EN IEC 61643-332:2024

SLOVENSKI STANDARD
01-julij-2024
Sestavni deli za nizkonapetostne naprave za zaščito pred prenapetostnimi udari -
332. del: Izbira in načini uporabe za kovinsko-oksidne varistorje (MOV)
Components for low-voltage surge protection - Part 332: Selection and application
principles for metal oxide varistors (MOV)
Bauelemente für den Überspannungsschutz bei Niederspannungen - Teil 332: Auswahl-
und Anwendungsgrundsätze für Metalloxidvaristoren (MOV)
Composants de protection contre les surtensions basse tension - Partie 332 : Principes
de sélection et d'application pour les varistances à oxyde métallique (MOV)
Ta slovenski standard je istoveten z: EN IEC 61643-332:2024
ICS:
29.120.50 Varovalke in druga Fuses and other overcurrent
nadtokovna zaščita protection devices
31.040.20 Potenciometri, spremenljivi Potentiometers, variable
upori resistors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61643-332

NORME EUROPÉENNE
EUROPÄISCHE NORM May 2024
ICS 31.040.20
English Version
Components for low-voltage surge protection - Part 332:
Selection and application principles for metal oxide varistors
(MOV)
(IEC 61643-332:2024)
Composants pour parafoudres basse tension - Partie 332: Bauelemente für den Überspannungsschutz bei
Choix et principes d'application des varistances à oxyde Niederspannungen - Teil 332: Auswahl- und
métallique (MOV) Anwendungsgrundsätze für Metalloxidvaristoren (MOV)
(IEC 61643-332:2024) (IEC 61643-332:2024)
This European Standard was approved by CENELEC on 2024-05-22. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61643-332:2024 E

European foreword
The text of document 37B/243/FDIS, future edition 1 of IEC 61643-332, prepared by SC 37B
"Components for low-voltage surge protection" of IEC/TC 37 "Surge arresters" was submitted to the
IEC-CENELEC parallel vote and approved by CENELEC as EN IEC 61643-332:2024.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2025-02-22
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2027-05-22
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61643-332:2024 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60364-5-51:2005 NOTE Approved as HD 60364-5-51:2009 +A11:2013 +A12:2017
IEC 61000-4-2:2008 NOTE Approved as EN 61000-4-2:2009 (not modified)
IEC 60068-1:2013 NOTE Approved as EN 60068-1:2014 (not modified)
IEC 60068-2-20:2021 NOTE Approved as EN IEC 60068-2-20:2021 (not modified)
IEC 60068-2-21:2021 NOTE Approved as EN IEC 60068-2-21:2021 (not modified)
IEC 60068-2-78:2012 NOTE Approved as EN 60068-2-78:2013 (not modified)
IEC 60721-3-3:2019 NOTE Approved as EN IEC 60721-3-3:2019 (not modified)
IEC 61643-21:2000 NOTE Approved as EN 61643-21:2001 (not modified)
IEC 61643-21:2000/A1:2008 NOTE Approved as EN 61643-21:2001/A1:2009
IEC 61643-21:2000/A2:2012 NOTE Approved as EN 61643-21:2001/A2:2013 (not modified)
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60664-1 2020 Insulation coordination for equipment EN IEC 60664-1 2020
within low-voltage supply systems - Part 1:
Principles, requirements and tests
IEC 61051-1 2018 Varistors for use in electronic equipment - EN IEC 61051-1 2018
Part 1: Generic specification
IEC 61051-2 2021 Varistors for use in electronic equipment - EN IEC 61051-2 2021
Part 2: Sectional specification for surge
suppression varistors
IEC 61643-11 (mod) 2011 Low-voltage surge protective devices - Part EN 61643-11 2012
11: Surge protective devices connected to
low-voltage power systems - Requirements
and test methods
- - + A11 2018
IEC 61643-331 2020 Components for low-voltage surge EN IEC 61643-331 2020
protection - Part 331: Performance
requirements and test methods for metal
oxide varistors (MOV)
IEC 62368-1 2023 Audio/video, information and EN IEC 62368-1 2024
communication technology equipment -
Part 1: Safety requirements
IEC 61643-332 ®
Edition 1.0 2024-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Components for low-voltage surge protection –

