Ferraz Shawmut
TPMOV Principles and Operation
®
Application Note
TPMOV – Origin
Due to recent changes in the Underwriters Laboratories
Standard for Safety for Surge Protective Devices – UL
1449 – additional requirements have been instituted to
maintain listings of Surge Protective Devices (SPDs)
with which some SPD designs and components could
not comply. A number of new designs for both SPDs
and SPD components have been released to overcome
the additional objectives presented by the updated UL
1449 standard – ANSI/UL 1449-2006 (commonly
referred to as “Third Edition”). One such SPD
component is the TPMOV (thermally protected metal
oxide varistor) provided by Ferraz Shawmut (Mersen).
The early development of the TPMOV extends back to
the 1990s. Forms of the basic theory and operation of
Page 1 of 5
888-987-8877
www.surgesuppression.com
February 2011 (Revised November 2013)
AN110215
the TPMOV have been used in IEC (mostly European)
designs for a number of years. One of the differences
between the TPMOV and the European designs is that
the TPMOV design has received much focus as an offthe-shelf solution to the recently established UL 1449
requirements.
TPMOV Components and Operation
The figures below are of the external appearance of the
TPMOV (middle), a view of the internal components
separated (right), a TPMOV with the top of the enclosure
removed (left top) and a TPMOV with the top of the
enclosure removed and the thermal element operated
(bottom left).
Copyright © 2013 Surge Suppression Incorporated
®
TPMOV is a registered trademark of Mersen/Ferraz-Shawmut
Ferraz Shawmut
TPMOV Principles and Operation
®
Application Note
To aid in the description of the components and operation
of the TPMOV refer to the figures herein and on Page 1.
[Note: (#) references the numbered circles from Page 1.]
The plastic, composite enclosure (1) of the TPMOV
provides a degree of protection for the internal components
of the device; however, its design and purpose are integral
to the success of the overall device as well. Beyond
providing a cover for the internal components, it also forms
the cylindrical channels that house the compressed springs
(3) and act as guides for positioning the insulating screen
(5) between the shaped metallic contact (6) and the metal
oxide varistor (MOV) body (2). The insulating screen (5)
rests within the cylindrical channels above the compressed
springs (3). The insulating screen (5) and compressed
springs (3) are held in place by the solder joint between the
MOV body (2) and the shaped metallic contact (6) that
forms the thermal element (4).
The figure below shows the channels that guide and house
the insulating screen (5) and compressed springs (3).
February 2011 (Revised November 2013)
AN110215
The figure below shows a top view of the contacts for the
micro-switch (7) and the openings in the top of the
enclosure for the visual indicators to protrude when
activated (visual indicators shown in activated state below).
Contacts for the micro-switch – Visual indicators
Note that the visual indicators are an integral part of the
insulating screen (5) and must be pushed through the
openings by the compressed springs (3) once the insulating
screen (5) has been forced up the channels in the
enclosure and in between the body of the MOV (2) and the
shaped metallic contact (6). Note that the visual indicators
may be excluded as an option.
MOV – Thermal Element and Metallic Contact – Insulating Screen
Note the shape of the cylindrical channels. They are
generally round (to house the round compression spring)
with a rectangular cutout that channels or guides the
insulating screen (5).
Openings for the visual indicators – Micro-switch
The opposing portion of the enclosure (1) – shown above –
serves as a mechanical mount for the normally open microswitch (7) that is supplied with the TPMOV. Further, it
provides the openings for the visual indicators that are
integral to the insulating screen (5) to protrude through the
top of the enclosure. The figure shows a view from the
inside of the enclosure (1). The micro-switch (7) is visible
along with the openings for the visual indicators.
Page 2 of 5
888-987-8877
www.surgesuppression.com
Opening for MOV contact – Shaped metallic contact – Enclosure
The base of the enclosure (1) is shown above. The MOV
contact protrudes through the opening on the left. The
shaped metallic contact (6) protrudes on the right. Although
the opening around the shaped metallic contact (6) forms a
close fit, the opening around the MOV contact does not –
leaving roughly 10 mm x 3.8 mm (0.4” x 0.15”) opening.
The openings in base of the enclosure (1) and the
mechanical nature of the TPMOV prevent the component
from being sealed. This limitation may create an
opportunity for contaminants to enter the device. Further, it
prevents the device from being encapsulated which may aid
in the improvement of the dielectric strength and protection
against the effects of shock and vibration.
