Designing with thermally protected TMOV
Varistors in TVSS Applications
Metal Oxide Varistors (MOVs) are
commonly used to suppress transients in
many applications such as: Transient Voltage
Surge Suppressors (TVSS), Uninterruptible
Power Supplies (UPS), AC Power Taps, AC
Power Meters or other products. Lightning,
inductive load switching, or capacitor bank
switching, are often the sources of these
over-voltage transients. Under normal operating conditions, the AC line voltage applied
to an MOV is not expected to exceed the
MOV’s Maximum ACRMS Voltage Rating or
Maximum Continuous Operating Voltage
(MCOV). Occasionally, over-voltage transients may occur that exceeds these limits.
These transients are clamped to a suitable
voltage level by the MOV provided the transient energy does not exceed the MOV’s
maximum rating.
MOVs can also be subjected to continuous
abnormal voltage conditions rather than
short duration transients. If an MOV is
subjected to a sustained abnormal over-voltage, limited current condition (as is required
in UL1449), the MOV may go into thermal
runaway resulting in overheating, smoke, and
potentially fire. For end products to comply
with UL1449, some level of protection must
be afforded to the MOV to prevent this failure mode. That protection has traditionally
been a thermal fuse or Thermal Cut-Off
(TCO) device.
UL1449 Abnormal
Overvoltage, Limited
Current Requirements
In AC line applications, the loss of a NeutralGround connection may occur in such a way
that there exists a risk that a sustained overvoltage may be applied to an MOV that is
rated for a much lower continuous voltage.
In an unlimited current condition the MOV
will first fail to a low impedance (few Ohms),
but due to the high amount of energy available, it most often ruptures instantaneously.
If, however, there are loads tied to the AC
line that limit current flow, the MOV can
overheat and potentially cause the TVSS
device to overheat resulting in smoke, outgassing and eventually fire.
For example, in a standard U.S. 120V AC
Line application, two 120V AC power lines
(180° out of phase) are commonly fed from
a center-tapped 240V transformer. See
Figure 1. Let’s assume a 150V rated MOV is
present in the top 120V circuit, and some
load exists on the bottom 120V circuit. Both
the MOV and load share the center tap
which is the Neutral-Ground Connection. If
a break occurs on the center tap (X—X),
then the load in the bottom phase acts as a
current limiter and the line fuse may not
clear. In this scenario, the 150V rated MOV
is subjected to 240V at a limited current
potentially resulting in thermal run away for
the MOV.
MOV rated for 150V rms
continuous voltage
Figure 1. Possible Fault Condition for a limited current
abnormal overvoltage event
requires that end-product manufacturers
include a thermal protection element for an
Table 1. defines the test voltage that should
be applied to various TVSS devices depending on the designer’s desired device1 rating.
Each test voltage is applied across each
conductor pair with a short circuit current of
5A, 2.5A, 0.5A and 0.125A respectively
across each of four TVSS devices. Since this
test is destructive, four devices are needed
to test for each of the four short circuit
currents. The four devices must be energized for 7 hours, or until current or
temperatures within the TVSS device attain
equilibrium, or until the TVSS becomes
disconnected from the AC Line.
For example shown in Figure 1, in a standard
120V AC Line application, the requirement is
for a 240VACRMS test voltage to be applied
across all conductor pairs. There are three
pairs; Line-Neutral (L-N), Line-Ground (LG), and Neutral-Ground (N-G). Again, this
test voltage is chosen because in the U.S.,
120V AC power is commonly fed from a
center-tapped 240V transformer. Thermally
unprotected MOVs for this application are
typically rated from 130Vacrms to
150Vacrms and will heat up, out-gas and may
catch fire in such circumstances.
This potential condition is specifically identified and addressed in the UL1449 TVSS
Standard. See Table 1. In many cases, it
Device Rating
High Leg Delta
High Leg Delta
Test Voltage(a)
Voltage Rating of
Conductor Pair
Table 1. Test voltage Selection Table
(a.) For device ratings not specified in this table, the test voltage shall be the maximum phase voltage (if available) or twice
the conductor pair voltage ratings up to 600V max.