Part 332: Selection and application principles for metal oxide varistors (MOV)

Composants pour parafoudres basse tension –

Partie 332: Choix et principes d'application des varistances à oxyde métallique

(MOV)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.040.20  ISBN 978-2-8322-8594-7

– 2 – IEC 61643-332:2024 © IEC 2024
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviated terms . 7
3.1 Terms and definitions . 8
3.1.1 Ratings . 8
3.1.2 Characteristics. 9
3.2 Symbols and abbreviated terms . 12
3.2.1 Symbols . 12
3.2.2 Abbreviated terms . 13
4 General . 13
5 Construction . 13
6 Function . 14
6.1 Theory of operation . 14
6.2 Thermal protection of MOVs . 15
6.3 Failure modes . 16
6.3.1 General . 16
6.3.2 Short-circuit failure mode . 16
6.3.3 Degradation failure mode . 16
6.3.4 Open-circuit and high clamping voltage failure mode . 17
7 Application . 17
7.1 MOVs basic application . 17
7.1.1 Application circuit . 17
7.1.2 Operational compatibility . 18
7.1.3 Voltage limiting . 18
7.1.4 Selection of MOVs . 18
7.1.5 Mitigating the consequences of failure . 27
7.1.6 Operations to failure . 30
7.1.7 Earthing and bonding . 30
7.1.8 Location of MOVs . 31
7.1.9 Applications for MOVs . 31
7.1.10 Parallel connections . 32
7.1.11 Series connections . 33
7.2 Thermally protected metal oxide varistor . 33
7.2.1 Introduction . 33
7.2.2 Selection of thermally protected MOV . 34
7.2.3 Time to open characteristics . 34
7.3 ESD . 34
7.3.1 Background . 34
7.3.2 Standards . 35
7.3.3 Application example 1 . 35
7.3.4 Application example 2 . 35
7.4 Consideration for TOV . 35
7.4.1 Failure of the low-voltage power supply circuit . 35
7.4.2 Failure of high voltage or medium voltage power supply circuit . 36
8 Safety and hazard information for MOVs. 36

IEC 61643-332:2024 © IEC 2024 – 3 –
8.1 Overview. 36
8.1.1 General . 36
8.1.2 Confirmation of rated performance . 36
8.2 Fire risks . 36
8.2.1 General . 36
8.2.2 Use between lines . 36
8.2.3 Use between line and earth . 37
8.2.4 Shatter-proof . 37
8.2.5 Prevention of burning . 37
8.2.6 Environmental condition . 37
8.3 Electrical shock risks . 37
8.4 Typical precaution statement for the use of MOVs . 37
8.4.1 Information related to degradation and failures of MOVs . 37
8.4.2 Information related to scattering of MOVs . 38
8.4.3 Information related to equipment damage or malfunction . 38
8.4.4 Information related to accidents caused by unexpected phenomena . 38
Annex A (informative) Terms and explanations . 39
A.1 Single-impulse peak current I . 39
TM
A.2 Maximum continuous voltage V . 39
M
A.3 Standby current I . 39
D
A.3.1 AC Standby current . 39
A.3.2 DC Standby current I . 40
DC
A.4 varistor voltage V . 42
V
A.5 Clamping voltage V . 42
C
A.6 Capacitance C . 44
V
Annex B (informative) MOV durability evaluation under DC bias condition . 45
B.1 Introduction . 45
B.2 Durability test . 45
B.3 Typical performances in MOV durability evaluation . 46
B.3.1 Ambient temperature: 85 °C . 46
B.3.2 Ambient temperature: 105 °C . 47
B.4 Conclusion . 47
Annex C (informative) Typical application circuits of thermally protected MOVs . 48
Annex D (informative) MOV application for wind turbine systems . 50
Annex E (informative) 5G powering surge protection . 51
E.1 AC power protection . 51
E.2 DC power protection . 51
Annex F (informative) Comparison of MOV terms with other standards . 53
Annex G (informative) How to select MOV/thermally protected MOV for equipment . 55
Annex H (informative) How to select an MOV/thermally protected MOV for an SPD . 57
Bibliography . 59

