Transient Control Levels:

A Proposal for
Insulation Coordination in Low-Voltage Systems

F.A. Fisher François Martzloff

General Electric Company General Electric Company
Pittsfield MA Schenectady NY
fafisher@lightningtech.cpm f.martzloff@ieee.org

© 1976 IEEE
Reprinted, with permission, from
IEEE Transactions on Power Apparatus and Systems, Vol. PAS-95, No.1, Jan/Feb 1976

Significance:
Part 2 Development of standards – Reality checks
Part 6 Textbooks and tutorial reviews

One of the first papers addressing the issues of surge protection in low-voltage AC power circuits, making
a proposal for a departure from the traditional unidirectional and separate 1.2/50 and 8/20 waveforms, on
the basis of the results of monitoring the occurrence of surges in these circuits. Nevertheless, the concept
is emphasized that surge test waveforms should not attempt to duplicate the environment, but only to apply
“representative” waveforms and levels that will demonstrate the equipment withstand capability.

The proposal also included the concept of establishing first a level of surges that will not be exceeded,
thanks to the application of appropriate SPDs, and only then designing equipment that will withstand level
higher than the allowable level of surges. This was nothing new, having been applied successfully in the
high-voltage utility environment. However, the proposal was new for the low-voltage community.

Unfortunately, the fait accompli of equipment being designed and placed on the market without such
coordination prevented application of that proposal. Thus, industry is left with the situation where equipment
failures under surge conditions can occur, after which remedies must be found as retrofits.

In 1975, the following statement appeared in the paper and should be kept in mind when questions arise on
the selection of “representative waveforms” in IEEE Std C62.41.2:
These BIL amplitudes, while assigned somewhat arbitrarily, were (and are) kept in touch with reality by the
fact that equipment designed in accordance with standards do 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 BILs.
 IEEE Transactions on Power Apparatus and Systems, Vol. PAS-95, no. 1, January/February 1976
TRANSIENT CONTROL LEVELS
A Proposal for Insulation Coordination in Low-Voltage Systems
F. A. Fisher F. D. Martzloff
General Electric Company General Electric Company
Pittsfield, Mass. Schenectady, N.Y.

ABSTRACT The purpose of this paper is to promote a concept of

transient coordination for electronic and other low-voltage
Failure and circuit upset of electronic equipment equipment through the establishment of a system of Tran-
due to transients is a problem now and is one which sient Control Levels (TCL’s), similar to the concept of
has promise of becoming more of a problem in the BIL’s so successfully used for many years in the electric
future as trends continue toward miniaturization and power industry. In the following sections, specific sugges-
circuit complexity. Protection methods are used more tions for possible standard Transient Control Levels and
or less extensively and often haphazardly. standard test wave shapes will be made. While the wave-
At present, there does not appear to be a clear approach forms here suggested are chosen somewhat arbitrarily, they
toward achieving compatibility between the transient with- are well grounded in physical reality. The purpose of mak-
stand capability of devices and the transients to which such ing such suggestions is to promote wide discussion as to
devices are exposed. A more scientific approach is needed to whether these waveforms and levels are the best that can be
guide manufacturers and users of equipment. developed, or if indeed the establishment of such standards
is the best way to promote good transient coordination for
The purpose of this paper is to promote a concept the electronics industry. The ultimate purpose of any system
of transient coordination for electronic and other low- of transient coordination would be to achieve greater
voltage equipment through the establishment of a sys- product reliability at minimum cost to the user.
tem of Transient Control Levels, similar to the con-
cept 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 pro-
mote 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 achiev-
ing 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 light-
ning frequently caused failure of such vital and expensive
power equipment as transformers and generators. Those
transient problems were solved by engineering design Fig. 1. The present situation.
guided by the concept of insulation coordination and the
establishment of a series of Basic Insulation Levels (BIL’s). 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 in-
ternal (switching of inductive loads, contactor restrikes
Paper F 75 466-3, recommended and approved by the IEEE Surge or cross talk from adjacent circuits). While the con-
Protective Devices Committee of the IEEE Power Engineering Society for cept of TCL’s is intended to apply to the full spec-
presentation at IEEE PES Summer Meeting, San Francisco, Calif., July
20-25, 1975. Manuscript submitted February 3, 1975; made available for trum of frequencies and voltages (DC, 120 V, 60 Hz
printing April 28, 1975. AC, 400 Hz) the problem of transient coordination will

