acceptable lifetime for repetitive surges of magnitude less than the maximum singleshot rating often 

emphasized in application and marketing information. 

In a second part, the paper describes experiments conducted on commercial fuses to subject them to 

repetitive current surges as defined in the first part. Empirical information is then suggested on how the 

rating of a fuse should be modified to account for the effect of repetitive surges. 

At the time of publication, the paper was recognized by an IEEE award. Twenty years later, as the 

importance of the role of SPD disconnectors and their proper design is gaining momentum, the concerns 

expressed in this paper still appear timely. 
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS. VOL IA-21, NO. 1, JANUARYIFEBRUARY 1985 99 

Matching Surge Protective Devices to Their Environment 

FRANCOIS D. MARTZLOFF, FELLOW, IEEE 

Abstract-A method is described for the rational selection of metal 

oxide varistors as surge protective devices in low-voltage applications, 

combining data on the surge environment and device characteristics. 

Fuses that may be connected in series with the varistors will be exposed to 

current surges resulling from the varistor operation. Experimental results 

on the effect of repetitive current surges on such fuses a-e described, and 

some guidance is provided on the necess ry derating of fuses for series 

applications with varistors. 

INTRODUCTION 

S URGE CURRENTS diverted from sensitive loads into a 

protective low-voltage varistor will deposit energy into the 

varistor material, resulting in a long-term shift in the standby 

characteristics of the varistor. This behavior has been well 

documented [I], [2]. The proper application of varistors, 

nevertheless, prevents any adverse consequences of this 

behavior. 

Pulse lifetime curves published by manufacturers to describe this behavior show families of repetitive pulses at equal 

amplitude; however, the real world involves a full range of 

amplitudes, as described in [3]. Analytical and numerical 

methods have been suggested [4]-[6] to combine the statistical information on surges of [3] with pulse lifetime curves in 

order to estimate the time required for a given varistor in a 

given environment to reach its rated life. In this manner, a 

rational selection can be made by matching the capability of 

the protective varistor to the known or postulated characteristics of the surge environment. The first part of this paper 

describes the numerical method, with examples for 1201240-V 

circuits. 

Both the statistical distribution of surges in the environment 

and the legitimate economic wish not to overprotect call for an 

awareness that a varistor might fail when subjected to a surge 

with an energy content greater than the rating of the device. 

The usual practice is to connect a fuse in series with the 

varistor in order to clear the shorted varistor from the circuit. 

The fuse, then, is subjected to a current surge each time the 

varistor diverts a surge. Data published by fuse manufacturers 

do not provide guidance on the selection of fuses for 

applications where surges of microseconds' duration are 

encountered. To fill this void, a set of tests was performed on 

various types of fuses. The second part of this paper presents 

the results of these tests, with suggestions on the necessary 

derating for the fuses. 

Paper ICPSC 83-71, approved by the Power Systems Protection Committee 

of the IEEE Industry Applications Society for presentation at the 1983 

Industry Applications Society Annual Meeting, Mexico City, Mexico, 

October 3-7, 1983. Manuscript released for publication May 7, 1984. 

The author is with the General Electric Company, Buidling 37-442, P. 0. 

Box 43, Schenectady, NY 12301. 

MATCHING THE VARISTOR CAPABILITY TO THE 

SURGE ENVIRONMENT 

Two examples of surge environments will be used to 

evaluate the effect of repetitive surges on typical varistors. In 

the first, the varistor is installed at the semice entrance to a 

building; in the second, the varistor is installed somewhere in 

the building, with no protection provided at the service 

entrance. Other types of applications can be evaluated in a 

similar manner. 