Figure 1 – V-I characteristic of an MOV . 10
Figure 2 – Symbol for an MOV . 12
Figure 3 – Symbol for a thermally protected MOV . 12
Figure 4 – Schematic depiction of microstructure of MOV . 13

– 4 – IEC 61643-332:2024 © IEC 2024
Figure 5 – Typical varistor V-I curve plotted log-log scale . 14
Figure 6 – MOV equivalent circuit model . 15
Figure 7 – Possible connection of MOVs (simplified) . 17
Figure 8 – Overvoltage categories . 20
Figure 9 – Test data example of impulse current vs repetitions for 14 mm MOVs . 21
Figure 10 – Example of 10 mm, 14 mm and 20 mm MOV voltage current characteristics . 22
Figure 11 – K value for various waveforms . 24
Figure 12 – 5/50 exponential waveform as an example . 25
Figure 13 – MOV pulse energy versus pulse width for various pulse repetitions . 25
Figure 14 – Options for MOV fuse connection . 28
Figure 15 – Time-current characteristic of fast acting and time delay fuse . 29
Figure 16 – Parallel connection of MOVs . 32
Figure 17 – Example of V-I characteristics for two parallel MOVs . 32
Figure 18 – Operating Time . 34
Figure 19 – Example of MOV application for ESD . 35
Figure 20 – Example of 4 ports application using MOVs for ESD. 35
Figure 21 – Combination an MOV with a GDT . 36
Figure A.1 – Short term effect of temperature, frequency, and voltage on standby
power of a typical 20 mm MOV . 40
Figure A.2 – Typical temperature coefficient of voltage versus current, 14 mm size,
−55 °C to 125 °C . 41
Figure A.3 – Typical clamping voltage response to 8/20 test current impulse . 42
Figure A.4 – Illustration of static (DC) I–V characteristics on linear scale . 43
Figure B.1 – Durability test result at 85 °C . 46
Figure B.2 – Durability test result at 105 °C . 47
Figure C.1 – AC Application Circuit . 48
Figure C.2 – DC Photovoltaic Application circuit . 49
Figure E.1 – AC power feed protection according to ITU-T K.120 . 51
Figure E.2 – DC power feed protection according to ITU-T K.97 and Diode steering . 52
Figure G.1 – Flow chart of MOV/thermally protected MOV selection for equipment . 56
Figure H.1 – Flow chart of MOV/thermally protected MOV selection for an SPD . 58

Table D.1 – Example of characteristics of the generator alternator excitation circuit and
selected SPD . 50
Table F.1 – Comparison of MOV terms/symbols with other standards for MOV voltages . 53
Table F.2 – Comparison of MOV terms/symbols with other standards for impulse
current ratings . 53
Table F.3 – Comparison of MOV terms/symbols with other standards for TOV and
abnormal voltage testing . 54

IEC 61643-332:2024 © IEC 2024 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
COMPONENTS FOR LOW-VOLTAGE SURGE PROTECTION –

Part 332: Selection and application principles
for metal oxide varistors (MOV)

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
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Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61643 has been prepared by subcommittee 37B: Components for low voltage surge
protection, of IEC technical committee 37: Surge arresters. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
37B/243/FDIS 37B/245/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

– 6 – IEC 61643-332:2024 © IEC 2024
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61643 series, published under the general title Components for
low-voltage surge protection, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

IEC 61643-332:2024 © IEC 2024 – 7 –
COMPONENTS FOR LOW-VOLTAGE SURGE PROTECTION –

Part 332: Selection and application principles
for metal oxide varistors (MOV)

1 Scope
This part of IEC 61643 describes the theory of operation, principles for the selection and
application of MOVs to be connected to power lines or telecommunication or signalling circuits,
up to 1 000 V AC or 1 500 V DC. These SPCs are designed to protect apparatus or personnel,
or both, from high transient voltages.
This document applies to MOVs having two electrodes and voltage dependent elements with or
without disconnectors. It does not apply to assemblies that include MOVs and their influence
on the MOV's characteristics.
This standard specifically discusses the zinc-oxide type of MOVs.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60664-1:2020, Insulation coordination for equipment within low-voltage supply systems –
Part 1: Principles, requirements and tests
IEC 61051-1:2018, Varistors for use in electronic equipment – Part 1: Generic specification
IEC 61051-2:2021, Varistors for use in electronic equipment – Part 2: Sectional specification
for surge suppression varistors
IEC 61643-11:2011, Low-voltage surge protective devices – Part 11: Surge protective devices
connected to low-voltage power systems – Requirements and test methods
IEC 61643-331:2020, Components for low-voltage surge protection – Part 331: Performance
requirements and test methods for metal oxide varistors (MOV)
IEC 62368-1:2023, Audio/video, information and communication technology equipment – Part 1:
Safety requirements
3 Terms, definitions, symbols and abbreviated terms
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp

– 8 – IEC 61643-332:2024 © IEC 2024
3.1 Terms and definitions
3.1.1 Ratings
3.1.1.1
rating
limiting capability or limiting condition beyond which damage to the MOV may occur
Note 1 to entry: A limiting condition may be either a maximum or a minimum.
3.1.1.2
single-impulse [transient] maximum current
I
TM
rated maximum value of current which may be applied for a single impulse of specified
waveform
Note 1 to entry: For power distribution SPDs, IEC 61643-11, Maximum Discharge Current I is used.
max
[SOURCE: IEC 61643-331:2020, 3.1.1.2]
3.1.1.3
nominal discharge current
I
n
crest value of the current through the MOV having a current waveshape of 8/20
[SOURCE: IEC 61643-331:2020, 3.1.1.3]
3.1.1.4
maximum continuous voltage
V
M
maximum voltage that may be applied continuously at a specified temperature
Note 1 to entry: May also be called U or MCOV (Maximum continuous operating voltage).
C
Note 2 to entry: See Figure 1.
[SOURCE: IEC 61643-331:2020, 3.1.1.7, modified (addition of "Maximum continuous operating
voltage" to Note 1 to entry)]
3.1.1.5
maximum continuous AC voltage
V
M(AC)
maximum value of RMS power frequency voltage (less than 5 % total harmonic distortion) that
may be applied continuously at a specified temperature
[SOURCE: IEC 61643-331:2020, 3.1.1.8]
3.1.1.6
maximum continuous DC voltage
V
M(DC)
maximum value of DC voltage that may be applied continuously at a specified temperature
[SOURCE: IEC 61643-331:2020, 3.1.1.9]

IEC 61643-332:2024 © IEC 2024 – 9 –
3.1.1.7
maximum discharge current
I
max
crest value of a current through the SPD having an 8/20 waveshape and magnitude according
to the manufacturer's specification.
Note 1 to entry: I is equal to or greater than I .
max n
[SOURCE: IEC 61643-11:2011, 3.1.48]
3.1.1.8
impulse discharge current for class I test
I
imp
crest value of a discharge current through the SPD with specified charge transfer Q and
specified energy W/R in the specified time
[SOURCE: IEC 61643-11:2011, 3.1.10]
3.1.1.9
rated average dissipation power
P
M
maximum average dissipation power of repetitive pulses allowed to be applied to the varistors
at ambient temperature of 25 °C
[SOURCE: IEC 61051-1:2018, 3.23]
3.1.2 Characteristics
3.1.2.1
characteristics
inherent and measurable properties of an MOV
[SOURCE: IEC 61643-331:2020, 3.1.2.1]
3.1.2.2
standby current
I
D
current passing through MOV at maximum continuous voltage V
M
Note 1 to entry: The current passing through the MOV at less than V is called leakage current.
M
[SOURCE: IEC 61643-331:2020, 3.1.2.2]
3.1.2.3
varistor voltage
V
V
voltage across the MOV measured at a specified current (typically 1 mA) for a specific duration
Note 1 to entry: The MOV manufacturer specifies the current. Otherwise, 1 mA DC for a duration of 20 to 100 ms
is normally used.
Note 2 to entry: See Figure 1.
[SOURCE: IEC 61643-331:2020, 3.1.2.3]

– 10 – IEC 61643-332:2024 © IEC 2024

Figure 1 – V-I characteristic of an MOV
3.1.2.4
Clamping voltage
V
C
peak voltage across the MOV measured under conditions of a class current (I ) and specified
CLS
waveform
3.1.2.5
class current
I
CLS
peak value of current, which is 1/10 of the maximum peak current for 100 pulses for the 8/20
current pulse with a time interval of 30 s
[SOURCE: IEC 61051-1:2018, 3.21]
3.1.2.6
Capacitance
C
V
capacitance across the MOV measured at a specified frequency, voltage and time
[SOURCE: IEC 61643-331:2020, 3.1.2.6]

