TRANSIENT CONTROL LEVEL PHILOSOPHY AND IMPLEMENTATION 

11. Techniques and Equipment for Making TCL Tests 

F.A. Fisher and F.D. Martzloff 

General Electric Company, Corporate Research and Development, 

Pittsfield, Massachussetts, and Schenectady, New York 

Abstract 

This is the second of a pair of papers 

describing how better transient protection might 

be achieved through the use of a Transient Control Level philosophy. The first paper deals 

with questions of why tests should be made and 

what should be the specifications governing such 

tests. This paper amplifies somewhat on those 

themes, but is primarily concerned with questions 

as to how such tests might be made. 

Direct Injection of Transients 

Equipment 

Surge generators capable of producing the 

types of transients involved in the Transient 

Control Level (TCL) phi losophy general ly employ 

capacitors that are discharged into a wave-shaping circuit. That circuit would be resistive if 

unidirectional transients are to be produced, and 

inductive if oscillatory transients are to be 

produced. 

It should be understood that the TCL philosophy is not written around any particular test 

circuit. Any circuit that produces open-circuit 

voltages and short-circuit current of the required characteristics will be satisfactory. 

Nevertheless, it does seem appropriate to give 

some specific guidance as to the types of generators that might be used. 

Some representative examples of circuits 

are shown on Fig. 1. The switch may be either 

triggered or untriggered, depending upon the degree of sophistication desired. Components 

should be chosen and laid out in such a way as to 

minimize undesired.residua1 inductance and radiated interference. Circuit voltages are frequent ly high enough to be hazardous; therefore, 

appropriate safety precautions must be taken. 

The two circuits shown on Figs. la and lb are 

designed to produce the 100 kHz oscillatory TCL 

test wave and the unidirectional 1.2 x 50 microseconds ANSI test wave. 

In Fig. la, the oscillatory frequency is 

determined primarily by C1 and L1 with the front 

time determined by R, and C,. In Fig. lb, the 

front and tail timesLare apfioximately: 

With different component values, this same type 

of circuit would then produce longer duration 

transients. Photographs of the waveforms produced by these generators are shown on Figs. 2 

and 3. A significant point to note is that the 

shapes of the current and voltage are different. 

This is particularly true of the oscillatory 

generator, Fig. la. It is characteristic of 

inductive transient sources that the shortcircuit current takes longer to reach its peak 

value than does the open-circuit voltage. 

Specifications should allow for this; it is generally unrealistic to require surge currents to 

rise to crest as fast as surge voltages, at least 

from inductive sources or into inductive loads. 

VOLTAGE 

p-rL [ 

CHAR""'" 

Fig. 1: Elementary Circuit Diagrams 

A) Oscillatory Generator 

8) Exponential Generator 

(Values in parentheses were used for transformer 

injection) 

The generators of Fig. 1 may be operated to 

produce either single or repetitive pulses. A 60 

(or 50) pulse per second rate is easily achieved 

by charging the storage capacitor from an ac 
OPEN CIRCUIT VOLTAGE - VOc SHORT CIRCUIT CURRENT- ISC 

0 20 40 0 20 

MICROSECONDS 40 MICROSECONDS 

0 2 4 0 2 4 

MICROSECONDS MICROSECONDS 

Fig. 2: Wave shapes produced by oscillatory 

generator (direct injection) 

OPEN CIRCUIT VOLTAGE- VOC SHORT CIRCUIT CURRENT-I sc 

0 40 80 0 40 80 

MICROSECONDS MICROSECONDS 

0 4 8 0 4 8 

MICROSECONDS MICROSECONDS 

Fig. 3: Wave shapes produced by exponential 

generator (direct injection) 

source and closing the output switch on the halfcycle following that which charged the capacitor. 

The type of discharge switch depends on the 

voltage rating and the degree of sophistication 

required. Low-voltage thyratrons and thyristors 

are suitable up to about 500 volts, series 

strings of thyristors to about 2000 volts, and 

spark gaps at higher voltages. This type of 

circuit lends itself readily to superimposition 

of the transient to the power frequency voltage. 

