LINE CONDUCTED DISTURBANCES — ORIGINS AND CONTROL

François D. Martzloff

Corporate Research and Development

General Electric Company

Schenectady, New York 12345

Reprint of handout for the Forum “Power and Grounding for a Computer Facility”

National Bureau of Standards, May 1984

Significance

Part 6: Tutorials, Textbooks, and Reviews

This invited lecture was presented at a forum organized by the National Bureau of Standards (NBS) 

under the auspices of the same organization that developed the publication FIPS PUB 94 “Guideline on 

Electrical Power for ADP Installations,” a widely disseminated and often-cited tutorial document in the 

eighties and nineties. My subjects were presented within the broad scope of the forum, with emphasis 

naturally given to surge protection.

Delivering this lecture at NBS marked a turning point in my career because it catalyzed the establishment 

of a program aimed at surge protection and EMI control which I joined in 1985, allowing me to continue 

my work in this area within the supporting environment of NBS (soon to become NIST) until my retirement 

in 2003. Such support for 18 years of original research and dissemination of results via contributions to 

IEEE, IEC, and UIE standards is gratefully acknowledged.
Power and Grounding for a Computer Facility 

National Bureau of Standards 

May 1984 

LINE CONDUCTED DISTURBANCES - 

ORIGINS AND CONTROL 

Fraqois D. Martzloff 

Corporate Research and Development 

General Electric Company 

Schenectady, New York 12345 
THE ORIGINS OF TRANSIENT OVERVOLTAGES 

Transient overvoitages in power systems originate from one 

cause. energy being injected into the power system, but from two 

sources: lightning discharges, or switching within the power system. However, in communication and data systems, there is another 

source of transients: the coupling of power system transients into 

the data system. 

Lightning discharges may not necessarily mean direct termination of a lightning stroke onto the power system. A lightning stroke 

term~nattng near a power line, e~ther by hitting a tree or the bare 

earth, will create a very fast-changmg magnet~c field that can 

induce voltages - and mject energy - into the loop formed by the 

conductors of the power system. Lightning can also inject 

overvoltages in a power system by raising the ground potential on 

the surface of the earth where the stroke term~nates, while more 

distant "ground" points remain at a lower voltage. closer to the 

potential of "true earth." The literature and other presentations in 

this conference provide information on the characteristics of lightning discha~ges.('-~) 

Surges from power system switching create overvoltages as a 

result of either trapped energy in loads being switched off. or restrikes in the switchgear. These will be examined in greater detail in 

the following paragraphs. 

SWITCHING TRANSIENTS 

A transient is created whenever a sudden change occurs in a 

power c~rcuit. especially during power switching - either the closing 

or openlng of a clrcuit. It is important to recognize the difference 

between the intended switching (the mechanical action of the 

switch) and the actual happenmg in the circuit. Dur~ng the closing 

sequence of a switch. the contacts may bounce, producing openings of the clrcuit with reclosing by restrikes and reopening by 

clearmg at the high-frequency current zero. In medlum-voltage 

sw~tchgear, pre-strlkes can occur just before the contacts close. 

with a succession of clearings at the high-frequency current zero. 

followed by restrike. Similarly, during an opening sequence of a 

switch, restrikes can cause electrical closing(s) of the circuit 

Simple Switching Transients 

Simple sw~tching transient^'^) include circuit closing transients. 

transients initiated by clearing a short circuit, and transients produced when the two clrcuits on either side of the- switch being 

opened oscillate at different frequencies. In circuits having inductance and capacitance (all phys~cal circuits have at least some in 

the form of stray capacitance and inductance) with little damping. 

these smple switching transients are inherently limited to twice the 

peak amplitude of the steady-state sinusoidal voltage. Another limit 

to remember when analyzing transients associated with current 

interruption (circu~t opening) is that the circuit inductance tends to 

maintam the current constant. At most, then, a surge protective 

device provided to divert the current will be exposed to that initial 

current. Without a surge protective device the current is available 

to charge the circuit capacitances at whatever voltage IS requlred 

to store the inductive energy from the current by converting it into 

capacitive energy. 

Abnormal Switching Transients 

Several mechanisms are encountered in practical power clrcuits. 

These mechan~sms can produce translent overvoltages far in 

excess of the theoretical twice-normal limit mentioned above. Two 

such mechan~srns occur frequently: current chopping and restrikes. 

the latter being espec~ally troublesome when capacitor switchmg is 

involved. 

Current chopp~ng is the name given to the rapid current reduction which fuses or which c~rcult breakers can force when clear~ng 

a circuit prlor to the natural current zero of the power system. 

When there is inductance In the c~rcuit. this rapid current change 

can produce high overvoltages - some 10 times the normal circuit 

voltage. A classical example is an unloaded transformer where the 

magnetizing inductance is high and the energy stored in the magnetic core can charge only the winding capacitance. 

Capacitor switching can be troublesome if the switch restrikes 

after current interruption. Restrike occurs followmg the initial interruption: the capacitor voltage remains nearly constant at maximum 

system voltage. since the interruption occurred at zero current. 

which is 90. apart from the voltage zero. wh~le the system voltage 

follows the normal sine wave (Figure 1). At 180' after mterruptlon. 

the sw~tch has to support twice the amount of system voltage, a 

stress it mrght not be able to withstand with its contacts not completely separated. A restr~ke can occur under these circumstances. 

In such a case, the capacitance of the clrcuit will tend to drive the 

voltage not toward the system voltage but beyond 11 - theoretically 

up to twice the difference. Such an overshoot means a poss~ble 

VOLTAGE 

ACROSS 

CONTACT CAPACITOR CONTACTS 

CLEARS VOLTAGE 1 

t t 

VOLTAGE 

CAPACITOR 

VOLTAGE 13E 

Figure 1. Restrike Mechanism on Capacitor Switching 
Origins 

voltage of three tlmes the system voltage Wh~le th~s h~ghfrequency osclllatlon takes place, the sw~tch may clear at a h~ghfrequency current zero, only to restr~ke agaln later w~th an even 

greater difference of voltage and then escalating to an even hlgher 

overshoot The outcome w~ll be e~ther a breakdown ~n one of the 

components. or. lf the swltch eventually recovers enough d~electrlc 

w~thstand In ~ts openlng gap, no further restr~kes But severe overvoltages w~ll have been impressed on the system. 

A similar scenarlo can unfold when an ungrounded power system experiences an arcing ground fault. The switching actlon is 

then not the result of a deliberate partlng of contacts but the intermittent connection produced by the arc. 

These sw~tching overvoltages. h~gh as they may be. are somewhat predictable and can be estimated with reasonable accuracy 

from the circuit parameters. once the mechanism involved has been 

identified. There IS still some uncertainty as to where and when 

they occur because the worst offenders result from some abnormal 

behawor of a clrcult element. L~ghtning-induced translents are 

much less predictable because there is a w~de range of coupling 

possibilities. Moreover. one user, assuming that his system will not 

be the target of a direct hit may take a casual view of protection. 

while another, fearlng his system will experience a "worst case." 

may demand the utmost protection. 

In response to these Concerns. various comrnlttees and working 

groups have anempted to describe ranges of transtent occurrences 

or maximum values occurring in power circuits. Three such 

attempts will be discussed now. 

STANDARDS ON TRANSIENT OVERVOLTAGES 

Several standards or gutdes have been issued or proposed in 

Europe by VDE, IEC. CECC. Pro-Electron. CClfT, and in the USA by 

IEEE NEMA. UL, REA. and the military, specifying a surge withstand 

capability for specific equipment or devices and specific condit~ons 

of transients in power or communication systems. Some of these 

specifications represent early attempts to recognize and deal with 

the problem In spite of insufficient data As a growing number of 

organizations address the problem and as exchanges of informat~on 

take place. improvements are being made in the approach. The 

IEEE has recently published a new standard describmg the environment in low-voltage ac power circuit^.'^' The Low-Voltage Insulation Coordination Subcomm~ttee SCJ28A of IEC has also completed 

a report IEC 664. listing the maximum values of translent overvoltages to be expected in power systems under controlled conditions 

and for specified system characteristic^.'^' For some time now. a 

document prepared by a Relaying Committee of IEEE under the title 

Surge Withstana Capability"oJ has been ava~lable. These three 

documents are reviewed in the pages that follow. Greater emphasls 

will be placed on the first cited above because it describes the 

transient environment: me other two assume an environment for the 

purpose of specifying tests. 

The specified waveshape of the voltage is osc~llatory waveshape, not the h~stor~cal untdirect~onal waveshape. 

A Source impedance, a characterlstlc undefined in many 

other documents. IS defined. 

The concept that all lines to the device under test must be 

subjected to the test is spelled out 

Because this useful document was released at a time when little 

other guidance was available. users attempted to apply the recommendations of this document to situations where the environment of 

a high-voltage substation did not exist. Thus. an important consideration in the writing and publishing of documents dealing with 

transients is a clear definition of the scope. and limitations of the 

application. 

The IEC 664 Report 

The Insulation Coordination Committee of IEC, following a comprehens~ve study of breakdown characteristics ~n alr gaps, included 

~n its repon a table ~ndlcating the voltages that equipment must be 

capable of withstanding in varlous system voltages and lnstallation 

categories (Table 1). The table specifies that it is applicable to a 

controlled VOltag9 situation, which implies that some surgelim~ting 

device will have been provided - presumably a typical surge 

arrester with charactaristlcs matching the system voltage in each 

case. The waveshape specified for these voltages is the 1.2150 ps 

wave. a specification constotent with the lnsulation w~thstand concerns of the group that prepared thct document No source 

impedance is indicated but four 'installation categories" are specified. each with decreasing voltage magnitude as the installation is 

further removed from the outdoor environment. Thus, this document 

addresses primarily the concerns of insulation coordlnatlon. and the 

specification it implles for the envlronment is more the result of 

efforts toward coordinattng levels than efforts to describe the 

environment and the occurrence of translents. The latter approach 

has been mat of the IEEE Working Group on Surge Voltages ~n 

Low-Voltage AC Power Circuits. whlch we shall now review In some 

detail. 

