the complicated (not necessarily much better) and
expensive. The field has also seen a number of devices claiming energy
savings in conjunction with transient suppression; there is no
foundation for such a claim, and the issue hopefully has been settled in
a study published by EPRI \[23).

Line conditioners using a ferro-resonant transformer are offered
primarily as line-voltage regulators, but also provide high attenuation
of transient overvoltages, a performance that isolating transformers do
not provide. Example No.2, in the Appendix, gives examples of the poor
performance of isolating transformers and of the effective performance
of line conditioners .in dealing with normal mode transients.

**6.6 Failure Modes**

Failure of an electrical component can occur because its capability was
exceeded by the applied stress or because some latent defect in the
component went unnoticed in the quality control processes. While this
situation 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 others,
fail-safe means that the junction 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 design of a protection scheme.
With nonlinear devices, an error made in the assumed value of the
current surge produces little error on the voltage developed across the
protective device and thus applied to the protected 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, e.g., an
error made in the assumption, a human error in the use of the device, or
because nature tends to support Murphy\'s law, the circuit in need of
protection can generally be protected at the price of failure in the
short-circuit mode of the protective device. However, if substantial
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).

With the failure mode of a suppressor being of the fail-short type, the
system protection with fuses can take two forms (Figure 17). For the
user concerned with maintaining the protection of expensive equipment,
even if failure of the protector means the loss of the function,
Alternative A must be selected. Conversely, if the function is
paramount, Alternative B must be selected.

**7. Conclusions**

Power system disturbances can inject damaging overvoltages in power
lines as well as data lines. Lightning surges can be equally damaging,
by direct termination of a stroke, or by induction or even differences
in ground potential caused by the flow of the current. Beware of
differential ground potential rise!

Fundamental precautions, best applied in the design and construction
stages, can provide effective protection at a small cost compared to the
alternative of failures and later retrofits. The cost of insurance
premiums always seems high before the accident.

Shielding, bonding, and grounding are the classical preventive methods
at the system and component leve1. Conflicts between traditional
grounding practices for noise reduction can be reconciled with the
requirements of surge protection. Grounding the shields at one end only
invites trouble.

A combined approach of fundamental precautions and protective devices
can provide effective protection over the range of natural and man-made
disturbances. However, these devices must be applied as part of a
concerted effort. Coordination of protective devices is the key to
functional and cost-effective protection.

**Acknowledgments**

Motivation for presenting this paper was provided by the reported case
histories and the penetrating questions raised by students at the
University of Wisconsin annual conferences on surge protection, as well
as by discussions with members of the IEEE Surge Protective Devices
Committee. Catharine Fisher and Elizabeth Zivanov of CRD contributed
valuable reviews and comments toward development of the final text.

**References**

1\. J.H. Hagenguth and J.G. Anderson, \"Lightning to the Empire State
Building,\" Part III, AlEE Transactions 7I PAS, August 1952.

2\. N. Cianos and E.T. 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,\" AlEE Transactions 64, 1945.

> 4\. World Distribution of Thunderstorm Days, WMO-OMM #21 TP6. Geneva
> World Meteorological Organization, 1953.
>
> 5\. R.H. Golde, \"The Lightning Conductor,\" Lightning II, R.H. Golde,
> ed. New York: Academic Press, 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 Transactions
of the Royal Society 63, London, 1773, and 68, 1777.

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

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

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

10\. IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits,
ANSI/IEEE Std C62.41-1980.

11\. J.E. Lenz, \"Basic Impulse Insulation Levels of Mercury Lamp
Ballast for Outdoor Applications,\" Illuminating Engrg., pp. 133-140,
February 1964.

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

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

14\. P. Chowdhuri, \"Transient Voltage Characteristics of Silicon Power
Rectifiers,\" IEEE Transactions on Industry Applications IA-9,
September/October 1973.

15\. F.D. Martzloff, The Propagation and Attenuation of Surge Voltages
and Surge Currents in Low-voltage AC Circuits, IEEE Transactions on
Power Apparatus and Systems PAS-102, May 1983, pp. 1163-1170.

16\. Transient Voltage Suppression Manual, Third Edition, General
Electric Company, Auburn, New York, 1982.

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

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

19\. M. Matsuoka, T. Masa Yama, and Y. Lida, \"Nonlinear Electrical
Properties of Zinc Oxide Ceramics,\" in Proc. of First Conf. Solid State
Devices, Tokyo, 1969, J. Japan Soc. Appl. Phys., vol. 39, 1970.

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

21\. F.D. Martzloff, \"Matching Surge Protective Devices to Their
Environment,\" Proc. of IEEE-I AS Annual Meeting. October 1983, pp.
387-392. Also available as Report 83CRD169, General Electric Company,
Schenectady, New York.

> 22\. F.A. Fisher, \"Overshoot - A Lead Effect in Varistor
> Characteris­tics,\" Report 78CRD201, General Electric Company,
> Schenectady, New York.
>
> 23\. Alexander Kusko, Inc., Evaluation of Transient Voltage
> Suppressors for Saving Electric Energy, Electric Power Research
> Institute Report EM-1722, Palo Alto, California, 1981.
>
> 24\. National Electrical Code 1981, National Fire Protection
> Association, Boston, Massachusetts.
>
> 25\. K.E. Crouch, Lightning Technology Inc., Pittsfield,
> Massachusetts, private communication.

**APPENDIX - EXAMPLES**

To illustrate the preceding discussion, some practical examples are
given in this appendix as the basis for sound design approaches.

***Example No.1 - Does an isolating transformer help?***

The author has witnessed and engaged in many discussions on the merits
of isolating transformers, sparked by the misconception that \"spikes
are attenuated by transformers\" or \"spikes do not pass through
transformers.\" Figures 18 and 19 are offered to support the position
that these quotations are misconceptions. When properly applied,
isolating transformers are useful to break ground loops and reduce
common mode effect, but .they do not by themselves attenuate spikes that
occur line-to-line in the normal mode.

Figure 18 shows the propagation - or worse, the enhancement - of a
voltage impulse in a 1:1 isolating transformer. The 6 kV, 0.5 μs-100 kHz
impinging wave of ANSI/IEEE Std C62.41-1980 is applied to the primary of
the transformer, H1H3 to H2H4. The output voltage, measured at X1X3 to
X2X4, appears as a 7 kV crest on the secondary side of this
\"isolating\" transformer. on the secondary side of a transformer.

Figure 18 was recorded with no load on the transformer secondary, which
represents the extreme case of the low-power electronic control in the
standby mode. Figure 19 shows the primary and secondary voltages of the
transformer with a 10 W (1500 Ω) and a 100 W (150 Ω) load on the
secondary side, at the same generator setting as Figure 18. With the 10
W load that might be typical of an electronic control in standby mode,
the combined series reactance of the transformer and shunt resistance of
the load produce the output shown in Figure 19A, still slightly higher
than the input.

With the l00W load shown in Figure 19B, the attenuation is now apparent,
but is only 2:1. Capacitive loads would, of course, produce a greater
attenuation than resistive loads for the inductive series impedance of
the transformer, at the frequency spectrum of this fast 2 μs-wide spike.
For surges of longer duration, the attenuation would be even smaller.

These examples show that, unless a well-defined load is connected to the
transformer, expecting attenuation from the transformer may prove to be
hazardous to the health of low-power electronics connected on the
secondary side of a transformer.

In contrast, decoupling is possible with a ferro-resonant line
conditioner which is primarily intended for line voltage regulation, but
which also provides a high degree of surge suppression. Figure 20 shows
the 6 kV incoming wave being attenuated to 60 V (100:1) on the secondary
side, of the unloaded line conditioner, and to 40 V (150:1) with a load
of only 10%; at full load, an attenuation to less than 10 V was
observed. The nature of the ferro-resonant line conditioner is such that
the decoupling improved with loading, while the simple transformer of
Figure 18 can only act as linear dividers with load changes. Conversely,
the decoupling between primary and secondary sides of the line
conditioner is further seen on the oscillogram recorded on the input
side of the line conditioner. This oscillogram is, in fact, a photograph
of two successive measurements, one with no load on the line conditioner
and one with a 100 W load. The input waves are exactly superimposed.

This decoupling reflects the nonlinear behavior of the ferro-resonant
line conditioner, which is significant in this case, compared to the
linear behavior of transformers: For surge sources of lower impedance
than the generator used in these tests, or for frequencies lower than
that contained in the 0.5 μs - 100 kHz spike, the transformer
attenuation would become lower, in direct proportion to the
corresponding impedance change, while the ferro-resonant line
conditioner would keep the decoupling unchanged.

For worst-case demonstration, the two oscillograms of the output were
recorded with the spike timed to occur at the peak of the 60 Hz line
voltage demonstration The peak-to-peak amplitude of the line voltage is
indicated by the gray band recorded on the oscillograms by
photographically superimposing repetitive traces of the line voltage.
For timings other than .the peak, the small voltage oscillation on the
output voltage would be completely contained within the normal
peak-to-peak band of the 60 Hz line voltage.

There is at present a trend to an upward spiral in specifying dB\'s of
attenuation in line conditioners. In the author\'s opinion, the point of
citing a 120 dB attenuation is moot because typical installation
practice will degrade that level of decoupling.

***Example No.2 - Connections options for suppressors and effects on
residual voltages.***

The author has witnessed lively controversies among various application
information sources on the most effective transient suppression
configuration to be applied. Taking, as a simplified example, the task
of specifying the protection of a single. phase equipment connected at
the end of a line with no opportunity to divert the transient closer to
the source (for instance; at the service entrance), the options would be
to connect one, two, or three varistors between the three wires (black,
white, and green) at the end of the line. However, additional
information needs to be known: Will the impinging surge be in the normal
mode (black-to-white) or in the common mode
(\[black-and-white\]-to-green)? Where in the equipment is the most
sensitive component:. line-to-line (most likely) or line (black OR
white)-to-green? Clearly, the situation is confusing, and there will not
be a single, simple answer applicable indiscriminately to all cases. The
National Electrical Code \[24\] specifically allows the connection of
surge arresters, (Article 280-22) if the interconnection occurs only by
operation of the surge arrester during the surge. Since the standby
current of varistors is very low, this requirement can be met;
furthermore, there will not be any interference with the operation of
Ground Fault Circuit Interrupters if there is only a small number of
protectors.

The set of measurements recorded in Figure 21 shows an example of these
many options with increasing protection, albeit at increasing cost, from
a single varistor to three varistors. The selection would depend on the
vulnerability level and location of the equipment to be protected. The
impinging surge is assumed to be black-to-\[white-and-green\], since
white and green are tied together at the service entrance. The line is a
75 m line and the surge is that available from the generator set for a
2000 A, 8/20 μs short-circuit impulse. Rather than attempt to modify the
setting of the generator for each case in order to maintain constant
current crest for the various configurations (an impossible task if wave
form is also to be maintained), the generator was left unchanged, to
discharge a constant total energy in the system - not a bad hypothesis
for the real world. The current crests are all in the range of 300 to
380 A, which is not a significant change for comparing varistor clamping
voltages.

If only one varistor is allocated to protect the equipment, the
black-to-white varistor connection (first row) affords maximum
protection for the electronics which are also likely to be connected
black-to-white. However, the voltages between either black or white and
green are large; this is the stress that will be applied to the
clearances of the equipment. This is a good example of conversion of a
normal mode transient into a common mode.

The configuration with varistor black-to-green (second row) does not
afford very good protection for components connected black-to-white;
therefore, it should be used only if there is a special need to clamp
black-to-green at a low voltage with only one varistor available.

An improved protection is obtained with a varistor connected
black-to-white, complemented by a second varistor connected
white-to-green (third row). The ultimate protection is, of course, one
varistor in every position (fourth row), but this should be required
only for exceptionally sensitive loads.

***Example No.3 - Ground potential rise on data lines.***

A distributed computer system had been installed with a central
processing unit and remote terminals located in separate buildings. In a
span of 5 weeks during the first summer, after commissioning the system,
three lightning storms occurred in the area; no direct strikes were
reported on the buildings, but extensive damage was done to the circuit
boards on terminals and CPU inputs.

After the first occurrence, power line surges were suspected and some
precautions were applied, when access to the hardware was possible, by
pulling out the ac power plugs from the CPU or terminal at the onset of
a lightning storm. This did not help. Next, isolating transformers were
considered but, again, did not help. At this point, the author was
called in for consultation, and the following proposed diagnostic was
established: the surges were not coming from the ac lines, but rather
were due to differences in the ground potential existing between the
separate buildings during flow of lightning currents. The data cables
had been run in PVC conduit, buried between the buildings and, true to
one tenet of steady-state noise prevention, only one end of the shield,
of the wire pairs had been grounded, with the, other left floating.
Figure 22 shows how this arrangement can produce high voltage between a
floating end of the shield and the local ground, a practice which is
bound to produce a flashover and flow of surge currents along unwanted
paths, in the circuit components. Thus, the problem was not power line
surges, but differential ground potential. Worse, by pulling out the ac
line plugs but leaving the incoming data cables connected, the operators
had unwittingly removed the local grounding connection from the hardware
frame, leaving only the data cables coming in, with a possibility of
raising the complete hardware several thousand volts above local grounds
in the room - a dangerous condition.

A solution to the problem could take several approaches. Radical
solutions, such as a microwave link or a fiber optics cable, would
indeed have eliminated the differential ground potential problems, but
were considered too expensive or too long to install.

Incidentally, part of the original puzzlement at the failures was the
notion that opto-isolators provided in the data link route should have
avoided problems. Close scrutiny of the circuits, however, disclosed
that the opto-isolators had been provided for some other purpose; in
fact, the ground potential loop was closed by the power supply to the
opto-isolator feeding the amplifiers from a local source rather than the
remote source, negating the isolation function.

Another solution, really the most simple and effective, would have been
the replacement of the plastic underground conduit by a continuous steel
conduit linking the steel conduits used inside the buildings. This would
have provided equalization of the ground potentials along the data
cable, while allowing the desired use of shields with one end only at
ground. However, that solution was not acceptable to the plant facility
organization.

In this particular location, a spare conduit had been buried next to the
data line conduit. This offered the possibility of pulling a heavy
ground cable in this conduit, close to the data cables, which could then
tie the two corners of the buildings at the point of entry of the data
cables, as a first step toward reducing differential ground potentials.
At first, this concept was somewhat difficult to sell to plant
facilities: because there is a ground grid tying to two buildings for 60
Hz faults, further ties between the two buildings did not seem
necessary. However, our thesis was that this grid would have too high an
impedance to serve the purpose, and furthermore that the data cable run,
located away from the ground grid, would form a flux-collecting loop
with the ground grid. After lengthy discussions, the thesis was accepted
and the cable was installed.

Simultaneously, the concept of tying the two ends of the cable shields
to ground was proposed, with the provision of a barrier that would avoid
the circulation of power-frequency currents \[25\]: inserting an array
of diodes (Figure 23) at one end of the shields reconciled the needs of
noise prevention under normal operation and the requirement of grounding
at both ends during lightning events. The forward drop of the two diodes
in series (1.5 V) was enough to block any 60 Hz circulating current that
would inject noise in the data cables. During a lightning strike,
however, the diodes would allow flow of current to compensate and cancel
the ground potential differences.

These two cures were implemented during the first summer, and no further
problems occurred for the rest of the lightning season. While these two
solutions might have been sufficient, the concern over another possible
failure of the system was sufficient to motivate the design of further
protection, the insertion of a voltage clamp in each data pair. This
solution required some design and acceptance testing from the system
manufacturer, so that it was not implemented until the next lightning
season. Thus, the system survived the remainder of the first lightning
season with only the first two remedies.

Experience has shown that conclusions on the effectiveness of lightning
protection schemes should wait perhaps as much as 10 years before being
proclaimed, because of the large variations in lightning activity.
However, after three summers of trouble-free operation compared to three
major failures in 5 weeks, the cure would seem effective. Hopefully,
these words will not have to be eaten by the author in a few years.

> In retrospect, then, the following recommendations can be drawn from
> this horror tale, for retrofits or new installations:

1\. Data cables linking separate buildings or spanning beyond a single
room within one building should have a shield tied to local ground at
both ends of the cable. If the first shield provided with the cables
must be left with one end floating by diktat of the system vendor, then
these cables should be installed within a continuous metal shield. This
continuous shield can be either a double shield of the flexible cable,
or simply a metal conduit, with both ends grounded.

2\. Substantial relief can be obtained in retrofits by grounding both
ends of existing shields through a low-voltage clamp, such as a diode
array, that will block noise-inducing power frequency currents, but will
allow the flow of ground-potential equalizing currents during surges.

3\. The ultimate protection may be the insertion of surge-protective
devices in each line. However, this solution requires careful design so
that degradation of the signals does not occur, and residual spikes are
not allowed to pass through.

**The Protection of Computer and Electronic Systems**

**Against Power Supply and Data Lines Disturbances**

François D. Martzloff

General Electric Company

Schenectady NY

**1. INTRODUCTION**

The irresistible trend toward greater numbers of distributed computing
systems may be a source of difficulties which the earlier, centralized
systems had overcome: surge voltages and surge currents injected or
induced into power supply lines and communication (data) lines. These
surges may be produced by lightning, by power system switching, or by
differences in the actual potential of points expected to be at ground
potential but which are, in fact, driven apart by a surge current.
Uninformed users of terminal equipment or personal computers might also
connect their equipment in a manner inviting difficulties. Another
source of transients, not covered in this report, is static discharge.

A different source of disturbances for computers is, of course,
undervoltage transients, sometimes called \"sags.\" These are largely
due to power system switching or faults, some being the consequence of
lightning. Unfortunately, it is more difficult to fill a void in the
power delivery than to divert and block an excess energy, so that the
techniques needed to protect computer systems against these sags require
approaches and equip­ment different from those discussed in this report,
whose scope is limited to overvoltage or overcurrent surges. A number of
approaches for mitigation of externally caused sags have been
successfully implemented by computer manufacturers and users. These
approaches range from power supplies with sufficient storage capacity or
motor generator sets with sufficient mechanical inertia to ride through
short sags, to uninterruptible power supplies with storage batteries and
solid-state inverters being instantaneously switched to take over a
failing utility power supply. These are now classical remedies and, when
properly applied, eliminate the problems of sags, while the problems
associated with surges still seem to be with us. This report presents a
summary of present knowledge on the occurrence of surges and on
mitigating measures, with recent case histories illustrating the
pitfalls of inappropriate applications.

**2, THE ORIGINS OF TRANSIENT OVERVOLTAGES**

Transient overvoltages in power systems originate from one cause --
energy being injected into the power system -- but from two sources:
lightning discharges or switching within the power system. In
communication or data systems there is another source of transients: the
coupling of power system transients into the system. Further­more, all
systems involving several connections to external equipment face the
risk of transient overvoltages associated with ground potential rise
during the flow of surge currents. As stated pre­viously, static
discharge problems are not treated in this report.

Lightning discharges may not necessarily mean direct termination of a
lightning stroke onto the system. A lightning stroke terminating on some
object near a power or data line will create a very fast-changing
magnetic field that can induce voltages - and inject energy - into the
loops formed by the conductors of the system. Lightning can also inject
overvoltages in a system by raising the ground potential on the surface
of the earth where the stroke terminates, while more distant \"ground\"
points remain at a lower voltage, closer to the potential of \"true
earth.\" The literature provides information on the characteristics of
lightning discharges. (1-6)

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

1.  **Transients in power systems**

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

Simple switching transients (7) include circuit closing transients,
transients initiated by the clearing of a short circuit, and transients
produced when the two circuits on either side of the switch, being
opened, oscillate at different frequencies. On the load side of the
switch, for circuits having inductance and capacitance (all physical
circuits have at least some, in the form of stray capacitance and
inductance) with little damping, these simple switching transients are
inherently limited to twice the peak amplitude of the steady-state
sinusoidal voltage. On the load side of the switch and without a surge
protective device, the current flowing just before switching is
available to charge the circuit capacitances at whatever voltage is
required to store the inductive energy of the current by converting it
into capacitive energy; voltages can reach high levels, such as ten
times the normal level.

Several mechanisms are encountered in practical power circuits that can
produce large transient overvoltages. Two such mechanisms occur
frequently: current chopping and restrikes, the latter being especially
troublesome when capacitor switching is involved.

A similar scenario can unfold when an ungrounded power system
experiences an arcing ground fault. The switching action is then not the
result of a deliberate parting of contacts but the intermittent
connection produced by the arc.

These switching overvoltages, high as they may be, are somewhat
predictable and can be estimated with reasonable accuracy from the
circuit parameters, once the mechanism involved has been identified.(8)
There is still some uncertainty as to when and where they occur because
the worst offenders result from some abnormal, and thus rare,\* behavior
of a circuit element. Lightning-induced transients are much less
predictable because there is a wide range of possibilities for

\* This rarity can take two different aspects: 1. In the vast majority
of circuits, these abnormal transients can some­times occur, but rarely
(\"when\"). 2. Among all circuits, a few rare ones are frequently and
consistently afflicted with such abnormal behavior (\"where\").

In response to these concerns, various committees and working groups
have attempted to describe ranges of transient occurrences or maximum
values occurring in power circuits. Three such attempts are described in
the next section. Figures 1, 2, and 3 show typical examples of
transients recorded in power systems.

**2.2 Transients in data lines**

Data lines are different from power lines in the occurrence of surges:
power systems can generate their own transients, as seen above, in
addition to receiving injected surges. Data lines are only subjected to
what the environment will inject. However, the operating voltages and
the overvoltage tolerance of signal-processing components in these data
systems are generally much lower than those of power system components.
Thus, damage (not to discount problems of misoperation) is more likely
to occur to data lines than to power system components from the same
exposure to injected transients.

The systematic effort at characterizing surges in power circuits,
exemplified by the IEEE standard described in the next section, has not
been extensively reported by users of data lines, with the exception of
the telephone industry. (9- 13) In the telephone environment, much
emphasis is placed on lightning effects on long overhead or buried
cables and lines, as well as on induced noise from adjacent power
systems. The same IEEE group that produced the Guide on surge voltages
is now addressing the lack of knowledge on data lines, such as
inter-building computer links and process control lines in chemical
plants, which will be more relevant to computer users than are telephone
environment data. (14) Figures 4 and 5 show transients recorded on
typical data lines exposed to lightning effects.

**3. STANDARDS ON TRANSIENT OVERVOLTAGES IN POWER LINES**

Several standards or guides have been issued or proposed in Europe and
in the United States specifying a surge withstand capability for
specific equipment or devices and specific conditions of transients in
power or communication systems. Some of these specifications represent
early attempts to recognize and deal with the problem in spite of
insufficient data. As a growing number of organizations address the
problem and as exchanges of information take place, improve­ments are
being made in the approach. Three of these are briefly discussed here.

1.  **The IEEE Surge Withstand Capability Test (SWC) (15)**

One of the earliest published documents which addressed new problems
facing electronic equipment exposed to power system transients was
prepared by an IEEE committee dealing with the exposure of power system
relaying equipment to the harsh environment of high-voltage substations.

Because this useful document was released at a time when little other
guidance was available, users attempted to apply the recommendations of
this document to situations where the environment of a high-voltage
substation did not exist. The revised version of this standard, soon to
be issued, recognizes the problem and attempts to be more specific (and
restrictive) in its scope. Thus, an important consideration in the
writing and pub­lishing of documents dealing with transients is a clear
definition of the scope and limitations of the application.

