Transient Control Levels:

A Proposal for 

Insulation Coordination in Low-Voltage Systems

F.A. Fisher

General Electric Company

Pittsfield MA

fafisher@lightningtech.cpm

François Martzloff

General Electric Company

Schenectady NY

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.
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 lowvoltage 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).

Paper F 75 466-3, recommended and approved by the IEEE Surge

Protective Devices Committee of the IEEE Power Engineering Society for

presentation at IEEE PES Summer Meeting, San Francisco, Calif., July

20-25, 1975. Manuscript submitted February 3, 1975; made available for

printing April 28, 1975.

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.

The purpose of this paper is to promote a concep

transient coordination for electronic and other low-volt

equipment through the establishment of a system of Tr

sient Control Levels (TCL’s), similar to the concept 

BIL’s so successfully used for many years in the elec

power industry. In the following sections, specific sugg

tions for possible standard Transient Control Levels 

standard test wave shapes will be made. While the wa

forms here suggested are chosen somewhat arbitrarily, t

are well grounded in physical reality. The purpose of m

ing such suggestions is to promote wide discussion a

whether these waveforms and levels are the best that can 

developed, or if indeed the establishment of such standa

is the best way to promote good transient coordination 

the electronics industry. The ultimate purpose of any sys

of transient coordination would be to achieve gre

product reliability at minimum cost to the user.

AN EXAMPLE OF THE PROBLEM

TCL concepts would be of benefit to all users 

electronic and other low voltage equipment, such 

railroad, telephone, power, oil industry, aircr

and high frequency communications. The source 

transients to which equipment is exposed may be ei

external (lightning and power system switching) or 

ternal (switching of inductive loads, contactor restri

or cross talk from adjacent circuits). While the c

cept of TCL’s is intended to apply to the full sp

trum of frequencies and voltages (DC, 120 V, 60 

AC, 400 Hz) the problem of transient coordination 

Fig. 1. The present situation.

120
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 transients1 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 study2 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 experience1 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 study3 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’ Laboratory4

, when correlated with the known

nuisance trip level of the devices and the observed number

of trips5

, 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

Fig. 2. Exposure of residential circuits to surge (Number of surge

highest surge at any one location)

that transients of a low level interfere with the op

tion of the mini-computer, causing it to give incor

results without causing permanent physical dama

The vulnerability level of such a mini-computer w

be higher than the susceptibility level. Both lev

must be higher than the normal operating level of 

computer logic elements or input/output terminals.

The transient breakdown level or vulnerability of se

conductors is not presently a part of any industry accep

rating system. The vulnerability level is furthermore 

inherently related to the normal operating voltage or p

inverse voltage (PIV) level. As examples, consider the d

of Table I. During this investigation, power diodes w

subjected to unidirectional transient voltages cresting i

few microseconds. The voltages at which failure occur

are seen to have little correlation to the nominal PIV rat

Similar data have been accumulated for many se

conductors, particularly when semiconductors 

exposed to very short transients, characteristic 

those produced by nuclear weapons (NEMP). Such 

formation has not been widely reported.

TABLE I

Transient Vulnerability Levels

Typical 1A Silicon Diodes

Diode PIV Failure Level Unde

Number Rating Reverse Impulse*

Volts Volts

1 200 1100 – 1500

2 400 1400 – 1500

3 600 1400 – 1600

*Breakdown observed when exposed to a unidirectional surge r

at 1000 volts per microsecond.

121
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 INDUSIRY

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.6 Yet, on

the basis of very limited testdata, a standard voltage test

wave was derived, the familiar 1.5  40 s wave. Similar

investigations in other countries led to the establishment in

Europe of the 1  50 s impulse wave. International

standardizing activities have now produced the 1.2  50 s

impulse wave, a test wave used throughout the world for

coordination of insulation protection. It was never p

tended, however, that naturally occurring surges were of 

type, only that the rise and fall times of the natural sur

were in the vicinity of the above values.