The procedure for evaluating the effect of repetitive surges 

involves developing a table for computing the effects at the 

various levels of surges, such as those shown in Fig. 1, on the 

basis of the pulse lifetime data published by the manufacturers 

of varistors. In this manner, an estimate can be made of the 

time required for a given varistor to reach its rated life in a 

given surge environment. These calculations are based on 

graphical data which do not allow three-digit accuracy, even 

though the numbers shown on the table are carried to three 

digits for the sake of checking the arithmetic. Futhermore, 

reaching the "rated life" does not mean that the device has 

reached the end of its service life. Pulse lifetime ratings are 

based on the arbitrary definition of rated life as the point at 

which the nominal voltage of the varistor [I] has decreased by 

ten percent. This decrease indicates some physical change in 

the varistor structure but does not affect the ability of the 

varistor to clamp surges. For properly applied varistors, a 

change of 20 percent in the nominal voltage would not be 

objectionable. Thus the rated life of a varistor is a very 

conservative designation. 

The first task in this evaluation is to convert the voltage 

surge density probability of [3] (Fig. 1) into a histogram of 

surge occurrences. From Fig. 1, selecting a given exposure, 

we read the number of occurrences between two voltage 

levels, defining a cell of the histogram. 

In Fig. 2, the medium exposure line of Fig. 1 has been 

redrawn. Voltage cells of approximately equal width (on the 

logarithmic scale) have been drawn. The center values of the 

cells are shown to the left of Fig. 2: 13 000, 10 000, 8000, 

etc. From the medium exposure line, the corresponding 

number of occurrences can be read on the vertical scale. These 

numbers are shown to the right of each cell: 0.03, 0.07, 0.18, 

etc. These numbers represent the number of occurrences 

exceeding the voltage level, so that the number in the cell itself 

is the number for the level, minus the number for the level to 

the right. For instance, the number of occurrences in the 

10 000-V level cell is 0.07 - 0.03 = 0.04 occurrences, and 

so forth for the other cells. 

Surge recordings, such as those reported in [3], show that 

switching transients are not likely to exceed 3000 V. We can 

0093-9994/8510100-0099$01.00 O 1985 IEEE 
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. IA-21, NO. I, JANUARYIFEBRUARY 1985 

SURGE CREST kV 

*In some locations, sparkover of clearances may limit 

the overvoltages 

Fig. 1. Rate of surge occurrence versus voltage level 131. 

Number Due to Lightning 

Voltage Level of Cell Number Computed 

I Number of Occurrences for Each Cell 

Exceeding Level 

160 80 

Fig. 2. Derivation of cell histogram. 

then am-ibute all occurrences above 3000 V to lightning; hence 

we can assign an 8120-ps waveform to the resulting current 

flowing in a varistor connected at the service entrance, or at a 

point where no protective device has been installed upstream, 

which could divert part of the surge. Conversely, occurrences 

below 3000 V can be considered as an Undefined combination 

of switching transients and lightning transients. It seems 

reasonable and conservative to consider that half of these may 

be lightning, with the corresponding high energy of the 8120- 

ps wave occurrences computed for each cell. Furthermore, 

continuing the histogram beyond the limits of Fig. 1 by 

extrapolating the lines above lo3 occurrences per year would 

imply more than 1000 lightning discharges per year at one 

location. Because this situation is unlikely, the histogram is 

truncated at that level. 

The first application example is a 32-mm varistor, rated 150 

V, which is assumed to be connected at the entrance of a 

building and thus would be subjected to the Category B [3] 

surges. As shown in Fig. 3, these surges are defined as 

having 6-kV open-circuit voltage and 3-kA short-circuit 

current. The ratio of voltage to current can be considered as 

the source impedance of the surge, which is 2 Q in this 

example. Another definition of the source impedance is 

implied in a requirement of the Federal Communications 

Commission [7] applicable to the ac input of devices interfacing with the telephone plant. In that requirement, a 1000-A 

short-circuit current is specified for the test generator having 

an open-circuit voltage of 2500 V, hence a source impedance 

of 2.5 3. Thus the selection of 2 Q in this example is based on 

accepted industry standards. 

In Table I, the first column shows the voltage level of each 

cell; the number of occurrences for that level, as derived from 

Fig. 2, is shown in the second column. From the varistor V-I 

characteristics (Fig. 4), an expected clamping voltage can be 

determined by iteration, for the current corresponding to that 

voltage level, as shown in the third column. The voltage of the 

cell, minus the expected clamping voltage, is the driving 

voltage shown in the fourth column. With the 2-3 source 

impedance derived from Fig. 3, this driving voltage, divided 

by two, yields the resulting current shown in the fifth column. 