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here be illustrated by discussion of 120 volt AC systems
intended for consumer and residential use. During the intro-
duction of electronic equipment into consumer appliances
and other residential use, the importance of transient coordi-
nation was not always sufficiently recognized. In some
cases, excessive failure rates occurred as a result of tran-
sients 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 Fig. 2. Exposure of residential circuits to surge (Number of surges vs
1000 volts. Field experience 1 indicated that a 100:1 drop highest surge at any one location)
occurred in the failure rate of clock motors when the with-
stand level was increased from 2000 to 6000 volts. These
data indicate that the exposure rate to surges of 2000-volt that transients of a low level interfere with the opera-
amplitude was sufficient to be of concern, but that surges tion of the mini-computer, causing it to give incorrect
exceeding 6000 volts were quite rare, at least on a national results without causing permanent physical damage.
basis. Another study 3 showed that during two weeks of The vulnerability level of such a mini-computer will
monitoring in a lightning-prone area, several surges exceed- be higher than the susceptibility level. Both levels
ing 2000 volts were recorded, with the maximum recorded must be higher than the normal operating level of the
being 5600 volts. Experience with field trials of Ground computer logic elements or input/output terminals.
Fault Circuit Interrupters sponsored by NEMA and the
Underwriters’ Laboratory 4, when correlated with the known The transient breakdown level or vulnerability of semi-
nuisance trip level of the devices and the observed number conductors is not presently a part of any industry accepted
of trips 5, would indicate an occurrence frequency of perhaps rating system. The vulnerability level is furthermore not
one surge per 7 years above 2000 volts per household. inherently related to the normal operating voltage or peak
inverse voltage (PIV) level. As examples, consider the data
Most residential wiring systems are constructed in such a of Table I. During this investigation, power diodes were
manner that the various wiring boxes will flash over if they subjected to unidirectional transient voltages cresting in a
are exposed to surges greater than 5 to 10 kV. This means few microseconds. The voltages at which failure occurred
that the amplitude distribution will be chopped at 5 to 10 kV. are seen to have little correlation to the nominal PIV rating.
Based on these admittedly scattered and tentative
numbers, it appears that the typical residential circuit will be
Similar data have been accumulated for many semi-
exposed to surges of magnitude and frequency of occurrence
conductors, particularly when semiconductors are
as illustrated in Figure 2.
exposed to very short transients, characteristic of
The magnitude of the transients produced on 120 those produced by nuclear weapons (NEMP). Such in-
volt power lines, however, is not of importance ex- formation has not been widely reported.
cept as it relates to the vulnerability level of the equip-
ment connected to such lines. “Vulnerability” is defined TABLE I
here as the level that causes an irreversible and un- Transient Vulnerability Levels
desirable change (usually failure) in a device. A Typical 1A Silicon Diodes
corollary term is susceptibility, or that level which Diode PIV Failure Level Under
causes temporary malfunction of the device. The Number Rating Reverse Impulse*
susceptibility level cannot, by definition, be higher Volts Volts
than the vulnerability level. Rectifier diodes and
1 200 1100 – 1500
similar semiconductors do not have any particular 2 400 1400 – 1500
susceptibility level; they either fail or do not fail when 3 600 1400 – 1600
exposed to transients. Active semiconductor devices
or a control system operated by a mini-computer *Breakdown observed when exposed to a unidirectional surge rising
system might be a different story. It is quite possible at 1000 volts per microsecond.