Some investigations, such as circuits involving 

semiconductors, require that the timing of the 

transient with respect to the power frequency 

wave be controlled. 

Types and conduct of tests 

Experience has shown that great amounts of 

confusion can surround two seemingly simple questions: for what purpose does one wish to make a 

TCL test, and how does one define the severity of 

the test? With respect to the first of these 

questions, TCL tests fall into one of two categories: those aimed at determining what TCL is 

appropriate for a particular piece of equipment, 

and those to determine whether a particular piece 

of equipment meets the requirements of a certain 

TCL specification. While the subject may seem 

elementary, experience has shown that the distinction between the two is often lost. These 

distinctions are illustrated in Figs. 4 and 5. 

OPEN CIRCUIT VOLTAGES 

15000 TRANSIENT CONTROL 

LEVEL 9 

6000 TCL 8 

3000 TCL 7 

1500 TCL 6 

FAILURE LEVEL 600 TCL 5 

UPSET LEVEL 300 TCL 4 ------ 150 TCL 3 

NO PROBLEM BELOW 

THIS LEVEL 60 TCL 2 

30 TCL I 

15 

Fig. 4: Determining upset or failure levels 

of a piece of equipment 

SURGE 

GENERATOR 

POWER 

SETTING "7 

TEST MADE 4. 

AT THIS LEVEL 

DEVICE EITHER 

PASSES OR 

DOES NOT PASS - A YES OR NO 

ANSWER 

3 METER -, 0 

'- 

- 600 -TCL 5 -ASSIGNED TRANSIENT 

CONTROL LEVEL 

(ASSUMED) 

Fig. 5: Acceptance tests on a piece 

of equipment 

In Fig. 4 the problem is to determine what 

the intrinsic withstand or upset level of the 

piece of equipment might be. In this type of test 

the surge generator is set to some level, the 

transient is applied to the terminals of the 

equipment under test, and the results are observed. The output level of the surge generator 

is then measured, probably in terms of the open- 
circuit voltage produced by that power setting. 

The power setting of the surge generator is then 

increased in steps until the device under test 

either upsets or fails catastrophically. The 

actual voltage or current amplitudes and wave 

shapes at the terminals of the equipment depend 

upon the input impedance of the equipment; and 

these might be unduly complicated to specify. 

The failure, however, will occur at some definite 

setting of the transient generator. If the sett i n g o f the generator (position of the knobs) was 

left unchanged, the energy level of the transient 

that produced failure or upset could then be 

described in terms of the voltage or current that 

the generator produced when connected to an open 

circuit or to a short circuit. This level will 

seldom be a nice round number, either so many 

volts or so many amperes. If one were to quote a 

level of transient that a device could withstand, 

that level would be the next lowest of the 

agreed-upon sequence of levels. If the levels 

from which the choice were to be made were as 

shown on Fig. 4, the appropriate level to quote 

would be level 3. 

The alternative problem is that of Fig. 5, 

in which the aim is to determine whether or not 

the piece of equipment under test is able to 

withstand the transients associated with the 

specified transient control level. In this type 

of test, the transient generator is set to deliver a specified current into a short circuit 

and to deliver a specified voltage to an open 

circuit. If the source impedance levels have 

been properly chosen and implemented, these two 

conditions wi 11 be met simultaneously at one 

power setting of the transient generator. With 

the setting of the transient generator left unchanged, the output terminals of the generator 

are then connected to the input terminals of the 

device under test. An appropriate number of 

transients are applied to the equipment, and the 

equipment is monitored for correct operation during the test or monitored after the test to 

determine whether it works or not. 

The preceding material has touched upon the 

question of how one defines the level of transient. Superficially, this might seem a trivial 

question to which an easy answer can be given -- the level of the transient is measured in terms 

of the voltage applied to the terminals of the 

device under test. However, this simple answer 

is deficient in that it neglects the input impedance of the device under test. If the device 

under test has a low input impedance or is fitted 

with surge protective devices, it might not be 

possible to develop a specified voltage without 

injecting excessively high currents into the device, currents higher than the naturally occurring transient source would be able to supply. 