Thermally Protecting
A simple block diagram of a typical line voltage transient protection scheme used to
meet the sustained abnormal over-voltage,
limited current test requirements of UL1449
is shown in Figure 2. An MOV or several
MOVs in parallel are each placed across each
of the three conductive pairs; L-N, L-G, and
N-G. This offers the utmost protection for
any possible line transient. A standard fuse is
placed in series with the line to protect the
system from an over-current condition that
exceeds a predetermined level. Typically, the
current rating of this fuse is higher than the
limited current flowing through the circuit
during UL1449 testing. This requires the
addition of a TCO that is placed in series
with each MOV or Parallel combination of
heat transfer, the MOV and TCO are not in
full contact in most cases.The position of the
terminal leads of the TCO makes it difficult
for the TCO to be located closely enough to
the MOV for effective heat transfer. The
result is less than efficient conduction from
Figure 3. Typical Arrangement of TCOs with MOVs
** one of the MOVs has been removed for clarity
while eliminating most of the problems associated with other methods. This technology
is a fully integrated, thermally self-protected
MOV - TMOV varistor Series. This new
device uses a patent pending thermal
element internal to the MOV so that it is in
direct contact with the metal oxide disk,
allowing for optimum heat transfer. Because
of the proximity of the thermal element to
the MOV body, a higher opening temperature element can be used. This allows the
thermally self-protected MOV to be wave
soldered simplifying the assembly process.
The construction method also allows the
new device to perform to standard MOV
Integrated Thermal
case to case. An example of a typical
arrangement of MOVs and TCOs is shown
in Figure 3. Note the TCO does not touch
the case of the MOV.
MOV Disk
Figure 2. Typical offline protection scheme
MOVs to protect it from a thermal event.
Often, the MOVs used are of the radial
leaded 14mm or 20mm disk diameter variety.
TCOs are available in a variety of different
opening temperatures. The position and
orientation of the TCO is important if it is to
be effective in thermally protecting an MOV.
When subjected to a sustained over-voltage,
MOVs will short at a random point on the
disk and will rapidly begin to self-heat if a
limited current is maintained.TCOs are activated by a combination of conducted,
converted and radiated heat from the MOV,
although the majority of the heat is transferred via conduction. The position of the
TCO in relation to the heat source at this
shorting point has a considerable effect on
the speed of operation of the TCO. The
most effective heat coupling has been
observed to be via conduction through the
varistor terminal lead to the insulated terminal of a metal jacket TCO. Thermal
convection and radiation processes are
effective when the heat source is immediately beside or below the TCO. Although
conduction is the most effective means of
The response time of this arrangement can
be disproportionately increased if the TCO
is not placed in close enough proximity to
the MOV and/or the punch-through point
on the MOV occurs remotely from the
TCO’s insulated terminal. In such cases,
considerable charring of the MOV can occur
and fire is a real possibility. Shrink-wrap or
other bonding materials can aid coupling, but
in adverse circumstances they are a source
of combustible material and may actually
make things worse.
While this scheme is generally effective in
removing the MOV from the circuit during
abnormal over-voltage testing such that the
MOV does not reach critical temperatures,
the downside to this method is that TCOs
can be difficult to handle during the assembly process. Because of the low opening
temperatures,TCOs must be soldered carefully. When hand soldering, the iron cannot
remain in contact with the lead of the TCO
for prolonged periods. Another option is to
use clips or pliers as a heat-sink.. TCOs with
useful opening temperatures for the MOVs
typically cannot be wave soldered, as the
device will clear in the solder bath. In
general, the use of TCOs in these types of
applications becomes largely a hand assembly process.
A new technology has been developed that
will aid the designer in meeting UL1449
requirements including the sustained abnormal over-voltage limited current testing,
Figure 4. TMOV varistor offline protection scheme
ratings with regards to peak current, peak
energy, voltage clamp levels, etc. while
providing the safety of a thermally protected
device. Figure 4. Illustrates the integrated
Comparing Methods of
Thermally Protecting
The internally thermally protected TMOV
varistor overcomes most the disadvantages
of the MOV/TCO combination method.
Placing the thermal element inside the epoxy
coating and close to the center of the disk
provides several benefits. 1) It optimizes
heat transfer between the MOV disk and the
thermal element by placing the thermal
element as close to the point of failure as
possible. This greatly improves clearing
(opening) times. 2) Allows for the thermal
element to have a higher opening temperature than most TCOs used while being
protected from external heat sources. This
allows the device to be wave soldered. See
Section 6.