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 Clearly, surges occur with amplitudes greater than the coordination of insulation protection. It was never pre-
vulnerability of the indicated semiconductors. The tended, however, that naturally occurring surges were of this
frequency of occurrence of such damaging surges, type, only that the rise and fall times of the natural surges
while small on an individual basis, may be unac- were in the vicinity of the above values.
ceptably high on a product line. The transient ampli-
tudes, of course, could be reduced by the use of suit- The next stage in the process of insulation coordination
able protective devices. Likewise, the vulnerability was the establishment of a series of standard test and design
levels of the diodes to transients could be raised. Some levels, BIL’s. For example, equipment designed for opera-
questions now present themselves, all having to do with tion on 115-kV systems was assigned a BIL of 550 kV. The
the question of who should assume what part of the job designer of equipment to be used on 115 kV systems then
of providing transient coordination. was required to provide an insulation structure that would
withstand 550 kV. The level of 550 kV was derived on the
a) Should it be the responsibility of the user to control premise that existing lightning arresters could be used to
transients to levels that do not damage equipment control the transients applied to that apparatus to less than
supplied by vendors? 550 kV. The proper design of the insulation system was next
b) Should it be the responsibility of the manufacturer to demonstrated by subjecting the apparatus in the laboratory
provide equipment that will not be damaged by the to a surge of 1.5 ⫻ 40 ␮s wave shape and a peak amplitude
naturally occurring transients? of 550 kV . Frequently it was part of the purchase agreement
that the equipment had to successfully pass the laboratory
c) If it is the responsibility of the user to control tran- test. If the equipment failed, it had to be rebuilt or re-
sients, to what level should he control them — the designed. Conversely, it became the responsibility of the
published operating levels (in this case the published user to insure that no surge greater than 550 kV was ever
PIV levels) or some other level higher than the applied to the apparatus.
operating level but below the vulnerability level?
As a result, power equipment achieves its resistance to
d) If it is the responsibility of the vendor to provide lightning-induced transients not so much by being designed
surge-proof equipment, what level of transient to the threat that might be posed by lightning, but by the
voltage and transient energy must he anticipate? threat that will be posed by an acceptance test. This accep-
Similar questions can be asked for all product lines: tance test does not subject the equipment to transients hav-
consumer, industrial, and military, and at all levels of ing the complex wave shapes produced by lightning, but
operating voltage. instead to transients having elementary wave shapes that can
be produced by basically simple test apparatus. Neither does
INSULATION COORDINATION the acceptance test subject the equipment to transients of the
IN THE ELECTRIC POWER INDUSIRY amplitude produced by lightning. However, it subjects the
equipment to transients of amplitude consistent with the
Similar questions occurred many years ago during the capabilities of existing surge-protective devices.
development of the electric power industry at a time when These amplitudes, the BIL’s while assigned somewhat
the art of designing equipment to withstand the effects of arbitrarily, were (and are) kept in touch with reality by the
lightning was in its infancy. The nature of the transients, the fact that equipment designed in accordance with standards
level of insulation to be used, or what should be expected of does not fail when exposed to surges produced by lightning,
the designers of transmission lines and lightning arresters in contrast to equipment designed prior to the development
was not clear. of the philosophy of insulation coordination and the estab-
Those transient problems have largely been eliminated lishment of standard BIL’s.
today by proper engineering design on a system-wide basis. The test and design levels, the BIL’s, are not necessarily
The evolution of insulation coordination in the electric fixed. As better protective devices are developed, the levels
power industry, while it can be only very briefly described may be lowered so that reliable equipment can be built at
here, may be of benefit to the electronic industry. lower cost.
First, the type of transients produced by lightning on Electronic and control equipment, on the other hand, is all
transmission lines, their magnitude and wave shape were too often designed, built, and delivered before the existence
measured. This was not easy in the days of cold-cathode of a transient threat is recognized. If transients turn out to
oscilloscopes employing 50 kV accelerating voltages. Even endanger the equipment, there may be no adequate surge
today with vastly improved instrumentation, such investiga- protective devices. There may, in fact, not be any satisfac-
tions are expensive and time-consuming to make. 6 Yet, on tory answer to the problem posed by transients.
the basis of very limited testdata, a standard voltage test
wave was derived, the familiar 1.5 ⫻ 40 ␮s wave. Similar THE TRANSIENT CONTROL LEVEL CONCEPT
investigations in other countries led to the establishment in
Europe of the 1 ⫻ 50 ␮s impulse wave. International One way in which transient compatibility might be
standardizing activities have now produced the 1.2 ⫻ 50 ␮s achieved in the electronics industry is to establish a
impulse wave, a test wave used throughout the world for transient coordination system similar in concept to the BIL