**3.2 The IEC 664 Report (16)**

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

**3.3 The IEEE Guide on Surge Voltages (ANSI/IEEE Std C62.41-1980)
(17)**

***Voltages and rate of occurrence***

Data collected from a number of sources led to plotting a set of lines
representing a rate of occurrence as a function of voltage for three
types of exposures (Figure 6). These exposure levels are defined in
general terms as follows:

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

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

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

The two lower lines of Figure 6 have been drawn at the same slope
because the data base shows reasonable agreement among several sources
on that slope. All lines may be truncated by sparkover of the
clearances, at levels depending on the withstand voltage of these
clearances. The high exposure line needs to be recognized, but it should
not be indiscriminately applied to all sys­tems. Such application would
penalize the vast majority of installations where the exposure is lower.

The voltage and current amplitudes presented in the Guide attempt to
provide for the vast

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

***Waveshape of the surges***

Many independent observations (17- 21 ) have established that the most
frequent type of tran­sient overvoltage in ac power systems is a decaying
oscillation, with frequencies between 5 and 500 kHz. This finding is in
contrast to earlier attempts to specify the unidirectional
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
proposes two representative waveshapes: an oscillatory waveshape inside
buildings, a unidirectional waveshape outside buildings, and both at the
interface (Figure 7).

The oscillatory waveshape simulates those transients affecting devices
that are sensitive to dv/dt and to voltage reversals during conduction,
(22) while the unidirectional voltage and current waveshapes, based on
long-established ANSI standards for secondary valve arresters, represent
an equivalent of the transients where energy content is the significant
parameter. Recent concerns on the occurrence of longer duration surges,
or lower frequencies, such as the 5 kHz lower limit cited, will probably
be reflected in the updating of this standard over the next several
years.

From a pragmatic point of view, the realization that oscillating waves
are unavoidably produced by practical test systems will also be a
driving force toward specification of oscillatory waveforms rather than
unidirectional impulses in future standards.

Increased recognition of the upset aspect of transient overvoltages is
also likely to result in the inclusion of a \"fast transient\" with rise
time and duration in the nanosecond range.

***Energy and source impedance***

The energy involved in the interaction of a power system with a surge
source and a surge protective device will divide between the source and
the protective device in accordance with the characteristics of the two
impedances. With a 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, or in the impedance of the circuit
upstream of the protective device. With an energy-absorber gapless
protective device, a substantial share of the surge energy is dissipated
in the suppressor, but its clamping action does not involve the
power-follow energy resulting from the short-circuit action of a gap. It
is therefore essential to the effective use of surge protective devices
that a realistic assumption be made about the source impedance of the
surge whose effects are to be duplicated.

Unfortunately, not enough data have been col­lected on what this
assumption should be for the source impedance of the transient.
Standards and recommendations, such as DOD STD-1399 or the IEC 664
report, either ignore the issue or indicate values applicable to limited
cases, such as the SWC test for high-voltage substation equipment.
ANSI/IEEE C62.41 attempts to relate impedance to categories of locations
but unavoidably remains vague on their definitions (Table 2).

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 (24- 27) for surge protective devices will be helpful in this
selection process. Other groups in the USA, as well as the international
bodies of the IEC and the Comité Consultatif International Télégraphique
et Téléphonique (CCITT), are now working toward further refinements and
the reconciliation of different approaches.

**3.4 Previous and future surge recordings**

The supporting data cited in Appendix A of ANSI/IEEE C62.41 are based on
voltage surge recordings made in the 1962-1975 period. In that period,
digital instrumentation for surge monitoring was not as readily
available as it is now, and, most significantly, the proliferation of
surge protective devices, such as metal oxide varistors, had not reached
the present level.

Measurements, limited to voltage, were conducted with
oscilloscope/camera systems or with peak-recording instruments. Voltages
were generally recorded between the line(s) and the neutral of a
single-phase or polyphase power system. No measurements had been
reported as neutral-to-ground; some may have been between line and
ground. Of course, that distinction is moot for measurements made at the
service entrance where neutral and ground are bonded.

Prior to the proliferation of varistors, a limitation had been
recognized for peak voltages: the flashover of clearances, occurring
typically between 2 and 8 kV for low-voltage wiring devices. For that
reason, the curves of Figure 1 in the Guide include the indication of a
possible truncation of the distribution around 6 kV. Recent studies,
still in progress, have indicated that benign flashover of clearances,
without power follow and therefore not readily detectable, may be more
prevalent than was previously believed.

An estimate of the number of low-voltage surge protective devices such
as varistors used in the United States since 1972 on ac power circuits
is in the order of 500 million. An undefined but substantial portion of
that number is installed in permanently connected equipment. Therefore,
it is now very likely that a new limitation exists in the recording of
voltage surges. A surge recording instrument installed indiscriminately
at a random location may have a varistor connected across the line near
the point being recorded.(28) This situation will have several
implications for the recordings obtained in present and future
measure­ments, as contrasted to those of previous measurement campaigns.

1\. Locations where voltage surges were previously identified - assuming
no change in the source of surges - are now likely to experience lower
voltage surges, while current surges will occur in the newly installed
protective devices.

2\. Not only will the peaks of the observed voltages be changed, but
also their waveforms will be affected by the presence of nearby
varistors as follows:

> a\. If a varistor is located between the source of the surge and the
> recording instrument, the instrument will record the clamping voltage
> of the varistor. This voltage will have lower peaks but longer time to
> half-peak than the original surge.
>
> b\. If the instrument is located between the source of the surge and a
> varistor, or if a varistor is installed in a parallel branch circuit,
> the instrument will record the clamping voltage of the varistor,
> preceded by a spike corresponding to the inductive drop in the line
> supplying surge current into the varistor.
>
> c\. If a varistor is connected between line and neutral with a surge
> impinging
>
> between line and neutral at the service entrance, a new situation is
> created: the line-to-neutral voltage is indeed clamped as intended,
> but the inductive drop in the neutral conductor between the point of
> connection of the varistor and the service entrance creates a spike
> voltage between the neutral and the grounding connector at the point
> of connection of the varistor and downstream points supplied by the
> same neutral. Because this spike will have a short duration, it will
> be enhanced by the open-end transmission line effect between the
> neutral and grounding con­ductors.(29)

3\. The surge voltage limitation function performed by flashover of
clearances is more likely to be assumed by new surge protective devices
that are constantly being added to the systems.

4\. The considerations discussed in paragraphs 1, 2, and 3 above will
produce a significant reduction in the mean of recorded voltage surges
in a population of different locations. This reduction will continue as
more and more varistors are installed. The upper limit, however, will
remain the same for locations where no varistor has yet been installed.
A sense of false security and an incorrect description of the
environment might be created by attention given only to the average of
voltage surges presently recorded in power systems. Furthermore, the
need for adequate surge current handling capability of a new candidate
surge suppressor might be underestimated if partial surge diversion is
already being performed by a nearby varistor. This risk will be
exacerbated if an attempt is made to clamp at lower voltages by the
installation of a new protective device with a clamping voltage lower
than that of the device already installed.(8)

**4. SURGE PROPAGATION**

**4.1 The limitations of arbitrary division into categories**

The standards cited in the preceding paragraphs describe surges which
may be expected at specific points of a wiring system; the implication
is that the surges will proceed downstream, at the same amplitude and
waveform, until some interface somehow produces a staircase-like
decrease in amplitude. IEC 664 proposes a voltage staircase with its
overvoltage categories (Table 1), while ANSI/IEEE C62.41 proposes a
current staircase with its location categories (Figure 8).

This staircase representation is useful to simplify the real world into
a manageable set o assumptions, but it is a simplification that ca mask
the reality. Surges will propagate in the system starting at the point
of entry; voltage surge will be attenuated to the extent that the series
impedance between the point of interest and the source (ZI, Figure 9) on
one hand and the shun impedance (Z2) on the other hand, form a voltage
divider. If the series impedance is low and the shunt impedance is high
(light loading of the system), the voltage divider does not produce high
attenuation of the voltage surges. In addition to an attenuation of the
amplitude, a waveform change takes place which is most apparent for fast
front and short duration pulses. (29) Conversely, current surges, if
they are the result of a current source such as a lightning strike, will
produce high voltages unless a low-impedance diverting path is offered
to the flow of current. If the current surge in a system is the result
of a combined current source and multipath to ground (Figure 10), there
is then a division of the current among the path that is governed by the
inverse ratio of the impedances. If a user has control over only on of
the paths, he can decrease the amplitude of the current surge in his
path only by forcing a greater share of the total current to flow
through the other paths; thus, surge blocking is likely to be an
exercise in passing the problem from one point of the system to another.
The solution lies in surge diversion, offering to the surge a path where
the current flow can occur harmlessly.

**4.2 The limitations of transmission line analysis**

In qualitative discussions of surge propagation, the classical behavior
of a transmission line is often called upon to provide explanations of
the situation. In particular, the reflections occurring at the end of a
line are cited in accordance with the theory that the impulse is doubled
if the line is open-ended, and that an inverse impulse is returned if
the line is shorted. However, these discussions sometimes lose sight of
the fact that the concept is applicable only if the line length is
sufficient to contain all of the surge front. If the surge has a rise
time longer than the propagation time along the line, the point is moot
and, by the time the surge reaches its peak, the voltage at the
receiving end of the line does not differ from the voltage applied at
the sending end.

Independently from the location of a device or equipment In the above
figure, It should remain safe (no fires, no personnel hazard) over the
full range of available surges at any point within the Installation. It
may also be desirable, under particular circumstances and for specific
devices, to proscribe damage as a result of testing at higher levels
than might be suggested by its typical location.

The effects of reflections and attenuations along a line are further
illustrated in Figures 12, 13, and 14, excerpted from Reference 29. A
three-wire, conduit-enclosed line, 225 m long was subjected to short
pulses applied through an impedance-matching network to provide minimum
interaction between the surge generator and the line. Access points at
75 and 150 m, in addition to the far end at 225 m, were provided within
reach of the measuring oscilloscope probes by folding the line in a
zig-zag configuration.

Figure 12 shows the propagation of a 200 ns-wide pulse when a matching
impedance is connected at the receiving end. The attenuation of this
pulse is quite apparent, with an average ratio of 0.7 between the
voltages at points 75 m apart in the line. The travel time can also be
seen as 1.1 μs from sending to receiving and, corresponding to 225 m/1.1
/μs = 204 m/μs, or two-thirds the speed of light in vacuum.

Figure 13 shows the same pulse propagating in the same line, but with a
doubling of the voltage at the receiving end, which was left open. In
spite of this doubling affect, there has been enough attenuation of the
short spike over the 225 m that the pulse at the receiving end is lower
than the pulse at the sending end. However, for shorter lines, the
attenuation would not have taken its toll before doubling at the end:
doubling the pulse which appears at the intermediate 75 m of the 225 m
line implies that, at the end of an open-ended 75 m, one could have a
pulse 20% greater than at the sending end. Note also that, while the
amplitudes are attenuated, the rise time tends to be increased;
therefore, the time integral of the pulse (and therefore its potential
for damaging energy-sensitive components) is not attenuated as quickly
as is the amplitude along the line length.

Figure 14 shows a 2 μs-wide pulse propagating along the same line, with
matched termination. The attenuation is lower than for the 200 ns pulse,
with an average ratio of 0.9 between the voltages of points 75 m apart
in the line. This lower attenuation at longer pulse duration (lower
frequencies) results from the decreased effect of the shunt capacitance
of the line.

These propagation characteristics will be discussed again in conjunction
with the performance of protective devices, in Section 6 of this report.
Thus, for many installations contained in a building, the line lengths
are short compared to the length required to contain, say, a 1 μs front
traveling at the speed of 200 m/μs. A more detailed report describing
some of the aspects of the propagation of surges and their implications
is given in References 29 and 30.

**4.2 Common Mode or Normal Mode?**

Another aspect of the propagation of surges concerns the dichotomy
normal mode/common mode. In power lines, the issue is whether
significant surges occur line-to-line (black-to-white, or
phase-to-neutral), the situation described by \"normal mode,\" or
whether they occur between any - or all - of the lines and ground
(black-to-green, white-to-green, or \[black-and-white\]-to-green), the
situation described by \"common mode.\" These terms were first defined
in the context of signal lines, where the concern for balanced circuits
reflects the fact that apparently innocuous common mode noise can be
converted into objectionable normal mode when circuit impedances along
the two signal-carrying wires are not symmetrical with respect to the
ground (common) conductor (Figure 15).

Conversely, Figure 16 shows how an attempt to protect against a normal
mode surge can produce a harmful common mode-like surge: at the end of a
branch circuit, the user installs a surge-protective device connected
line-to-neutral, in order to protect his load equipment against a normal
mode surge impinging the origin of the branch circuit, where the user in
this assumed scenario has no access to provide a surge protective
device. Common knowledge that the neutral and grounding conductors are
bonded at this origin might lead the user to believe that no harmful
voltage can occur at his end between neutral and grounding conductors.
Yet, the very installation of the protective device between line and
neutral, results in a significant L di/dt voltage developed along both
line and neutral conductors when the surge current flows into the
protective device; half of the total voltage appears at the user\'s end
between the neutral and the grounding conductors. Because there is
generally no admittance between the neutral and grounding conductors at
the user\'s end, the situation is then that of an open-ended line, which
produces a doubling of the impinging neutral-to-ground voltage. An
example of this situation is discussed in Section 6.5.

Thus, the answer to the normal/common mode dichotomy is that, in most
cases, both modes must be considered, because one can convert into the
other, depending upon the coupling, the wiring practices, and the
attempts made at suppressing the mode perceived as the greatest threat.
Here again, the pervasive and pervert reality is too often that a
solution aimed at suppressing one effect only displaces the problem.
Case History No. 5, later in this report, shows quantitative measurement
results of the effects of various methods of connecting a surge
protective device at the end of a line.

**5. FUNDAMENTAL PROTECTION TECHNIQUES**

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

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

**5.1 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 and power system faults
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 using low-voltage signals, high-impedance circuits,
and installed over a wide area such as a chemical plant spread over
several kilometers, would present much more serious problems than the
same system confined to a single building. As discussed in Case History
No. 1, the installation of remote terminals in separate buildings is a
prime candidate for trouble unless some basic precautions are observed.

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 or diversion of surges at their
origin.

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

**5.2 Shielding, Bonding, and Grounding**

Shielding, bonding, and grounding are three interrelated methods for
protecting a circuit from external transients. Shielding is the practice
of enclosing the circuit components in a conductive enclosure, which
theoretically cancels out any electromagnetic field inside the
enclosure-, actually, it is more an attenuation than a cancellation
because the enclosure is rarely complete and perfect. Bonding is the
practice of providing low-impedance connections between adjacent metal
parts, such as the panels of a shield, cabinets in an electronic rack,
or rebars in a concrete structure. Grounding is the practice of
providing a low impedance to earth or a well-defined reference
ultimately con­nected to earth,\* through various methods of driving
conductors into the soil. Each of these techniques has its limitations,
and each can sometimes be overemphasized.

### Shielding

Shielding conductors by wrapping them in a grounded sheath or shielding
an electronic circuit by enclosing it in a grounded conductive box is a
defensive measure that occurs very naturally to the system designer or
the laboratory experimenter anticipating a hostile electromagnetic
environment. Difficulties arise, however, when the concept of
\"grounded\" is examined in detail. Difficulties also arise when the
goals of shielding for noise immunity conflict with the goals of
shielding for surge immunity.

A shield can be the size of a matchbox or an airplane fuselage; it can
cover a few centimeters of wire or kilometers of buried or overhead
cables. Effective grounding of these diverse shields is not always an
easy thing to do because the impedance to earth of the grounding
connection must be acknowledged. The situation is made even more
controversial because of the conflict between the often-proclaimed
design rule \"ground cable shields at one end only\" - a rule justified
by noise immunity performance, in particular common mode noise
reduction - and the harsh reality of current flow and Ohm\'s law when
lightning strikes or when power systems faults occur.

The difficulty may be caused by a perception on the part of the noise
prevention designers that the shield serves as an electrostatic shield
in which longitudinal currents associated with common mode noise
coupling should not flow. This concept is exemplified in the terminology
of shielded cable users, when they describe the shield construction of
some cable design as having a foil plus \"drain wire,\" as if there were
electrostatic charges that needed to be removed (drained). Indeed,
electrostatic charges can be drained by connecting only one end of the
shield. Further-more, if the two ends of the shield of a cable spanning
some distance are connected to the local ground at each end, there is a
definite possibility that some power frequency current may flow in the
shield. For low-level signals, this current could produce noise (hum) in
the signals. For that reason, many system designers will insist on the
one-end-only grounding rule, and they are correct from that point of
view. Sometimes the shield is used as a return path for the signal
cir­cuit, in which case shield currents will 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. Worse yet, in the case of a lightning
stroke or of a power system fault injecting current into the earth in
the area spanned by the one-end-only grounded shield, the potential of
one end of the cable defined as \"ground\" is not the same as the \"
ground\" at the other end of the cable. Very high voltages can be
developed (Figure 17) between the floating end of the shield and the
local ground. No practical insulation can with -stand these levels, and
breakdown will occur, allowing surge currents to flow in spite of the
designers\' intent to prevent them. The path of these currents will be
determined by the components most likely to fail when the voltage
rises - the low-level logic circuits, of course. In contrast, by
deliberately allowing a small part of these surge currents to flow in
the shields, one obtains a cancellation of the voltages that connection
of the reference to earth is a general safety requirement involving low
frequency fault currents for which adequate low-impedance connection is
a matter of conductor resistance. At the high frequencies involved with
transient disturbances, the inductance of the connection becomes
significant.

This conflict is actually very simple to resolve if recognized in time:
provide an outer shield, grounded at both ends (and at any possible
intermediate points); inside this shield the electronic designer is then
free to enforce his single-point grounding rules. The only drawback to
this approach is the hardware cost of double shields. In many
installations, however, there is a metallic conduit through which the
cables are pulled; with simple but close attention to maintaining the
continuity of this conduit path, through all the joints and junction
boxes, a very effective outer shield is obtained at negligible
additional cost. In the case of underground conduit runs, the most
frequent practice is to use plastic conduit, which unfortunately breaks
the continuity. System designers would be well advised to require metal
conduits where the circuits are sensitive or, at a minimum, to pull a
shielded cable in the plastic conduit where the shield is used to
maintain continuity between the above-ground metal conduits. That
additional cost, then, is the insurance premium, which is well worth
accepting. Case History No. 1, given later in this report, illustrates
the penalty inflicted by nature when the one-and-only one-ground rule
was misapplied by the designer.

### Bonding

We have already mentioned one aspect of bonding in describing the
continuity of the outer shield. Another instance of bonding occurs where
the shield of an incoming cable is connected to an equipment cabinet in
order to allow shield current flow. The shield current flows in the
connecting pigtail and creates electromagnetic radiation at the point of
cable entry.

Adjacent cabinets in a lineup must be bonded together for safety as well
as transient and noise immunity. In principle, a flat strap has a lower
inductance than a round wire of the same area. This concept may be
somewhat overused; actually several strategically located smaller wires
provide a much more effective bond than one massive strap, either round
or flat. The difficulty lies in imple­menting this alternate view, and
overcoming the comforting sight of a large grounding strap at the bottom
of the cabinet lineup. Such a strap does no harm and is a good safety
practice, but it may not do as much good as expected from the point of
view of surge protection, compared to multiple point bonds.

A significant subset of the general subject of bonding is the
termination of cable shields by connectors at the junction to an
equipment cabinet. The search for low-cost construction often results in
the shield being connected through one pin of the multiple-pin
connector. Add to this construction the misguided concept that all
grounding connections in a cabinet should be made to a single point, and
the result will be the worst possible practice, as shown in Figure 18A:
the shield current is injected inside the cabinet along a tortuous path,
creating interference in all circuits. Figure 18B shows a tolerable
practice, where the connection is still made through the connector, for
convenience, but a very short lead makes the connection to the cabinet
frame inside the cabinet. Figure 18C shows an improvement, with the
connection made outside the cabinet. A variation of this arrangement is
encountered in RS232 connectors with a metal shell where the two
securing screws provide a double (and symmetrical) bond to the equipment
chassis. The ultimate and best bond, of course, is obtained by using a
connector with a continuous bond provided by a cable connector with
metallic shell and ring screwed to the chassis connector all around the
cable (Figure 18D).

### Grounding

Grounding, or earthing, has different meanings as well as different
roles. The primary definition is the connection of the circuit, shield,
or refer­ence to earth. But what is earth? System designers, construction
crews, inspectors, and technical conference authors are concerned with
establishing, measuring, and maintaining a low ground resistance, often
determined by dc measurements on rods driven into the ground. Driving
many rods into the ground at great expense does not ensure a low
impedance under the transient conditions of a high rate of current
change associ­ated with lightning discharges.

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, acting as a zero-reference. 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
(short of a flying aircraft, which does quite well, thank you, without
an earth connection), 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 surges or power system faults, high vol­tages will appear
across these isolated wires and local ground - voltages which, in some
cases, are far beyond the withstand capability of insulation. That
insulation, then, must be protected by suitable devices which, in fact,
do connect the wires to the local ground, but only for the duration of
the transient. This type of momentary grounding is one function of
transient suppressors.

An effective approach to limiting the adverse effects of ground
potential differences is the enforcement of a \"ground window\"\*
arrangement of all conductors entering a system, as shown in Figure 19.
The system can be a single cabinet, room-size equipment, or a complete
floor in a building - the principle remains the same. This ground window
must be specified from the beginning, as retrofits are generally
difficult to make.

**5.3 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 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 between the subsystems.

Where moderately high voltages only can occur, effective isolation can
be accomplished by the insertion of an isolating transformer or an
opto-isolator; if metallic isolation is not required, a filter can also
be used if it does not degrade data pulse shapes or system frequency
response.

Where the voltages will reach levels exceeding the withstand capability
of economically or technically feasible insulation, two possible
solutions exist. The first, already mentioned, and applicable to power
as well as data systems, is to bond the grounds or references of the two
systems during the transient by means of a surge protective device,
which returns to a high level of insulation after the transient has
subsided. The second, applicable only to low-power data transmission, is
to use other methods, such as insertion of audio couplers or a fiber
optics link. Complete decoupling of electrical transients and noise
resulting from ground potential differences can be achieved in this
manner; however, these techniques will not guard against noise collected
by the circuits themselves and faithfully transmitted by the link to the
other end.

**6. TRANSIENT SUPPRESSORS**

Various devices have been developed for protecting electrical and
electronic equipment against transients. They are often called
\"transient suppressors\" although, for accuracy, they should be called
\"transient limiters\...\.... clamps,\" or \"diverters\" because they
cannot really suppress transients; rather, they limit transients to
acceptable levels or make them harmless by diverting them to ground
around the sensitive equipment.

