François Martzloff

General Electric Company

Schenectady NY

f.martzloff@ieee.org

F.A. Fisher

General Electric Company

Pittsfield MA

fafisher@lightningtec.com

Reprinted, with permission, from 

Proceedings, 2nd International Symposium on Electromagnetic Compatibility, Montreux, 1977

Significance:

Part 2 – Development of standards – Reality checks

Part 5 – Monitoring instruments, laboratory measurements and test methods

Part 6 – Textbooks and tutorial reviews

Presentation to the EMC community in a European forum of the Transient Control Level concept being proposed in

the US via the IEEE Power Engineering Society (See Fisher and Martzloff in IEEE Transactions PAS 95, 1976).

A companion paper on implementation is reprinted in Parts 5 and 6 (See Fisher and Martzloff in the same forum).

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.

Filename: TCL Philosophy
TRANSIENT CONTROL LEVEL PHILOSOPHY AND IMPLEMENTATION 

I. The Reasoning Behind the Philosophy 

F.D. Martzloff and F.A. Fisher 

General Electric Company, Corporate Research and Development, 

Schenectady, New York, and Pittsfield, Massachusetts 

Abstract 

This is the first of a pair of papers 

describing how better transient protection might 

be achieved through the use of a Transient Control Level (TCL) philosophy. The authors have 

developed and are proposing this TCL philosophy 

because damage to and upset of electronic and 

other low-voltage equipment by transients seems 

to be a never-ending problem, and one that is 

likely to get worse in the future as electronic 

controls permeate even more of the products which 

affect our lives. A number of proposals have 

been made - some already incorporated into standards - on various test wave shapes and specifications. The authors propose an approach integrating many of these proposals while focusing 

attention on significant parameters. 

Introduction 

An area where present standards do not seem 

to offer sufficient guidance to designers and 

manufacturers of electronic equipment is in what 

types of transients to consider and how to prove 

that equipment works in the presence of transients. This situation is perhaps under better 

control in the electric power field than it is in 

the fields of aerosapce, general indu-stry, housewares, and the military. For instance, the insulation of high-voltage apparatus is coordinated 

to the threats that nature provides to that 

insulation through the philosophy of insulation 

coordination as expressed in the Basic Insulation 

Level (BIL) system. The BIL system provides for 

a standardized series of levels being coordinated 

with the protective abilities of existing protective devices. On the other hand, electronic and 

control equipment is all too often designed, 

built, and delivered before the existence of a 

transient threat is recognized. If transients 

turn out to endanger the equipment, there may be 

no adequate surge protective devices. In fact, 

there may not be any satisfactory answer to the 

problem posed by transients. 

The authors1 TCL philosophy is aimed at 

achieving better coordination than now exists 

between the transients to which equipment is 

exposed and the abilities of equipment to withstand the transients. It is patterned after the 

BIL approach to insulation coordination so successfully used in the electric power field. 

The purposes of this first paper are t

explain the reasoning behind the different ele

ments of the BIL system of insulation coordina

tion, and to explain how similar reasoning ha

led to the formulation of the TCL philosophy

Some observations on how to perform TCL tests ar

given in a companion paper [I]. 

Proposal for TCL 

This proposal can be summarized by sayin

that we want to: 

Establish the concept that equipment shall b

rated in terms of its ability to withstand 

limited set of transient proof tests, rathe

than in terms of its ability to withstan

unknown "actual" transients. 

Establish the concept that transient specif

cations apply to power and signal lines. 

the past, only power mes have been con

s i dered . 

Establish a set of levels (limited in numbe

to which equipment is designed and tested. 

Establish a set of standard test waves (lim

ited in number) to which low-voltag

equipment will be subjected. 

Establish standardized relationships betwee

voltage and current (source impedance). 

