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Surge Suppression Technologies for AC Mains Compared

(MOVs, SADs, Gas Tubes, Filters and Transformers)

Application Note

January 1998

AN9310.1

Synopsis

[ /Title

(AN93

10)

/Sub-

ject

(Surge

Sup-

pres-

sion

Tech-

nolo-

gies

for AC

Mains

Com-

pared

(MOV

SADs,

Gas

Tubes,

Filters

and

Trans-

form-

ers))

/Autho

r ()

/Key-

words

(Surge

Sup-

pres-

sion

Tech-

nolo-

gies

for AC

Protection on the low voltage AC mains system from

transient overvoltages is now a fundamental power quality

concern. The use of correctly selected and installed surge

protectors have a long and proven history of successful ﬁeld

performance. The expected transient environment is

addressed, along with various types of surge suppression

components are compared to MOVs.

outages or combinations of the above and are generically

known as transients. A common result of encountering these

overvoltages is the early failure of semiconductor

components and other sensitive electrical components. An

equally serious effect is the loss of control in solid-state logic

systems that may think surges are legitimate signals and

thus endeavor to react to them.

Numerous studies have been performed which indicate that

the causes of the surges in an electrical system can be

attributed to one of the following causes:

•

Lightning

Opening or closing of switch contacts under load

Propagation of surges through transformers

Severe load changes in adjacent systems

Power line ﬂuctuations and pulses

Short circuits or blown fuses

Introduction

The increasing use of semiconductors and other solid state

components in modern electrical systems has resulted in a

growing awareness about system reliability. This is a

consequence of the fact that solid state devices are very

susceptible to stray electrical transients which may be

present in the low voltage AC distribution system. The initial

use of semiconductors resulted in a large number of

unexplained failures. Investigation into these failures

revealed that they were caused by a number of diverse

overvoltage conditions which were present in the distribution

system. Transient voltages result from the sudden release of

previously stored energy from overstress conditions such as

lightning, inductive load switching, electromagnetic pulses or

electrostatic discharges. The severity of, and hence the

damage caused, by the transient depends on their frequency

of occurrence, their peak values and their waveshapes.

Electrical overvoltages on AC mains can cause either

permanent deterioration, or temporary malfunctions in

electronic components and systems. Protection from

transients can be obtained by using specially designed

components which will, either limit the magnitude of the

transient using a large series impedance or by diverting the

transient using a low value shunt impedance.

A prudent designer will consider the need for transient

protection in the early stages of the design. Too many times

it has been necessary to retroﬁt existing equipment with

transient suppressors. This is expensive in terms of ﬁeld

failures, customer downtime and potential loss of business.

In some systems retroﬁtting becomes cumbersome, as the

space required was not planned for in the initial design. The

device selected as the system protector must have the

capability to dissipate the impulse energy of the transient at

a sufﬁciently low voltage so that the capabilities of the

system being protected are not affected.

The power system is made up of a large network of cross

connected transmission lines. This power system is often

interfered with by transients originating from one of the

aforementioned sources.

Transients caused by lightning can inject very high currents into

the system. These lightning strikes, usually to the primary

transmission lines, may result in coupling to the secondary line

through mutual inductive or capacitive coupling. Even a

lightning hit that misses the line can induce substantial voltage

onto the primary conductors, triggering lightning arresters and

creating transients. Man-made switching transients can be of a

lesser, but more frequent threat. Switching of the power grid

can cause transients which may damage down line equipment.

The use of thyristors in switching circuits or power control can

also create such transients.

Studies and laboratory investigation of residential and

industrial low voltage AC power systems have shown that

the amplitude of the transient is proportional to the rate of its

occurrence, i.e., lower magnitude transients occur most

often. Governing standards bodies, in particular lEEE and

ANSl, have established a document which gives practical

guidelines of the transient environment one may expect to

encounter in a low voltage AC power system. This document

is called the ANSl/lEEE standard C62.41 and was developed

in 1980. Since its inception, more accurate information has

become available on the transient environment and this has

led to the generation of an updated standard, which should

be available later this year.

Problem Deﬁnition

(The Transient Environment)

[1, 2, 3, 4]

Primarily the problem is that of the enigmatic presence of

overvoltage surges, above the normal system voltage.