There are two categories of transient suppressors: those that block
transients, preventing their propagation toward sensitive circuits, and
those that divert transients, limiting voltages to an acceptable
residual level. Because many of the transients originate from a current
source, the blocking of a transient may not always be possible; thus,
diverting the transient is more likely to find general application. A
combination of diverting and blocking can be a very effective approach.
This approach generally takes the form of a multistage circuit, where a
first device diverts the transient toward ground, a second device - an
impedance or resistance - offers a restricted path to the transient
propagation but an acceptable path to the signal or power, and a third
device clamps the residual transient (Figure 20). Thus, we are primarily
interested in the diverting devices. These diverting devices can be of
two kinds: voltage-clamping devices or short-circuiting devices
(crowbar). Both involve some nonlinearity, either frequency nonlinearity
(as in filters) or, more usually, voltage nonlinearity. Depending on the
type of device, this voltage nonlinearity is the result of two different
mechanisms - a continuous increase in the device conductivity as the
current increases, or an abrupt switching as the voltage increases.

Because the technical and trade literature con­tains many articles on
these devices, a discussion of the details will be limited and review of
the references is suggested. Some comparisons will be made, however, to
point out the significant differences in performance; clarification of
some issues resulting from unwarranted concern will also be given. We
will first examine the basic principles of single-component suppressors,
then the application of these devices to protect circuits, as single- or
multiple-component packaged devices.

1.  **Crowbar devices**

The principle of crowbar devices is simple: upon occurrence of an
overvoltage, the device changes from its normal 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 the recently
introduced two-terminal multi-junction semiconductors. Some applications
have also been made of externally 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 surge-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 dissipation of
high energy within the device itself-, the energy has to be spent
elsewhere in the circuit. This so-called \"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
communication circuits, the spark gap has the advantage of very simple
construction with potentially low cost.

The crowbar device, however, has three major limitations. The first
limitation concerns the volt-time sensitivity of the breakdown process.
As the voltage increases across a spark gap, a significant conduction of
current - and hence the voltage limitation of a surge - cannot take
place until the transition occurs to the arc mode of conduction by
avalanche breakdown of the gas between the electrodes. The load is left
unprotected during the initial rise because of this delay time
(typically in microseconds). Considerable variation exists in the
sparkover voltage achieved in successive operations, because the process
is statistical in nature. In addition, this sparkover voltage can be
substantially higher after a long period of rest than after successive
discharges. From the physical nature of the process, it is difficult to
produce consistent sparkover voltage for low-voltage ratings. This
difficulty is increased by the effect of manufacturing tolerances on
very small gap distances, but it can be alleviated by filling the tube
with a gas having a lower breakdown voltage than air. However, if the
enclosure seal is lost and the gas is replaced by air, this substitution
creates a reliability problem due to the substantially higher sparkover
of the air gap.

In communication circuits using a pair of conductors, protection is
often provided by connecting a gas tube between each conductor and
ground. Upon occurrence of a common mode surge, which would leave the
input circuits unaffected, an nature of the process, it is difficult to
produce consistent sparkover voltage for low-voltage rat­ings. This
difficulty is increased by the effect of manufacturing tolerances on
very small gap distances, but it can be alleviated by filling the tube
with a gas having a lower breakdown voltage than air. However, if the
enclosure seal is lost and the gas is replaced by air, this substitution
creates a reliability problem due to the substantially higher sparkover
of the air gap.

In communication circuits using a pair of conductors, protection is
often provided by connecting a gas tube between each conductor and
ground. Upon occurrence of a common mode surge, which would leave the
input circuits unaffected, an undesirable condition results from the
volt-time variation between the two devices: unavoidable manufacturing
tolerances between the two devices, plus the statistical variation for
each tube breakdown cause one tube to fire before the other. During the
time separating the two firings, a substantial normal surge is applied
to the input circuitry, with possible destructive effects (Figure 21).
This problem can be avoided by using three-electrode tubes where firing
of the first gap causes firing of the second gap with no delay (Figure
22).

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 illustration of this problem is found in
oscillograms recording the sparkover of a gap where the trace exhibits
an anomaly before the sparkover (Figure 23). This anomaly is due to the
delay introduced into the oscilloscope circuits to provide an advanced
trigger of the sweep.

What the trace actually shows is the event delayed by a few nanoseconds,
so that in real time, the gap sparkover occurs and noise enters the
oscilloscope by stray coupling, while the electron beam is still writing
the pre-sparkover rise. Another, more objectionable, effect of this fast
current change can be found in some hybrid protective systems. The
circuit of one such commercial device is shown in Figure 24. The gap
does a very nice job of discharging the impinging high-energy surges,
but the magnetic field associated with the high di/dt induces a voltage
in the loop adjacent to the second suppressor, adding what can be a
substantial spike to the expected clamping voltage provided by the
second device. An illustration of this effect is discussed in Case
History No. 1.

A third limitation occurs when a power current from the steady-state
voltage source follows the surge discharge (follow-current or
power-follow). In ac circuits, this power-follow current may or may not
be cleared at a natural current zero. In dc circuits, clearing is even
more uncertain. Additional means, therefore, must be provided to open
the power circuit if the crowbar device is not designed to provide
self-clearing action within specified limits of surge energy, system
voltage, and power-follow current. This combination of a gap with a
current-limiting, nonlinear varistor has been very successful in the
utility industry as a surge arrester, often referred to as a
\"valve-type arrester.\"

2.  **Voltage-clamping devices**

Voltage-clamping devices exhibit a variable impedance, depending on the
current flowing through the device or the voltage across its terminals.
These components show a nonlinear characteristic -- that is, Ohm\'s law
can be applied, but the equation has a variable R. Impedance variation
is monotonic and does not contain discontinuities, in contrast to the
crowbar device, which shows a discontinuity by turn-on action. As far as
volt-ampere characteristics are concerned, these components are
time-dependent to a certain 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 in a circuit, the circuit
remains essentially unaffected by the device before and after the
transient for any voltage below clamping level. Increased current drawn
through the device as a surge voltage attempts to rise results in
voltage-clamping action. Nonlinear impedance means that this current
increases more than the voltage. The increased voltage drop (IR) in the
source impedance due to higher current results in the apparent clamping
of the voltage. It should be emphasized that the device depends on the
source impedance to produce clamping. A voltage divider action is at
work where the ratio of the divider is not constant, but changing. If
the source impedance were very low, the ratio would be low, and
eventually the suppressor could not work at all with a zero source
impedance (Figure 25). In contrast, a crowbar type of device effectively
short circuits the transient to ground; once established, however, this
short circuit will continue until the current (the surge current as well
as any power­-follow current supplied by the power system) is brought to
a low level.

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

3.  **Avalanche diodes**

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

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

Silicon avalanche diodes are available with characteristics tailored to
transient suppression. These should not be confused with regulator-type
Zener diodes although many engineers tend to use the generic term
\"Zener diode.\" May Zeus help them if they misapply a regulator-type
Zener, expecting to achieve good protection!

Since the junction is very thin, the capacitance of an avalanche diode
is appreciable. This can be a concern. The effect of capacitance can he
minimized by using series combinations with low-capacitance diodes
(Figure 26).

4.  **Varistors**

The term varistor is derived from the function of the device as variable
resistor. This device has also been called a voltage-dependent resistor,
but that description tends to imply that voltage is the independent
parameter in surge protection, while in fact surge current is the given
parameter. Two very different devices have been successfully developed
as varistors: silicon carbide blocks have been used for years in the
surge arrester industry, and more recently, metal oxide varistors have
become widely used.

Metal oxide varistors depend on the conduction process occurring at the
boundaries between grain 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. (31-35)

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 27. The
major element is the varistor proper, Rave, whose V-I characteristic is
assumed to be the perfect power law, I = kV\". In parallel with this
varistor, there is a capacitor, C, and a leakage resistance, Rap. In
series with this three-component group, there is the bulk resistance of
the zinc oxide grains, Rest, 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 not dealt with property;
and the capacitance can offer either a welcome additional path for fast
transients or an objectionable loading at high frequency, depending on
the application.

When the V-I characteristic is plotted on a log-log graph, the curve of
Figure 28 is obtained. Three regions result from the dominance of Rap,
then Rave, and finally R, as the current in the device increases from
Nan amperes to kilo amperes.

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, discussed in 7.6.

5.  **Packaged suppressors**

The need for protection and the opportunity to provide packaged
protective devices to concerned computer users has prompted the
marketing of many packaged suppressors, ranging from the very simple and
inexpensive to the complicated (not necessarily much better) and
expensive. The field has also seen a number of devices claiming energy
savings in conjunction with transient suppression; there is no
foundation for such a claim, and the issue, hopefully, has now been
settled in a study published by EPRI. (36)

Component surge protective devices such as gaps, varistors, or avalanche
diodes are used by manufacturers for incorporation into the circuitry of
their products. In contrast, packaged suppressors are applied by prudent
users as preventive and complementary protection, or by aggrieved users
as retrofit protection. These packaged suppressors may contain only a
single protective device or a combination of devices; they are available
for power-line protection, for data-line protection, and also in
combined power/data lines protection.

The combined power/data lines protection packages offer not only
convenience for protecting both lines of peripheral or remote equipment
but also, and very important, permit the implementation of a common
reference (ground) between the power and the data line. This common
reference can be located right at the point of installation, and thus
realize the ground window approach discussed in the preceding section.

A new type of suppressor has also appeared on the market, the so-called
\"tracking protectors.\" This type of device provides a voltage-limiting
action over a narrow band of deviation from the power-frequency sine
wave, rather than the fixed, absolute voltage limit of clamping or
crowbar devices. Typical circuits involve the switching on of a shunt
capacitor when the instantaneous voltage deviation exceeds a preset
limit.

Packaging of the suppressors accomplishes two desirable goals:
convenience of insertion by the user and coordination of the design for
multiple-component protective schemes. Unfortunately, this packaging
sometimes intentionally obscures the principles of protection being
offered, making an evaluation of performance claims difficult. The
competitive nature of these products is an unavoidable reality, which
does not justify obscuring performance characteristics, even if some
users are only interested in simple assurances that the devices packaged
will provide them with adequate protection.

One reason for the frequent lack of information on the performance of
the packages being offered is a lack of standards that would provide
manufacturers and users with realistic and uniform application
requirements. Component protective devices have the benefit of presently
available test specification standards, (24-27) but standards-writing
groups have not yet completed their projects on packaged suppressors. In
particular, the IEEE has an ongoing project that will take several years
of work before publication; the Underwriters\' Laboratories (UL) are
approaching release of a document (36) that will provide not only safety
guidance but also a more uniform basis of comparison of the packages
being offered in the trade.

As an example of power line packaged suppres­sors, the recently
introduced General Electric VSS device offers plug-in protection with
both line-to-neutral and neutral-to-ground protective devices (Figure
29). Further protection against low-level, high-frequency disturbances
is obtained with the VNS device, which has an L-C filter added to the
package (Figure 30).

The presence of neutral-to-ground protection in these two packages is a
definite improvement over the simpler packages, which provide only
line-to-neutral or neutral-to-line protective devices. Each of these two
simpler devices may leave parts of the \"protected\" load unprotected
during the occurrence of certain types of surges, as men­tioned in the
common mode/normal mode discussion of the preceding section. Figure 31
illustrates the complete protection provided by the dual devices, and
Figure 32 shows the incomplete protection of the simple devices, where a
normal mode surge is converted into a voltage transient between the
neutral and the grounding conductor.

6.  **Line conditioners as surge suppressors**

The need to provide \"clean power\" to sensitive electronic equipment
has promoted the development of a wide variety of devices generally
described as \"line conditioners.\" Depending upon the design and
principle involved, these devices can perform several of the following
functions: line isolation, voltage regulation (medium- and long-term),
noise suppression, surge suppression, common mode suppression, and
back-up power supply. Close inspection of the specifications is required
to distinguish the details of the perfor­mance of these devices. Their
principle of operation can be used to categorize the types:

-   Ferro resonant transformers (primarily regulation)

-   Isolation transformers with filters (primarily common-mode
    decoupling)

-   Motor-generator sets (primarily decoupling and short-time
    ride-through)

-   Magnetic synthesizers (primarily regulation)

-   Electronic synthesizers (primarily regulation)

-   Electronic rectifiers/battery/inverters (primarily uninterruptible
    power supplies)

When properly understood and applied, these devices can provide not only
their desired primary functions, but surge suppression as well - a
welcome side effect. A notable exception, resulting from
misunderstanding of the application, is discussed in Case History No. 4.

1.  **Failure modes**

Failure of an electrical component can occur because its capability was
exceeded by the applied stress or because some latent defect in the
com­ponent went unnoticed in the quality control processes. While this
situation is well recognized for ordinary components, a surge protective
device, which is no exception to these limitations, tends to be expected
to perform miracles, or at least to fail graciously in a fail-safe mode.
The term \"fail-safe,\" however, may mean different failure modes to
different users and, therefore, should not be used. To some users,
fail-safe means that the protected hardware must never be exposed to an
overvoltage, so that failure of the protective device must be in the
fail-short mode, even if it puts the system temporarily out of
operation. To others, fail-safe means that the function must be
maintained, even if the hardware is left temporarily unprotected, so
that failure of the protective device must be in the open-circuit mode.
It is more accurate and less misleading to describe failure modes as
fail-short or fail-open, as the case may be.

When the diverting path is a crowbar-type device, little energy is
dissipated in the crowbar, as noted earlier. In a voltage-clamping
device, because more energy is deposited in the device, the
current-handling capability of a candidate pro­tective device is an
important parameter to consider in the design of a protection scheme.
With nonlinear devices, an error made in the assumed value of the
current surge produces little error on the voltage developed across the
protective device and thus does not affect the protective function
(Figure 33). However, the error is directly reflected in the amount of
energy which the protective device has to absorb. At worst, when surge
currents in excess of the protective device capability are imposed by
the environment (for example, an error made in the assumption, a human
error in the use of the device, or nature\'s tendency to support
Murphy\'s law), the circuit in need of protection can generally be
protected at the price of failure of the protective device in the
short-circuit mode. However, if substantial 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).

With the failure mode of a suppressor being of the fail-short type, the
system protection with fuses can take two forms (Figure 34). For the
user concerned with maintaining the protection of expensive equipment,
even if failure of the protector means the loss of the function,
Alternative A must be selected. Conversely, if the function is
paramount, Alternative B must be selected.

**7. EXAMPLES AND CASE HISTORIES**

To illustrate the preceding considerations, some practical examples are
given in this section as the basis for sound design approaches, or as
horror tales where the names of the \"guilty\" have been withheld: it is
always easy, with hindsight, to see what should have been done, but less
easy at the outset for engineers unfamiliar with surge protection and
more concerned with other system considerations.

These case histories show how unintentional but clear violations of the
principles described in this report resulted in the problems that
follow:

Case 1: Ground potential differences on data lines caused by
discontinuous shield

Case 2: Ground potential differences on power lines caused by ignorance
of the ground window approach

Case 3: Insufficient and poor utilization o protection

Case 4: Misunderstanding of common versus normal mode

Case 5: Controversies on connection options

Case 6: Measurement problems

Case 7: Unwanted surge suppression by a instrument

Case 8: Undersized protective device

The detailed explanations of the problems and cures in these case
histories are offered as a help to avoid these types of pitfalls.

**Case History No. 1 - Computer graphics system: Ground potential
difference on data lines**

A CAD/CAM graphics system had bee installed by a computer graphics
vendor linking central processing unit to remote terminals locate in
separate buildings. In a span of 5 weeks during the first summer after
the system was commissioned, three lightning storms occurred in the
area; no direct strikes were reported on the buildings, but extensive
damage was done to the circuit boards on terminals and central
processing unit (CPU) inputs.

After the first occurrence, power line surges were suspected and some
precautions were applied, when access to the hardware was possible, by
pulling out the ac power plugs from the CPU or terminals at the onset of
a lightning storm. This did not help. Next, isolating transformers were
installed but, again, did not help. At this point, the author was called
in for consultation, and the following proposed diagnosis was
established: the surges were not coming from the ac lines but, rather,
were due to differences in the ground potential existing between the
separate buildings during flow of lightning currents. The data cables
had been run in plastic conduit buried between the buildings and, true
to the controversial tenet of steady-state noise prevention, only one
end of the shield of the wire pairs had been grounded, with the other
left floating. Figure 35 shows how this arrangement can produce high
voltages between a floating end of the shield and the local ground, an
arrangement that is bound to produce a flashover and flow of surge
currents along unwanted paths in the circuit components. Thus, the
problem was not power line surges but differential ground potential.
Worse, by pulling out the ac line plugs but leaving the incoming data
cables connected, the operators had unwittingly removed the local
grounding connection of the hardware frame, leaving only the data cables
coming in, with a possibility of raising the complete hardware several
thousand volts above local grounds in the room a dangerous condition.

A solution to the problem could take several approaches. Radical
solutions, such as a micro­wave link or a fiber optics bundle, would
indeed have eliminated the differential ground potential problems, but
were considered too expensive or too long to install.

Incidentally, part of the original bewilderment at the failures was the
notion that opto-isolators provided in the data link route should have
served to avoid problems. Close scrutiny of the circuits, however,
disclosed that the opto-isolators had been provided for some other
purpose; in fact, the ground potential loop was closed by the power
supply to the opto-isolator feeding the amplifiers from a local source
rather than the remote source, negating the isolation function.

Another solution, really the most simple and effective in principle,
would have been the replacement of the plastic underground conduit by a
continuous steel conduit linking the steel conduits used inside the
buildings. This additional metal would have provided equalization of the
ground potentials along the data cable, while allowing, within the
conduit, the desired use of shields with one end only at ground.
However, that solution was not acceptable to the plant facility
organization. (One does not dig up the front lawn two times within a few
months at an industrial park!)

In this particular location, a spare conduit, buried next to the data
line conduit, offered the possibility of pulling a heavy ground cable in
the spare conduit,. close to the data cables. This cable the computer
vendor, so that it was not implemented until the next lightning season,
the spring of 1981. Thus, the system survived all of the remainder of
the 1980 lightning season with only the first two remedies.

Voltage clamps were designed for insertion at each end of the data
cable, at the point where the cables were terminated prior to connection
to the CPU or to the terminal (Figure 37). The objective was to clamp
any transient developed in the pair, with respect to ground, at less
than 25 V, without introducing excessive degradation of the pulses\'
fronts in the signals transmitted by the cables. Vendors of hybrid
protection packages were consulted and samples were obtained for
evaluation. All these samples consisted of a gas tube protective device
connected between the line and ground and followed by some resistance,
and a silicon avalanche suppressor between line and ground (Figure 38).
The criteria were the optimization of the RC parameters of the
suppressor R being the series resistance, and C the capacitance of the
silicon diode - for maximum suppression with limited and acceptable
decrease in the steepness of the data pulses.

During evaluation testing of the various candidate suppressors submitted
by prospective vendors, it became apparent that the physical layout of
the components had an effect on the clamping voltage obtained: when the
gas tube gap sparked over, the high di/dt in the gap produced a high
rate of flux change in adjacent loops, in particular the loop involving
the second-stage avalanche diode and the load (Figure 39). While the
clamping voltage of the avalanche diode was, in fact, 15 V, as much as
45 V spikes were recorded across the output of the packaged hybrid
suppressors, raising doubts about the effectiveness of the protection.
These spikes were extremely short (nanoseconds), but the vendor of the
integrated circuits that had failed in the initial problems would not
agree to consider more than the specified maximum 25 V overvoltage, even
for these very short spikes.

Since the proposed connection between the interface box in the terminal
or CPU rooms and the terminal equipment involved a flexible connection
by shielded pairs, a simple solution was possible. The inherent
capacitance of the pairs to their shield, combined with an additional
resistance at the output of the hybrid suppressor could reduce this
induced spike below 25 V (Figure 40). Thus, an acceptable package was
designed, providing for the clamping of any surge to a level below the
tolerance level of the integrated circuits of the line drivers or line
receivers, but still not producing an objectionable degradation of pulse
fronts. The complete protection scheme was installed before the 1981
lightning season, and no further prob­lems have been reported.

Experience has shown that conclusions on the effectiveness of lightning
protection schemes should wait perhaps as much as 10 years before being
proclaimed, because of the large variations in lightning activity.
However, after several years of trouble-free operation compared to three
major failures in 5 weeks, the cure would seem effective.

In retrospect, then, the following recommendations can be drawn from
this horror tale,\' for retrofits or new installations:

1\. Data cables linking separate buildings or spanning beyond a single
room within one building should have a shield tied to local ground at
both ends of the cable. If the first shield provided with the cables
must be left with one end floating by diktat of the system vendor, then
these cables should be installed within a continuous metal shield. This
continuous shield can be either a double shield of the flexible cable or
simply a metal conduit with both ends grounded and proper attention to
maintenance of its continuity.

> \*Which, in the last several years, was found repeated at several
> other facilities involving different systems but the same basic
> problem. Thus, this case history has achieved the status of
> \"classic\" or \"textbook\" importance.

2\. Substantial relief can be obtained in retrofits by grounding both
ends of existing shields through a low-voltage clamp, such as a diode
array, that will block noise-inducing power frequency currents but will
allow the flow o ground-potential equalizing currents during surges.

3\. The ultimate protection may be the insertion of surge-protective
devices in each line. However, this solution requires careful design so
that degradation of the signals does not occur and residual spikes are
not allowed to pass through.

4\. While damage prevention can be accomplished by these approaches,
data errors may still be produced. If data integrity is an absolute
requirement, metallic connections should be avoided for data links
spanning remote terminals.

**Case History No. 2 - Computer-aided industrial control: Ground
potential differential on the**

**power lines (absence of ground window)**

In this situation, a novel adaptive control using sophisticated
microprocessor-based sensors and phase-control of power thyristors,
limited to a single room in a building, had suffered system crashes and
memory component damage on repeated occasions. Suspicions developed that
there was some correlation between the crashes or damage and the
operation of another developmental power system in an adjacent
laboratory. The software engineers working on the control system were
attempting to continue their work by staggering the schedule with their
neighbors; they were also considering installation of an independent
power feeder to their system.

From another point of view, focused by the author, this case presented
an opportunity to learn, while still in the laboratory, what the real
world can inflict on unprotected computerized power systems. Rather than
blame the power supply, a more fruitful approach for the long run would
be to develop immunity to interference and damage. In defense of the
system designers, it should be pointed out that their system had enough
challenges to be tackled in the main objectives that it might have
seemed reasonable to overlook the interference problems in the initial
stages. Sooner or later, however, the ignored problems will crop up,
and, sooner is better than later when fundamental design concepts are
involved.

A review of the total system revealed the existence of ground loops. On
one side, the power supply for the computer and some signal processing
circuits were obtained from the room outlets of the laboratory 120 V
system, including the grounding conductor (green safety wire). On the
other side, the power supply for the high-power circuit was obtained
from a feeder coming directly from the building power center, including
again a grounding conductor run alongside this power line, properly
installed by electricians, and bonded to the frame of the machine being
con-trolled. One of the signals used for controlling the process was
derived from a voltage pickup referred to the frame of the machine,
while the chassis of the computer and its zero reference were bonded to
the grounding conductor of the 120 V room supply. Thus, a double ground
loop was formed: one between the grounding conductor of the 120 V room
supply and the power-feed grounding conductor, and the other between the
overall zero reference of the signal processing and the voltage pickup
with its separate ground reference (Figure 41).

