SURGE SUPPRESSORS AND CLAMPS 

Franpois D. Martzloff 

National Bureau of Standards 

Gaithersburg MD 20899 

INTRODUCTION 

Various devices have been developed for the 

protection of electrical and electronic 

equipment against transient overvoltages. 

They are often called Ittransient suppressors1I although, for accuracy, they should 

be called "transient limitersIt1 wclamps,ll 

or "divertersI1 because they cannot really 

suppress the transients; rather, they limit 

the transients to acceptable levels or make 

them harmless by diverting them to ground. 

The IEEE dictionary has selected the more 

generic but lengthy term of Itsurge protective device. 

There are two categories of transient suppressors: those that block transients, preventing their propagation toward sensitive 

circuits, and those that divert transients, 

limiting residual voltages. Since many of 

these transients originate from a current 

source, blocking them may not always be 

possible because the current forced into 

the high-impedance blocking path would only 

result in higher voltages and breakdown. 

Therefore, diverting of the transient is 

more likely to find general application. A 

combination of diverting and blocking can 

be a very effective approach. This approach 

generally takes the form of a multistage 

circuit, where a first device diverts the 

transient current to ground; a second device offers a restricted path for transient 

propagation but an acceptable path for the 

signal or power, and a third device clamps 

the residual transient (Figure 1). Thus, 

we are primarily interrested in diverting 

devices. 

RESTRICT 

4 DIVERT PROTECTED 

CIRCUIT 

Figure 1. Multistage protection 

The diverting device can be of two kinds: 

voltage-clamping or short-circuiting devices (the latter called "cr~wbars*~). Both 

of them involve some nonlinearity, either 

frequency nonlinearity (as in filters) or, 

more usuallv. voltaae nonlinearitv. This 

voltage noniinearity is the result of two 

different mechanisms -- a continuous change --- -~ - 

in the device conductivity as the current 

increases, or an abrupt switching action as 

the voltage increases. 

Because the technical and trade literature 

contains many articles on these devices, we 

shall limit the discussions of the details 

but make some comparisons to point out the 

significant differences in the performance. 

Understanding the relative merits and the 

limitations of each technology will allow 

informed choice and recognition that there 

is room for each approach in the wide range 

of needs and applications. 

CROWBAR DEVICES 

The principle of crowbar devices is quite 

simple: upon occurrence of an overvoltage, 

the device changes from a high-impedance 

stat8 to a low-impedance state, offering a 

low-impedance path to divert the surge to 

ground. This switching can be inherent to 

the device, as in the case of spark gaps 

involving the breakdown of a gas or be the 

recently introduced combination of multijunction solid state devices. 

The major advantage of the crowbar device 

is that its low impedance allows the flow 

of substantial surge currents without the 

development of high energies within the 

device itself; the energy has to be spent 

elsewhere in the circuit. This I1reflec

tion" of the impinging surge can also be a 

disadvantage in some circuits if the transient disturbance associated with the gap 

firing is being considered. Where there is 

no power- f ollow (discussed below) , such as 

in some communication circuits, the spark 

gap has the advantage of very simple construction with potentially low cost. 
The crowbar device, however, has three limitations. One is the volt-time sensitivity 

of the breakdown process in air gaps or gas 

tubes. As the voltage increases across a 

gap, significant conduction of current -- 

therefore voltage limitation for the surge 

-- cannot occur until the transition to the 

arc mode of conduction, by avalanche breakdown of the gas between the two electrodes. 

The load is unprotected during the initial 

rise because of this delay time (typically 

in microseconds). Large variations exist 

in sparkover voltage attained in successive 

operations since the process is statistical 

in nature. This sparkover voltage can also 

be substantially higher after a long period 

of rest than after successive discharges. 

Because of the physics of the process, it 

is difficult to produce consistent sparkover voltage for low voltage ratings. This 

difficultly is increased by the effect of 

manufacturing tolerances on very small gap 

distances, but can be alleviated by filling 

the tube with a gas having lower breakdown 

voltage than air. The technology developed 

by manufacturers of gas tubes has minimized 

these effects. One advantage of the solidstate crowbars over the gas breakdown tubes 

is the absence of any practical time delay 

in firing. 