In order to compare the clearing times of
both methods, several standard MOVs
(Littelfuse 20mm, 130Vacrms, UltraMOV
varistors) in combination with TCOs of various opening temperatures, Tf, were tested
and compared with several thermally self
protected MOVs (Littelfuse 20mm,
Both methods were
subjected to a sustained abnormal over-voltage of 240V at 5A. As can be seen in Table
2a and as expected, the TCOs with higher Tf
take longer to clear. The 73ºC TCO proved
difficult to hand solder without clearing the
Clearing Time (s)
Tf (°C)
Table 2a. MOV/TCO observed clearing times for 5A
limited current test
captured for each method. As can be seen,
the case temperature of a standard MOV
rated for 130VRMS will continue to rise (to
the point of combustion) if no thermal
protection is used. The MOV/TCO combo
performs better reaching temperatures of
220°C before the TCO clears. The internally
protected MOV has a faster response time,
clearing at temperatures of around 150°C in
less than 20 seconds. Note that the temperature continues to rise once the thermal
fuses have cleared. Heat generated within
the zinc oxide disk is at a higher temperature
than the outer epoxy coating. Heat continues to flow outward to the epoxy for some
time before finally cooling down.
Figures 6a – 6c illustrate the effects of the
temperature rise on each MOV. As can be
seen, the new technology eliminates much of
the charring when compared with a standard MOV or MOV/TCO combination.
Wave Soldering the
TMOV Varistor
Figure 7 shows a suitable wave solder profile
that can be used for the TMOV varistor. The
profile temperatures are very typical to
those found in general wave solder methods.
In contrast, the solder profile shown for the
TCO shows temperatures much lower than
those found in a typical solder bath. In fact,
the profile shown for the TCO actually
depicts a profile at which the TCO fails
(opens) generally indicating that a TCO
(even one with a high Tf (142°C) cannot be
wave soldered.
Generally, here will be a cost benefit associated with eliminating the TCO which must
be hand soldered carefully to avoid opening
the element.
Wavesolder trials on TMOV varistor vs. TCO
142 deg TCO
Clearing Time (s)
Tf (°C)
14/20mm TMOV varistor
Figure 6a. Standard MOV
Case Temperature (°C)
TMOV Varistor
Integrity of an Open
Thermal Element
Figure 6b. MOV/TCO combination
Once the thermal element of a TMOV varistor opens, it is important that the element
stays open and that a reconnection does not
occur. Remember, in order for the thermal
element to have cleared (opened), the varistor disk itself must have heated up due to
thermal run-away and the thermal runaway
began with a failed (shorted) varistor. It is,
therefore, undesirable to have a failed varistor placed back into the circuit, electrically
Figure 5. shows the effects of applying a
UL1449 abnormal over-voltage test
(240VRMS, 5A) on three devices or combination of devices - 1) MOV alone (20mm,
130Vacrms – V20E130) 2) MOV/TCO
combination (20mm, 130Vacrms MOV –
Figure 7. Wave solder profile of TMOV varistor vs. TCO
Table 2b. TMOV varistor observed clearing times for
5A limited current test
device despite the use of an appropriate
heat-sink. Table 2b shows the clearing times
for the internally protected MOV. Clearly,
the times are shorter than for any of the
MOV/TCO combinations tested.
Time (seconds)
Figure 5. Typical surface temperature vs. Time for several
protection schemes
V20E130 and TCO with Tf = 94°C), and 3)
TMOV varistor (20mm, 130Vacrms –
Epoxy surface temperature vs. time was
Figure 6c. TMOV varistor
In order to ascertain the integrity of an open
thermal element within a TMOV varistor,
devices were first subjected to an abnormal
over-voltage limited current event causing
the thermal element to clear. These devices
were then subjected to two tests. First, the
devices were subjected 6kV, 3kA 8x20µsec
pulses. The TMOV varistors were then
subjected to bias voltage and monitored for
leakage currents indicating a full or partial
reconnection. None were noted. Next,
1000Vrms was applied for several hours,
again with no connection as verified by the
leakage test.