Along with protection against the effects of shock and
vibration, the ability to encapsulate a circuit provides
protection against pollution and improves the performance
with regard to clearances and creepage distances. This is
evident in the UL 1449 standard as it allows encapsulated
Copyright © 2013 Surge Suppression Incorporated
®
TPMOV is a registered trademark of Mersen/Ferraz-Shawmut
Ferraz Shawmut
TPMOV Principles and Operation
®
Application Note
circuits to be considered to have an improved pollution
degree rating.
Although the openings in the top and bottom of the
enclosure (1) of the TPMOV serve specific purposes, they
may also create complications during the assembly
process. Many manufacturing facilities use automated
soldering processes that typically include a cleaning
process to remove contaminants and flux from the circuits
being assembled. The openings in the TPMOV may
prevent the cleaning process from being used or if it is
used, the openings may allow detergents, water or other
contaminants to enter the TPMOV mechanism if the
openings are not sealed in some fashion. These
contaminants may be corrosive in nature if not rinsed
properly and could potentially degrade the operation of the
compressed springs (3).
The metal oxide varistor or MOV (2) used within the
TPMOV has a 34 mm (1.34”) nominally square shape and
the thickness (of the actual MOV body) varies based upon
the voltage rating of the MOV/TPMOV. This size MOV is a
fairly common component used in Type 1 and Type 2
SPDs. By inspection, it can be shown that the MOV is
supplied by multiple approved (certified) suppliers.
The compressed springs (3) are key elements in the
operation of the TPMOV. Although, the composition of the
springs is not published, their construction and supply is
subject to the follow up services of Underwriters
Laboratories by way of the UL certification of the device.
The springs are housed within the channels of the
enclosure (1) and are the source of the mechanical force to
thrust the insulating screen (5) or arc shield into place
between the MOV body (2) and the shaped metallic contact
(6), then, up through the openings in the enclosure (1)
providing visual indication and activating the micro-switch
(7). See Endnote #1 for additional information regarding
springs.
Visual indicators
Insulating screen
Springs (notcompressed)
The thermal element (4) is the solder connection between
the shaped metallic contact (6) and the MOV body (2). The
thermal element (4) is critical to the operation of the
Page 3 of 5
888-987-8877
www.surgesuppression.com
February 2011 (Revised November 2013)
AN110215
interrupting mechanism. The melting temperature of the
solder must be precise to the point that the heat produced
by the MOV body during the abnormal operation
(overvoltage) is sufficient to cause the solder alloy to melt
so that the shaped metallic contact (6) can separate from
the MOV body (2). Further, the thermal transfer from the
MOV body (2) to the shaped metallic contact (6) must be
adequate to allow the heat transfer through the thermal
element (4) to quickly melt and separate to allow the
mechanism to safely operate.
The interrupting mechanism of the TPMOV solely relies
upon the operation of the thermal element (4). The
interrupting mechanism of the TPMOV operates differently
than that of systems that rely upon fuses, breakers or more
conventional means of interruption. Although the TPMOV
mechanism interrupts the flow of current through the MOV
body (2) at very low levels, it does not react to the actual
current flowing like a traditional fuse or breaker. The action
of the TPMOV mechanism depends on the heat generated
by the MOV body during an overvoltage and the conduction
of current through the MOV that follows. The thermal
element (4) is closely coupled to the MOV body (2). Over
time, this allows heat transfer to occur and the shaped
metallic contact (6) to release from the MOV body (2). See
Endnote #2 for additional detail.
The timing of these events is paramount to the operation of
the TPMOV mechanism. This system operates properly
when the TPMOV is exposed to overvoltages that are
roughly twice the operating voltage of the MOV – like the
voltages used in the Current Tests (exposure to abnormal
overvoltages) of ANSI/UL 1449-2006. The application of
this level of overvoltage allows for significant heat buildup in
the MOV and time for the heat to transfer to the thermal
element (4). In turn, the thermal element (4) opens and
interrupts the flow of current through the MOV body (2).
However, in circumstances where the TPMOV is exposed
to higher voltages (for example, the misapplication of lower
voltage device to higher voltage system), the heat buildup
in the MOV could be very fast and the rate of heat transfer
to the thermal element may be not be sufficient to allow for
proper operation of the thermal element and may result in
an undesired condition due to the excessive heating of the
MOV. In this circumstance since the TPMOV does not
incorporate any current limiting protection or backup fusing,
the interruption of current flow will depend on an upstream
fuse or breaker, if one exists (the TPMOV specification
sheet states that “no additional overcurrent protection
device (fuses) required” for available short-circuit currents
up to the short-circuit current rating or 200,000 Amps of
available fault current).