Table I 

IEC Report 664 

PREFERRED SERIES OF VALUES OF IMPULSE 

WITHSTAND VOLTAGES FOR RATED VOLTAGES 

BASED ON A CONTROLLED VOLTAGE SITUATION 

(V rms and dc) 1 I II 111 IV 

-- 

The IEEE Surge Withstand Capability Test 

Voltages Lineto-Earth 

Derived from Rated 

System Voltages. Up to: 

One of the earliest publishad documents to address new 

problems facing electronic equipment exposed to power system 

transients was Prepared by an IEEE committee dealing with the 

exposure of power system relaying equipment to the harsh environment of htgh-voltage substations. Thls document, whlch describes 

a translent generated by the arcrng that takes place when atr-break 

. disconnect swltches are opened or closed ln the power system. 

presents innovations In translent protection: 

Preferred Series of impulse 

Withstand Voltages in 

Installation Categor~es 
I Origins 

I 

The IEEE Guide on Surge Voltages (IEEE Std 587-1 980) 

Voltages and Rate of Occurrence 

Data collected from a number of sources led to plotting a set of 

lines representing a rate of occurrence as a function of voltage for 

~ three types of exposures (Figure 2). These exposure levels are 

defined In general terms as follows: 

I Low Exposure - Systems in geographical areas known for 

low lightning activity, with little load switching activity. 

i Medium Exposure - Systems in geographical areas known 

for high lightning activity, with frequent and severe switching 

transients. 

High Exposure - Rare but real systems supplied by long 

overhead lines and subject to reflecttons at line ends. where 

the characteristics of the installation produce htgh sparkover 

levels of the clearances. 

The two lower lines of F~gure 2 have been drawn at the same 

slope smce the data base shows reasonable agreement among 

several sources on that slope. All lines may be truncated by sparkover of the clearances. at levels depending on the withstand voltage 

of these clearances. The high exposure line needs to be recogn~zed. but it should not be indiscriminately applied to all systems. 

Such application would penalize the vast majority of installations 

where the exposure is lower. 

Figure 2. 

Rnte of Surge Oecumncea verrus Voltage 

Lcvd at Unprotected Lacations 

Rate of Surge Occurrence versus Voltage Level 

(Reprin ted with permission from IEEE Std 58% 1980. 

4EEE Guide for Surge Voltages in Low-Voltage AC 

Power Circuits. '? 

It IS essenttal- to recogntze that a surge voltage observed in a 

power system can be either the drtving voltage or the voltage 

lirn~ted by the sparkover of some clearance in the system. Hence. 

the term unprotected circuit must be understood to be a circu~t In 

which no iow-voltage protective device has been installed but one 

in which clearance sparkover will eventually limit the maximum voltage. The distribution of surge levels, therefore, is influenced by the 

surge-producing mechanisms as well as by the sparkover level of 

clearances in the system. Thts distinction between actual driving 

voltage and voltage limited by sparkover is particularly important at 

the interface between outdoor equipment and indoor equipment. 

Outdoor equipment has generally higher clearances. hence hlgher 

sparkover levels: 10 kV may be typical. but 20 kV is possible. In 

contrast. most indoor wiring dev~ces used in 120 to 240 V systems 

have sparkover levels of about 6 kV; this 6 kV level, therefore, can 

be selected as a typical cutoff for the occurrence of surges In 

indoor power systems. 

The voltage and current amplitudes presented in the Gutde 

attempt to provtde for the vast majority of lightning strikes but none 

should be considered "worst case." as this concept cannot be 

determined realistically. It is necessary to Mink in terms of the statlstical distribution of strikes. and to accept a reasonable upper limit 

tor most cases. Where the consequences of a failure are not catastrophic but merely represent an annoying economic loss, it is 

appropriate to make a tradeoff of the cost of protection against the 

likelihood of a failure caused by a high but rare surge. 

Waveshape ol the Surges 

Many independent obser~ations(ll-~~) have established that the 

most frequent type of transient overvoltage in ac power systems is 

a decaying oscillation. with frequencies between 5 and 500 kHz. 

This finding 1s in contrast to earlier attempts to apply the unidirectlonal double exponential voltage wave that is generally described 

as 12/50, Indeed. the unidirectional voltage wave has a long 

history of successful application in the field of dielectric withstand 

tests and is representative of the surges propagating in power 

transmission systems exposed to lightning. In order to combme the 

rnerlts of both waveshape definttions and to specify them where 

they are applicable, the Guide specifies an oscillatory waveshape 

~ns~de buildings, a unidirectional waveshape outside buildings, and 

both at the interface (Figure 3). 

The osc~llatory waveshape simulates those transients affecting 

devices that are sensitive to dvldt and to voltage reversals during 

conduction. while the unidirectional voltage and current 

waveshapes. based on long-established ANSI standards for secondary valve arresters, simulate the transtents where energy content is 

the significant parameter. 

Energy and Source Impedance 

The energy involved in the interaction of a power system with a 

surge source and a surge protective device will d~v~de between the 

source and the protectwe device In accordance with the charactenstics of the two ~rnpedances. In a gap-type protective device. 

the low ~mpedance of the arc after sparkover forces most of the energy to be disstpated elsewhere - for instance. in a resistor added 

in series with the gap for limiting the power-follow current. In an 

energy-absorber protective device. by its very nature. a substantial 

share of the surge energy is disstpated in the suppressor, but its 

clamping action does not involve the power-follow energy resulting 

from the short-circuit action of a gap. it is therefore essent~ai to the 

effective use ot surge protective devices that a reai~stic assumptton 

be made about the source impedance of the surge whose effects 

are to be duplicated. 
Origins 

Figure 3. IEEE Std 587 Transient Overvoltages and Discharge Currents 

Unfortunately. not enough data have beon collected on what this 

assumption should ba for Me source imwame of Me transient 

Standards and recommendations. such as MIL SlD-1399 or the 

IEC 664 report either ignore the issue or indicate values applicable 

to limited cases. such as the SWC test for high-voltage substation 

equipment The IEEE 587 standard attempts to relate impedance to 

categories of locations but unavoidably remains vague on their definitions mable 2). 

The 6 kV open-circuit voltage derives from two facts: the limiting action of w~ring device sparkover, and the unattenuated propagation of voltages in unloaded systems The 3 kA discharge current 

In Location Category B derives from experimental results: fie@ 

experience in surge protective dewce performance and simulated 

lightning tests. The two levels of discharge currents for Me 

0.5~s-100 kHz wave derive from the increasing ~mpedance 

expected in movlng from Category B to Category A. 

Category C is likely to be exposed to substantially h~gher voltages than Category B because the limiting effect of sparkover 

m~ght not be available. The high exposure rates of Figure 2 could 

apply. with voltage in excess of 10 kV and discharge currents of 

10 kA or more. Installing unprotected load equ~pment in Category C 

is not recommended; the installation of secondary arresters. however. can provide the necessary protection. 

Having defined the environment for low-voltage ac power circuits. the Working Group IS now preparing an Application Guide. 

where a step-by-step approach will outline the method for assessing the need for transient protection and selecting the appropriate 

device or system. Parallel work in other IEEE worklng groups 

preparing test specification standards("-'6) for surge protective de: 

vices will be helpful in thts select~on process. Other groups In the 

US, as well as the international bodies of IEC and CCITT. are now 

worklng toward further refinements and the reconciliation of different approaches. 

Table 2 

Surge Voltages and Currents Deemed to aepreSent the Indoor Environment 

and Recommended for Use in Deaigpins Protective Systems 

(Reprinted w~th permcssion from IEEE Std 587-1980. "IEEE Gu~de for Surge Voltages In Low-Voltage AC Power Circu~ts.") 
Origins 

PROPAGATION OF SURGES 

Voltage surges propagate in wiring at speeds in the order of 213 

the speed of light, i.e.. about 200 ml~s. Thus, for short lengths of 

wiring, the classical transmission line behavior of reflections IS not 

applicable: with surges having a rise time of 1 GS, many round-trips 

between the sending end and the far end can occur dur~ng the rlse, 

so that the final voltage 1s not affected by the reflections; only during the rise can one notice a small difference between the two ends 

(Figure 4). 

Figure 4 also illustrates the fact that a line with an unloaded end 

does not attenuate the voltage. This fact prevails over the misconception that the wiring system in a building, for instance. inherently 

produces a progressive attenuation of surges from the service entrance to the locations inside the bui~ding.'~' 

Likewise, suggestions are sometimes made that an "isolating 

transformer" can attenuate surges. This is true only if the transformer IS fully loaded. It 1s not difficult to imaglne a scenario where 

the transformer wdl operate at very light load, for instance, with only 

an electronic control energized during the off-cycle of the power circuit Figure 5 shows how an impinging 0.5 ~s-100 kHz transient of 

6 kV (IEEE 587) produces a 7 kV output at the unloaded secondary 

terminals. The second reprint in these notes. "The Propagation and 

Attenuation of Surge Voltage and Surge Currents in L&-voltage AC 

Circuits." presents more detailed discussions of these concerns. 

THE TRANSIENT CONTROL LEVELS CTCL) CONCEPT 

Until recently, there did not appear to be a clear approach for 

achieving compatibility between the transient withstand capability 

of devices and the transients to which such devices are exposed. 

This situation was somewhat like that which prevailed many years 

ago in the electric power industry. Transients produced by lightning 

frequently caused failure to such vital and expensive power equipment as transformers and generators. Those translent problems 

were solved by engineering design guided by the concept of ~nsulation coordination and the establishment of a series of Basic Insulatlon Levels (BlLs). At present the transient control levels concept 

of testing and coordination promises a solution to the problem of 

compatibility. 