122
system, but of a nature more adapted to the requirements of lower limit of 5 kHz might be more typical. 8 Thus, it appears
electronic and control equipment. that the observed transients are not at all typical of lightning
surges propagated directly into the system but are rather the
In this paper, such a concept is called the Transient
response of the power system to an initial excitation caused
Control Level (TCL)* concept. Specifically, it is hereby by a nearby lightning stroke. The internally generated tran-
proposed:
sients due to switching operations typically are of the same
a) That there be defined for electronic equipment (and basic type as those produced by the indirect effects of light-
other low-voltage equipment) a standard transient ning. The observed transients are in each case more nearly
voltage similar in concept to, but different in wave the result of the natural oscillatory response of the local
shape from the 1.2 ⫻ 50 ␮s wave used in coordina- wiring system, in this case the wiring system of typical
tion of insulation in high-voltage power apparatus. residences. Similar surge wave shapes have been encoun-
tered in a wide variety of other systems, ranging from air-
b) That there be defined for electronic (and other low-
planes to space booster rockets. 9, 10 Typical examples of
voltage) equipment a series of TCL’s similar in
concept to the BIL’s. recorded transient wave shapes are given in the Appendix.
The great bulk of the recorded transients exhibit a faster
c) That a start be made on assigning one of these front time and shorter decay time than do the transients
standard levels to individual electronic components produced by lightning on high-voltage power lines, the
and electronic devices. 1.2 ⫻ 50 ␮s type of wave.
d) That individual protective devices be rated in terms Switching transients in air break contacts (internally
of their ability to control transients to levels no generated transients) can produce rise times in the order of
greater than, and preferably lower than, one of the 10 to 100 ns. Although this steepness attenuates rapidly with
above levels. distance, the typical front time is still less than 1.2 ␮s. For
e) That equipment and procedures be developed by some types of devices (rectifier diodes) the wave shape is of
which equipment may be tested by vendors to secondary importance, with only the peak magnitude being
determine which TCL is appropriate to assign to important. For other types of apparatus (inductive devices
individual components and equipment. such as motors), the front time, or more correctly the rate of
change, is of importance equal to that of the peak magnitude.
f) That TCL’s begin to be used in purchase specifica- In still other types of devices (surge protective devices), the
tions. total energy content of the surge is of most importance.
g) That such equipment and procedures be used by
purchasers to evaluate vendor-supplied equipment to Current Wave Shapes and Source Impedances
determine its compliance with such purchase The characteristics of short-circuit current wave shapes
specifications. are less well known than those of open-circuit voltage. The
h) That such TCL’s begin to appear in regulatory short-circuit current is of importance both for evaluation of
specifications for consumer apparatus in which the surge protective devices and for equipment of low input
consumers cannot make the appropriate tests or impedance such as lower voltage semiconductor devices. In
prepare appropriate specifications. any discussion of test wave shapes and test levels, it is
important to recognize the natural response of the device in
Suggested TCL Voltage Wave Shape the test. It is inappropriate to prepare a specification that
implies that a specified voltage must be developed across a
The wave shape suggested for the TCL concept (with the device of low input impedance, such as a spark gap after it
understanding that discussion and presentation of alterna- has broken down, or to seemingly require that a specified
tives is actively encouraged) is shown on Figure 3. Shown short-circuit current be produced through a high input
are both proposed open-circuit voltage and short-circuit impedance, such as the line-to-ground insulation of a relay
current waveforms, since the question of the impedance of coil. The characteristics of short-circuit currents are poorly
the source from which voltage surges derive must ultimately defined because the impedance of the circuits from which
be considered. These shapes are different from the long- transients are produced is poorly defined or unknown.
established 1.2 ⫻ 50 ␮s wave employed in the BIL rating
system for electric power apparatus because none of the For purposes of discussion, it is suggested that
recorded transients exhibited this type of wave shape on two different types of impedance be considered, one
120-volt AC circuits. The type of transient most frequently independent of frequency (resistive source impedance
recorded appeared of an oscillatory nature, very strongly or classical surge impedance, Z = 兹L/C), and one of
damped, and in a frequency range between 100 and simple inductive source impedance. The waveform
500 kHz. shown on Figure 3b assumes a source impedance of
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 resi- * The TCL concept was first proposed by one of the authors (F. A. Fisher)
denial sytems. 7 Other investigations indicate that a in regard to electronic equipment on the Space Shuttle. 12

123
 Fig. 3. Proposed TCL wave shapes.

10 ␮H. 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 mH. Fig. 4. Test circuit for applying spikes on 120-volt. AC lines.

