GENERAL @ ELECTRIC 

General Electric Company 

Corporate Research and Development 

Schenecfody, New York 

TECHNICAl llVFORAATlON 

SERIES 

THOR 

dartzloff, FD 

Transient Control Level Test Generators 1- 

SUBJECT 

impulse generator for LTE77c~~241 

TCL testing ovemher 1m I - . - . - -- - .- - - - - 

IiINUTlNG 

,,,, ,,, Electronic Power Conditioning RESEARCH CORPORATE PND DEVELOPMEN 

and Control Laboratory SCHENECTPDI, N.Y. 

TLE 

MMARY 

The successful operation of consumer electronic devices requires that these devices survive in a hostile environment, sometimes not recognized, where overvoltages on a 120 V supply can 

exceed 5 kV in extreme cases, and frequently 1200 V in many locations. Therefore, a method of demonstrating withstand capability has to be developed along acceptable standards, or at least 

guidelines. 

As an example of the need for such a circuit, the Underwriters' 

Laboratories require a trip-free withstand of 3 kV, and a failurefree withstand of 6 kV for the recently introduced Ground Fault Circuit Interrupters. 

A test circuit which produces an impulse voltage superimposed 

on the 120 V supply has been described. The circuit approximates 

a typical waveshape as observed during several years of recording 

transients in residential circuits; the source impedance is believed 

to be representative of real-world conditions. 

In other reports the concept of Transient Control Levels (TCL) 

is proposed to users and manufacturers of equipment. It is hoped 

that the impulse generator circuits described in this report will be 

a useful tool in implementing the concept so that it will gain gen- eral acceptance. 

GE CL~SS 1 

iY WORDS 

transients, spikes, surges, withstand, standard, 

coordination overvoltage tests, design tests, Transient 

Control Levels (TCL) 

INFORMATION PREPARED FOR 

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Microfiche Copies Available From 

RD-54 (1W70) 

Corporate Research 8 Development Distribution 

P.O. Box 43 Bldg 5, Schenectady , N.Y. ,12301 

Technical Information Exchange 

P.O. Box 43 Bldg. 5, Schenectady , N.Y. ,12301 
F. D. Martzloff 

INTRODUCTION 

The successful operation of consumer electronic 

devices requires that these devices survive in a hostile environment, sometimes not recognized, where 

overvoltages on a 120 V supply can exceed 5 kV in 

extreme cases, and frequently 1200 V in many locations. Therefore, a method of demonstrating withstand capability has to be developed along acceptable 

standards, or at least guidelines. 

As an example of the need for such a circuit, the 

Underwriters1 Laboratories require a trip-free withstand of 3 kV, and a failure-free withstand of 6 kV 

for the recently introduced Ground Fault Circuit Interrupters. (2) 

A test circuit which produces an impulse voltage 

superimposed on the 120 V supply has been described 

(Ref. 3). This circuit approximates a typical waveshape as observed during several ears of recording 

transients in residential circuits. 8) The source irnpedance is believed to be representative($) of realworld conditions. 

In other reports(5. 6) the concept of Transient 

Control Levels (TCL) is proposed to users and manufacturers of equipment. It is hoped that the impulse 

generator circuits described in this report will be a 

useful tool in implementing the concept so that it will 

gain general acceptance. 

TEST WAVESHAPE 

The test waveshape proposed in Refs. 5 arid 6 

under the name of "Transient Control Level" has 

been selected as most representative of conditions 

encountered during actual measurements, It contains 

a fast rise that can stress inductive components, a 

slower oscillating tail which can be integrated by 

those circuits containing a rectifier circuit. However, it does not have the large energy contained in 

a 40 or 50 ps to half-value tail suggested by others, 

which appears excessive and would unduly stress 

energy-absorbing components in the test piece. 