For low impedance circuits, it is more appropriate to define the transient in terms of current. However, a specification considering only 

surge current is not sufficient in that the 

impedance of the circuit might be so high that a 

specified current might be developed only by 

applying excessive voltage. For these reasons it 

seems best not to define a test in terms of the 

voltage or current developed at the terminals of 

the device under test. Rather, in the TCL philosophy, the tests are defined in terms of the 

capabi 1 i ty of the surge generator to deliver 

specific voltages or currents into specific 

loads. If one is dealing with surge generators 

suitable for direct connection to the terminals 

of the device under test, the most convenient 

loads are open or short circuits. In cases where 

the transient is to be superimposed to the power 

frequency voltage, it will be necessary to disconnect the power supply for the short-circuit 

test. 

As examples of why these quantities are of 

importance, open-circuit voltage is of importance 

if one is studying the ability of insulation to 

withstand voltage, or if one is studying the 

ability of an isolated circuit to withstand 

interference. Short-circuit current is the 

quantity of most relevance if one is studying the 

ability of circuit protective devices to withstand surge currents, or if one is studying the 

interference produced on signal circuits by 

current flowing upon the shields of cables. 

Transformer Injection of Surqes 

Direct injection tests are most appropriate 

to determine whether or not a piece of equipment 

by itself would be damaged by a specific level of 

transient. A problem of greater complexity is 

determining whether a group of interconnected 

pieces of equipment will operate correctly in the 

presence of transients. Generally, the equipment 

cannot be tested for proper operation unless it 

is part of a completely interconnected system. 

This usually implies there must be a source, a 

load, and appropriate interconnected wiring. It 

would also seem to go without saying that the 

method of injecting the transient into the wiring 

should realistically duplicate the way in which 

the natural electromagnetic field induces transients into the wiring. One such test technique 

is shown on Fig. 6. In nature, an electromagnetic field might be set up between the 

conductors and ground, the field being distributed over the entire length of the interconnecting wiring. In Fig. 6, a similar magnetic 

field is produced, but confined within the core 

of a pulse injection transformer. If this transformer is placed around the wires under consideration, the current and voltage induced into those 

wires are nearly the same as that produced by the 

distributed magnetic field of nature. The magnetic field is set up in the core of the transformer by discharging a surge generator through 

an exciting winding on the core. 

IDC SURGE 

GENERATOR 

INJECTION 

Fig. 6: A magnetic field confined in the 

core of an injection transformer 
Some considerations regarding the 

capabilities of such transient injection follow. 

First, it should be remembered that the circuit 

under test is primarily exposed to the changing 

electromagnetic field contained in the core of 

the pulse injection transformer. If the source 

under test is a high impedance circuit, the 

natural effect of that changing magnetic field is 

to induce an open-circuit voltage, the magnitude 

and duration of which will depend upon the amplitude and rate of change of the magnetic field in 

the core, but which will not be significantly 

affected by the length of the circuit under test. 

However, if the circuit is of low impedance, the 

magnetic field will induce a circulating current 

in the circuit. The shape of this current wi 11 be 

of a duration longer than that of the opencircuit voltage, and will be nearly the same as 

the duration of the current produced by the pulse 

generator in the primary of the transformer. 

The magnitude of this current will depend 

upon the impedance of the circuit under test. 

That impedance is governed both by the internal 

impedance of the terminal equipment and the characteristics of the wiring used to interconnect 

those pieces of equipment. Just as with direct 

injection of transients, there can be ambiguities 

in specifications if the characteristics of these 

impedances are not taken into account. If, for 

example, the wiring under test is fitted with a 

shield grounded at each end, the natural measure 

of the transient is the current induced on the 

shield. If the wiring is unshielded and connected to high impedance loads the natural measure of the transient is the induced voltage. 

Again, a complete specification of the test 

circuit must deal both with the maximum voltage 

and the maximum circuit current that may be produced. 