As examples of an application that may use TPMOVs, Type
1 and Type 2 SPDs could be candidates for the use of the
TPMOV. Type 1 SPDs are permitted to be installed on the
line side of the service equipment; therefore, the upstream
Copyright © 2013 Surge Suppression Incorporated
®
TPMOV is a registered trademark of Mersen/Ferraz-Shawmut
Ferraz Shawmut
TPMOV Principles and Operation
®
Application Note
fuse could be located at the serving transformer and could
have a high current rating. Further, Type 1 or Type 2 SPDs
can be installed on the load side of the service equipment
which may make the serving fuse or breaker the main
disconnect/fuse which, again, may have a high current
rating. As with any electrical device, caution must be taken
to be sure the SPD is intended for the system to which it is
being installed. Without any overcurrent protection, this is
particularly important with the TPMOV.
Although not published, the insulating screen (5) appears to
be comprised of a fiberglass resin composition similar to or,
possibly, the same material used in fabricating most circuit
boards (FR-4 fiberglass). The primary purpose of the
insulating screen (5) is to provide a high dielectric insulator
between the shaped metallic contact (6) and the MOV body
(2) when the thermal element (4) is operated to interrupt
and prevent the flow of current. Secondarily, the insulating
screen (5) also activates the micro-switch (7) through
physical contact (electrically isolated) and provides visual
indication via the extended tabs that protrude through the
top of the standard TPMOV enclosure (1).
The dielectric strength of FR-4 fiberglass is roughly 400 to
800 Volts/mil (1 mil = 0.001 inches). The insulating screen
(5) of the TPMOV is approximately 30 mils thick. Thus, if
the insulating screen of the TPMOV is made of FR-4
fiberglass, the dielectric strength of the insulating screen (5)
is approximately between 12,000 and 24,000 volts.
The micro-switch (7) provides indication of the operation of
the TPMOV’s interrupting mechanism by changing state
from a normally open to a normally closed contact position
when the TPMOV interruption mechanism operates. The
micro-switch (7) may be utilized with low-voltage diagnostic
circuits. The visual indicators integral to the insulating
screen (5) also provide indication of the operation of the
TPMOV’s interrupting mechanism.
TPMOV Ratings and Certifications
The TPMOV is reported to be available in a range of 150 to
550 Vac. The TPMOV specification sheet indicates that the
TPMOV has a 50 kA peak surge current rating for an 8/20
μs current impulse. The TPMOV has a short-circuit current
rating (SCCR) of 200 kA. The operating and storage
temperature range of the device is -25 to 60 °C (-32 to 140
°F). The TPMOV is a recognized component under
ANSI/UL 1449-2006 as a Type 4 SPD used in Type 2 SPD
applications (surge protective devices employing integral
thermal-links, for SPD Type 2 applications).
The URL http://us.ferrazshawmut.com/oem/media/pdf/TPMOV.pdf
(accessed on February 16, 2011) was the source for the
ratings and certifications reported above.
Page 4 of 5
888-987-8877
www.surgesuppression.com
February 2011 (Revised November 2013)
AN110215
Summary
Considerations for the application of the TPMOV may be*:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
The proper design, compression and released
force of the two independent compressed springs
The retention of the initial designed force of the
compression springs as the springs age and
remain in a compressed state
The compressed springs not being subjected to
temperatures or corrosion that may negatively
impact their operation or spring rate
The coefficient of friction between the insulating
screen and the channels of the enclosure and the
shaped metallic contact
The proper alignment and motion (sliding) of the
insulating screen (to prevent the insulating screen
from “jamming” or locking against any of the sides
of either channel or the shaped metallic contact)
The proper operation of the thermal element
(releasing the shaped metallic contact from the
body of the MOV at the appropriate temperature)
The force being applied to the insulating screen to
push the shaped metallic contact away from the
body of the MOV
The ability of insulating screen to interrupt the flow
of current through the shaped metallic contact and
the MOV body
The ability of insulating screen to interrupt any arc
that may form as the shaped metallic contact
separates from the MOV body
The ability of the insulating screen to prevent an
arc from forming once it is positioned between the
shaped metallic contact and the body of the MOV
(act as an arc shield)
The ability of the MOV to produce sufficient heat at
an appropriate rate to activate (release) the
thermal element during an overvoltage event
Adequate rate of thermal transfer between body of
the MOV and the shaped metallic contact via the
thermal element to allow the interrupting
mechanism to operate in a timely manner during
an overvoltage event
The level of system overvoltage is limited to a
magnitude that will not cause the MOV to overheat
too quickly and, therefore, not allow the interrupting
mechanism to operate properly
The temperature of the application environment is
within the operating temperature range (-40 to 85
°C or -40 to 185 °F)
*It should be noted that the safe operation of the
TPMOV during the UL 1449 Overvoltage Tests is
not dependent on the arc shield and springs of the
TPMOV. The TPMOV safely disconnects without
the operation of these components.
