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| # Las | # Surge Protection Anthology                       | # Su   |
| t upd |                                                    | rges H |
| ated  | **Part 6 -- Tutorials, Textbooks, and Reviews**    | appen! |
| April |                                                    |        |
|  2004 | **Text of "Transient Control Levels" files**       |        |
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FOREWORD

This file contains the text part from six papers or reports on the
following subjects:

-   Transient control levels: A proposal for insulation coordination in
    low-voltage systems (1976)

-   Transient Control Levels: A better way to voltage ratings in power
    converter applications (1976)

-   Transient control level philosophy and Implementation (1977)

> Part 1: Reasoning behind the philosophy

-   Transient control level philosophy and implementation (1977)

> Part 2 Techniques and equipment for testing

-   Transient Control Levels: A new concept licks an old problem (1977)

-   Transient Control Level test generators (1977)

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
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![](media/image1.png){width="0.9576388888888889in"
height="0.4666666666666667in"}

**TRANSIENT CONTROL LEVELS**

**A Proposal for Insulation Coordination in Low-Voltage Systems**

F. A. Fisher

General Electric Company

Pittsfield, Mass.

F.D. Martzloff

General Electric Company

Schenectady, N.Y.

**ABSTRACT**

Failure and circuit upset of electronic equipment due to transients is a
problem now and is one which has promise of becoming more of a problem
in the future as trends continue toward miniaturization and circuit
complexity. Protection methods are used more or less extensively and
often haphazardly.

At present, there does not appear to be a clear approach toward
achieving compatibility between the transient withstand capability of
devices and the transients to which such devices are exposed. A more
scientific approach is needed to guide manufacturers and users of
equipment.

The purpose of this paper is to promote a concept of transient
coordination for electronic and other low-voltage equipment through the
establishment of a system of Transient Control Levels, similar to the
concept of Basic Insulation Levels so successfully used for many years
in the electric power industry. Specific suggestions for possible
Transient Control Levels and standard test wave shapes are made, in
order to promote wide discussion as to whether these waveforms and
levels are the best that can be developed toward good transient
coordination for the electronic industry.

**INTRODUCTION**

Failure and circuit upset of electronic equipment due to transients is a
problem now and is one which has promise of becoming more of a problem
in the future as trends continue toward miniaturization and circuit
complexity. At present, there does not appear to be a clear approach
toward achieving compatibility between the transient withstand
capability of devices and the transients to which such devices are
exposed. This situation appears somewhat as illustrated on Figure 1. A
similar situation prevailed many years ago in the electric power
industry. Transients produced by lightning frequently caused failure of
such vital and expensive power equipment as transformers and generators.
Those transient problems were solved by engineering design guided by the
concept of insulation coordination and the establishment of a series of
Basic Insulation Levels (BIL\'s).

The purpose of this paper is to promote a concept of transient
coordination for electronic and other low-voltage equipment through the
establishment of a system of Transient Control Levels (TCL\'s), similar
to the concept of BIL\'s so successfully used for many years in the
electric power industry. In the following sections, specific suggestions
for possible standard Transient Control Levels and standard test wave
shapes will be made. While the waveforms here suggested are chosen
somewhat arbitrarily, they are well-grounded in physical reality. The
purpose of making such suggestions is to promote wide discussion as to
whether these waveforms and levels are the best that can be developed,
or if indeed the establishment of such standards is the best way to
promote good transient coordination for the electronics industry. The
ultimate purpose of any system of transient coordination would be to
achieve greater product reliability at minimum cost to the user.

**AN EXAMPLE OF THE PROBLEM**

TCL concepts would be of benefit to all users of electronic and other
low voltage equipment, such as railroad, telephone, power, oil industry,
aircraft, and high frequency communications. The source of transients to
which equipment is exposed may be either external (lightning and power
system switching) or internal (switching of inductive loads, contactor
restrikes or cross talk from adjacent circuits). While the concept of
TCL\'s is intended to apply to the full spectrum of frequencies and
voltages (DC, 120 V, 60 Hz AC, 400 Hz) the problem of transient
coordination will here be illustrated by discussion of 120 volt AC
systems intended for consumer and residential use. During the
introduction of electronic equipment into consumer appliances and other
residential use, the importance of transient coordination was not always
sufficiently recognized. In some cases, excessive failure rates occurred
as a result of transients having amplitudes greater than the withstand
level of the equipment.

In residential circuits, transients can occur from two main sources:
internally, from the switching of appliances, and externally, most
typically from the effects of lightning. One study of internally
generated transients ^1^ has indicated that in about three percent of
U.S. households transients greater than 1200 volts occur one or more
times per week. Several studies have been made of externally generated
transients. One such study ^2^ indicates two percent of recorded
transients exceed 1500 volts. The data also indicate that at the
location studied, approximately two surges per year would exceed 1000
volts. Field experience ^1^ indicated that a 100:1 drop occurred in the
failure rate of clock motors when the withstand level was increased from
2000 to 6000 volts. These data indicate that the exposure rate to surges
of 2000-volt amplitude was sufficient to be of concern, but that surges
exceeding 6000 volts were quite rare, at least on a national basis.
Another study ^3^ showed that during two weeks of monitoring in a
lightning-prone area, several surges exceeding 2000 volts were recorded,
with the maximum recorded being 5600 volts. Experience with field trials
of Ground Fault Circuit Interrupters sponsored by NEMA and the
Underwriters\' Laboratory ^4^, when correlated with the known nuisance
trip level of the devices and the observed number of trips ^5^, would
indicate an occurrence frequency of perhaps one surge per 7 years above
2000 volts per household.

Most residential wiring systems are constructed in such a manner that
the various wiring boxes will flash over if they are exposed to surges
greater than 5 to 10 kV. This means that the amplitude distribution will
be chopped at 5 to 10 kV.

Based on these admittedly scattered and tentative numbers, it appears
that the typical residential circuit will be exposed to surges of
magnitude and frequency of occurrence as illustrated in Figure 2.

The magnitude of the transients produced on 120 volt power lines,
however, is not of importance except as it relates to the vulnerability
level of the equipment connected to such lines. \"Vulnerability\" is
defined here as the level that causes an irreversible and undesirable
change (usually failure) in a device. A corollary term is
susceptibility, or that level which causes temporary malfunction of the
device. The susceptibility level cannot, by definition, be higher than
the vulnerability level. Rectifier diodes and similar semiconductors do
not have any particular susceptibility level; they either fail or do not
fail when exposed to transients. Active semiconductor devices or a
control system operated by a mini-computer system might be a different
story. It is quite possible that transients of a low level interfere
with the operation of the mini-computer, causing it to give incorrect
results without causing permanent physical damage. The vulnerability
level of such a mini-computer will be higher than the susceptibility
level. Both levels must be higher than the normal operating level of the
computer logic elements or input/output terminals.

The transient breakdown level or vulnerability of semiconductors is not
presently a part of any industry accepted rating system. The
vulnerability level is furthermore not inherently related to the normal
operating voltage or peak inverse voltage (PIV) level. As examples,
consider the data of Table 1. During this investigation, power diodes
were subjected to unidirectional transient voltages cresting in a few
microseconds. The voltages at which failure occurs are seen to have
little correlation to the nominal PIV rating.

Similar data have been accumulated for many semiconductors, particularly
when semiconductors are

exposed to very short transients, characteristic of those produced by
nuclear weapons (NEMP). Such information has not been widely reported.

## Table I -- Transient vulnerability levels -- Typical 1A silicon diode

Clearly, surges occur with amplitudes greater than the vulnerability of
the indicated semiconductors. The frequency of occurrence of such
damaging surges, while small on an individual basis, may be unacceptably
high on a product line. The transient amplitudes, of course, could be
reduced by the use of suitable protective devices. Likewise, the
vulnerability levels of the diodes to transients could be raised. Some
questions now present themselves, all having to do with the question of
who should assume what part of the job of providing transient
coordination.

a\) Should it be the responsibility of the user to control transients to
levels that do not damage equipment supplied by vendors?

b\) Should it be the responsibility of the manufacturer to provide
equipment that will not be damaged by the naturally occurring
transients?

c\) If it is the responsibility of the user to control transients, to
what level should he control them -- the published operating levels (in
this case the published PIV levels) or some other level higher than the
operating level but below the vulnerability level?

d\) If it is the responsibility of the vendor to provide surge-proof
equipment, what level of transient voltage and transient energy must he
anticipate?

Similar questions can be asked for all product lines: consumer,
industrial, and military, and at all levels of operating voltage.

**INSULATION COORDINATION IN THE ELECTRIC POWER INDUSTRY**

Similar questions occurred many years ago during the development of the
electric power industry at a time when the art of designing equipment to
withstand the effects of lightning was in its infancy. The nature of the
transients, the level of insulation to be used, or what should be
expected of the designers of transmission lines and lightning arresters
was not clear.

Those transient problems have largely been eliminated today by proper
engineering design on a system-wide basis. The evolution of insulation
coordination in the electric power industry, while it can be only very
briefly described here, may be of benefit to the electronic industry

First, the type of transients produced by lightning on transmission
lines, their magnitude and wave shape were measured. This was not easy
in the days of cold-cathode oscilloscopes employing 50 kV accelerating
voltages. Even today with vastly improved instrumentation, such
investigations are expensive and time consuming to make Yet, on the
basis of very limited test data, a standard voltage test wave was
derived, the familiar 1.5 x 40 μs wave. Similar investigations in other
countries led to the establishment in Europe of the 1 x 50 μs impulse
wave. International standardizing activities have now produced the 1.2 x
50 μs impulse wave, a test wave used throughout the world for
coordination of insulation protection. It was never pretended, however,
that naturally occurring surges were of this type, only that the rise
and fall times of the natural surges were in the vicinity of the above
values.

The next stage in the process of insulation coordination was the
establishment of a series of standard test and design levels, BIL\'s.
For example, equipment designed for operation on 115-kV systems was
assigned a BIL of 550 kV. The designer of equipment to be used on 115 kV
systems then was required to provide an insulation structure that would
withstand 550 kV. The level of 550 kV was derived on the premise that
existing lightning arresters could be used to control the transients
applied to that apparatus to less than 550 kV. The proper design of the
insulation system was next demonstrated by subjecting the apparatus in
the laboratory to a surge of 1.5 x 40 μs wave shape and a peak amplitude
of 550 kV. Frequently it was part of the purchase agreement that the
equipment had to successfully pass the laboratory test. If the equipment
failed, it had to be rebuilt or redesigned. Conversely, it became the
responsibility of the user to ensure that no surge greater than 550 kV
was ever applied to the apparatus.

As a result, power equipment achieves its resistance to
lightning-induced transients not so much by being designed to the threat
that might be posed by lightning, but by the threat that will be posed
by an acceptance test. This acceptance test does not subject the
equipment to transients having the complex wave shapes produced by
lightning, but instead to transients having elementary wave shapes that
can be produced by basically simple test apparatus. Neither does the
acceptance test subject the equipment to transients of the amplitude
produced by lightning. However, it subjects the equipment to transients
of amplitude consistent with the capabilities of existing
surge-protective devices.

These amplitudes, the BIL\'s while assigned somewhat arbitrarily, were
(and are) kept in touch with reality by the fact that equipment designed
in accordance with standards does not fail when exposed to surges
produced by lightning, in contrast to equipment designed prior to the
development of the philosophy of insulation coordination and the
establishment of standard BIL\'s.

The test and design levels, the BIL\'s, are not necessarily fixed. As
better protective devices are developed, the levels may be lowered so
that reliable equipment can be built at lower cost.

Electronic and control equipment, on the other hand, is all too often
designed, built, and delivered before the existence of a transient
threat is recognized. If transients turn out to endanger the equipment,
there may be no adequate surge protective devices. There may, in fact,
not be any satisfactory answer to the problem posed by transients.

**THE TRANSIENT CONTROL LEVEL CONCEPT**

One way in which transient compatibility might be achieved in the
electronics industry is to establish a transient coordination system
similar in concept to the BIL system, but of a nature more adapted to
the requirements of electronic and control equipment.

In this paper, such a concept is called the Transient Control Level
(TCL) concept. (\* The TCL concept was first proposed by one of the
authors (F.A. Fisher) in regard to electronic equipment on the Space
Shuttle \[12\]. ) Specifically, it is hereby proposed:

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

b\) That there be defined for electronic (and other low-voltage)
equipment a series of TCL\'s similar in concept to the BIL\'s.

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

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

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

f\) That TCL\'s begin to be used in purchase specifications.

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

h\) That such TCL\'s begin to appear in regulatory specifications for
consumer apparatus in which the consumers cannot make the appropriate
tests or prepare appropriate specifications.

**Suggested TCL Voltage Wave Shape**

The wave shape suggested for the TCL concept (with the understanding
that discussion and presentation of alternatives is actively encouraged)
is shown on Figure 3. Shown are both proposed open-circuit voltage and
short-circuit current waveforms, since the question of the impedance of
the source from which voltage surges derive must ultimately be
considered. These shapes are different from the long-established 1.2 x
50 μs wave employed in the BIL rating system for electric power
apparatus because none of the recorded transients exhibited this type of
wave shape on 120-volt AC circuits. The type of transient most
frequently recorded appeared of an oscillatory nature, very strongly
damped, and in a frequency range between 100 and 500 kHz.

Independent work on the resonant frequency of power systems previously
indicated a range of 150 to 500 kHz as being the natural frequency of
typical residential systems ^7^. Other investigations indicate that a
lower limit of 5 kHz might be more typical ^8^.

Thus, it appears that the observed transients are not at all typical of
lightning surges propagated directly into the system but are rather the
response of the power system to an initial excitation caused by a nearby
lightning stroke. The internally generated transients due to switching
operations typically are of the same basic type as those produced by the
indirect effects of lightning. The observed transients are in each case
more nearly the result of the natural oscillatory response of the local
wiring system, in this case the wiring system of typical residences.
Similar surge wave shapes have been encountered in a wide variety of
other systems, ranging from airplanes to space booster rockets. Typical
examples of recorded transient wave shapes are given in the Appendix.
The great bulk of the recorded transients exhibit a faster front time
and shorter decay time than do the transients produced by lightning on
high-voltage power lines, the 1.2x50 μs type of wave.