Differentiate between the task of establish

ing the family of test levels and wa

shapes, and the task of actually selecting 

specific level. This means that: 

a We will propose to you a family 

levels and wave shapes 

0 You will select the specific level a

shape, based on your reliability goals

your costs, and your experience. 

This ~ro~osal ismade with awareness th

it may be one &re of an already confused array 

standards. However, if accepted by a large se

tion of industry and users, it could become 

unifying link and make the applications mo

successful. 
In the following paragraphs, we will attempt to present the background justifying our 

proposal, for each of the points listed above. 

1. Basis for ratinq equipment 

The concept that equipment be rated in 

terms of its ability to withstand a standard test 

rather than "actual" service conditions is not 

new. This is at the very heart of the system of 

BIL, which has been so successful in the field of 

electric utility equipment. 

Fortunately for the utilities, few parties 

were involved in making the decisions, and thus 

it was possible at an early stage to establish 

the BIL system and to enforce it because of the 

near total control of the engineering department 

of a utility over the system design. In the field 

of low-voltage systems, however, the selection 

and purchase of a multiplicity of components and 

equipment by a multiplicity of buyers from a 

multiplicity of vendors on behalf of a multiplicity of users have made it very difficult to 

maintain the organized systems approach which 

succeeded in the case of the electric utilities. 

A basic concept, which needs to be mutually 

accepted by users and manufacturers of equipment, 

is that it is impossible to simulate all possible 

transient overvoltages (and over-currents) that a 

product line might experience in service. However, by designing the equipment to a certain 

standard and controlling the level of transients 

by suitabrprotection, a much greater chance of 

successful operation in the cruel real world will 

be obtained. 

The task is then to establish a set of 

standard tests, acceptable to the vast majority 

of applications, reflecting the real world but 

not pretending to duplicate it, simple enough to 

be practical, conservative enough to ensure reliability, but realistic in terms of economics. 

Obtaining complete agreement from all is 

most unlikely an impossible goal, and thus the 

unsatisfactory situation endures. This stalemate 

can be broken by accepting a proposal which might 

not be perfect, but is better than many isolated 

standards or no standard at all. 

2. All lines subject to transient tests 

The existence of transients on power lines 

is by now a recognized and accepted fact, so that 

most applications will involve a certain amount 

of precautions in specifying transient withstand 

capability. However, in the case of signal 

lines, this recognition is less frequent, and 

there have been examples where a total lack of 

appreciation of the problem has led to the design 

and deployment of equipment that cannot be protected from transients. 

Transients can be introduced into a piece 

of equipment by the power lines from many 

sources, such as lightning, switching transients, 

fault clearing, and coupling from adjacent circuits. Signal lines, especially in the case of 

extensive systems covering a vast area, can also 

be subjected to induced transients by lightning, 

adjacent circuits, ground currents, etc. Since 

quite often the signal circuits tend to be 

lower voltage than the power circuits, the 

crepancy between the rated level in the cir

and the actual level of transients makes 

signal circuits more susceptible to trans

problems. 

A question related to which lines are 

subjected to transients is that of "comnon m

versus "transverse mode." This is not al

clear and must be addressed in a comprehen

specification. 

3. Test Levels 

An important feature of the BIL system 

that it involved a limited number of test le

graded to the operating voltage of the system 

which apparatus was being designed. A succes

TCL system should also be designed around a r

tively small number of levels. One who trie

establish levels is pulled in two directions

to avoid complexity by establishing a min

number of levels, and in another to pro

levels that accomnodate existing practices 

minimum disruption. 