With hindsight, it was not difficult to conclude that during transient
conditions involving the high-power feed to this system and the
neighboring system, substantial current could flow between the two
grounding wires linked by the computer reference wiring. An immediate
cure was to open this path for surge ground currents by inserting an
isolating transformer in the 120 V supply to the computer, and bonding
the secondary side of this transformer to the single ground point
derived from the high-power feed (a National Electrical Code
requirement). This correct application of an isolating transformer, to
open a ground loop, is in contrast to the misconception that isolating
transformers can eliminate line-to-line spikes, as discussed in Case
History No. 4.

Opening the second ground loop involving the voltage pickup could not be
as readily imple­mented because it required a differential voltage pickup
circuitry. Plans for further refinements of the system included this
change, but even with only the first ground loop opened, the major
crashes and damage stopped; only occasional interference occurred,
probably not associated with the second group loop but, rather, with a
more subtle software problem associated with sensing the power flow in
the system. Clearly, the first ground loop was one of the major sources
of the problem, a problem that would have been avoided if the system had
been arranged with a single ground window.

**Case History No. 3 - Outdoor recorder retrofit: Insufficient surge
protection against**

**line-to-ground surges**

In this case history, a field failure problem was caused by a lack of
awareness (on the part of the circuit designer) of the degree of
hostility in the environment where the circuit was to be installed. A
varistor had been provided to protect control circuit components on the
printed circuit board, but its capability was exceeded by the surge
currents occurring in the particular location. In defense of the circuit
designer, however, it must be stated that he was unaware of the data
published in IEEE Std 587 (now ANSI/IEEE C62.41).

Since 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 42).
Laboratory proof-testing of the retrofit demonstrated the capability of
the combined scheme to withstand 6 kA crest current surges (Figure 43),
which is a 200% margin from the suggested IEEE/ANSI C62.41 Category B
level. Furthermore, it demonstrated reproduction of the field failure
pattern (Figure 43). The latter is an important aspect of any field
problem retrofit. By simulating in the laboratory the assumed surges
occurring in the field, verification of the failure mechanism is the
first step toward an effective cure. Figure 43 illustrates the effect of
improper installation of the suppressor in a first retrofit attempt with
8 inches of leads instead of a direct connection across the input
terminals of the circuit. The author has observed far too many
applications of varistors with excessive lead lengths, to the point that
the protection is substantially reduced for fast rising surges, the
present case being typical.

## Case History No. 4 - Does an Isolating transformer help?

The author has witnessed and engaged in many discussions on the merits
of isolating transformers, sparked by the misconception that \"spikes
are attenuated by transformers\" or \"spikes do not pass through
transformers.\" Figures 44 through 46 are offered to support the
position that these quotations are misconceptions. When properly
applied, isolating transformers are useful to break ground loops, but
they do not by themselves attenuate spikes that occur line-to-line, the
so-called \"normal mode.\"

Figure 44 shows the propagation - or worse, the enhancement - of a
voltage impulse in a 1:1 isolating transformer. The 6 kV, 0.5 μs-100 kHz
impinging wave of ANSI/IEEE C62.41 is applied to the primary of the
transformer, H1H3 to H2H4. The output voltage, measured at X1X3 to X2X4,
appears as a 7 kV crest on the secondary side of this \"isolating\"
transformer.

Figure 45 shows the similar behavior of a transformer offered as a line
isolator. This transformer is intended to provide low effective
capacitance and ground loop isolation between primary and secondary
windings, but here again, the author has observed that users of this
device expect attenuation of spikes. The response of this isolator, due
to its internal construction, is different from that of the simple
two-winding transformer of Figure 44, but we also note that a crest of 8
kV occurs on the secondary side during the second half-cycle. Hardly an
improvement.

Figures 44 and 45 were recorded with no load on the transformer
secondary, which represents the extreme case of a low-power electronic
control in standby mode. Figure 46 shows the primary and secondary
voltages of the transformer with a 10 W (1500 Ω) and a 100 W (150 Ω)
load on the secondary side, at the same surge generator setting as
Figure 44. With the 10 W load that might be typical of an electronic
control in standby mode, the combined series reactance of the
transformer and shunt resistance of the load produce the output shown in
Figure 46, still slightly higher than the input.

With the 100 W load shown in Figure 46, the attenuation is now apparent,
but is only 2:1. Capacitive loads would, of course, produce a greater
attenuation than resistive loads for the inductive series impedance of
the transformer, at the frequency spectrum of this fast, 2 μs, wide
spike. For surges of longer duration, the attenua­tion would be even
smaller.

These examples show that, unless a well-defined load is connected to the
transformer, expecting attenuation from the transformer may prove to be
hazardous to the health of low-power electronics connected on the
secondary side of a transformer.

By contrast, decoupling is possible with a ferro-resonant line
conditioner which is primarily intended for line voltage regulation but
which also provides a high degree of surge suppression. Fig­ure 47 shows
the 6 kV incoming wave being attenuated to 60 V (100:1) on the secondary
side of the unloaded line conditioner, and to 40 V (150:1) with a load
of only 10%; at full load, an attenuation to less than 10 V was
observed. The nature of the ferro-resonant line conditioner is such that
the decoupling improves with loading, while the simple transformers of
Figures 44, 45, and 46 can only act as linear dividers with load
changes. Conversely, the decoupling between primary and secondary sides
of the line conditioner is further seen on the oscillogram recorded on
the input side of the line conditioner. This oscillogram is, in fact, a
photograph of two successive measurements, one with no load on the line
conditioner and one with a 100 W load. The input waves are exactly
superimposed.

This decoupling reflects the nonlinear behavior of the ferro-resonant
line conditioner, which is significant in this case, compared to the
linear behavior of transformers: For surge sources of lower impedance
than the generator used in these tests, or for frequencies lower than
the frequency contained in the 0.5 μs - 100 kHz spike, the transformer
attenuation would become lower, in direct proportion to the
corresponding impedance change, while the ferro-resonant line
conditioner would keep the decoupling unchanged. See also Case History
No. 7 for an application of a ferro-resonant line conditioner to
decouple a surge protective device from the power supply, the inverse
situation of what is described here.

For worst-case demonstration, the two oscillograms of the output were
recorded with the spike timed to occur at the peak of the 60 Hz line
voltage demonstration. The peak-to-peak amplitude of the line voltage is
indicated by the gray band recorded on the oscillograms by
photographically superimposing repetitive traces of the line voltage.
For timings other than at peak, the small voltage oscillation on the
output voltage would be completely contained within the normal
peak-to-peak band of the 60 Hz line voltage.

## Case History No. 5 - Connections options for suppressors and effects on residual voltages

The author has witnessed lively controversies over the most effective
transient suppression configuration to be applied. Taking, as an
example, the task of specifying the protection of an appliance or
equipment connected at the end of a line with no opportunity to divert
the transient closer to the source (for instance, at the service
entrance), the options would be to connect one, two, or three varistors
between the three wires (black, white, and green) at the end of the
line. However, additional information needs to be known: Will the
impinging surge be in the normal mode (black-to-white) or in the common
mode (\[black-and-white-to-green)? Where in the equipment is the most
sensitive component: line-to-line (most likely) or line (black or
white)-to-green? Clearly, the situation is confusing, and there will not
be a single, simple answer applicable indiscriminately to all cases. The
National Electrical Code (39) specifically allows the connection of
surge arresters between neutral and grounding conductors (Article
280-22) if the interconnection occurs only by operation of the surge
arrester during the surge. Since the standby current of varistors is
very low, this requirement can be met; furthermore, there will not be
any interference with the operation of Ground Fault Circuit Interrupters
if there are only a small number of protectors.

The set of measurements recorded in Figure 48 shows an example of these
many options with increasing protection, albeit at increasing cost, from
a single varistor to three varistors. The selection would depend on the
vulnerability level and location of the equipment to be protected. The
impinging surge is assumed to be black-to-\[white-and-green\], since
white and green are tied together at the service entrance. The line is a
75 m line and the surge is that available from the generator set for a
2000 A 8/20 μs short-circuit impulse. Rather than attempt to modify the
setting of the generator for each case in order to maintain a constant
current crest for the various configurations (an impossible task if
waveform is also to be maintained), the generator was left unchanged, to
discharge a constant total energy into the system - not a bad hypothesis
for the real world. The current crests are all in the range of 300 to
380 A, which is not a significant varia­tion for comparing varistor
clamping voltages.

Only one varistor is allocated to protect the equipment, the
black-to-white varistor connection (first row) affords maximum
protection for the electronics, which are also likely to be connected
black-to-white. However, the voltages between either black or white and
green are large; that voltage is the stress that will be applied to the
clear­ances of the equipment. This situation is a good example of the
conversion of a normal mode tran­sient into a common mode, as discussed
in Section 6.5.

The configuration with varistor black-to-green (second row) does not
afford very good protection for components connected black-to-white;
therefore, it should be used only if there is a special need to clamp
black-to-green at a low voltage with only one varistor available or
allowed.

An improved protection is obtained with a varistor connected
black-to-white, complemented by a second varistor connected
white-to-green (third row). The ultimate protection is, of course, one
varistor in every position (fourth row), but this should be required
only for exceptionally sensitive loads.

## Case History No. 6 - Measurement problems

Considerable controversy has been raised on an \"overshoot\" associated
with the performance of varistors under fast pulses. Actually, this
overshoot is primarily a measurement problem associated with lead
effects. To illustrate the effect of lead length on the overshoot, two
measurement arrangements were used. As shown in Figures 49(a) and 49(b),
respectively, 0.5 cm^2^ and 22 cm^2^ of area were enclosed by the leads
o the varistor and of the voltage probe.

The corresponding voltage measurements are shown in the oscillograms of
Figures 49(c) and 49(d). With a slow current front of 8 As, 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 in Figure 49(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.

Other measurement errors can be introduced by the connection of the
voltage probes, as illustrated by the following experiment. When making
voltage measurements across a clamping device for evaluating its
performance, one must recognize possible difficulties requiring special
precautions. Two precautions must be taken:

1\. Use two probes in a differential mode to make a measurement directly
at device terminals. Commercial oscilloscope preamplifiers offer a wide
choice of differential mode operation, either through a mode (add
Channel A + invert Channel B) with 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.

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

3.  To illustrate the second point, the measurement circuit shown in
    Figure 50 was set up in the output circuit of a generator producing
    a 8/20 μs impulse. The \"device\" was a hollow conductor, with a
    hole at the center through which a twisted pair was fed, one wire of
    the pair branching out to each end of the conductor, separated by
    10 cm. At the same 10 cm separation, but outside of the hollow
    conductor, two thin wires were attached and brought -to the midpoint
    of the hollow conductor, in close contact with the conductor; from
    the midpoint outward, they were twisted in the same manner as the
    inside pair. A third set of wires was soldered at the end points of
    the hollow conductor, and arranged to form a rectangle, the hollow
    conductor being one side of that rectangle. Several widths could be
    set up for the rectangle, and each time the measured voltage was
    recorded. Figure 52 shows the measured voltage versus radial
    distance of the opposite side of the rectangle, plotted from the
    oscillograms of Figure 51.

> This experiment shows that not only must one connect the probes as
> close as possible to the terminals of a clamping device but still
> strive to minimize the area established by the probes close to the
> device. In the case of a low-voltage suppressor, it would be better to
> solder short leads to the device terminals, bring them together while
> tightly hugging the device, then twist them in a pair and connect the
> oscilloscope probes some distance away from the device.

This experiment also shows the importance of wire layout in making the
connections of a protective device in an actual circuit. As discussed in
Case History No. 1, creating a loop near the pro­tective device is an
invitation to induce additional voltages in the output of the protective
device, thus losing some of its effectiveness.

Hence, when one is making measurements as well as when one is designing
a circuit for a pro­tection scheme, it is essential to be alert to the
effects of lead length (or more accurately of loop area) for connecting
the varistors. This warning is especially important when the currents
are in excess of a few amperes with rise times of less than 1 μs.

## Case History No. 7 - The best surge suppressor is a surge monitor!

Increasing recognition of the existence of transients on power lines has
encouraged extensive use of commercial disturbance analyzers as a first
step in identifying a potential transient problem. Surveys have also
been made on the quality of the available ac power, based on recordings
obtained from such analyzers. However, the results of such measurements
may be ambiguous as a result of the design of at least one of these
instruments, as made by Dranetz Technologies, Inc., until recently.

The problem arises from a characteristic of the Dranetz equipment, which
exists for both Models 606 and 626, and which was not recognized at the
time some measurements were made but is now pointed out in more recent
Dranetz instruction manuals.

In order to protect the electronics of the Disturbance Analyzer from
damage by overvoltages in the power supply to these internal
electronics, a surge suppressor has been provided in the input to the
power supply - not the monitoring input of course. However, if the ac
power system being monitored is the same as the power system in which
the instrument power supply cord is plugged - a likely possibility in
the general case and precisely the situation of some reported
measurements - then the observations of surge occurrences on that power
system are those of a system whose transients have been suppressed!

To support this claim, Figure 53 shows an oscillogram recorded at the
output of a surge gen­erator which provides both 120 V ac power and the
IEEE/ANSI C62.41 Ring Wave, Category B. The oscillogram shows the surge
without the Dranetz analyzer plugged into the test system out-put, and,
superimposed, the effect of plugging the power cord only of the Dranetz
analyzer into the test system output. Without the analyzer, the
open-circuit voltage is 3 kV; with the analyzer plugged in, the output
voltage is reduced to 1.1 kV.

The ANSI/IEEE C62.41 Category B characteris­tics are 6 kV open-circuit
voltage, 500 A short­circuit current, therefore a source impedance of
6000 : 500 = 12 Ω. From the circuit values of Figure 54, the unknown
effective impedance o the analyzer, Z, can be computed to be only 7 Ω.
That low impedance, when connected in parallel with the voltage
measurement leads, will load the source of the transient and yield lower
voltage recordings than the actual occurrence would have been without
the analyzer connected. This situation makes the facetious remark in the
1970 paper come true (\"the best surge suppressor is a surge
monitor!\").

While it is too late to correct data already recorded, there is a very
simple solution to the problem. Ferro-resonant line conditioners not
on\] provide surge isolation at their output but also decoupling of the
input from the output. (30) Figure 55 shows the open-circuit output of
the surge generator at 6 kV (upper trace) and the output with the line
conditioner feeding the analyze plugged in (lower trace). There is no
detectable effect on the impinging surge. Thus, by merely inserting the
line conditioner in the power cord o the analyzer, the issue disappears,
and measurements can be obtained without the ambiguity which can cause a
sense of false security in the relatively low levels of impulse cited in
some o the published reports.

**Case History No. 8 - Varistor versus environment: Winning the
rematch**

During the initial startup of a solid-state motor drive in a chemical
processing plant, difficulties arose with the varistor and its
protective fuse at the input of the thyristor circuits. Frequent blowing
of the fuse was observed, with occasional failure of the varistor. The
plant substation, fed at 23 kV from the local utility, included a large
capacitor bank with one-third of the bank switched on and off to provide
power factor and system voltage regulation. These frequent switching
operations were suspected of generating high-energy transients that
might be the cause of the failure of the fuses and varistors, because
literally thousands of similar drive systems have been installed in
other locations without this difficulty.

On-site measurements performed after repeated blowing of fuses and
occasional failure of varistors connected at the input to the thyristor
drive indicated that indeed the devices were not matched to their
environment. From this point on, specifying larger sizes, sizes
appropriate to the environment, solved the problem. Immediate relief was
secured by the installation of a larger varistor at the same point of
the circuit; long-term protection was obtained by the addition of a
gapless metal-oxide varistor arrester on the primary side of the
step-down transformer feeding the drive. The situation has been changed
from failures occurring every few days to no further problems in the 3
years since the larger varistor was installed. A complete description of
this case history is given in Refer­ence 8; a summary is given in this
report.

This case history illustrates how surge protective devices that are
successfully applied for the majority of cases can occasionally suffer
failure when exposed to exceptionally severe surge environments. It also
shows how little attenuation occurs, at the frequencies produced by
switching surges, between the distribution level (23 kV) and the
utilization level (460 V), even though a long line and two step-down
transformers exist between the source of the transient and the point of
measurement.

A typical total event recorded on one of the phases during a capacitor
bank closing is shown in Figure 56 A. A low-frequency oscillation with a
period of 3 ms (330 Hz) and initial peak-to-peak amplitude of 450 V
decayed in about 10 ms. The high-frequency oscillations are resolved in
the recording of Figure 56B (recorded during a similar switching
sequence). This high frequency has an initial peak-to-peak amplitude of
2000 V, decaying in about 5 ms. The period is 180 μs (5.5 kHz), A
similar, third event is shown in Figure 56C. For scaling the amplitudes,
the steady-state voltage is shown in Figure 56D.

Figure 57 shows recordings of transient currents in all of the three
varistors. Figure 57A shows a train of current pulses in the range of 10
to 40 A. In the burst of Figure 57B, the recorded current pulses range
from 5 A to 200 A.

Conclusive evidence, therefore, was obtained that substantial current
pulses were absorbed by the varistors during capacitor switching. The
magnitude and duration of these pulses were excessive for the capability
of a 20 mm disc used originally; many similar drives installed elsewhere
do not experience the failures encountered at that particular location.
Another significant finding from these measurements is the fact that the
switching transients, generated at the 23 kV level, propagate down to
the point of utilization at the 460 level.

An obvious remedy would be to use a varistor with greater
current-handling capability. The 32 mm size offers such a possibility.
The improvement in the number of pulses is 50 times more pulses until
pulse rating is reached. The improvement in the number of pulses until
varistor failure occurs, however, is not necessarily 50 times more
pulses. Because of the imprecision in the margin between end of pulse
rating an ultimate failure, that margin is not necessarily the same for
the two sizes, 20 mm and 32 mm, but it is reasonable to expect the same
order of magnitude improvement in the ultimate failure as in the pulse
rating. This expectation of a 50-time improvement would change the time
between failures from the few days observed with the 20 mm size to
perhaps 1 year with the 32 m size, providing immediate relief and time
to make further changes for the long term. Therefore, the change to a 32
mm size, connected at the same point of the circuit, was immediately
implemented for that particular environment.

In addition to the proposed upgrading of pro­tection at the 460 V level,
three other remedies could be considered: installation of surge
arresters at the 2300 V level, installation of surge arresters at the
23 kV level, or a change in the circuits involved in the capacitor
switching, designed to reduce the severity of the transients at their
origin. In a second phase of the retrofit described here, 2300 V
arresters were installed at the transformer primary.

**8. CONCLUSIONS**

Power system disturbances can inject damaging overvoltages into power
lines as well as data lines. Lightning surges can be equally damaging,
by direct termination of a stroke, by induction, or especially, by
differences in ground potential caused by the flow of the current into
earth. Beware of differential ground potential rise!

Fundamental precautions, best applied in the design and construction
stages, can provide effective protection at a small cost compared to the
alternative of failures and retrofits. The cost of insurance premiums
always seems high before the accident.

Shielding, bonding, and grounding are the classical preventive methods
at the system and component level. Conflicts between tradition grounding
practices for noise reduction can b reconciled with the requirements of
surge protection. Grounding the shields at only one en invites trouble.

A combined approach of fundamental precautions and protective devices
can provide effective protection over the range of natural and man made
disturbances. However, these devices must be applied as part of a
concerted effort. The coordination of protective devices is the key t
functional and cost-effective protection.

**9. ACKNOWLEDGMENTS**

Motivation for preparing this report was pro vided by the reported case
histories and the penetrating questions raised by students at the
University of Wisconsin annual conferences of surge protection of
computers and electronic systems, as well as by discussions with members
of the IEEE Surge Protective Devices Committee Maurice Tetreault of
Digital Equipment Corporation graciously made available the recordings
of transients on data cable. Virginia Barnum, Catharine Fisher, and
Elizabeth Zivanov of Corporate Research and Development contributed
valuable review and comments toward development of the final text.

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1980.

13\. R.L. Carroll, \"Loop Transient Measurements in Cleveland, South
Carolina,\" Bell System Technical Journal 59 (9), pp. 1630-1645,
November 1980.

14\. M. Tetreault and F.D. Martzloff, \"Characterization of Disturbing
Transient Waveforms on Computer Data Communication Lines,\" Proc. of the
6th Symposium on Electromagnetic Compatibility, Zurich, pp. 423-428,
1985.

15\. Guide for Surge Withstand Capability (5WC) Tests, ANSI Std C37.90a,
1974; IEEE Std 472-1974.

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

17\. IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits,
ANSI/IEEE Std C62.41­1980.

18\. J.E. Lenz, \"Basic Impulse Insulation Levels of Mercury Lamp
Ballast for Outdoor Applica­tions,\" Illumin. Eng., pp. 133-140, February
1964.

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

20\. 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, pp. 445-448, May
1979.

21\. H. Wernsto6m, M. Broms, and S. Boberg, Transient Overvoltages on AC
Power Supply Systems in Swedish Industry, National Defence Research
Institute, Linkoping, Sweden, 1984.

22\. P. Chowdhuri, \"Transient Voltage Character­istics of Silicon Power
Rectifiers,\" IEEE Tran­sactions on Industry Applications IA-9,
September/October 1973.

23\. P. Richman, \"Changes to Classic Surge Test Waves Required by Back
Filters Used for Testing Powered Equipment, Proc. of the 6th Symposium
on Electromagnetic Compatibility, Zurich, pp. 413-418, 1985.

24\. IEEE Standard Test Specifications for Gas Tube Surge-Protective
Devices, ANSI/IEEE C62.31­1981.

25\. IEEE Standard Test Specifications for Low­Voltage Air Gap
Surge-Protective Devices (Excluding Valve and Expulsion Type Devices),
ANSI/IEEE C62.32-1981.

26\. IEEE Standard Test Specifications for Varistor Surge-Protective
Devices, ANSI/IEEE C62.33­1982.

27\. IEEE Standard Test Specifications for Avalanche Junction
Semiconductor Surge-Protective Devices, IEEE C62.34-1984.

28\. Martzloff, F.D., Discussion of paper \"Rural Alaska Electric Power
Quality,\" IEEE Transactions on Power Apparatus and Systems PAS-104,

> No. 3, pp. 618-619, March 1985.

29\. F.D. Martzloff and H.A. Gauper, Surge and High Frequency
Propagation in Industrial Power Lines, 85CRD087, General Electric
Company, Schenectady, New York, 1985.

30\. F.D. Martzloff, \"The Propagation and Attenuation of Surge Voltages
and Surge Currents in Low-Voltage AC Circuits,\" IEEE Transactions on
Power Apparatus and Systems, PAS-102, pp. 1163-1170, May 1983.

31\. Transient Voltage Suppression Manual, Fourth Edition, General
Electric Company, Syracuse, New York, 1983.

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

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

34\. 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., 94.

35\. M. Matsuoka, \"Non-Ohmic Properties of Zinc Oxide Ceramics,\" Jpn.
J. AppL Phys. 10, p.736, 1971.

36\. Alexander Kusko, Inc., \"Evaluation of Tran­sient Voltage
Suppressors for Saving Electric Energy,\" Electric Power Research
Institute Report EM-1722, Palo Alto, California, 1981.

37\. Standard for Voltage Surge Suppressors, UL 1449, Underwriters\'
Laboratories, 1985.

38\. K.E. Crouch, Lightning Technology, Inc., Pittsfield, Massachusetts,
private communica­tion.

39\. National Electrical Code, National Fire Protection Association,
Boston, Massachusetts.