According to the transient control levels concept, Instead of 

retrofitting protective devices into a finished product or design, the 

first step is to determine at what level the transients occurring in 

the system can be limited by suitable protective devices. It is then 

sufficient to build the equipment to withstand only that level established by the protective device. The result is a well-defined situation, with adequate margins between the maxlmum level of transients and the demonstrated w~thstand capability of the equipment. 

The specific proposals made in that concept are the following: 

That there be defined for electronic equipment (and other 

low-voltage equipment) a standard transient voltage s~milar 

in concept to. but different In waveshape from. the 1.2150 ps 

wave used in the coordination of insulation in h~gh-voltage 

power apparatus. 

That there be defined for electronic equipment (and other 

low-voltage equipment) a series of TCLs slrnilar in concept 

to the BILs. 

That a start be made on assigning one of these standard 

levels to individual electronic components and electronic 

devices. 

That ~ndivtdual protective dev~ces be rated in terms of their 

ability to control transients to levels no greater than, and 

preferably lower than, one of the above levels. 

That equipment and procedures be developed by which 

equipment may be tested by vendors to determine which 

TCL is appropriate to assign to individual components and 

equipment 

That TCLs begin to be used in purchase specifications. 

That such equipment and procedures be used by purchasers to evaluate vendor-supplied equipment to determine 

its compliance w~th such purchase specifications. 

That such TCLs begin to appear In regulatory specifications 

for consumer apparatus in which the consumers cannot 

make the appropriate tests or prepare appropriate 

specifications. 

The engineering community in the United States responded 

favorably to the proposal, and some of the concepts found their way 

into the IEEE Guide discussed In the first sectlon of these notes. 

With the writing of Application Guldes by the IEEE as well as by the 

IEC, there is an opportunity to advance the proposal further. A 

reprint of the original paper presenting this proposal is included in 

the notes. 

1 kVA CIENERAL PURPOSE 1M/120 TRANSFORMER IIIIIIII 

Figure 4. Sending and receiving'end voltage with 1.2/50 ps 

impulse applied to wiring in metal conduit. 

Figure 5. Propagatlon of a 0.5 rs-100 kHz ring wave 

through an isolating transformer. 
Origins 

CONCLUSIONS 

The two major causes of transient overvoltages, lightning surges 

and sw~tching surges. have been ident~fied with greater precision in 

low-voltage ac clrcu~ts as well as In cornrnun~cation systems. 

While standardizing the definition of the environment will not 

change the environment itself, the emergence of realistic standards 

will enable designers to increase the reliability of the~r products. 

Likewise. users will be able to protect their equipment more 

effectively. 

REFERENCES 

1. J.H. Hagengum and J.G. Anderson, 'Lightning to the Empire 

State Building," Part Ill. AlEE Transactions 71 PAS, August 1952. 

2. N. Cianos and ET. Pierce. A Ground Lightning Environment for 

Engrneering Usage. Stanford Research Institute. Technical 

Report August 1972. 

3. R.H. Golde. "The Frequency of Occurrence and the Distribution 

of L~ghtning Flashes to Transm~ssion Lines." AIEE Transactions 

64. 1945. 

4. World Distribution of Thunderstom Days WMO-OMM #21 

TP6. World Meteorological Organization. Geneva. 1953. 

5. R.H. GoMe. "Tho Lightning Conductor." Lightning 11. R.H. Golde. 

ed.. Academic Press. New Yo* 1977. 

6. H. Cavendish. W. Watson, B. Franklin, and J. Robertson. 'Report 

of the Committee Appointed by the Royal Society to Consider a 

Method of Securing the Powder Magazines at Pumeet" Philosophical Transactions of the Royal Society. London. 63. 1773. 

and 68. 1777. 

7. A. Greenwood. "Electr~cal Trans~ents In Power Systems." Wiley 

Intersclence. New York. 1971. 

8. IEEE Gurde for Surge Voltages in Low-Voltage AC Power Circuits, IEEE Std 587-1980. 

9. "Insulation Coordination Within Low-Voltage Systems Including 

Clearances and Creepage Distances for Equipment" International Electrotechnical Commiss~on Report 664. 1980. 

10. Guide for Surge Withstand Capability (SWC) Tests, ANSI Standard C37.90a 1974; IEEE Std. 472-1974. 

11. J.E. Lenz 'Basic Impulse Insulation Levels of Mercury Lamp 

Ballast lor Outdoor Applications." llluminatrng Engrg, 133-140 

(February 1964). 

12. F.D. Mamloff and G.J. Hahn. "Surge Voltage in Residential and 

Industrcal Power Circuits." IEEE PAS 89 6. 1049-1056 

(July/August 1970). 

13. R. Hasler and R. Lagadec. "Digital Measurement of Fast Transients on Power Supply Lmes." Proc. 3rd Symposrum and Technical Exhibition on Electro-Magnetic Compatibility. Rotterdam. 

Holland. 445-448 (May 1979). 

14. Test Specifications for Gas Tube Surge Protective Devices. 

IEEE Std C62.31-1982. 

15. Test Specifications for Low-Voltage Air Gap Surge Protective 

Devices. IEEE Std C62.32. 1981. 

16. Test Specifications for Varistor Surge Protective Devices, IEEE 

Std C6233. 1982. 
Origins 

1 

SWITCHING TRANSIENTS 

SIMPLE: 2X NORMAL 

ABNORMAL 

I - CURRENT CHOPPING: 10X NORMAL 

- RESTRIKES: 

CURRENT CURRENT 

FORCED 

TO ZERO 

u 

OPENS 

NATURAL ZERO 

SLIDE NOTES 

I 

The traditional "cone of protection." revised with the 

"rolling ball" concept based on striking distance. 

There are many ways to get into trouble from the 

"benign" to the horrendous. 

A simple switching transient in an LC circuit. undamped. 

can reach 200%. or 2 per unit, for instance during fault 

clearing by a circuit breaker. 

Current chopping produces steep ditdt, thus high L dildt 

voltages. 
Origins 

SLIDE NOTES 

STANDARDS ON 

TRANSIENT OVERVOLTAGES 

2.5 - 3 kV 

1 - 1.5 WX swc 

The restrike phenomenon can escalate to very high voltages. Here is the mechanism for going from one per unit to 

three per unit. The next go-round can reach 5, then 7 . . . 

All these various mechanisms produce all kinds 

of transient waveforms -- which one to choose? 

Many national and international groups are concerned. 

and prepare standards; some of these are not always 

compatible. 

The "Surge Withstand Capability" test was among the first 

published for low-voltage equipment -- in high-voltage 

substations. 
Origins 

-587 

GUIDE ON 

SURGE VOLTAGES 

IN LOW VOLTAGE 

AC POWER CIRCUITS 

GUIDELINE 

~ SCOPE 

ORIGIN OF SURGES 

RATE OF OCCURENCE AND LEVELS 

W AVESHAPE 

ENERGY AND SOURCE IMPEDANCE 

SUPPORTING DATA 

SLIDE NOTES 

The International Eiectrotecnnical Commission is 

recommending the application of a staircase of 

descending voltages from the servlce entrance 

onward. 

Military precision in an imprecise world: 

4-digit specifications. 

One of the more recent attempts to describe the real 

world. 

The contents of IEEE Std 587-1980. 
Origins 

VOlTage at mt1et Indicated: 500 Yldtv 

SLIDE NOTES 

The occurrences of surges has statistical distribution at 

any location, and locations and exposures vary. 

An extreme case of surge occurrences over a period of 24 

hours, due to switching surges in a house. 

Scenario for the division of lightning current toward ground. 

along ground conductors. 

The result of a ground current of only 1.5 kA is 2200 V 

induced line-to-neutral on unloaded circuits. 

With a moderate load, the voltage is reduced to 1400 V. 

Note the oscillatory voltages from a unidirectional current 

pulse. 
Origins 

SLIDE NOTES 

Is the source impedance of the above scenario 75 ? 

First representative waveform from IEEE Std 587 - voltage and current. The "0.5 ps-lOO kHz Ring Wave." 

Second IEEE Std 587 waveform - voltage for high 

impedance - 1.2150 ps. 

Third IEEE Std 587 waveform - current for low impedance 

- 8/20 WS. 
Origins 

SLIDE NOTES 

Clamping surge protectrve dev~ces work only if there is a 

finite source impedance. 

Location categories in an industrial system. 

Location categories in a residential system. 

Now we know which one to pick. 
PROPAGATION 

OF SURGES 

1 More about this in the reprint section. 

Reflections at line ends are not significant if the travel time 

is shorter than the rise time. 

Clean test waves are quickly confused by complex circuits. 

Typical wiring impedance. 
Origins 

TRANS-IENT 

CONTROL 

LEVELS 

I HOW THINGS STAND NOW 

I r EQUIPMENT IS BUILT 

r INSTALLED .,. FAILS 

r AH - HA!! - TRANSIENTS - 

r ADD SUPRESSORS 

SLIDE NOTES 

A noble objective, but perhaps the world is not quite ready, 

or the presently scrambled situation can no longer be 

unscrambled? 

is a backward situation. 

The electric utilities long ago developed the Basic 

Insulation Level (BIL) concept. 

Fundamentals 

shtelds." However, in many ~nstallatlons there is a metallic condu~t 

through which the cables are pulled; w~th simple but close attent~on 

to malntalnlng the continu~ty of this conduit path, through all the 

joints and iunctron boxes. a very effective outer shield is obtained at 

negligible additional cost In the case of underground conduit runs. 

the most frequent practice is to use plastic conduit. which unfortUnateiy breaks the continuity. System designers would be well 

advlsed to requlre metal conduits where the circu~ts are sensitive or, 

at a mtnlmum to pull a shielded cable in the plastic conduit where 

the shield is used to maintain continuity between me abovaground 

metal conduits. That additional cost then. is the insurance premium. which is well worth accepting. 