As discussed in the TCL paper, there is no way 

to select a waveshape that will represent all possible 

conditions. Rather, the approach is to agree on a 

standard waveshape anchored in reality and, by experience, to correlate the fact that devices which 

can pass the TCL waveshape have successful field 

performance. To represent conditions realistically, 

the proposed test waveshape had a 0.5 )IS rise time, 

a period of 10 (is during later oscillations, and a decrement of 60% between successive peaks. Further, 

the effective source impedance is set at 50 n, as determined by a 50% decrease from open-circuit voltage to a 50 0 resistive load condition. 

After this waveshape had been established, discussions within the IEEE and IEC community on the 

desirable value of source impedance led to the proposal of a parallel combination of 50 61 and 50 UH as 

a representative source impedance. The initial generator circuit was then modified to provide this new 

source impedance, and the results were compared to 

the initial circuit(3) performance. This report describes in some detail the principle, design, and results of the two circuits. The generator presented 

here, while not offering the ruggedness or convenience 

of commercial products, (I) can be assembled by a 

reasonably competent technician and used in a laboratory environment by qualified individuals. 

PRINCIPLE 

Energy is stored in a capacitor and discharged 

into an isolated 120 V supply by means of a simple 

electromechanical relay. The rate of rise of the voltage is produced by a first resonating L-C circuit, 

which is damped sufficiently to let a second resonating 

circuit control the decay and subsequent oscillations 

of the voltage, according to the proposed standard 

waveshape. The 120 V upstream supply is protected 

against overvoltages by a large capacitor connected 

across the secondary of the isolating transformer. 

The use of a low-voltage (120 V rating) relay as 

the high-voltage switch might seem surprising. However, this is a very effective and low-cost approach, 

as long as it is recognized that conventional clearances 

and withstand voltages are obviously not observed in 

this circuit. However, in the case of breakdown, the 

worst that can happen is a premature exposure of the 

test piece to the spike that would normally be manually triggered by the "fire" button. Compared to other 

approaches, using a high-voltage switch or a stepup 

transformer, (3) this approach using a relay makes 

the circuit inexpensive and easily assembled. 

The complete circuit is contained in a metal cabinet, requiring only a 120 V. 60 Hz supply (Fig. 1). 

This cabinet includes the necessary DC supply for 

charging the capacitor, an isolating transformer for 

providing the 120 V supply to the test piece, and the 

closing relay and its control circuit. 

BASIC IMPULSE GENERATOR CIRCUIT 

The schematic of the basic impulse in its first 

form is shown in Fig. 2. The variable voltage obtained from the autotransformer T3 is applied to the 

primary of the stepup transformer T2 (a 15 kV neon 

tube supply transformer). The secondary of T2 is 

connected in a full-wave, center-tapped rectifier configuration. The storage capacitor C1 is charged 

through the resistor R3, until the relay CR-1 will 

discharge it into the 120 V test circuit, supplied by 

Manuscript received October 18, 1977. 
Fig. 1 Transient control level test generator. 

by the isolating transformer TI. The relay control 

circuit is described in detail in the next section. 

The rise time of the wave is controlled by the 

series resonance of L1 with C1-C2 in series, with 

onate with the inductance L2, as C3 offers a low impedance to the surge, at the same time protecting the 

upstream power supply from the surge. L2 then holds 

off the surge voltage while carrying the full-load 60 Hz 

current (and is designed accordingly). The resistance 

R2 is selected to provide the damping of the tail oscillation, which has been set at a decay of 60% between 

successive peaks in the proposed "standard" TCL 

waveshape. 

The output voltage, brought out to terminals of 

the front panel, can be monitored through the approximate divider R5/(~4 + R5). Alternately, the opencircuit voltage produced by the generator can be preset by calibrating the autotransformer T3. However. 

neither method can be sufficiently accurate for precise 

determination of the spike voltage; final determination 

should be made by direct measurement at the output 

terminals, using a suitable probe and oscilloscope. 

