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Combining GDTs and MOVs for Surge Protection
of AC Power Lines
Application Note
July 1999
AN9732.1
[ /Title
(AN97
32)
/Sub-
ject
(Com-
bining
GDTs
and
MOVs
for
Surge
Protec-
tion of
AC
Power
Lines)
/Autho
r ()
/Key-
words
(Har-
ris
Corpo-
ration,
Sup-
pres-
sion
Prod-
ucts)
/Cre-
ator ()
/DOCI
NFO
pdf-
mark
AC power line disturbances are the cause of many
equipment failures. The damage can be as elusive as
occasional data crashes or as dramatic as the destruction of
a power supply, computer terminal, or television set. Power
line disturbances go by many names -- transients, surges,
spikes, glitches, etc. -- but regardless of the name, an
understanding of their characteristics and the operation of
the various protection devices available is necessary to
design an effective protection circuit. This Application Note
will illustrate how to design high-performance, cost-effective
surge protection for equipment connected to AC power lines.
The role of gas discharge tube (GDT) surge arresters
specifically designed for AC power line protection will also be
discussed.
The first step in providing an effective defense against power
line transients is to accurately characterize the transients.
One good reference is IEEE C62.41-1991 entitled “IEEE
Recommended Practice on Surge Voltages in Low-Voltage
AC Power Circuits” (formerly IEEE Standard 587). This
standard defines the open circuit voltage and short circuit
current waveforms which can be expected to occur on AC
power lines of 1000 Volts (RMS) or less. The standard
defines three levels of increasing transient activity, labeled
Location Categories A through C, dependent on the distance
of the equipment from the service entrance. Line cord-
connected equipment will usually be covered by Location
Category A or, occasionally, Location Category B. There are
two standard waveforms which define the types of transients
expected in these Location Categories:
• 0.5µs-100kHz Ring Wave (Figure 1A) — an oscillatory
waveform having a peak open circuit voltage of up to
6kV (Note 1), a risetime of 0.5ms, a ring frequency of
100kHz, and a “Q” of three. Though a short-circuit cur-
rent is not specified, peak currents of up to 0.5kA can
be expected (Note 1).
• 1.2/50µs-8/20µs Combination Wave (Figure 1B) — a
unidirectional impulse waveform having a peak open-
circuit voltage of up to 6kV (Note 1) with a rise time of
1.2µs and a duration of 50µs (Note 2) AND a peak
short-circuit current of up to 3kA (Note 1) with a rise
time of 8µs and a duration of 20µs (Note 3).
Test waveforms for evaluation of a surge protection system
should conform to these standard waveforms as closely as
possible to ensure valid results. The use of test waveforms
having slower rise times or lower peak currents/voltages
may result in a false sense of security concerning the level of
protection actually provided under field conditions.
1000
VOLTAGE (V)
0
0.5µs to 100kHz
0.5µs
10µs
100kHz
TIME
FIGURE 1A. IEEE-587 RING WAVE
3000
CURRENT (A)
50%
PEAK
0
8
20
TIME (µs)
FIGURE 1B. IEEE-587 COMBINATION
The second step in designing an effective surge protection
circuit is to choose which type(s) of surge protector to use.
Surge protection devices can be divided into two basic
types: Crowbar-type devices such as gas tube surge
arresters, spark gaps, and SCRs; and Clamp-type devices
such as avalanche diodes, transient absorption zener
diodes, and metal oxide varistors.
The clamp-type devices have faster response times but are
limited in their current handling ability because most of the
energy of the transient must be dissipated by the clamping
device. Also, the voltage drop across a clamp-type surge
protector increases with the conducted current as shown in
Figure 2A.
Crowbar-type devices such as gas tube surge arresters have
slightly slower response times but can handle much higher
current because they act as a low impedance switch which
diverts the transient energy away from the protected
equipment to be dissipated externally. While the peak
1-800-4-HARRIS or 407-727-9207
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Copyright
10-73
©
Harris Corporation 1999
Application Note 9732
voltage experienced by the protected circuit during the
leading edge of some transients may be higher than with a
clamp-type device; the duration, and thus the total energy
delivered to the protected circuit, is much lower when using
a crowbar-type device as shown in Figure 2B.
1000
METAL OXIDE VARISTOR
1000A
750
VOLTAGE (V)
traditional crowbar type devices because their designs were
optimized for the ability to turn off in the presence of a low-
current DC bias. While that is appropriate for protecting a
telecom line, additional components (such as a series resistor
or parallel-connected series-RC network) were required to
ensure that the gas tube surge arrester would extinguish
when placed across an AC power line with its relatively low
source impedance and the resultant follow-on currents (Note
4). These components invariably decreased the performance
of the protector while increasing its installed cost.
Gas tube surge arresters specifically designed for AC power
line applications provide the low impedance switching action
and high peak current capabilities of traditional gas tube
surge arresters while optimizing the ability to extinguish in
the presence of AC follow-on currents in excess of 300A. In
most applications, no additional components are required
other than those of the basic surge protection circuit. A
surge protection circuit with sub-nanosecond response time,
precise control of transient energy let-through, and a peak
current rating of 20,000A is now practical even for cost-
sensitive applications such as power supplies, home
stereos, monitors, and printers.
A typical installation is illustrated in Figure 3A (next page).
This is a two stage hybrid circuit consisting of a gas tube
surge arrester as the primary protector and a Metal Oxide
Varistor (MOV) as the secondary protector. These elements
must be separated by an isolating impedance. This
impedance may be either resistive (>10Ω) or inductive
(>0.1mH) to ensure proper coordination of the protective
devices. Most AC applications utilize an inductive element to
minimize power dissipation and voltage drop during normal
operation.