Overvoltages are sometimes explainable or sometimes they

just mysteriously appear in the electrical system; they take

the form of disturbances, notches, swells, sags, brownouts,

10-41

Rate of Occurrence

The rate of occurrence of surges varies quite a lot and is

dependent upon the particular power system. Rate is related

to the level of surges and low magnitude surges are more

common than high level surges. Data from many sources

have shown that surges of 1kV or less are relatively

common, while surges of 3kV are more rare. The data

1-800-4-HARRIS or 407-727-9207

Harris Corporation 1998

Application Note 9310

generated from the studies was used to generate the curve

shown in Figure 1. This curve shows with certainty only a

relative frequency of occurrence, while the absolute number

of occurrences can be described in terms of “low exposure”,

“medium exposure” and “high exposure”.

NUMBER OF SURGES PER YEAR EXCEEDING

HIGH

EXPOSURE

SURGE CREST OF ABSCISSA

MEDIUM

EXPOSURE

recommended that the device selected have, as a minimum,

the capability to handle the conditions called out in location

Category A of Table 1. The optimum device would preferably

have a minimum capability of surviving the transient

occurrences called out in location Category B.

TABLE 1. SURGE VOLTAGES AND CURRENTS DEEMED TO

REPRESENT THE INDOOR ENVIRONMENT

IMPULSE

LOCATION

CATEGORY

Long Branch Circuits

and Outlets

Major Feeders and

Short Branch Circuits

WAVEFORM

0.5µs

100kHz

1.2/50µs

8/20µs

0.5µs

100kHz

MEDIUM

EXPOSURE

6kV

200A

6kV

3kA

6kV

500A

(SEE NOTE) SPARKOVER

OF CLEARANCES

-1

LOW

EXPOSURE

-2

0.3

0.5

SURGE CREST (kV)

NOTE: In some locations, sparkover of clearances may limit the

overvoltages.

FIGURE 1. RATE OF SURGE OCCURRENCES vs VOLTAGE

LEVEL AT UNPROTECTED LOCATIONS

An area described as a “low exposure” area would have very

little lightning activity and few switching loads on the AC power

system. A “medium exposure” area is known for high lightning

activity, with frequent and severe switching transients. When

designing equipment for the global environment it is expedient

that it be, at least, designed for use in an area with “medium

exposure” transient occurrences. “High exposure” areas are

rare but real systems supplied by long overhead transmission

lines and subject to reflections at line ends, where the

characteristics of the installation produce high sparkover

levels of the clearances.

All indoor low-voltage AC power systems have an inherent

protection system built into the wiring of the building. Wiring

systems used in 120V - 240V systems have a natural

sparkover level of 6000V. This 6kV level has therefore been

selected as the worst case cutoff for the occurrence of

transients in the indoor power system. The transient

generated by sparkover creates a high energy, low impedance

pulse. The further away from the source of the transient the

protected equipment is located, the more the energy is

absorbed in the wiring impedance and the more the

equipment is protected. This, therefore, allows different size

suppressors to be used at different locations in the system.

The investigation into the indoor low voltage system revealed

that location Category A encounters transients with

oscillatory waveshapes with frequency ranges from 5kHz to

more than 500kHz; the 100kHz being deemed most

common (Figure 2). Surges recorded at the service

entrance, location Category B, are both oscillatory and

unidirectional in nature. The typical “lightning surge” has

been established as 1.2/50µs voltage wave and 8/20µs

current wave (Figure 3).

PEAK

0.9 V

PEAK

T = 10µs (f = 100kHz)

0.1 V

PEAK

0.5µs

60% OF V

PEAK

FIGURE 2. 0.5µs - 100kHz RING WAVE (OPEN CIRCUIT VOLTAGE)

0.9V

PEAK

0.3V

PEAK

0.5V

PEAK

Representative Transients

Table 1 reﬂects the surge voltages and currents deemed to

represent the indoor transient environment in a low-voltage

AC power system. When deciding on the type of device to

use as a transient voltage surge suppressor, it is

50µs

x 1.67 = 1.2µs

FIGURE 3A. OPEN-CIRCUIT WAVEFORM

10-42

Application Note 9310

0.9 I

PEAK

Filters

The installation of a filter in series with the equipment seems an

obvious solution to overvoltage conditions. The impedance of a

low pass filter, e.g., a capacitor, forms a voltage divider with the

source impedance. As the frequency components of a transient

are several orders of magnitude above the power frequency of

the AC circuit, the inclusion of the filter will result in attenuation

of the transient at high frequencies. Unfortunately, this simple

approach may have some undesirable side affects; a)

unwanted resonances with inductive components located

elsewhere in the system leading to high voltage peaks, b) high

inrush currents during switching and c) excessive reactive load

on the power system voltage. These undesirable effects can be

reduced by adding a series resistor, hence the popular use of

RC snubber networks. However, the price of the added

resistance is less effective clamping.