Bonding 

We have already mentioned one aspact of bonding in describing 

the continuity of the outer shield. Another instance of bonding 

occurs where the shield of an incoming cable is connected to thecurrent flow. with me shield current now flowing in me pigtail and 

the creation of the corresponding electromagnetic radiation at the 

polnt of cable entry. 

Adjacent cabinets m a lineup must be bonded together for safety 

as well as translent and noise immunity. In principle. a flat strap 

has a lower inductance than a round wire Of Me same area. This 

concept may be somewhat overused: actually several strategically 

located smaller wires provide a much more effective bond than one 

massive strap either round or flat The difficulty lies in 

tmplementing this alternate view. and overcoming the comforting 

scght of a large grounding strap at the bottom of me cabinet lineup. 

Such a strap does no harm and is a good safety practice, but it may 

not do as much good as expected from Me point of view of surge 

protection. 

Grounding 

Grounding. which is also referred to as "earthing." has different 

meanlngs as well as different roles. The primary definition is the 

connection of the circult sh~ela or reference to eanh. But what is 

"earth"? System des~gners. construction crews. inspectors, and 

technical conference authon are concerned with establishmg. 

measuring. and maintaining a low ground resistance. often determined by dc measurements on rods dnven Into the ground. Drivlng 

rods into the ground does not ensure a low impedance under the 

transient conditions of high rate of current change assoc~ated with 

lightning discharges. This remark is not intended as a criticism of 

the efforts golng into achieving a low resistance but. rather, to alert 

the system designer that there is more to it than just low reststance. 

and that one can overdo the act of burying copper In the ground. 

When one deals with a reasonably compact System, be it 

cabinet-size. room-size. or building-size. it is more effective to view 

the grounding as a well-bonded connection to the outer shield (if 

any), building frame. or cabinet enclosure. The resistance (impedance) from that reference to 'earth' is not very significant as long 

as other wlres at "ground" potential are not brought to the system. 

Since there IS little chance of dealing with an abSOlUtely Isolated 

system, the question is: What should be done with incoming 

wires? These wires can be isolated from the local ground during 

normal operatlon. but one must recognize that during transient conditions of lightning surge or power system faults. high Voltages will 

appear across these isolated wires and local ground. voltages 

whlch, in some cases. are totally beyond the withstand capab~lity of 

~nsulation. That Insuiatlon. then, must be protected by su~table 

devices which In fact do connect the wlres to the local ground for 

the duration of the translent. Th~s type of grounding 1s one function 

of translent suppressors. 

Power System Grounding 

In the context of grounding 11 IS appropriate to mention the questions raised by proponents of ungrounded and grounded power systems. Indeed, a discussion of grounding practices would not be 

complete without reference to these questions. 

It has been a long-established practice to operate some threephase power systems without an intentional ground connectlon of 

box of the circuit or to the building ground. The princlple is simple: 

the shield can be viewed as an extension of the box. and thus 

bonding ,of the shield to the box should be continuous over 360 - 

degrees. In practice. unless special connectors are used. this is difficult to achieve. tor often a shlelded cable is terminated at a connection board with the shleld peeled back and turned into a pigtail. 

whlch in turn is connected to the "ground" termtnal of the connection board. One can imagme the many possible variations of 

the neutral. The intent is to increase the reliability of service continuity. in principle allowing the system to continue operating with 

one ground fault. as opposed to grounded systems. where an 

outage will be the result of the first ground fault to occur. Examples 

of Such concems are found In military power systems ('ride through 

the first shell") and industr~al low-voltage systems. 

Closer analysis of the consequences of this choice indicates. 

however. Mat the overall performance of the ungrounded systems 

may not necessarily be improved over the safer. more predictable 

grounded system. An excellent discussion of the advantages and 

disadvantages can be found in IEEE Standard 142-1 972. A detailed 

description is presented of multiple faults to ground. arcing faults to 

ground (which can be caused by sparkowr after a transient overvoltage and result in massive equipment damage), location of faults. 

personnel safety, pertormance with overvoltages. and system costs. 

The effects of these concepts on the roles performed by sh~elding. bonding. and grounding can be summarized in a set of relatively 

simple deslgn guidelines, which are stated in Me appendix. 

Iaolatlon ot Subsystems 

In the case of systems involvlng separate buildings. remote sensors, or the interconnection of a power system with a communlcation system, other requirements may dictate the isolation of the subsystems. creating the illusion that protection agalnst overvoltages 

has also been accomplished. And yet we haw seen that. durrng 

transient conditions. high Voltages can occur. 

Where moderately high voltages only can occur. effective lsoiation can be accomplished by the insertion of isolating transformers 

or. when metallic isolation is not required. by insertion of a filter. 

Where the voltages will reach levels exceeding the w~thstand 

capability of economically or technically feasible ~nsulation, two 

possible solutions exist. The first, already mentioned. is to bond the 

two systems durlng the translent by means of a nonlinear surge protectlve device, whrch returns to a high level of insulation after the 

transient has subs~ded. Another method. and one which is becoming increas~ngly attractive, is the insertion of a fiber optics link into a 

control or data system. Complete decoupling of. electrical transients 

and nolse can be achieved in this manner. 

CONCLUSIONS 

A number of fundamental protectron technlques are ava~lable to 

llmlt the penetration of ilghtnmg surges Into a system These are 

best ~mplemented at the early stages of deslgn They tnclude the 

provtslon of pomts where a itghtnlng stroke can attach and be 

dlverted w~thout harm to earth. shleldlng of clrcults. bondtng of 

enclosures. and grounding of the shlelds and reference polnts. 
It is a dangerous illusion to believe that lightning effects can be 

eliminated by the isolation of conductors or subsystems. It is much 

safer and quite acceptable, if included in the design, to provide 

bonding during transient conditions by suitable protective devices. 

The important point to remember is that lightning is a fairly welldefined phenomenon, with known characteristics and effects in 

general, but its probability of occurrence at a particular location is 

unknown. For the successful operation of a system, foresight 1s 

needed in applying fundamental protection techniques at the beginning. 

FOR FURTHER READING 

Boehne. EW.. and R.H. Golde, eds., "Research on the Lightning 

Phenomenon." Journal of the Franklin Institute 283 (6). June 1967. 

pp. 45 1-478. 

Fisher, FA.. and J.A. Plumer. Lightning Protection of Aircraft 

NASA RP 1008. National Aeronautrcs and Space Administration. U.S. 

Government Printing Office. Washington. DC 20402. 1977. 

Gary. C.. "Les Principes Phys~ques Gu~dant la Protection des Batiments Contre la Foudre," Revue Generale de LElectricite 89, May 

1980. pp. 351-367. 

Greenwood. A.. Electr~cal Transients in Power Systems, Wiley Intersclence. New York. London. 1971. 

IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems. IEEE Standard 142-1972. 197 1. 

Lee. R.H.. "Lightnmg Protection of Buildings." IEEE Transactions on 

Industry Applications 1A-15. MayIJune 1979. pp. 236-240. 

Shelkunoff. SA.. "Electromagnetic Theory of Coaxial Transmission 

Lines and Cylindr~cal Shields." Bell System Technical Journal 13, 

1934. pp. 532-579. 

"The Protection of Telecommunication Lines and Equipment Against 

L~ghtnlng Discharges." Comitb Consultatif International de 

TeIEtgraphe et Telephone, International Telecommunication Union. 

Geneva. Switzerland. 1974. 

Uman, MA, Understanding L~ghtning, Bek Technical Publications. 

Carnegia. PA 15106. 1971. 

Valazquez. R.S.. V. Gerez. D. Mukhedar, and Y. Gervais, "Probabilistic 

Calculat~ons of Lrghtn~ng Protect~on for Tall Buildings." IEEE Transactions on Industry Applications 1A-18 (3)- ~ayl~une 1982. pp. 

252-259. 

Golde. R.H, ed.. Lightning, 2 vols, Academic Press. London and New 

York. 1977. 

Appendix 

BASIC CONSIDERATIONS ON SURGE CONTROL FOR ELECTRONIC SYSTEMS 

(From Surge Protection of Electronics. University of Wisconsin. March 1979. Conference Notes) 

Frank A. Fisher. General Electric Company 

PROTECTION FROM THE DIRECT EFFECTS OF LIGHTNING 

Try to divert the stroke away from the system, the farther Me better. 

If the stroke cannot be diverted away completely, it must be carried to ground along a path where it does the least damage. 

This basically means providing a system of lightning rods and lightning conductors to a low-resistance ground. 

Reduce the resistance along the current-carrying path as much as feasible. This is particularly true of the ground resistance 

where the lightning conductor is grounded. 

In buildings housing electronic equipment. establish a uniform potential ground plane over as much of the building as possible. In new buildings. all reinforcing steel in concrete and structural steel members should be bonded together and connected to ground rods. These should be located around the periphery.of the building. Bonding should be done at many 

points. All water pipes and utility conduits should be bonded to this ground system where they enter and at frequent mtervals 

within the building. 

Insofar as feasible. the building should be arranged to form a grounded metal enclosure (Faraday cage). External magnetic 

and electric fields will not penetrate into a perfect Faraday cage. 

Power systems should be protected with commercially available surge arresters. 

Avo~d the use of ground systems for electronic equipment that are isolated from building or power system grounds. This 

means uslng a multiple ground system rather than a single-point ground system. 

Connect the cases of all electronic equipment to the nearest building ground point. This ensures that. at a minimum, the 

cases will not assume a high potential under lightn~ng flash conditions. relative to the surrounding structure, and so will not 

present an electr~cal hazard to people operating the equipment. Ground leads should be as short and direct as possible, possessing a minimum of resistance and (especially) inductance. 