Switching transients in air break contacts (internally generated
transients) can produce rise times in the order of 10 to 100 ns.
Although this steepness attenuates rapidly with distance, the typical
front time is still less than 1.2 μs. For some types of devices
(rectifier diodes) the wave shape is of secondary importance, with only
the peak magnitude being important. For other types of apparatus
(inductive devices such as motors), the front time, or more correctly
the rate of change, is of importance equal to that of the peak
magnitude. In still other types of devices (surge protective devices),
the total energy content of the surge is of most importance.

**Current Wave Shapes and Source Impedances**

The characteristics of short-circuit current wave shapes are less well
known than those of open-circuit voltage. The short-circuit current is
of importance both for evaluation of surge protective devices and for
equipment of low input impedance such as lower voltage semiconductor
devices. In any discussion of test wave shapes and test levels, it is
important to recognize the natural response of the device in the test.
It is inappropriate to prepare a specification that implies that a
specified voltage must be developed across a device of low input
impedance, such as a spark gap after it has broken down, or to seemingly
require that a specified short-circuit current be produced through a
high input impedance, such as the line-to-ground insulation of a relay
coil. The characteristics of short-circuit currents are poorly defined
because the impedance of the circuits from which transients are produced
is poorly defined or unknown.

For purposes of discussion, it is suggested that two different types of
impedance be considered, one independent of frequency (resistive source
impedance or classical surge impedance, Z = (L/C)^1/2^, and one of
simple inductive source impedance. The waveform shown on Figure 3b
assumes a source impedance of 10 μH.

Again, for purposes of discussion, it is proposed that a resistive
source have an impedance of 50 ohms, and an inductive source have an
impedance of 10 μH.

**Voltage and Current Levels**

Central to the success of the BIL system of insulation coordination is
the fact that only a limited number of BIL\'s were established, arranged
in a generally geometric order of progression. For purposes of
discussion, we therefore propose that there be established a series of
TCL\'s progressing in the approximate ratio of 10^1/3^ or 3 values per
decade. Such possible TCL\'s, as rounded to convenient voltages, then
appear as shown on Table II.

The subject of source impedance and short-circuit current needs to be
further discussed since the concept of constant surge impedance, and
particularly constant inductive surge impedance, may not be valid.
Transients of high voltage and large energy content tend to be produced
by physically large systems, whose inductance tends to be larger than
that of the systems producing lower voltage or lower energy transients.

**Proof Test Techniques**

The generation of surge voltages in the laboratory is well known to
manufacturers and users of high power equipment. However, producing a
test wave of the shape and levels proposed here may present some
difficulty for the small equipment manufacturer. To answer this need, a
previously developed circuit, as shown in Figure 4, may be applicable.

The objective of this design was to superimpose on a 120-volt, 60-Hz
power line a transient having a rise time to first peak of 0. 5 μs,
followed by a damped ringing at 100 kHz, in which each successive peak
is 60% of the preceding peak amplitude. The amplitude of the first peak
is adjustable from 0 to 8000 volts. The source impedance for the
high-voltage transient is 50 ohms.

The 0.5 μs rise characteristic is obtained by the series resonance of L1
and the capacitance of C1 and C2 in series. Component values were
selected to make L/C approximately 50 ohms, and RI was selected to
provide heavy damping for a smooth transition to the following wave.

The 100 kHz damped ring results from the parallel resonance of L2 with
the parallel capacitance of C1 plus C2. Again, (L/C)^1/2^ is about 50
ohms. The series damping resistor R2 was selected to produce the decay
to 60% amplitude between successive peaks.

**CONCLUSIONS**

1\. The present lack of transient coordination methods in low-voltage
systems does not allow the user of electronic equipment to obtain the
best reliability at lowest cost.

2\. Manufacturers, vendors, and users could benefit from a systematic
approach to transient coordination similar in concept to the BIL used
for many years in high-voltage systems. This is illustrated in Figure 5.

3\. A concept of Transient Control Level (TCL) is proposed by the
authors. This involves discrete steps of withstand level and proof tests
based on the capability of available s urge protective devices and
reflecting the occurrence of surges in the real world.

4\. Discussion is earnestly invited on the parameters to be considered
in defining TCL\'s such as:

· - voltage waveform of the transients

· - source impedance of the transients

· - current waveform of the transients

· - levels to be assigned - current and voltage

· - proof-test techniques.

Successful application of the TCL concept will require careful study of
these factors, so as to develop a valid consensus among all interested
parties.

**REFERENCES**

\(1\) F. D. Martzloff and G. J. Hahn, \"Surge Voltages in Residential
and Industrial Power Circuit, \"IEEE Trans. Power Apparatus Systems
PAS-89, No. 6, 1049-1056 (July-Aug. 1970).

\(2\) P. M. Speranza (Bell Laboratories); Oral Communication to IEEE-SPD
WG #3. 4. 4 (Oct. 1974).

\(3\) J. E. Lenz, \"Basic Impulse Insulation Levels in Mercury Lamp
Ballast for Outdoor Appreciation, Illum. Eng. 133-140 (Feb. 1964).

\(4\) A. Martin and A. W. Smoot, \"Fact Finding Report on Ground Fault
Circuit Interrupters,\" Underwriters Laboratories File E45269 (March
1972).

\(5\) D. W. Nestor (Rucker Electronics), private communication (1972).

\(6\) IEEE Committee Report, \"Bibliography on Surge Voltages in AC
Power Circuits Rated 600 Volts or Less,\" IEEE; Trans. Power Apparatus
Systems PAS-89, No. 6, 1056-1061 (July-Aug. 1970).

\(7\) H. A. Gauper Jr., General Electric Co.; oral communication to
NEMA, Ground Fault Protection Section (Aug. 1973).

\(8\) P. M. Speranza and L. H. Sessler (Bell Laboratories); oral
communication to NEMA, Ground Fault Protection Section (Aug. 1973).

(9) J. A. Plumer, F. A. Fisher, and L. C. Walko, \"Lightning Effects on
    the NASA F-8 Digital-Fly-by-Wire Airplane,\" Contract NAS 4-2090,
    NASA Scientific and Technical Information Facility, College Park,
    Md. 20740.

\(10\) J. A. Plumer, \"Lightning Effects on General Aviation Aircraft,\"
Report No. FAA-RD-73-99, National Technical Information Service,
Springfield, VA. 22151.

\(11\) E. K. Howell and F.D. Martzloff, \"High Voltage Impulse
Testers,\" Report 75CRD039, General Electric Company, Corporate Research
and Development Center, Schenectady, N. Y.

\(12\) \"Space Shuttle Lightning Protection Criteria Document,\" NASA,
Lyndon B. Johnson Space Center, JSC-07636, September 1973.

**DISCUSSION**

**S.M. Harvey** (Ontario Hydro Research Division, Toronto, Canada): This
paper provides a clear presentation of the case for a transient
interference immunity standard applicable to residential and,
presumably, light commercial electronic equipment. Designing transient
or surge withstand compatibility into low-voltage equipment is not, of
course, a new concept. The telephone companies have been doing it for
years. However, the authors have commendably proposed their Transient
Control Level concept in the context of a general and down to earth
philosophy of testing that should encourage informed discussion.

Following the establishment of Basic Insulation Levels, the electric
power industry has not been idle in the area of overvoltage testing of
low-voltage equipment. A number of committees, including the Power
System Relaying Committee of the IEEE Power Engineering Society and
Technical Committee No 41 of the International Electrotechnical
Commission have been working for years on the surge testing of static
relays used for transmission line protection.

The Swedish Electrical Commission has prepared a draft proposal for
interference withstand capability testing of apparatus used in power
stations and industrial installations. These committees have proposed a
range of test waveforms including the familiar 1.2/50 impulse at peak
voltages of 1, 3, and 5 kV, a moderately damped I MHz oscillatory wave
at peak voltages of 0.5, 1, and 2.5-3.0 kV, and a high-frequency spark
test at 2 - 4 and 4 - 8 kV.

In 1974, Ontario Hydro introduced a uniform transient immunity test
specification for relays and other equipment intended for substation
relay or control buildings. The test waveform is a moderately damped
oscillatory transient whose frequency can be specified in the range of
100 kHz to 2 MHz. One of four test levels, specified in Table 1, can be
called for. The test is supervised by our Supply Division, and
manufacturers are encouraged to supply their own test equipment.
However, it is still frequently necessary for Ontario Hydro to make its
own test generators available.

**Table I \-- Transient Test Levels**

Note that these levels when specified at 100 kHz are very similar to
tests 6 and 9 in Table II of the present paper. Level B, incidentally,
when specified at 1 MHz is equivalent to the IEEE Relay Test \[1\].

Our experience with the tests, although limited, suggests that minor
circuit deficiencies leading to operational upsets are common but that
damage is relatively rare. Probably the marginally greatest value of the
tests at this time lies in their potential for creating an awareness of
the transient problem.

A number of questions being considered at this stage of our transient
test program can be rephrased to apply also to the proposals in this
paper. Perhaps the authors could comment on the following:

1\. What is the advisability of introducing a new test waveform or test
procedure in addition to those already in circulation?

2\. Would it be necessary to shield the test circuit of Fig. 4 or to
locate it, say, 4-6 meters from the equipment under test? In the latter
case, should the voltage and current waveforms be measured at the near
end or the far end of the connecting cable?

3\. Can the test circuit of Fig. 4 correctly simulate transient
disturbances that occur when the white wire neutral and the green wire
ground are connected together a quarter wavelength from the device under
test?

4\. Can a reliable certification procedure, particularly in terms of
energy deliverable to a load, be established for test generators
differing in design from the one shown?

5\. Finally, what is the incidence of damage or significant upset to
equipment now used in resident at or light commercial environments and
does it justify the introduction of transient testing to this class o
apparatus? If applied, in view of the data contained in Fig. 2 of the
paper, what criterion would be used to select a test level of less than,
say 500 volts?

REFERENCES

\[11 ANSIC37.90a-1974(EEE Std472-1974) Guide for Surge Withstand
Capability (SWC) Tests.

**E.J. Cohen** (U.S. Dept. of Agriculture, Washington, D.C.): We feel
the concept expressed in this paper is long overdue in the field of
electrical protection of electronic equipment. Experience within the
telephone industry has already demonstrated that, with present trends to
ever-smaller equipment, protection problems can be severely aggravated.
The over voltage and current tolerance of microelectric circuits has
decreased to the point where protection should be major consideration in
circuit design.

Added to this increased equipment vulnerability, we have found a
"communications gap\" between the manufacturers of electronic equipment,
and the producers of protection devices. When a protection defect is
uncovered, we frequently encounter disagreements between the equipment
and arrester manufacturers. By establishing \"Transient Control
Levels,\" as proposed by this paper, much of this \"finger pointing\"
could be eliminated. As both equipment and arrester manufacturers should
know precisely what the other adequate protection should be minimized.

It is felt that while the concept expressed here is valid, further
consideration should be given to the levels and waveshapes involved in
the tests. As these parameters may be critical to the workability of
this proposal, every effort should be made to generate realistic values.

**Richard F. Hess** (Sperry Flight Systems, Phoenix, Arizona): I agree
that some form of action is needed to properly assess and overcome the
adverse effects of power transients on military and commercial
equipment. Assuming a consensus is reached concerning the need for
transient control and the adoption of Transient Control Levels (TCL),
the following comments are intended to complement the proposal for
transient control in low voltage systems.

The voltage specification is based upon measurements which are
appropriate to present and past equipment designs. For the most part
these designs use devices which present a relatively high impedance to a
source of transient energy.

Damage occurs during a power transient when the device breaks down and
high to medium voltages are developed across the device while large to
medium currents are flowing through it. Standard components are not
normally tested under transient conditions, therefore it may be
difficult to determine whether they would break down or to assign a
confidence level that they would survive such a transient. When a device
breaks down, either a voltage or a current viewpoint could be assumed
when describing the threat of the power transient to the device.

If in order to conform to a specified TCL a device has been designed to
withstand a specified voltage level, then the voltage specification is
appropriate. However, a manufacturer designing equipment to meet a
specific TCL could adopt an approach which calls for the use of
transient power suppression devices (tranzorbs, metal oxide varistors,
etc). In this case, transient power surges are manifested as large
current surges into equipment (through the protection device) rather
than a large voltage transient across the equipment. Even when passing
large currents, the network impedances (suppression devices, etc.) will
probably be significant enough to produce a natural mode current
response within the total network. Thus, current measurement of such a
network would contain a significant oscillatory component similar to
that present in the voltage measurements. Two types of TCL
specifications should be provided:

1\. Voltage

2\. Current

Like the voltage specification, the waveform and magnitude of the
current specification at each TCL would be based upon the measurement of
the current response modes of networks containing power suppression
devices and excited by a power transient.

With the two types of specifications, equipment could be designed and
tested to withstand a power transient by safely withstanding specified
voltage levels or by safely passing specified currents levels. The test
equipment for, the voltage specification would be calibrated under open
circuit conditions and would be designed to deliver current (in the
event of device breakdown) at a level at least as large as that
specified in the current specification. The test equipment for the
current specification would be calibrated under short circuit conditions
and would be designed to provide voltage (in the event of a high
impedance) at a level at least as large as that specified in the voltage
specification.