The second limitation is associated with 

the speed of the sparkover, which produces 

fast current rise in the circuits and, thus 

objectionable noise. An example is found in 

hybrid protective systems. Figure 2 shows 

the circuit of such a commercially available device. The gap does a very nice job 

of diverting impinging high energy surges, 

but the magnetic field associated with the 

high di/dt induces a voltage in the loop 

adjacent to the second suppressor, adding a 

substantial spike to what was expected to 

be a low clamping voltage. Some product 

literature advocates locating solid-state 

crowbars on the very circuit board where 

components to be protected are mounted -- a 

perfect recipe for injecting interference 

kght into the circuits! 

A third limitation occurs if power current 

from the steady-state voltage source can 

follow the surge discharge (hence the term 

"power-follown). In ac circuits, this power 

- a 

dl 

Figure 2. Hybrid protector with gap 

follow current may or may not be cleared at 

a natural current zero. Additional means, 

therefore, must be provided to open the 

power circuit if the crowbar device is not 

designed to provide self-clearing action 

within specified limits of surge energy, 

system voltage, and power-follow current. 

This combination of a gap with a currentlimiting, nonlinear varistor has been very 

successful in the utility industry as high 

voltage surge arresters. 

VOLTAGE-CLAMPING DEVICES 

Voltage-clamping devices have variable impedance, depending on the current flowing 

through the device or the voltage across 

its terminal. Impedance variation is monotonic and does not contain discontinuities, 

in contrast to crowbar devices, which show 

a turn-on action. As far as their voltampere characteristics are concerned, these 

devices are time-dependent to a certain degree. However, unlike the sparkover of a 

gap or the triggering of a thyristor, time 

delay is not involved. 

When a voltage clamping device is applied, 

the circuit remains essentially unaffected 

by the device before and after the transient for any steady-state voltage below 

clamping level. Increased current drawn 

through the device as the surge voltage 

attempts to increase results in voltageclamping action. Nonlinear impedance is 

the result if this current rise is greater 

than the voltage increase. The increased 

voltage drop in the source impedance (Zs in 

Figure 3) due to higher current results in 

the apparent clamping of the voltage. It 

must be emphasized that the device depends

on the source im~edance to produce this 

clamping. The circuit behaves as a voltage 

divider where the source impedance (high 

side of the divider) is constant but the 

clamping device impedance (low side of the 

divider) is changing. If the impedance of 

the source is very low, the ratio is low, 

and eventually the suppressor could not 

work at all with a zero source impedance. 

In contrast, a crowbar-type device effectively short circuits the transient toward 

ground, but, once established, this shortcircuit will remain until the current (th

surge current as well as any power-follow 

current supplied by the power system) is 

brought to a low level. 

Figure 3. Voltage-clamping action 
The action of voltage clamping can be performed by any device exhibiting a nonlinear 

impedance. Two categories of such devices, 

having the same effect but operating quite 

different physical processes, have found an 

acceptance in the industry: polycrystalline 

varistors and single-junction avalanche diodes. Another technology, using selenium 

rectifiers, has been practically eliminated 

because of the improved characteristics of 

modern varistors. 

avalanche diodes 

Avalanche diodes were initially applied as 

voltaue clamps in the form of Zener diodes, 

a natural outgrowth of their application as 

voltage regulators. Improved construction, 

specifically aimed at surge absorption, has 

made them very effective suppressors. Large 

diameter junction and low thermal impedance 

connections are used to deal with the problem of dissipating the heat of the surge, 

inherent in a thin single-layer junction. 

The advantage of the avalanche diode is the 

possibility of obtaining quite low clamping 

voltages and nearly flat volt-ampere characteristics over its useful power range. 

Therefore, these diodes are widely used in 

low-voltage electronic circuits to protect 

5 V or 15 V logic circuits, as an example. 

At higher voltages, the problem of the heat 

generation associated with single junctions 

can be overcome by stacking several lower 

voltage junctions. 

