essful eld
performance. The expected transient environment is
addressed, along with various types of surge suppression
components are compared to MOVs.

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 retrot existing equipment with
transient suppressors. This is expensive in terms of eld
failures, customer downtime and potential loss of business.
In some systems retrotting 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 sufciently low voltage so that the capabilities of the
system being protected are not affected.

Problem Denition
(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,
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 uctuations and pulses
Short circuits or blown fuses
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.

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

[ /Title
(AN93
10)
/Sub-
ject
(Surge
Sup-
pres-
sion
Tech-
nolo-
gies
for AC
Mains
Com-
pared
(MOV
s,
SADs,
Gas
Tubes,
Filters
and
Trans-
form-
ers))
/Autho
r ()
/Key-
words
(Surge
Sup-
pres-
sion
Tech-
nolo-
gies
for AC
10-42

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.
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 reections 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.

Representative Transients

Table 1 reects 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
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.
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 1.2/50

µ

s voltage wave and 8/20

µ

s
current wave (Figure 3).
0.3
0.5
1
2
5
10
20
SURGE CREST (kV)
NUMBER OF SURGES PER
YEAR EXCEEDING
SURGE CREST OF ABSCISSA
10
3
10
2
10
1
1
10
-1
10
-2
MEDIUM
EXPOSURE
(SEE NOTE) SPARKOVER
OF CLEARANCES
LOW
EXPOSURE
HIGH
EXPOSURE
NOTE: In some locations, sparkover of clearances may limit the
overvoltages.
FIGURE 1. RATE OF SURGE OCCURRENCES vs VOLTAGE
LEVEL AT UNPROTECTED LOCATIONS

TABLE 1. SURGE VOLTAGES AND CURRENTS DEEMED TO
REPRESENT THE INDOOR ENVIRONMENT
LOCATION
CATEGORY
IMPULSE
WAVEFORM
MEDIUM
EXPOSURE

A
Long Branch Circuits
and Outlets
0.5

µ

s
100kHz
6kV
200A
B
Major Feeders and
Short Branch Circuits
1.2/50

µ

s
8/20

µ

s
6kV
3kA
0.5

µ

s
100kHz
6kV
500A

FIGURE 3A. OPEN-CIRCUIT WAVEFORM
0.9 V
PEAK
V
PEAK
0.1 V
PEAK
0.5
µs
T = 10
µs (f = 100kHz)
60% OF V
PEAK
FIGURE 2. 0.5
µs - 100kHz RING WAVE (OPEN CIRCUIT VOLTAGE)
0.9V
PEAK
V
PEAK
0.3V
PEAK
50
µs
0.5V
PEAK
t
1
x 1.67 = 1.2
µs
t
1
V

Application Note 9310
10-43

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