There is a fundamental limitation to the use of a ﬁlter for

transient suppression. Filter components have a response

which is a linear function of current. This is a big

disadvantage in a situation where the source of the transient

is unknown and it is necessary to assume a source

impedance or an open-circuit voltage. If the assumption of

the characteristics of the impinging transient are incorrect

the consequences for a linear suppressor is dramatic. A

slight change in the source impedance can result in a

disproportionately increase in clamping voltage [6].

0.5 I

PEAK

0.1 I

PEAK

20µs

x 1.25 = 8µs

FIGURE 3B. DISCHARGE CURRENT WAVEFORM

FIGURE 3. UNIDIRECTIONAL WAVESHAPES (OUTDOOR

LOCATIONS)

Transient Protection

Once it has been decided to include transient suppression in

the design of equipment, the next stage in the process is to

decide on what protection technology to use and on how to use

it. The transient suppressor selected must be able to suppress

surges to levels which are below the failure threshold of the

equipment being protected, and the suppressor must survive a

definite number of worst case transients. When comparing the

various devices available considerations must be given to

characteristics such as protection levels required, component

survivability, cost, and size.

There are a number of different technologies available for

use as a transient suppressor in the low voltage AC mains

system. Generally speaking, these can be grouped into two

major categories of suppressors: a) those that attenuate

transients, thus preventing their propagation into the

sensitive circuit; and b) those that divert transients away

from sensitive loads and so limit the residual voltage.

Attenuating a transient - that is, keeping it from propagating

away from its source or keeping it from impinging on a sensitive

load - is accomplished with by placing either filters or isolating

transformers in series within a circuit. The

isolator

attenuates

the transient (high frequency) and allows the signal or power

flow (low frequency) to continue undisturbed.

Diverting the transient can be accomplished with a crowbar

type device or with a voltage clamping device.

Crowbar

device types involve a switching action, either the breakdown

of a gas between electrodes or the turn on of a thyristor.

After switching on, they offer a very low impedance path

which diverts the transient away form the parallel-connected

load.

Clamping

devices have a varying impedance which

depends, either, on the current ﬂowing through the device or

on the voltage across the terminals. These devices exhibit a

nonlinear impedance characteristic. The variation of the

impedance is monotonic; that is, it does not contain

discontinuities in contrast to the crowbar device, which

exhibits a turn on action.

10-43

Isolation Transformers

Isolation transformers generally consist of a primary and

secondary windings with an electrostatic shield between the

windings. The isolation transformer is placed between the

source and the equipment requiring protection. As its name

suggests, there is no conduction path between the primary and

secondary windings. A widely held belief is that “isolation

transformers attenuate voltage spikes” and “that transients do

not pass through the windings of the transformer”. When

properly applied, the isolation transformers is useful to break

ground loops, i.e., block common-mode voltages.

Unfortunately, a simple isolation transformer provides no

differential-mode attenuation [7]. Thus, a differential-mode

transient will be transmitted through the windings of the

device. Also, an isolation transformer will not provide any

voltage regulation.

Spark Gaps and Gas Tubes

Spark gap suppression is a crowbar suppression technology.

During an overvoltage condition, a crowbar device changes

from being an insulator to an almost ideal conductor.

Crowbars suppress transients by brute force, (they have the

effect of dropping a metal crowbar across the system). The

main type of crowbar device is the gas tube surge arrester.

The original offering in the spark gap surge suppression family

was a carbon blocks. The carbon block suppressor used the

principle of a voltage arcing across an air gap. The air gap

breaks down at approximately 150V per thousands of an inch.

The minimum size gap was used to provide the lowest level of

protection without disturbing regular system operation. When a

Application Note 9310

transient overvoltage occurred in the system, the air gap in the

carbon block would ionize and break down. The breakdown of

the gap forms a very low impedance path to ground thus

diverting the surge away from the equipment. As soon as the

overvoltage condition was removed, the air gap is restored and

system operation is continued.

The disadvantage of carbon block spark gap technology was

that short duration pulses “pitted” the surface of the carbon

blocks, thus removing small pieces of the face material. This

material builds up after a number of surges, eventually

causing a permanently shortened gap resulting in the need

for protector replacement. This had a very adverse effect on

the maintenance and replacement costs of the protection

circuit. Another disadvantage of this technology was the

difﬁculty in exactly controlling the breakdown characteristics

over a wide variety of operating conditions.

