Littelfuse SPD, AC Application Note

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

1

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