Tests for semiconductor vulnerability (damage) levels using square pulse
waveform are common practice with the military. The damage level of many
discrete components has been determined an recorded. However, the damped
sinusoid pulse is more appropriate to susceptibility testing (transient
upset). Depending upon the type of equipment being tested and the
frequency content of expected transients, it may be desirable to test
using more than one wave-form. lower frequency, high amplitude sinusoid
(100 KHz) would be used to vulnerability testing and a higher frequency
sinusoid (500 KHz, I MHz or 10 MHz depending upon the bandwidth of the
equipment) would be used for susceptibility testing. At each frequency
the equipment should be subjected to at least two pulses:

1\. Maximum pulse is positive

2\. Maximum pulse is negative

As a final observation, testing and test equipment should be kept a
simple as possible to avoid adding inordinate costs to the equipment
Ideally, the degree of confidence obtained by such testing should result
in a net reduction in equipment costs (manufacture plus maintenance).

**F.A. Fisher and F. D. Martzloff:** We appreciate the response of the
discussors and will attempt to both respond to their questions and
expand somewhat on the protection philosophy we propose. First of all, i
should be pointed out that while this paper was written using house hold
appliances as an example and presented before a group largely concerned
with utility relaying, the problems of transients pervade the en tire
field of low voltage electrical and electronic apparatus, including the
communication (telephone) industry. One of the areas where the authors
have seen a great need for better transient compatibility is in the
Aerospace field. Much of the background upon which the TCL concept is
based comes from consideration of the transients induced in aerospace
vehicles by lightning and other energetic discharges. Designers in the
Aerospace community tend not to have had the problem of transients
brought as forcibly to

their attention as have the designers of relay devices intended to work
in the harsh electrical environment of a utility substation. With
reference to Mr. Harvey\'s first question, we feel that it is advisable
to introduce new test procedures because the specialized test procedures
adapted in the electric utility field may no meet the needs of users in
other fields.

Each of the discussors mentions the subject of levels and wave shapes.
We suggested the voltage waveshape of Figure 3 of the paper because
measurements have indicated that most transients to which electronic
equipment is exposed are oscillatory in nature and generally of faster
front and tail times than the 1.2 X 50 microsecond test wave common in
the electric power industry. Several other factors influence our choice.
One was that the proposed wave is of long enough duration that breakdown
of semiconductor junctions would not be greatly influenced by deviations
from the specified waveshape. With much shorter waveshapes, the
resistance of semiconductor junctions to bum out becomes strongly
influenced by waveshape. Another is that transients of this nature can
be injected into wires by rather simple trans former-coupled
pulse-injection generators, whereas transformer injection of higher
frequency oscillatory voltages and currents is more difficult.
Transformer injection of transients has not been discussed in this paper
but is sometimes an appropriate means of evaluating the resistance of a
device to circuit upset. Mr. Hess mentions the need for two types of TCL
specifications: voltage and current. We agree. We have seen instances of
groups worrying wastefully about specifications that call for a specific
voltage transient to be developed at the terminals of a device when that
device had properly been fitted with a low-pass filter, a low impedance
suppressor, or transient suppression spark gap. Specifications that do
not recognize that one can neither develop a voltage across a short
circuit nor circulate a current through an open circuit are not only
incomplete but mischievous and counterproductive.

With reference to more of Mr. Harveys questions, we feel that any test
circuit should be built in a sufficiently well-shielded cabinet so that
there is no need to physically separate the test circuit from any device
under test. If a test circuit must be located away from the device under
test and an interconnecting cable be used, we would think that the
generator open-circuit voltage and short-circuit current should be
measured at end of the cable nearest the device under test.

We do not really know what would be the interaction between a white wire
neutral and a green wire ground if the two were connected together a
quarter wavelength away from the generator. We take refuge in the
observation that transient coordination is more likely to be achieved
through the successful passing of even an imperfect test than it is in
the avoidance of all but perfect tests.

We hold no special faith in the virtues of the test circuit shown on
Figure 4 of the paper and show it only as one example of various test
circuits that might be produced. We feel that a reliable certification
procedure not only can be, but must be, based on specifications that are
not unique to any one test circuit. It is for this reason that we
propose specifications be written in terms of open-circuit voltage and
short circuit currents; a concept that implies a fixed generator
impedance. Care must be taken that the voltage and current
specifications not be incompatible with the generator impedance. Since
the writing of this paper another paper discussing the impedance of AC
wiring circuits has been published \[1\]. Based on this paper, we would
now propose that the internal impedance of a transient generator be 50
ohms paralleled by 50 microhenries.

Figure 1, reproduced from the referenced paper with the permission of
the author, shows how the impedance of the line (\"the mains\") can be
closely approximated by the parallel combination of 50 ohms and 50
microhenries. Levels and waveshapes appropriate to such an impedance
might then appear as shown in Figure 2 and Table 1.

As Messrs. Cohen, Harvey and Hess emphasize, the choice of appropriate
levels is crucial to the successful implementation of a TCL philosophy.
While a TCL of 5000 or 6000 volts might be appropriate to high
reliability utility relays or a safety-oriented consumer product such as
the Ground Fault Circuit Interrupter, it might impose an unnecessary
economic hardship on a high volume item intended for routine household
use. Likewise, while a TCL of 500 volts might be too low for residential
purposes, it might be appropriate for the power inputs of electronic
equipment used in aircraft, and excessively high for the signal inputs
of data processing equipment intercommunicating through well-shielded
signal wires.

Since of the major purposes of this paper is to promote discussion, it
is appropriate to list some of the questions the authors have posed to
themselves during the formulation of this proposal:

\- Are there sufficient problems relating to transient coordination to
warrant an effort, likely to be major and long term, to achieve better
coordination between the transients to which equipment is exposed, and
the ability of equipment to withstand such transients?

\- Would transient control level (or some other) specifications and
standards help achieve successful transient coordination between
equipment manufacturers, utilities and equipment users?

\- Should there be a limited number of fixed levels? The authors feel
that it is essential that the number of levels be limited, perhaps to
9-15 levels distributed in a geometric progression over the range
10-5000 volts. The assignment of the levels may have to be done
arbitrarily. This need not be cause for alarm. The electronic industry
for years has worked successfully with resistor and capacitor values
produced according to an arbitrarily selected geometric progression.

\- Should these levels reflect the system voltage, the expected
reliability of the equipment function, the environment?

\- What kind of source impedance is appropriate? As mentioned above, an
impedance of 50 ohms paralleled by 50 microhenries may be appropriate.

\- Should open-circuit voltage and impedance be stated or,
alternatively, should open-circuit voltage and short-circuit current be
specified?

\- Is one impedance value suitable for the majority of the systems?

\- What waveshape is appropriate, for voltage as well as current? For
damage, we are mostly concerned with energy and front-of-wave but if
upset (interference) is to be included in TCL, then do we need to
specify a frequency spectrum?

Reference

\[1\] \"Impedance of the Supply Mains at Radio Frequencies,\" J. H.
Bull, Proceeding of 1^st^ Symposium on EMC, Montreux, May 1975.

TRANSIENT CONTROL LEVELS

## A BETTER WAY TO VOLTAGE RATINGS

######### IN POWER CONVERTER APPLICATIONS

+------------------------------------+---------------------------------+
| K.E. Crouch and F.A Fisher         | François D. Martzloff           |
|                                    |                                 |
| General Electric Company           | General Electric Company        |
|                                    |                                 |
| Pittsfield, MA                     | Schenectady, NY                 |
+------------------------------------+---------------------------------+

### **Abstract**

Failure and circuit upset of electronic equipment caused by transients
has now been recognized as a significant obstacle to the development of
new systems. While designers have recognized the problem and are taking
steps to deal with it in their own systems there is a lack of overall
coordination beyond the confines of a given black box. The total
environment must be considered in contrast to the present approach where
electronic power and control equipment is all too often designed, built,
and delivered before the transient threat is dealt with.

The authors transient control level (TCL) philosophy provides an
approach toward coordination of the intrinsic capability of equipment to
withstand overvoltages, the capability of existing or future protective
devices for limiting overvoltages, and the known or assumed
characteristics of surge voltages in power systems. Examples of this
coordination, for general applications as well as for a specific power
converter circuit, are given at the end of the paper. The paper also
discusses the evolution of the TCL philosophy since its initial
disclosure.

### **INTRODUCTION**

Present standards do not offer sufficient guidance to designers and
manufacturers of electronic equipment regarding what types of transients
to consider and how to prove that equipment works in the presence of
transients. This situation is perhaps under better control in the
electric power field than it is in the fields of aerospace, general
industry, housewares, and the military. For instance, the Working Group
on Surge Voltages in AC Power Circuits Rated 600 Volts and Less, of the
IEEE Surge Protective Devices Committee, has been collecting data on the
occurrence of surges for several years. This group is now in the process
of preparing a guideline document describing the transient environment
\[1\]. The IEEE Power System Relaying Committee also has collected
information on the surges found in power substations and has developed
the Surge Withstand Capability (SWC) test \[2\]. Transient insulation
coordination is well developed with regard to high voltage apparatus.
Insulation structures are designed to meet specific industry standard
transient levels, and the equipment is subjected to proof tests. The
design levels are coordinated with the capabilities of existing surge
protective devices. Electronic and control equipment, on the other hand,
is all too often designed. built, and delivered before the existence of
a transient threat is recognized. If transients turn out to endanger the
equipment, there may be no adequate surge protective devices. In fact,
there may, not be a satisfactory answer to the problem posed by
transients when retrofit situations are involved.

In an attempt to improve the situation, where existing standards do not
offer sufficient guidance, the authors have proposed the concept of
transient control levels \[33, patterned after the basic insulation
level (BIL) approach which has been utilized very successfully in the
electric utility field to coordinate the withstand capability of
equipment, the surge-limiting ability of suppressors (surge arresters),
and the overvoltage/overcurrents encountered in power systems.

Therefore, it will be useful to briefly review the concept of insulation
coordination, and the procedures developed to verify satisfactory
performance, in the power/utility industry.

# THE CONCEPT OF INSULATION COORDINATION

As a result of the mature philosophy of insulation coordination, power
equipment achieves its resistance to lightning-induced transients not so
much from being designed to the threat posed by lightning, but by the
threat posed by acceptance test. This acceptance test does not subject
the equipment to transients having the complex waveshapes produced by
lightning. Instead, the equipment is subjected to transients which have
elementary waveshapes that can be produced by basically simple test
apparatus. Furthermore, the acceptance test does not subject the
equipment to transients of the amplitude produced by lightning. However,
it does subject the equipment to transients having an amplitude
consistent with the capabilities of existing surge-protective devices.
If it is intended that suppression devices be used to control the
magnitude of the voltage transient, one should be sure there are
suitable devices before the level is selected.

The basic concept, which needs to be mutually accepted by both
manufacturers and users of equipment, is that it is impossible to
simulate all of the possible transient overvoltages a given product line
might experience. However, by designing the equipment to a certain
standard and controlling the occurrence of overvoltages by suitable
protection, a much greater chance of successful operation in the real
world is obtained.

The challenge is to select a standard for the withstand capability of
the equipment that will reflect the real world and yet be simple enough
to be practical. The other difficulty is to trade off the specification
of ultra-conservative standards that insure a very high reliability for
the equipment, but impose on the manufacturer (and therefore on the user
who pays for the equipment) unnecessary costs,.

# THE TRANSIENT CONTROL LEVEL CONCEPT

Three basic parameters that relate to the amplitude of the surge, the
waveshape of the surge, and the energy content of the surge must be
considered. The amplitude and energy content depend, to some degree, on
the nominal operating voltage of the system on which the surge may
appear. Higher amplitude and higher energy surges are found on higher
voltage systems. The waveshape and amplitude of the surge depend, to
some extent, on the origin of the surge and how far away the point of
observation is from the origin. Surges produced by arcing contacts tend
to involve faster rates of change than surges produced by lightning. The
latter, in turn, tend to have slower rates of change than surges
produced by arcless switching of highly inductive circuits. Surges
originating remote from an observation point tend to have slower rates
of change than surges originating close to the point of observation.
There [is no \"typical\" surge]{.underline}.

On the premise that even an imperfect approach to transient coordination
is better than none, two of the authors of this paper recently proposed
the transient control level philosophy. In the paper \[3\] in which the
philosophy was presented, a request was made for feedback about some of
the details of the philosophy. Some of the feedback indicates that
modifications to the test waveshapes and levels originally presented
might be in order. The essential elements of the philosophy remain the
same and are reiterated below.

The amplitude and energy content of the surge are defined in terms of
the voltage produced by the surge across an open circuit and the current
produced by the surge through a short circuit. A specified open circuit
voltage and short circuit implies a definite impedance of the source
from which the surge originates. This impedance is usually reactive.
Observations by Bull \[4\] and others suggest that a reasonable
impedance for 115-VAC circuits is 50 Ω paralleled by 50 μH (Figure 1).
In the absence of other information, the authors suggest that a TCL
philosophy be built around this impedance. This combination of elements
has a breakpoint (ωL = R) at 159 kHz, which may be difficult to
implement in a simple surge generator. The important points about the
source impedance are that the impedance should be in the order of 50 Ω
resistive at high frequencies and 50 μH inductive at low frequencies.

Consideration of waveshapes is of great importance. Two types of
waveshapes enjoy some prominence. The firs is the 1.2 x 50 μs
unidirectional voltage which is the basis of the BIL system of
insulation coordination and whose historical origin comes from studies
of the effect of lightning on high voltage power lines. The second is
the highly oscillatory (1.5 MHz) SWC voltage waveform. The latter
waveshape has its origins in the transients produced by switching
operations in high voltage substations. Experience has proven that
neither of these types of waveforms is a good representation of the
types of transients found in 115-VAC circuits and other circuits of
similar physical construction. The typical surge is oscillatory, but of
a frequency lower than that specified for the SWC test.

