Application Note EC635:
Fuse
MOV rated
for 150V rms
continuous
voltage
Load
120V
120V
Designing with Thermally Protected
®
TMOV
Varistors in SPD and AC Line Applications
Introduction
Metal Oxide Varistors (MOVs) are commonly used to
suppress transients in many applications such as: Surge
Protection Devices (SPD), 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 overvoltage 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 overvoltage, 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.
(a)
Device
110-120V / 220-240V Split 240 110-120V
220-240V / 380-415V 3-wye 4 15 220-240V
254-277V / 440-480V 3-wye 480 254-277V
Rating Phase
110-120V Single 240 All
120V / 208V 3-wye 208 120V
220-240V Single 415 All
240V
254-277V Single 480 All
480V
347V Single 600 All
347V / 600V 3-wye 600 347V
Table 1. Test voltage Selection Table
Notes: (a)“Device” is defined as the end SPD product - example: UPS, SPD Strip etc.
(b) 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 1000V max.
©2011 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
High Leg
Delta
High Leg
Delta
Test
Voltage
240 120V
480 254-277V
Voltage Rating of
(b)
Conductor Pair
UL1449 Abnormal Overvoltage, Limited Current
Requirements
In AC line applications, the loss of a Neutral-Ground
connection may occur in such a way that there exists a
risk that a sustained over-voltage 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 SPD device to overheat resulting in
smoke, out-gassing 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.
This potential condition is specifically identified and
addressed in the UL1449 SPD Standard. See Table 1.
In many cases, it requires that end-product manufacturers
include a thermal protection element for an MOV.
Table 1. defines the test voltage that should be applied to
various SPD devices depending on the designer’s desired
device rating. Each test voltage is applied across each
conductor pair with a short circuit current of 10A for Type 1
& 2 SPD, and 5A, 2.5A, 0.5A and 0.125A for Type 3 SPD
respectively across each of five SPD devices. Since this
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Revision: May 31, 2011
Figure 1. Possible Fault Condition for a limited
current abnormal overvoltage event
Designing with Thermally Protected
Fuse
Line
TCO
TCO
MOV
MOV
MOV
Neutral
Ground
TCO
120VAC
TMOV® Varistors in SPD and AC Line Applications
test is destructive, five devices are needed to test for each
of the five short circuit currents. The five devices must be
energized for 7 hours, or until current or temperatures
within the SPD device attain equilibrium, or until the SPD
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 (L-G), 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.
Thermally Protecting MOVs
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 overcurrent 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 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
©2011 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
Figure 2. Typical offline protection scheme
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 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
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
M O V.
Figure 3. Typical Arrangement of TCOs with MOVs
** one of the MOVs has been removed for clarity
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 punchthrough 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.
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Revision: May 31, 2011
Designing with Thermally Protected
Fuse
Line
TMOV™
Varistor
TMOV
TMOV
Neutral
Ground
120VAC
Integrated
Thermal
Element
MOV Disk
0 10 20 30 40 50 60 70 80 90 100 110
0
50
100
150
200
250
300
350
400
450
Time (seconds)
Case Temperature (C)
MOV/TCO
TMOV Varistor
MOV
TMOV® Varistors in SPD and AC Line Applications
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,
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 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 function.
Figure 4. TMOV varistor offline protection scheme
UltraMOV varistors) in combination with TCOs of various
opening temperatures, Tf, were tested and compared
with several thermally self protected MOVs (Littelfuse
20mm, 130Vacrms, TMOV® varistor – TMOV20R130).
Both methods were subjected
to a sustained abnormal overvoltage 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
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.
TCO
Tf (°C)
73
94
121
Table 2a. MOV/TCO observed clearing
times for 5A limited current test
Tf (°C)
TMOV
Clearing Time (s)
Mean
30
34
36
Clearing Time (s)
Mean
13
Range
11-52
20-46
16-56
Range
2-25
varistor
Table 2b. TMOV varistor observed clearing
times for 5A limited current test
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 – V20E130 and TCO with Tf =
94°C), and 3) TMOV varistor (20mm, 130Vacrms –
TMOV20R130).
Comparing Methods of Thermally Protecting MOVs
The internally thermally protected TMOV varistor
overcomes most of 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,
©2011 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
Figure 5. Typical surface temperature vs. time for several protection schemes
Epoxy surface temperature vs. time was 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
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Revision: May 31, 2011
Designing with Thermally Protected
Wavesolder trials on TMOV varistor vs. TCO
142 deg TCO
14/20mm TMOV varistor
0
500
1000
1500 2000
TMOV® Varistors in SPD and AC Line Applications
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
Figure 6a. Standard MOV
seen, the new technology
eliminates much of the
charring when compared
with a standard MOV or
MOV/TCO combination.
Since the same size zinc
oxide disks are being used
for both MOV and TMOV,
the TMOV has the same
Figure 6b. MOV/TCO combination
surge perfomance as a
same size MOV, and
complies with IEC 60950-1,
Annex Q. The TMOV device
is a fail-safe product, so no
additional interruptor device
is needed in series to
protect, as required in
clause 1.5.9.2, IEC60950-1.
Figure 6c. TMOV varistor
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
Figure 7. Wave solder profile of TMOV varistor vs. TCO (Tf=142°C)
TCO fails (opens)
generally indicating that a TCO (even one with a high Tf
(142°C) cannot be wave soldered.
Generally, there will be a cost benefit associated with
eliminating the TCO which must be hand soldered
carefully to avoid opening the element.
Integrity of an Open Thermal Element
Once the thermal element of a TMOV varistor opens, it is
important that the element stay open and that a
reconnection not occur. The thermal element cleared
(opened) because the varistor disk heated due to thermal
run-away, and the thermal runaway began with a failed
(shorted) varistor.
©2011 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
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 to 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.
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Revision: May 31, 2011
Designing with Thermally Protected
Fuse
Line
TMOV™
Varistor
TMOV20R130M
Neutral
120VAC
D1
LED
Normally
On
R1
47k,0.5W
TMOV® Varistors in SPD and AC Line Applications
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.
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 is 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 design.
Figure 8. Indicator circuit using the iTMOV
varistor (LED normally on)
Conclusion
The UL1449 standard was created to protect the end
product and users from a loss of neutral situation where
an abnormal 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.
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 outgassing 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.
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 power.
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.
©2011 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
5
Revision: May 31, 2011
Designing with Thermally Protected
TMOV® Varistors in SPD and AC Line Applications
Note:
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 should be such
as to contain this possibility. Application testing is
strongly recommended.
References:
1. Surge Protective Devices - UL1449, April 19, 2010
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
5. Information Technology Equipment - Safety, IEC60950-1,
Amendment 1, December 2009
©2011 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
6
Revision: May 31, 2011
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