One way to achieve this is through the 

of major and minor intervals in the lev

Figure 1 shows several possible level ser

The scales show the range 30 to 3000 volts di

ed into intervals based on 10lb, and 

The physical positioning of the numbers on 

figure shows how those numbers match the pro

tionate interval scales. In the past, we 

proposed that there be three levels per de

with the s acing between levels being appr

mately lo1/. The factors 1.5, 3, and 6 

appropriate, particularly since such a set c

include the voltage levels 600, 1500, and 

volts in some existing specifications. 

widely used specification MIL-704 includes 

600-volt level for transients, and it w

appear that this number, at least, should ap

in any set of TCL levels. Levels based on 

above progression appear in the left-hand col

TRANSIENT LEVELS CoNmoL SUGGESTED PROPOSED 

LEVELS 

PROPOSED IEC TC 28A 

7-n 

- 2000 

- 1200 

- BOO 

- 500 

- (3001 

- (200) 

- (1201 

- I801 

- (50) 

- (301 

- (201 

10"~ 

Fig. 1: Proposed levels for TCL voltages 

compared to existing level system
A progression proposed in IEC TC 28A, Low 

Voltage Insualtion Coordination, is shown in the 

right-hand column. The levels that have been 

proposed range from 500 to 12000 volts. On Fig. 

1, the levels in parentheses are inserted only to 

indicate the sequence. This progres ion, which 

seems to be based on the factor 10li, does not 

include the 600-volt level. 

Levels as arranged in the center column 

might appear to provide an appropriate compromise. We propose that the levels in boldface 

print be the recommended levels while those in 

lighter print be used, preferably sparingly, when 

intermediate levels are needed. Associated with 

each of these levels would be a short-circuit 

current level, the magnitude of which is related 

to the voltage levels through defined source 

impedances. Source impedance wi 11 be discussed 

further below. 

Some of the levels will seem very low, 

particularly to those accustomed to dealing with 

transients on power lines. They may not be 

unrealistic for some low-voltage signal circuits. 

A more important point, however, is that the 

establishment of a series of levels, from which a 

choice may be made, is a task separate and 

distinct from that of deciding to what level a 

piece of equipment should be designed. This 

latter point is discussed in more detail later. 

4. Wave shape 

Many test waves have been proposed in the 

past. Table I shows some that have been proposed. 

These wave shapes range from the very fast 

rise, short duration, to the slow-rise, long 

duration, with oscillatory or unidirectional 

voltages. Each of these is based on practical 

considerations for specific applications; but the 

total picture is then one of confusion and discouraging attempts at standardization. 

Observations of oscilToscope recordings 

and independent work on the resonant frequency of 

power systems [2] have shown that most transient 

voltages in low-voltage systems have an oscillatory wave shape, in contrast to the well-known 

and generally accepted unidirectional wave used 

in high-voltage insulations standards. Frequencies are typically in the range of 5 kHz to 

500 kHz, with the majority of the transients 

having frequencies above 100 kHz L31 . 

On the basis of these observations, the 

authors have proposed the voltage wave shape of 

Fig. 2, as being most representative of transients in low-voltage systems. 

This wave is a composite. One component is 

aimed at producing the effects associated with 

fast rise times. Coupled interference and the 

response of inductive devices are examples. 

Another component is aimed at producing the effects associated with the more slowly changing, 

and osci 1 latory, tai 1. Voltage summation in 

capacitive circuits coupled by rectifiers is an 

example. Energy handling capability of surge 

protective devices is another. 

Fig. 2: Proposed TCL voltage wave 

While this wave may then appear art

ficially contrived, it will subject test sample

to the two most significant effects of voltage

circuit upset, and circuit damage. Since th

wave may be produced by simple laboratory cir

cuits, comparison tests ma be easily done b

different organizations. [41 

This wave shape was first defined by 

consensus at a meeting of the Ground Fault Pro

tection Section of NEMA, in August 1973, and ha

since received increasing acceptance, notably a

the Underwriter's Laboratories. Recently

independent considerations [Slhave given furthe

support to a 0.5 ps rise time and 5 ps duratio

impulse. 

However, in all probability this on

osci 1 latory TCL wave will not meet the needs o

all users. Therefore, we propose that the wav

of Fig. 2 be supplemented by two unidirectiona

voltage waves: the classic ANSI 1.2x50p.

impulse wave and a 10 x 1000 us wave [6,7]. 