From the pulse lifetime curves of Fig. 5, the rated number of 

pulses for each current level is entered in the sixth column. 

Finally, the last column shows the percentage of rating 

consumed for the number of pulses at that level; for instance, 

at the 3200-V level, with four occurrences at a current of 1360 

A, hence a rating of 500 pulses, the percentage consumed is 

4500 = 0.80 percent. The sum of the percentages of all levels 

is the percentage of rating consumed in one year. The number 

of years required to reach the rated value, shown at the bottom 

of the table, is the inverse of the yearly percentage. 

For the selected example of a 32-mm varistor, rated 150 V 

and connected in a Category B location at medium exposure, 

16 years would be needed to reach the rated pulse life of the 

device under the conservative assumptions made in this 

example. Once again, reaching rated life does not mean 

reaching a device failure level. 

Another estimation example will be useful to illustrate the 

process and the types of reasonable assumptions that can be 

made. Consider a varistor installed in an appliance connected 

at some distance from the service entrance, still in Category B, 

medium exposure. Table I1 shows the corresponding pairs of 

occurrence numbers and levels, from which computations 

similar to those of Table I1 can be derived. We add some 

impedance to the 2-Q value used in the preceding example, say 
MARTZLOFF: MATCHING SURGE PROTECTIVE DEVICES TO ENVIRONMENT 

Energy (joules) 

Impulse Type Deposited in a Suppressor* 

Comparable to of Specimen with Clamping Voltage of 

Location IEC No 664 Mediilm Exposure or Load 500V 1 OOOV 

Category Category Waveform Amplitude Circuit (120 V System) (240 V System) 

A Long branch 

Circuits and 

outlets 

I1 6 kV High impedancet - 0.5 ps-100 kHz 200 A Low impedance?. 5 0.8 

B Major feeders, 1.2 X 50ps 6 kV High impedance+ - - --. 8 X 20 pr 3 kA Low 40 80 impedance+ 

circuits, and 111 6 kV High lmpedancet - - I load center 0.5 p-100 kHz 500 A Low impedanceT. 5 2 4 

*Other suppressors having different clamping voltages would receive different energy levels. 

t~or hlgh-impedance test specimens or load clrcuts, the voltage shown represents the surge voltage. In making simulation tests, use 

that value for the open-ircut voltage of the test generator. 

T~or low-impedance test specimens or load circuits. the current shown represents the discharge current of the surge (not the shoncircuit current of the power system). In making simulation tests, use that current for the short-circuit current of the test generator. 

 he maximum amplitude (200 or 500 A) is specified, but the exact waveform will be influenced by the load characteristics. 

I 

Fig. 3. Voltages and currents shown in ANSIIIEEE Std. C62.41-1980 (formerly IEEE 587) [3] as representative for Locations 

Categories A and B. 