The next stage in the process of insulation coordina

was the establishment of a series of standard test and des

levels, BIL’s. For example, equipment designed for op

tion on 115-kV systems was assigned a BIL of 550 kV. 

designer of equipment to be used on 115 kV systems t

was required to provide an insulation structure that wo

withstand 550 kV. The level of 550 kV was derived on 

premise that existing lightning arresters could be used 

control the transients applied to that apparatus to less t

550 kV. The proper design of the insulation system was n

demonstrated by subjecting the apparatus in the laborat

to a surge of 1.5  40 s wave shape and a peak amplit

of 550 kV . Frequently it was part of the purchase agreem

that the equipment had to successfully pass the laborat

test. If the equipment failed, it had to be rebuilt or 

designed. Conversely, it became the responsibility of 

user to insure that no surge greater than 550 kV was e

applied to the apparatus.

As a result, power equipment achieves its resistance 

lightning-induced transients not so much by being desig

to the threat that might be posed by lightning, but by 

threat that will be posed by an acceptance test. This acc

tance test does not subject the equipment to transients h

ing the complex wave shapes produced by lightning, 

instead to transients having elementary wave shapes that 

be produced by basically simple test apparatus. Neither d

the acceptance test subject the equipment to transients of 

amplitude produced by lightning. However, it subjects 

equipment to transients of amplitude consistent with 

capabilities of existing surge-protective devices.

These amplitudes, the BIL’s while assigned somew

arbitrarily, were (and are) kept in touch with reality by 

fact that equipment designed in accordance with standa

does not fail when exposed to surges produced by lightn

in contrast to equipment designed prior to the developm

of the philosophy of insulation coordination and the es

lishment of standard BIL’s.

The test and design levels, the BIL’s, are not necessa

fixed. As better protective devices are developed, the lev

may be lowered so that reliable equipment can be buil

lower cost.

Electronic and control equipment, on the other hand, i

too often designed, built, and delivered before the existe

of a transient threat is recognized. If transients turn ou

endanger the equipment, there may be no adequate su

protective devices. There may, in fact, not be any satis

tory answer to the problem posed by transients.

THE TRANSIENT CONTROL LEVEL CONCEP

One way in which transient compatibility might 

achieved in the electronics industry is to establis

transient coordination system similar in concept to the B

122
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. Specifically, it is hereby

proposed:

a) That there be defined for electronic equipment (and

other low-voltage equipment) a standard transient

voltage similar in concept to, but different in wave

shape from the 1.2  50 s wave used in coordination of insulation in high-voltage power apparatus.

b) That there be defined for electronic (and other lowvoltage) 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 longestablished 1.2  50 s 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 residenial sytems. 7 Other investigations indicate that a

lower limit of 5 kHz might be more typical.8 Thus, it appe

that the observed transients are not at all typical of lightn

surges propagated directly into the system but are rather 

response of the power system to an initial excitation cau

by a nearby lightning stroke. The internally generated tr

sients due to switching operations typically are of the sa

basic type as those produced by the indirect effects of li

ning. The observed transients are in each case more ne

the result of the natural oscillatory response of the lo

wiring system, in this case the wiring system of typ

residences. Similar surge wave shapes have been enco

tered in a wide variety of other systems, ranging from 

planes to space booster rockets.9, 10 Typical examples 

recorded transient wave shapes are given in the Appen

The great bulk of the recorded transients exhibit a fa

front time and shorter decay time than do the transie

produced by lightning on high-voltage power lines, 

1.2  50 s type of wave.