It seems appropriate to measure these voltages and currents on a dummy circuit of fixed 

dimensions. The dimensions must be fixed since 

length, diameter, and height all affect the inductance, the most important component of the 

impedance. The inductance of a conductor is 

almost directly proportional to its length, but 

proportional only to the logarithms of the diameter and of the height above the ground plane. In 

the absence of any other specifications, it is 

suggested that the surge generator and injection 

transformer be such that the specified shortcircuit current be induced on a conductor 3 meters long, having a diameter of 0.41 centimeter, 

and spaced 5 centimeters above the ground plane. 

Such a conductor may be provided by the shield of 

RG-58 coaxial cable. 

An example of a test structure having these 

dimensions is shown on Fig. 7. Representative 

examples of the transients that may be produced 

are shown on Figs. 8 and 9. 

It is harder to inject a transient indirectly into a cable than it is to produce a 

transient for direct injection tests, because of 

the limitations of the injection transformer. If 

Fig. 8 is compared with Fig. 2, it will be noted 

that both the current and the voltage are less, 

and that the r a t i o o f voltage to current is less 

when the transient is injected via a transformer 

than when it is injected directly. This results 

from the characteristics of the injection transformer. If the transformer has high magnetizing 

V TRANSFORMER CONDUCTOR 

UNDER TEST 

Fiq. 7: Transformer injection of transients: 

A) Conductor geometry 

B) Injection transformer 

0 4 8 0 4 8 

MICROSECONDS MICROSECONDS 

Fiq. 8: Waveforms produced by oscillatory 

generator (transformer inject ion) 

0 40 80 0 40 80 

MICROSECONDS MICROSECONDS 

0 4 8 0 4 8 

MICROSECONDS MICROSECONDS 

Fiq. 9: Waveforms produced by unidirectional 

generator (transformer injection) 
inductance and low leakage inductance, the 

exciting current -- and hence, magnetic flux -- will be proportional to the integral of the 

exciting voltage. In such a case, the opencircuit voltage induced on the cable will be of 

the same shape and amplitude as the driving voltage from the surge generator. 

As a practical consideration, it is often 

desirable that the cable joining the two pieces 

of equipment under test be threaded through the 

core only once, either because of the physical 

characteristics of the cable or because of the 

effect on the transient response of the transformer of multiple turns. This limits the magnetizing inductance to relatively low values. 

The current in the exciting winding -- and hence 

the flux in the core -- is then proportional more 

nearly to the generator voltage than to the integral of that voltage, particularly for surge 

generators of internal impedance high compared to 

the impedance of the transformer. As a consequence, open-circuit voltage induced onto the 

cable may be of shorter duration than desired, 

approaching the derivative of the driving voltage. The amplitude of the voltage might also be 

limited by saturation of the core. These effects 

are more troublesome for longer duration transients, as may be noted by comparing Fig. 9 to 

Fig. 3. 

Two measures were taken to improve the wave 

shape of the open-circuit voltage. The first of 

these was that some resistive loading, via an 

auxiliary winding, was used on the injection 

transformer to provide partial integration of the 

flux. The second was that the front time of the 

open-circuit voltage from the exciting pulse generator was made longer through the use of higher 

values of wave shaping capacitance. While these 

compensating techniques were partially successful, there is still ample room for improvement in 

the art of making equipment for transformer injection tests. 

Injection transformers are not constrained 

to be operated with only one turn, or even equal 

turns, on the primary and secondary. Either 

voltage or current may be emphasized by operating 

the transformer with unequal turns, although such 

operation should be recognized as affecting the 

impedance of the test circuit. In general terms, 

the effects of unequal turns are: 

a) More turns on secondary gives (for a fixed 

number of turns on the primary): 

o Less short-circuit current 

o More open-.circuit voltage 

o A decrease in the frequency of the 

natural oscillatory mode of the cable 

under test. 

b) More turns on primary gives (for a fixed 

number of turns on the secondary): 

o More short-circuit current 

o Less open-circuit voltage 

o A decrease in the frequency of the 

natural osci 1 latory mode of the pulse 

generator 

o A longer rise time of the current 

pulse. 