The objective of this design was to super-impose on a

Voltage and Current Levels 120-volt, 60-Hz power line a transient having a rise time to
first peak of 0. 5 uus, followed by a damped ringing at
Central to the success of the BIL system of insulation
100 kHz in which each successive peak is 60% of the
coordination is the fact that only a limited number of BIL’s
preceding peak amplitude. The amplitude of the first peak is
were established, arranged in a generally geometric order of
adjustable f r o m 0 to 8000 volts. The source impedance for
progression. For purposes of discussion, we therefore pro-
the high-voltage transient is 50 ohms.
pose that there be established a series of TCL’s progressing
in the approximate ratio of 3兹10 or 3 values per decade. The 0.5 ␮s rise characteristic is obtained by the series
Such possible TCL’s, as rounded to convenient voltages, resonance of L1 and the capacitance of C1 and C2 in
then appear as shown on Table II. series. Component values were selected to make 兹L/C
approximately 50 ohms, and R1 was selected to provide
The subject of source impedance and short-circuit current heavy damping for a smooth transition to the following
needs to be further discussed since the concept of constant wave.
surge impedance, and particularly constant inductive surge
impedance, may not be valid. Transients of high voltage and The 100 kHz damped ring results from the parallel
large energy content tend to be produced by physically large resonance of L2 with the parallel capacitance of C1 plus C2.
systems, whose inductance tends to be larger than that of the Again, 兹L/C is about 50 ohms. The series damping resistor
systems producing lower voltage or lower energy transients. R2 was selected to produce the decay to 60% amplitude
between successive peaks.
Proof Test Techniques
CONCLUSIONS
The generation of surge voltages in the laboratory is well 1. The present lack of transient coordination methods in
known to manufacturers and users of high power equipment. low-voltage systems does not allow the user of electronic
However, producing a test wave of the shape and levels equipment to obtain the best reliability at lowest cost.
proposed here may present some difftculty for the small
equipment manufacturer. To answer this need, a previously 2. Manufacturers, vendors, and users could bene-
developed circuit 11, as shown in Figure 4, may be applicable. fit from a systematic approach to transient coordina-

124
tion similar in concept to the BIL used for many years (2) P. M. Speranza (Bell Laboratories); Oral Com-
in high-voltage systems. This is illustrated in Figure 5. munication to IEEE-SPD WG Ⲇ3. 4. 4 (Oct. 1974).
3. A concept of Transient Control Level (TCL) is
proposed by the authors. This involves discrete steps of (3) J. E. Lenz, “Basic Impulse Insulation Levels in
withstand level and proof tests based on the capability of Mercury Lamp Ballast for Outdoor Appreciation,
available s urge protective devices and reflecting the occur- Illum. Eng. 133-140 (Feb. 1964).
rence of surges in the real world. (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 commu-

nication (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).
Fig. 5. Well-coordinated low voltage system.
(9) J. A. Plumer, F. A. Fisher, and L. C. Walko,
4. Discussion is earnestly invited on the parameters to “Lightning Effects on the NASA F-8 Digital-Fly-by-
be considered in defining TCL’s such as: Wire Airplane,” Contract NAS 4-2090, NASA
Scientific and Technical Information Facility,
• voltage waveform of the transients College Park, Md. 20740.
• source impedance of the transients
• current waveform of the transients (10) J. A. Plumer, “Lightning Effects on General Avia-
• levels to be assigned — current and voltage tion Aircraft,” Report No. FAA-RD-73-99, National
• proof-test techniques. Technical Information Service, Springfield, VA.
22151.
Successful application of the TCL concept will require
careful stud yof these factors, so as to develop a valid (11) E. K. Howell and F.D. Martzloff, “High Voltage
consensus among all interested parties. Impulse Testers,” Report 75CRD039, General
Electric Company, Corporate Research and
REFERENCES Development Center, Schenectady, N. Y.
(1) F. D. Martzloff and G. J. Hahn, “Surge Voltages in (12) “Space Shuttle Lightning Protection Criteria
Residential and Industrial Power Circuit, “IEEE Document,” NASA, Lyndon B. Johnson Space
Trans. Power Apparatus Systems PAS-89, No. 6, Center, JSC-07636, September 1973.
1049-1056 (July–Aug. 1970).