In the previous paper \[3\], the authors suggested a voltage test wave
of the characteristics shown in Figure 2. That particular test wave had
its origins in the Space Shuttle program and was originally viewed as a
short duration surge predominantly unidirectional. It was not intended
that the surge be oscillatory; but some degree of oscillation was
considered allowable. If treated as a damped oscillatory surge, Figure 2
implies an oscillatory frequency of about 50 kHz. After considering
other studies and feedback from readers of the original paper \[3\], it
seems more appropriate to emphasize the oscillatory nature of the surge,
rather than to downplay it, and to specify the surge in terms of its
oscillatory frequency rather than in terms of the duration of the first
quarter cycle. A specification that may more nearly meet the
requirements of the industry is shown in Figure 3.

The important characteristics include an initial rate of change
sufficient to stimulate the dv/dt effects in semiconductors and
inductive components with an oscillatory decay, a decrement typical of
that observed in low voltage circuits, and a duration comparable with
clock cycles in digital equipment . The duration of the original and
revised waveforms have also been chosen after consideration of the
failure modes of semiconductors. Failure characteristics tend to be
different if the semiconductors are exposed to much shorter duration.
Finally, the waveforms can be produced with simple test equipment. An
example of a test circuit is shown in Figure 4. An example of the open
circuit voltage and short circuit current produced by this circuit is
shown in Figure 5.

It should be understood that the TCL philosophy is not written around
any particular test circuit. Any circuit that produces open circuit
voltage and short circuit current of the characteristics shown in Figure
3 and has an internal impedance similar to that of Figure 1 would be
equally satisfactory. The calculated impedance characteristics of the
circuit shown in Figure 4 are shown in figure 6. If losses had been
considered in the calculations, the height of the resonance peak would
have been reduced to 100 or 200 Ω.

For a standard to be practical, there must be a limited number of test
levels reflecting system voltages and application requirements. The
authors propose that the levels shown in Table I be defined. The table
is based on a geometrical progression and deliberately includes, but is
not limited to, levels having the magnitudes 600, 3000 and 6000 V
specified in other standards \[2,5, and 7\]. Intermediate levels, if
needed, should also be based on a geometric progression.

A concept requiring emphasis is that the defini­tion of a series of
levels from which a choice of a level may be made is a task separate and
distinct from the task of choosing which level is appropriate for any
particular application. This latter choice must reflect the nature of
the application and the consequences of a failure, as well as the
exposure of the power system. For instance, a low cost domestic
appliance, produced under strong competitive pressure, cannot be
expected to offer the same degree of transient withstand capability as a
critical industrial system.

Table I

> POSSIBLE TRANSIENT CONTROL LEVELS
>
> Open Circuit Short Circuit\
> Level Voltage Level Current Level\
> Number. (volts) (amperes)\
> 1 60 2.2\
> 2 150 5.4\
> 3 300 11\
> 4 600 22\
> 5 1500 54\
> 6 3000 110\
> 7 6000 220

It is unlikely that a single surge waveshape, such as that of Figure 3,
can meet all of the needs of a mature TCL system any more than the 1.2 x
50 μs waveshape meets all of the needs of the BIL system. For many
applications other waveshapes may be needed. The authors suggest that
the 1.2 x 50 μs unidirectional and the SWC waveshapes be considered as
part of the TCL family of waveshapes. The family would then consist of:

> 1\. A relatively slow, low amplitude overvoltage, representing
> switching transients and some fault clearing transients. This may be
> the unidirectional 1 .2 x 50 μs waveform from the BIL system.
>
> 2\. A composite waveshape, including a fast rise time and oscillatory
> decaying tail. This will test some dv/dt effects as well as the energy
> handling capability of suppressors. This would be the waveshape shown
> in Figure 3.
>
> 3\. A very fast burst-type oscillation, such as the current SWC test.
> This will test devices against dielectric failures as well as
> electro­magnetic interference.

Depending upon the application, the degree of reli­ability required, and
the available knowledge of the environment, one or more of these three
tests could be specified for a particular system.

### **EXAMPLES OF TCL SELECTION**

Two examples of TCL selection are presented: one leads to the selection
of one of the proposed levels of Table 1 and the other does not. The
first example deals with ground fault circuit interrupter (GFCI)
devices. A combination of field experience, qualification testing \[6\],
and a priori requirements has indicated that these devices should not
functionally fail at 6 kV and should not experience nuisance tripping
when exposed to 3 kV transients \[7\].

The type of transients to which these devices are most apt to be exposed
are shown in Figure 5. A TCL of 6 kV (Level 7) would be appropriate for
qualification testing of devices unprotected by circuit protection
devices. However, protection devices that can clamp transients to 3 kV
or less are available. A production test might be run at the 3 kV level
on interrupters fitted with protective devices. The purpose of the test
would be to demonstrate that the protective devices could withstand the
current surge associated with a TCL of 6, rather than demonstrate that
the GFCI could withstand 3 kV at its terminals.

An example of a specialized system in which the proposed subdivision of
levels shown in Table 1 may be too coarse in the static power converter
shown in Figure 7. In a static power converter, the voltage transients
are best separated into three frequency ranges or transient durations.
High frequency transients (tens of microseconds duration) are basically
lightning-related transients and some load switching transients. Medium
frequency transients (hundreds of mi­croseconds duration) include power
system switching surge transients as well as SCR commutating transients.
Low frequency transients (milliseconds duration) are confined to 60-Hz
overvoltages and leg fuse clearing voltages.

The high frequency transients enter the system by capacitive coupling
through the transformer and have relatively high source impedance (50 to
100 Ω). The waveshape of Figure 3 is very appropriate for simulation of
these effects. The medium and low frequency transients are inductively
coupled or generated in the secondary winding and have fairly low source
impedances (0.1 to 0.001 Ω). Such waves primarily stress insulation and
semiconductors. The 1.2 x 50 μs waveshape would be appropriate for
simulation of these effects.

Basically, the TCL selected for the power converter will be an economic
compromise between the cost of high transient withstand capability in
the equipment and the cost of suppressor devices. Two limiting
possibilities exist: 1) design the equipment (basically SCR reverse
voltage ratings) to withstand the highest transient expected or; 2)
design the equipment to withstand only the peak reverse operating
voltage and clamp all transients to that level.

Nominal peak reverse voltage for the bridge is 1000 V. Commutating
voltage spikes would normally reach 1300 V. If these spikes were
suppressed to 1200 V with a nonlinear varistor material, the varistors
would be subjected to transient currents of 10 to 20A, and there would
be about 50 W of continuous power released in the varistors. Single
pulse transients, such as these generated when a leg fuse is called upon
to interrupt a short circuit, can reach 1600 to 1800 V. (Figure 8).
Suppressing these transients to a 1200 V level would require that the
suppressors handle 10 to 20 kA. High frequency transients, such as those
produced by lightning or arcing contacts on switches, can reach 5000
These transients can be easily suppressed to a 2000 V level with
suppressors capable of handling about 100 A.

The proposed standard TCL\'s of 1500, 3000, and 6000 (based on a 3
\--/\--10 progression) are not well adapted for this application. If
SCR\'s of 1300 PIV were used, the transients associated with a 1500 V
TCL would exceed the PIV and transients could only, with difficulty, be
suppressed to 1500 V in the first place. Transients could easily be
suppressed to a 3000 V level, but the cost of SCR\'s with a PIV of 3000
V would be very high. Silicon controlled rectifiers with a PIV of 1800 V
are another possibility. These could be tied into a TCL progression
based on 6\--/\--10. Appropriate levels, rounded to convenient values,
would be 600, 1000, 1500, 2000, 3000, and 4500 V. An economically
appropriate TCL for the power converter might then be 2000 V.

### **ACKNOWLEDGMENTS**

Helpful comments and suggestions on the TCL proposal have been received
since the closing of the reference 3 paper. The feedback from Messrs.
H.A. Gauper of Corporate Research and Development, General Electric
Company, P. Richman of KeyTek Instrument Corp., A.W. Smoot of
Underwriter\'s Laboratories, and P.D. Speranza of Bell Telephone
Laboratories are gratefully acknowledged.

### **REFERENCES**

1\. IEEE, SPD Committee, W.G. #3.4.4. \"Guideline - Surges in AC Power
Systems Rated 600 V and Less\" -- Project 587.1. Inquiries and
contributions concerning this project are invited; contact F.D.
Martzloff, General Electric Co. 37-442, P.O. Box 43, Schenectady, NY.

2\. ANSI C37.90a-1974 (IEEE Std 472-1974). \"Guide for Surge Withstand
Capability.\"

3\. F.A. Fisher, F.D. Martzloff, \"Transient Control Levels - A Proposal
for Insulation Coordination in Low-Voltage Systems,\" IEEE Trans.
PAS-95, No. 1, Jan/Feb 1976, pp. 120-129.

4\. J.H. Bull, \"Impedance of the Supply Mains at Radio Frequencies.\"
First Symposium on EMC, Montreux, 1975. IEEE 75CH1012-4 Mont.

5\. MIL-STD-461A. \"Electromagnetic Interference Characteristics
Requirements for Equipment.\"

6\. D.W. Nestor (Rucker Electronics), private communication (1972).

7\. Underwriters Laboratories, Inc., \"Proposed Requirements for Surge
Tests of Ground Fault Circuit Interrupters.\" Melville, NY, July 1975.

TRANSIENT CONTROL LEVEL

PHILOSOPHY AND IMPLEMENTATION

I.  **The Reasoning Behind the Philosophy**

F.D. Martzloff and F.A. Fisher

General Electric Company, Corporate Research and Development,

Schenectady, New York, and Pittsfield, Massachusetts

## Abstract

This is the first of a pair of papers describing how better transient
protection might be achieved through the use of a Transient Con­trol
Level (TCL) philosophy. The authors have developed and are proposing
this TCL philosophy because damage to and upset of electronic and other
low-voltage equipment by transients seems to be a never-ending problem,
and one that is likely to get worse in the future as electronic controls
permeate even more of the products which affect our lives. A number of
proposals have been made - some already incorporated into stan­dards - on
various test wave shapes and specifi­cations. The authors propose an
approach inte­grating many of these proposals while focusing attention on
significant parameters.

# Introduction

An area where present standards do not seem to offer sufficient guidance
to designers and manufacturers of electronic equipment is in what types
of transients to consider and how to prove that equipment works in the
presence of tran­sients. This situation is perhaps under better control
in the electric power field than it is in the fields of aerospace,
general industry, house­wares, and the military. For instance, the
insu­lation of high-voltage apparatus is coordinated to the threats that
nature provides to that insulation through the philosophy of insulation
coordination as expressed in the Basic Insulation Level (BIL) system.
The BIL system provides for a standardized series of levels being
coordinated with the protective abilities of existing protec­tive
devices. On the other hand, electronic and control equipment is all too
often designed, built, and delivered before the existence of a transient
threat is recognized. If transients turn out to endanger the equipment,
there may be no adequate surge protective devices. In fact, there may
not be any satisfactory answer to the problem posed by transients.

The authors\' TCL philosophy is aimed at achieving better coordination
than now exists between the transients to which equipment is exposed and
the abilities of equipment to with­stand the transients. It is patterned
after the BIL approach to insulation coordination so suc­cessfully used
in the electric power field.

The purposes of this first paper are to explain the reasoning behind the
different ele­ments of the BIL system of insulation coordina­tion, and to
explain how similar reasoning has led to the formulation of the TCL
philosophy. Some observations on how to perform TCL tests are given in a
companion paper \[l\].

**[Proposal for TCL]{.underline}**

This proposal can be summarized by saying that we want to:

1.  Establish the concept that equipment shall be rated in terms of its
    ability to withstand a limited set of transient proof tests, rather
    than in terms of its ability to withstand unknown \"actual\"
    transients.

2.  Establish the concept that transient specifi­cations apply to power
    and signal lines. In the past, only power lines have been
    con­sidered.

3.  Establish a set of levels (limited in number) to which equipment is
    designed and tested.

4.  Establish a set of standard test waves (lim­ited in number) to which
    low-voltage equipment will be subjected.

5.  Establish standardized relationships between voltage and current
    (source impedance).

6.  Differentiate between the task of establish­ing the family of test
    levels and wave shapes, and the task of actually selecting a
    specific level. This means that:

> o We will propose to you a family of levels and wave shapes
>
> o You will select the specific level and shape, based on your
> reliability goals, your costs, and your experience.

This proposal is made with awareness that it may be one more of an
already confused array of standards. However, if accepted by a large
sec­tion of industry and users, it could become a unifying link and make
the applications more successful.

In the following paragraphs, we will at­tempt to present the background
justifying our proposal, for each of the points listed above.

***1. [Basis for rating equipment]{.underline}***

The concept that equipment be rated in terms of its ability to withstand
a standard test rather than \"actual\" service conditions is not new.
This is at the very heart of the system of BIL, which has been so
successful in the field of electric utility equipment.

Fortunately for the utilities, few parties were involved in making the
decisions, and thus it was possible at an early stage to establish the
BIL system and to enforce it because of the near total control of the
engineering department of a utility over the system design. In the field
of low-voltage systems, however, the selection and purchase of a
multiplicity of components and equipment by a multiplicity of buyers
from a multiplicity of vendors on behalf of a multi­plicity of users have
made it very difficult to maintain the organized systems approach which
succeeded in the case of the electric utilities.

A basic concept, which needs to be mutually accepted by users and
manufacturers of equipment, is that it is impossible to simulate all
possible transient overvoltages (and over-currents) that a product line
might experience in service. How­ever, by designing the equipment to a
certain standard and controlling the level of transients by
suitable-protection, a much greater chance of successful operation in
the cruel real world will be obtained.

The task is then to establish a set of standard tests, acceptable to the
vast majority of applications, reflecting the real world but not
pretending to duplicate it, simple enough to be practical, conservative
enough to ensure reli­ability, but realistic in terms of economics.

Obtaining complete agreement from all is a most unlikely an impossible
goal, and thus the unsatisfactory situation endures. This stalemate can
be broken by accepting a proposal, which might not be perfect but is
better than many isolated standards or no standard at all.