TABLE I 

TIME TO REACH RATING FOR A 32-mm 150-V VARISTOR IN MEDIUM 

EXPOSURE, CATEGORY B LOCATION 

It Number I-xpccted Av,~iliihle Resultmg R;iled Percent 

level ofRl20 cl.trnp~np drn~ng current numhcr oiritt~ne 

of cclh occurrences uolt.cgc volt.tpe ill 2 I1 nl' pul\c\ conwmed 

13 000 0 03 620 12380 6190 h 0 5 

10 000 0 04 580 9420 4710 I? 0 27 

8 000 0 11 550 7450 3 725 25 0 44 

6 000 0 32 520 5 480 2 740 60 0 53 

4 500 I 560 4000 2 000 IiO O 67 

3 200 4 480 2720 1360 500 O RO 

2 400 h 460 1940 970 1 000 0 hO 

1 700 2 2 440 I260 h30 3000 n 73 

l 200 80 430 770 385 9000 O 89 

800 390 420 3x0 190 60000 Oh5 

Total Percenl of Rdt~ng (onsumed ~n One Yexr 6.08'K 

2 Cl for an 8/20-ps surge propagating in a few meters of wiring 

[8]. In this example, we compare a 20-mm varistor to a 14-mm 

varistor, both rated 150 V; the same value of expected 

clamping voltage is used for both devices (Fig. 4), and the 

resulting surge currents are computed, as shown in the fifth . - 

column of Table 11. Referring to the respective pulse lifetime 

rating curves for the 20- and 14-mm devices (Fig. 5), we 

obtain the number of surges needed to reach the rating, shown 

in the next two columns, and compute the yearly consumption 

for each device at each level. Again, the totals are shown, with 

the resulting number of years required to reach the ratings: 20 

years for the 14-mm device and 33 years for the 20-mm 

device, in the medium exposure environment. 

Thus we have a method for estimating the ability of a 

varistor to function reliably in a particular surge environment. 

Clearly, the estimate is only as good as the assumption made 

about the actual environment: for instance, an assumption 

based on the generalized description given in [3]. Nevertheless, this method of estimating represents a reasonable and 

conservative approach to selecting device ratings for reliable 

operation, and to comparing between different device sizes. 

FUSE CONNECTION OPTIONS 

Extreme surge values can occur which will exceed the 

predictions of limited-base statistics, so that one must take into 

PEAK AMPERES 

PEAK AMPERES 

Fig. 4. V-I characteristics for 14-, 20-, and 32-mm varistors rated 1SV m. 

6000 

consideration the case in which excess energy might be 

deposited in the varistor. Like any other electronic component 

abused in this manner, a varistor will fail, generally in a shortcircuit mode. Depending on the available short-circuit current 

from the power system, the current flowing in the failed 

varistor can melt soldered connections or shatter the device. 

5000 

aoeo 

7000 

I 

1 

1, I1 11 I 

T1 = 40 TO 85'C - MAXIMUM CLAMPING VOLTAGE - 130 TO 320 VAC RATING -. MODEL SIZE 32mm 
102 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. IA-21, NO. 1, JANUARYIFEBRUARY 1985 

IMPULSE DURATION - ,a IMPULSE DURATION - t6 

NOTE: End of lifetime is defined as a degradation failure which 

occurs when the device exhibits a shift in the varistor voltage at 

one (1) milliampereexceeding k 10%. i 1V of the initial value. 

This type of failure is normally a result of a decreasing V, value. 

but does not prevent the device from continuing to function. 

However, the varistor will no longer meet the original specifications. 

IMPULSE DURATION - ps 

Fig. 5. Pulse lifetime ratings for 14-, 20-, and 32-mm varistors rated 150-V rms. 

TABLE I1 

TIME TO REACH RATING FOR 20- AND 14-mm 150-V VARISTORS IN MEDIUM EXPOSURE, CATEGORY B LOCATION 

Voltage Number Expected 

level of 8/20 clamp~ng 

of cells occurrences voltage 

6 000 0.5' 550 

4 500 I 500 

3 200 4 460 

2 400 6 430 

1700 22 400 

I200 80 380 

800 390 370 

Available 

dr~ving 

voltage 

5 450 

4 000 

2 740 

1870 

1300 

820 

430 

- 

14 mm 20 mm 

Resulting Ratzd Percent Rated Percent 

currenl number of rating number of ratlng 

at 4 n of pulses consumed of pulses consumed 

1360 40 1.3 80 0.6 

l 000 80 1.3 200 0 5 

685 3 00 13 800 0.5 

465 1000 0 6 1 500 0.4 

325 10000 0.2 10000 04 

205 60000 0 1 20000 0.4 

107 150000 0.3 200000 0 2 

Total Percent of Rating Consumed in One Year 

Years Reauired to Consume 100Y0 of Rat~ne 

'Surges above 6000 V are not likely to propagate In low-voltage wlrlng w~thout causlng operatton of a protective device, or flashovei of a clearance upstream [91, so that the varlslor InsValled downstream ~n the Category B locat~on will not be subjected to the energy asoclated with levels above 6000 V. 