A second circuit approach, suggested by Crouch 

and ~isher(8) is shown in Fig. 3. In this circuit, the 

basic oscillation is produced by the Cl-L1 circuit when 

the CR-1 switch is closed, and applied to the test piece 

through the 50 uH/50 i2 network. The rise time is controlled by the R1-C2 time constant. This second circuit has an impedance which is closer to the theoretical 50 uH/50 source impedance than the first. a surge impedance of approximately 50 R, damping 

being provided by the resistance R1. Following this However, the actual difference in the performance. rise, the two capacitors, C1 and C2, now tied together as well as the measured impedance, is small, as by the closed contact and the low-inductance L1, res- 

~1 - 22Q, lw, comp. c1 - 0.025pF, lOkv (0.02 + 0.0051 Plastic Film 

R2 - lZn, 1W. Cmp. c2 - 0.00611F, 7.5kV ceramic (Spraguel 

R3 - 1.3MQ (12 X 1lOkn. 1/2W1 C3 - 1OvF. 330VAC Pyrano: (GE) 

p.4 - 47kn (10 r 4.7kQ. l/ZW) ~1 - 15uH. (35 turns, X23 wire 

R5 - 470n, 1/2W 0.75" dia. Air core1 

CR-1 - Relay 2 N.O. poles in series L2 - 70pH. (44 turns, X14 wire 

GE CR2790B100 A2 2.25" dia. Air Core) 

T1 - 2 KYA 120-240/120-240 Volts D3.W - DLodes UD: 10 

T2 - 90 VA 120/15000 Volts 60 MA (Neon Tubel 

$1 - Line Switch 

Fig. 2 Basic impulse generator circuit, first design. 
CR-I 

y gg

impedance plot of Fig. 6. 

While the initially developed circuit used a gate 

output from an oscilloscope to produce a timed closing VOLTAGE of the relay (with respect to the 60 Hz line voltage), OUTPUT the new imdementation includes a timer circuit pro- 

+ 

+ 

A C 

t 

C1 

C2 

CR 

Fig. 3 C 

wave. 

=o.z5$ RI =25n LI =lo@ = 0.005 uF R2 = 25 n L2 = 50 pH 

-1--Same as Fig. 2 

Iutput stage of the second circuit for TCL 

ducing a single-shot pulse of current driving the relay 

coil at a preselected time of the supply sinusoid, so 

that the spike voltage can be applied at any desired 

instant of the 60 Hz wave, by setting a built-in control 

(Fig. 7). 

A built-in time delay lockout in the trigger circuit 

has been provided to allow the capacitor driving the 

relay coil to reach a uniform voltage from one test to 

the next, in order to reduce variability in the closing 

time of the relay and thus to promote more consistent 

timing of the spike. If a repetition rate faster than 

the built-in 6-second interval were necessary, the 

time delay could be reduced, at the cost of less accurate timing of the spike. 

Fig. 4 Output of the first circuit. Fig. 5 Output of the second circuit. 
- CIRCUIT NO. l 

CIRCUIT NO. 2 - 

FREQUENCY -MHz 

Fig. 6 Impedance of the two TCL circuits. 

The amplitude of the pulse is adjusted simply by 

means of a control autotransformer in the primary 

of the high-voltage transformer, which can be precalibrated for open-circuit voltage setting. 

The isolating transformer, a conventional general-purpose transformer with two sets of windings 

in the primary and secondary, can be connected for 

a 240 V output if required; as built, the transformer 

Fig. 7 Control circuit for synchronous trigger. 

can deliver 2 kVA at 120 V into the test piece to be 

subjected to the spikes. Of course, the connection 

of a 2 kVA resistive load, on the 60 Hz/impulse output of the generator, will produce a large voltage drop 

in the pulse applied to the test piece. This voltage 

drop, caused by the regulation of the generator, must 

be recognized and addressed when specifying the test 

conditions. 

The front panel controls (Fig. 8) are then limited 

to a voltage setting, a timing setting, and a fire button. 

The output is available on a standard three-prong outlet, with one side of the outlet grounded to the cabinet 

chassis. In turn, the cabinet chassis is connected to 

the ground wire of the power cord. Thus, the cabinet 

is normally grounded to the power supply ground. If 

a single point ground is required by the instrumentaFig. 8 Front panel controls. 