All wiring between different locations in the system should be carrred cn shielded cables. wlth sh~elds grounded at both ends. 

Electron~c equipment should be designed to withstand surge voltages. Surges can be carrred Into such equipment on input 

and output leads and on power supply leads, and consideration should be given to the use of protective devices on these clrCuits. 
Fundamentals 

PROTECTION COMPATIBLE WITH NOISE ATTENUATION DESIGN 

An approach whtch has been used successfully to prov~de ltghtntng protection of communication systems without compromising 

the systems' steady-state noise performance IS shown In Figure A-1. The important elements of the system are as follows: 

Electrical cables carrying signals between sensitive or crtttcal apparatus in separated locations must have an overall shield. 

This shield must be continuous and must be grounded at each end (A and 8) to the building ground systems. 

The indivtdual cables within this bundle should be shtelded, but this is not an absolute requirement. 

The shields on mesa individual cables should be grounded at each end (C and D), but may. at the circuit designer's discretion. be grounded at only one end (E and F) if such a practlce is preferable for the control of steady-state noise. As long as 

the designer leeves the overall sh~eld alone, he can do whatever he w~shes with any internal shields. 

The electronic equipment to which the cables connect should have the housings (G and H) connected to the building ground 

system (I). 

A ground bus within the individual pieces of electronic equipment. but tsolated from the equipment case. IS often desirable. If 

such ground busses are provided (J and K). it Is preferable from the viewpoint of lightntng protection that they be connected 

to the building ground system at the polnt of entry (L) of the building ground system. 

If direct connection of the ground systems cannot be tolerated. a separate electronic ground (M) can be provtded. A Spark 

gap or other voltage-l~miting device (N) should then be prov~ded to limit the surge voltages that can be developed between 

the two ground systems. 

If the electronic ground system is very extensive, it may be desirable to provide other voltage-limiting elements (0) between 

the electronic ground system and me building ground system. 

Electronic cables are generally carried between locations in cable trays These trays, although not shown on the figure. 

should be connected to the buildlng ground system at each end. The cable trays should be electrically continuous over the~r 

entire length. Cable trays, if used, do not elimtnate the need for an overall shield on the cables within the tray. 

If cables are carried between two locations in electrically contlnuous metallic conduits. such conduits being connected to the 

building ground system at each end. the overall shield on the cables may be eliminated. smce the conduits take the place of 

the shield. Cables cam in nonmetallic conduits. however. must have the overall shield. 

Locatlon 1 Building 

1 Bui'ding 1, Locatlon 2 

Equipment, 

Figure A-1. Lightning Protection Measures Which Provide Minimum Disruption to Steady-State Operation 
Fundamentals 

FUNDAMENTAL 

PROTECTION 

TECHNIQUES 

' What can we do to prevent problems? 

PROTECTED 

CIRCUIT 

Lightning and electronics do not mix with good results, but 

simple precautions can go a long way. 

The basic protection philosophy 

Surge protective devices at the black box level can deal 

with limited energy. 
Fundamentals 

SHIELDING 

BONDING 

GROUNDING 

SLIDE NOTES 

Three very powerful protective measures. 

The first technique. 

The 13th Commandment of "electronikers." 

The consequence of the 13th Commandment 
Fundamentals 

I GROUNDING I 

SLIDE NOTES 

A simple solution to reconcile two contradictory 

requirements. 

The second technique. 

Last, but not least, the third technique. 

Grounding alone is not enough 

2 1 
Fundamentals 

SLIDE NOTES 

Ground potential differential me. 

Dangerous scene. 

One way to reconcile everybody. 

The total picture. 
TRANSIENT SUPPRESSORS 

INTRODUCTION 

Various devices have been developed for protecting electr~cal 

and eiectron~c equipment against translents. They are often called 

"translent suppressors" although, for accuracy, they should be 

called "transient lim~ters." "clamps." or "diverters" because they cannot really suppress trans~ents: rather, they limit transients to acceptable levels or make ethem harmless by diverting them to ground. 

There are two categories of transient suppressors: those that 

block transients, preventing the~r propagation toward sensitive circults. and those that divert translents. limtting restdual voltages. 

Smce many of the translents orlglnate from a current source, the 

blocklng of a transient may not always be posslbie; the dlvertrng of 

the transrent is more likely to find general application. As we shall 

see In the section on coordination, 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 toward ground. a second device - impedance or 

resrstance - offers a restricted path to the translent propagation but 

an acceptable path to the signal or power. and a third dev~ce 

clamps the residual translent (Figure 1). Thus, we are pr~marily 

interested in the diverting devices. These diverting devices can be 

of two kmds: voltage-clamp~ng devces or short-circuit~ng devices 

(crowbar). Born involve some nonlinearity. either frequency nonlinearity (as in filters) or, more usually. voltage nonlinearity. Th~s 

voltage nonlinearity is the result of two different mechan~sms - a 

continuous change ~n the device conductivity as current Increases. 

or an abrupt switchmg as voltage increases. 

Figure 1. Multi-stage Protection 

RESTRICT 

~olvEm cLAMpo 

Because the techn~cal and trade literature contams many artlcles on these devtces. we shall llm~t the discuss~on of the detads 

and refer the reader to the bibhography at the end of this section. 

We shall, however. make some comparisons to polnt out the s~gnificant differences In performance. 

pCROWBAR DEVICES 

The pr~nc~ple of crowbar dev~ces IS qulte simple. upon occurrence of an overvoltage, the dev~ce changes from a h~gh-mpedance 

state to a low-impedance state, offerlng a low-~mpedance path to 

dlvert the surge to ground. Thls swltchmg can be Inherent to the 

devlce, as ~n the case of spark gaps lnvolv~ng the breakdown of a 

gas Some appl~cat~ons have also been made of triggered dev~ces 

such as tr~ggered vacuum gaps In h~gh voltage technology or thyrlstors In low-voltage c~rcults where a control clrcult senses the rwng 

voltage and turns on the power-rated devce to dtvert the surge 

The major advantage of the crowbar dev~ce 1s that 11s low 

Impedance allows the flow of substantlal surge currents wtthout the 

development of h~gh energy wcthln the devce Itself, the energy has 

to be spent elsewhere In the clrcu~t. Th~s "reflex~on" of the tmplngIng surge can also be a disadvantage ~n some clrcults when the 

translent dtsturbance assoc~ated wlth the gap fmng IS bemg cons~dered Where there 1s no problem of power-follow (discussed 

below), such as ~n some communlcatlon clrcu~ts. the spark gap has 

the advantage of very slmple construction w~th potent~ally low cost 

The crowbar dev~ce. however, has three major Ilm~tatlons One IS 

the volt-t~me sensltlvlty of the breakdown process As the voltage 

Increases across a spark gap slgnlflcant conductton of current - and hence the voltage llm~tatlon of a surge - cannot take place 

untll the transltlon occurs to the arc mode of conduct~on, by 

avalanche breakdown of the gas between the electrodes The load 

IS left unprotected durmg the lnlt~al rlse because of th~s delay tme 

(typ~caily In m~croseconds) Cons~derable var~at~on ex~sts In the 

sparkover voltage ach~eved In successwe operations. slnce the process IS stattstlcal In nature Thts sparkover voltage In addltlon. can 

be substant~ally h~gher after a long pertod of rest than after successlve discharges From the physical nature of the process. tt IS d~fflcult to produce consustent sparkover voltage for low voltage ratlngs 

This dlfftculty IS Increased by the effect of manufacturing tolerances 

on very small gap distances, but rt can be allev~ated by f~ll~ng the 

tube w~th a gas havlng a lower breakdown voltage than alr However. ~f the enclosure seal IS lost and the gas IS replaced by air. th~s 

subst~tut~on creates a rel~ab~l~ty problem because the sparkover of 

the gap IS then substant~ally h~gher 

The second l~m~tat~on IS assoc~ated wlth the sharpness of the 

sparkover, whlch produces fast current rlses In the clrcu~ts and 

thus, objectionable nouse A classlc example IS found ~n osc~llograms recordmg the sparkover of a gap where the trace exh~b~ts an

anomaly before the sparkover (F~gure 2) Thls anomaly IS due to 

the delay Introduced In the osc~lioscope clrcu~ts to prov~de an 

advanced trtgger of the sweep What the trace shows IS the events 

delayed by a few nanoseconds. so that ln real me, the gap sparkover occurs wh~le the trace IS still wrltlng the pre-sparkover rlse 

Figure 2. Anomaly in Oscilloscope Recording 
Suppressors 

Another, more oblectionable effect of this fast current change can 

be found In some hybrtd protectlve systems. Figure 3 shows the 

ctrcuit of such a device. as found in the commerce. The gap does a 

very nice job of discharging the impingtng h~gh energy surges, but 

the magnetic field associated with the high dildt induces a voltage 

in the loop adjacent to the secondary suppressor, adding what can 

be a substantial spike to the expected secondary clamping voltage. 

Figure 3. Hybrid Protector with Gap 

A third limitation occurs when a power current from the steadystate voltage source follows the surge discharge (follow-current. or 

power-follow). In ac circuits, this power-follow current may or may 

not be cleared at a natural current zero. In dc circuits, clearing is 

even more uncertain. Additional means, therefore, must be provided 

to open the power Circuit if the crowbar device is not designed to 

prw~de self-clearing action within specifled limits of surge energy. 

system voltage, and power-follow cumnt This combination of a 

gap with a current-lim~ting. nonlinear varistor has been very successful In Me utility ~ndustry as a surge arrester or surge diverter. 

The prtnc~ples and applicattons of these devices will be examlned in 

some detail in later paragraphs. 

VOLTAGE-CLAMPING DEVICES 

Voltage-clamp~ng devices have variable R. impedance. depending on the current flowing through the device or the voltage across 

rts terminal. Them components show a nonlinear characteristic - 

Mat is. Ohm's law can be applied. but the equation has a vartable. 