Since these ultimate failure modes are not generally desirable, 

the common practice is to provide a fuse in series with the 

varistor, in order to clear the fault from the circuit. 

Two approaches can be considered for disconnecting a 

failed varistor by melting a fuse. In Fig. 6(a) the fuse in series 

with the load ensures that power is removed from the load in 

case of varistor failure. The load, then, will not be exposed, 

unprotected, to further surges. In Fig. 6(b) the varistor fuse in 

shunt ensures that the power supply to the load will not be 

interrupted by a spurious varistor failure. Thus the load is left 

undisturbed but unprotected from further surges. As pointed 

out in industry standards [I], one or the other of these 

scenarios may be preferred by users, so that defining a 

particular one as fail safe would appear misleading to those 

taking the opposite view. For instance, an application can 

dictate that maximum service continuity be ensured; Fig. 6(b) 

would then be labeled fail safe by those requiring this mode of 

failure. Other users, eager to protect their hardware even at 

the price of losing the function, would label Fig. 6(a) fail safe. 

For the Fig. 6(b) configuration, the only requirement for the 
MARTZLOFF: MATCHING SURGE PROTECTIVE DEVICES TO ENVIRONMENT 

Fig. 6. Options for varistor fuse connection. (a) Protection maintained, 

function interrupted. (b) Function maintained, protection lost. 

fuse is to ensure fast removal of a failed varistor, without 

blowing under the surges expected to flow from time to time in 

the varistor. The selection of this fuse, therefore, should 

reflect the expected surge currents. Unfortunately, data 

presently available from fuse manufacturers do not extend the 

i2t ratings into the microsecond range, nor do they provide 

guidance on any necessary derating of the fuse in order to 

account for the possible effect of repetitive surges at levels 

slightly below the melting level. The second part of this paper, 

therefore, aims at providing some information on this selection 

process. This information is based on a limited test program 

conducted on various types of low-voltage fuses that might be 

considered for a shunt-connected varistor with a fuse in series 

with the varistor. 

It should be noted, of course, that while the operation of the 

fuse in the series connection (Fig. 6(a)) generally has an 

obvious effect on the system operation (shutdown), operation 

of the fuse in the shunt branch is not immediately apparent. 

Depending upon the application and the economics of the 

situation, it may be desirable to provide some indication that 

the varistor is no longer protecting the circuitry. 

PERFORMANCE OF FUSES WITH REPETITIVE 

SURGE CURRENTS 

An experimental program was conducted on various fuses to 

determine the minimum ampere rating required to prevent the 

fuse from being blown by the surge currents diverted by the 

varistor at its expected point of installation. This minimum 

limit involves a current level for one surge and another level, 

presumably lower, for repetitive surges. Fuse sensitivity to the 

number of surges is parallel to the situation discussed for 

varistors, where the pulse lifetime is also a function of the 

number of surges. 

Results of tests made on three types of commercial currentlimiting fuses and one circuit board fuse wire are presented 

here to illustrate the effects of repetitive surges and to motivate 

a discussion of the issues and the sharing of information. The 

fuses were all rated 250 V, with an interrupting capacity of 10 

kA, representative of what might be used in an application 

with moderate fault currents. The construction of fuses rated 

for higher prospective fault currents, such as 100 kA, differs 

primarily in the current-limiting characteristics. Here, we are 

concerned only with the element-melting characteristics. One 

fuse was a "fast fuse," rated 3 A, with a ratio of 13:l for the 

melting currents at 1000 and at 0.01 s. The second fuse was a 

"time delay fuse," also rated 3 A but with a ratio of 37: 1. The 

third, rated 10 A, has a ratio of 13: 1. Fig. 7(a) shows an X-ray 

photograph of the 3-A fast fuse, Fig. 7(b) shows the 3-A timedelay fuse, and Fig. 7(c) shows the 10-A fuse. 