4 
tion system, resulting in the removal of the cabinet 

ground, adequate safety precautions must be taken. 

As an alternate panel connection, the 120 V terminals 

and the impulse terminals can be brought out on studs 

and connected externally to suit the needs of the test. 

A connector on the front panel also brings out a 

signal for oscilloscope trigger, occurring at the time 

the relay coil is energized. This trigger signal is useful for displaying the full 60 Hz sinusoid with the superimposed spike. For resolution of the spike at faster 

sweep rates, this trigger signal would be prema:ure; 

therefore, the oscilloscope should be set on internal 

trigger. Another connector on the front panel brings 

out a signal with an approximate voltage divider reading of 10 V per kilovolt of output voltage (within 10%). 

More accurate voltage readings require the use of a 

calibrated probe, such as Tektronix P6015, connected 

at the actual output terminals. 

The original circuit specified for the waveshaping 

capacitor was a type that was found to have excessive 

(and inconsistent) internal inductance. This excess 

produced a very high frequency ring on the front of the 

wave, which in extreme cases could have an amplitude 

exceeding the crest of the desired wave. The problem 

was eliminated by substituting ceramic capacitors to 

the initial plastic-film design. 

As a further refinement for laboratory use, the 

two basic impulse circuits were built as plug-in subcircuits so that they could be substituted for one 

another during the investigation of output waveshape 

and currents, as well as during later development of 

other circuitry. Figures 9, 10, and 11 show the 

plug-in arrangement of this circuitry. 

Fig. 9(a) Interior, top view. 

Fig. 11 Second circuit plug-in. 

Fig. 9(b) Interior view, back of panel. 

TRIGGER CIRCUIT 

Figure 12 is a schematic of the timing circuit 

used to drive the relay at any time in the 60 Hz wave 

by a simple front panel setting. 
RESISTORS 1/4W 2% 

Rl1 lOkR R21 lOkn Dl,D2 IN5062 

Rl2 5kn (4W) R22 lkn SCRl C-122B 

R13 lOkn R23 5.6kQ C11 0.0047pF M 

P.14 lOkR 

R15 3.3kn 

Rl6 6.8kfi 

~17 33kn 

R18 lOkR 

R19 lOkR 

RZO lOOkfi 

RZ4 33kn C12 

R25 lOkQ 

10 Turn Pot C13 

R26 68n 

R27 loon C14 

10VF A 

50 WDC 

30uF E 

450 VDC 

Relay 

Coil (~ottom View) Mounting Tab 

WLPACIMRS 

A AluSl!dlytic 

B Black Hawk 

M Mylar 

E Electrolytic 

M Mica 

T Tantalum 

Fig. 12 Synchronous trigger circuit. 
The circuit utilizes a zero crossing detector and 

a controlled time delay circuit. This arrangement 

permits zero time to be coincident with the 60 Hz 

wave as it crosses zero while going positive. 

The circuit functions in the following manner. 

The 60 Hz AC from the secondary of TI is applied to 

IC1 through R12. The circuit normally provides an 

output pulse every zero crossing of the 60 Hz wave. 

A resistive bridge network, consisting of R13, R14, 

and two resistors internal to IC1, is unbalanced by 

R15. In the unbalanced state the circuit is inhibited. 

Activation of the switch S1 causes the circuit to operate. 

A pulse is fed internally to an AND gate which outputs only on positive-going zero crossings. The output of IC1 is applied to Q1, which inverts the pulse and 

applies it to IC2. IC2 is used as a time delay. The 

bias level of an internal comparator circuit is controlled by R25. Varying R25 changes the delay time 

and permits selection of the time of application of the 

pulse with respect to the 60 Hz wave. (Because delay 

time also includes the relay operation time, calibration must be made for each unit.) The output of IC2 

is applied to IC3, which produces an output pulse. 