Impedance varration IS monotonic and does not contain discontinuities. rn contrast to the crowbar devce. which shows a turn-on 

action. As far as their volt-ampere characteristics are concerned. 

these components are timedependent to a certain degree. However, unlike the sparkover of a gap or the trlggerlng of a thyr~stor. 

time delay is not involved. 

When a voltage-clamping devlce is installed. 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 dev~ce as the surge voltage attempts to rise 

results In voltage-clampmg action. Nonlinear Impedance is the 

result if this current rlse IS greater than the voltage increase. The 

increased voltage drop (IR) in the source impedance due to htgher 

current results in the apparent clamping of the voltage. It should be 

emphasized that the devtce depends on the source impedance to 

produce the clamping. A voltage div~der actlon is at work where 

one sees the ratio of the divider as not constant but changmg. If 

the source impedance is very low, the ratio is low, and eventually 

the suppressor could not work at all with a zero source Impedance 

(Figure 4). In contrast a crowbar-type device effectively short circuds the transtent to ground but once established, thts short circuit 

w~ll contlnue until the current (the surge current as well as any 

power-follow current supplted by the power system) IS brought to a 

low level. 

Figure 4. Voltage Clamping Action of a Suppressor 

The principle of voltage clamping can be achieved with any 

device exhibiting thls nonlinear impedance. Two categories of 

devices, having the same effect but operating on very different 

physical processes. have found acceptance In the industry: the 

polycrystalline varistors and the smgle-junction avalanche diodes. 

Another technology, the selen~um rectifier, has been practically 

eliminated from the field because of me mproved characteristtcs of 

modern varistors. 

AVALANCHE DIODES 

Avalanche diodes. the Zener diodes. were inittally applied as 

voltage clamps. a natural outgrowth of their application as voltage 

regulators. Improved construction, specifically aimad at surge 

absorption. has made these diodes very effective suppressors. 

Largediameter junctions and low thermal impedance connections 

are used to deal with the inherent problem of dissipating the heat of 

the surge in a very thin single-layer junction. 

The advantage of Me avalanche diode. generally a P-N stlicon 

junction. is the possibility of achievmg low clamping voltage and a 

nearly flat volt-ampere charactertstic over its useful power range. 

Therefore, these diodes are w~dely used In low-voltage electrontc 

ctrcuits for the protection of 5 or 15 V logic circuits. for Instance. 

For higher voltages. the neat generation problem assoclated w~th 

single junctions can be overcome by stacktng a number of lower 

voltage junctions. admittedly at some extra cost. 

Characteristics of Avalanche Diodes 

Slllcon avalanche dtodes are avatlable w~th character~sttcs 

ta~lored to translent suppresston. These should not be confused 

w~th regulator-type Zener drodes although many engtneers tend to 

use the generlc term "Zener dlode." May Zeus help them ~f they 

m~sapply a regulator-type Zener, expectrng to ach~eve good 

protectton1 

Figure 5 shows a typ~cal famtly of V-1 character~st~cs for one 

product The parameter l IS the peak of a specified current 

waveform. generally the 10/1000 rs double exponenttal prefenea by 

communtcatlon englneers In contrast to the 8/20 rs wave preferred 

by power englneers. These curves are remarkably flat from 1 to 

20 amperes. with clamptng voltages low enough to protect sens~t~ve 

electronics. 

Manufacturers of avalanche diodes generally rate their devices 

in terms of maximum peak power for a specified pulse duration. 

Although the power decreases wlth ~ncreasing pulse duration, thts 

does not occur at constant energy (Figure 6). A two-decade 

increase In pulse duration reduces the power by only about one 

decade, primarily because of increasing heat transfer for longer 

pulses. 

Since the junctton IS very thtn. the capacftance of an avalanche 

dtode 1s apprec~able. Thts can be a concern. It IS posstble to rntntmlze thts effect by using sertes combtnattons w~th low capacitance 

diodes (Figure 7). 
Suppressors 

L . . . . i -.--,. r.. .. ...... I . L- , , -+-+J 

SILICON TRANSIENT SUPPRESSOR 

TRANSIDIT 

SUPPRESSOR 

- CAPACITANCE DIODE 

LOW CAPACITANCE 

MODES 

(a J (b) 

DW w h*u c.,-~ - .- Figure 7. Examples of Low-Capacitance Protectors 

Figure 5. Typical V-l Characteristics 

Figure 6. Peak Pulse Power Curve versus Pulse Time 

Properly packaged and wired avalanche diodes exhibit a quick 

response to steep, front pulses, and have been widely used for 

NEMP protection of electronic equipment. However, this quick 

response can be completely obliterated by Improper wirmg (lead 

length). The effect of lead length, discussed in detail in the next 

section, is applicable to any transient suppressor. 

VARISTORS 

The term varistor is derived from its function as a variable 

resistor. it is also called a voltage-dependent resistor, but that 

description tends to imply that the voltage is the independent 

parameter In surge protection. Th~s concept will be contested 

repeatedly during the conference. Two very different dev~ces have 

been successfully developed as varlstors: silicon carblde disks have 

been used for years In the surge arrester industry, and, more 

recently, metal ox~de varlstor technology has come of age. 

In silicon carbide varistors. as well as In metal oxide varistors. 

the relationship between the current flowmg in the devce and the 

voltage appearing across its terminals can be represented approximately by a power function I - kVa, where 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 

hlgher values of a. We will see, however, that there are practical 

limits to this race and that. in fact. better performance can be 

obtamed at hlgher current densities by departing somewhat from 

the large values of the exponent a. 

In silicon carbide varlstors, the phys~cal process of nonlinear 

conduction is not completely understood, and the manufacturing of 

Figure 8. Equwalent Circuit of a Varistor 

the mater~al. successful as ~t IS, has remamed an an It appears 

that the process takes piace at the tlps of the gralns of s~l~con carb~de wh~ch are held together by a bmder The story goes that the 

dev~ce action was found acc~dentally by havmg a grlnd~ng wheel, on 

a disorderly work bench, accrdentally connected to an expermental 

clrcutt; for many years stlcon carb~de var~stors mdeed looked hke 

grtndmg wheels, each complete wlth a hole In the center 

Metal oxide varistors depend on the conductlon process occurring at the boundaries between the large gralns of oxide (typically 

zinc ox~de) grown In a carefully controlled smtering process. The 

phys~cs of the nonlinear conductlon rnechanlsm have been 

described in the ~iterature:('-'~' In these appllcat~on notes, we will 

be more concerned with the behavtor of the varistors as a twoterminal electrical component. 

Electrical Characteristics of Varistors 

Because the prlme functlon of a varlstor is to provide the nonlinear effect other parameters are generally the result of tradeoffs in 

design and inherent characteristics. The electrical behavior of a 

varistor can be understood by exammation of the equivalent circuit 

of Figure 8. The major element is the varlstor proper. Rv. whose V-l 

characterist~c is assumed to be the perfect power law l=kP. In 

parallel with thls varistor, there is a capacitor. C, and a leakage 

resistance. R,. In serles with this three-component group, there IS 

the bulk resistance of the zlnc oxlde gralns. R,, and the ~nductance 

of the leads. L. 

Under dc conditions (at low current densities, because obv~ously 

no var~stor could stand the h~gh energy deposited by dc currents of 
Suppressors 

htgh density), only the varistor element and the parallel leakage 

reslstance are significant. Under pulse condittons at high current 

densities, all but the leakage reststance are significant: the varlstor 

provldes low impedance to the flow of current but eventually the 

series resistance will produce an upturn in the V-l characteristic: 

the lead inductance can give rtse to spurious ovenhoot problems if 

it is not dealt with properly: the capacitance can offer either a weicome additional path with fast transients or an objectionable loading at high frequency. depending on the applicat~on. 

V-1 Characteristic When the V -1 characteristic is plotted on a 

log-log graph, the curve of Figum 9 is obtained. with three regions 

as shown, resulting from the dominance of R,. R,. R, as the current 

in the device goes from nanoamperes to kiloamperes. 

10 Y I I I I I I I lod to* 10' lo2 td to' lo4 

CURW-bMmms 

Figum 9. Typical V4 Characteristic 

The V -1 characteristic is Men the basic application design tool 

for selecting a device in order to perform a protective function. For 

a successful application, however. other factors. which are discum In detail in the information available from manufacturers. 

must also b taken into consideration. Some of these factors are: 

. Selection of the appropr~ate nominal voltage for the line voltage of the application 

. Selection of energy-handling capability (including consideration of the source tmpedance of the transient the waveshape. 

and the number of occurrences) 

Heat dissipation 

Proper installation in the circuit (lead length). 

In fact enough instances of poor installation practices have 

been ~bSeN€?d and enough questions have been raised on alleged 

"overshoot" that a br~ef discussion of lead effects is in order. 

Overshoot: A Lead Effect To illustrate the effect of lead length 

on the overshoot two measurement arrangements were used. As 

shown In Figures 10(a) and 10(b). 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 10(d and 10(d). With a slow current front 

of 6 ps. there IS little difference in the voltages occurrtng wlth a 

small or large loop area. even with a peak current of 2.7 kA. With 

the steep front of 0.5 as, the peak voltage recorded with the large 

loop is nearly twice the voltage of the small loop. Note In 

Figure 10(d), that at the current peak, L dildt = 0, and the two voltage readings are equal; before the peak. L di/dt is p~sttl~e, and 

after, it IS negative. 

nnm = as d 

OUTPUT LEAD FUOM 

TRANSIENT GENERATOR 

PAM 

la) Minimal Lo00 Arm Ib) Ercsgive Loop Area 

Ict Current Rise of 8 us Id1 Current Rise of 0.5 us 

Flgure 10. Effect of Lead Length on Overshoot 

PROBE CONNECTIONS 

When making voltage measurements across a clamping devtce. 

e.g- for evaluating its performance. one must recognize possible difficulties requiring special precautions. Two precautions must be 

taken: 

1. Use two probes in a differentla1 mode to make a mesurernent 

directly at device termmais. 