As an alternative to the steep melting time-current characteristic (Fig. 8) of a fast fuse, the 'time-delay' fuse offers the 

possibility of surviving larger impulse currents. The larger 

ratio between 1000 and 0.01-s currents of the time-delay fuse 

maintains a comparable overcurrent protective capability at 

current values between the minimum melting current and the 

high currents associated with a short circuit, where the fuse 

may be required to melt in a relatively short time. 

The test current waveform was the standard 8/20-ps 

impulse used for characterizing varistors and other surge 

protective devices. Fig. 9 shows such an impulse, for which 

the i2t integral can be computed. For a 1000-A crest, the i2t 

value of the 8120-ps impulse is 19.5 A2s. 

Test Results on 3-A Fuses 

Melting curves for the 3-A fuse show a melting current of 

46 A rms at 0.01 s, which corresponds to an i2t of 21 A's. By 

subjecting each of ten fuses to a single-shot impulse in the 

range of 1000-1 100 A crest, a one-shot melting current crest 

of 1050 A was established. This current impulse has an i2t 

content of 21.5 A's, compared to 21 A2s at 0.01 s with.sine 

wave current. We will designate this observed single-impulse 

melting i2t as 12T. Likewise, the 3-A time delay fuse with a 

published 170 A2s at 0.01 s was found to have an Z2T of 

190 A2s. 

Repetitive impulses, at a rate sufficiently low to avoid 

accumulation of heat in the fuse, were applied to several 

groups of fuses. The impulses ranged from 90 to 30 percent of 

the melting current, corresponding, respectively, to i2t contents of 81 to 9 percent of the observed 12T. Table 111 shows 

the results for these two fuses. 

Test Results on 10-A Fuse 

The 12Tof this fuse, as defined earlier, was found to be 180 

A's, somewhat less than the value expected from the published 

melting current at 0.01 s which would have been 220 A2s. The 

purpose of the test at that point of the experiment was to find 

the amount of derating from the 8120-ps 12T needed to account 

for repetitive pulses, not to compare i2t values at 8/20 ps to i2t 

values at 0.01s. That would be an interesting further investigation. 

Table IV summarizes the results of repetitive tests performed 

on three groups of ten each of these 10-A fuses. By 

comparison with the results on the 3-A fuses, the aging 

process is less noticeable: no effect is detectable at 36 percent 

12Tand only a slight effect at 50 percent 12T. One each of the 

fuses aged at 50 and 36 percent of 12T was submitted to X-ray 

photograph; no visible sign was found of an alteration in the 

ribbon structure. 
104 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. IA

21, NO. I, IANUARYIFEBRUARY 1985 

Fig. 9. Curent ~rnpulse of 8/20 ps. 

(c) 

Fig. 7. X-ray photographs of test fuses. 

RMS AMPERES 

Fig. 8. Time-current characteristic of fast fuse and of time-delay fuse. 

Test Results on Circuit Board Fuses 

In some electronic printed circuit boards, a bare wire is 

sometimes used to provide a fusible link (with low interrupting 

capacity) in the circuitry. The same concerns on the effect of 

repetitive current surges that led to tests on commercial fuses 

also apply to these fuses. Some of the parameters influencing 

the behavior of such bare wire fuses were also investigated. 

Two silver wire sizes were selected as representative of 

what might be used: 0.005- and 0.010-in diameter. The Z2T of 

these wire was first determined with an 8120-ps current pulse. 

For the 0.005-in wire, the IZTis 16 A2s, for the 0.010-in wire, 

it is 240 A2s. This is consistent with the fact that the 12T of a 

wire is proportional to the square of its cross section, but with 

a slightly lower value than expected for the 0.010-in wire. 

Since there was a suspicion that electromagnetic forces 

associated with higher surge currents might contribute to an 

earlier breaking of the wire than with lower surge currents, an 

experiment was conducted to vary this electromagnetic force. 

In one test configuration, the fuse wire was spanning the 

space between two pads of the printed circuit, with the return 

path of the current flowing in an adjacent run only 5 mm away 

from the fuse wire. In the other configuration, the fuse wire 

was part of a straight line conductor with the return path 20 cm 

away. In this manner, two conditions could be evaluated, an 

unusually high and a negligible electromagnetic force effect. 