We believe that most applications can b

treated by one of these three wave shapes, onc

the concept is accepted that a perfect match o

"actual" wave shape and "test" wave shape is no

essential. The first wave, fast rise and 100 kH

ring, would be more applicable for circuits ex

posed to 'lightning remnants" (the natural osci

lation of a power system excited by a lightnin

discharge or switching transient at some remo

point) as well as control circuitry exposed t

induced transients. The second wave shape, th

familiar 1.2 x 50 unidirectional, would be appl

cable to circuits where direct exposure t

lightning. strokes is likely; while the thir

(long tail) would be applicable to situation

involving lightning current discharge on lon

cables. The second and third wave shapes ar

also representative of transients produced by th

switching of inductive circuits. 

Special applications, such as NEMP (Nuclea

Electromagnetic Pulse) hardening, or high-volt

age substation supervisory equipment, wou

rather retain their own well-documente

standards. 

5. Source impedance and enerqy 

In some types of tests, the object is t

determine what level of voltage will cause fail

ure (permanent or temporary) of insulation. Th
TABLE I - PARTIAL LISTING OF EXISTING OR PROPOSED TEST WAVES 

- ORIGIN 

ANSI, IEC 

DESCRIPTION 

Wave Shape Amp1 i tude 

Specified voltage Specified current 2.5 kV peak 

AP

Power ap

Low-volta

and co

substa

IEEE Std. 472 

Guide for Surge Withstand 

Capability (SWC) 

- 1.25 MHz repetitive 

at 60 Hz 

- 6 ps decay to 50X - 150n source impedance 

Specified 

levels 

Low-volta

and si

Fisher-Martzloff [a] - 0.25 ps rtse 

- 5 us to zero - Unspecified ring 

GE Transient Suppression Manual 141 - 500 kHz rise 

- 100 kHz ring 

- 40% decay 

Low-volt

Crouch-Fisher-Martzloff 1101 - 0.5 us rise 

- 100 kHz ring U.L. Ground Fault Interrupters - 2nd peak 2 60% first 

- 50n source impedance 

Specified 

levels Low-volta

IEEE Std. 465.1 Three requirements: 

Test Specifications for Gas Tube Surge Protective Devices : ~oxX2~o~~ ::rF:iFe"t Telephon

Linear loltage ramp 

of 100, 500, 5000, 10,000 V/gs 

unti 1 sparkover 

FCC Docket 19528 Comnunl

equipm

- 10 x 560 ps - 100 A short circutt current 800 V peak 

. Longitudinal 

- 10 x 160~s 

- 200 A short circuit current 1500 V peak 

Rural Electrlf ication 

Administration Spec. PE-60 

- 10 x IDDO ps voltage - 100 V/us rise 

3o of protector 

level 

Telephon

Rectangular Evaluati

pulse 3 ns to 10 ps NEMP Hardening 

* Damped sinewave 

10' to lo8 HZ 

NASA Space Shuttle Damped stnewave 

125 kHz 

a Unidirectional 

-2x100us 

Space sh

Mi 1 itary 

Envelope specif fed, 

max. duration 50 gs 
nature of the transient following breakdown is 

not of much concern. The typical test piece is of 

high impedance (except after breakdown), and thus 

does not load the generator. People have tended 

to overlook the source impedance of the generator, even in applications where that impedance 

is important. 

However, with the development of voltage 

suppression devices, the source impedance becomes 

an integral part of the suppression scheme. Some 

types of devices (spark gaps) function by 

switching into a low impedance state and reflecting the energy associated with the transient back 

from whence it came. Other devices (varistors, 

selenium, and Zener type diodes) clamp the 

voltage across their terminals while conducting 

the surge current and thus dissipate the surge 

energy in the protective device. The a b i l i t y o f 

the device to handle that energy becomes of importance. In either case, the test generators 

must be capable of supplying an appropriate 

amount of current, but should not supply too much 

current. 