Fiq. 10: Equivalent circuits of injection 

transformers 

A) Basic transformer 

B) Equivalent circuit referred 

to secondary 

C) Equivalent circuit if Np = LS 

and Lp = s 

If one wishes to determine by analysis the characteristics of the test circuit, one may derive 

equivalent circuits for the injection transformer 

and the conductor under test. For the injection 

transformer, the Tee equivalent shown on Fig. 10 

is generally satisfactory. 

In Fig. 10 

M = 1/4 (LA - LB) . . . (3) 

LA and LB are the inductances measured with the 

primary and secondary windings connected first in 

series aiding and then in series bucking. 

The cable under test could probably be 

treated equally satisfactorily as a Tee equivalent of inductance and capacitance. A complete 

equivalent circuit, less the resistive loading 

shown on Fig. 7, would then be as on Fig. 11. 

Numerical analysis of this circuit with the aid 

of a computer program such as ECAP is entirely 

feasible. While the task has not been done, such 

analysis might point the way to test circuits 

better adapted to producing the desired TCL test 

waves. 

Equipment for Generation of 

Electric and Maqnetic Fields 

The test equipment described so far has 

been aimed at the injection of current or voltage 

pulses into the terminals of electronic equipment. There is also sometimes a need to subject 

equipment to an engulfing electric or magnetic 

field, principally to check for magnetic field 

leakage of the cabinet. Sometimes these effects 
- SURGE GENERATORJ INJECTION I CONDUCTOR UNDER 

I TRANSFORMER I TEST 

I- 

- 

I I 

I I 

Fiq. 11: Equivalent circuit of a surge generator connected to cable 

through an injection transformer: 

o the compensating loop of Fig. 7 is not included 

o component values shown are those appropriate for 

the test ferrites of Fig. 7. 

are checked with a wire-wrapped technique in 

which a coil of wire is wrapped around the item to 

be tested and the coil then excited from a pulse 

generator. The type of magnetic field so produced is perhaps not the best that may be derived 

for direct effects testing. In the study of 

nuclear electromagnetic pulse (NEMP) effects, use 

is often made of stripline simulators in which a 

pulse generator is matched to a large open transmission line. In such simulators attention is 

given to insuring that there is a suitable transition from the small geometry of the pulse 

generator to the large geometry of the test chamber. The pulse generators and working chambers 

are also carefully matched to each other to a1 low 

the production of the required short-duration, 

rapidly changing fields. For the production of 

more slowly varying electromagnetic fields, such 

as those resulting from lightning, the physical 

design of such chambers can frequently be simplified. The rise times involved are not as fast as 

those in the NEMP studies and the requirement of 

plane wave propagation in the test chamber is not 

as important. A test chamber suitable for many 

types of equipment might be like that of Fig. 12. 

It would basically consist of a one-turn loop 

antenna of rectangular cross section enclosing a 

volume of about 1 m3. Such a coil, when excited 

by the pulse generator sketched, would produce in 

its working volume a magnetic field of approximately 1000 amperes per meter or an electric 

field of about 20 kV per meter. 

Conclusions 

The equipment capable of performing the 

types of tests implied in the TCL concept is 

basically simple in nature. If only the lower 

levels need be generated, and if the equipment 

Fiq. 12: Possible test chamber for evaluating 

electric and magnetic field effects 

need not be very refined, it can be built in 

virtually any laboratory. 

Ultimately, of course, one might expect 

suitable test equipment to become available commercially, and indeed some equipment is already 

available. This is particularly true of equipment suitable for direct injection of transients. 

Techniques for indirect injection have not yet 

been as well worked out. There is a need for 

further development in that area, supported by 

comnercial test equipment. 

The true need for subjecting electronic 

hardware to electric and magnetic fields has not 

yet been determined, and accordingly very little 

has been done toward producing equipment for such 

tests.