Switching transients in air break contacts (intern

generated transients) can produce rise times in the orde

10 to 100 ns. Although this steepness attenuates rapidly w

distance, the typical front time is still less than 1.2 s. 

some types of devices (rectifier diodes) the wave shape i

secondary importance, with only the peak magnitude be

important. For other types of apparatus (inductive dev

such as motors), the front time, or more correctly the rat

change, is of importance equal to that of the peak magnitu

In still other types of devices (surge protective devices), 

total energy content of the surge is of most importan

Current Wave Shapes and Source Impedances

The characteristics of short-circuit current wave sha

are less well known than those of open-circuit voltage. 

short-circuit current is of importance both for evaluation 

surge protective devices and for equipment of low in

impedance such as lower voltage semiconductor devices

any discussion of test wave shapes and test levels, i

important to recognize the natural response of the devic

the test. It is inappropriate to prepare a specification 

implies that a specified voltage must be developed acro

device of low input impedance, such as a spark gap afte

has broken down, or to seemingly require that a speci

short-circuit current be produced through a high in

impedance, such as the line-to-ground insulation of a re

coil. The characteristics of short-circuit currents are po

defined because the impedance of the circuits from wh

transients are produced is poorly defined or unknown.

For purposes of discussion, it is suggested t

two different types of impedance be considered, 

independent of frequency (resistive source impeda

or classical surge impedance, Z = L/C), and one 

simple inductive source impedance. The wavefo

shown on Figure 3b assumes a source impedance 

* The TCL concept was first proposed by one of the authors (F. A. Fi

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.

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 3

10 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 difftculty for the small

equipment manufacturer. To answer this need, a previously

developed circuit11, as shown in Figure 4, may be applicable.

Fig. 4. Test circuit for applying spikes on 120-volt. AC lines.

The objective of this design was to super-impose o

120-volt, 60-Hz power line a transient having a rise tim

first peak of 0. 5 uus, followed by a damped ringing 

100 kHz in which each successive peak is 60% of 

preceding peak amplitude. The amplitude of the first pea

adjustable f r o m 0 to 8000 volts. The source impedance 

the high-voltage transient is 50 ohms.

The 0.5 s rise characteristic is obtained by the se

resonance of L1 and the capacitance of C1 and C2 

series. Component values were selected to make L

approximately 50 ohms, and R1 was selected to prov

heavy damping for a smooth transition to the follow

wave.

The 100 kHz damped ring results from the para

resonance of L2 with the parallel capacitance of C1 plus 

Again, L/C is about 50 ohms. The series damping resi

R2 was selected to produce the decay to 60% amplit

between successive peaks.

CONCLUSIONS

1. The present lack of transient coordination method

low-voltage systems does not allow the user of electro

equipment to obtain the best reliability at lowest cost.

2. Manufacturers, vendors, and users could be

fit from a systematic approach to transient coordi

124
Fig. 5. Well-coordinated low voltage system.

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 stud yof 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).

nication (1972).

(6) IEEE Committee Report, “Bibliography on Su

Voltages in AC Power Circuits Rated 600 Volt

Less,” IEEE; Trans. Power Apparatus Syste

PAS-89, No. 6, 1056-1061 (July–Aug. 1970).

(7) H. A. Gauper Jr., General Electric Co.; o

communication to NEMA, Ground Fa

Protection Section (Aug. 1973).

(8) P. M. Speranza and L. H. Sessler (Bell Laboratori

oral communication to NEMA, Ground Fa

Protection Section (Aug. 1973).

(9) J. A. Plumer, F. A. Fisher, and L. C. Wal

“Lightning Effects on the NASA F-8 Digital-Fly-

Wire Airplane,” Contract NAS 4-2090, NA

Scientific and Technical Information Facil

College Park, Md. 20740.

(10) J. A. Plumer, “Lightning Effects on General A

tion Aircraft,” Report No. FAA-RD-73-99, Natio

Technical Information Service, Springfield, V

22151.

(11) E. K. Howell and F.D. Martzloff, “High Volt

Impulse Testers,” Report 75CRD039, Gene

Electric Company, Corporate Research 

Development Center, Schenectady, N. Y.

(12) “Space Shuttle Lightning Protection Crite

Document,” NASA, Lyndon B. Johnson Sp

Center, JSC-07636, September 1973.

125
TYPICAL WAVE SHAPESS

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

box.

Fig. A2. Transient recorded during lightning storm on street pole. Fig. A4. Composite recording of furnace ignition transformer transi

over 24 hours at service box.