2. Avoid contaminating the true devlce voltage by the additional 

voltage caused by magnetic coupling. 

Commercial oscilloscope preamplifiers offer a wtde chotce of differential mode operation, e~ther through an [add + Invert one1 mode 

of two-channel preaplifiers. or through a differential amplifier built 

specifically for high common-mode rejection, sometimes at the 

expense of bandwidth. Thus. careful attention must be glven to this 

aspect of measurements. 

The voltage measured by the two probes IS the sum of the 

actual clamptng voltage exlstlng across the device and a spurtous 

voltage caused by magnetlc coupltng. Thls spurtous voltage IS 

tnduced Into the loop formed by the clarnprng dev~ce length and the 

two probes by the changlng magnetlc field of the current flowlng In 

the dev~ce. 

TO further illustrate thls sltuatlon. the measurement clrcult shown 

In Figure 11 was set up In the output crrcu~t of a generator producIng a 6/20 @ ~mpulse. The "devtce" was a hollow conductor, wtth a 

hole at the center through whtclr a twlsted palr was fed. one wtre of 

the palr branchtng out to each end of the conductor, separated by 

10 cm At the same 10 cm separatton. but outstde of the hollow 

conductor, two thm wires were also soldered. brought to the m~dpolnt of the hollow conductor and in close Contact wtth the conductor; from the mldpomt outward, they were twfsted In the same 

manner as the rnslde parr A thtrd set of wlres were soldered at the 

end polnts of the hollow conductor. and arranged to form a 
Suppressors 

rectangle, the hollow conductor being one side of that rectangle 

Several widths could be set up for the rectangle, and each time the 

measured voltage was recorded. Figure 13 shows the measured 

voltage versus radlal distance of the opposite side of the rectangle, 

plotted from the oscillograms of Figure 12. 

This example shows that not only one must connect the probes 

as close as possible to the terminals of a clamping device. but still 

strive to minimize the area established by the probes close to the 

device. 

In this case of a low-voltage suppressor, it would be better to 

solder short leads to the device terminals, bring them together while 

tightly hugging the device, then twist them in a pair and connect the 

oscilloscope probes some distance away from the device 

Current in Conductor, 80A/ws Voltage by coaxial probes 

Voltage by side probes Voltage 1x10 cm loop 

Figure 12. Voltages Recorded for Various Probe Connections 

This example also shows the Importance of wlre layout in making the connections of a protective device in an actual circuit As 

discussed in Case History No. 1, creating a loop near the protective 

device IS an invitation to Induce additional voltages in the output of 

the protective device, thus losing some of its effect~veness. 

Hence, when one is making measurements as well as when one 

is designing a clrcuit for a protection scheme, it is essental 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 1 WS. 

TO CRO 

l7A13) 

Figure 11. Circuit Configuration 

COMPARISONS OF PROTECTIVE DEVICES 

Linear Versus Nonlinear Devices 

When 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 transients expected to occur. In particular, if an error is made in 

assuming the source impedance of the transient, the consequences 

are dramatically different between a linear protective device and a 

nonlinear protective device, as illustrated in a simplified comparison 

of the two suppressor devices (see box). 

Spark Gap Versus Varistor 

The cholce between these two devices will be influenced by the 

inherent characteristics of the application. Where power-follow is 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 "protected" circuit until the voltage is limited 

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

the low inherent capacitance of a gap seems attractive. If very high 

energy levels, compared to the lower levels inherent with the 

crowbar action of a gap, can be deposited in a varistor, then a high 

capacity surge arrester near the service entrance may be combined 

with a lower clamping voltage varistor installed farther into the circuit. This combined protection, however, requires adequate coordination between the two suppressors (see reprint F79 635-4 in this 

section). 

Figure 13. Voltage versus Area 
Suppressors 

A SIMPLIFIED COMPARISON BETWEEN PROTECTION 

WITH LINEAR AND NONLINEAR SUPPRESSOR DEVICES 

Assume an open-circuit voltage. V,. of 3000 V (see the 2. Now. assuming that the source impedance is only 

figure). 4 5 fl (a 10:l error in the original assumption). the 

voltage across the same linear 8 fl suppressor is: 

- 1850 V 

1. ~ssufning a source impedance of Z, = 50 0 

However, the nonlinear varistor has much lower imWith a suppressor impedance of Z, = 8 fl. the pedance. Again, by iteration from the characteristic 

expected current is curve, use 450 V at 500 A. which is correct for the 

Therefore. 

VR-8x 52A 

Working back from an assumed 52 A current but 

now considering a varistor, the maximum voltage 

appearing across the terminals of a typical nonlinear V150LA20A varistor at 52 A is 285 V (from 

characteristic curve). 

Note that: 

2, x I - 50 x 52 - 2600 V 

Therefore. 

v, ;; 3000 V. 

The identity between this result for V, and the 

assumed 3000 V justifies the trial selection of 

52 A In the circuit. 

Therefore. 

v, z 3000 V. 

The identity between this result for V, and the assumed 3000 V justifies the trial selection of 500 A in 

the circuit. 

3. Conclusions: 

r With a linear resistor. a cham of source Impedance from 50 Q to 5 Sl would produce a 

change in the protactive Ievsi tmm 416 V to 

1850 V. 

. With a varistor, the same change of source Impedances produces a pmt~tion level change of 

only 385 V to 450 V. 

Avalanche Diode Versus Varistor 

The basic performance characteristics of these two devices are 

slmilar, and therefore the choice may be dictated by clamplng voltage requirements (the avalanche diode is available at lower clamping voltages), by energy-handling capabilities (the avalanche diode 

is generally lower in capability per unit of cost), and by packaging 

requirements (the varistor material is more flexible and does not 

requlre hermetic packagmg). 

Conventional Surge Arresters Versus Gapless Arresters 

Surge arresters (diverters) have reached a high degree of 

sophistication over the years by uslng precision gaps in series with 

sliicon carb~de varistors. For high-voltage applications, the arrester 

1s made of a stack of modules. generally of 3 or 6 kV each. With 

the use of current-limiting gaps, the clearing of power-follow current 

In dc applicatron was made possible. A series gap was required in 

ail these arresters using silicon carbide because. for the specified 

discharge voltage and discharge current, the standby current at the 

normal voltage would be excessive. 

With the advent of metal oxide varistors, the high exponent of 

the V-l characteristic reduced the standby current to a very low 

level, one that can be tolerated by Wa varistor under steady-state 

conditions. The series gap can mus b. eliminated. thereby producing three considerable improvements: 

Performance (elimination of abrupt sparkover) 

Reliability (elirnination not only Of the gap and all trigger circuitry but also of the parallel voltage-grading varistors) 

Contamination withstand (elimination of effects of leakage 

current on the outer shell). 

Previously, where low-voltage secondary arresters using gaps 

were the only devices capable of meeting the ANSI requirement of 

a 10 kA. 8/20 ps discharge currant highamrgy varistors are now 

capable of meeting this requirement without the problems 

assocrated w~th series gaps. 

MATCHING VARISTOR CAPABILITY TO THE ENVIRONMENT 

Manufacturers of metal oxide vuktors publish ratlngs which include a family of curves, generally dHICdbetd as "Pulse Lifetime." 

that show the number of surges whbh a given varistor can absorb 

before some arbitrary limit of characteristic shift is reached. These 

curves show numbers of surges of equal amplitudes for a set of 
Suppressors 

~p-p 

waveshapes. The actual occurrence of surges, however. IS a full 

range of values, from the lowest to the h~ghest. and also of different 

waveshapes. 

Attempts to simplify the description of surge occurrences have 

resulted in the statistical information of IEEE Std 587-1980, where a 

family of curves has been obtained from actual surge recordings; 

these curves show the frequency of occurrence of surges as a function of the voltage surge level, for various exposures (Figure 14). 

Carroll first developed a method(12) whereby the statistical information of IEEE 587 can be combined to predict the effect of the range 

of surge, as opposed to constant amplitude surges, on the varistor 

Pulse Lifetime. Korn has shownu3) how a simple set of assumptions and computations based on Figure 14 and Pulse Lifetime 

curves can provlde an estimate of the time required to reach the 

varistor rated Pulse Lifetime. The last reprint. "Matching Surge Protective Devices to Their Environment." provides a detailed explanation of the method, with two specific examples. 

*In some loatiom, apukover of cle~ca my limit the overvoltaeea (see 8.3). 

Figure 14. Rate of Surges Occurrence vs. Voltage Level 

(From IEEE Std 587-1980) 

Failure Modes 

Fa~lure of an electr~cal component can occur e~ther because ~ts 

capablltty was exceeded by the apphed stress or because some 

latent defect In the component went by unnot~ced In the quallty 

control processes. Wh~le thls s~tuat~on 1s well recogn~zed for ordlnary components. a surge protectwe dev~ce, whlch IS no exception 

to these Ilm~tat~ons. tends to be expected to perform miracles, or at 

least to fall grac~ously In a "fall-safe" mode. The term "fall-safe." 

however, may mean drfferent failure modes to different users and. 

theretore, should not be used. To some users. tall-safe means that 

the protected hardware must never be exposed to an overvoltage. 

so that fa~lure of the protectlve devlce must be In the fall-short 

mode. even 11 ~t puts the system out of operatlon To other users, 

fall-safe means that the function must be mamtalned, even ~f the 

hardware IS left temporarily unprotected, so that fallure of the protectlve devlce must be In the open-clrcu~t mode. It IS more accurate and less mlsleadmg to descrlbe fa~lure modes as "fall-short" or 

"fall-open" as the case may be 

When the d~vertmg path IS a crowbar-type devlce. l~ttle energy IS 

dtsslpated ~n the crowbar as noted earl~er In a voltage-clamp~ng 

dev~ce, because more energy IS depos~ted In the devtce, the energyhandllng capabll~ty of a candidate protectlve devlce IS an Important 

parameter to cons~der In the deslgnmg of a protectlon scheme 

W~th nonlmear devlces. an error made In the assumed value of the 

current surge produces llttle error on the voltage developed across 

the protectlve devce and thus apphed to the protected clrcu~t, but 

the error IS dlrectly reflected ~n the amount of energy whtch the protectlve device has to absorb At worst, when surge currents In 

excess of the protectwe devce capab~l~ty are unposed by the envlronment, such as an error made In the assumption, human error In 

the use of the devlce, or because nature tends to support Murphy's 

law, the clrcult In need of protectlon can generally be protected at 

the prrce of fallure tn the short-c~rcu~t mode of the protectlve devcce. 