***2. [All lines subject to transient tests]{.underline}***

The existence of transients on power lines is by now a recognized and
accepted fact, so that most applications will involve a certain amount
of precautions in specifying transient withstand capability. However, in
the case of signal lines, this recognition is less frequent, and there
have been examples where a total lack of appreciation of the problem has
led to the design and deployment of equipment that cannot be pro­tected
from transients.

Transients can be introduced into a piece of equipment by the power
lines from many sources, such as lightning, switching transients, fault
clearing, and coupling from adjacent circuits. Signal lines, especially
in the case of extensive systems covering a vast area, can also be
subjected to induced transients by lightning, adjacent circuits, ground
currents, etc. Since quite often the signal circuits tend to be at a
lower voltage than the power circuits, the discrepancy between the rated
level in the circuit and the actual level of transients makes the signal
circuits more susceptible to transient problems.

A question related to which lines are to be subjected to transients is
that of "common mode" versus "transverse mode." This is not always clear
and must be addressed in a comprehensive specification.

***3. [Test Levels]{.underline}***

An important feature of the BIL system was that it involved a limited
number of test levels graded to the operating voltage of the system for
which apparatus was being designed. A successful TCL system should also
be designed around a relatively small number of levels. One who tries to
establish levels is pulled in two directions; one to avoid complexity by
establishing a minimum number of levels, and in another to provide
levels that accommodate existing practices with minimum disruption.

One way to achieve this is through the use of major and minor intervals
in the levels. Figure 1 shows several possible level series. The scales
show the range 30 to 3000 volts divided into intervals based on 10^1/3^,
10^1/5^, and 10^1/6^. The physical positioning of the numbers on the
figure shows how those numbers match the proportionate interval scales.
In the past, we have proposed that there be three levels per decade with
the spacing between levels being approximately 10^1/3^. The factors 1.5,
3, and 6 seem appropriate, particularly since such a set could include
the voltage levels 600, 1500, and 3000 volts in some existing
specifications. The widely used specification MIL-704 includes the
600-volt level for transients, and it would appear that this number, at
least, should appear in any set of TCL levels. Levels based on the above
progression appear in the left-hand column.

### **Fig. 1: Proposed levels for TCL voltages compared to existing level systems**

A progression proposed in IEC TC 28A, Low Voltage Insulation
Coordination, is shown in the right-hand column. The levels that have
been proposed range from 500 to 12000 volts. On Fig. 1, the levels in
parentheses are inserted only to indicate the sequence. This
progression, which seems to be based on the factor 10^1/5^, does not
include the 600-volt level.

Levels as arranged in the center column might appear to provide an
appropriate com­promise. We propose that the levels in boldface print be
the recommended levels while those in lighter print be used, preferably
sparingly, when intermediate levels are needed. Associated with each of
these levels would be a short-circuit current level, the magnitude of
which is related to the voltage levels through defined source
impedances. Source impedance will be discussed further below.

Some of the levels will seem very low, particularly to those accustomed
to dealing with transients on power lines. They may not be unrealistic
for some low-voltage signal circuits. A more important point, however,
is that the establishment of a series of levels, from which a choice may
be made, is a task separate and distinct from that of deciding to what
level a piece of equipment should be designed. This latter point is
discussed in more detail later.

***4. [Wave shape]{.underline}***

Many test waves have been proposed in the past. Table I shows some that
have been pro­posed. These wave shapes range from the very fast rise,
short duration, to the slow-rise, long duration, with oscillatory or
unidirectional voltages. Each of these is based on practical
considerations for specific applications; but the total picture is then
one of confusion and dis­couraging attempts at standardization.

Observations of oscilloscope recordings and independent work on the
resonant frequency of power systems \[2\] have shown that most transient
voltages in low-voltage systems have an oscilla­tory wave shape, in
contrast to the well-known and generally accepted unidirectional wave
used in high-voltage insulations standards. Frequencies are typically in
the range of 5 kHz to 500 kHz, with the majority of the transients
having frequencies above 100 kHz \[3\].

On the basis of these observations, the authors have proposed the
voltage wave shape of Fig. 2, as being most representative of tran­sients
in low-voltage systems. This wave is a composite. One component is aimed
at producing the effects associated with fast rise times. Coupled
interference and the response of inductive devices are examples. Another
component is aimed at producing the ef­fects associated with the more
slowly changing, and oscillatory, tail. Voltage summation in capacitive
circuits coupled by rectifiers is an example. Energy handling capability
of surge protective devices is another.

### **Fig. 2: Proposed TCL voltage wave**

While this wave may then appear arti­ficially contrived, it will subject
test samples to the two most significant effects of voltage, circuit
upset, and circuit damage. Since the wave may be produced by simple
laboratory cir­cuits, comparison tests may be easily done by different
organizations. \[4\].

This wave shape was first defined by a consensus at a meeting of the
Ground Fault Pro­tection Section of NEMA, in August 1973, and has since
received increasing acceptance, notably at the Underwriter\'s
Laboratories. Recently, independent considerations \[5\] have given
further support to a 0.5 μs rise time and 5 μs duration impulse.

However, in all probability this one oscillatory TCL wave will not meet
the needs of all users. Therefore, we propose that the wave of Fig. 2 be
supplemented by two unidirectional voltage waves: the classic ANSI 1.2 x
50 μs impulse wave and a 10 x 1000 μs wave \[6,7\].

We believe that most applications can be treated by one of these three
wave shapes, once the concept is accepted that a perfect match of
\"actual\" wave shape and \"test\" wave shape is not essential. The
first wave, fast rise and 100 kHz ring, would be more applicable for
circuits exposed to \"lightning remnants\" (the natural oscil­lation of a
power system excited by a lightning discharge or switching transient at
some remote point) as well as control circuitry exposed to induced
transients. The second wave shape, the familiar 1.2 x 50 unidirectional,
would be applicable to circuits where direct exposure to lightning
strokes is likely; while the third (long tail) would be applicable to
situations involving lightning current discharge on long cables. The
second and third wave shapes are also representative of transients
produced by the switching of inductive circuits.

Special applications, such as NEMP (Nuclear Electromagnetic Pulse)
hardening, or high-volt­age substation supervisory equipment, would
rather retain their own well-documented standards.

***5. [Source impedance and energy]{.underline}***

In some types of tests, the object is to determine what level of voltage
will cause fail­ure (permanent or temporary) of insulation.

### **Table I - Partial listing of existing or proposed test waves**

\[See pdf file\]

The nature of the transient following breakdown is not of much concern.
The typical test piece is of high impedance (except after breakdown),
and thus does not load the generator. People have tended to overlook the
source impedance of the gener­ator, even in applications where that
impedance is important.

However, with the development of voltage suppression devices, the source
impedance becomes an integral part of the suppression scheme. Some types
of devices (spark gaps) function by switching into a low impedance state
and reflect­ing the energy associated with the transient back from whence
it came. Other devices (varistors, selenium, and Zener type diodes)
clamp the voltage across their terminals while conducting the surge
current and thus dissipate the surge energy in the protective device.
The ability of the device to handle that energy becomes of importance.
In either case, the test generators must be capable of supplying an
appropriate amount of current, but should not supply too much current.

Test specifications should reflect the fact that, in some cases, voltage
is the appropriate measure of the transient, and in other cases current
is the appropriate measure. Above all, they must avoid wording that
leads the inex­perienced to struggle valiantly, with ever-larger surge
generators, to develop a specified [voltage]{.underline} across a
correctly functioning spark gap or varistor. This has occurred.

In the original formulation of the TCL concept, the authors proposed,
and still do propose, that the generator impedance associated with the
100 kHz oscillatory test wave be an impedance representative of that
measured on a-c supply mains. Such an impedance can be represented as 50
ohms in parallel with 50 micro­henrys \[8\].

The ANSI specifications dealing with the long-established 1.2 x 50 μs
unidirectional wave do not treat source impedance directly, but
recognize its existence by providing a separate current test wave for
surge arresters or other surge protective devices. In the TCL concept as
we now visualize it, this same approach would be followed: separate
voltage and current levels.

One of the applications where the 10 x 1000 μs unidirectional test wave
might be appro­priate would be those involving switching of inductive
circuits. The impedance associated with such transients can vary over
wide limits and may be quite low. We do not feel there is yet a
sufficient engineering consensus as to what a suitable standard source
impedance might be. Ac­cordingly, we made no recommendations for such
impedance, feeling that the evaluation of such impedance must be done on
an individual basis for the specific application at hand.

***[7. Selection of specific levels]{.underline}***

The task of selecting the transient control level appropriate for any
one piece of equipment, or any one application, is one of engineering
and cannot be fully dealt with in this paper. How­ever, some discussion
of the task is necessary to show how that task fits into the overall TCL
philosophy. The BIL system provides some guidance. A fundamental tenet
of the BIL system is that the insulation structure of apparatus is not
designed until after the required insulation level is agreed upon, and
that this insulation level is not chosen until one is sure that there
are voltage-limiting devices (surge arresters) that can control natural
transients to levels lower than those to which the factory proof test
will subject the apparatus under design.

On the other hand, low-voltage and elec­tronic equipment is all too often
designed with­out consideration of transients or whether protective
devices might even be available if needed. One guideline is then that
equipment should not be designed until an appropriate design level has
been chosen. This choice should be made after consideration of the
distribution of naturally occurring transients.

The occurrence of transients is a statis­tical process, both in voltage
levels and energy content. Low levels are common while high levels occur
rarely. Figure 3 shows the relationship between voltage level and
frequency of occurrence on 120-volt residential circuits, from
observations made in the United States \[9\]. While this type of
information cannot serve to predict the occurrences at individual
locations, it is of interest if one is concerned with the overall
statistics of transients. For instance, a manufacturer can select a
withstand level (or conversely, a failure level) by trading off the
tangible and intangible cost of failures for the cost of the added
protection required to achieve that level. From the graph of Fig. 3, we
can see that decreasing the withstand level from, say, 4 kV to 2 kV is
likely to increase the failure rate of a product by a factor of 10.

### **Fig. 3: Exposure of residential circuits to surges**

Selection of the most appropriate level for a specific application
should remain the preroga­tive of the parties directly interested. This
choice will be based on a number of factors such as the circuit rated
voltage, the exposure of the circuit to induced transients, the presence
or absence of a mandatory suppressor in the circuit, the risk analysis
(probability of failure, conse­quence of a failure, cost-trade off), etc.

T**able II -- Proposed impulse levels by IEC TC 28A**

*\[See pdf file\]*

An example of such a selection process is found in current proposals of
IEC 28A for low-­voltage insulation coordination. This proposal includes
a matrix of voltage levels depending on one hand on the system voltage
and on the other hand on a level category, which is left to the users to
choose but implies some recognition of exposure factors. This proposed
table is repro­duced here as Table II with the permission of the IEC TC
28A Chairman.

**[Conclusion and Recommendation]{.underline}**

Acceptance of the TCL concept by manufac­turers and users of equipment,
as well as standardizing and regulatory agencies, would be a great step
toward simplification of specifications and toward more reliable system
per­formance.

This paper has incorporated the feedback received after several
proposals made at IEEE meetings, and at this point represents the
position of the authors, supported and amended by the comments received.
Further feedback from the EMC community is earnestly invited and
welcome.

To summarize our proposal, we recommend consideration and eventual
acceptance of the following:

1.  Major voltage levels of 300, 600, and 1250 volts, with intermediate
    levels of 450, 850, and 2000 volts used if necessary; the levels to
    be scaled upwards or downwards by the appropriate powers of ten.

2.  A voltage wave shape of 0.5 μs rise x 100 kHz ring with current
    related to voltage by a source impedance of 50 Ω and 50 μH. This
    wave shape would be supplemented by 1.2 x 50 μs and 10 x 1000 μs
    unidirectional waves.

3.  All terminals, power and signal, are to be subjected to TCL tests.

4.  For any particular piece of equipment, an appropriate level would be
    chosen from the above series, by mutual agreement between supplier
    and user.

**[References]{.underline}**

1\. Fisher, F.A. and Martzloff, F.D., \"Transient Control Level
Philosophy and Implementation; Part II. Techniques and Equipment for
Making TCL Tests,\" Proceedings, 2nd EMC Symposium, June 1977.

2\. Speranza, P.D. and Sessler, L.H. (Bell Tele­phone Laboratories), Oral
Communication to NEMA, Ground Fault Protection Section, Aug. 1973.

3\. Martzloff, F.D. and Hahn, G.J., \"Surge Volt­ages in Residential and
Industrial Power Cir­cuits"

4\. Transient Voltage Suppression Manual, Gen­eral Electric Company,
Syracuse, N.Y., 1976.

5\. Lerstrup, K., \"Atmospheric Overvoltages on Low-voltage
Installations,\" Document 28A/WGl 02/76.

6\. IEEE Standard 465.1, \"Test Specifications for Gas Tube Surge
Protective Devices,\" 1977.

7\. Rural Electrification Administration (REA) Specification PE-60,
Washington, DC, 20250.

8\. Bull, J.H., \"Impedance of the Supply Mains at Radio Frequencies,\"
Proceeding lst EMC Symposium, 1975.

9\. Fisher, F.A. and Martzloff, F.D., \"Transient Control Levels, a
Proposal for Insulation Coordination in Low-voltage Systems,\" IEEE
Transactions PAS-95 No.1, 120-129 (Jan.-Feb. 1976).

10\. Crouch, K.E., Fisher, F.A. and Martzloff, F.D., \"Transient Control
Levels, A Better Way to Voltage Ratings in Power Converter
Appli­cations,\" Proceedings of 1976 Annual Meeting of the IEEE Industry
Application Society.

**TRANSIENT CONTROL LEVEL**

**PHILOSOPHY AND IMPLEMENTATION**

**II. Techniques and Equipment for Making TCL Tests**

## F.A. Fisher and F.D. Martzloff

General Electric Corporate Research and Development

# Abstract

This is the second of a pair of papers describing how better transient
protection might be achieved through the use of a Transient Control
Level philosophy. The first paper deals with questions of why tests
should be made and what should be the specifications governing such
tests. This paper amplifies somewhat on those themes, but is primarily
concerned with questions as to how such tests might be made.

