ceramic material that could eventually result in thermal runaway (see Selecting Varistor Clamping

Voltage: Lower is not Better!” in Part 7). That concern led to the experimental work reported in “The Ef

of Repetitive Swells on Metal-Oxide Varistors” in 1992, also available in Part 7 of this Anthology
A GLIMPSE AT LONG-TERM EFFECTS OF MOMENTARY 

OVERVOLTAGES ON ZINC OXIDE VARISTORS 

Fransois D. Martzloff and Thomas F. Leedy 

National Institute of Standards and Technology 

Gaithersburg, Maryland 20899 

ABSTRACT 

Because the prime function of varistors is the diversion of high energy surges, 

most of the attention is directed toward selecting the appropriate device rating 

to ensure long life under surge conditions. Some attention is also given to 

matching steady-state rating of the device to the power system voltage. 

However, during abnormal (and not well defined) power system conditions, the 

line voltage can reach values that will cause substantial current in the varistor. 

Until the effects of these momentary overvoltages are better identified and 

understood, there will be a risk of near-term failure at worst and accelerated 

aging at best. 

SELECTION OF VARISTOR RATINGS 

Selection of a varistor rating is essentially a process of matching the capabilities 

of the device to the power system environment, while obtaining the desired low 

clamping voltage during surge events. Low clamping voltage of a varistor may 

be obtained in one of two ways, or both: low current density at the surge current, 

which means a relatively large area, or low intrinsic voltage, which means a 

relatively thin disc. Increasing the area of a disc increases the device cost so that 

the natural tendency of designers will be to seek the thinner disc rather that the 

larger diameter disc. Therefore, this paper addresses the concerns raised by 

excessive reduction of disc thickness, that is, the use of varistors with rated line 

voltage lower than what prudent practice would indicate. 

In matching a varistor to its environment, the two principal environment 

characteristics to be considered are: (1) the surges expected to occur on the 

system -- the primary reason for using the varistor -- and (2) the system rated 

voltage. In the application of surge arresters to electric utility systems, 

considerable attention is given to abnormal power system conditions, as evident 

in the rating of these arresters which takes into consideration the maximum 

continuous operating voltage (MCOV) permissible for an arrester.[l] 

Varistor Technology 
In the the case of low-voltage electronic circuits, equipment designers generally 

rely on the manufacturer's rating of a device for the stated nominal system 

voltage. This system voltage is defined with some expected tolerance, such as 

1596, on the maximum voltage that can occur. 

Surge ratings - Specification sheets published by varistor manufacturers 

generally include a "Pulse Rating" family of curves showing the number of 

permissible surges (pulses) as a function of current amplitude and duration. 

This pulse rating makes it possible to select a device of appropriate dimensions 

-- that is, current handling capability -- to ensure survival in the known or 

postulated surge environment [2]. Actually, survival mkans more than the 

non-occurrence of a catastrophic failure. For varistors used in electronic 

equipment, the criterion for end of life, or total consumption of the Pulse Rating, 

is a change of more than 10% in the device nominal voltage. The actual process 

leading to this change in device characteristic is not clearly described by the 

manufacturers literature, but is generally believed to involve microscopic 

damage to the structure. 

Steady-state vs. momentary ratings - The steady-state rating is generaIIy 

determined by the manufacturers, on the basis of in-house experience. This 

process is reflected in the wording of an IEEE Standard on varistors defining 

"Rated RMS Voltage" as follows: There is no single test that can determine the 

voltage rating but rather an evaluation process taking into consideration .... mese 

considerations are within the realm of the manufacturer's and user's application 

engineering functions. [3] In plain English, this means that there is no clear-cut 

guidance available; rather, designers are left on their own to make the hard 

choices. The manufacturers, however, do publish an Arrhenius plot of mean 

life versus temperature, implying high activation energy levels in the processes 

leading to the end of life under steady-state line voltage. 