Lightning and Surge Protection of Photovoltaic Installations

# Two Case Histories: Vulcano and Kythnos

François D. Martzloff

National Institute of Standards and Technology

## Abstract

Two installations of photovoltaic (PV) systems were damaged during
lightning storms. The two sites were visited and the damaged equipment
that was still available on the site was examined for analysis of the
suspected lightning-related damage. The evidence, however, is
insufficient to conclude that all the observed damage was caused by the
direct effect of lightning. A possible scenario may be that
lightning-induced overvoltages caused insulation breakdown at the edges
of the photovoltaic modules, with subsequent damage done by the dc
current generated by the array. Other surge protection considerations
are also addressed, and suggestions are presented for further
investigations.

**1. Introduction**

Photovoltaic systems are inherently exposed to direct and indirect
lightning effects. For high-capacity systems, the deployment of solar
cell arrays requires a large area with commensurate exposure to direct
lightning strikes at the local annual rate of ground strikes per unit
area. The presence of a ground grid related to the PV system in an
otherwise isolated area may act as a collector of lightning
ground-current from nearby strikes. For PV systems tied to a local power
grid, the exposure also includes surges coming from the power grid and
the possible differences in the ground potential of the ac power system
and that of the dc array system.

In the present development state of photo-voltaic systems, occurrences
of lightning strikes have been rare, thus field experience is still
limited. Nevertheless, justifiable concerns exist, both from the
economic point of view of damage versus cost of protection and from the
less tangible impact on the perceptions of reliability for a technology
still in the early stages of commercial utilization.

The Sandia National Laboratories, sponsored by the U.S. Department of
Energy, are developing a Recommended Practice document for the
electrical design of photovoltaic systems. As part of that project, the
National Institute of Standards and Technology is contributing the
lightning, surge protection, and grounding recommendations for these
systems, based on known characteristics T of surge protective devices
and on field experience. By this means, a review of the circumstances
and effects of lightning in the few known or suspected cases of
lightning damage to worldwide photovoltaic installations will contribute
to more effective design and application of future systems.

In this report, two case histories are examined. These include the
photovoltaic installations at Vulcano Island (Italy) and at Kythnos
Island (Greece). Following the description of these two case studies, a
discussion is presented, leading to firm conclusions when the evidence
is sufficient, and allowing conjectures when the evidence is less
conclusive. Both should serve as an indication of the need for further
investigations, laboratory work, or theoretical study.

**2. Surge Protection at the Vulcano Island Installation**

**2.1 Background**

Vulcano is one of the islands in the Aeolian Group in the Tyrrhenian
Sea, north of Sicily. The photovoltaic system in this island was
designed by ENEL, the Italian national electric utility, as a research
and demonstration facility and was commissioned in 1984 (Photograph 2-1)
[^1] ENEL has been operating this facility since the commissioning. The
visit, arranged by Dr. A. Previ of ENEL, took place in November 1988.

One case of damage attributed to lightning has been reported, with
damage to only one panel (Photograph 2-2). No other damage occurred in
the system, not even to the protective varistors provided at each
junction box in the array field. This history makes that site an
interesting case study, considering the scarcity of documented lightning
occurrences on photovoltaic systems.

**2.2 System Configuration**

The Vulcano photovoltaic system (see Figure 2-1) includes the following
major components: the array (1); a storage battery (2); one self
commutated, stand-alone inverter (3); one line-commutated inverter (4);
a rectifier for charging the battery (5); and a static switch (6).

Photograph 2-3 shows the block diagram of the system provided on the
control cubicle. interface with the 20 kV ac grid of the island is
obtained by the three-winding 150/150/20 000 V transformer which is an
integral part of the output circuit of the line-commutated inverter. A
group of 40 local domestic users was originally supplied at 380 V by an
existing substation connected to the 20 kV grid. The 380 V bus of the
substation was modified to allow power flow from the output of the
stand-alone inverter, through the static switch, to the local users.

With this configuration, the system can operate in two modes:
grid-connected, and stand-alone. In the grid-connected mode, the tie to
the grid is obtained through the 150/150/20 000 V transformer, absorbing
all of the plant output. In that mode, the storage battery is not in the
circuit, and the local loads are supplied by the ac grid. In the
stand-alone mode, the local loads are supplied at 380 V directly from
the self-commutated inverter. in that mode, the storage battery is
connected to the dc bus and it can either absorb power from the array or
deliver power to the inverter. The local loads can also be supplied, if
necessary, from the island ac grid through a back-up transformer.

Individual strings from the array can be switched by dc contactors
located in the control room, to be connected to the dc bus or
disconnected from the dc bus according to the charge state of the
battery. For maintenance purposes, a dc disconnect is located in
terminal boxes next to the respective strings of the array (Photograph
2-4). Mechanical interlock is provided between the contactor and the
cover of the terminal box, which prevents accidental opening of the
disconnect under load.

**2.3 Grounding Practices**

A major design decision in a photovoltaic system is whether to ground or
not to ground the dc side. In contrast to ac power systems, which are
grounded in most cases (by generally accepted practice or by mandate,
depending on the country), no general agreement has been reached on
grounding practices for photovoltaic systems. Two reasons are generally
cited for an ungrounded system:

\(1\) the possibility to continue operating with one ground fault on the
system, and

\(2\) some limitation of single L-G fault currents and hence reduction
of damage in case of a fault, because two ground faults are then
required to produce a significant dc fault current.

In the Vulcano system, the dc system is not grounded. A ground fault
detection system is provided (Figure 2-2), with alarm indication on the
control panel (Photograph 2-3) but no automatic trip nor remote
indication of the fault condition (the system is unattended). Experience
with this system is described as satisfactory after an initial period of
reported difficulties associated with insulation deficiencies in the
panels. (These were eventually corrected by field or factory rework on
the panels.)

While the dc system is not grounded, a ground grid has been installed at
the site, for safety, surge protection, and grounding of the ac side. In
addition to a grid of ground cables running along the dc cables in the
array (but outside of the plastic conduits containing the dc cables, see
Photograph 2-5), ground rods (16 rods, each 2 m long) were driven into
the earth. Considerable care was given to the implementation of this
ground grid. For instance, the integrity and effectiveness of the
grounding system for protection against step voltage and touch voltage,
in case of a ground fault on the 20 kV system, were the subject of
well-documented tests. Providing low impedance earthing was made easier
by the volcanic nature of the soil, which resulted in the unusually low
value of 1.8 Ω for the earth resistance. The lower leg of each panel
frame is bonded to the ground grid (Photographs 2-6 and 2-7).

Concerns frequently associated with grounding practices are corrosion of
connections and leakage of the insulation from energized parts to ground
At this site, the ground grid was implemented with direct-burial copper
cables with welded connections (Photographs 2-5 and 2-7), an effective
assurance against corrosion problems. Some corrosion. problems occurred
in the original metal boxes containing the module by-pass diodes
(Photograph 2-8). The problems were corrected by improving the
insulation to ground with a better sealing of the metallic frames.

The significance of a history of corrosion/ insulation tracking is that
these insulation problems may be a clue to a scenario other than that of
simple direct lightning damage. One may speculate on a scenario
involving a double ground fault that could have resulted in panel
damage; this scenario will be presented in the discussion of the
observations of Section 2.5.

**2.4 Surge Protection**

Overvoltage protection for the Vulcano system is provided at three
interfaces, as sketched in Figure 2-2:

\(1\) At the terminal box of each pair of strings (Photograph 2-9),
between each of the two dc lines and ground, by one 32-mm diameter
varistor (4 total) rated 560 V dc (GE Cat. No. V420HE400). No further
protective devices are provided at the entrance of the dc cables to the
power conditioner house (the capacitor bank at the input of the
inverters can serve as overvoltage limiter for any impinging surge
because the front time is relatively long as a result of the cable
impedance). The blocking diode for each string, located in the field
terminal box, is protected by one 32-mm diameter varistor (2 total),
rated 560 V dc (GE Cat. No. V420HE400). This varistor has a clamping
voltage of 1200 V for a 300 A peak surge current. The repetitive peak
voltage rating of the diode (IR Cat. No. SD 70N12P) is 1200 V.

\(2\) At the 380 V ac interface of the output of the inverters, by three
varistors connected line­to-ground (Photographs 2-10 and 2-11). These are
also 32-mm diameter varistors, with a 420 V ac rms rating (GE Cat. No.
V420HE400). A fuse rated 8 A, 500 V, 100 kA interrupting capacity is
provided in series with each varistor (Photograph 2-12). About 50 cm of
leads are used to connect the varistors to the 380 V terminals at the
base of each inverter cabinet. (In this case, this length is not
significant because of the front time limitation discussed above.)

\(3\) At the 20 kV interface with the island system, by \'conventional\"
surge arresters installed at the potheads of the underground connection,
and connected line to ground (Photograph 2-13). The 20 kV overhead line
stops about 200 m from the control room, with the final connection to
the plant made by underground cable (Photograph 2-14).

**2.5 Discussion**

***2.5.1 Lightning damage report***

The damage caused to the PV panel by the presumed lightning strike is
shown in Photograph 2-2. (The photograph was supplied by Previ as part
of the background history; the damaged module was not available for
examination.) This damage occurred during the commissioning period of
the plant in the autumn of 1984; it was found early in the morning by
the ENEL staff after a thunderstorm occurred during the night. The glass
and part of the cells were described as \'melted near the metallic frame
of the module.\" No failure of the blocking diode nor of the varistor of
that string was found as a result of that incident.

***2.5.2 Lightning damage scenarios***

The damage to the module is located at the lower part of the array, as
shown in Photograph 2-2. Postulating a scenario of a direct strike to
the array, the point of attachment of the lightning would be the point
of origin of the rising streamer that meets the descending stepped
leader (Figure 2-3).

This position at the lower part of the array is rather unexpected for
the point of initiation of the streamer. A more likely point for
streamer initiation - and resulting termination of the strike - would be
the upper edge of the array, which is 2 m above grade level (Photograph
2-15). Thus, there is some doubt on drawing a conclusion that the damage
was the result of a direct strike terminating at the array.

In view of the reported insulation problems that occurred during the
initial period of operation, one might ask whether the damage to that
panel might be the result of a leakage of dc current to the frame,
rather than the simple direct effect of a lightning strike. This dc
leakage might be the consequence of a lightning induced overvoltage
stress that created a double fault in one single event, or that created
the second fault after the first had previously occurred but remained
uncorrected. The scenario could unfold as follows:

Assume that two independent ground faults, (A) and (B) have occurred on
the system (Figure 24). When the first, say (A), occurs, the fault
detection system indicates that event but no immediate action is taken
because of the unattended status of the system, and there is no ground
fault current resulting from that first fault (except the insignificant
current passing through the detection circuit). A ground fault current
can exist only after the second fault occurs, establishing the path
through (A) and (B).

Assume now that one of the two faults, say (B), involves a very low
resistance. Then, even for substantial fault currents, little heat is
generated at fault (B). Assume further that (A) has a low enough
resistance to produce a \"sufficient\" current in the fault path, where
\"sufficient\" is defined as a level which, combined with the low but
finite resistance of fault (A), will create heat dissipation in (A), in
contrast with the negligible heat dissipation in (B).

In this manner, we have the elements that could create the observed
effects, that is, an obvious fault with burning at (A), and a less
obvious fault at (B), with a low resistance that may be eliminated
during emergency maintenance work following the occurrence of the
incident. The likelihood of such a double fault is admittedly low, but
cannot be ruled out in view of the design of the ground fault detection
system which indicates faults locally only. This scenario, still
associated with lightning, would not be in contradiction with the
observed low position of the damage since it does not require
termination of the strike at that low point of the panel. Furthermore,
the low point on the sloped array is also a place where moisture is more
likely to accumulate and thus create a good candidate for a contributing
cause in the scenario of two-stage insulation breakdown.

A variation on the theme of the double fault might even be that the
fault was entirely caused by long-term insulation breakdown, without the
\"coup de grace\" administered by the lightning incident. However, the
observation of a damaged module soon after a lightning storm would point
to the lightning-induced overvoltage scenario.

One significant aspect of the failure mode is the reported shattering of
the glass cover of that module. The question is whether it could have
been produced by the less violent action of a dc fault (glass breakage
has been reported in the United States during dc ground faults), or
could be explained only by the mechanical shock associated with a
lightning strike. The reported melting of the glass is also a clue that
could be investigated further.

If data were available on the failure modes of [this type of
module]{.underline}, some of the conjectures proposed in this discussion
might be replaced by more positive conclusions. The incentive for
reaching such positive conclusions is not merely one of intellectual
curiosity. if overvoltages induced by indirect lightning are sufficient
to cause insulation breakdown, then the provision of lightning air
terminals is irrelevant - and thus becomes an unjustifiable cost - while
improving the insulation levels in the modules would yield better
results for the added expense.

***2.5.3 Insulation coordination***

Coordination of the protective devices with the withstand capability of
the equipment to be protected is sometimes overlooked in system designs.
At the Vulcano site, this coordination was presumed to have been
incorporated in all the system design and was not audited during a visit
aimed primarily at a review of the lightning incident.

However, given the concerns on the protection afforded by the varistors,
the coordination for one example of protection can be evaluated by a
simple comparison: From their catalog description, the blocking diodes
of the array strings have a repetitive peak voltage rating of 1200 V
(albeit not a perfect assessment of their transient withstand
capability). Therefore, the maximum clamping voltage for the protective
varistor should not exceed 1200 V. For a varistor rated 560 V dc, this
maximum allowable clamping voltage of 1200 V corresponds to a 300 A
surge crest current. In other words, protection can be expected as long
as surge currents do not exceed 300 A in that string.

At first glance, this 300 A allowable level of surge current may appear
low. However, when postulating a lightning-induced surge current level
in the wiring, one should not be influenced by the thousands of amperes
of the direct stroke, but rather consider the voltage required to drive
the postulated current wave­form along the inductance of the wiring: a
high rate of current change means a high driving voltage. However, in
this case, high driving voltage would not be possible because sparkover
of the insulation would occur. Thus, the 300 A crest of an 8/20 μs
postulated wave­form appears an appropriate order of magni­tude. In this
example, therefore, insulation coordination was in fact achieved for
voltage levels that might be induced in the wiring.

**2.6 Suggestions on the Design**

In his role of sponsor of the visit, Previ asked for comments on the
surge protection provided at this site. Accepting for the moment the
hypothesis that lightning was the cause for damage to the panel, the
successful operation of the installation and survival of the electronics
through one lightning occurrence are already a testimonial of the
adequacy of the protection system.

Taking a devil\'s advocate view in search of greater protection, a more
conservative approach could have been to provide additional surge
protection for the incoming dc cables at the interface with the inverter
inputs, but expe­rience so far has indicated survival without these
additional protective devices. This observation, however, does not
necessarily guarantee that another lightning strike scenario, with a
different point of termination or higher amplitude, could not induce
some damaging overvoltage along the cables between the array protections
and the inverter input.

A concern expressed by Previ was the failure mode of the varistors
installed at the base of the electronic cabinets at the ac interface.
These varistors can be expected, in case of failure, to be promptly
isolated from the power source by operation of their series-connected
fuses (that have ample interrupting capacity). Therefore, the generation
of hot gases during the short-circuit following failure of the varistor
would be brief. Again, as an exercise in very conservative design, a
further step could be applied to limit the consequences of a varistor
failure by providing a partial metal shielding around the varistors to
deflect any evolving gas away from the rest of the circuit. The 8 A
rating of the fuses seems adequate to avoid premature aging of the fuses
caused by repetitive surges \[3\], should such repetitive surges occur
at that site.

Previ also asked about the possibility of monitoring the condition of
the varistor aging for the purpose of anticipating an impending failure.
This question has been raised by many users, sensitized to the issue by
competitive claims from advocates of silicon avalanche diodes. At this
time, no easy method has been proposed for field measurements
(especially in dc circuits where a clamp-on transformer is not suitable
\[4\]).

Increasing concerns on the issue are likely to catalyze the development
of such measurements. For the moment, the only technically simple but
operationally difficult method would be to remove each varistor from the
circuit and compare its present nominal voltage to its original nominal
voltage. In existing installations, that information is not likely to be
available. An intermediate solution for this installation would be to
implement monitoring the varistors, albeit at a late stage of the
project, and watch for trends, even though the initial value is not
available. As a last resort, a surface temperature measurement on the
varistor might give a warning of impending failure.

This discussion of varistor failure scenarios should not be interpreted
as an inference that the varistors are in fact in jeopardy. It is only
an exercise in asking and answering conserva­tive \"what-if\' questions.

**2.7 Specific Conclusions from the Vulcano Case**

The experience accumulated at the Vulcano site indicates no major
problem of surge occurrences, with only one reported case of damage to
one panel among several hundred. This one case of damage is not
conclusively attributable to lightning.

Furthermore, even if the damage were caused by lightning, then a
partially satisfactory conclusion would be that sufficient protection
could be provided for the electronic components in the power
conditioning system, at least for that particular case. Power
conditioning equipment is the most expensive part of the system and
cannot be considered \"expendable\" in contrast with a few modules being
lost with the rest of the system remaining operational. The ambiguity in
attributing the damage to direct or indirect lightning might be resolved
by further study of the failure modes of a panel (a module within its
frame). one failure mode to be investigated would be under simulated
light­ning strikes; the other failure mode would be under dc stress with
surface contamination. Further discussion of this issue, from the
technical as well as economic and intangible aspects, is offered in the
general discussion of Section 5.

**3. Surge Protection at the Kythnos Island Installation**

**3.1 Background**

Kythnos is one of the islands in the Cyclades Group, in the southern
part of the Aegean Sea. The photovoltaic system on this island was
designed and implemented in 1983 by Siemens. It is operated by the Greek
Public Power Corporation. The visit, which was arranged by Dr. J.
Chadjivassiliadis of Public Power Corporation, took place in November
1988.

Several panel failures have occurred in 1986, 1987, and 1988, which have
been attributed to lightning. Lightning rods and surge arresters are
provided at this site, making it an interesting subject of study, both
for an explanation of the presumed direct strikes occurring in spite of
the lightning rods, and for a study of the protection afforded by the
surge protective devices installed in the circuits, as well as their
failure modes.

**3.2 System Configuration**

The installation was designed and implemented by Siemens, as one of the
experimental facilities coordinated by the European Economic Community
(Photograph 3-1). The plant has a nominal output capacity of 100 kW.
Figure 3-1 shows a schematic of the system components. The modules are
grouped in arrays formed by a series string of 20 modules, each
producing a dc bus voltage of 160 V. Each of these 43 arrays is
terminated in a junction box in the field, where two or three strings
are connected in parallel to bring the dc power to the power conversion
cabin (Photograph 3-2).

In the power conversion circuitry, the variable 160 V dc is raised and
regulated to 250 V by a dc/dc converter to match the battery voltage for
optimum charging conditions and operation of the solar cells. The dc/dc
conversion is per-formed by four units, each rated 25 kW. Depending on
the instantaneous power transfer, one to four converters are in service.

Conversion to ac power is performed by three inverters, each rated 50
kW. The output voltage of 380 V is stepped up to 15 kV for connection to
the island power grid. Although the arrays and conversion equipment are
located adjacent to the Diesel generating plant of the island, operation
of the photovoltaic system can be automatic, and does not require daily
supervisory. Extensive monitoring and control of operating parameters is
provided by a \"Logistronic\" control system and other controls
incorporated in the design.

**3.3 Grounding Practices**

This site is located in the center part of the island next to the Diesel
power plant, but with its ground grid isolated from that of the Diesel
plant. This grid consists of several loops encircling each of the four
groups of arrays. Part of the each loop follows the routing of the dc
cables between the array junction boxes and the power conditioning cabin
(Figure 3-2).

The perimeter of the field is defined by stone walls, in keeping with
the prevailing island practice for marking boundaries between pastures
and cultivated fields. Consequently, there is no metal fence around the
photovoltaic field, and thus no perimeter grounding cable. The
conductors are made of 10-mm diameter galvanized steel, buried directly
at the bottom of a trench, with the dc cables above the ground
conductors. There are no driven ground rods added to this grid. The
choice of galvanized steel probably reflects the German practice, where
concerns over corrosion effects by buried copper seem to deprecate the
use of copper.

All the metal structures of the system, including the array supporting
beams, junction boxes, lightning rods, and housings for the power
conditioners and battery, are bonded to the ground grid. Connections are
made using bolted con­nectors above ground (Photographs 3-3, 3-4, and
3-5), as well as under ground, with protection against corrosion being
provided in accordance with the normal practices of the various
manufacturers and contractors (these were not discussed during the
visit).

The dc system is not grounded, but includes a ground fault detection
circuit with fault indica­tion available only in the control cubicle of
the system. The separation of the photovoltaic ground grid from the
Diesel plant ground grid raises the question of a possible difference of
ground potential between these two systems during a lightning strike. If
instrumentation or telemetering equipment spans across the two systems,
the difference in ground potentials might become a problem. However, no
such problem was identified at that site.

**3.4 Surge Protection**

This installation presents an interesting case his­tory because it
includes both lightning rods (air terminals) in the array and surge
arresters in the circuits. Damage to several panels, presumably as a
result of lightning over a period involving three separate occurrences,
raises questions on the effectiveness of the protection against direct
strikes. Damage to the surge arresters also occurred in one of the field
junction boxes, but no damage occurred on the power conversion units.
Some damage occurred in the control circuits of the battery charger
during the initial period, when they did not have surge arresters at
their ac power input. After arresters were added to this ac input, no
further damage events occurred but some upsets did still occur in the
control system.

***3.4.1 Air terminals***

Air terminals (lightning rods) have been installed between rows of the
array as shown on Photograph 3-6. The height of these air terminals is
10.5 m above grade level; the upper edge of the panels is 2 m above
grade level, thus leaving a net elevation of the air terminals 8.5 m
above the upper edges of the panels. Considering a 45o cone of
protection, one of the classical criteria, the panel upper edges would
then be \"protected\" within a radius of 8.5 m from each air terminal.
Those panels located beyond that radius would be left unpro­tected.
Reviewing the location of panels involved in the damage (Figure 3-2)
shows the following horizontal distance from the nearest air terminal:

\- Module E2/B4/1 - Location 10m\
- Module E2/B6/5 - Location II 10m\
- Module E3/B4/13 - Location III 10m\
- Module E3/B4/19 - Location IV 12m\
- Module E3/B5/18 - Location V 8m\
- Module E4/B3/20 - Location VI 10m

Thus, five of the six damaged modules were beyond the 45^o^ cone, and
the sixth was on the fringe of the cone. Some panels in the array, not
impacted by lightning, are further away from an air terminal, the
greatest distance in the field being 15 meters. Another interesting
statistic is the distribution of the panels with respect to being within
a protected area of 8.5 meters radius (approximately 75%) or outside the
protected area (25%).

Another protection criterion has been developed, that of the \"rolling
ball\' \[5\], as discussed in section 3.5. According to that criterion,
the protection radius would extend to 12 m so that all panels would have
been expected to be in the protected zone.

According to yet another definition of the cone of protection, sometimes
cited by less conserva­tive designers, a 2:1 instead of a 1:1 ratio of
radius to height may be considered. in such a case, one would expect all
of the panels to be \"protected\" as the distance from the mast would
increase to 17 m.