### **Direct Injection of Transients**

### **Equipment**

Surge generators capable of producing the types of transients involved
in the Transient Control Level (TCL) philosophy generally employ
capacitors that are discharged into a wave-shaping circuit. That circuit
would be resistive if unidirectional transients are to be produced, and
inductive if oscillatory transients are to be produced.

It should be understood that the TCL philosophy is not written around
any particular test circuit. Any circuit that produces open-circuit
voltages and short-circuit current of the required characteristics will
be satisfactory . Nevertheless, it does seem appropriate to give some
specific guidance as to the types of generators that might be used.

Some representative examples of circuits are shown on Fig. 1. The switch
may be either triggered or untriggered, depending upon the degree of
sophistication desired. Components should be chosen and laid out in such
a way as to minimize undesired residual inductance and radiated
interference. Circuit voltages are frequently high enough to be
hazardous; therefore, appropriate safety precautions must be taken. The
two circuits shown on Figs. 1a and 1b are designed to produce the 100
kHz oscillatory TCL test wave and the unidirectional 1.2 x 50
microseconds ANSI test wave.

In Fig. 1a, the oscillatory frequency is determined primarily by
C**~1~** and L**~1~** with the front time determined by R**~2~** and
C**~2~**. In Fig. 1b, the front and tail times are approximately:

> t**~tail~** = 0.7 R**~1~**C**~1~** (1)\
> t**~front~** = 3.0 R**~2~**C**~2~** (2)

With different component values, this same type of circuit would then
produce longer duration transients. Photographs of the waveforms
pro­duced by these generators are shown on Figs. 2 and 3. A significant
point to note is that the shapes of the current and voltage are
different. This is particularly true of the oscillatory generator, Fig.
1a. It is characteristic of inductive transient sources that the
short-circuit current takes longer to reach its peak value than does the
open-circuit voltage. Specifications should allow for this; it is
generally unrealistic to require surge currents to rise to crest as fast
as surge voltages, at least from inductive sources or into inductive
loads.

## Figure 1 -- Elementary Circuit Diagrams

The generators of Fig. 1 may be operated to produce either single or
repetitive pulses. A 60 (or 50) pulse per second rate is easily achieved
by charging the storage capacitor from an ac source and closing the
output switch on the half-cycle following that which charged the
capacitor.

The type of discharge switch depends on the voltage rating and the
degree of sophistication required. Low-voltage thyratrons and thyristors
are suitable up to about 500 volts, series strings of thyristors to
about 2000 volts, and spark gaps at higher voltages. This type of
circuit lends itself readily to superimposition of the transient to the
power frequency voltage. Some investigations, such as circuits involving
semiconductors, require that the timing of the transient with respect to
the power frequency wave be controlled.

**Figure 2 -- Waveshapes produced by oscillatory generator (direct
injection)**

**Figure 3 -- Waveshapes produced by exponential generator (direct
injection)**

### Types and conduct of tests

Experience has shown that great amounts of confusion can surround two
seemingly simple questions: for what purpose does one wish to make a TCL
test, and how does one define the severity of the test? With respect to
the first of these questions, TCL tests fall into one of two categories:
those aimed at determining what TCL is appropriate for a particular
piece of equipment, and those to determine whether a particular piece of
equipment meets the requirements of a certain TCL specification. While
the subject may seem elementary, experience has shown that the
distinction between the two is often lost. These distinctions are
illustrated in Figs. 4 and 5.

**Figure 4 -- Determining upset or failure levels of a piece of
equipment**

**Figure 5 -- Acceptance test on a piece of equipment**

In Fig. 4 the problem is to determine what the intrinsic withstand or
upset level of the piece of equipment might be. In this type of test the
surge generator is set to some level, the transient is applied to the
terminals of the equipment under test, and the results are observed. The
output level of the surge generator is then measured, probably in terms
of the open-circuit voltage produced by that power setting.

The power setting of the surge generator is then increased in steps
until the device under test either upsets or fails catastrophically. The
actual voltage or current amplitudes and wave shapes at the terminals of
the equipment depend upon the input impedance of the equipment; and
these might be unduly complicated to specify. The failure, however, will
occur at some definite setting of the transient generator. If the
setting of the generator (position of the knobs) was left unchanged, the
energy level of the transient that produced failure or upset could then
be described in terms of the voltage or current that the generator
produced when connected to an open circuit or to a short circuit. This
level will seldom be a nice round number, either so many volts or so
many amperes. If one were to quote a level of transient that a device
could withstand, that level would be the next lowest of the agreed-upon
sequence of levels. If the levels from which the choice were to be made
were as shown on Fig. 4, the appropriate level to quote would be level
3.

The alternative problem is that of Fig. 5, in which the aim is to
determine whether or not the piece of equipment under test is able to
withstand the transients associated with the specified transient control
level. In this type of test, the transient generator is set to deliver a
specified current into a short circuit and to deliver a specified
voltage to an open circuit. If the source impedance levels have been
properly chosen and implemented, these two conditions will be met
simultaneously at one power setting of the transient generator. With the
setting of the transient generator left unchanged, the output terminals
of the generator are then connected to the input terminals of the device
under test. An appropriate number of transients are applied to the
equipment, and the equipment is monitored for correct operation during
the test or monitored after the test to determine whether it works or
not.

The preceding material has touched upon the question of how one defines
the level of transient. Superficially, this might seem a trivial
question to which an easy answer can be given -- the level of the
transient is measured in terms of the voltage applied to the terminals
of the device under test. However, this simple answer is deficient in
that it neglects the input impedance of the device under test. If the
device under test has a low input impedance or is fitted with surge
protective devices, it might not be possible to develop a specified
voltage without injecting excessively high currents into the device,
currents higher than the naturally occurring transient source would be
able to supply. For low impedance circuits, it is more appropriate to
define the transient in terms of current. However, a specification
considering only surge current is not sufficient in that the impedance
of the circuit might be so high that a specified current might be
developed only by applying excessive voltage. For these reasons it seems
best not to define a test in terms of the voltage or current developed
at the terminals of the device under test. Rather, in the TCL
philosophy, the tests are defined in terms of the capability of the
surge generator to deliver specific voltages or currents into specific
loads. If one is dealing with surge generators suitable for direct
connection to the terminals of the device under test, the most
convenient loads are open or short circuits. In cases where the
transient is to be superimposed to the power frequency voltage, it will
be necessary to dis­connect the power supply for the short-circuit test.

As examples of why these quantities are of importance, open-circuit
voltage is of importance if one is studying the ability of insulation to
withstand voltage, or if one is studying the ability of an isolated
circuit to withstand interference. Short-circuit current is the quantity
of most relevance if one is studying the ability of circuit protective
devices to with­stand surge currents, or if one is studying the
interference produced on signal circuits by current flowing upon the
shields of cables.

### **Transformer Injection of Surges**

Direct injection tests are most appropriate to determine whether or not
a piece of equipment by itself would be damaged by a specific level of
transient. A problem of greater complexity is determining whether a
group of interconnected pieces of equipment will operate correctly in
the presence of transients. Generally, the equipment cannot be tested
for proper operation unless it is part of a completely interconnected
system. This usually implies there must be a source, a load, and
appropriate interconnected wiring. It would also seem to go without
saying that the method of injecting the transient into the wiring should
realistically duplicate the way in which the natural electromagnetic
field induces transients into the wiring. One such test technique is
shown on Fig. 6. In nature, an electromagnetic field might be set up
between the conductors and ground, the field being distributed over the
entire length of the interconnecting wiring. In Fig. 6, a similar
magnetic field is produced, but confined within the core of a pulse
injection transformer. If this trans­former is placed around the wires
under consideration, the current and voltage induced into those wires
are nearly the same as that produced by the distributed magnetic field
of nature. The magnetic field is set up in the core of the transformer
by discharging a surge generator through an exciting winding on the
core.

## Figure 6 -- Magnetic field confined in the core of an injection transformer

Some considerations regarding the capabilities of such transient
injection follow. First, it should be remembered that the circuit under
test is primarily exposed to the changing electromagnetic field
contained in the core of the pulse injection transformer. If the source
under test is a high impedance circuit, the natural effect of that
changing magnetic field is to induce an open-circuit voltage, the
magnitude and duration of which will depend upon the ampli­tude and rate
of change of the magnetic field in the core, but which will not be
significantly affected by the length of the circuit under test. However,
if the circuit is of low impedance, the magnetic field will induce a
circulating current in the circuit. The shape of this current will be of
a duration longer than that of the open-circuit voltage, and will be
nearly the same as the duration of the current produced by the pulse
generator in the primary of the transformer.

The magnitude of this current will depend upon the impedance of the
circuit under test. That impedance is governed both by the internal
impedance of the terminal equipment and the characteristics of the
wiring used to interconnect those pieces of equipment. Just as with
direct injection of transients, there can be ambiguities in
specifications if the characteristics of these impedances are not taken
into account. If, for example, the wiring under test is fitted with a
shield grounded at each end, the natural measure of the transient is the
current induced on the shield. If the wiring is unshielded and con­nected
to high impedance loads the natural measure of the transient is the
induced voltage. Again, a complete specification of the test circuit
must deal both with the maximum voltage and the maximum circuit current
that may be produced.

It seems appropriate to measure these voltages and currents on a dummy
circuit of fixed dimensions. The dimensions must be fixed since length,
diameter, and height all affect the inductance, the most important
component of the impedance. The inductance of a conductor is almost
directly proportional to its length, but proportional only to the
logarithms of the diam­eter and of the height above the ground plane. In
the absence of any other specifications, it is suggested that the surge
generator and injection transformer be such that the specified
short-circuit current be induced on a conductor 3 meters long, having a
diameter of 0.41 centimeter, and spaced 5 centimeters above the ground
plane. Such a conductor may be provided by the shield of RG-58 coaxial
cable.

An example of a test structure having these dimensions is shown on Fig.
7. Representative examples of the transients that may be produced are
shown on Figs. 8 and 9. It is harder to inject a transient indirectly
into a cable than it is to produce a transient for direct injection
tests, because of the limitations of the injection transformer. If Fig.
8 is compared with Fig. 2, it will be noted that both the current and
the voltage are less, and that the ratio of voltage to current is less
when the transient is injected via a transformer than when it is
injected directly. This results from the characteristics of the
injection transformer. If the transformer has high magnetizing
inductance and low leakage inductance, the exciting current -- and
hence, magnetic flux -- will be proportional to the integral of the
exciting voltage. In such a case, the open-circuit voltage induced on
the cable will be of the same shape and amplitude as the driving voltage
from the surge generator.

**Figure 7 -- Transformer injection of transients**

**Figure 8 -- Waveforms produced by oscillatory generator (transformer
injection)**

**Figure 9 -- Waveforms produced by unidirectional generator
(transformer injection)**

As a practical consideration, it is often desirable that the cable
joining the two pieces of equipment under test be threaded through the
core only once, either because of the physical characteristics of the
cable or because of the effect on the transient response of the
transformer of multiple turns. This limits the magnetizing inductance to
relatively low values. The current in the exciting winding -- and hence
the flux in the core -- is then proportional more nearly to the
generator voltage than to the integral of that voltage, particularly for
surge generators of internal impedance high compared to the impedance of
the transformer. As a conse­quence, open-circuit voltage induced onto the
cable may be of shorter duration than desired, approaching the
derivative of the driving voltage. The amplitude of the voltage might
also be limited by saturation of the core. These effects are more
troublesome for longer duration transients, as may be noted by comparing
Fig. 9 to Fig. 3.

Two measures were taken to improve the wave shape of the open-circuit
voltage. The first of these was that some resistive loading, via an
auxiliary winding, was used on the injection transformer to provide
partial integration of the flux. The second was that the front time of
the open-circuit voltage from the exciting pulse generator was made
longer through the use of higher values of wave shaping capacitance.
While these compensating techniques were partially successful, there is
still ample room for improvement in the art of making equipment for
transformer injection tests.

Injection transformers are not constrained to be operated with only one
turn, or even equal turns, on the primary and secondary. Either voltage
or current may be emphasized by operating the transformer with unequal
turns, although such operation should be recognized as affecting the
impedance of the test circuit. In general terms, the effects of unequal
turns are:

a\) More turns on secondary gives (for a fixed number of turns on the
primary):

-   Less short-circuit current

-   More open-circuit voltage

-   A decrease in the frequency of the natural oscillatory mode of the
    cable under test.

b\) More turns on primary gives (for a fixed number of turns on the
secondary):

-   More short-circuit current

-   Less open-circuit voltage

-   A decrease in the frequency of the natural oscillatory mode of the
    pulse generator

-   A longer rise time of the current pulse.

If one wishes to determine by analysis the characteristics of the test
circuit, one may derive equivalent circuits for the injection
transformer and the conductor under test. For the injection transformer,
the Tee equivalent shown on Fig. 10 is generally satisfactory

## Figure 10 -- Equivalent circuits of injection transformers

L**~A~** and L**~B~** are the inductances measured with the primary and
secondary windings connected first in series aiding and then in series
bucking.

The cable under test could probably be treated equally satisfactorily as
a Tee equiva­lent of inductance and capacitance. A complete equivalent
circuit, less the resistive loading shown on Fig. 7, would then be as on
Fig. 11. Numerical analysis of this circuit with the aid of a computer
program such as ECAP is entirely feasible. While the task has not been
done, such analysis might point the way to test circuits better adapted
to producing the desired TCL test waves.

## Figure 11 -- Equivalent circuit of a surge generator

## connected to cable through injection transformer

### Equipment for Generation of Electric and Magnetic Fields

The test equipment described so far has been aimed at the injection of
current or voltage pulses into the terminals of electronic equipment.
There is also sometimes a need to subject equipment to an engulfing
electric or magnetic field, principally to check for magnetic field
leakage of the cabinet.