Field experience has shown that the expectation of a + 1570 maximum deviation 

of line voltage is optimistic. The term "swell" has been proposed [4] to describe 

the occurrence of a momentary increase of the sinusoidal line voltage amplitude, 

lasting from a few cycles to a few seconds. During such a swell, the normal 

standby current of the varistor will increase in accordance with the varistor 

power law. For instance, a 20% increase in line voltage (a factor of 1.2) rather 

than the 1570 often considered as the "high-line limit" by designers, with a 

varistor exponent of 32, produces an increase of the standby current of 1.2 

elevated to the 32th power -- a 340-fold increase ! This current level can no 

longer be considered a benign "standby current." 

Varistor Technology 
Thus, the selection process of a varistor must also include proper attention to 

the long-term effects of repeated swells. A search of the literature reveals that 

no work has been published on these effects. 

Perhaps one of the reasons for this lack of information is that very little specific 

information has been collected on the severity of swells occurring in power 

systems. The electric utilities and their customers have recently increased their 

awareness of the so-called Power Quality issue, as a result of concerns over the 

performance of increasingly sophisticated electronic equipment supplied by a 

disturbance-prone power system. One of the effects of these concerns has been 

the development of disturbance monitors with graphics capability. As more and 

more of these monitors are applied, the occurrence and characteristics of swells 

will become much better known. 

Thus, between the well-defined bounds of short-time surges and the less defined 

steady-state voltage rating, neither the occurrence nor the effects of repeated 

swells are well defined. Faced with this lack of information, the authors have 

initiated a joint project of experimental and theoretical evaluation of the effect 

of repeated swells on zinc oxide varistors. The experimental work consists 

essentially in applying repeated swells of various amplitudes and durations, 

seeking to detect some change in the electrical characteristics of the varistor 

that would herald the onset of objectionable degradation. The theoretical work 

consists in modeling the behavior of the varistor, essentially its temperature, in 

response to a variety of swells. To fully understand the science involved, 

however, this work should be complemented by fundamental studies on the 

degradation mechanisms leading to any observed change in the electrical 

characteristics. One of the purposes of this paper is to motivate ceramists 

toward looking into the effects of swells from the point of view of ceramic 

structural changes. In the meantime, the experimental work, supported by 

modeling to identify the impact of various levels of swells, wiIl draw the attention 

of equipment designers to the pitfalls of selecting excessively thin discs.[5] 

MODELING THE EFFECT OF SWELLS 

A circuit analysis model of a metal-oxide varistor was used to investigate the 

behavior of varistors when exposed to swells . The model consists of two parts: 

The electrical part of the model predicts the current-voltage behavior of the 

varistor and the thermal part of the model predicts the temperature changes 

caused by the energy deposited in the varistor during the swell. The electrica1 

and thermal simulations were performed using a commercial software system 

which uses behavioral equations to mimic nearly any physical device or process. 

Varistor Technology 
The model computations were performed for avaristor rated at 130 V rms. Such 

a varistor is the lowest rating offered by manufacturers for a 120 V circuit. To 

represent a possible worst case, the varistor characteristics used for the model 

computations were set at the lower limit of the tolerance band. To simplify the 

model, no allowance is made for heat loss by convection, the situation that can 

be expected in a tight packaging without any encapsulation. Heat losses by 

radiation are negligible at the temperatures involved. These postulates provide 

worst case results, a justifiable approach when considering reliability. 

Figure 1 shows an example of the output plot of the model computations, for a 

206 V peak line voltage (145 V rms, a 20% swell). The current exhibits the highly 

nonlinear response of a varistor, with 20 mA peaks, The resulting temperature 

increase steps are also plotted for each successive pulse, occurring at the 120 

pulse-per-second rate. As the temperature increases, so will heat losses, until 

a thermaI equilibrium is reached, unless thermal runaway would occur. 

Figure 2 shows the temperature rise predicted for the application of swells at 

two voltage levels beyond the 20% swell of Figure 1, each with a duration of 

200 s. This 200 s duration was selected because it approximates the time 

required to reach thermal equilibrium in this model, not because of an inference 

that power system swells last 200 s. The lower curve shows the temperature rise 

of approximately 130 "C expected from 156 V rrns (a 30% swell). The upper 

curve shows a temperature rise of nearly 300 "C for 160 V rms (a 33% swell)

These curves illustrate that a relatively small change in the swell level, from 

156 V to 160 V rms, produces a drastic increase in the power dissipation and the 

temperature rise of a varistor. The 300 "C temperature rise means assured 

destruction of the varistor, the 130 "C temperature could mean some accelerated 

aging, if repeated enough times. Thus, the cumulative effect of repeated swells 

needs to be evaluated. This evaluation is the subject of the experimental work. 