Fig. A5. Typical transients recorded during lightning injection tests on

fighter-type aircraft (amplitudes are relative).

Fig. A6. Typical transients recorded during lightning injection test

small general aviation aircraft (amplitudes are relative).

126
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 1 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 I, 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

Test Peak Amplitude (Volts) Source Impedance (ohms)

A 5000 100-500

B 2500 100-150

C 1000 30-50

D 500 30-50

Note that these levels when specified at I 00 kHz are very similar to tests 6 and

9 in Table II of the present paper. Level B, incidentally, when specified at I

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

[1] ANSIC37.9Oa-1974(IEEE Std 472-1974)

Guide for Surge Withstand Capability (SWC) Tests.

Manuscript received August 13, 1975.

E.J. Cohen (U.S. Dept. of Agriculture, Washington, D.C.): We feel th

cept expressed in this paper is long overdue in the field of electrical pro

of electronic equipment. Experience within the telephone industry has a

demonstrated that, with present trends to ever smaller equipment, pro

problems can be severely aggravated. The over voltage and current tol

of microelectric circuits has decreased to the point where protection sho

major consideration in circuit design.

Added to this increased equipment vulnerability, we have fo

.,communications gap” between the manufacturers of electronic equip

and the producers of protection devices. When a protection defect is unco

we frequently encounter disagreements between the equipment and a

manufacturers. By establishing “Transient Control Levels,” as proposed 

paper, much of this “finger pointing” could be eliminated. As both equi

and arrester manufacturers -should know precisely what the other ad

protection should be minimized.

It is felt that while the concept expressed here is valid, further conside

should be given to the levels and waveshapes involved in the tests. As 

parameters may be critical to the workability of this proposal, every 

should be made to generate realistic values.

Manuscript received August 13, 1975.

Richard F. Hess (Sperry Flight Systems, Phoenix, Arizona): I agree tha

form of action is needed to properly assess and overcome the adverse 

of power transients on military and commercial equipment. Assuming 

sensus is reached concerning the need for transient control and the adop

Transient Control Levels (TCL), the following comments are intended to 

plement the proposal for transient control in low voltage systems.

The voltage specification is based upon measurements which are appro

to present and past equipment designs. For the most part these desig

devices which present a relatively high impedance to a source of tra

energy.

Damage occurs during a power transient when the device breaks dow

high to medium voltages are developed across the device while la

medium currents are flowing through it. Standard components are not no

tested under transient conditions, therefore it may be difficult to dete

whether they would break down or to assign a confidence level that they 

survive such a transient. When a device breaks down, either a voltag

current viewpoint could be assumed when describing the threat of the 

transient to the device.

If in order to conform to a specified TCL a device has been desig

withstand a specified voltage level, then the voltage specification is app

ate. However, a manufacturer designing equipment to meet a specifi

could adopt an approach which calls for the use of transient power suppr

devices (tranzorbs, metal oxide varistors, etc). In this case, transient 

surges are manifested as large current surges into equipment (throu

protection device) rather than a large voltage transient across the equip

Even when passing large currents, the network impedances (suppr

devices, etc.) will probably be significant enough to produce a natural 

current response within the total network. Thus, current measurement o

a network would contain a significant oscillatory component similar 

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 c

specification at each TCL would be based upon the measurement of the c

response modes of networks containing power suppression devices and e

by a power transient.

With the two types of specifications, equipment could be designed and 

to withstand a power transient by safely withstanding specified voltage 

or by safely passing specified currents levels. The test equipment fo

voltage specification would be calibrated under open circuit conditio

would be designed to deliver current (in the event of device breakdow

level at least as large as that specified in the current specification. T

equipment for the current specification would be calibrated under 

circuit conditions and would be designed to provide voltage (in the eve

high impedance) at a level at least as large as that specified in the v

specification.

Manuscript received August 14, 1975.