Sometimes these effects are checked with a wire-wrapped technique in
which a coil of wire is wrapped around the item to be tested and the
coil then excited from a pulse generator. The type of magnetic field so
produced is perhaps not the best that may be derived for direct effects
testing. In the study of nuclear electromagnetic pulse (NEMP) effects,
use is often made of stripline simulators in which a pulse generator is
matched to a large open transmission line. In such simulators attention
is given to ensuring that there is a suitable tran­sition from the small
geometry of the pulse generator to the large geometry of the test
chamber. The pulse generators and working chambers are also carefully
matched to each other to allow the production of the required
short-duration, rapidly changing fields. For the production of more
slowly varying electromagnetic fields, such as those resulting from
lightning, the physical design of such chambers can frequently be
simplified. The rise times involved are not as fast as those in the NEMP
studies and the requirement of plane wave propagation in the test
chamber is not as important. A test chamber suitable for many types of
equipment might be like that of Fig. 12. It would basically consist of a
one-turn loop antenna of rectangular cross section enclosing a volume of
about 1 m**^3^**. Such a coil, when excited by the pulse generator
sketched, would produce in its working volume a magnetic field of
approximately 1000 amperes per meter or an electric field of about 20 kV
per meter.

## Figure 12 -- Possible test chamber for evaluating electric and magnetic field effects

### **Conclusions**

The equipment capable of performing the types of tests implied in the
TCL concept is basically simple in nature. If only the lower levels need
be generated, and if the equipment need not be very refined, it can be
built in virtually any laboratory.

Ultimately, of course, one might expect suitable test equipment to
become available com­mercially, and indeed some equipment is already
available. This is particularly true of equipment suitable for direct
injection of transients. Techniques for indirect injection have not yet
been as well worked out. There is a need for further development in that
area, supported by commercial test equipment.

The true need for subjecting electronic hardware to electric and
magnetic fields has not yet been determined, and accordingly very little
has been done toward producing equipment for such tests.

TRANSIENT CONTROL LEVELS

A new concept licks an old telco problem

François D. Martzloff

General Electric Company

Overvoltages are here to stay, and they cause more problems than ever
with sensitive electronic gear. The industry, can, and must cope.

Transient overvoltages are here to stay; we can only learn to live with
them. The electrical and electronic equipment of yesterday could survive
the effect of these transients because of conservative designs with
built-in capabilities. But today and tomorrow^\'^s equipment depends on
sophisticated, miniaturized solid state devices. Because they are very
sensitive to overvoltages, these devices cannot survive transient
overvoltages if not protected.

The transients we are discussing here occur on power systems as well as
on signal lines. They are overvoltages caused by natural events such as
lightning or high-voltage wires falling on lower voltage lines, and by
such man-made events as load switching, intermittent contacts, or
coupling from adjacent circuits.

While transient duration is extremely short---a few millionths of a
second (microseconds)---transient amplitude can exceed 10 to 20 times
that of the normal circuit voltage (see photograph). This overvoltage
can cause component failure, sparkover between wires followed by a power
arc, and mistriggering of solid state devices. This damage to equipment,
on the power supply side as well as on the signal lines side of the
telephone plant, is the main issue here rather than the nuisance
effects.

Successful survival depends on a technically sound, cost-effective
coordination among the environment (transient occurrence), the equipment
design (withstand capability), and the use of protective suppressors
(voltage clamping). Such a coordination has been developed in the
electric utility industry with great success, but the electronics
industry as a whole, including the communications people, has not yet
been able to develop, to accept and to enforce a rational system of
coordination. The Transient Control Level (TCL) is a plan for
systematically coordinating the three parameters of environment,
equipment and protection.

The TCL approach proposes four specifications:

1.  Equipment will be de-signed to withstand well-defined ^\"^proof
    tests,^\"^ rather than inconsistent simulations of an uncertain
    environment.

2.  Both the power lines and the signal lines of an electronic black box
    will be subjected to proof tests.

3.  A limited number of tests levels will be established; from the range
    of these the manufacturer and the user together will select a
    particular level for a given application.

4.  The waveshape (the value of the overvoltage as a function of. time)
    and energy content of the transient will be defined by established
    guidelines.

These four specifications for TCL are rooted in several hard facts.
Users and manufacturers must accept the impossibility of simulating all
possible transient overvoltages encountered by equipment in service.
Nevertheless, equipment can be designed to withstand a standard
transient, and experience has shown that if the standard is set
properly, the field experience of the approved equipment is far more
acceptable than that of equipment which has not met the standard.

Clearly, users would like to have high standards of withstand
capability, while manufacturers, facing competitive pressures in an
environment of undefined transient standards, would like to reduce costs
by limiting with-stand capability.

Actually, producing electronic equipment with high inherent transient
withstand capability would be economic heresy. To allocate resources to
a balance between moderate capability and moderate protection cost is,
in fact, the rational approach to the problem of equipment protection.
The key concept of TCL is that the coordination of environment,
equipment, and protection not only can be best accomplished but will be
most cost-effective if it is planned ahead rather than retrofitted---if
a retrofit is possible.

In defining the standard test conditions, then, there must be a balance
between the desire to produce high reliability and the desire to
minimize cost. There is nothing new in this aspect of engineering. The
novelty here is the fast pace of new equipment growth and of the
progress in making transient suppressors available.

Transient suppressors which can be applied in the telephone plant are of
two types: the spark-gap devices and the energy-absorber devices. Spark
gaps have been used for a long time in the forms of the familiar
carbon-block and the more modem gas tube. In both devices the gas (or
air) between two electrodes connected to the line and ground breaks down
when an overvoltage occurs. The level of voltage together with the speed
at which this breakdown occurs is a characteristic of the design. After
the breakdown, a low volt-age is maintained across the gap, which allows
a high current flow without large power dissipation (hence heating) in
the device. Spark gaps have the limitation that the breakdown requires
some time to occur; until then the transient is not limited. While the
delay may be no longer than a microsecond or less, there may still be
time for a fast-rising transient to destroy a component before the spark
gap responds.

Zener diodes, especially in their improved versions designed for
transient suppression, and varistors (variable nonlinear resistors) are
energy-absorber devices. In these the resistance of the device to the
flow of current decreases as the current it-self increases, a
counteraction that produces a clamping effect on the voltage transient
but that dissipates a large part of the transient energy within the
device. Energy-absorber protectors do not have the delay limitation of
spark gaps; they are permanently connected to the circuit and begin
acting as the voltage rises, without waiting for a breakdown. Their
limitation is the fact that the energy of the transient is dissipated in
the source as well as in the device. The sharing of the energy is in
inverse proportion to the impedance: for a given device, a low source
impedance forces more of the energy to be dissipated in the device,
which has a capacity limited by its dimensions (and dimensions are
generally limited by cost and by available space).

Knowledge of the capability of suppressors to limit the voltage under
defined conditions of voltage enables the equipment designer to build
into his circuits the matching withstand capability. Survival is then
assured. This is another key concept of TCL. Equipment is designed to
fit the protective level that practical suppressors can guarantee not to
be exceeded rather than to retrofit suppressors after the equipment is
found to have transient problems. Of course this is an idealized
situation because uncertainties of the environment, practical
manufacturing tolerances, and the fundamental refusal of Nature to be
coerced into neat human systems will still allow some failures to occur.
But imperfect as the implementation of the TCL might be, it will still
be better than to wait for the perfect system to come along.

The TCL approach is being proposed to the industry through the medium of
engineering society meetings, technical papers, and standards
committees. Engineers are receptive to the concept, for the standard
test conditions will enable them to compare results and to evaluate
performance under consistent conditions. While the concept has not yet
been elevated to the status of an industry standard, acceptance is
increasing.

An example of possible coordination for cost-effectiveness can be found
in the power supplied for telephone electronics connected to the AC
power grid. Typically, rectifiers in these supplies are exposed to all
the transients occurring in the power system; for central locations
these can be mild, but for line equipment in isolated locations exposure
can be severe. Requiring the rectifiers to withstand the several
thousand volts which can occur on the lines would force the designer to
select expensive rectifier with high Peak Inverse Voltage ratings (PIV).
(This PIV rating affects directly the price of the device.) It is much
more effective to specify a rectifier with a PIV rating consistent with
the normal voltages of the power circuit, at reasonable cost, and then
to pro-vide protection ahead of the rectifiers with a device whose cost
is lower than the price differential between the high PIV rectifier and
the low PIV rectifier.

Another example, a potentially long retrofit problem, will illustrate
what can happen when coordination is not designed at the outset of a
system. Telephone switching circuits using hybrid relay and
semiconductor circuits are typically protected by gas tube primary
protectors. A situation can develop where lack of coordination between
the withstand capability of the components that immediately follow the
protector and the clamping capability of the protector will cause
failure of the component. One obvious ^\"^fix^\"^ is to increase the
withstand capability of the component and make this increase a new
specification. However, the failing component may in fact have acted,
while failing, as a protector for the rest of the system, and raising
its withstand level would let the transient go farther into the system,
causing more extensive damages. What is needed is a systematic appraisal
of the total capabilities and withstand levels---that is, a control of
the transient levels allowed to occur in the system, starting with
protectors installed where the lines enter the plant, possibly with
additional protectors deeper into the system.

Protection for the more sensitive equipment being installed today is
possible. Acceptance of the Transient Level Control concept throughout
the electronics industry would enable telephone system manufacturers and
operators to enjoy at reasonable cost a high degree of reliability.

TRANSIENT CONTROL LEVEL TEST GENERATORS

F.D. Martzloff

General Electric Company

Schenectady NY

**INTRODUCTION**

The successful operation of consumer electronic devices requires that
these devices survive in a hostile environment, sometimes not
recognized, where overvoltages on a 120 V supply can exceed 5 kV in
extreme cases, and frequently 1200 V in many locations. \[1\] Therefore,
a method of demonstrating withstand capability has to be developed along
acceptable standards, or at least guidelines.

As an example of the need for such a circuit, the Underwriters\'
Laboratories require a trip-free withstand of 3 kV, and a failure-free
withstand of 6 kV for the recently introduced Ground Fault Circuit
Interrupters. \[2\]

A test circuit which produces an impulse voltage superimposed on the 120
V supply has been described (Ref. 3). This circuit approximates a
typical wave-shape as observed during several years of recording
transients in residential circuits. \[1\] The source impedance is
believed to be representative(4) of real-world conditions.

In other reports \[5, 6\] the concept of Transient Control Levels (TCL)
is proposed to users and manufacturers of equipment. It is hoped that
the impulse generator circuits described in this report will be a useful
tool in implementing the concept so that it will gain general
acceptance.

# 

# TEST WAVESHAPE

The test waveshape proposed in Refs. 5 and 6 under the name of
\"Transient Control Level\" has been selected as most representative of
conditions encountered during actual measurements. It contains a fast
rise that can stress inductive components, a slower oscillating tail
which can be integrated by those circuits containing a rectifier
circuit. However, it does not have the large energy contained in a 40 or
50 μs to half-value tail suggested by others, which appears excessive
and would unduly stress energy-absorbing components in the test piece.

As discussed in the TCL paper, there is no way to select a waveshape
that will represent all possible conditions. Rather, the approach is to
agree on a standard waveshape anchored in reality and, by experience, to
correlate the fact that devices which can pass the TCL waveshape have
successful field performance. To represent conditions realistically, the
proposed test waveshape had a 0. 5 μs rise time, a period of 10 μs
during later oscillations, and a decrement of 600% between successive
peaks. Further, the effective source impedance is set at 50 Ω, as
determined by a 50% decrease from open-circuit voltage to a 50 Ω
resistive load condition.

After this waveshape had been established, discussions within the IEEE
and IEC community on the desirable value of source impedance led to the
proposal of a parallel combination of 50 Ω and 50 μH as a representative
source impedance. The initial generator circuit was then modified to
provide this new source impedance, and the results were compared to the
initial circuit \[3\] performance. This report describes in some detail
the principle, design, and results of the two circuits. The generator
presented here, while not offering the ruggedness or convenience of
commercial products, \[7\] can be assembled by a reasonably competent
technician and used in a laboratory environment by qualified
individuals.

# 

# PRINCIPLE

Energy is stored in a capacitor and discharged into an isolated 120 V
supply by means of a simple electromechanical relay. The rate of rise of
the voltage is produced by a first resonating L-C circuit, which is
damped sufficiently to let a second resonating circuit control the decay
and subsequent oscillations of the voltage, according to the proposed
standard waveshape. The 120 V upstream supply is protected against
overvoltages by a large capacitor connected across the secondary of the
isolating transformer

The use of a low-voltage (120 V rating) relay as the high-voltage switch
might seem surprising. How-ever, this is a very effective and low-cost
approach, as long as it is recognized that conventional clearances and
withstand voltages are obviously not observed in this circuit. However,
in the case of breakdown, the worst that can happen is a premature
exposure of the test piece to the spike that would normally be manually
triggered by the \"fire\" button. Compared to other approaches, using a
high-voltage switch or a step-up transformer, \[3\] this approach using
a relay makes the circuit inexpensive and easily assembled.

The complete circuit is contained in a metal cabinet, requiring only a
120 V, 60 Hz supply (Fig. 1). This cabinet includes the necessary DC
supply for charging the capacitor, an isolating transformer for
providing the 120 V supply to the test piece, and the closing relay and
its control circuit.

## Figure 1 -- Transient control level test generator

# BASIC IMPULSE GENERATOR CIRCUIT

The schematic of the basic impulse in its first form is shown in Fig. 2.
The variable voltage obtained from the autotransformer T3 is applied to
the primary of the step-up transformer T2 (a 15 kV neon tube supply
transformer). The secondary of T2 is connected in a full-wave,
center-tapped rectifier configuration. The storage capacitor Cl is
charged through the resistor R3, until the relay CR-1 will discharge it
into the 120 V test circuit, supplied by the isolating transformer Tl.
The relay control circuit is described in detail in the next section.