It is not known whether such a 2:1 cone, or the 1:1 (45) cone, or the
rolling ball with a 30-m radius was used in the initial layout of the
air terminals. The design has been described as \"installed according to
VDE standards\" (VDE is the acronym for Verein Deutscher
Electrotechniker).

***3.4.2 Overvoltage protection***

Overvoltage protection at the Kythnos installation is provided at four
interfaces, numbered (1) through (4) in Figure 3-3:

\(1\) At the Junction boxes In field - There are several slightly
different types of junction boxes in the field. Some include termination
for two or for three strings, while some also contain additional
circuitry for the data collection system. Photograph 3-7 shows a typical
three-circuit box (undamaged). One surge arrester is connected between
each of the floating dc lines (+) and (-) and a ground bus inside the
box. In turn, this bus is bonded to the footing by a copper cable (in
parallel with the inherent bonding between the metallic junction box and
the I beam of the footing).

These arresters appear similar to those for which the voltage response
had been documented in a paper presented at the 1981 EMC Zurich
Symposium \[8\]. From the voltage response characteristic reported in
that paper, it appears that the surge arrester con­sisted of a
silicon-carbide varistor with a series gap. The presence of a series gap
is significant in discussing the upset events cited for the control
circuits at this site.

The string blocking diodes are mounted in the junction box and are
protected by a metal-oxide varistor connected in parallel with each
-diode (Photograph 3-8). Photograph 3-9 shows another junction box with
the additional data collection circuitry installed in the box cover.
This particular box is the one where the lightning-suspected damage
occurred, as shown in the close-up views of Photographs 3-10 and 3-11.

\(2\) At the power conversion units - The dc lines from the array are
brought to the cabinets of the dc-dc converters where each of the four
converter inputs is protected by two surge arresters (2a) (Photograph
3-12) connected between the (+) and (-) lines, and ground. This arrester
is of the same type as that described for the array junction boxes.

Similarly, the ac outputs of the inverters are protected against surges
from the ac grid by four arresters (2b); one is connected between each
line (a,b,c) and ground, and one between neutral and ground (N)
(Photograph 3-13). While the grounding connection of the 220/380 V
system was not reviewed, presumably it follows the European practice of
bonding to earth only at the secondary transformer, in this case the
step-up transformer of the grid interface. This practice, different from
that used in the United States, motivates and justifies the provision of
the arrester between neutral and ground.

\(3\) At the Logistronic circuit power supply -The \'Logistronic\'
circuit controlling the battery charger is powered from the 220 V ac
line in the battery cabin. Thus, its power supply is exposed to surges
that may occur on that sup­ply. Initially, there was no protection on
this ac supply; perhaps as a consequence, damage occurred three times in
the early years of the system (Appendix B). Subsequently, two arresters
were installed on the ac supply line ahead of the Logistronic input
terminals (Photograph 3-14). After these ac arresters were installed,
only upsets were recorded (four occurrences). This behavior is
consistent with the voltage-limiting effect of the arresters but at the
price of a steep voltage collapse when the gaps fires. This
electromagnetic disturbance is a likely source of interference in nearby
digital circuits.

\(4\) At the ac grid interface - Protection against surges coming from
the island ac power grid is provided by the three distribution-type
arresters mounted on a cross-arm above the transformer (Photograph
3-15). No information was available on these arresters; they are likely
to be of the conventional design using a silicon carbide varistor with a
series gap. This type of arrester is perfectly adequate for protecting
transformers against surges, but might not be sufficient for the
electronic components on the 220/380 V side. For that reason, the
secondary arresters described above are a good idea. However, gapless
secondary arresters are now available that can offer a more
comprehensive protection, including some degree of upset protection.

***3.4.3 Examination of the damaged modules***

1.  *Summary*

At the date of the visit, the three modules damaged in 1986 had been
replaced in the array. These modules were still kept in storage at the
site, so that it was possible to examine them closely. The two modules
damaged in 1987 and the one module damaged in 1988, however, were still
in position in the array, as no spares were available. Detailed
photographs and observations for each panel are given in the following
paragraphs, in chronological order.

At this site, the arrays are only one module high, so that the long edge
of the module reaches from the highest to the lowest edge of the array.
In all six failed modules, there is damage evident at one or both upper
corners, along one or both long edges, and at the bottom of the module.
The panel is completely separated from the frame in some cases, while in
other cases, only partial separation occurred. One of the modules has
severe bums marks on the top corner of the frame, while on the other
modules the damage ranges from none to some readily visible burn marks.

*3.4.3.2 Detailed examinations*

MODULE - 307 0423 This module was in storage and had been at location E2
B4 I (\"I\" on Figure 3-2), 10 m from the nearest air terminal. There
are burn marks along both long edges, but not the complete length
(Photograph 3-16). On the right side, the burns are mostly at the lower
part of the edge, away from the most damaged corner (Photograph 3-17).
On the left side, the burns are mostly in the upper part, with
intriguing spots over some of the cells (Photograph 3-18). The top right
comer shows some marks on the frame, with the most extensive damage at
that corner (Photograph 3-19).

MODULE - 303 0267 This module was in storage and had been at location E3
B5 18 (\"V\" on Figure 3-2), 8 m from the nearest air terminal. There
are burn marks along both vertical edges, but not over the complete
length (Photograph 3-20). On the right side, the burns are mostly at the
upper part of the module, with damage at both corners (Photographs 3-21
and 3-22). The top right corner (Photograph 3-21) shows heavy burn marks
on the frame, while the top left cor­ner (Photograph 3-23) shows light
marks on the frame. It should be noted that this module, which has the
heaviest burn marks on its frame among the six modules, is the only
module that was located within the \"cone of protection\" of an air
terminal. This remark will be discussed further in the next section.

MODULE - 304 0294 This module was in storage and had been at location E3
B5 19 (\"IV\" on Figure 3-2), 12 m from the nearest air terminal. There
are burn marks along all of the right side, and part of the left side
(Photograph 3-24). Both top corners show damage (Photographs 3-25 and
3-26). The top right corner (Photo­graph 3-25 shows light burn marks on
the frame, while the top left corner (Photograph 3-26) hardly shows any
burn marks on the frame. There is extensive separation of the panel from
the frame along the right side (Photograph 3-27)

MODULE - 306 04l7 This module is still in the array at location E2 B6 5
(\"II\" on Figure 3-2), 10 m from the nearest air terminal, and was
found damaged on February 5, 1987. The bypass diode in the string allows
the array to remain operational. The right edge shows burns (Photograph
3-28). Both right side corners show extensive destruction of panel
material (Photographs 3-29 and 3-30), but the upper corner has no burn
marks on the frame (Photograph 3-29).

MODULE - 304 0300 This module is still in the array at location E3 B4 13
(\"III\" on Figure 3-2), 10 m from the nearest air terminal, and was
found damaged on February 5, 1987. The bypass diode in the string allows
the array to remain operational. There is damage on three of the comers
and some of the edges (Photo­graphs 3-31, 3-32, and 3-33), but the
heaviest damage is on the lower left corner (Photograph 3-34). The two
upper corners shows surface degradation on the frame, but these do not
appear to be burn marks (Photographs 3-32 and 3-33).

MODULE - 310 0592 This module is still in the array at location E4 B3 20
(\"VI\" on Figure 3-2), 10 m from the nearest air terminal, and was
found damaged on February 25, 1988. The bypass diode in the string
allows the array to remain operational. The damage is concen­trated on
the left edge of the module (Photographs 3-35 and 3-37). The panel is
sep­arated from the frame (Photograph 3-38). The apparent discoloration
of the frame at the top left corner does not seem attributable to burns
(Photograph 3-36).

**3.5 Discussion**

***3.5.1 Effectiveness of air terminals***

Lightning protection of solar arrays by air terminals is still a subject
of debate (effectiveness, shadow effects, cost, appearance). The
observations made at the Kythnos site do not bring conclusive evidence
for or against the effective-ness of correctly designed air terminals,
although they tend to weaken the case for providing air terminals.

The Kythnos experience involves points of (presumed) lightning
termination that are at the edges of the zone of protection of several
criteria, where this protection becomes more uncertain. ironically, the
most severe burn mark is found on the frame of the module that was
closest to an air terminal, and within the zone of protection as
detailed in paragraph 3.4.1. Thus, a brief review of the uncertainties
of the zone-of-protection concepts will provide the necessary
perspective on the issue.

Indiscriminate application of the 45^o^ cone of protection criterion to
tall structures has led to contradictions. An example is occurrence of
lightning strikes terminating on the side of tall buildings, within the
cone of \"protection\". The original concept of a cone of protection is
now generally replaced by the rolling ball criterion, based on the
striking distance theory. Accord­ing to this striking distance theory
\[6\], the striking distance at the tip of the descending stepped leader
increases with the amount of charge in the leader. Thus, the leaders
having the highest potential current level have the longest striking
distance (Figure 3-4). Conversely, leaders having the lowest potential
current level have the shortest striking distance. The point of
termination of a lightning strike can be anywhere within the striking
distance from the last point of advance of the descend­ing stepped
leader. This fact can be repre­sented by imagining a sphere with a radius
equal to the striking distance, which is determined by the charge in the
lower part of the leader. Any point at ground potential penetrat­ing that
sphere is a candidate for emitting a upward streamer that will complete
the path for the return stroke. Thus, points at ground potential outside
of the sphere are still \'protected' while the points inside the sphere
are not.

Considering now the configuration of a vertical mast on the ground plane
(Figure 3-5), rolling a ball on the ground until it touches the tip of
the mast defines the limiting condition when the descending leader will
terminate at the tip of the mast, thus leaving other points below the
sphere uninvolved. Figure 3-6 shows graphically the configuration for
the 10.5 m masts used in Kythnos, with the upper edge of the panels at
2 m above the ground plane.

Figure 3-6 shows the zone of protection as defined by the traditional
45o cone of protection, as well as that defined by a rolling ball of
30.m radius, as specified in the Lightning Protection Code \[5\]. Simple
geometry shows a dis­tance of 8.5 m from the mast for the 45o cone, while
the graphical solution for the rolling ball shows a distance of 12 m
from the mast. It should be emphasized that the selection of a 30 m
radius for the ball is somewhat arbitrary, in view of the data shown in
Figure 3-4. From Figure 3-6, it is apparent that a pessimistic
assumption would be a smaller radius for the rolling ball: such a
smaller ball would roll closer to the mast and thus would reduce the \"
protected\" distance from the mast.

This observation needs to be combined with the statistical distribution
of lightning current amplitudes as stated by Cianos & Pierce \[2\] to
appreciate that the 30 m radius is only a pragmatic choice, not an
absolute criterion. Therefore, observing points of presumed lightning
termination at distances of 8.5 m to 12 m from the base of an air
terminal is not startling, especially for low-current strokes. This
observation shows how precarious the assurance of protection can be when
only sparsely distributed air terminals are provided. In other words,
increasing the degree of confidence that sufficient protection zones are
established might require such a density of masts (or overhead wires)
that the cost, appearance, and shadow effects would loom large in the
overall trade-off.

***3.5.2 Lightning current path***

The resulting return stroke would then draw charges from the earth via
the grounded structure, that is, the return current would come out of
the grounding cable at the base of the column, and proceed by the
shortest route toward the upper edge of the panel. This shortest path
does not include the lower half of the panel edges, as it would require
the lower panel brace plus the panel edge to become involved. While this
path may still be somewhat involved, the major part of the current
should only involve the upper half of the panel, a situation which is
not reflected in the more or less even (or. random) distribution of the
damage observed on both upper and lower halves of the long edges of the
modules.

3.  ***Direct versus consequential effects***

In the absence of definitive knowledge on the direct effect of a
lightning current involving a module, only conjectures can be made on
the failure mode of the panel. As discussed in the preceding paragraph,
the presence of damage at the lower half of the panels is somewhat
con­tradictory to the hypothesis of all the damage being done by the
lightning current. This con­tradiction adds weight to the argument (also
presented in the case of the Vulcano incident), that the observed damage
may be the result of a dc fault current occurring after an initial
insulation breakdown caused by an indirect lightning overvoltage induced
in the de circuit. The insulation breakdown would occur at the point of
lowest withstand, not necessarily in the upper half of the panel, and
the ensuing dc fault would proceed along the edge as the blow-torch
effect associated with the high temperatures of the dc arc, lingering at
the fault, would cause burning along the edges, similar to what was
observed.

On the other hand, the extent of the damage in the E2 B5 box
(Photographs 3-10 and 3-11) appears to be greater than what could be
expected from the dc current alone. Damage caused by the occurrence of a
lightning surge current is a more likely scenario in this case.

***3.5.4 Mechanical effects***

The top glass cover plates of the damaged modules generally had several
cracks, but do not have the frosty appearance associated with the
tempered glass used in the Vulcano module. This difference may provide
some clue about the sequence of the scenario, if it could be correlated
with the mechanical characteristics of the glass. Damage to the glass
during dc faults has been reported in the United States. How­ever, no
further detailed information is available in either case to pursue this
line of thought. This subject could be part of a test program aimed at
finding failure modes of PV modules related to dc faults and lightning
(both direct and indirect).

6.  ***Integrity of the grounding system***

The grounding system has been implemented with galvanized steel
conductors, in keeping with the standard German practice where concerns
over cathodic corrosion have steered designers away from copper. in the
salty environment of an island, questions may be raised on the long-term
integrity of buried galvanized steel conductors. Even in the dry
environment of the array footings, some signs of corrosion are apparent
(Photograph 3-40).

**3.6 Specific Conclusions from the Kythnos Case**

The observed damage to the panels cannot be conclusively attributed to a
direct lightning strike. The six reported incidents might involve a
combination of effects, with one case involving a direct strike, and the
others being an indi-rect effect. in other words, the evidence that
might point to invalidating a particular scenario might not apply to the
scenarios of other inci­dents. The surge-protective devices provided at
the site performed well since no damage was inflicted to the
electronics. Failure of one surge arrester in the performance of its
protective duty can be viewed as the ultimate sacrifice of the device
fulfilling its mission - but it raises the question of monitoring for
failure of protective devices.

**4. General Discussion**

**4.1 To protect or not to protect ?**

The debate on whether to provide protection by air terminals or suffer
the consequences of a direct strike is not settled by these case
histories. In spite of the presence of air terminals at Kythnos, damage
occurred. This damage my be a direct effect, or may be an indirect
effect, or a combination of both. At Vulcano, with no air terminals,
only one case of lightning-related damage has occurred, and this single
case may be an indirect effect rather than a direct effect. Indirect
effects are not eliminated by air terminals. A better argument could be
made if a firm conclusion were reached on whether the damage was a
direct or indirect effect.

If the damage is attributed to direct effects, then the conclusion is
that the air terminals, at the spacing and height used at Kythnos, were
ineffective. However, precisely because air terminals were distributed
perhaps too sparsely, the Kythnos case history does not invalidate
protection if it were ensured by appropriate air terminals with adequate
height and density.

If the damage is attributed to an indirect effect, then one would argue
that the air terminals cannot serve any useful purpose - the
counter-argument being that the direct damage would have been even worse
than what actually occurred.

**4.2 Grounding practices**

Differences in grounding practices leave many questions unanswered. On
the materials aspects, there is the different approach of using copper
or of using galvanized steel. On the circuitry aspects, there is the
issue of grounding the dc circuit or leaving it floating (but with a
ground fault detection scheme). This latter choice, however, raises
questions on the implementation of a ground fault indication which is
available only to local operators. That design may raise concerns in the
context of long-term operation where immediate action on a ground fault
may not be perceived as important. This postponing of action may then
lead to the occurrence of a second fault caused by light­ning or by
further pollution of insulation, with damage to components at that time.

**4.3 Suggestions for further investigations**

The ambiguity on the interpretation of the reported damage gives added
weight to the desirability of consolidating all available data on panel
failure modes, and eventually performing lightning simulation tests, as
well as insulation failure (tracking) tests. Evidence from the lightning
damage incident that occurred at the photovoltaic installation of the
Sacramento Municipal Utility District (SMUD PV1) in California \[7\]
should be compared to the damage observed at Vulcano and Kythnos.

The conjectural scenario of a nearby lightning strike inducing sparkover
at points of weak insulation, followed by damage caused by the dc
current, could be more credible if knowledge were available on two
parameters: (1) dielectric withstand of the insulation between the
modules and their frame, under various conditions of contamination, and
(2) levels of the overvoltages that could be induced in the circuits.
The first parameter would require tests on the actual configurations,
and might be impractical in view of the large number of possible
configurations. The second parameter might be evaluated by theoretical
analysis, such as that reported Stolte \[9\].

The ambiguity in the postmortem may be resolved by further study of the
failure modes of a panel (module within its frame) under simulated
lightning and under dc stress with surface contamination. The value of
such tests would be to determine the need of further protection or
design improvements in the panels to avoid damage, or to better
understand the mechanism of the failure in order to settle the dilemma
on the exact scenario leading to the observed damage. Ultimately, the
knowledge would also provide the basis for an informed decision on the
cost-effectiveness of air terminals.

**5. General Conclusions**

The two case histories presented in this report demonstrate that it is
possible to provide protection for the power conversion electronics in
the face of inescapable lightning strikes to the array field. in several
instances, damage was limited to the modules; the surge protective
devices performed their function with no damage to themselves. In one
instance, damage was inflicted to the surge protective devices, but even
while failing, they protected the expensive downstream circuitry.
Depending on the point of view, achieving protection at the cost of a
failed protective device may be considered successful, while an
alternate view might be to expect protection with no sacrifice of the
protective device.

The observations made at these two sites, the evidence collected before
the visits, and the preceding discussions lead to a set of conclu­sions,
some still in the form of conjectures, some in the form of firm
conclusions. Further­more, implementation of the recommendations
presented here may validate the conjectures and elevate them to the
status of firm conclusions. A most important point to bear in mind,
however, is that the unpredictability of lightning occurrences make it a
risky business to draw sweeping conclusions based on only a few years of
observation \[6\].

Protection against lightning damage to the array modules is a more
difficult and less clear-cut issue than operation and survival of
protective devices incorporated in the circuit:

-   First, there is still some ambiguity in attributing all of the
    observed damage either to a direct effect of lightning, or to an
    indirect effect.

-   Second, there is no sufficient evidence and long-term data on the
    effects and costs of a presumed direct strike to rule out air
    terminals, although their cost-effectiveness appears questionable.

**5.1 Conjectural conclusions**

A likely scenario to explain the observed effect is a combination of
lightning-induced overvoltages with low insulation withstand. This low
withstand may be an inherent limitation of the photovoltaic module
layout, or may be the result of pollution or moisture.

The evidence at Vulcano tends to point away from a simple direct
lightning strike because the reported damage was limited to the lower
part of the array. However, no direct inspection of the failed module
was possible in this case.

The overvoltages associated with the one incident at Vulcano were
successfully suppressed as no damage was inflicted to either the surge
suppressor themselves, the first line of defense, or to the power
conversion electronics, the poten­tial victim equipment. However, since
the amplitude of the lightning stroke in that incident is not known, the
conclusion should not be that protection has been achieved for level of
severity.

The effectiveness of lightning rods appears questionable in view of the
several incidents at Kythnos. However, a higher density of rods, or
greater height, might have reduced the damage. Nevertheless, the
scenario of possible damage by indirect effects leaves in doubt the
justification for the expense and disadvantages of providing lightning
rods.

**5.2 Firm conclusions**

The one obvious conclusion, not unexpected, is that lightning does
represent a threat to photo-voltaic arrays, either by direct damage or
by indirect damage.

Good evidence has been provided that surge-protective devices with
appropriate ratings (coordinated protection with the equipment to be
protected, adequate surge current handling capability, and not
excessively low clamping voltage for the systems voltage conditions) can
protect the electronic equipment.

The one case of failure of a surge protective device that occurred shows
that with suitable failure mode (i.e., short-circuit), protection of the
electronics can be obtained for the first incident. However, if the
protective devices are associated with fuses, as in the case of Vulcano,
failure of the protective device would result in blowing the fuse and,
unless an indication of that situation were provided, the equipment
would then be left unprotected for the next occurrence.

**5.3 Recommendations**

The ambiguity in attributing the damage to a direct lightning strike may
be reduced if the suggestions proposed in this report for simulated
lightning tests and study of failure modes were implemented:

-   Establish a common, world-wide data base summarizing all
    observations of documented or suspected lightning damage to panels.

-   Establish a common, world-wide data base summarizing all
    observations of damage to surge protective devices

-   Establish a common, world-wide data base summarizing all
    observations of documented dc insulation faults on panels.

-   Perform laboratory simulation of lightning attachment to panel
    frames and to module surfaces.

-   In view of the prevalent practice, with apparent success, among
    European designers of not grounding the dc system, the
    quasi-axiomatic practice by U.S. designers of multiple-point
    grounding should be re‑examined, and a dialogue initiated between
    the two parties.

-   Operating procedures associated with the occurrence of the first
    fault in an isolated dc system should be reviewed and clearly
    defined.

An intriguing although not crucial question is that of the nature (and
thus cost) of the materials used for the ground grid. The Italian
practice calls for copper, while the German practice applied in Kythnos
calls for galvanized steel. The question of copper versus galvanized
steel in this context should be re-examined by specialists of cathodic
protection schemes.

**6. Acknowledgements**

The two site visits described in this report were made possible by the
cooperation and hospitality of A. Previ and V. Messina of ENEL, and of
J. Chadjivassiliadis and A. Grielas of Public Power. T.S. Key provided
insights in reviewing the draft of this report. Funding for this project
was provided by the Sandia National Laboratories for the U.S. Department
of Energy.

**7. References**

\[1\] Previ , A., \'The Vulcano Project, \"Int. J. Solar Energy, 1985,
Vol 3, pp 124-141.

\[2\] Cianos, N. and Pierce, E.T. \"A ground-lightning environment for
engineering usage.\" Stanford Research Institute, Menlo Park, Ca, 1972.

\[3\] Martzloff, F.D., \"Matching Surge Protective Devices to their
Environment,\" IEEE Transactions, Vol IA-21, No.1, January/ February
1985.

\[4\] Shirakawa, S., F. Endo, H. Kitajima, S. Kobayashi, and K. Kurita,
"Maintenance of Surge Arrester by a Portable Arrester Leakage Current
Detector," IEEE Transactions, Vol PD-3, No. 3, July 1988.

\[5\] NFPA 78 \'Lightning Protection Code, \"National Fire Protection
Association, Boston, MA, 1986.

\[6\] Golde, R.H., \"Lightning Protection, \"Chemical Publishing
Company, New York, NY, 1975.

\[7\] Collier, D.E., and Key, T.S., \"Electrical Fault Protection for a
Large Photovoltaic Plant Inverter,\" Proceedings, 20th IEEE Photovoltaic
Specialist Conference, September 1988.

\[8\] Martzloff, F.D., "Transient Overvoltage Protection: The
Implications of New Techniques," Proceedings, 4^th^ Symposium on EMC,
Zurich, 1981.

\[9\] Stolte, W.J., \"Photovoltaic System Grounding and Fault Protection
Guidelines," Contractor Report SAND83-7025, National Technical
Information Service, 1985.