IEC 61643-332:2024 © IEC 2024 – 11 –
3.1.2.7
SPD
device that contains at least one nonlinear component that is intended to limit surge voltages
and divert surge currents
[SOURCE: IEC 61643-11 Clause 3.1.1]
3.1.2.8
SPC
discrete component whose primary function is to divert or limit excessive voltage and current
surges to protect sensitive equipment and circuits from potential damage. Examples of SPCs
are MOVs, GDTs, SITs, ABDs, and thyristors.
3.1.2.9
metal oxide varistor (MOV)
non-linear resistor made of a sintered mixture of metal oxides whose conductance, at a given
temperature, increases rapidly with voltage
Note 1 to entry: This is also known as a voltage dependant resistor (VDR).
[SOURCE: IEC 61643-331:2020, 3.1.2.7]
3.1.2.10
thermally protected metal oxide varistor
varistor which includes a series non-resettable element that will disconnect the MOV when it is
overheated due to excessive dissipation
[SOURCE: IEC 61643-331:2020, 3.1.2.8]
3.1.2.11
nonlinearity current index
β
starting from Formula (1) of 3.3, it is defined by the formula
IUd
β × (1)
UId
Note 1 to entry: For the convenience of calculation, the following formula may be used:
log (UU )
10 1 2
β =
(2)
log (I I )
10 1 2
β is always less than 1.
[SOURCE: IEC 61051-1:2018, 3.4]
=
– 12 – IEC 61643-332:2024 © IEC 2024
3.1.2.12
non-linearity voltage index
γ
reciprocal of non-linearity current index β
Note 1 to entry: γ is always greater than 1.
Note 2 to entry: In varistor industry and literature, the non-linearity voltage index is usually denoted by α rather
γ.
than
[SOURCE: IEC 61051-1:2018, 3.5]
3.1.2.13
AC standby current
I
AC
current passing through MOV at maximum continuous voltage AC V
M(AC)
3.1.2.14
DC standby current
I
DC
current passing through MOV at maximum continuous voltage DC V
M(DC)
[SOURCE: IEC 61643-331:2020, 3.1.2.9]
3.2 Symbols and abbreviated terms
3.2.1 Symbols
Figure 2 and Figure 3 show the symbols for an MOV and a thermally protected MOV,
respectively.
Figure 2 – Symbol for an MOV
Figure 3 – Symbol for a thermally protected MOV
NOTE IEC 60027 recommends the letters V and v only as reserve symbols for voltage; however, in the field of MOV
components, these are so widely used that in this publication they are preferred to U and u.

IEC 61643-332:2024 © IEC 2024 – 13 –
3.2.2 Abbreviated terms
ESD Electrostatic Discharge
GDT Gas Discharge Tube
IC Integrated Circuit
IT Information Technology
IT Isole-Terre (French for Isolated-Earth)
MCOV Maximum Continuous Operating Voltage
MOV Metal Oxide Varistor
SMD Surface Mount Device
SPC Surge Protective Component
SPD Surge Protective Device
TCO Thermal Cut off
TOV Temporary Overvoltage
TN Terre-Neutre (French for Earth-Neutral)
TT Terre-Terre (French for Earth-Earth)
4 General
Due to the high complexity of the ceramics on which the functioning of the MOV is based, the
performance of the MOV depends on the technology and processes used. Thus, the electrical
properties and characteristics (tolerances, impulse withstand capability, etc.) may vary among
manufacturers. The explanations of terms related to electrical properties and characteristics
are described in Annex A.
5 Construction
Typically, MOVs consist of a round disc-shaped body of sintered zinc-oxide with suitable
additives. Other types in use include rectangular and tubular shapes and multilayer structures.
MOVs have metal particle electrodes consisting of a silver alloy or other metal. The electrodes
may have been applied to the body by screen printing and firing or by other processes
depending on the metal used. MOVs also often have wire leads or some other type of
termination that may have been soldered to the electrode.
The basic conduction mechanism of MOVs results from semiconductor barriers at the boundary
of the zinc-oxide grains formed during a sintering process. The MOV may be considered a multi-
barrier component with many grains acting in series-parallel combination between the terminals.
A schematic cross-sectional view of a typical MOV is shown in Figure 4.