In an effort to overcome the disadvantages of the carbon

block, a sealed spark gap was developed which uses an

inert gas in a sealed ceramic envelope. This technology is

known as a gas tube surge arrester. In a non conducting

mode the impedance of the gas is in the gigiohms region.

The gas is set to ionize at a predetermined voltage and

offers an extremely low impedance path to ground. Once the

overvoltage condition is removed the gas deionizes and the

circuit restores itself to its normal operating condition.

The gas tube arrester is an inherently bidirectional device and

is comprised of either two or three electrodes lying opposite

each other in the sealed chamber. When the voltage across the

arrester terminals exceeds a certain limit, known as the firing or

breakdown voltage, it triggers an electric arc. This arc limits the

voltages applied to the connected equipment. Gas tubes have

typical DC firing voltages between 150V and 1000V. They have

the smallest shunt resistance of all nonlinear transient

suppressors, typically in the milliohm range. Their capacitance

is low, between 1pF and 5pF, and they are commonly found in

high frequency transmission applications, such as telephone

systems. Another advantage of this technology is its ability to

handle large currents (up to 20kA).

In applications where there is a normal operating voltage, as

in the AC mains, there is a possibility that the gas tube will not

reset itself once it has fired and suppressed the transient. This

condition is known as follow on current and is defined by ANSl

“as the current that passes through a device from the

connected power source following the passage of discharge

current”. Follow on current will maintain conduction of the

ionized gas after the transient has disappeared and the

concern is that the follow on current may not clear itself at a

natural current zero. A gas tube specifically designed for AC

line operation should be used in this type of application.

the tightest clamping voltage of available devices. When a

voltage greater than the device breakdown is applied, the diode

will conduct in the reverse direction.

A peak pulse power rating is usually given on diode data

sheets. Common values are 600W and 1500W. This peak

pulse power is the product of the maximum peak pulse current,

, and the maximum clamping voltage, V

, at a current of

during a 10/1000µs transient duration. Use of peak power

ratings may be confusing when transients of other than

10/1000µs are to be considered. A maximum energy rating for

non-repetitive, short duration transients, similar to that supplied

with MOVs, may be of more benefit to design engineers.

The V-l characteristics are the best features of the avalanche

diode. Low voltage devices look extremely good. The

avalanche diodes has an excellent clamping voltage

capability, but only over a small range of current (1 decade).

The biggest disadvantage to using the avalanche diode as a

transient suppressor on an AC mains line is its low peak

current handling capability. Due to their being, at most, only

two P-N junctions in a device their is very little material

available for the dissipation of the peak power generated

during high energy pulses.

Metal Oxide Varistor (MOV)

[6]

A metal oxide varistor (MOV) is a nonlinear device which has

the property of maintaining a relatively small voltage change

across its terminals while a disproportionately large surge

current ﬂows through it. This nonlinear action allows the

MOV to divert the current of a surge when connected in

parallel across a line and hold the voltage to a value that

protects the equipment connected to that line. Since the

voltage across the MOV is held at some level higher than the

normal line voltage while surge current ﬂows, there is energy

deposited in the varistor during its surge diversion function.

The basic conduction mechanism of a MOV results from

semiconductor junctions (P-N junctions) at the boundaries of

the zinc oxide grains. A MOV is a multi junction device with

millions of grains acting as a series-parallel combination

between the electrical terminals. The voltage drop across a

single grain in nearly constant and is independent of grain size.

The material of a metal oxide varistor is primarily zinc oxide

with small additions of bismuth, cobalt, manganese and

other metal oxides. The structure of the body consists of a

matrix of conductive zinc oxide grains separated by grain

boundaries, which provide the P-N junction semiconductor

characteristics. When the MOV is exposed to surges, the

zinc oxide exhibits a “bulk action” characteristic permitting it

to conduct large amounts of current without damage. The

bulk action is easily explained by imagining this material to

be made up of an array of semiconducting P-N junctions

arranged electrically in series and parallel so that the surge

is shared among all of the grains. Because of the ﬁnite

resistance of the grains, they act as current limiting resistors

and, consequently current ﬂow is distributed throughout the

bulk of the material in a manner which reduces the current

concentration at each junction.

Silicon Avalanche Diodes

Although rarely used on AC mains application, due to their very

low transient surge capability, silicon avalanche diodes are an

excellent surge suppressor in low voltage DC applications.