For the close configuration at 95 percent IZT, a visible 

stretching of the wire occurred after each successive shot. The 

wire broke at its middle point during the fifth to the seventh 

shot during several tests. Just prior to the last shot, the 20-mm 

long wire had been bowed away from the straight line with a 

maximum permanent deflection of about 5 mm. 

For the remote return configuration, at the same 95 percent 

12T, no visible stretching of the wire occurred, and the wire 

withstood 14-25 shots before melting. Melting occurred at a 

point apparently randomly located along the wire length, 

probably due to a local wire defect which concentrated the 

heating process. At 90-percent 12T, the wire withstood 200 

shots before melting at a random point along the length. 

These results indicate that-indeed there is some contribution 

from electromagnetic forces at the high surge currents of 

interest associated with surge suppression. Therefore, mechanical fatigue of the fuse element can be expected after a 

high number of repetitive surges. 

Discussion of Test Results 

The results of these tests indicate that in order to avoid 

premature blowing of a fuse, the i2t of the fuse should be 

selectedsuch that the expected surge current will not exceed 

25 percent of the observed 12T required to produce one-shot 

melting of the fuse. Previous experience in semiconductor fuse 

applications [lo] for semiconductor protection and cyclic 

duties, where the duration of the surges ranges from a fraction 
MARTZLOFF: MATCHING SURGE PROTECTIVE DEVICES TO ENVIRONMENT 

TABLE 111 

EXPERIMENTAL RESULTS FOR 3-A FUSES 

Number of Number or Shots 

'%I of Fuses On Each Fuse Effect Observed On Surv~vors 

Fuae Typc i'r In Group Before Melt~ng 

FAST B 1 10 M~n~mum. 3 No rurvivor5 

Max~mum 63 

36 10 M~nimuni. 5Nom~nal PI unchanged 

Max~niurn 4 

6 survived 200 shots 

9 20 All \urvlved 200 sho~r Noni~nsl l'r unchanged 

TIME 81 5 All at 2 

DELAY 64 5 All rt 2 or 4 

50 5 All wrwved 10 sholr 

3 6 10 All \urwved 100 aholr Avcr~ge ohrcrved izr of dged fuse 

Ir 85'%> of nomlndl PT 

9 10 All rurwved SO0 ahol\ Avsrage obrerved tar of aged fuae 

15 901% of nonilnal IaT 

TABLE IV 

EXPERIMENTAL RESULTS ~0~10-A FUSES 

Number of Shots 

on Each Fuse Effect Observed 

I'T Before Melt~ng on Surwvors 

81 Mlnlmum 2 No rurvlvors 

Max~murn 12 

50 1 a1 45 3 fuses out of 8 

I at 650 me11 al 95% I'T 

8 surwved 800 shots 

36 All I0 survwed 800 shol, Observed 8'1 15 

unchanged from I'T 

used in power semiconductor applications, is also applicable to 

wire-type fuses used with a varistor. Fuses using more rugged 

construction, or less severe notching of the elements, such as 

motor-starting fuses, have been successfully applied with a 50 

percent derating. The results on the ribbon type 10-A fuse 

reported here tends to confirm this rule. This conclusion was 

not obvious a priori, since the dynamics of wire melting, 

vaporizing, and breaking may differ when they occur in a few, 

rather than thousands of, microseconds. Eventually, we might 

see manufactures provide data comparable to the varistor pulse 

lifetime, for which surge current peak, waveform,- and of a second to a few seconds, had indicated that the i2t of the numbers of occurrences will be taken into consideration. fuse for these applications should be selected such that the - - expected surge i2t would be less than 25 percent of the 

observed 12T of the fuse. We find good agreement between 

this established empirical rule and the new observations in the 

microsecond range. 

There are good reasons for this agreement. What we have 

called "observed 127"' for a fuse is actually the sum of three 

parts of the heat deposition in the fuse: 

1) heat necessary to bring the solid wire to melting 

temperature (this heat is about 70 percent of the total Z2T 

for typical metals used in fuse manufacturing at short 

melting times), 

2) latent heat required to change the wire to the liquid state, 

3) vaporization heat to cause rupture of the wire. 

