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AC Line Voltage Transients and
Their Suppression
Application Note
January 1998
AN9308.2
[ /Title
(AN93
08)
/Sub-
ject
(AC
Line
Volt-
age
Tran-
sients
and
Their
Sup-
pres-
sion)
/Autho
r ()
/Key-
words
(TVS,
Tran-
sient
Sup-
pres-
sion,
Protec-
tion,
ESD,
IEC,
EMC,
Elec-
tro-
The increasing usage of sensitive solid state devices in
modem electrical systems, particularly computers,
communications systems and military equipment, has given
rise to concerns about system reliability. These concerns
stem from the fact that the solid state devices are very
susceptible to stray electrical transients which may be
present in the distribution system.
The initial use of semiconductor devices resulted in a
number of unexplained failures. Investigation into these
failures revealed that they were caused by transients, which
were present In many different forms in the system.
Transients in an electrical circuit result from tile sudden
release of previously stored energy. The severity of, and
hence the damage caused by transients depends on their
frequency of occurrence, the peak transient currents and
voltages present and their waveshapes.
In order to adequately protect sensitive electrical systems,
thereby assuring reliable operation, transient voltage
suppression must be part of the initial design process and
not simply included as an afterthought. To ensure effective
transient suppression, the device chosen must have the
capability to dissipate the impulse energy of the transient at
a sufficiently low voltage so that the capabilities of the circuit
being protected are not affected. The most successful type
of suppression device used is the metal oxide varistor. Other
devices which are also used are the zener diode and the
gas-tube arrestor.
NUMBER OF SURGES PER YEAR EXCEEDING
SURGE CREST OF ABSCISSA
Introduction
1.
10
3
HIGH
EXPOSURE
10
2
MEDIUM
EXPOSURE
10
1
1
(SEE NOTE) SPARKOVER
OF CLEARANCES
10
-1
LOW
EXPOSURE
10
-2
0.3
0.5
1
2
5
SURGE CREST (kV)
10
20
NOTE: In some locations, sparkover of clearances may limit the
overvoltages.
FIGURE 1. RATE OF SURGE OCCURRENCES vs VOLTAGE
LEVEL AT UNPROTECTED LOCATIONS
The low exposure portion of the graph Is derived from data
collected in geographical areas known for low lightning
activity, with little load switching activity. Medium exposure
systems are geographical areas known for high lightning
activity, with frequent and severe switching transients. High
exposure areas are rare, but real systems, supplied by long
overhead lines and subject to reflections at line ends, where
the characteristics of the installation produce high sparkover
levels of the clearances.
Investigations into the two most common exposure levels,
low and medium, have shown that the majority of surges
occurring here can be represented by typical waveform
shapes (per ANSI/IEEE C62.41-1980). The majority of
surges which occur in indoor low voltage power systems can
be modeled to an oscillatory waveform (see Figure 2). A
surge impinging on the system excites the natural resonant
frequencies of the conductor system. As a result, not only
are the surges oscillatory but surges may have different
amplitudes and waveshapes at different locations in the
system. These oscillatory frequencies range from 5kHz to
500kHz with 100kHz being a realistic choice.
The Transient Environment
The occurrence rate of surges varies over wide limits,
depending on the particular power system. These transients
are difficult to deal with, due to their random occurrences
and the problems in defining their amplitude, duration and
energy content. Data collected from many independent
sources have led to the data shown in Figure 1. This
prediction shows with certainty only a relative frequency of
occurrence, while the absolute number of occurrences can
be described only in terms of low, medium or high exposure.
This data was taken from unprotected circuits with no surge
suppression devices.
10-36
1-800-4-HARRIS or 407-727-9207
|
Copyright
©
Harris Corporation 1998
Application Note 9308
V
PEAK
0.9 V
PEAK
Transient Energy and Source Impedance
Some transients may be intentionally created in the circuit
due to inductive load switching, commutation voltage spikes,
etc. These transients are easy to suppress since their
energy content is known. It is the transients which are
generated external to the circuit and coupled into it which
cause problems. These can be caused by the discharge of
electromagnetic energy, e.g., lightning or electrostatic
discharge. These transients are more difficult to identify,
measure and suppress. Regardless of their source,
transients have one thing in common - they are destructive.
The destruction potential of transients is defined by their
peak voltage, the follow-on current and the time duration of
the current flow, that is:
τ
T = 10µs (f = 100kHz)
0.1 V
PEAK
0.5µs
60% OF V
PEAK
FIGURE 2. 0.5µs - 100kHz RING WAVE (OPEN CIRCUIT
VOLTAGE)
E
=
∫
Vc(t)
•
I(t) dt
0
In outdoor situations the surge waveforms recorded have
been categorized by virtue of the energy content associated
with them. These waveshapes involve greater energy than
those associated with the indoor environment. These
waveforms were found to be unidirectional in nature (see
Figure 3).
where:
E
I
= Transient energy
= Peak transient current
V
C
= Resulting clamping voltage
t
= Time
= Impulse duration of the transient
τ
V
0.9 V
PEAK
V
PEAK
0.5 V
PEAK
0.3 V
PEAK
It should be noted that considering the very small
possibilities of a direct lightning hit it may be deemed
economically unfeasible to protect against such transients.