Copyright © 2013 Surge Suppression Incorporated
®
TPMOV is a registered trademark of Mersen/Ferraz-Shawmut
Ferraz Shawmut
TPMOV Principles and Operation
®
Application Note
February 2011 (Revised November 2013)
AN110215
The combination of MOV and fuse allowed a peak value of
2,442 amps of current to flow before interrupting. These
values are shown in the upper right of each oscillogram.
Endnotes
#1 - Generally, compression springs are considered reliable
and the performance of the materials used are consistent –
particularly when subject to inspection and qualification of a
certifying body. However, many spring materials are
susceptible to temperature and oxidation (corrosion) that
can impact their consistent operation. In addition, oxidation
or corrosion can impact the spring constant of a
compression spring. The spring constant determines the
amount of force the spring will apply to a load.
Temperature and oxidation can negatively impact these
properties, changing the performance of the spring.
These oscillograms are shown to demonstrate the
difference in operation between a TPMOV and an MOV
protected by a fuse. The TPMOV conducts at a consistent
current level near the peak value of the current for a period
of time until the MOV body transfers enough heat to the
thermal element to interrupt the current flow.
With the MOV and fuse combination, a current that is much
smaller than the peak current is conducted for a short
period of time. Then, the MOV ruptures or tunnels through
causing the current to spike. The fuse operates which
interrupts the current flow.
#2 – The interrupting mechanism of the TPMOV, when
subjected to overvoltages within its ability to interrupt,
operates at very low currents. The mechanism allows a
relatively low current to develop before the thermal transfer
between the MOV body and the shaped metallic contact
releases the thermal element. This differs from other
methods of interruption such as fuses or breakers. The
large diameter MOV used in the construction of the TPMOV
aids in this action. The large diameter MOV can conduct
low level currents for (relatively) long periods of time before
rupturing or tunneling through which forms a spike in the
current due to the lower impedance of the rupture or tunnel.
MOV and Fuse Oscillogram – 240 Vac with 100,000 Amps available
The time required for the TPMOV to interrupt the current
flow during this level of overvoltage and available shortcircuit current is much longer than that of the MOV and fuse
combination. In this example, the TPMOV conducted
current for about 1.98 seconds while the MOV and fuse
combination conducted current for about 0.2 seconds. In
the TPMOV, during this time of current flow, the heat that is
generated by the conducting MOV is transferred to the
thermal element and shaped metallic contact causing the
thermal element to operate.
TPMOV Oscillogram – 240 Vac with 100,000 Amps available
As shown in the oscillogram for the TPMOV, the TPMOV
tested in this example only allowed a peak value of 83.5
amps of current to flow into the MOV before interrupting.
This particular test was performed on a 150 Vac TPMOV
with 240 Vac applied and 100,000 Amps of available shortcircuit current. Further, the oscillogram from a 150 Vac
MOV with a protecting fuse tested with 240 Vac applied and
100,000 Amps of available short-circuit current is shown.
Page 5 of 5
888-987-8877
www.surgesuppression.com
In this example, it is shown that the TPMOV interrupts only
a relatively small amount of current while the fuse used in
conjunction with the MOV interrupts a much larger peak
value of current (2,442 amps versus 83.5 amps). Because
the interrupting mechanism of the TPMOV is integral to its
package, it does not offer overcurrent protection to the
surrounding assembly (circuit board, wiring, diagnostics,
etc.).
Copyright © 2013 Surge Suppression Incorporated
®
TPMOV is a registered trademark of Mersen/Ferraz-Shawmut