This pulse is applied to the gate of the thyristor SCRl 

and to the scope trigger output. The thyristor SCRl 

discharges C1 through the relay coil, thereby applying 

the high-voltage pulse to the line. 

RELAY MODIFICATION 

To maintain cost as low as possible, the CRl relay 

is operated as a high-voltage switch. In the normal 

relay construction, the switch leads are attached to 

the case and connected by a braid to the contacts. 

The proximity of the braid connection to the relay 

coil on some relays may cause flashover to the coil 

at about 7 kV, applying undesirable pulses to the circuitry. To avoid this problem the relay was modified to bring the high voltage into the top contacts of 

the relay, and the braid connection to the terminals 

was removed. The two contacts on the armature 

bridge were then joined to obtain a series connection of the two contacts gaps, with input terminals 

at the stationary contacts. This configuration can 

withstand 14 kV between contacts and coil without 

breakdown. 

The relay is further modified by removing the 

stationary contact of the normally closed pair and 

twisting the moving contact slightly to increase the 

contact gap, until only a barely perceptible wiping 

action occurs upon closing. This modification allows 

the two series-connected contacts to withstand 10 kV. 

ISOLATING TRANSFORMER 

T1 is a 120/240 V input, 120/240 V output transformer that may be used for 240 V AC output operation by changing the connection internally. The highvoltage circuit to the test device does not require 

change. The input to the timing circuit must bemaintained at 120 V AC by using only one-half (XI-X2) of 

the T2 secondary when the two halves are connected 

in series to produce 240 V. This will maintain the 

timing characteristics relative to the zero crossing 

intact. Care must be taken to ensure that the ground 

connection (which is so marked) of the timing circuit 

remains connected to the grounded side of the TI secondary. 

SAFETY 

The metallic enclosure of the cabinet provides 

a generally safe construction, consistent with typical laboratory-type equipment. Of course, the output outlet will supply 120 V to the test piece, with superimposed high-voltage spike. This will then require 

safety precautions consistent with general laboratory 

practices. As stated earlier, the cabinet is normally 

grounded to the power supply ground by means of the 

third wire of the three-wire line cord. There may 

be circumstances where a single-point ground, such 

as the chassis of the oscilloscope used for monitoring, 

may be required to minimize extraneous signals resulting from ground loops. In such cases, the line 

cord may be connected through a two-wire adapter, 

but suitable precautions must be taken to maintain 

the single-point ground connected at all times. 

A warning sign cautions against opening the cabinet by uninformed personnel; high-voltage components, including capacitors that can retain a charge 

even after disconnecting the power cord, are not protected after the opening of any of the cabinet panels. 

Because no interlock is provided, the user must be 

thoroughly familiar with the design of this unit before 

attempting to open the cabinet, and must exercise all 

appropriate caution in servicing the circuit. 

PERFORMANCE 

The oscillograms of Figs. 13 through 17 (recorded 

with a Tektronix 7623A storage oscilloscope and a 

Tektronix P6015 1000:l probe) illustrate the performance of the test generator. Figures 13 through 15 

show the output voltage at different sweep speeds, in 

the open-circuit condition. The 100 kHz oscillation, 

or 10 ps period in the tail. can be seen in Fig. 13, as well as the 60% decrement factor between successive 

peaks. Figures 14 and 15 show the front of the wave, 

including some fine detail of the residual oscillations 

in the front, which are due to stray capacitance in the 

inductors and inductance in the capacitors. The frequencies are high, but the amplitudes, compared to 

the crest voltage, are small. A rise time of about 

250 ns can be observed on Fig. 15. 

Figure 16 shows the output of the generator when 

feedinga 50 0 resistive load, at the same voltage setting as the open-circuit condition of Fig. 13. The approximate 50% reduction in output voltage indicates an 

internal impedance slightly below 50 61, providing a 

conservative margin over the design objective of a 50 R 
Fig. 13 Open-circuit voltage at maximum setting: 

2 kV/div, 2 Cls/div. 

Fig. 16 Output voltage with 50-ohm load at maximum setting: 2 kV/div, 2 ps/div. 