Test specifications should reflect the fact 

that, in some cases, voltage is the appropriate 

measure of the transient, and in other cases 

current is the appropriate measure. Above all, 

they must avoid wording that leads the inexperienced to struggle valiantly, with everlarger 

surge generators, to develop a specified voltaqe 

across a correctly functioning spark gap or 

varistor. This has occurred. 

In the original formulation of the TCL 

concept, the authors proposed, and still do 

propose, that the generator impedance associated 

with the 100 kHz oscillatory test wave be an 

impedance representative of that measured on a-c 

supply mains. Such an impedance can be t-gW$~erIzy! as 50 ohms in parallel with 50 microThe ANSI specifications dealing with the 

long-established 1.2 x 50 ~s unidirectional wave 

do not treat source impedance directly, but 

recognize its existence by providing a separate 

current test wave for surge arresters or other 

surge protective devices. In the TCL concept as 

we now visualize it, this same approach would be 

followed: separate voltage and current levels. 

One of the applications where the 10 x 

1000 us unidirectional test wave might be appropriate would be those involving switching of 

inductive circuits. The impedance associated 

with such transients can vary over wide limits 

and may be quite low. We do not feel there is yet 

a sufficient engineering consensus as to what a 

suitable standard source impedance might be. Accordingly, we made no recommendations for such 

impedance, feeling that the evaluation of such 

impedance must be done on an individual basis for 

the specific application at hand. 

6. Selection of specific levels 

The task of selecting the transient control 

level appropriate for any one piece of equipment, 

or any one application, is one of engineering and 

cannot be fully dealt with in this paper. However, some discussion of the task Is necessary to 

show how that task fits into the overall TCL 

philosophy. The BIL system provides som

guidance. A fundamental tenet of the BIL system 

is that the insulation structure of apparatus i

not designed until after the required insulatio

level is agreed upon, and that this insulatio

level is not chosen until one is sure that ther

are voltage-limiting devices (surge arresters

that can control natural transients to level

lower than those to which the factory proof tes

will subject the apparatus under design. 

On the other hand, low-voltage and elec

tronic equipment is all too often designed without consideration of transients or whether pro

tective devices might even be available i

needed. One guideline is then that equipmen

should not be designed until an appropriate 

design level has been chosen. This choice shoul

be made after consideration of the distributio

of naturally occurring transients. 

The occurrence of transients is a statistical process, both in voltage levels and energ

content. LOW levels are comnon while high level

occur rarely. Figure 3 shows the relationshi

between voltage level and frequency of occurrence 

on 120-volt residential circuits, from 

observations made in the United States 191

While this type of information cannot serve t

predict the occurrences at individual locations, 

it is of interest if one is concerned with th

overall statistics of transients. For instance, 

a manufacturer can select a withstand level (o

conversely, a failure level) by trading off th

tangible and intangible cost of failures for th

cost of the added protection required to achieve 

that level. From the graph of Fig. 3, we can se

that decreasing the withstand level from, say, 4 

kV to 2 kV is l i k e l y t o increase the failure rat

of a product by a factor of 10. 

I I I I I I .I 

2 4 6 8 ID kV 

SURGE CREST 

Fig. 3: Exposure of residential circuits to 

surges (number of surges vs highest 

surge at any one location) 

Selection of the most appropriate level fo

a specific application should remain the preroga

tive of the parties directly interested. Thi

choice will be based on a number of factors suc

as the circuit rated voltage, the exposure of th

circuit to induced transients, the presence o

absence of a mandatory suppressor in the circuit, 

the risk analysis (probability of failure, conse

quence of a failure, cost-trade off), etc. 
TABLE I1 - PROPOSED IMPULSE LEVELS BY IEC-TC 28A 

I Rated Voltages 

Line-toEarth 

up to 

Volts 

1-phase 

or d.c. 