Clearly, a fuse wire that has been brought to its melting 

temperature will have experienced some change in its physical 

condition. Furthermore, the last restriction in the cross 

section, such as nicks or pinches, will accelerate this process 

of changed condition. 

Another effect, one likely to become more significant with 

repetitive pulses, is that of alloying the fuse wire at its soldered 

connection to the fuse ferrule, if a soldered construction is 

used. Other tests made on fuse wires soldered directly in 

printed circuit boards have also shown this tendency. For this 

reason, some fuses are assembled by welding rather than by 

soldering. The exact behavior of course depends on the 

specific metals used and the geometries of the fuses, so that we 

should not attempt to make sweeping generalities. Rather, we 

suggest that the empirical 25 percent derating practice, long 

CONCLUSION 

Guides on the surge environment in low-voltage ac power 

circuits, such as [3], can be used to generate a reasonable 

definition of the duty imposed on surge protective devices. 

Varistors can be selected and compared for predictable 

performance by combining their pulse lifetime cha&xteristics 

with the surge environment characteristics. Fuses connected in 

series with surge protective devices and therefore subjected to 

repetitive current surges will exhibit some aging process; 

selection of a fuse for a particular application requires an 

appropriate derating. The rule of thumb of selecting fuses for 

an i2t rating of three to four times the i2t of the seconds-long 

surges seems equally applicable in cases of microseconds-long 

surges. Small fuses will require a four times derating; larger 

fuses might be applied with a three times derating only. 

ACKNOWLEDGMENT 

The author gratefully acknowledges J. G. Leach's insight 

into the dynamics of fuse melting. 

REFERENCES 

[I] Test Specifications for Varistor Surge Protective Devices, IEEE 

Standard C62.33, 1982. 

[21 Transient Voltage Suppression, Electronic Data Library, General 

Electric Co., Syracuse, NY, 1982. 

[31 Guide on Surge Voltages in Low- Voltage AC Power Circuits, IEEE 

Standard 587, 1980. (Now designated ANSIlIEEE C62.41-1980). 

141 E. Carroll, "Transient attenuation with metal-oxide varistors for ac 

mains powered equipment," presented at the COMEL Conf., 1980. 

[S] S. Korn, "Transients and transient suppressors for power conversion 
equipment," presented at the INTELEC Conf., 1982. Franqois D. Martzloff (M'56-SM'80-F'83) 

[6] F. D. Martzloff, "The effect of repetitive surges on transient completed his undergraduate studies in France an

overcurrent and overvoltage protective devices," Proc. 5th Symp. on his graduate studies in electrical engineering in th

EMC, Mar. 1983. U.S. 

(71 Regulations, Section 68.302, Title 37, Telecommunications, His experience with General Electric (GE) starte

Washington, DC: U.S. Government Printing Oftice, 1977. in 1956 and includes five years in the Transforme

[8] F. D. Martdoff, "The propagation and attenuation of surge voltages and Switchgear Departments. He joined the G

and surge currents in low-voltage ac power circuits," IEEE Trans. Corporate Research and Development in 196

Power App. Syst., vol. PAS-102, pp. 1163-1 170, May 1983. Since that time, he has been primarily involved wit

(91 F. D. Martzloff, "Coordination of surge protectors in low-voltage ac identifying and solving problems associated wit

power circuits," IEEE Trans. Power App. Syst., vol. PAS-99, pp. transient overvoltages. He holds 1 1 U.S. patents. 

129-133, Jan.lFeb. 1980. Mr. Martzloff is active in committee and standards work on surg

[lo] J. G. Leach, "Advances in the protection of semiconductors by fuses," protective devices in the IEEE, the American National Standards Institute, th

presented at the Int. Conf. on Electric Fuses and their Applications, National Electrical Manufacturers Association, and the International Electro

Liverpool Polytechnic, Great Britain, 1976. technical Commission.