However, to protect against transients generated by line
switching, ESD, EMP and other such causes is essential,
and if ignored will lead to expensive component and/or
system losses.
The energy contained in a transient will be divided between
the transient suppressor and the line upon which it is
travelling in a way which is determined by their two
impedances. It is essential to make a realistic assumption
of the transient's source impedance in order to ensure that
the device selected for protection has adequate surge
handling capability. In a gas-tube arrestor, the low
impedance of the arc after sparkover forces most of the
energy to be dissipated elsewhere - for instance in a
power-follow current-limiting resistor that has to be added
in series with the gap. This is one of tile disadvantages of
the gas-tube arrestor. A voltage clamping suppressor (e.g.,
a metal oxide varistor) must be capable of absorbing a
large amount of transient surge energy. Its clamping action
does not involve the power-follow energy resulting from the
short-circuit action of the gap.
The degree to which source impedance is important
depends largely on the type of suppressor used. The surge
suppressor must be able to handle the current passed
through them by the surge source. An assumption of too
high an impedance (when testing the suppressor) may not
T
1
50µs
T
1
x 1.67 = 1.2µs
FIGURE 3A. OPEN-CIRCUIT WAVEFORM
I
0.9 I
PEAK
I
PEAK
0.5 I
PEAK
0.1 I
PEAK
T
2
20µs
T
2
x 1.25 = 8µs
FIGURE 3B. DISCHARGE CURRENT WAVEFORM
FIGURE 3. UNIDIRECTIONAL WAVESHAPES (OUTDOOR
LOCATIONS)
10-37
Application Note 9308
subject it to sufficient stresses, while the assumption of too
low an impedance may subject it to unrealistically large
stress; there is a trade off between the size/cost of the
suppressor and the amount of protection required.
In a building, the source impedance and the load impedance
increases from the outside to locations well within the inside
of the building, i.e., as one gets further from the service
entrance, the impedance increases. Since the wire in a
structure does not provide much attention, the open circuit
voltages show little variation. Figure 4 illustrates the
application of three categories to the wiring of a power
system.
These three categories represent the majority of locations
from the electrical service entrance to the most remote wall
outlet. Table 1 is intended as an aid in the selection of surge
suppressors devices, since it is very difficult to select a
specific value of source impedance.
Category A covers outlets and long branch circuits over 30
feet from category B and those over 60 feet from category C.
Category B is for major feeders and short branch circuits
from the electrical entrance. Examples at this location are
bus and feeder systems in industrial plants, distribution
panel devices, and lightning systems in commercial
buildings. Category C applies to outdoor locations and the
electrical service entrance. It covers the service drop from
pole to building entrance, the run between meter and the
distribution panel, the overhead line to detached buildings
and underground lines to well pumps.
TABLE 1. SURGE VOLTAGES AND CURRENTS DEEMED TO REPRESENT THE INDOOR ENVIRONMENT AND RECOMMENDED FOR
USE IN DESIGNING PROTECTIVE SYSTEMS
ENERGY (JOULES)
DEPOSITED IN A
SUPPRESSOR WITH
CLAMPING VOLTAGE
TYPE OF SPECIMEN
OR LOAD CIRCUIT
CIRCUIT
IMPULSE
COMPARABLE
TO IEC
664 CATEGORY
MEDIUM
EXPOSURE
AMPLITUDE
LOCATION CATEGORY
CENTER
WAVEFORM
500V
(120V Sys.)
-
0.8
-
40
-
2
1000V
(240V Sys.)
-
1.6
-
80
-
4
A.
Long branch circuits and
outlets
II
0.5µs - 100kHz
6kV
200A
High Impedance (Note 1)
Low Impedance (Note 2)
High Impedance (Note 1)
Low Impedance (Note 2)
High Impedance (Note 1)
Low Impedance
B.
Major feeders short
branch circuits, and load
center
III
1.2/50µs
8/20µs
0.5µs - 100kHz
6kV
3kA
6kV
500A
NOTES:
1. For high-impedance test specimens or load circuits, the voltage shown represents the surge voltage. In making simulation tests, use that value
for the open-circuit voltage of the test generator.
2. For low-impedance test specimens or load circuits, the current shown represents the discharge current of the surge (not the short-circuit current
of the power system). In making simulation tests, use that current for the short-circuit current of the test generator.
3. Other suppressors which have different clamping voltages would receive different energy levels.
10-38
Application Note 9308
SERVICE
ENTRANCE
METER
SERVICE
ENTRANCE
METER
OUTBUILDING
UNDERGROUND SERVICE
SERVICE
ENTRANCE
OUTBUILDING
TRANSFORMER
METER
UNDERGROUND SERVICE
FIGURE 4. LOCATION CATEGORIES
A
Outlets and long branch circuits.
All outlets at more than 10m (30ft.) from
Category B.
All outlets at more than 20m (60ft.) from
Category C.
B
Feeders and short branch circuits.
Distribution Panel Devices Bus and feeder in
industrial plants.