~ine-to-line 

L-M or L-L 

volts 

150- 300 

300- 600 

600-1200 

3-Phase 

L-N or L-L 

up to 

Volts 

System Voltages 

According to 

IEC-Publ. 38 

Volts 

Preferred Series of Impulse 

Withstand Voltages in Volts 

Category 

Note: The values of impulse withstand voltage given in columns I through V are a preferred series of values to be 

used by the Technical Cormittees for the purpose of insulation coordination. Products subjected in the field to 

the salne conditions of overvoltages or rated to withstand the same overvoltages are to be assigned values from 

the same column. While it might be useful to describe products and specify a preferred column for such products, 

SC 28A has refrained from doing so. 

An example of such a selection process is 

found in current proposals of IEC 28A for lowvoltage insulation coordination. This proposal 

includes a matrix of voltage levels depending on 

one hand on the system voltage and on the other 

hand on a level category, which is left to the 

users to choose but implies some recognition of 

exposure factors. This proposed table is reproduced here as Table I1 with the permission of 

the IEC TC 28A Chairman. 

Conclusion and Recomnendation 

Acceptance of the TCL concept by manufacturers and users of equipment, as well as standardizing and regulatory agencies, would be a 

great step toward simplification of specifications and toward more reliable system perf ormance. 

This paper has incorporated the feedback 

received after several proposals made at IEEE 

meetings, and at this point represents the position of the authors, supported and amended by 

the comnents received. Further feedback from the 

EMC comnunity is earnestly invited and welcome. 

To sumnarize our proposal, we recomnend 

consideration and eventual acceptance of the 

following: 

Major voltage levels of 300, 600, and 1250 

volts, with intermediate levels of 450, 850, 

and 2000 volts used if necessary; the levels 

to be scaled upwards or downwards by the 

appropriate powers of ten. 

A voltage wave shape of 0.5~s rise x 100 kHz 

ring with current related to voltage by a 

source impedance of 50n and 50 pH. This wave 

shape would be supplemented by 1.2 x 50 ys 

and 10 x 1000 t~s unidirectional waves. 

All terminals, power and signal, are to be 

subjected to TCL tests. 

For any particular piece of equipment, an 

appropriate level would be chosen from the 

above series, by mutual agreement between 

supplier and user. 

References 

Fisher, F.A. and Martzloff, F.D., "Transi

Control Level Philosophy and Implementati

Part 11. Techniques and Equipment for Mak

TCL Tests, " Proceedings, 2nd EMC Symposi

June 1977. 

Speranza, P.D. and Sessler, L.H. (Bell Te

phone Laboratories), Oral Comnunication 

NEMA, Ground Fault Protection Section, A

1973. 

Martzloff, F.D. and Hahn, G.J., 'Surge Vo

ages in Residential and Industrial Power C

cuits." 

Transient Voltage Suppression Manual, G

eral Electric Company, Electronics Pa

Syracuse, N.Y., 1976. 

Lerstrup, K., "Atmospheric Overvoltages 

Low-voltage Installations," Document E

28A/WG1 02/76. 

IEEE Standard 465.1, "Test Specifications 

Gas Tube Surge Protective Devices," 1977. 

Rural Electrification Administration (R

Specification PE-60, Washington, DC, 2025

Bull, J.H., "Impedance of the Supply Main

Radio Frequencies," Proceeding 1st EMC S

posium, 1975. 

Fisher, F.A. and Martzloff, F.D., "Trans

Control Levels, a Proposal for Insula

Coordination in Low-voltage Systems," 

Trans. Power Apparatus System PAS-95 No. 

120-129 (Jan.-Feb. 1976). 

Crouch, K.E., Fisher, F.A. and Martzlo

F.D., "Transient Control Levels, A Better 

to Voltage Ratings in Power Converter App

cations," Proceedings of 1976 Annual Meet

of the IEEE Industry Application Society