Indication of an Open
Thermal Element:
iTMOV Varistor
The benefits of the TMOV varistor have
been thoroughly discussed, but one question
remains: How do I know when the thermal
element has cleared? By design, MOVs
exhibit a very high impedance when
subjected to voltages below its MCOV
(Maximum Continuous Operating Voltage).
So, once installed into an end product, how
do you know if the TMOV varistor is still
operational? Enter the iTMOV varistor.
The iTMOV varistor adds an additional
third indicator lead that provides access to
the connection between the thermal
element and the MOV electrode. Having
access to this point of the circuit makes indication of the thermal element a simple
procedure. Figures 8 shows a simple application circuit with indication.
LED Normally On
Figure 8. Indicator circuit using the iTMOV varistor (LED
normally on)
In Figure 8, an iTMOV varistor is used to
protect the L-N connection of a typical U.S.
120Vac line. An AC rated LED in placed
across the iTMOV varistor’s indicator lead
and the Neutral line. A series resistor, R1, is
added to limit current through the LED. A
47kΩ, 0.5W resistor is shown, but the
designer should review the LED’s ratings to
choose the correct value. A series diode,
D1, may be needed if the reverse voltage
rating of the LED is insufficient to handle
negative voltages from the AC line.
Additionally, a Littelfuse 3AG, 10A (313010)
fuse is shown to protect against excessive
over-current into the load, but the designer
should choose a value specific to his/her
Under normal conditions, the LED is forward
biased from the line voltage through the
thermal element to Neutral. If the thermal
element opens current will be interrupted
and the LED will go off. The LED will also go
off if the Line Fuse opens indicating a loss of
When paralleling TMOV varistors, the
iTMOV varistor can be used for several or
all parallel devices. That is, one may wish to
indicate when a certain percentage or
TMOV varistors fail. Generally, once some
of the MOVs in parallel begin to fail, all begin
to fail.
The UL1449 standard includes a requirement that was created to protect the end
product and users from a loss of neutral situation
over-voltage/limited current condition could
be applied to Metal Oxide Varistors. This
event would cause an MOV to have a
sustained voltage applied in excess of its
maximum working voltage, which in turn
would cause the MOV to enter a thermal
runaway condition.
Several methods exist to prevent the MOV
from reaching combustible temperatures the most common of which is to use TCOs.
While TCOs perform adequately in limiting
MOVs from reaching very high temperatures, there are limitations. Out-gassing and
some charring are evident when the test is
applied. Additionally, the assembly process is
difficult to automate, as wave soldering is
typically not an option.
1. Transient Voltage Surge Suppressors UL1449, June 25, 1998
2. Littelfuse Datasheet, Thermally Protected
Metal Oxide Varistor (TMOV Varistor),
March 2001
3. Littelfuse Datasheet, High Surge Current
Radial Lead Metal Oxide Varistor
(UltraMOV Varistor Series), March 2001
4. Paul Traynham and Pat Bellew, Using
Thermally Protected MOVS in TVSS or
Power Supply Applications, Power
Systems World, Intertec Exhibition
Proceedings, September 2001
1 In Table 1., “device” is defined as the end
TVSS product - example: UPS,TVSS Strip
Overall, the new integrated TMOV varistor
thermal fuse technology reduces part count,
saves space and is UL1449 recognized. It
performs better than other methods of
protection when subjected to a limited
current over-voltage condition, by clearing
more quickly at a lower temperature to
reduce the potential for out-gassing or charring. It has all the performance capability of
a standard MOV, including peak pulse
current capability, energy rating and clamping
voltage. The new device can also be wave
soldered which saves on assembly costs and
simplifies the assembly process by eliminating
most of the hand assembly required with
other methods.
All data was taken with a limited sample size.
Results may vary due to normal variations in
electrical and mechanical parameters.
Designers are encouraged to evaluate their end
design with a large enough sample size to
ensure consistent results. In some instances
TMOV varistors may exhibit substantial heating
and venting prior to opening. Module design
Specifications descriptions and illustrative material in this literature are as accurate as known at time of publication, but are
subject to change without notice. Please visit for the most up-to-date product and application literature.
Copyright © 2001 Littelfuse, Inc., All Rights Reserved
should be such as to contain this possibility.
Application testing is strongly recommended.
Littelfuse, Inc.
800 E. Northwest Highway
Des Plaines, IL 60016