125
 APPENDIX
TYPICAL WAVE SHAPESS

Fig. A1. Transient recorded during starting of a furnaceblower at service Fig. A3. Transient recorded during unidentified disturbance at service box.
box.

Fig. A2. Transient recorded during lightning storm on street pole. Fig. A4. Composite recording of furnace ignition transformer transients
over 24 hours at service box.

Fig. A5. Typical transients recorded during lightning injection tests on Fig. A6. Typical transients recorded during lightning injection tests on
fighter-type aircraft (amplitudes are relative). small general aviation aircraft (amplitudes are relative).

126
Discussion E.J. Cohen (U.S. Dept. of Agriculture, Washington, D.C.): We feel the con-
cept expressed in this paper is long overdue in the field of electrical protection
S.M. Harvey (Ontario Hydro Research Division, Toronto, Canada): This of electronic equipment. Experience within the telephone industry has already
paper provides a clear presentation of the case for a transient interference demonstrated that, with present trends to ever smaller equipment, protection
immunity standard applicable to residential and, presumably, light commercial problems can be severely aggravated. The over voltage and current tolerance
electronic equipment. Designing transient or surge withstand compatibility into of microelectric circuits has decreased to the point where protection should be
low-voltage equipment is not, of course, a new concept. The telephone compa- major consideration in circuit design.
nies have been doing it for years. However, the authors have commendably Added to this increased equipment vulnerability, we have found a
proposed their Transient Control Level concept in the context of a general and .,communications gap” between the manufacturers of electronic equipment,
down to earth philosophy of testing that should encourage informed discussion. and the producers of protection devices. When a protection defect is uncovered,
Following the establishment of Basic Insulation Levels, the electric power we frequently encounter disagreements between the equipment and arrester
industry has not been idle in the area of overvoltage testing of low-voltage manufacturers. By establishing “Transient Control Levels,” as proposed by this
equipment. A number of committees, including the Power System Relaying paper, much of this “finger pointing” could be eliminated. As both equipment
Committee of the IEEE Power Engineering Society and Technical Committee and arrester manufacturers -should know precisely what the other adequate
No 41 of the International Electrotechnical Commission have been working for protection should be minimized.
years on the surge testing of static relays used for transmission line protection. It is felt that while the concept expressed here is valid, further consideration
The Swedish Electrical Commission has prepared a draft proposal for interfer- should be given to the levels and waveshapes involved in the tests. As these
ence withstand capability testing of apparatus used in power stations and parameters may be critical to the workability of this proposal, every effort
industrial installations. These committees have proposed a range of test wave- should be made to generate realistic values.
forms including the familiar 1.2/50 impulse at peak voltages of 1, 3, and 5 kV,
a moderately damped 1 MHz oscillatory wave at peak voltages of 0.5, 1, and Manuscript received August 13, 1975.
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 speci-
fication for relays and other equipment intended for substation relay or control
buildings. The test waveform is a moderately damped oscillatory transient Richard F. Hess (Sperry Flight Systems, Phoenix, Arizona): I agree that some
whose frequency ’can be specified in the range of 100 kHz to 2 MHz. One of form of action is needed to properly assess and overcome the adverse effects
four test levels, specified in Table I, can be called for. The test is supervised of power transients on military and commercial equipment. Assuming a con-
by our Supply Division and manufacturers are encouraged to supply their own sensus is reached concerning the need for transient control and the adoption of
test equipment. However, it is still frequently necessary for Ontario Hydro to Transient Control Levels (TCL), the following comments are intended to com-
make its own test generators available. plement the proposal for transient control in low voltage systems.
The voltage specification is based upon measurements which are appropriate