However. ~f substant~al power-frequency currents can be supplled 

by the power system the fall-short protectlve dev~ce generally termlnates as farl-open when the power system fault In the fa~led 

dev~ce ts not quickly cleared by a serles overcurrent protectlve 

dev~ce (fuse or breaker) 

When there IS a need to prov~de ellmmat~on of a failed protector 

at the speclflc equlprnent level, lnsertlon of the fuse In the llne prov~des protectlon of the equ~pment (F~gure 15a). whlle msertlon of the 

fuse In serles wtth the shunt-connected protector provldes protectlon of the funct~on Albe~t w~th loss of overvoltage protectlon 

(Figure 15b) 

. a. PfOt.~ll~" m.,*t.,ned 

lunctlon ~~HND~W D.Funct>m mamtamao 

DlOIKllOn IOSI 

Figure 15. Fus~ng Opt~ons for Suppressors. 

CONCLUSIONS 

Surge protectlve devlces are ava~lable for protecting low-voltage 

electron~cs. Two baslc types offer d~fferent character~stlcs. 

crowbar dev~ces have hlgh-current capab~l~ty but generally lnvolve 

power-follow when applled on a power system; voltage clamplng 

devlces, elther srltcon avalanche or varistors, are free from the 

power-follow problem. 

Avalanche dlodes offer low clampmg voltage. which makes them 

most sultable for low-voltage, low-power electron~cs Metal oxide 

vartstors are now ava~lable In a wlde range of clampmg voltages 

and energy-handllng capaclt~es Each of these dev~ces has its own 

best f~eld of appl~cat~on, lnsurlng greater rel~ab~l~ty of the clrcu~ts In 

the not-qu~te-defined electromagnet~c environment of power and 

communlcatlon systems. 

FOR FURTHER READING 

Barksdale. C.B.. "New Gas Tube Protector Techn~que for Circuits." 

T818phOny 171. 1970. pp. 21-28. 

Bazar~an, A,, "When Should You Use Gas-Discharge Protectors?" 

Nectronrc Design News. December 20. 1973, pp. 58-62. 

Clark. O.M.. and J.J. Piulcaroli. "Eifect of Lead Wlre Lengths on Protector Clampmg Voltages." Fed8raI Aviation Admfnistration-flonda 

Institute of Technology Workshop on Grounding and Lightning 

Technology. FAA-RD-79-6. March 6-8. 1979. Melbourne. Flor~da. US. 

Dept. of Transportation. Washlngton. DC 20590. 
Suppressors 

Kothelmer, W C.. "Control Circuit Transients." Power Engineermg 73. 5. Matsuoka. M.. "Non-Ohmc Propertres of Zinc Oxlde Ceramics." 

1969. Jap. J. Appl. Phys 10. 1971, p. 736. 

Richman. P.. "Diagnostrc Surge Testmg. Part I." Solid State Power 

Conversron. Septemoer/Octocmr 1979. 

Smith. M.N, Practical Applicatron and Effectweness of Commercially Avarlable Pulse Volage Transient Suppressors Naval Civil Englneertng Laboratory. Port Hueneme. CA 1973. 

"Surge Voltage Protectors - Application Note and Suggested Applications." SERT. The Journal of the Society of Electronic and Radio 

Technrclans 9. (4). April 1975. pp. 85-91. 

Vadicka, V.W.. "Voltage Translents Tamed by Spark-Gap Arresters," 

Elecrroncs 39. 1966. pp. 109-1 13. 

6. Morns. W.G.. and F.D. Mamloff, "VARPAR - A Program for Calcuiating Parallel Varlstor Character~st~cs" 74CRD278. General 

Electrlc Company. Schenectady, NY. 1974. 

7 Phllipp. H.R.. and LM. Levinson.. "High-Temperature Electrical 

Studies on Metal Oxide Varlstors." 7BCR0131. General Electric 

Company. Schenectady. NY, 1978. 

8. Philipp. H.R. and LM. Levinson. "Long Time Polarization Currents in Metal Oxide Varistors,'' 76CR0026, General Electric 

Company. Schenectady, NY. 1976. 

9. Sakshaug. EC, J.S. Kresge, and S.A. Mlske. "A New Concept ~n 

REFERENCES Station Arrester Des~gn." "IEEE PAS-96. (2). MarchlApril 1977. 

Pp. 647-656. 1. Harden. J.D.. F.D. Marhloff. W.G. Morris. and F.B. Golden. 'The 

GE-MOV Varistor - The Super Alpha Varlstor,w Nectronics 45 10. Transient Voltage Suppress~on Manual. Third Editlon. General 

(21). 1972. p. 91. Eiectrlc Company. Auburn. NY. 1962. 

2. Levlnson. LM, and H.R. Philipp. Low Temperature AC Properties of Metal-Oxide Varistors. 78CRD102. General Electric Company. Schenectady, NY. 1978. 

3. Mahan. G.D.. LM. Levinson. and H.R. Philipp. 'Theory of Conduction in ZnO Varlstors." 78CR0205. General Electric Company. 

Schenectady. NY. 1978. 

4. Matsuoka. M.. T. Masa Yama. and Y. Lida "Nonlinear Electrical 

Properties of Zinc Oxlde Ceramlcs." PrOc. of First Conf. Solid 

State Devices. Tokyo. 1969. J. Japan Soc. Appl. Phys 39. 1970. 

Suppl.. p. 94. 

11 'Test Speclflcatlons for Varlstor Surge-Protective Devices." IEEE 

Std C6233. 1982 

12. Carroll. E. "Transient Attenuation wcth Metal Oxlde Vartstors for 

AC Malns Powered Egu~pments." presented at "Cornel." Unlverslty of Twente. Holland. November 1980 

13. Korn. S, "Transients and Transcent Suppressors FOP Power 

Convers~on Equipment." INTELEC Conference. Washington. D.C., 

1982. 
I Suppressors 

DIVERT CLAMP PROTECTED 

CIRCUIT 

TRANSIENT SUPPRESSORS 

SUPPRESSORS 

LIMITERS 

CLAMPS 

DIVERTERS 

ARRESTERS 

PROTECTORS 

PROTECTIVE DEVICES 

GAPS CARBON BLOCKS 

0 GAS TUBES 

ROD GAPS 

0 .... CLEARANCES ! 

NON LINEAR 0 SILICON CARBIDE 

RESISTORS 0 METAL OXIDE VARISTORS 

SEMI- 0 SELENIUM RECTIFIERS 

CONDUCTORS 0 SILICON AVALANCHE DIODES 

LINEAR 0 RESISTORS 

CAPACITORS 

CROWBAR 

SLIDE NOTES 

The basic approach to application of protective devices. 

They come under all names 

. . . and are of many types 

First family: gaps, semiconductor switches. 
Suppressors 

ion T 

SLIDE NOTES 

One of the problems introduced by the solution. 

Another family: clamps. 

"Zener diode" response. 

Parameters of an avalanche diode. 
Suppressors 

SLIDE NOTES 

The newest, but well-established. 

1 High exponent I-V characteristic. 

More joules. 

33 
Suppressors 

TRACE 1 

LOAO VDLTlCGE 

wmw1 VbRlSm8 

- TrUCE 2 

COlO VnTm CLAMPED 

BY VUItSTOR 

EFFECT 

OF 

CONNECTIONS 

StlM 700TES *r"*"* + 3 .an 

The "upturn" at high cwrent density 

makes current sharing possible. 

.Ni Pa-,-.*' ii r 

A a~\ *4e-*- r4 

in a very low special mductance (VLSI) 

Two effect% 

Memurinq device characterlstics 

Ap&ykyl the devices correctly 

Connection options for 3-wire service. 
Suppressors 

CIRCUIT F=T 

The clamping voltages and voltages between other conductors for various connection options 

(see propagation paper in reprint section). 
Suppressors 

MATCHING 

THE 

ENVIRONMENT 

Published pulse lifetime characteristics. 

Calculating the current in the varistor. 

36 
Suppressors 

1 NUMBER OF OCCURRENCES AT 8/20 

SUBTRACT CLAMPING VOLTAGES 

DIVIDE BY IMPEDANCE 

LOOK UP RATED NUMBER OF PULSES 

COMPUTE YEARLY CONSUMPTION 

AND TOTAL 

FAILURE MODES 

FAIL-SAFE - NO NO 

FAlL OPEN - J 

FAlL SHORT - 4 

SLIDE NOTES 

Simplified pulse lifetime. 

' The sequence of operations. 

Word usage. 

Two options for fusing. depending on rniss~on objective. 
Suppressors 

SLIDE NOTES 

How a perfectly good isolating transformer -- for common mode -- fails to decouple normal mode 

surges. 

Low interwinding capacitance does not do any better -- 

perhaps worse. 

Loading the transformer secondary helps, 

but is not sufficient. 

The ferroresonant line conditioner not only regulates and 

provides 112 cycle ride-through, but decouples the surge: