GENERAL @ ELECTRIC 

General Electric Company 

Corporate Research and Development 

Schenectady, New York 12345 

nTLE I GE CLASS 

Lightning and NEMP Transient Protection with 1 

Metal Oxide Varistors I YO. PA6ES 10 

TECHNICAL INFORMATION 

SERIES 

\UTIIOR 

F.D. Martzloff 

I -. 

IRSMT~YG WRPORLTE 

mmNENT Automation and Control Laboratory RESEAM AND DEVELOPMENT 

SCHEWECTADV Y.Y. 

Protection of electronic components against the transient overvoltages 

SUWECT 

varistors for NEMP 

hardening 

produced by lightning or nuclear electromagnetic pulses (NEMP) involves 

several techniques, starting with hardening and shielding the circuits and, 

finally, with protecting individual components. Metal oxide varistors, in 

their conventional form or in unconventional packages described in this 

82CRD084 

DATE March 1982 

report, can offer protection at various stages of the circuit, in bulkhead connectors, in printed circuit cards, or in substrate form. 

In this report, a brief overview is given on varistor fundamentals, including the limitations brought about by leads in NEMP protection. Several 

examples of novel, unconventional approaches are then described. These 

approaches have been proposed and reduced to practice as far as basic feasibility is concerned, but the details have not yet been developed. 

LEY WORDS 

NEMP, varistors, GE-MOV@ varistors, hardening, 

transients 

INFORMATION PREPARED FOR 

Additional Hard or Microfiche Copies 

Available from 

Technical Information Exchange 

Bldg. 81 Room A133, Schenectady, N.Y. 12345 
LIGHTNING AND NEMP TRANSIENT PROTECTION 

WITH METAL OXIDE VARISTORS 

F.D. Martzloff 

FUNDAMENTALS ON VARISTORS 

The term varistor is derived from its function as 

a variable resistor. It is also called a voltagedependent resistor, but that description misleadingly 

implies that the voltage is the independent parameter in surge protection. Two very different devices 

have been successfully developed as varistors: silicon carbide discs have been used for years in the 

surge arrester industry, and, more recently, metal 

oxide varistor technology has come of age. 

Structure and Packaging 

Composition of the varistor consists primarily of 

zinc oxide with small additions of selected metal 

oxides. These materials are sintered at high temperature to produce a polycrystalline ceramic body. 

The structure of the body is a matrix of conductive 

zinc oxide grains separated by a highly resistive 

intergranular boundary (Figure 1). 

Figure 1. Photomicrograph of a polished and etched 

section of a metal oxide varistor. 

Since electrical conduction occurs between zinc 

oxide grains distributed throughout the bulk of the 

device, a varistor is inherently more rugged than its 

single junction counterparts, such as Zener diodes. 

In the varistor, energy is absorbed throughout the 

body of the device with the resultant heat spread 

evenly through its volume. Electrical properties are 

Manuscript received March 18, 1982 

controlled mainly by the physical dimensions of the 

varistor body, which is generally sintered in the 

shape of a disc. Energy rating is determined by 

volume, voltage rating by thickness, and curren

capability by area. 

In conventional applications, a varistor device 

consists of a disc of the material, with electrode

applied by silk screen, metal spray, etc.. to oppo

site, parallel faces. A connection is made to the 

electrodes for feeding current in and out of the 

vajstor by attachment of the device leads to the 

disc, and thence to the circuit. The attachment can 

take many forms (Figure 2),(') but all involve the 

insertion of some lead length, a potential problem 

that will be discussed later. 

Electrical Parameters 

Varistor V-I Characteristics 

Varistor electrical characteristics are conveniently displayed using a log-log format in order to 

show the wide range of the V-I curve. The log format is also clearer than a linear representation 

which tends to exaggerate the nonlinearity in proportion to the current scale chosen. A typical V-I 

characteristic curve is shown in Figure 3. In order 

to illustrate three distinct regions of electrica

operation, this plot shows a wider range of current 

than is normally provided on varistor data sheets. 

In the leakage region (Figure 3). the predominant parameter is the ROm resistance (Figure 41

shown as the lo9 ohm line in the figure. As the 

current density increases, normal varistor operation 

occurs, with R, being the predominant parameter

Finally, in the upturn region, where R, has become 

very small compared to Ron, the latter dominates

Two other parameters, the capacitance C (reflecting 

a high effective dielectric constant of the material) 

and the lead inductance L are significant at high 

frequencies. 

The leakage region is of little interest to the 

user; in the normal operating region, the flatness of 

the characteristic is a criterion of clamping 

effectiveness. Departure from this flat characteris

tic in the upturn region could be viewed as undesirable; however, this upturn is useful in providing 
CROSS SECTION OF 

MOLDED AXIAL PACKAGE 

CROSS SECTION OF 

RADIAL LEAD PACKAGE 

PICTORIAL VIEW OF PICTORIAL VIEW OF 

POWER MOV PACKAGE HIGH ENERGY PACKAGE 

Figure 2. Typical lead configurations. 

Figure 3. Typical varistor V-I curve plotted on log-log 

scale. 

(LEAD 

INDUCTANCE) 

Figure 4. Varistor equivalent circuit model. 
current sharing across the section of the device at 

high currents. 

The capacitance may be objectionable at high 

frequencies. Adding a low capacitance avalanche 

diode in series is a method of reducing the effective capacitance. The unconventional examples 

described in the next section also show means of 

reducing capacitive effects. 

The lead inductance, already mentioned, will be 

discussed at greater length in the following subsections. 

The V-I characteristic, then, is the basic application design tool for selecting a device to perform a 

protective function. For a successful application, 

however, other factors, which are discussed in 

detail in the information available from manufacturers, must also be taken into consideration. 

Some of these factors are: 

Selection of the appropriate nominal voltage 

for the line voltage of the application 

Selection of energy-handling capability 

(including source impedance of the transient, 

waveshape, and number of occurrences) 

Heat dissipation 

Proper installation in the circuit (configuration, lead length) 

"Overshoot" - A Lead Effect 

Enough instances of poor installation practices 

have been observed, and enough questions have 

been raised on alleged "overshoot," that a brief 

discussion of lead effects is in order.(*) To illustrate the effect of lead length on the overshoot, two 

measurement arrangements were used. As shown 

in Figures 5a and 5b, respectively, 0.5 cm2 and 

22 cm2 of area were enclosed by the leads of the 

varistor and of the voltage probe. 

The corresponding voltage measurements are 

shown in the oscillograms of Figures 6a and 6b. 

With a slow current front of 8 ~s, there is little 

difference in the voltages occurring with a small or 

large loop area, even with a peak current of 2.7 kA. 

With the steep front of 0.5 ps, the peak voltage 

recorded with the large loop is nearly twice the 

voltage of the small loop. Note (Figure 6b) that at 

the current peak, L di/dt = 0, and the two voltage 

readings are equal; before the peak, L di/dt is positive, and after, it is negative. 

Hence, when one is making measurements as 

well as when one is designing a circuit for a protection scheme, it is essential to be alert to the effects 

of lead length (or, more accurately, of loop area) 

for connecting the varistors. This warning is especially important when the currents are in excess of 

a few amperes with rise times of less than one 

microsecond. 

Speed of Response 

The varistor action depends upon a conduction 

mechanism similar to that of other semiconductor 

devices. For this reason, conduction occurs very 

rapidly, with no time lag apparent, even into the 

nanosecond range. Figure 7 shows a composite 

photograph of two voltage traces with and without a 

varistor inserted in a very low inductance impulse 

generator.c3) The second trace (which is not synchronized with the first, but merely superimposed 

on the oscilloscope screen) shows that the voltage 

clamping effect of the varistor occurs in less than 

one nanosecond. 

In the conventional lead-mounted devices, the 

inductance of the leads would completely mask the 

fast action of the varistor. Therefore, the test circuit for Figure 7 required insertion of a small piece 

of varistor material in a coaxial line to demonstrate 

the intrinsic varistor response. 

Tests made on lead-mounted devices, even with 

careful attention to minimizing lead length, shqw 

that the voltages induced in the loop formed by the 

leads contribute a substantial part of the voltage 

appearing across the terminals of a varistor at high 

current and fast current rise. In typical power 

applications where the threat is lightning or switching surges, the impressed current rise time is selAREA = 0.5 crn2 

AREA = 22 crn2 n 

r VOLTAGE PROBE 

OUTPUT LEAD FROM 

TRANSIENT GENERATOR j SHIELD 

-- - CURRENT VARISTOR GROUND 

PATH 

(a) Minimal Loop Area (b) Excessive Loop Area 

Figure 5. Measurement circuit for varistors. 
;cJ 

0.2 ps I 

(c) Current Rise of 8 ps (d) Current Rise of 0.5 ps 

Figure 6. Effect of lead length on "overshoot." 

Figure 7. Response to a fast current rise. 

dom short enough for this effect to become 

significant. Competitive technology will make 

claims that an "overshoot" occurs, but that has 

been shown to be primarily a lead effect as discussed in the preceding subsection. 

Voltage rate-of-rise is not the best term to use 

when discussing the response of a varistor to a fast 

impulse, unlike spark gaps where a finite time is 

involved in switching from a non-conducting to a 

conducting state. The response of the varistor to 

the transient current delivered by a circuit is the 

appropriate characteristic to consider. 

Applications involving NEMP protection will be 

fast-rising voltages and current surges, so that the 

packaging will become very significant, and a departure will be required from the conventional packages currently found in commercial devices. In the 

next section, these unconventional applications will 

be briefly presented. 

Failure Modes 

An electrical component is subject to failure 

either because its capability was exceeded by the 

applied stress or because some latent defect in the 

component went by unnoticed in the quality control 

processes. While this situation is well recognized 

for ordinary components, a surge protective device, 

which is no exception to these limitations, tends to 

be expected to perform miracles, or to at least fail 

graciously in a "fail-safe" mode. The term "failsafe," however, may mean different failure modes 

to different users and, therefore, should not be 

used. To some users, fail-safe means that the protected hardware must never be exposed to an overvoltage, so that failure of the protective device 

must be in the fail-short mode, even if it puts the 

system out of operation. To other users, fail-safe 

means that the function must be maintained, even if 

the hardware is left temporarily unprotected, so that 

failure of the protective device must be in the 

open-circuit mode. It is more accurate and less 

misleading to describe failure modes as "fail-short" 

or "fail-open," as the case may be.(4' 

Metal oxide varistors, when subjected to excessive energy (high surge current), will fail in the 

"fail-short" mode. If the varistor is connected to a 

power system with large short-circuit capability, the 

fault current may produce melting of a soldered 
connection or shattering of the ceramic, ultimately 

producing a "fail-open" condition. 

UNCONVENTIONAL APPLICATIONS 

For the protection of fast rise transients occurring with NEMP, it is apparent that lead-mounted 

devices are likely to be unsatisfactory. To still 

benefit from the inherent advantages of metal oxide 

varistors, novel packaging is required. In this section, several examples are given: bulkhead connectors and coaxial connectors, each with conventional 

sintered blocks or with reconstituted material. Both 

have been shown to be feasible, although the complete details have not yet been fully developed. 

Two further applications are also described, 

which are still in the conceptual stage, but show 

promise of useful application in the integrated protection of circuits: as thick film layers between conductor runs, and as substrates. 

Bulkhead Connectors 

The inductance of the leads whose effects have 

been discussed can be essentially eliminated by 

construction in a coaxial configuration. This type 

of packaging would be especially effective and convenient in conjunction with connectors for cable 

entry into shielded boxe~.'~' 

Figure 8 shows a configuration for a single wire, 

and Figure 9 for multiple wires or pairs. In both 

figures, a varistor bead is inserted between the 

conductor(s) and an opening of the bulkhead plate, 

which is part of the grounded shell. In this 

manner, any impinging surge current flowing in the 

conductor is diverted to the grounded chassis of the 

enclosure, with minimum inductance inserted. 

1 Chassis 6 Varistor bead 

2 Grounded shell 7 Outside electrode 

3 Conductor 8 Inside electrode 

4 lnsulating insert 9 Electrical connection 

5 Shoulder 

Figure 8. Single, coaxial connector with integral 

protection. 

1 Grounded shell 4 Varistor bead 

2 Conductor 5 Electrodes 

3 Bulkhead plate 6 Insulating insert 

Figure 9. Connector for multiple conductors. 

In principle, this construction is quite simple. 

The details of the bonding between conductors and 

varistor will require development, which should be 

within the skills of connector manufacturing. 

A possible objection to this construction would 

be- that the whole volun~e of the varistor bead acts 

as the dielectric of a capacitor inserted between the 

conductor and its sheath. With an effective dielectric constant of 1000, and the small radial distance 

required for low-voltage clamping, the resulting 

capacitance might be excessive for high-frequency 

applications. We note, however, that some 

advanced varistor materials have been fabricated 

with lower dielectric constants. 

An alternate construction is shown in Figure 10, 

where the varistor disc has two concentric electrodes that are silk-screened on the side of the disc, 

rather than on the inside and outside surfaces, as in 

Figures 8 and 9. In this manner. the disc can have 

a substantial distance between the inside and outside surfaces, reducing the resulting capacitance. 

The clamping voltage is then determined by the 

distance between the edges of the two electrodes. 

This arrangement offers the advantages of lower 

1 Grounded shell 5 Shoulder 

2 Conductor 6 Inside electrode 

3 Insulating msert 7 Outs~de electrode 

4 Varistor disc 

Figure 10. Connector with edge electrodes. 
capacitance and greater accuracy in the distance 

between electrodes, albeit at the cost of reducing 

the energy handling capability since only a thin surface layer of the varistor disc will conduct the surge 

current. 

Coaxial Mounting 

A varistor can also be inserted in a shielded 

cable by a coaxial "Tee" configuration. In fact, 

that configuration is required for making valid measurements of the varistor performance in the submicrosecond time range.(3) 

Here again, the capacitance of the varistor might 

be objectionable at high frequencies. A possible 

remedy to this situation is shown in the 

configuration of Figure 11, where the capacitance of 

the varistor is series-tuned to the inductance of the 

clamping spring. This achieves high impedance to 

the UHF signal frequency, thus low loading, but 

offers a lower impedance and good suppression at 

the lower frequencies associated with lightning 

surge currents.(6) This would not, of course, be 

suitable for NEMP protection. 

controlled dimension, mixed with a suitable binder 

in proper proportions, and molded into the finished 

part. By this method, it is possible to embed electrodes and conductors directly into the varistor 

material, as well as to fill a cavity such as a conductor shell.(') 

Figure 12 shows the structure of a portion of a 

device using reconstituted material (tracing of a 

photomicrograph), where the three constituents 

interface: electrode, particle of varistor material, 

and plastic binder. Here, the particle of varistor 

material is formed of several grains of ZnO with the 

varistor boundary layer between the individual ZnO 

grains. If the particles of varistor material were 

smaller than the ZnO grains, there would not be 

any barrier layer left and the varistor action would 

be lost. Thus, there is a need to optimize the grain 

size, the particle size, and the proportion of binder, 

depending on the specific application. 

A1 

ELECTROD

CERAMIC 

PLASTIC 

Figure 12. Tracing of photomicrograph showing 

boundaries. 

1 Shell 6 Spring 

2 Conductor 7 Button 

3 Insulating insert 8 Plug 

4 Side shell 9 Electrode 

5 Varistor disc 

Figure 11. Tuned combination varistor-inductance. 

Reconstituted Material 

In the two preceding examples, the description 

of the varistor simply stated "varistor disc," with 

the assumption that the material had been prepared 

in the conventional sequence of pressing a powder 

into shape and sintering the piece in its final form. 

There is another method of producing varistor 

components, especially attractive for parts shaped in 

a form different than conventional mass-produced 

discs. 

In this method, the varistor material, obtained 

by the usual sintering, is ground into particles of 

Figure 13 shows a photograph of a connector 

made by molding the reconstituted varistor material 

inside the shell of a connector provided with two 

pins. The resulting varistor characteristic is shown 

in Figure 15 for pin-to-pin and pin-to-case. With a 

shell that would be especially made for this application (the sample shown here was a standard shell), 

the pin-to-case characteristic would be controlled 

for lower voltage. 

Another shape demonstrated is the plug of Figure 14, where a flat-headed stud is embedded in a 

metal cup, illustrating how electrodes can be 

molded directly into the reconstituted material. 

The resulting characteristic is shown in Figure 16. 

Thick Film 

The reconstituted material just described might 

also be useful for producing varistor layers with 

thick-film technology. Currently, the reconstituted 

material is obtained by pressing the particles of 

varistor material in a cavity. Producing a paint-like 

film would require some development in order to 

maintain the particlelbinder proportion leading to 
7
Figure 15. V-I characteristic of two-pin connector. 

AMPERES 

AMPERES 

Figure 16. V-I characteristic of plug-type varistor. 
the behavior as a varistor. Should the particles 

become isolated from one another by an excess of 

binder, the varistor action would be lost. 

With a suitable paint-like compound, it would 

be possible to silk screen varistor layers directly 

between conductor runs. The clamping voltage 

would be controlled by the distance between the 

runs, while the current density would be controlled 

by the combined thickness of the layer and length 

along the run. 

As an alternate to painting layers of reconstituted varistor material between runs, a more 

advanced concept would include the sintering of 

varistor layers on a suitable substrate, on which 

runs and other components could be added in subsequent processing. This concept has not been 

reduced to practice, but illustrates the wide range of 

options that may be available beyond the commercial packaging of discs. 

Substrates 

Another approach to the integration of varistor 

protection to circuits would be the use of substrate 

slabs of sintered varistor material, on which the circuit runs could be metallized. Two possible concepts are proposed. In Figure 17, the substrate acts 

only along a thin layer between two runs located on 

the same side of the substrate. Because of the 

highly nonlinear property of the material, there is 

little fringing of current into the depth of the substrate. 

,...--.-- 

,CIRCUIT RUN /IN CONDUCT'ON /CIRCUIT RUN 

Figure 17. Varistor substrate with surface conduction 

between runs. 

In Figure 18, the substrate acts as an insulating 

barrier between the runs on one side and the 

ground plane on the other side. Under normal 

operating voltages, the substrate layer offers a very 

large resistance and, thus, is nearly an insulator. 

Upon development of a high voltage between any 

part of the circuit and ground, the surge is immediately diverted to the ground plane without any additional lead length. 

In the latter approach, development of optimum 

compositions would allow producing a substrate 

IN CONDUCTION 

CWlCutT RUN. n 

Figure 18. Varistor substrate with bulk conduction t

ground plane. 

with sufficient mechanical strength and a voltage 

which is still low enough. Here we are dealing with 

the suppression of transients of moderate energy a

the point where they may be induced (presumabl

as the circuit would be illuminated by an EM field)

Varistor Materials Technology 

Many of the unconventional varistor applica

tions listed above required tailoring the varisto

materials properties for optimum implementation o

the system requirements under consideration

Using technology available or under development

we can modify the varistor material in a variety o

ways. For example, we may: 

Increase the varistor breakdown field, thereby 

decreasing the space necessary for a given 

breakdown voltage. 

Decrease (or increase) the varistor dielectri

constant. 

Fabricate screen-printable thin film varistors. 

Fabricate large (up to about lo3 g weight and 

30 cm diameter) devices for extremely larg

energy or current carrylng capacltles. 

Directly press varistors in special shapes (e.g.

rings or slabs) to fit specific connector sys

tems. 

Fabricate low-voltage varistors with break

down voltages as low as 15 to 20 volts. 

Vary the degree of varistor nonlinearity. - 

In general, optimization of a particular varisto

material parameter will involve a trade-off wit

other material properties. The proper componen

design will involve the synergistic combination o

materials technology and circuit design expertise. 

CONCLUSIONS 

Varistor materials have been shown suitable fo

applications other than the conventional lead

mounted device used in electronic circuit boards o

the large blocks used in power system surg

arresters. The feasibility of some approaches ha

been shown, with details of implementation to b

developed. For other approaches, the developmen

is still at the conceptual stage.