Protecting Computer Systems against Power Transients

François Martzloff

National Institute of Standards and Technology

Because small systems have moved from computer rooms into offices,
factories, and homes, users and systems designers must deal with the
subtle dangers the machines encounter

For the third time in less than three weeks, the sky darkened and
thunder rumbled in the distance. With that ominous warning, the
appointed \"thunder scout\" decided it was time to pull the plugs of the
central unit and remote terminals of his CAD/CAM computer system. Better
to shut down the operation than risk damage to the system, as had
occurred in the two previous storms.

But pulling the plugs did not help. When the storm was over and the
system was restarted, permanent damage had been done to it---to the
chagrin of both the operator and the service contractor.

In this common example, the damage was caused not by power-line surges
but by differences in ground potential at various terminals of the
system. The oversimplified assumption that power-line surge problems
could be eliminated had led the uninformed operator to attempt a simple
prevention step. Not only did it not work, but it created a safety
hazard: unplugging the line cords removed the safety ground, leaving the
equipment still connected to the data lines where the surges were
occurring.

Understanding the general causes of, and remedies for, power transients
can help users of small computer systems, especially stand-alones,
protect their systems with do-it-yourself methods. More complex systems
may need the attention of a specialist. Systems designers should also be
aware of the way users hook up their systems, the potential damage that
could be caused by power transients, and side effects of incorrectly
applied measures.

Growing concern among computer users that power-line sur­ges may damage
equipment or cause loss of data has created a market for surge
suppressors. But clear performance standards in the industry are
lacking, and several standards-writing groups are striving to develop
adequate ones. To make a difficult choice among these devices, the
consumer should learn some basic rules about selecting and installing a
suitable surge suppressor. Even the best surge suppressor, if
incorrectly applied, might not work and could cause adverse side
effects.

# Transient origins

While the term \"transient\" is often understood as a transient
overvoltage, it is also more broadly interpreted as the occurrence of
any disturbance, either on the power line or the computer system\'s data
line.

The most obvious source of an electrical disturbance is a lightning
strike, but the lightning bolt need not hit power lines to cause damage.
Because the electromagnetic field radiated by the lightning current
couples into the conductors of power lines or data lines, it induces
transient voltages along these conductors. Also, as the lightning
current spreads into the ground, it produces differences in potential at
points that are normally at \"ground\" potential. Conductors spanning
some distance between their ends in the area where the lightning current
is spreading will be exposed to these differences of potential, or to a
transient overvoltage.

Though the direct effects of lightning can be dramatic, their relatively
low rate of occurrence can lull one into complacency, and most of their
widespread indirect effects can be overcome through sound protection
practices. On the other hand, electrostatic discharges, which could be
considered miniature lightning discharges, require only the fingertips
of mortals rather than an Olympian fistful of lightning bolts to have
very serious effects

A less obvious but more frequent source of transients is switching
sequences in the power system. Switching can be a normal, recurrent
operation such as turning a local load on and off, or it could entail
occasionally clearing an overload or short circuit.

These switching transients cover a wide range of frequencies and
amplitudes. Some have a brief duration (nanoseconds) and involve little
energy (millijoules). While they present little risk of damage, their
high-frequency spectrum makes them likely sources of interference.
Others have a longer duration (microseconds or even milliseconds) and
involve greater energy (up to hundreds of joules) with lower
frequencies. They have the opposite trait of low risk of interference
because of the relatively low frequencies, but because of the longer
duration and increased energy, they have a higher damage risk.

Another source of disturbance to computer systems is the occurrence of
an undervoltage that could be caused by a nearby startup of heavy loads
or by distant faults, such as lightning-induced line flashover, falling
trees, or utility lines downed by runaway vehicles. While transient
overvoltages can be easily suppressed---a more correct description would
be \"mitigated\"---by a simple added device that diverts the excess
energy, the reduced energy associated with an undervoltage cannot be
supplemented by a simple device. Different methods are needed for a
solution of that problem.

Over the years, the need to learn more about the characteristics of
these transients has led to various projects aimed at monitoring
power-line disturbances. One result of these projects---which are
performed by isolated researchers, sometimes with equipment designed by
the researchers rather than commercial equipment---is that their reports
are based on different assumptions and definitions of disturbances.
Comparing results can thus become difficult and confusing \[see table,
overleaf\].

Leaving the problems of monitoring transients and designing protective
devices to the specialists, an informed user can take several steps
toward buttressing the reliability and integrity of the system. The
first step is to distinguish between mere temporary upset and permanent
damage, each of which has a different impact on the user, depending on
the relative importance of the operation. For a commercial setup,
disrupting the operation can be more expensive than repairing the damage
so that protecting data integrity ranks high. For an engineer working at
his home computer, however, damage protection may be more important than
some data loss. In this case, limiting the protection expense to
avoiding damage and accepting interruptions may be preferred.

# Vulnerable stand-alones

Small computer \"systems\" can be categorized as stand-alone systems or
distributed systems. A stand-alone system is typically a one-operator
setup consisting of a desktop computer coupled with a printer, or any
microcomputer not linked to a net-work. Distributed systems range from a
simple stand-alone augmented by a telephone or other network link to
multiple-station systems or process control systems with remote sensors
and actuators.

Found in offices, laboratories, and homes, stand-alone systems can be
disrupted or damaged by two possible causes. First, transients with low
amplitudes (less than 1000 volts) are buffered by the computer\'s power
supply but might still couple into circuits and cause glitches.
Transients of high amplitudes (over 1000 V) may at worst damage the
power input components and are likely to cause glitches at best. Second,
power interruptions (sags or outages) can cause a momentary shutdown.

Transient damage protection for these systems is simple to achieve and
is probably built in to some degree. However, until the day arrives when
equipment has its transient capability stated on the nameplate (which
may be sooner than expected because the Europeans are increasingly
concerned with electromagnetic compatibility issues), the user has no
way to know the extent of that protection. The European approach,
motivated by a 1989 Directive on Electromagnetic Compatibility
promulgated by the European Community Council, requires that equipment
must operate satisfactorily in a specified environment without
introducing intolerable disturbances into that environment. Thus, this
ability is likely to be stated explicitly, in addition to the usual
volt-age, frequency, and current ratings now required.

So far, the approach has been one of purchasing additional peace of mind
by inserting a separate surge suppressor (also called spike protector
and transient voltage suppressor) on the power cord. Prices for these
devices range over an order of magnitude, and claims of performance may
include the fastest response (an irrelevant issue) and the lowest
clamping voltage (a reliability risk because the transient protector may
fail under abnormal power fluctuations).

Though its basic technology does not change rapidly, details of the
rating and packaging of a surge suppressor are driven by market
competition. Ideally, its rating should reflect three basic
requirements: the nominal line voltage, the surge current capability,
and the clamping voltage during the surge. All of these should be stated
by the maker of the device with due consideration to the user\'s needs
for protection and long-term reliability.

At this time, there is only one performance standard in the United
States for transient voltage surge suppressors, UL 1449, which was
developed by the Underwriters Laboratories. This standard specifies
primarily the safety aspects of the product, but does contain some
performance specifications. The UL label on a surge suppressor means
that a test has been applied to the device, reflecting industry
consensus standards on the severity of the environment. In the UL test,
a specified surge current is applied to the device and the maximum
resulting voltage is measured; this is then indicated on the product.

Product literature for some devices, however, makes claims of response
time in nanoseconds---even picoseconds---a feature that is not important
in a power system. Nanosecond pulses do not propagate very far in power
systems, and measuring a picosecond response time in support of the
claim would be a technical challenge. Like-wise, emphasis on achieving
the lowest clamping voltage only demonstrates imbalance in the design
goals: the object of a surge suppressor is to lessen the surge level
from the thousands of volts that can occur occasionally; it is not to
shave off the last tens of volts from the protection level in a \"lower
is better\" bid for ranking in the purchaser\'s choice. An exces­sively
low clamping voltage in­troduces the risk of premature aging, even
failure, of the device when the power line goes through repeated
momentary overvoltages, or \"swells.\"

The second type of disturbance, a sag or outage, cannot be corrected by
a surge suppressor. The computer operation is interrupted when the sag
or outage exceeds the capability of the internal dc supply to power the
logic and memory circuits. Most computers have a built-in capability to
maintain operation for a short time when this power is lost, but that
supply is drained out if the interruption is long enough. if the
computer is using a disk drive when the sag occurs, a shut-down is
likely; in an office using several identical machines, some ride through
a disturbance while others, especially those reading from a disk, shut
down and have to be restarted. Protection against such sags and outages
requires an uninterruptible power supply (UPS), which is now readily
available. In fact, the volume of UPS production as well as competition
has brought prices down so low that purchasing one becomes a viable
solution and, for users de-pendent on the continuity of their operation,
a must.

**Unexpected problems**

A power outage or sag on distributed systems has the same effect as for
stand-alone systems. A more subtle problem, however, has crept in for
some sophisticated systems that include automatic restart, or rebooting,
after a power interruption. Anecdotes have circulated of damage caused
by repeated sags during the automatic rebooting sequence, typically
occurring because of multiple lightning strokes or during fault clearing
with automatic reclosing by the utility system.

In the case of surges, as soon as a simple stand-alone system is
augmented by peripherals, additional remote terminals, networking, and
sensors that require a data link, the threat that the system will be
affected increases. Even what may appear as a stand-alone system, such
as a simple desktop pair of a PC linked with a printer, might be at risk
if the two units are plugged into different power receptacles fed by
separate branch circuits from the breakers.

In addition to the risk of interference or damage from surges on the
power line, the data-line input and output ports are also vulnerable.
Several mechanisms can inject interfering or damaging transients into
the data lines of distributed systems. First, a problem could result
because data lines act as antennas that can collect energy from
electromagnetic fields and feed it, as noise or surges, to the data
port\'s input or output, the driver or the receiver of the computer, or
its peripherals.

The problem\'s severity increases with the length of the data link
Within the same room, the risk of damage is minimal. But as the
communication link reaches farther out, the risk increases that a surge
would not only interfere with a system but could damage it. Though there
may be an unknown (to the user) built-in protection or inherent
capability of the data port components to with-stand these surges,
little is known about the occurrence of surges on data lines, compared
to that on ac power lines, which makes the task of designing protection
difficult.

For users of computer systems in the same room or the same corner of a
building, the built-in capability probably suffices. For systems with
longer reach, the ultimate protection is an optical-fiber link with no
metallic jacket, which provides immunity against noise collection as
well as possible surge damage. For these complex systems, however, the
do-it-yourself approach should be replaced by one that has been designed
by a specialist.

Another mechanism that could cause trouble is the difference in the
potential of objects at nominal \"ground\" potential occur-ring during
surge events. Most data links operate with the signal reference
conductor (shield or one wire of a group) connected to the chassis of
the equipment. This chassis is in turn connected to the grounding
conductor of the power cord supplying the equipment, a requirement of
the National Electrical Code. Thus, if lightning or power system faults
inject a high current in the sites ground conductors, the potential of
the \"grounded\" points at the two ends of the data link differs. This
potential difference causes a current to flow into the data link,
possibly exceeding the capability of the input or output components.

The user can stay with conductors for the data link or convert (or
initially design) it to an optical-fiber link, an approach that is
becoming increasingly popular as hardware costs fall with economics of
scale. However, if the conversion electronics at the ends of the fiber
link are disturbed by electrical noise, that noise will be faithfully
transmitted, not blocked.

If a conductive data link is to remain, the remedy is to insert
protective devices that are complementary for the power line and data
line. These devices typically operate by limiting the overvoltage or
attenuating the higher frequencies by filtering, which works effectively
on the power line but not on the data link. Here, filtering is not
possible because it would affect the signals; limiting the overvoltages
will eliminate that damage risk, but might still let through a spurious
signal. Thus, data integrity may be more difficult to achieve unless the
software includes inherent immunity or fault tolerance.

# Side effects

Avoiding damage with protective devices may then seem to re-quire only
the insertion of a power-line surge suppressor at the wall receptacle
and a data-line surge suppressor at the input to the computer. This
apparent simplicity, however, is deceptive because the very operation of
this device, if incorrectly installed, can have a side effect that would
put the data link components at risk, a mechanism that is only beginning
to be fully recognized.

Still another mechanism can be demonstrated by a scenario that can occur
in any building with power and telephone service. The incoming telephone
line is provided with surge suppressors (car-bon blocks or gas tubes)
that divert surges to the nearest grounded conductor, generally a nearby
water pipe. The manufacturer of the computer or modem used for the
computer-telephone-line linkup may have provided a protective device
within the equipment. Alternately, the surge-conscious user may have
inserted a protective device in the power cord. But should a surge occur
on either the data line or the power line, the corresponding protective
device will dutifully divert that surge to the nearest ground. Since the
\"nearest ground\" may not be the same for the connection of the two
suppressors, the surge current in the ground connection raises the
potential of one side with respect to the other, placing the data input
at risk.

The solution is a miniature setup of the \"ground window\" concept
developed by telephone companies in protecting their central station
switches: all cables entering a room or a complete floor in a building
are routed through a single \"window\" where grounding conductors,
shields, and ground connections of protective devices are bonded
together. In this manner, there cannot be any potential difference
between the various ground reference points within the room or floor.

Some surge suppressor manufacturers have adapted that concept to a
portable version of the ground window, a device consisting of a
suppressor for the power line and one for the data line, but packaged in
a single box most likely sharing the same ground connection. This local
ground window is now found in computer or hobby stores and is easily
recognizable because it features both a power connection (male plug for
connection to a wall receptacle and female receptacles for powering the
loads) and a pair of data link connectors (input and output). Depending
on what is needed, these connectors can be a standard telephone jack, a
multipin RS232, or a cable television coaxial connector. The device is
then inserted near the computer, with the power cord and data link
routed through its connectors.

Another protection scheme is always available: disconnect the system
when not in use! In fact, some of the consumer guidance folders inserted
by the utilities with their monthly bills mention that approach. That
option may not be practical for commercial operations, where some link
could be left connected, creating the risk of ungrounded equipment.
Thus, if applied, every link to the outside world must be disconnected.

**To probe further**

A good source of information on the basics of lightning is the book
Understanding Lightning by Martin A. Uman, available from Academic
Press, New York, 1971. Solutions to noise problems are given a general
treatment in the second edition of Noise Reduction Techniques in
Electronic Systems by Henry W. Ott, available from John Wiley & Sons,
New York, 1989. Fundamentals of surge protection techniques are treated
in Protection of Electronic Circuits from Overvoltages by Ronald B.
Standler, also available from John Wiley & Sons, 1989. Another useful
reference is Uninterruptible Power Supplies by David C. Griffith,
published by Marcel Dekker, New York, 1989.

Guidance on the nature and severity of transients (not specifications
for protective devices) is given in the IEEE Guide for Surge Voltages in
Low Voltage AC Power Circuits, American National Standards Institute,
C62.41-1980, available from the IEEE Service Center, 445 Hoes Lane, Box
1331, Piscataway, N.J.; 800-678-IEEE.

A paper by François Martzloff and Thomas Gruzs titled \"Power Quality
Site Surveys: Facts, Fiction and Fallacies\" in the November 1988 IEEE
Industry Applications Society Transactions presents a review of
recording, analyzing, and reporting transient disturbances on power
lines. Another paper, \"Coupling, Propagation, and Side Effects of
Surges in an Industrial Wiring System,\" by Martzloff in the same
Transactions is in press. The journal is available from the IEEE Service
Center.

**Number range CHAPTERUpdate on a Consumer-Oriented Guide**

**for Surge Protection**

+--------------------------+----------------------------+--------------+
| Thomas Key, Doni         | François Martzloff         | Jim May      |
| Nastasi, Kermit Phipps   |                            |              |
|                          | National Institute of      | Illinois     |
| EPRI-PEAC                | Standards and Technology\* | Power        |
|                          |                            | Company      |
| 10521 Research Dr.       | Gaithersburg MD 20899-8113 |              |
|                          |                            | 500 South    |
| Knoxville, TN 37932      | <f.martzloff@ieee.org>     | 27^th^       |
|                          |                            | Street       |
|                          |                            |              |
|                          |                            | Decatur, IL  |
|                          |                            | 62525        |
+--------------------------+----------------------------+--------------+

# Abstract

Caught among contradictory stories on the need for surge protection as
well as unsupported anecdotes of surge-related failures found in some
editorial advertising, the typical consumer is in a quandary on how to
best allocate personal resources to protect the expensive electronic
equipment found in a modern household. To help provide some answers to
this quandary, a team of experts had previously developed a basic
engineering Recommended Practice on surge protection of residential
electronics. The value of the theoretical concepts presented in the
Recommended Practice is illustrated by two case histories where such
concepts were not applied. In addition, "post mortem" examinations have
been performed on appliances turned in as having failed as a result of a
lightning surge. Those appliances that were in fact not damaged by
lightning, although part of a claim settlement, were tested for surge
immunity.

1\. Introduction

**1.1 The need for a Guide on Surge Protection**

As home appliances and electronics have become more sophisticated they
have also become more susceptible to damage from energy anomalies. These
anomalies are most often caused by switching surges and lightning
strikes. They are also causing more frequent homeowner insurance claims.
While there is a considerable amount of knowledge about the various
components of the issue (lightning, surge damage and protection, and
grounding practices), there is no single definitive source for combined
information. Therefore, a partnership was formed between State Farm,
Illinois Power, the Electric Power Research Institute, and the National
Institute of Standards and Technology to develop a document that would
fill the need for a consumer-oriented, reader-friendly guide on surge
protection. The initial goal of this partnership was to develop a
recommended practice for protecting homes against surges and lightning.
From this, the partners expect to convey best surge mitigation
practices, to decrease appliance damage and the number of loss claims to
insurance companies, and to reduce consumer worry about surge-related
damage to their electronic appliances.

Like many electric utilities, Illinois Power, with over 500,000
customers, understood the need to provide an authoritative and
well-documented reference document applicable to protecting property and
equipment. With such a reference, electric power producers can more
easily answer the many customer inquiries about surge protection as well
as to better communicate with customers via specific power quality and
equipment protection programs. Promoting such customer-oriented service
programs is expected to be a win-win situation. In the case of Illinois
Power, sponsoring this project was also an opportunity to help one of
their large customers, State Farm Insurance in Bloomington, IL.

**1.2 From engineering writing to general-public writing**

As a first phase of this project, a group of experts had developed a
Recommended Practice document \[EPRI Project 01 39-8002, 1997\][^2] on
the origins and mitigation of surges in the residential environment.
This 90-page document was written from the engineering point of view to
document the consensus reached among these experts and serve as the
basis for the next step, a consumer-oriented publication available to
the general public. The need for a consumer-oriented guide stems from
the practical fact that, for most situations, a generic surge protection
system costs less than an engineering analysis to determine exact
requirements. Consequently the partners recognized the value of
developing a version of the guide written so the general public could
understand and apply the basic concepts. The idea is that individual
homeowners, who are concerned about their appliances, will better
understand what to look for and how to correct deficiencies so that they
might take sound and cost-effective actions.

To accomplish this objective, NIST, in concert with the co-authors of
the present paper, is undertaking the writing of a shorter,
reader-friendly guide on surge protection. One of the important
theoretical points made in the engineering report was the need to
provide coordinated grounding practices among the utilities (electric
power, telephone, cable and satellite TV) serving a residence. To
further document the importance of this coordination and the dire
consequences of not following them, two case studies were performed on
lightning-related incidents. Bench tests were performed on damaged
equipment obtained by agents of State Farm, with the purpose to
determine typical failure modes and assess surge vulnerabilities and
immunity.

Three critical elements for the guide are recognized in this task:

1\. To be clearly written in layman\'s language --- a challenge to the
editors

2\. To originate from an objective source --- hence the participation of
NIST as the "publisher."

3\. To provide simple instructions on how to reduce surge
vulnerabilities in the home --- another editorial challenge for
engineers accustomed to write engineering instructions.

2\. Lightning-Damage Case Studies

In general much more technical information had been collected in the
Recommended Practice development effort than a typical, or even highly
motivated, homeowner could use. Even so, some voids in understanding
appliance failures still existed, namely exactly why some appliances
fail and other do not. Both field experience and lab evaluations of
appliances show that susceptibility to surges varies by appliance type,
make, and model. According to insurance claim records, the most commonly
damaged appliances include telephones and modems, computerized
equipment, televisions, VCRs, and satellite receiver systems, which are
all generally "multi-port" appliances.

The theory for failures presented in the recommended engineering
practice document was the likelihood that voltage potential differences
between ports was the Achilles' heal of many residential appliances and
systems. To back up this theory, a sampling of multiple-port appliances
was obtained from appliances surrendered as part of insurance claims in
the state of Tennessee. Postmortems and laboratory experiments were
conducted to better understand the failure modes of these appliances.

Two known lightning incidents occurred recently in the Knoxville area,
providing a unique opportunity to document the mechanisms and failure
modes of residential electronics. The first incident involved a small
house made primarily of wood where the power service entrance and the
cable TV entrance were at opposite ends of the house -- the classic
error in installation practice. The second incident involved an
expansive residence with an elaborate audio-video system with a
centralized equipment rack distributing signals to speakers and video
monitors.

**2.1 The case of the Cozy Cabin**

At this rural site in Eastern Tennessee, a wood-structure residence
suffered two successive failures of video equipment during lightning
storms, a few months apart. The owners allowed PEAC personnel to visit
the site and acquire the second failed TV receiver for a post-mortem.

**2.1.1 Site configuration**

The power service entrance is located at one end of the house, while the
cable TV service entrance is located at the opposite end. A visit to the
site revealed that the cable TV shield was grounded only by a
questionable ground rod next to the house foundation (within the drip
line, and thus in dry soil, see Figure 1). Furthermore, this grounding
connection was not bonded to the grounding connection of the power
service entrance, a clear violation of the National Electrical Code,
according to the current edition as well as several, if not all, earlier
editions \[NFPA 70, 1999\].

\[See pdf file\]

## Figure 1 -- Cable TV service entrance and 'ground bonding'

The local cable TV company was informed of this situation and took what
they believed an appropriate action: the incoming cable was first routed
under the house, in the crawl space, to allow bonding the shield to the
power service grounding connection. From there, the cable was returned
to its original point of entry into the house, at the opposite end. It
would seem that under such a configuration, a lightning current surge
traveling along the cable shield (instead of a proper bonding
conductor), might create sufficient voltage drop along the path under
the house to create problems. However, the interaction was not pursued
further with the cable company. A recommendation was made to the owner
of the house to install a surge reference equalizer near each of the
video equipment in the house.

**2.1.2 Post mortem and surge tests**

One of the failed TV receivers was made available to PEAC for
examination. No evidence of damage was found on the power side of the
chassis, but a clear indication of surface flashover was observed
between the cable input 'ground' termination (connected to the incoming
cable shield) and the shielding can of the tuner (Figure 2). After
cleaning as best as possible the carbonized path of the flashover, a
surge voltage was applied between the power cord of the receiver and the
'ground' of the cable input: flashover occurred for a 2 kV Ring Wave. By
removing the material to a greater depth and covering it with epoxy, the
gap did withstand a greater level, and failed at 2.5 kV. That flashover
occurred at another part of the original insulation, thus providing
valid information on the original withstand capability.

\[See pdf file\]

## Figure 2 -- Cable TV input termination and UHF tuner at rear of receiver

**2.1.3 Conclusions from the Cozy Cabin**

This case study illustrates the classic situation of separate service
entrances, compounded with incorrect bonding. The examination and test
demonstrate that at least 2.5 kV can be developed across the power port
and the cable TV port of the receiver under a condition of distant or
nearby lightning strike. This finding is consistent with the results of
other tests reported under Section 4.