Figure 4 – Schematic depiction of microstructure of MOV

– 14 – IEC 61643-332:2024 © IEC 2024
6 Function
6.1 Theory of operation
MOVs have the property of maintaining a relatively small voltage change across their terminals
while the surge current flowing through them varies over several decades. This non-linear action
allows them to divert the current of a surge when connected in shunt across the line, and hold
the voltage across the line to values that protect the equipment connected to that line. Since
the voltage across the MOV is held at some level higher than the normal circuit voltage (but still
protecting) while surge current flows, there will be energy dissipated in the MOV during its surge
diversion function.
The MOV material consists of zinc-oxide grains separated by a thin intergranular material.
Bismuth oxide and other metal oxides comprise the boundary between grains, and these form
semiconducting barriers with the grains. A fundamental property of the material is that the
voltage drop across a single interface between the grains is nearly constant, and is independent
of the grain size. Figure 5 shows a typical V-I characteristic of an MOV in one direction of
conduction, and the opposite direction would be similar.
The voltage and current values of the curve in Figure 5 were measured by using DC current
-2
when the current being less than 10 A, or by using 8/20 current when the current being greater
than 100 A.
Figure 5 – Typical varistor V-I curve plotted log-log scale
The electrical behaviour of an MOV can be understood by reference to the V-I characteristic of
Figure 5 and the equivalent circuit components of Figure 6 as follows:
1) Leakage Region: When the voltage across the MOV is below its varistor voltage (V ), the
V
non-linear resistance R approaches a high ohmic state and can be disregarded, hence the
x
parallel resistance R , is the prevailing component. In the example of Figure 6, R is such
off off
a high value that the MOV is essentially in a nearly open-circuit state.

IEC 61643-332:2024 © IEC 2024 – 15 –
2) Normal Varistor Operation Region: The non-linear resistance R becomes so small that it
x
is much lower than the linear leakage resistance R , so that the R is ignored but R
off off x
remains larger than R . The variable resistance R takes on continuously decreasing
on x
values, according to the power law relating current and voltage. In this region, current
increases in several orders of magnitude as V remains rather constant.
V
3) Upturn Region: At large currents, the series resistance R becomes a significant part of the
on
total device resistance, causing an upturn with the value of R as an asymptote. The MOV
on
is essentially in a short-circuit state during this region.
The parametric values given in Figure 5 and Figure 6 are shown for illustration purposes only.

Figure 6 – MOV equivalent circuit model
Under AC or signal operating conditions as well as under surge conditions the reactive
components (L and C) of Figure 6 may significantly affect the behaviour of the MOV. The parallel
capacitance C can pass a current that may be larger than the DC standby current. The series
inductance L, resulting from the leads can increase the voltage appearing across the component
terminals when it passes surge currents with steep wave fronts. This inductive voltage drop
may appear as an overshoot on the voltage waveform and appear to be a delay in MOV
response.
Ambient temperature, and/or the temperature rise caused in the MOV by operating conditions
or by a surge, produces little effect on the clamping voltage. However, the standby current will
increase with increasing temperature. Therefore, if conditions result in temperatures that may
exceed the component rating, consideration will have to be given to thermal design in the
application.
6.2 Thermal protection of MOVs
Thermal protection can be effective in cases where current through an MOV might be so low as
to not result in the operation of the overcurrent protection, and where currents could be high
enough to cause an MOV to reach extremely high temperatures sufficient to result in extreme
overheating. The following examples describe how thermal protection means could be applied
to disconnect the MOV:
1) If a power frequency overvoltage occurs, AC standby power dissipation will rise. This
could result in heating of an MOV.
2) If an MOV is exposed to stresses beyond its rating, it could be subject to a degradation
failure mode. With severe degradation of an MOV, standby power dissipation could rise
enough to cause heating.
3) If the power source impedance is relatively high, and the fault current available is too
low to operate an overcurrent means, an MOV failed in the short-circuit mode could
cause heating.
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