Table I to present and past equipment designs. For the most part these designs use
Transient Test Levels devices which present a relatively high impedance to a source of transient
Test Peak Amplitude (Volts) Source Impedance (ohms) energy.
A 5000 100-500 Damage occurs during a power transient when the device breaks down and
B 2500 100-150 high to medium voltages are developed across the device while large to
C 1000 30-50 medium currents are flowing through it. Standard components are not normally
D 500 30-50 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
Note that these levels when specified at I 00 kHz are very similar to tests 6 and survive such a transient. When a device breaks down, either a voltage or a
9 in Table II of the present paper. Level B, incidentally, when specified at I current viewpoint could be assumed when describing the threat of the power
MHz is equivalent to the IEEE Relay Test [1]. transient to the device.
Our experience with the tests, although limited, suggests that minor circuit If in order to conform to a specified TCL a device has been designed to
deficiencies leading to operational upsets are common but that damage is withstand a specified voltage level, then the voltage specification is appropri-
relatively rare. Probably the marginally greatest value of the tests at this time ate. However, a manufacturer designing equipment to meet a specific TCL
lies in their potential for creating an awareness of the transient problem. could adopt an approach which calls for the use of transient power suppression
A number of questions being considered at this stage of our transient test devices (tranzorbs, metal oxide varistors, etc). In this case, transient power
program can be rephrased to apply also to the proposals in this paper. Perhaps surges are manifested as large current surges into equipment (through the
the authors could comment on the following: protection device) rather than a large voltage transient across the equipment.
1. What is the advisability of introducing a new test waveform or test Even when passing large currents, the network impedances (suppression
procedure in addition to those already in circulation? devices, etc.) will probably be significant enough to produce a natural mode
2. Would it be necessary to shield the test circuit of Fig. 4 or to locate it, current response within the total network. Thus, current measurement of such
say, 4-6 meters from the equipment under test? In the latter case, should the a network would contain a significant oscillatory component similar to that
voltage and current waveforms be measured at the near end or the far end of present in the voltage measurements.
the connecting cable? Two types of TCL specifications should be provided:
3. Can the test circuit of Fig. 4 correctly simulate transient disturbances 1. Voltage
that occur when the white wire neutral and the green wire ground are connected 2. Current
together a quarter wavelength from the device under test? Like the voltage specification, the waveform and magnitude of the current
4. Can a reliable certification procedure, particularly in terms of energy specification at each TCL would be based upon the measurement of the current
deliverable to a load, be established for test generators differing in design from response modes of networks containing power suppression devices and excited
the one shown? by a power transient.
5. Finally, what is the incidence of damage or significant upset to equip- With the two types of specifications, equipment could be designed and tested
ment now used in resident at or light commercial environments and does it to withstand a power transient by safely withstanding specified voltage levels
justify the introduction of transient testing to this class o apparatus? If applied, or by safely passing specified currents levels. The test equipment for, the
in view of the data contained in Fig. 2 of the paper, what criterion would be voltage specification would be calibrated under open circuit conditions and
used to select a test level of less than, say 500 volts? 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
REFERENCES equipment for the current specification would be calibrated under short
circuit conditions and would be designed to provide voltage (in the event of a
[1] ANSIC37.9Oa-1974(IEEE Std 472-1974) high impedance) at a level at least as large as that specified in the voltage
Guide for Surge Withstand Capability (SWC) Tests. specification.