Avalanche diodes are designed with a wider junction than a

standard zener diode. This wide junction gives them a greater

ability than a zener to dissipate energy. Avalanche diodes offer

10-44

Application Note 9310

The MOV has many advantages which make it ideal for use

as a suppressor on the low voltage AC power line. The bulk

nature of its construction gives it the required energy

handling capability to handle the secondary level transients

resulting from indirect lightning hits.

MOVs are both cost and size effective, are widely available

and do not have a signiﬁcant amount of overshoot. The

ﬂexibility available in the manufacturing of these devices

means that different size varistors are available for transient

suppression in all categories of the ANSl/lEEE C62.41

standard. They have no follow on current and their response

time is more than sufﬁcient for the types of transients

encountered in the AC mains environment.

A common misconception is that the device is irreversibly

damaged every time it has to suppress a transient. Under

high energy transient conditions in excess of the device

ratings, the V-I characteristics of the varistor are seen to

change. This change is reﬂected in a decrease in the

nominal varistor voltage. After applying a second or third

pulse the nominal varistor voltage can be seen to return to

its original value (Figure 4). To be conservative, peak pulse

limits have been established which, in many cases, have

been exceeded many fold without causing harm to the

device. Field studies and laboratory tests have shown that

the degradation which may result, after a number of pulses

outside the ratings of the device, is safe for the equipment

being protected. This does not mean that the established

limits should be ignored but rather viewed in the perspective

of the deﬁnition of a failed device. A “failed” device is deﬁned

by a

±10%

change in the nominal varistor voltage at the 1mA

point. This does not imply a non-protecting device, but rather

a device whose clamping voltage has been slightly altered.

TABLE 2. COMPARATIVE PERFORMANCE DATA [8]

PROTECTION

TECHNOLOGY

Metal Oxide

Varistor

DEVICE

PART

NUMBER

V130LA1

V130LA5

V130LA10A

Silicon Avalanche

Diode

Gas Tube Surge

Arrester

Filter Capacitor

1.5KE200C

CG2-230

C280A-

EA4K7

AVERAGE

PROTECTION

LEVEL (kV)

0.51

0.50

0.49

0.48

0.67

1.30

FAILS/

SAMPLE

SIZE

0/10

2/10

0/10

Device Selection

To select the correct varistor for a speciﬁc application,

determine the following information:

1. The maximum system RMS voltage.

2. How is the MOV to be connected?

3. The MOV with a voltage 10% - 25% above system

voltage.

4. The worst-case transient energy that will need to be

absorbed by the MOV. (Use the guidelines called out in

ANSI/IEEE C62.41 -1980).

5. The clamping voltage required for system protection (As

device size increases, for a given voltage family, the

clamping voltage gets better).

References

For Harris documents available on the web, see

http://www.semi.harris.com/

Harris AnswerFAX (407) 724-7800.

[1]

“ANSl/IEEE C62.41 -1980 Guide On Surge Voltages In

Low-Voltage AC Power Circuits”.

[2]

“AC Line Voltage Transients and Their Suppression”,

Corbett, M., Application Note AN9308, Harris

Suppression Products, AnswerFAX Doc. No. 99308.

VOLTS AT 1mA

[3]

“Understanding AC Line Transient Immunity”,

Dash, G.

and Straus, l.

NUMBER OF PULSES

[4]

“Suppression Of Voltage Transients Is An Art Trying To

Be A Science”,

GE # 660.33.

[5]

“Surge Protection of Electronics”,

Haskell, Jr., N.H., P.E.

[6]

“Transient Voltage Suppression Devices”,

Harris

Suppression Products DB450.

[7]

“The Propagation and Attenuation of Surge Voltages

and Currents in Low Voltage AC Circuits”,

Martzloff,

F.D., - 1984.

[8]

“Protective Level Comparison of Voltage Transient

Suppressors”,

Hopkins, D.C.

[9]

“Comparison of Transient Suppressors!”,

Korn, S.

FIGURE 4. REPETITIVE PULSE WITHSTAND CAPABILITIES

Device Comparisons

A range of standard varistors, avalanche diodes, gas tube

arresters and filter capacitors were evaluated under a 6kV,

0.5µs x 100kHz ring wave. This transient replicates that called

out in location Category A of the ANSl/lEEE C62.41 and is the

most benign condition expected in this location. All of the

selected devices are rated for use on a 120V

line. The

results obtained from this evaluation are per Table 2.

10-45

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