EXPERIMENTS WITH REPEATED SWELLS 

Several factors are known or suspected in the behavior of actual varistors 

exposed to repeated stress, complicating the apparent simplicity of varistor 

modeling. Aging -- a change in the varistor characteristics in response to 

overstresses -- is only one of them. Even when considering only aging, there is 

no universally recognized criterion of degradation. Most varistor specifications 

' refer to a 10% limit in changes of the nominal voltage of the varistor. However, 

since this nominal voltage is defined arbitrarily for a 1 mA current regardless of 

the varistor cross section, the criterion varies with varistor sizes. Another 

proposed criterion, power dissipation at some ac voltage, is complicated by the 

fact that the power dissipation changes with the duration of application of the 

steady-state as well as swell ac voltages. 

Varistor Technology 
This effect, which permanently changes the characteristics of the varistor, 

sometimes called "formation," was quite apparent when successive swells were 

applied. During application of one swell, for instance, the increasing 

temperature of the varistor is reflected in the increased peak amplitude of the 

current, as shown in Figure 3k However, when comparing the pattern of 

current peaks within the first swell to the pattern of a later swell, after many 

additional swells (Figure 3B), the most obvious change is a reduction of the 

current peaks between the original swell and the later swell. Thus, a systematic 

method to account for that "formation" effect must be developed to define a 

valid criterion of aging before conclusions can be drawn on quantifying the aging 

effect of swells. 

CONCLUSIONS 

Attempts at improving surge protection by selecting varistors with low voltage 

ratings increase the risk of premature aging, even early failure, of varistors 

exposed to repeated momentary overvoltages ("swells"). 

The effects of such repeated swells have not been documented by the 

manufacturers, although it is a subject of investigation that would provide useful 

results for greater reliability. Additional theoretical and experimental work is 

necessary to develop guidance useful to prudent equipment designers. 

While the occurrence of these swells has not been well characterized in the past, 

the increasing availability of power line disturbance monitors will improve this 

characterization in the future. Combined with a better understanding of the 

effects of swells, this knowledge will enhance the reliable application of 

metal-oxide varistors. 

REFERENCES 

[I] Sakshaug, E.C., Burke, J.J., and Kresge, J.S., Metal Oxide Arresters on 

Distribution Systems - Fundamental Considerations, IEEE Winter Power 

Meeting Paper, 1989. (To be published in IEEE Trcutsuctions on Power Delivery.) 

[2] Martzloff, F.D., Matching Surge Protective Devices to their Environment, 

IEEE-US Transactions, Vol. IA-21, No.1, pp. 99-106, 1986. 

[3] IEEE Standard Test Specifications for Varistor Surge-Protective Devices, 

ANSmEEE C62.33- 1982. 

[4] Martzloff, F.D., and Gruzs, T.M., Power Quality Site Surveys: Facts, Fiction 

and Fallacies, IEEE-US Transactions, Vol. 24, Noh, pp. 1005- 1017, 1988. 

[5] Martzloff, F.D., and Leedy, T.F., Selecting Varistor Clamping Voltage: 

Lower is not Better ! Proceedings, EMC Ziirich Symposium, pp. 137-142, 1989. 

Varistor Technology 
30 0 

9 200 

.- 3 L 

E 

3 

C 2 roo 0 

Q 

E + 

0 

I 

0 50 100 150 200 250 300 

TIME (s) 

Figure 1. Computed instantaneous 

temperature increase of a varistor 

during two cycles of a 20% swell 

Figure 2. Computed integrated 

temperature increases during a 

30% swell and a 33% swell 

(A) Fist swell applied to varistor (B) Tenth swell applied to same varistor 

Note: These recordings are obtained by taking eight samples, each with a 

duration of one cycle, at Zsecond intervals during the 16-second swell, 

and displaying these samples as a single record of the digital oscilloscope. 

Figure 3. Samples of varistor current during a 16-second swell 

with peak voltage at 117% of the varistor nominal voltage 

Varistor Technology