Manuscript received August 13, 1975. Manuscript received August 14, 1975.

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 Tests for semiconductor vulnerability (damage) levels using square pulse achieved through the successful passing of even an imperfect test than it is in
waveform are common practice with the military. The damage level of many the avoidance of all but perfect tests.
discrete components has been determined an recorded. However, the damped We hold no special faith in the virtues of the test circuit shown on Figure 4
sinusoid pulse is more appropriate to susceptibility testing (transient upset). of the paper and show it only as one example of various test circuits that might
Depending upon the type of equipment being tested and the frequency content be produced. We feel that a reliable certification procedure not only can be, but
of expected transients, it may be desirable to test using more than one wave- must be, based on specifications that are not unique to any one test circuit. It
form. lower frequency, high amplitude sinusoid (100 KHz) would be used to is for this reason that we propose specifications be written in terms of open-cir-
vulnerability testing and a higher frequency sinusoid (500 KHz, 1 MHz or cuit voltage and short-circuit currents; a concept that implies a fixed generator
10 MHz depending upon the bandwidth of the equipment) would be used for impedance. Care must be taken that the voltage and current specifications not
susceptibility testing. At each frequency the equipment shoul be subjected to at be incompatible with the generator impedance. Since the writing of this paper
least two pulses: another paper discussing the impedance of AC wiring circuits has been pub-
1. Maximum pulse is positive lished [1]. Based on this paper, we would now propose that the internal
2. Maximum pulse is negative impedance of a transient generator be 50 ohms paralleled by 50 microhenries.
As a final observation, testing and test equipment should be kept a simple as Figure 1, reproduced from the referenced paper with the permission of the
possible to avoid adding inordinate costs to the equipment ideally, the degree author, shows how the impedance of the line (“the mains”) can be closely
of confidence obtained by such testing should result in a net reduction in approximated by the parallel combination of 50 ohms and 50 microhenries.
equipment costs (manufacture plus maintenance). 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
F.A. Fisher and F. D. Martzloff: We appreciate the response of the discussors Interrupter, it might impose an unnecessary economic hardship on a high
and will attempt to both respond to their questions and expan somewhat on the volume item intended for routine household use. Likewise, while a TCL of 500
protection philosophy we propose. First of all, it should be pointed out that volts might be too low for residential purposes, it might be appropriate for the
while this paper was written using household appliances as an example and power inputs of electronic equipment used in aircraft, and excessively high for
presented before a group largely concerned with utility relaying, the problems the signal inputs of data processing equipment intercommunicating through
of transients pervade the entire field of low voltage electrical and electronic well-shielded signal wires.
apparatus, including the communication (telephone) industry. One of the areas Since of the major purposes of this paper is to promote discussion, it is
where th authors have seen a great need for better transient compatibility is i appropriate to list some of the questions the authors have posed to themselves
the Aerospace field. Much of the background upon which the TCL concept is during the formulation of this proposal:
based comes from consideration of the transients induced in aerospace vehicles
by lightning and other energetic discharges. Designers in the Aerospace com-
munity 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 th 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 waveshapes. We
suggested the voltage waveshape of Figure 3 of the pape because measure-
ments 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 ⫻ 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 duratio 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 burn out
becomes strongly influenced by waveshape. Another is that transients of this
nature can be injected into wires by rather simple transformer-coupled pulse-in-
jection generators, whereas transformer injection of higher frequency oscilla-
tory 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 Fig. 1. Comparison of impedance measurements made by the Electrical
need for two types of TCL specifications: voltage and current. We agree. We Research Association (ERA) on the impedance of power systems with a net-
have seen instances of groups worrying wastefully about specifications that call work of 50 ohm & 50 ␮H in parallel
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 mis-
chievous 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 Fig. 2. Short-circuit current (I SC) resulting from a transient source with VOC
open-circuit voltage and 50 ⍀/ /50 ␮H source impedance.
Manuscript received October 10 1975.

128
 TABLE 1 — Would transient control level (or some other) specifications and standards
help achieve successful transient coordination between equipment manufac-
Proposed Transient Open-Circuit Short-Circuit turers, utilities and equipment users?
Control Level Voltage Level Current Level — Should there be a limited number of fixed levels? The authors feel that it is
Number (volts) (amperes) essential that the number of levels be limited, perhaps to 9-15 levels dis-
tributed in a geometric progression over the range 10-5000 volts. The
1 10 0.68
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
2 25 1.7
with resistor and capacitor values produced according to an arbitrarily
selected geometric progression.
3 50 3.4 — Should these levels reflect the system voltage, the expected reliability of the
equipment function, the environment?
4 100 6.8
— What kind of source impedance is appropriate? As mentioned above, an
impedance of 50 ohms paralleled by 50 microhenries may be appropriate.
5 250 17 — Should open-circuit voltage and impedance be stated or, alternatively,
should open-circuit voltage and short-circuit current be specified?
6 500 34
— Is one impedance value suitable for the majority of the systems?
— What waveshape is appropriate, for voltage as well as current? For damage,
7 1000 68 we are mostly concerned with energy and front-ofwave but if upset (interfer-
ence) is to be included in TCL, then do we need to specify a frequency
8 2500 170
spectrum?
9 5000 340
REFERENCE
— Are there sufficient problems relating to transient coordination to warrant an
effort, likely to be major and long term, to achieve better coordination
[1] “Impedance of the Supply Mains at Radio Frequencies”, J. H. Bull,
between the transients to which equipment is exposed, and the ability of Proceeding of 1st Symposium on EMC, Montreux, May 1975.
equipment to withstand such transients?

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