Characteristics of Avalanche Diodes --- 

Silicon avalanche diodes are available with 

characteristics especially tailored to providing transient suppression. These special 

diodes must not be confused with regulatortype Zener diodes although many engineers 

tend to use the generic term "Zener diode." 

May Zeus help them if they were to misapply 

a regulator-type Zener, expecting effective 

protection! 

Figure 4 shows a typical family of the V-I 

characteristics for one product. The parameter I is the peak of a specified current 

Figure 4. Typical V-I characteristic 

waveform, generally the 10/1000 p s doubleexponential test waveform preferred bv com- 1 --- 

munication engineers, rather than a 8/20 ps 

waveform preferred by power engineers. The 

V-I curves are remarkably flat from 1 to 20 

amperes, with clamping voltages low enough 

to protect sensitive electronics. 

Manufacturers of avalanche diodes generally 

rate their devices in terms of maximum peak 

power for a specified pulse duration. Even 

though the rated power will decrease with 

increasing pulse duration, it is not a case 

of constant total energy (see Figure 5). A 

two-decade increase in pulse duration will 

reduce the power by only about one decade, 

primarily because of increasing heat transfer for longer pulses. 

Since the junction is very thin, the capacitance of an avalanche diode is appreciable. This capacitance can be a concern in 

high-frequency circuits where it would produce an undesirable insertion loss. It is 

possible to minimize this effect by using a 

combination with a low-capacitance ordinary 

diode in series with the avalanche diode. 

Properly packaged and installed avalanche 

diodes exhibit a quick response to steepfront pulses, and have been used for NEMP 

protection. However, this quick response 

can be completely obliterated by improper 

wiring. The effect of lead length, discussed in detail in the next section, is 

applicable to any transient suppressor. 

Varistots 

The term varistor is derived from its funcction as a -able resim. It is also 

called a voltage-dependent resistor, but 

that description implies that the voltage 

is the independent parameter in surge protection, an incorrect perception. Two very 

different devices have been successfully 

developed as varistors: 1) silicon carbide 

disks have been used for years in the surge 

arrester industry and 2) metal oxide varisristors are now widely used. 

kW peak 

1m 

la 

0 

1 

Figure 5. Peak pulse power vs. pulse time 
In silicon carbide varistors, as well as in 

metal oxide varistors, the relationship between the current flowing in the device and 

the voltage appearing across its terminals can be represented approximately by a power 

function I = kV a. In this equation, the 

exponent a, can be considered as a figure 

of merit: the higher the value of a, the more effective the clamping. Hence, there 

has been a race between manufacturers and 

specification writers for higher and higher 

values of a. We will see, however, that 

there are practical limits to this race and 

that better performance can be obtained at 

higher current densities by departing somewhat from large values of the exponent a. 

In silicon carbide varistors, the physical 

process of nonlinear conduction is not com- pletely understood, and the manufacturing 

of the material, successful as it is, has 

remained an art. It appears that the proc- ess takes place at the tips of the grains 

of silicon carbide which are held together 

by a binder. The story is told that this 

device's action was found accidentally by 

having a grinding wheel, on a disorderly 

work bench, inadvertently connected to an experimental circuit; for many years the 

silicon carbide varistors indeed looked 

like grinding wheels, each complete with a hole in the center. 

Metal oxide varistors depend on the conduction process occurring at the boundaries 

between the grains of oxide (typically zinc 

oxide) grown during a carefully controlled 

sintering process. The physics of the nonlinear conduction have been described in 

the literature [I-41; in these application 

notes, we will be more concerned with the 

behavior of the varistors as two-terminal 

electrical components. 

Electrical Characteristics of Varistors--- 

Because the prime function of a varistor is 

to provide the nonlinear effect, the other 

Figure 6. Varistor equivalent circuit 

parameters are generally the result of a tradeoff in design and inherent characteristics. Electrical behavior of a varistor can be understood through the equivalent 

circuit of Figure 6. The major element is 

the varistor proper, Rv, whose V-I characteristic is assumed to be the perfect power 

law I = kva. In parallel with this varistor, there is a capacitor, C, and a leakage 

resistance, R In series with this threecomponent grocp, there is the bulk resistance of the zinc oxide grains, Rs, and the 

inductance of the leads, L. 

Under dc conditions (at low current densities, because obviously no varistor could 

stand a high energy produced by dc currents 

of high density), only the varistor element 

and the leakage resistance are significant. 

Under pulse conditions at high current densities, all but the leakage resistance are significant: the varistor provides low impedance to the current flow, but eventually 

the series resistance produces an upturn in 

the V-I characteristic; the lead inductance 

can produce spurious - overshoot problems if 

it is not dealt with properly; the capacitance can offer either a welcome additional 

path for fast transients or an obectionable 

loading for high-frequency signals, depending on the application. 

V-I Characteristics --- When the V-I characteristics are plotted on a log-log graph, 

the curve of Figure 7 is obtained. It has 

three regions as shown, resulting from the 

dominance of Rp, h, Rs as the current in 

the device goes from nanoamperes to kiloamperes. 

The V-I characteristic is the basic application design tool for selecting a device 

in order to perform a protective function. 

For successful application, however, other 

factors, which are discussed in detail in 

information available from manufacturers, 

must also be taken into consideration. 

CU(IIIINT AMCEIYS 

Figure 7. Theoretical V-I characteristic 

Eol A 
Some of these factors are: 

Selection of the appropriate nominal 

voltage for the line voltage 

(Avoid unnecessarily low clamping.) 

Selection of energy-handling capability 

(Consideration of the source impedance, 

waveshape, and number of occurrences of 

the transient [5].) 

Heat dissipation 

(Ambient temperature, steady state and 

transient energy.) 

Proper installation in the circuit 

(Lead length). 

In fact, enough instances of poor'installation practices have been observed that a 

brief discussion of lead effects is quite 

in order. 

When making voltage measurements across a 

clamping device for evaluating its performance, one must recognize possible instrumentation artifacts which require special 

precautions to avoid errors: 

1. Use two probes in a differential mode 

to make a measurement directly at device 

terminals, and not a single-ended system. 

2. Avoid contaminating the true device 

voltage by the additional voltage caused by 

magnetic coupling into the probes loop. 

Commercial oscilloscope preamplifiers offer 

a wide choice of differential mode operation, either through an [add + invert] mode 

of two-channel preamplifiers, or through a 

differential amplifier built specifically 

for high common-mode rejection (sometimes 

at the expense of bandwidth). Thus, careful attention must be given to these two 

aspects of measurements. 

U CLOSE 

Figure 8. Circuit configuration for 

measurement of voltage drop 

along conductor with various 

probe geometries 

The voltage measured by the two probes (or 

the voltage that would be applied to a protected load downstream from the device) is 

the sum of the actual clamping voltage at 

the device terminals and a spurious voltage 

caused by magnetic coupling. This spurious 

voltage is induced into the loop formed by 

the clamping device leads and the probes 

(or the downstream wiring in the case of an 

actual protective scheme) by the changing 

magnetic field of the surge current flowing 

in the device. 

To illustrate this situation, the measurement circuit shown in Figure 8 was set up 

in the output circuit of a generator producing a 8/20 ps impulse. The "devicem was 

a hollow conductor, with a hole at the center through which a twisted pair was fed, 

one wire of the pair branching out to each 

end of the conductor, separated by 10 cm. At the same 10-cm separation, but outside 

of the hollow conductor, two thin wires 

were also soldered, brought to the midpoint 

of the hollow conductor and in close contact with the conductor; from the midpoint 

outward, they were twisted in the same manner as the inside pair. A third pair of 

wires was soldered at the end points of the 

hollow conductor, and arranged to form a 

rectangle, the hollow conductor being one 

side of that rectangle. Various widths were 

set up for the rectangle, and each time the 

measured voltage was recorded. Figure 9 

shows measured voltage vs. radial distance 

of the opposite side of the rectangle. The 

effect is present even for close proximity, 

and reaches a saturation beyond 10 cm. 

This example shows that one must not only 

connect the probes as close as possible to 

the clamping devices terminals, but also 

strive to minimize the area established by 

the probes close to the devices. 

RADIAL DISTANCE - cm 

Figure 9. Voltage observed by probes 

versus radial distance from 

conductor to opposite side 

of rectangular loop 
In this case of a low-voltage suppressor, 

it would be better to solder short leads to 

the device terminals, bring them together 

while hugging the device, then twist them 

into a pair and connect the oscilloscope 

probes some distance away from the device. 

The same conclusion is applicable to the 

wiring layout in actual hardware: Creating 

a loop near the protective device is an invitation to induce additional voltages in 

the output of the protective device, thus 

losing effectiveness (Figure 3). 

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 (more 

accurately of loop area) for connecting the 

protective devices. This warning is especially important when the currents are in excess of a few amperes with rise times of 

less than 1 us. 

COMPARISONS OF PROTECTIVE DEVICES 

LineWhen a protection scheme is designed for an electronic system operating in an environment which is not completely defined, it is 

often necessary to make an assumption about 

the parameters of the transient expected to occur. In particular, if an error is made 

in assuming the source impedance of this 

transient, the consequences are dramatically different between a linear device for 

which the protective level will be affected 

in proportion to the error, and a nonlinear 

device where the protective level will be 

affected approximately in inverse ratio to 

the exponent of the V-I characteristic. 

&ark GaD Versus Varistor 

The choice between these two devices will 

be influenced by the inherent characteristics of the application. Where powerfollow may be a problem, there is little 

opportunity to apply a simple gap. Where 

very steep-front transients occur, the gap 

alone may let an excessive voltage go by 

the 19protectedn circuit until the voltage 

is limited by sparkover. Where the capacitance of a varistor is objectionable, the 

low inherent capacitance of a gap will seem attractive. If very high energy levels can be deposited in a varistor (the power dissipated remains as heat energy trapped in 

the bulk material), compared to the lower 

levels inherent to the crowbar action of a gap, then a surge arrester of any type with 

high capacity connected at the service 

entrance may be combined with a varistor of 

lower clamping voltage installed further in 

the circuit. This combined protection, however, requires proper coordination between 

the two suppressors [6]. 

Fvalanche Diode Versus V&sto~ 

The basic performance characteristics of 

these two devices are similar, and therefore the choice may be dictated by clamping 

voltage requirements (avalanche diodes are 

available at lower clamping voltages), by 

energy-handling capabilities (avalanche 

diodes are generally lower in capability 

per unit of cost), or by packaging requirements (varistor material is more flexible 

and does not require hermetic packaging). 

FAILURE MODES 

Failure of electrical components can occur because their capability was exceeded by 

the applied stress or because some latent 

defect in the component went by unnoticed 

in the quality control processes. This 

situation is well recognized for ordinary 

components but a surge protective device, 

which is no exception to these limitations, 

tends to be expected to perform miracles, or at least to fail graciously in a "failsafet1 mode. 

The term "fail-safe, l9 however, means dif - 

ferent failure modes to different users, 

therefore it should not be used. To some users, fail-safe means that the protected 

hardware must never be exposed to an over- voltage, so that failure of the pr0tectiv.e 

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 hard- ware 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 a 

failure mode wfail-shortn or nfail-open,9

as the case may be. 

When the diverting path is a crowbar-type 

device, little energy is dissipated in the 

crowbar, as noted earlier. In a voltageclamping device, because more energy is 

deposited in the device, the energy handling capability of a candidate protective 

device is an important parameter to con- sider in the design of a protection scheme. 

With nonlinear devices, an error made in 

the assumed value of the current surge produces little error on the voltage developed across the protective device and applied to 

the protected circuit (the error, however, 

affects directly the amount of energy which 

the protective device has to absorb. At 

worst, when surge currents in excess of the 

protective device capability are imposed by 

the environment, such as an error made in 

the assumptions, a human error in the use of the device, or because nature tends to 

support Murphy l s law, the circuit in need 

of protection can generally be protected at 

the price of failure in the short-circuit 

mode of the protective device. However, if 

substantial power-frequency currents can be 
supplied by the power system, a fail-short 

protective device generally terminates as fail-open when the power system fault in 

the failed device is not quickly cleared by 

a series overcurrent protective device (a 

fuse or a breaker). 

When there is a need to eliminate a failed 

protector at the specific equipment level, 

insertion of the fuse in the line provides 

protection of the equipment (Figure lOa), 

while insertion of the fuse in series with 

the shunt-connected protector provides protection of the function, albeit with loss 

of overvoltage protection (Figure lob . A 1 fuse must be selected with suitable i t to 

withstand the effect of repetitive surges 

expected to flow in the application. 

REFERENCES 

1. Matsuoka, M., Masa Yama, T. and Lida.,Y. 

"Nonlinear Electrical Properties of Zinc 

Oxide Ceramics," Proc. First Conf. Solid 

State Devices, Tokyo, 1969, J. JaDan Soc. 

 ADD^. PhVS., 39 1970, Suppl. p.94. 

2. Harnden, J.D., Martzloff, F.D., Morris, 

W.G. and Golden, F. B., "The GE-MOV varistor 

-- The Super Alpha Varistor," Electronics, 

45, no.21, 1972, p.91. 

3. Sakshaug, E.C., Kresge, J.S. and Miske, 

S.A., "A New Concept in Station Arrester 

De~ign,~~ IEEE PAS-96 No.2, March-April 

1977, p.647. 

4. Mahan, G.D., Levinson, L.M., and 

Philipp, H.R.,llTheory of Conduction in ZnO 

Varist~rs,~~ Report 78CRD205, General 

Electric Company, Schenectady, NY, 1978. 

5. Martzloff, F.D., "Matching Surge 

Protective Devices to Their Environment," 

JEEE IA-21 No.1, Jan/Feb 1985, p.99. 

Figure 10. Fusing options for suppressors 

6. Martzloff, F.D., "Coordination of Surge 

Protectors in Low-Voltage Power CircuitsI8l 

IEEE PAS-99 No.1, Jan/Feb 1980, p.129. 

CONCLUSIONS 

1. Surge protective devices are available 

for protecting low-voltage electronics. 

Two basic types offer different advantages: 

* Crowbar devices have a high current 

capability but they would produce a 

power-follow, if applied in a power 

system; their fast switching action 

can be a cause of problems if it is 

not recognized. 

Voltage clamping devices, either the 

avalanche junctions or the varistors, are free from the problems of powerfollow and interference caused by a fast switching, but the energy which 

is deposited in these must be recognized for long-term reliability. 

2. Avalanche diodes offer low clamping 

voltage, which makes them most suitable for 

low-voltage, low-power electronics. 

3. Metal-oxide varistors are now available 

in a wide range of forms, clamping voltages 

and energy-handling capacities. 

4. Each of these devices has its own best 

field of application for better reliability 

of the circuits in the not-quite-defined 

electromagnetic environment of power and 

communication systems. 

ABOUT THE AUTHOR - Fran~ois D. Martzloff is 

a graduate from French 

engineering schools and 

continued his studies at 

Georgia Tech for an MS, 

EE and at Union College 

for an MS in Industrial 

Administration. 

His 29-year career at 

General Electric Company 

included design of high 

voltage bushings, fuses, 

and 20 years in the area of transient overvoltages measurements and 

suppression, especially with varistors. 

In 1985, he joined the National Bureau of 

Standards, Electrosystems Division, with a charter of measurements and correction of 

Conducted Electromagnetic Interference. 

As a Fellow of the IEEE, he is active in 

the Surge Protective Devices Committee, as the Chairman of the Working Group in surge 

Characterization in Low-Voltage Circuits. 

He is also a member of the Surge Protection 

and Wave Distortion Subcommittee. 

He is serving in advisory capacity to two 

International Electrotechnical Commission 

Technical Committees and to the Industry 

Advisory Group of the Underwriters Laboratories on Transient Suppressors.