Heavy appliance outlets with “short”
connections to service entrance.
Lighting systems in large buildings.
C
Outside and service entrance
Service drop from pole to building.
Run between meter and panel.
Overhead line to detached building.
Underground line to well pump.
Transient Suppression
The best type of transient suppressor to use depends on the
intended application, bearing in mind that in some cases
both primary and secondary protection may be required. It is
the function of tile transient suppressor to, in one way or
another, limit the maximum instantaneous voltage that can
develop across the protected load. The choice depends on
several factors, but the decision is ultimately a trade-off
between the cost of the suppressor and the amount of
protection needed.
The time required for a transient suppressor to begin
functioning is extremely important when it is used to protect
sensitive components. If the suppressor is slow acting and a
fast-rise transient spike appears on the system the voltage
across the protected load can rise to damaging levels before
suppression begins. On AC power lines the best type of
suppression to use is a metal oxide varistor. Other devices
occasionally used are the zener diode and the gas-tube
arrestor.
Gas-Tube Arresters
This is a suppression device which finds most of its
applications in telecommunication systems. It is made of two
metallic conductors usually separated by 10mils to 15mils
encapsulated in a glass envelope. This glass envelope is
pressurized and contains a number of different gases. Types
specifically designed for AC line operation are available and
offer high surge current ratings.
Zener Diodes
One type of clamp-action device used in transient
suppression is the zener diode. When a voltage of sufficient
amplitude is applied in the reverse direction, the zener diode
is said to break down, and will conduct current in this
direction. This phenomenon is called avalanche. The voltage
appearing across the diode is therefore called the reverse
avalanche or zener voltage.
When a transient propagates along the line with a voltage
exceeding the reverse-based voltage rating of the diode, the
diode will conduct and the transient will be clamped at the
10-39
Application Note 9308
zener voltage. This clamping voltage is lower than that of an
equivalent varistor. Some manufacturers have claimed that
the response time of a zener diode is 1ps to 2ps. In practice,
the speed of response is greatly determined by the parasitic
inductance of the package and the manner in which the
device is connected via its leads. Although zener diodes can
provide transient protection, they cannot survive significant
instantaneous power surges. Larger diodes can be used to
increase the power rating, but this is only at the expense of
increased costs. Also, the maximum tolerable surge current
for a zener diode in reverse breakdown is small when
compared to tolerable surge currents for varistors. Due to
the fact that there is only the P-N junction in a zener diode, it
will need to have some additional heat sinking in order to
facilitate the rapid buildup of heat which occurs in the
junction after it has encountered a transient.
Summary
When designing circuits of the complex nature seen in
today’s electrical environment, the initial design must
incorporate some form of transient voltage surge
suppression. The expense of incorporating a surge
protection device in a system is very low when compared
with the cost of equipment downtime, maintenance and lost
productivity which may result as a consequence of not
having protection. When selecting surge suppressors for
retrofit to an existing design, one important point to
remember is that the location of the load to be protected
relative to the service entrance is as important as the
transient entrance which may be present in an overvoltage
situation.
References
For Harris documents available on the internet, see web site
http://www.semi.harris.com/
Harris AnswerFAX (407) 724-7800.
[1]
An American National Standard/IEEE Guide for Surge
Voltages in Low Voltage AC Power Circuits,
C62.41-
1980.
[2] Harris Suppression Products,
Transient Voltage
Suppression Devices,
DB450.
[3] Korn, Sebald,
Voltage Transients and Power Conversion
Equipment,
GE.
Metal Oxide Varistor
As the name suggests, metal oxide varistors (MOV) are
variable resistors. Unlike a potentiometer, which is manually
adjusted, the resistance of a varistor varies automatically in
response to changes in voltages appearing across it.
Varistors are a monolithic device consisting of many grains
of zinc oxide, mixed with other materials, and compressed
into a single form. The boundaries between individual grains
can be equated to P-N junctions with the entire mass
represented as a series-parallel diode network.
When a MOV is biased, some grains are forward biased and
some are reverse biased. As the voltage is increased, a
growing number of the reversed biased grains exhibit
reverse avalanche and begin to conduct. Through careful
control in manufacturing, most of the nonconducting P-N
junctions can be made to avalanche at the same voltage.
MOVs respond to changes in voltages almost
instantaneously. The actual reaction time of a given MOV
depends on physical characteristics of the MOV and the
wave shape of the current pulse driven through it by the
voltage spike. Experimental work has shown the response
time to be in the 500 picosecond range.
One misconception about varistors is that they are slow to
respond to rapid rise transients. This “slow” response is due
to parasitic inductance in the package and leads when the
varistor is not connected with minimal lead length. If due
consideration is given to these effects in its installation, the
MOV will be more than capable of suppressing any voltage
transients found in the low voltage AC power system.
The MOV has many advantages over the zener diode, the
greatest of which is its ability to handle transients of much
larger energy content. Because it consists of many
P-N junctions, power is dissipated throughout its entire bulk,
and unlike the zener, no single hot spot will develop. Another
advantage of the MOV is its ability to survive much higher
instantaneous power.
10-40

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