Fig. 17 Voltage across capacitor C3 at maximum 

output: 200 ~/div, 2 p/div. 

Fig. 14 Open-circuit voltage at maximum setting: 

2 k~/div, 0. 5 ps/div. 

Fig. 15 Open-circuit voltage at maximum setting: 

2 kV/div, 0.1 Cls/div. 

source impedance. This has not been raised to 50 61: 

first, in order to provide some margin and, second. 

in anticipation of the possible acceptance of J. H. Bull's 

(Ref. 4) value of 50 n, paralleled by 50 pH for a representative value of the impedance of the power system, 

which has an "effective impedance" of 30 R for the open. 

circuit voltage waveshape of interest. 

Figure 17 shows the voltage appearing across 

the capacitor C3 at full output (6.8 kV) setting of the 

pulse. The very moderate disturbance in the 120 V 

power supply indicates the adequacy of the filtering 

scheme used in this circuit in isolating the test piece 

supply from the bench supply. 

100% 2 kV/div 

2 p/div 0. 2 ps/div 

Fig. 18 Peak voltage and rise time. 
The ceramic capacitors used for the output waveshaping have a small capacitance/voltage coefficient, 

as do most ceramic capacitors. With a variable voltage, the resulting capacitance change could affect the 

wave shape. 

Admittedly, a search could be made for a capacitor with more constant dielectric. However, for the 

purpose of the test waveshape, the results obtained 

with the ceramic capacitors are satisfactory. 

Figure 18 shows how a voltage change from 25% 

to 100% produces a slight change in the front of the 

wave, but very little change in the subsequent oscillation. 

The most noticeable effect is the notch in the wave 

for lower voltages, resulting from the characteristics 

of the arc at the relay contacts. This small anomaly 

in the waveshape can be ignored because the real significance of the waveshape is the front of the wave and 

the total volt-seconds in the tail. 

ACKNOWLEDGMENTS 

The first basic impulse generator circuit approach 

derived from the work of E. K. Howell of the General 

Electric Circuit Protective Devices Products Department. The second circuit was developed by K. E. 

Crouch and F. A. Fisher of Corporate Research and 

Development (CRD). The synchronous trigger circuit 

was developed by M. D. Bloomer of CRD. Packaging 

of the circuit was accomplished by P.M. Szczesny of 

CRD. The improvements incorporated in this circuit 

were motivated by helpful discussions with A. W. Smoot 

and P. Richman, which are gratefully acknowledged. 

REFERENCES 

J. G. Hahn and F. D. Martzloff, "Surge Voltages 

in Residential Power Circuits, " IEEE Trans. 

PAS-89, No. 6, 1049-1056 (July-Aug. 1970). 

"Standard for Ground-Fault Circuit Interrupter, 

UL-943, Underwriters1 Laboratories, Melville, 

N. Y. 

E. K. Howell and F. D. Martzloff, "High-Voltage 

Impulse Testers, General Electric TIS Report 

75CRD039. 

J. H. Bull, 'Impedance of the Supply Mains at 

Radio Frequencies, Proc. of 1st Intern. Symp. 

on EMC, Montreux (May 1975). 

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

Control Levels -- A Proposal for Insulation 

Coordination in Low-Voltage Systems, Trans. 

Power Applications and Systems, PAS-95 No. 1 

(Jan-Feh 1976), pp. 120-129. 

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

Control Level Philosophy and Implementation, 

77CH1224-5 EMC, Proc. of the 2nd Symposium 

on EMC, Montreux (June 1977). 

Bulletins 424 and 281/261: "Model 424 Surge 

 en era tor / &Tonitor, * Keytek Instrument Corp.. 

Waltham, Mass. (1976). 

K. E. Crouch, F. A. Fisher, and F. D. Martzloff, 

"Transient Control Levels -- A Better Way to 

Voltage Ratings in Power Converter Applications, 

76CH1122-1-IA, IEEE-IAS 1976 Conferences nacuru, pp. JYV-JYY.