127
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 waveform. lower frequency, high amplitude sinusoid (100 KHz) would be used to

vulnerability testing and a higher frequency sinusoid (500 KHz, 1 MHz or

10 MHz depending upon the bandwidth of the equipment) would be used for

susceptibility testing. At each frequency the equipment shoul 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 expan somewhat on the

protection philosophy we propose. First of all, it should be pointed out that

while this paper was written using household appliances as an example and

presented before a group largely concerned with utility relaying, the problems

of transients pervade the entire field of low voltage electrical and electronic

apparatus, including the communication (telephone) industry. One of the areas

where th authors have seen a great need for better transient compatibility is i

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 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 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  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-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 shortcircuit 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

Manuscript received October 10 1975.

achieved through the successful passing of even an imperfect test than 

the avoidance of all but perfect tests.

We hold no special faith in the virtues of the test circuit shown on Fi

of the paper and show it only as one example of various test circuits that 

be produced. We feel that a reliable certification procedure not only can 

must be, based on specifications that are not unique to any one test cir

is for this reason that we propose specifications be written in terms of op

cuit voltage and short-circuit currents; a concept that implies a fixed gen

impedance. Care must be taken that the voltage and current specificatio

be incompatible with the generator impedance. Since the writing of this 

another paper discussing the impedance of AC wiring circuits has bee

lished [1]. Based on this paper, we would now propose that the in

impedance of a transient generator be 50 ohms paralleled by 50 microh

Figure 1, reproduced from the referenced paper with the permission 

author, shows how the impedance of the line (“the mains”) can be c

approximated by the parallel combination of 50 ohms and 50 microh

Levels and waveshapes appropriate to such an impedance might then app

shown in Figure 2 and Table 1.

As Messrs. Cohen, Harvey and Hess emphasize, the choice of appro

levels is crucial to the successful implementation of a TCL philosophy. 

a TCL of 5000 or 6000 volts might be appropriate to high reliability 

relays or a safety-oriented consumer product such as the Ground Fault C

Interrupter, it might impose an unnecessary economic hardship on 

volume item intended for routine household use. Likewise, while a TCL 

volts might be too low for residential purposes, it might be appropriate 

power inputs of electronic equipment used in aircraft, and excessively h

the signal inputs of data processing equipment intercommunicating th

well-shielded signal wires.

Since of the major purposes of this paper is to promote discussion

appropriate to list some of the questions the authors have posed to them

during the formulation of this proposal:

Fig. 1. Comparison of impedance measurements made by the Electrica

Research Association (ERA) on the impedance of power systems with 

work of 50 ohm & 50 H in parallel

Fig. 2. Short-circuit current (ISC) resulting from a transient source with 

open-circuit voltage and 50 / /50 H source impedance.

128
TABLE 1

Proposed Transient Open-Circuit Short-Circuit

Control Level Voltage Level Current Level

Number (volts) (amperes)

1 10 0.68

2 25 1.7

3 50 3.4

4 100 6.8

5 250 17

6 500 34

7 1000 68

8 2500 170

9 5000 340

— 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 sta

help achieve successful transient coordination between equipment ma

turers, utilities and equipment users?

— Should there be a limited number of fixed levels? The authors feel th

essential that the number of levels be limited, perhaps to 9-15 leve

tributed in a geometric progression over the range 10-5000 volt

assignment of the levels may have -to be done arbitrarily. This need 

cause for alarm. The electronic industry for years has worked succe

with resistor and capacitor values produced according to an arb

selected geometric progression.

— Should these levels reflect the system voltage, the expected reliability 

equipment function, the environment?

— What kind of source impedance is appropriate? As mentioned abo

impedance of 50 ohms paralleled by 50 microhenries may be appro

— Should open-circuit voltage and impedance be stated or, alterna

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 da

we are mostly concerned with energy and front-ofwave but if upset (in

ence) is to be included in TCL, then do we need to specify a freq

spectrum?

REFERENCE

[1] “Impedance of the Supply Mains at Radio Frequencies”, J. H

Proceeding of 1st Symposium on EMC, Montreux, May 1975.

129