## Figure 2 -- Basic impulse generator circuit, first design

The rise time of the wave is controlled by the series resonance of Ll
with Cl-C2 in series, with a surge impedance of approximately 50 Ω,
damping being provided by the resistance RI. Following this rise, the
two capacitors, Cl and C2, now tied together by the closed contact and
the low-inductance Ll, resonate with the inductance L2, as C3 offers a
low impedance to the surge, at the same time protecting the upstream
power supply from the surge. L2 then holds off the surge voltage while
carrying the full-load 60 Hz current (and is designed accordingly). The
resistance R2 is selected to provide the damping of the tail
oscillation, which has been set at a decay of 60% between successive
peaks in the proposed \"standard\" TCL waveshape.

The output voltage, brought out to terminals of the front panel, can be
monitored through the approximate divider R5/(R4 + R5). Alternately, the
open circuit voltage produced by the generator can be preset by
calibrating the autotransformer T3. However, neither method can be
sufficiently accurate for precise determination of the spike voltage;
final determination should be made by direct measurement at the output
terminals, using a suitable probe and oscilloscope.

A second circuit approach, suggested by Crouch and Fisher \[8\] is shown
in Fig. 3. In this circuit, the basic oscillation is produced bv the
Cl-Ll circuit when the CR-1 switch is closed, and applied to the test
piece through the 50 μH/50 Ω network. The rise time is controlled by the
Rl - C2 time constant. This second circuit has an impedance which is
closer to the theoretical 50 μH/50 Ω source impedance than the first.
However, the actual difference in the performance, as well as the
measured impedance, is small, as indicated by the oscillograms of Figs.
4 and 5 and the impedance plot of Fig. 6.

**Figure 3 -- Output stage of the second circuit for TCL wave**

**Figure 4 -- Output of the first circuit**

**Figure 5 -- Output of the second circuit**

**Figure 6 -- Impedance of the two TCL circuits**

While the initially developed circuit used a gate output from an
oscilloscope to produce a timed closing of the relay (with respect to
the 60 Hz line voltage), the new implementation includes a timer circuit
producing a single-shot pulse of current driving the relay coil at a
preselected time of the supply sinusoid, so that the spike voltage can
be applied at any desired instant of the 60 Hz wave, by setting a
built-in control (Fig. 7).

**Figure 7 -- Control circuit for synchronous trigger**

A built-in time delay lockout in the trigger circuit has been provided
to allow the capacitor driving the relay coil to reach a uniform voltage
from one test to the next, in order to reduce variability in the closing
time of the relay and thus to promote more consistent timing of the
spike. If a repetition rate faster than the built-.in 6-second interval
were necessary, the time delay could be reduced, at the cost of less
accurate timing of the spike.

The amplitude of the pulse is adjusted simply by means of a control
autotransformer in the primary of the high-voltage transformer, which
can be precalibrated for open-circuit voltage setting.

The isolating transformer, a conventional general-purpose transformer
with two sets of windings in the primary and secondary, can be connected
for a 240 V output if required; as built, the transformer can deliver 2
kVA at 120 V into the test piece to be subjected to the spikes. Of
course, the connection of a 2 kVA resistive load, on the 60 Hz/impulse
out-put of the generator, will produce a large voltage drop in the pulse
applied to the test piece. This voltage drop, caused by the regulation
of the generator, must be recognized and addressed when specifying the
test conditions.

The front panel controls (Fig. 8) are then limited to a voltage setting,
a timing setting, and a fire button. The output is available on a
standard three-prong outlet, with one side of the outlet grounded to the
cabinet chassis. In turn, the cabinet chassis is connected to the ground
wire of the power cord. Thus, the cabinet is normally grounded to the
power supply ground. If a single point ground is required by the
instrumentation system, resulting in the removal of the cabinet ground,
adequate safety precautions must be taken.

As an alternate panel connection, the 120 V terminals and the impulse
terminals can be brought out on studs and connected externally to suit
the needs of the test.

## Figure 8 -- Front panel controls

A connector on the front panel also brings out a signal for oscilloscope
trigger, occurring at the time the relay coil is energized. This trigger
signal is useful for displaying the full 60 Hz sinusoid with the
super-imposed spike. For resolution of the spike at faster sweep rates,
this trigger signal would be premature; therefore, the oscilloscope
should be set on internal trigger. Another connector on the front panel
brings out a signal with an approximate voltage divider reading of 10 V
per kilovolt of output voltage (within 10%). More accurate voltage
readings require the use of a calibrated probe, such as Tektronix P6015,
connected at the actual output terminals.

The original circuit specified for the waveshaping capacitor was a type
that was found to have excessive (and inconsistent) internal inductance.
This excess produced a very high frequency ring on the front of the
wave, which in extreme cases could have an amplitude exceeding the crest
of the desired wave. The problem was eliminated by substituting ceramic
capacitors to the initial plastic-film design.

As a further refinement for laboratory use, the two basic impulse
circuits were built as plug-in subcircuits so that they could be
substituted for one another during the investigation of output waveshape
and currents, as well as during later development of other circuitry.
Figures 9, 10, and 11 show the plug-in arrangement of this circuitry.

**Figure 9 --Interior view**

**Figure 10 -- First circuit plug-in**

**Figure 11 -- Second circuit plug-in**

# TRIGGER CIRCUIT

Figure 12 is a schematic of the timing circuit used to drive the relay
at any time in the 60 Hz wave by a simple setting.

## Figure 12 -- Synchronous trigger circuit

The circuit utilizes a zero crossing detector and a controlled time
delay circuit. This arrangement permits zero time to be coincident with
the 60 Hz wave as it crosses zero while going positive.

The circuit functions in the following manner. The 60 Hz AC from the
secondary of T1 is applied to IC1 through R12. The circuit normally
provides an output pulse every zero crossing of the 60 Hz wave. A
resistive bridge network, consisting of R13, R14, and two resistors
internal to IC1, is unbalanced by R15. In the unbalanced state the
circuit is inhibited. Activation of the switch S1 causes the circuit to
operate.

A pulse is fed internally to an AND gate which outputs only on
positive-going zero crossings. The out-put of IC1 is applied to Q1,
which inverts the pulse and applies it to IC2. IC2 is used as a time
delay. The bias level of an internal comparator circuit is controlled by
R25. Varying R25 changes the delay time and permits selection of the
time of application of the pulse with respect to the 60 Hz wave.
(Because delay time also includes the relay operation time, calibration
must be made for each unit. ) The output of IC2 is applied to IC3, which
produces an output pulse. This pulse is applied to the gate of the
thyristor SCR1 and to the scope trigger output. The thyristor SCR1
discharges C1 through the relay coil, thereby applying the high-voltage
pulse to the line.

# RELAY MODIFICATION

To maintain cost as low as possible, the CR1 relay is operated as a
high-voltage switch. In the normal relay construction, the switch leads
are attached to the case and connected by a braid to the contacts. The
proximity of the braid connection to the relay coil on some relays may
cause flashover to the coil at about 7 kV, applying undesirable pulses
to the circuitry. To avoid this problem the relay was modified to bring
the high voltage into the top contacts of the relay, and the braid
connection to the terminals was removed. The two contacts on the
armature bridge were then joined to obtain a series connection of the
two contacts gaps, with input terminals at the stationary contacts. This
configuration can withstand 14 kV between contacts and coil without
breakdown.

The relay is further modified by removing the stationary contact of the
normally closed pair and twisting the moving contact slightly to
increase the contact gap, until only a barely perceptible wiping action
occurs upon closing. This modification allows the two series-connected
contacts to withstand 10 kV.

# 

# ISOLATING TRANSFORMER

T1 is a 120/240 V input, 120/240 V output transformer that may be used
for 240 V AC output operation by changing the connection internally. The
high voltage circuit to the test device does not require change. The
input to the timing circuit must be maintained at 120 V AC by using only
one-half (Xl-X2) of the T2 secondary when the two halves are connected
in series to produce 240 V. This will maintain the timing
characteristics relative to the zero crossing intact. Care must be taken
to ensure that the ground connection (which is so marked) of the timing
circuit remains connected to the grounded side of the T1 secondary.

# 

# SAFETY

The metallic enclosure of the cabinet provides a generally safe
construction, consistent with typi­cal laboratory-type equipment. Of
course, the output outlet will supply 120 V to the test piece, with
superimposed high-voltage spike. This will then require safety
precautions consistent with general laboratory practices. As stated
earlier, the cabinet is normally grounded to the power supply ground by
means of the third wire of the three-wire line cord. There may be
circumstances where a single-point ground, such as the chassis of the
oscilloscope used for monitoring, may be required to minimize extraneous
signals resulting from ground loops. In such cases, the line cord may be
connected through a two-wire adapter, but suitable precautions must be
taken to maintain the single-point ground connected at all times.

A warning sign cautions against opening the cabinet by uninformed
personnel; high-voltage components, including capacitors that can retain
a charge even after disconnecting the power cord, are not protected
after the opening of any of the cabinet panels. Because no interlock is
provided, the user must be thoroughly familiar with the design of this
unit before attempting to open the cabinet, and must exercise all
appropriate caution in servicing the circuit.

# PERFORMANCE

The oscillograms of Figs. 13 through 17 (recorded with a Tektronix 7623A
storage oscilloscope and a Tektronix P6015 1000:1 probe) illustrate the
performance of the test generator. Figures 13 through 15 show the output
voltage at different sweep speeds, in the open-circuit condition. The
100 kHz oscillation, or 10 μs period in the tail, can be seen in Fig.
13, as well as the 60% decrement factor between successive peaks.
Figures 14 and 15 show the front of the wave, including some fine detail
of the residual oscillations in the front, which are due to stray
capacitance in the inductors and inductance in the capacitors. The
frequencies are high, but the amplitudes, compared to the crest voltage,
are small. A rise time of about 250 ns can be observed on Fig. 15.

Figure 16 shows the output of the generator when feeding a 50 Ω
resistive load, at the same voltage setting as the open-circuit
condition of Fig. 13. The ap­proximate 50% reduction in output voltage
indicates an internal impedance slightly below 50 Ω, providing a
conservative margin over the design objective of a 50 Ω source
impedance. This has not been raised to 50 Ω: first, in order to provide
some margin and, second, in anticipation of the possible acceptance of
J. H. Bull\'s (Ref. 4) value of 50 Ω, paralleled by 50 μH for a
representative value of the impedance of the power system, which has an
\"effective impedance\" of 30 Ω for the open circuit voltage waveshape
of interest.

Figure 17 shows the voltage appearing across the capacitor C3 at full
output (6. 8 kV) setting of the pulse. The very moderate disturbance in
the 120 V power supply indicates the adequacy of the filtering scheme
used in this circuit in isolating the test piece supply from the bench
supply.

## Figures 13-18 \-- Typical oscillograms of output

The ceramic capacitors used for the output wave­shaping have a small
capacitance/voltage coefficient, as do most ceramic capacitors. With a
variable volt­age, the resulting capacitance change could affect the
waveshape. Admittedly, a search could be made for a capaci­tor with more
constant dielectric. However, for the purpose of the test waveshape, the
results obtained with the ceramic capacitors are satisfactory.

Figure 18 shows how a voltage change from 25% to 100% produces a slight
change in the front of the wave, but very little change in the
subsequent oscilla­tion. The most noticeable effect is the notch in the
wave for lower voltages, resulting from the characteristics of the arc
at the relay contacts. This small anomaly in the waveshape can be
ignored because the real sig­nificance of the waveshape is the front of
the wave and the total volt-seconds in the tail.

# 

# ACKNOWLEDGMENTS

The first basic impulse generator circuit approach derived from the work
of E. K. Howell of the General Electric Circuit Protective Devices
Products Department. The second circuit was developed by K. E. Crouch
and F. A. Fisher of Corporate Research and Development (CRD). The
synchronous trigger circuit was developed by M. D. Bloomer of CRD.
Packaging of the circuit was accomplished by P. M. Szczesny of CRD. The
improvements incorporated in this circuit were motivated by helpful
discussions with A. W. Smoot and P. Richman, which are gratefully
acknowledged.

# REFERENCES

> 1\. J. G. Hahn and F.D. Martzloff, \"Surge Voltages in Residential
> Power Circuits, \" IEEE Trans. PAS-89 No. 6, 1049-1056 (July-Aug.
> 1970).

2\. \"Standard for Ground-Fault Circuit Interrupter, UL-943,
Underwriters\' Laboratories, Melville, NY.

3 . E. K. Howell and F.D. Martzloff, \" High-Voltage Impulse Testers, \"
General Electric TIS Report 75CRD039.

4\. J. H. Bull, \"Impedance of the Supply Mains at Radio Frequencies, \"
Proc. of 1 st Intern. Symp. on EMC, Montreux (May 1975).

5\. F. A. Fisher and F. D. Martzloff, \"Transient Control Levels \-- A
Proposal for Insulation Coordination in Low-Voltage Systems, \" Trans.
Power Applications and Systems, PAS‑95 No. 1 (Jan-Feb 1976), pp.
120-129.

6\. F. D. Martzloff and F. A. Fisher, \"Transient Control Level
Philosophy and Implementation, 77CH1224-5 EMC, Proc. of the 2nd
Symposium on EMC, Montreux (June 1977).

7\. Bulletins 424 and 281/261: \"Model 424 Surge Generator/Monitor, \"
KeyIek Instrument Corp.

8\. K. E. Crouch, F. A. Fisher, and F. D. Martzloff, \"Transient Control
Levels \-- A Better Way to Voltage Ratings in Power Converter
Applications, 76CH1122-1-IA, IEEE-IAS 1976 Conferences Record, pp.
940-944.

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| ## François Martzloff                                                 |
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| ## April 2004                                                         |
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