The inductor used in this example is part of the RFI filter
already required by the design. The output of the protection
circuit during a transient is illustrated in Figure 3B.
500
100A
25A
250
0.1A
0
0
5
10
TIME (µs)
15
20
25
FIGURE 2A. MOV CLAMPING VOLTAGE AT VARIOUS
CURRENT LEVELS
1000
GAS TUBE SURGE ARRESTER
750
VOLTAGE (V)
500
250
0
0
5
10
15
TIME (µs)
20
25
The following sequence of events is depicted in Figure 3B:
A. The leading edge of the transient is clamped by the MOV to
a value just above the normal operating voltage.
B. As the current through the MOV increases, a voltage is
developed across the inductor which causes the gas
tube surge arrester to fire. The energy of the transient is
now quickly shunted through the gas tube surge arrester
and away from the protected circuit.
C. The gas tube surge arrester remains in full conduction
for the duration of the transient.
D. When the transient has passed, the gas tube surge
arrester extinguishes—ready for the next transient.
This circuit uses each component to do what each does
best: the gas tube surge arrester diverts the high-energy
portion of the transient and the MOV provides the fast,
accurate clamping of the low energy leading edge.
FIGURE 2B. TRANSIENT RESPONSE OF PLASMA SURGE
ARRESTER
This peak voltage is a function of the rise time of the leading
edge of the transient. Faster rise times will result in higher
peak voltages due to the response time of the protector.
Although Zener-gated SCRs and thyristors are available
which offer faster response times, their use is limited to telecom
and signal line applications due to their relatively low peak
current ratings. A major benefit of the gas tube surge arrester is
that the voltage drop across the device remains essentially
constant (<20V) regardless of the conducted current.
The ideal surge protector would overcome the current
handling and energy diverting characteristics of the crowbar
type device with the speed of the clamp type device. This
approach has been difficult and expensive to realize with
10-74
Application Note 9732
HOT
LOAD
breakdown voltage is calculated by multiplying the normal
line voltage (120V
RMS
) by 1.414 to obtain the peak voltage
and then adding an appropriate guard band to allow for
normal variations in the supply voltage.
120V
RMS
×
1.414
=
170V
PEAK
170V
PEAK
×
(15% Guardband) = 196V Minimum
NEUTRAL
GROUND
PLASMA SURGE
ARRESTERS
RFI FILTER
METAL OXIDE
VARISTORS
The MOV should be selected using the same formula. When
used in a properly designed hybrid circuit, a 7mm or 10mm
device is normally adequate to handle the small leading-edge
currents until the gas tube surge arrester goes into
conduction.
The inductor should have a value of at least 0.1mH. If the
inductance is too low, the MOV may clamp the transient voltage
at a level that does not allow the gas tube surge arrester to go
into conduction. This would result in energy beyond the ratings
of the MOV. Tests have been conducted using several common
RFI filters having inductances or 1-2mH with excellent results.
Hybrid surge protection circuits incorporating HAC Series
gas tube surge arresters can provide cost-effective
protection against transients that exceed even the tough
guidelines of IEEE C62.41-1991 for Location Categories A
and B.
FIGURE 3A. HYBRID SURGE PROTECTION CIRCUIT
1000
750
VOLTAGE (V)
500
A
B
250
C
D
Notes
0
0
5
10
15
TIME (µs)
20
25
1. The exact value is a function of the Location Category and
System Exposure level. See the IEEE spec for more
detailed information.
2. The rise time for an open-circuit voltage waveform is
defined as 1.67 x (t
90
- t
30
) where t
30
and t
90
are the 30%
and 90% amplitude points on the leading edge of the
waveform. The duration is defined as the time from the
virtual origin, t
0
(where a line through t
30
and t
90
intersects the zero voltage axis), to the 50% amplitude
point of the trailing edge of the waveform, t
50
.
3. The rise time for an short-circuit current waveform is
defined as 1.25 x (t
90
- t
10
) where t
10
and t
90
are the 10%
and 90% amplitude points on the leading edge of the
waveform. The duration is defined as the time from the
virtual origin, t
0
(where a line through t
10
and t
90
intersects the zero current axis), to the 50% amplitude
point of the trailing edge of the waveform, t
50
.
4. If the current supplied by the AC power line exceeds the
maximum follow-on current of the gas tube surge arrester
(typically, ~20A), the device will continue to conduct, even
at a zero crossing of the AC voltage signal, causing the
gas tube surge arrester to overheat and fail.
FIGURE 3B. TRANSIENT RESPONSE OF HYBRID CIRCUIT
The cost effectiveness of this protection circuit is enhanced
by three factors:
1. The use of an AC line gas tube surge arrester eliminates
the need for additional components to ensure turnoff.
2. The isolating impedance is supplied by an existing
component (the RFI filter).
3. A small diameter MOV (LA or UltraMOV) is used since the
gas tube surge arrester handles the high-energy portion
of the transient.
The example duplicates the circuit between Neutral and
Ground in addition to the Hot-to-Neutral circuit. This provides
protection against Common Mode (both lines surged relative
to ground) as well as Normal Mode (Hot-to-Neutral)
transients. This is important because both types of
transients are frequent occurrences in the real world. The
failure to provide Common Mode protection is one of the
leading causes of failure in many otherwise solid designs.
The critical points in the selection of the gas tube surge
arrester are the minimum DC breakdown voltage (which
must be higher than the highest normal voltage expected on
the protected line) and the follow-on current rating (which
must be higher than the expected fault current of the
incoming supply line). In this example, the minimum DC
10-75

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