Another significant finding from this case history is the anecdotal
confirmation of allegations that cable TV installation practices
prevailing in many residential situations might be in violation of the
National Electrical Code (NEC). This violation makes even more hazardous
the now well-recognized occurrence of undesirable separation between
utilities entrances.

**2.2 The Case of the Rambling Residence**

A residential estate insured by State Farm was the scene of a lightning
incident where a tree adjacent to the house was struck (and subsequently
died). Extensive damage was inflicted to the audio-video components
distributed from a central rack throughout the residence and its
surroundings (large patio and swimming pool with outdoor lighting and
audio speakers).

Through the cooperation of the State Farm agents, the home owners
graciously allowed PEAC engineers to visit the residence and observe the
configuration of the system in an attempt to better understand the
mechanisms leading to the damage. State Farm also made available to PEAC
the complement of damaged or presumed damaged equipment that had been
promptly replaced as part of the claim settlement. Thus this case
history, unlike most lightning incidents, offered an unusual opportunity
for documenting the process and consequences. It was made very clear to
the owners that the visit was prompted by the curiosity of electrical
engineers and not by State Farm agents.

Three activities were undertaken for that purpose:

-   Bench examination of returned equipment and surge testing of
    undamaged equipment

-   Site visit of the residence

-   Laboratory coupling of electric field stress into a remote
    speaker-to-amplifier connection

    1.  **Bench Examination**

The complete central rack of audio-video entertainment equipment, as
well as the remote speakers had been replaced as part of the claim
settlement. Local State Farm agents were able to retrieve the damaged
entertainment equipment from the house and send it to PEAC for
examination. Bench tests for each electrical appliance began by plugging
it into a 120-V ac outlet. Physical signs of normal operation were noted
such as illuminated displays, response of controls, or audio/video
output. The equipment cover was removed for an internal inspection. In
most of the equipment, physical damage such as a burned-out transistor
was very apparent. Table 1 lists the home entertainment equipment that
was submitted, and their condition.

A significant finding was that all of the power supplies in the
equipment were functional, indicating that the lightning surge either
did not enter through the ac power port, or was not severe enough to
cause damage to the power supplies.

**Table 1 Equipment submitted for bench tests**

  -------- ----------------------------- ---------------------------------
  Qty.     Equipment                     Condition

  1        Stereo tuner/amplifier        Illuminated display was not
                                         working

  3        DSS receiver                  Damage to telephone circuit

  2        Twelve-channel integration    Damage to power transistors
           amplifier                     

  12       Indoor speaker                Woofer damaged

  4        Indoor/outdoor speaker        Woofer damage on all but two
                                         units

  1        Independent color TV receiver No damage
  -------- ----------------------------- ---------------------------------

The examination and, as appropriate, tests revealed the following
conditions:

-   Except for its non-operational display, the stereo tuner/amplifier
    appeared undamaged.

```{=html}
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```
-   The visible damage to the DSS receivers was nearly identical in all
    three units. Apparently, the surge had entered through the telephone
    port and had damaged the small surface-mount electronics that make
    up the modem circuit. However, because the satellite receivers could
    not be operated without their antenna and code-reading cards, the
    extent of the functional damage could not be determined

-   The two 12-channel amplifiers had sustained damage to their output
    transistors that drive the speakers. The power supplies and fuses
    were not damaged. Furthermore, it was found that some speaker
    outputs were still operational. This situation suggested a possible
    mechanism that damaged only some of the output transistors, and
    therefore not attributable to a "power-line surge." In turn, that
    hypothesis was later verified by a laboratory test, as described in
    2.2.3 below.

-   Speakers, tested by connecting them to an amplifier that was known
    to be functioning properly, were found to have their woofers
    damaged, but the tweeters still operational.

Other electrical appliances in the house including television sets and
VCRs not connected to the centralized system were not included in the
returned package and therefore presumed as not damaged. This finding
helps to confirm the conclusion that the long leads interconnecting the
distributed system components acted as energy collectors, feeding the
induced voltages or currents into the communications ports of the
equipment.

1.  **Site Visit**

The purpose of the visit was to look for clues to explain the specific
damage that was observed in some of the equipment. The following
observations were made:

-   Approximate distances between the lightning strike and affected
    equipment

-   Location and grounding at the service entrances of cable TV, power,
    and telephone

-   Other wiring and general installation practices

The overall physical layout of the house, damaged tree, and electronic
equipment are shown in Figure 3. The mature tree that was struck is much
taller than the house and is the tallest of the trees in the immediate
area. The bark was stripped from the tree trunk for most of its height.
As shown in the figure, the trunk is only approximately 15 meters from
the house and even closer to the patio, where some of the audio speakers
were installed. Outdoor speakers are also located in the pool area and
connecting walkways.

From this informal examination, it appears that no electrical codes were
violated. Furthermore, the cable TV and telephone entered the residence
together with the electrical service at the same location (garage), as
is the practice recommended in the engineering report.

One of the observations during the laboratory inspection of the audio
amplifiers was that the power supplies of the units were not damaged,
but their output transistors that drive the speakers and the speakers
themselves were damaged. Before arrangements could be made for the site
visit, it was speculated that the surge energy had been coupled into the
speaker wires, which are probably very long and very near to the
lightning strike, rather than as a "power-line surge." The visit to the
installation verified this theory. The home entertainment equipment,
including amplifiers, is centrally installed in the basement of the
house. Speakers are located throughout the house, the patio, and the
swimming pool area. The wire length between the outdoor speakers and the
central amplifier is about 30 meters, and some of the wires run within
10 meters of the tree trunk.

\[See pdf file\]

Figure 3 -- Schematic configuration of the Rambling Residence

1.  **Laboratory Coupling of Electric Field**

To validate the hypothesis of a failure mechanism involving the coupling
of electric field energy into the speaker wire run, a qualitative
laboratory test was staged, using the surviving channel of the amplifier
to drive a speaker. An audio signal from a tape deck was fed into the
amplifier input to monitor its operation during the test.

A conventional Marx impulse generator at the NIST High-Voltage
Laboratory was used to apply a 1.2/50 μs high-voltage field between two
parallel plates, each 2 m x 1 m. A length of 5 m of speaker wire feeding
the audio output from the system audio amplifier was sandwiched between
the two plastic foam sheets separating the lower (grounded) plate, to
which the amplifier chassis was bonded, from the upper (impulsed) plate.
A UPS was used to power the tape deck supplying the audio input signal
to the power amplifier, which was also powered by the floating UPS.

With increasing amplitude of the impulse, a total potential of up to 30
kV was developed between the speaker wire and the chassis of the
amplifier, but no distress was observed at that level. Note that the
actual field strength during the incident is unknown but suspected to be
quite high, so that this test does not pretend to duplicate the
incident, but only to illustrate a possible mechanism.

The effective length of the wire was then increased by connecting to
each wire a piece of foil of about 0.02 m^2^, simulating the increased
capacitance effect of about 50 m of wire. Again, the impulse was applied
in increasing steps. At 70 kV, a flashover occurred between the two
plate edges, but not involving the speaker wire. Immediate failure of
the amplifier output circuit was noted. From this anecdote, we conclude
that the rapid field change (not recorded during the test but certainly
faster than the standard 1.2/50 μs impulse) did have the capability to
couple enough surge energy into the capacitance divider of wire/ground
plane and cause destructive failure of the output transistor effectively
connected across that capacitance. Such a scenario can be considered a
reasonable emulation of the circumstances surrounding the "Lightning
Incident at the Rambling Residence."

Thus, this qualitative laboratory demonstration provides one more piece
of evidence that electronic appliances can be damaged by surges
impacting their communications port, to the point that expecting
protection by simple application of SPDs on the power port is not
enough, and comprehensive protection is a necessity.

2.  **Conclusions from the Rambling Residence**

Generally, electronic equipment can be protected from surges by proper
installation and with the use of surge suppressors. In the rare case of
a nearly direct strike, however, there is little that can be done in
widespread residential settings to prevent damage, unless a thorough
mitigation study and extensive protection implementation would be
performed. While that type of protection is theoretically possible and
has been implemented --- at considerable cost --- for facilities
involving national security, airports, communications centers, etc., it
is generally not considered as economically justifiable for residential
installations. A central home entertainment system with remote speakers,
such as the one installed at this house, is an easy victim for coupling
of surge voltages or surge currents in the long runs of cable.

Since the visit to the residence was motivated, as stated, by the
curiosity of PEAC engineers rather than by a customer complaint, there
was no need to offer a prescription for protection. Instead, the
visiting engineers offered the general statement that for a direct
strike such as the one that had occurred, providing a high level of
protection confidence by a comprehensive retrofit would probably cost
more than the equipment that was damaged. In contrast, the dead tree was
the one irreplaceable loss that was most painful for the owners.
Techniques for protecting trees exist (an extensive array of grounding
cables along the limbs, \[NFPA 780, 1997\] and are sometimes applied to
trees recognized as having great historical or esthetic value, but the
cost is generally a deterrent for typical residential surroundings.

3\. Post-Mortems on Returned Equipment

An assortment of residential equipment returned to State Farm as
settlement of lightning-related claims was made available to PEAC. These
included submersible well pumps, audio and video equipment, and
telephones. The goal was to determine the failure mechanisms of the
samples, whether it be failed power supply components, input/output
ports, or other types of equipment failures.

Individual pieces of equipment were examined for mechanical condition,
signs of electrical insulation breakdown, and if it seemed appropriate,
powered to test their possible functionality. Most were in fact damaged,
as tabulated below, but a few appeared still operational. This apparent
functionality may well be a case of "walking wounded" so that the
intention of the examination was not to contest the claim of damage, but
to gain insight into the failure levels and mechanisms.

**3.1 Equipment condition**

1.  **Well pumps**

Fifteen submersible well pumps in the size range of ½ hp to 1 hp were
examined. These pumps spend their lives underwater while connected to
the power system conductors, and are therefore susceptible to lightning
damage.

The units were visually inspected and resistance was measured between
pairs of motor terminals and between case and terminals. No conclusive
evidence was observed on most of the samples, however one sample had
very interesting evidence of lightning damage. It had burn marks present
at the point where the voltage is applied to the stator winding. It also
had a bulge in the stator that appeared to have been caused by excessive
current through the winding.

2.  **Television Sets**

Seven television sets were received for inspection. Of these units, the
failures most commonly observed were in the power supply. Four of seven
units had open fuses, suggesting that components inside had drawn
excessive current and had failed, causing a short circuit. Two of the
units had shorted rectifier diodes, further confirming this theory. One
television had an open fuse, but was fully functional after a fuse
replacement. One of the more interesting cases was a television that had
its tuner severely damaged, but the power supply was not damaged.

Television sets, being two-port devices, require some type of isolation
between antenna port and ac power port. Different protection schemes
were observed in the television sets depending on their age. Older sets
utilized an antenna isolation scheme, with television chassis connected
to the one of the line cord conductors. Newer designs had direct antenna
connections (no isolation), but had isolation built into their power
supplies.

**3.1.3 VCRs**

Five VCRs were inspected. Two had damaged power supply components. One
had apparent damage to the tuner. VCRs have the same two ports as
televisions and therefore require the same type of isolation. All of the
VCR samples studied at PEAC had the isolation scheme built into the
power supplies rather than in the antenna port.

**3.1.4 Stereo Equipment**

Nine individual stereo components were examined. One had damaged power
supply components. Three of the amplifiers had damaged output
transistors. Two of these were from the "Rambling Residence." Others
were functional.

**3.1.5 Telephones**

Five telephones were examined, from which three had damaged ICs. These
may have been damaged by lightning. Most electronic phones are equipped
with plug-in adapters. These adapters may sustain damage, but seem to
protect the telephone equipment from damage on the ac power port.

**3.2 Conclusions for Post Mortems**

There were almost as many types of equipment failure modes as there were
samples to examine. The samples that had apparent lightning damage
presented the signs one could expect from insulation failure, most often
with subsequent damage done by the power-frequency fault current
following the insulation breakdown.

4\. Surge Immunity Tests

The returned equipment that was found still operational was then used
for destructive surge testing by subjecting them to Ring Waves or
Combination Waves on their power port or on their communication port.
Two types of surge tests were conducted:

1.  Application of the surge to the power port only. This test is
    primarily a surge current withstand capability for the rectifier
    that connects the DC link capacitor to the ac mains. During a surge,
    this capacitor acts as a "surge absorber" and can draw a substantial
    charging current \[Mansoor et al., 1999\]. Ultimately, the voltage
    in the DC link might reach an excessive level for the downstream
    components, but from the few limited tests performed here, it
    appears that either the rectifier fails, followed by opening of the
    fuse, or that the fuse itself opens without downstream failure. In
    either case, however, the appliance is a candidate for a trip to the
    repair shop, or a total loss damage claim, as indicated in the
    post-mortems.

2.  Application of the surge between power port and communication port.
    This is the scenario illustrated by the Cozy Cabin case history in
    this paper. The tests served to document further the process and the
    levels at which failure can occur.

**4.1 Power port tests**

Combination wave surges were applied to the equipment between the two
power port conductors. Beginning with a peak voltage of 1 kV, the test
sample was surged and then checked for functionality. If the equipment
sustained no apparent damage, then the surge voltage was increased and
re-applied in 1 kV steps until either the equipment failed or until a
maximum of 6 kV was delivered.

Three well pumps from the original group were functional and therefore
were available for surge tests. The coil resistance was measured and
recorded before applying any surges. The well pumps demonstrated a
strong immunity to these surges. All three samples survived all tests to
and including 6. V and with no notable changes in performance or in coil
resistance. This remarkable withstand capability might be attributed to
the presence of a built-in SPD.

Two working televisions were available for surge tests. Each unit was
energized, and using an antenna, was tuned to a local station having a
strong signal. The two units failed at 3 kV. Upon examination of the
circuits, both had suffered open fuses and failed power supply
components. Rectifier diodes were found shorted inside one of the units.
Recall that in the post-mortem examinations, inside four of the sets,
there were open fuses and two samples had shorted rectifier diodes.

Three VCRs were functional and were subjected to surge tests. The first
sample failed at 3 kV.

An open fuse and failed rectifier diodes were found inside. The second
sample failed at 4 kV, but without opening fuses and with no visible
damage. A probable result was a damaged voltage regulator. The third
sample withstood surges up to 5 kV, where a open fuse resulted. The fuse
was replaced and the unit functioned normally. Surges were repeated,
opening the fuse again at 4 kV. The difference in survivability among
the VCRs seemed to be transformer size. Modern power supply transformers
are miniaturized, operating at higher frequencies than the 60 Hz power
frequency. The VCR samples that had physically larger transformers
survived the higher surge voltages. The sample that blew the fuses and
withstood the surge tests had the largest transformer of the three VCRs
tested.

Five individual stereo components, specifically two cassette decks, two
amplifiers, and one CD player were tested. This type of stereo equipment
is designed for use as interconnected systems of media-playing devices,
amplifier, and speakers. The goal of the surge tests was to assess the
immunity of individual components\' power supply circuits. Therefore,
each piece was tested without connection to other components. After each
surge, the tested sample was briefly connected to necessary components
to verify that it was working properly. A very consistent outcome was
observed. All of the samples survived the surge tests up to and
including 6 kV.

Only one telephone sample was available for surge tests. It was of the
cordless variety, with an external power supply adapter. This unit
survived all of the surge tests with no apparent damage.

**4.2 Two-port tests**

Televisions and VCRs are of a similar category of equipment in that they
both have the same input ports: ac power and antenna, for which the
'ground' reference can be raised to different potentials during a surge
event. The power port has no direct connection to the equipment
grounding conductor because a two-prong ac plug is used. However, at the
service entrance, the neutral --- one of the two conductors of the cord
--- is bonded to the ground bus of the service panel. The antenna input
is referenced to ground via a connection to a grounding rod outside the
residence and (per NEC), a bonding conductor. Manufacturers use various
techniques to isolate these two ports from each other inside the
equipment. The purpose of these tests was to determine the surge voltage
at which this isolation breaks down.

Most of the equipment was less than 6 years old and therefore represents
the current state of technology. By observation of a few samples, there
seem to be two isolation schemes. Older technology seemed to use a
potted ring (an amorphous, plastic insulating material) paralleled with
a resistor to isolate the shielded conductor of the antenna from the
chassis of the tuner. Newer sets have isolation in their switch-mode
power supplies and directly connect the shielded antenna conductor to
the tuner chassis. All VCRs that have been observed use the latter
technique.

Surge tests were conducted on five televisions and three VCRs to
determine the voltage level at which spark-over would occur between
these power and antenna ports. The two power port conductors (line and
neutral) were tied together because the two conductors act together to
create the power port, and because there is no rationale to support the
separate application of surges to each of the conductors of that port
individually. Using a 0.5 :s --- 100 kHz ring wave, a 500 V peak surge
was applied between power port and shield of the antenna port.
Subsequent tests were performed at incremental steps of 500 V until
spark-over occurred or until a maximum of 6 kV was reached. This voltage
level was recorded and an oscillogram was taken. A decision was then
necessary. If a surge voltage of 500 V less than the recorded value
would also produce sparkover, it is possible that a carbon trace exists
in the device, and tests were discontinued. If there was no sparkover at
the reduced voltage, the unit was subjected to a 1.2/50-8/20 μs
Combination Wave, starting at 500 V and increasing in increments of 500
V until breakdown or a maximum of 6 kV was reached.

The units that had isolation built into the antenna port sparked-over at
the series capacitor of the antenna port. The breakdown voltage level
averaged approximately 2.5 kV in these units. Units whose power supply
outputs were electrically isolated from the inputs via transformers
generally fared better. One sample survived and performed normally after
all surge tests. Physical size of the transformer seemed to have some
impact on the results. Larger transformers seemed to tolerate surges
better than smaller ones.

**4.3 Conclusions to two-port surge immunity tests**

Some type of isolation scheme is used in televisions and VCRs to
electrically isolate the antenna input from the power supply input. A
general observation based on the small number of test samples is that
older televisions (before VCRs were popularly used) seem to have their
isolation built into the antenna ports using an insulating plastic
material. Newer technology seems to have moved the isolation from the
antenna port to the power supply port by utilizing the high-frequency
chopper output transformer. During these surge tests, it was observed
that the power supply isolation method was more immune to damage than
the antenna port isolation method.

Based on this small number of VCR samples, a general observation is that
VCRs use power supply transformers as isolation, similar to the newer TV
sets, and have no isolation between antenna input to tuner chassis.
Generally, this type of isolation scheme withstood higher surge voltages
than the antenna port isolating ring that can be found in older
television sets, therefore VCRs seem to have the same (or slightly
greater) levels of immunity as any television set.

5\. From the Engineering Report to the Consumer-Oriented Publication

The engineering report completed as the first phase of the project
\[EPRI Project 01-39-8002, 1997\] and described in the PQA'97 summary
paper \[Key et al., 1997\] was written for engineers, not for the
general public. The ultimate goal of the project, however, has been from
the beginning to produce a self-standing but brief publication aimed at
the general public, in particular electrical contractors, builders,
insurance agents, and interested homeowners. In the 1980\'s a Federal
Information Processing Standards Publication \[FIPS Pub 94, 1983\] was
developed by the then National Bureau of Standards (NBS) for the
"computer room" environment. At that time, when the residential
electronics had not yet started their exponential growth, FIPS 94 became
--- and remains --- a widely read reference. It was first envisioned
that a similar publication, albeit less voluminous, might be issued by
the National Institute of Standards and Technology (NIST), the successor
of NBS. The project end-goal subsequently evolved to plan for a shorter
publication, limited to the perceived limits of a typical reader's
attention span, but still under the sponsorship of the generic
perspective of NIST.

The target date for general availability of this publication is now late
1999, giving interested parties an opportunity to implement feasible
retrofit recommendations in time for the Y2K lightning season.
Hopefully, the recommendations will also influence future installations.

**6. General Conclusions**

The combination of the theoretical considerations, presented in the
cited engineering report, with further documentation presented in this
paper, clearly demonstrates that the two-port interaction plays a large
role in appliance failures during surge events. That concept will be
emphasized in the final proposed NIST Special Publication, which remains
the ultimate goal of the project.

Nevertheless, readers of the present paper might be interested in a list
of technical and practical conclusions, leading to recommendations they
might wish to make to their colleagues or clients pending availability
of the NIST publication:

-   The expanding use of multi-port appliances is increasing surge
    vulnerability in residential environments. More attention to their
    design and installation wiring is needed. The concept of
    "compatibility levels" can help.

-   The power port might not even be one of the ports involved in the
    damage.

-   The typical homeowner is likely to assume the worst after a
    lightning event and mis-diagnose lightning damage. "If it works it
    is probably not damaged" may be a reasonable point to publicize.
    (Notwithstanding a justifiable concern about keeping a "walking
    wounded" in case of serious damage to other equipment in the
    residence.)

-   In fact, when lightning strikes, the configuration of the residence
    appliances and wiring may be more important than the level of the
    threat. This is particularly true for non-power born surges.

-   House wiring systems will play an increasing role in determining
    vulnerability of appliances as installed in specific locations, as
    more wires are run and more appliance ports are referenced to each
    other.

**7. Acknowledgments**

Roger Witt provided encouragement to the authors and comments on this
paper, as well as making arrangements for the return of equipment and
site visits.

The owners of the two residences, whose identity and exact location are
not given here to preserve their privacy, provided a rare opportunity
for documenting these two enlightening case histories, for which the
authors are indeed grateful.

8\. Bibliography

Bibliographic citations in the text are listed below by alphabetical
order of the lead author.

FIPS 94, Guideline on Electrical Power for ADP Installations, Federal
Information Processing Standards Publication 94, Sept. 21, 1983.

EPRI Report 01-39-8002, "Recommended Practice for Protecting Residential
Structures and Appliances Against Surges," Final Report, 1997.

IEEE Std 1100-1992, IEEE Recommended Practice for Powering and Grounding
of Sensitive Electronic Equipment, Emerald Book, 1992.

Key, T.S., Martzloff, F.D., Witt, R., May, J., and Black, S.,
"Developing a Consumer-Oriented Guide on Surge Protection," Proceedings,
EPRI PQA'97 Conference, Columbus, OH, March 1997.

Key, T.S. and Martzloff, F.D., "Surging the Upside-Down House: Looking
into Upsetting Reference Voltages," Proceedings of the 3rd International
Conference on Power Quality, Amsterdam, Holland, October 1994.

Mansoor, A., Martzloff, F.D., and Phipps, K.O., "The Fallacy of
Monitoring Surge Voltages: SPDs and PCs Galore !" Proceedings, EPRI PQA
'99 Conference, Charlotte, NC, May 1999.

NFPA 70, National Electrical Code, National Fire Protection Association,
1999.

NFPA 780, Standard for the Installation of Lightning Protection Systems,
National Fire Protection Association, 1997.

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| ## François Martzloff                                                 |
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| ## June 2004                                                          |
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[^1]:

[^2]: *Citations in the text appearing as \[Author, Date\] are listed in
    alphabetical order in Section 6, Bibliography*.