SIST EN IEC 61643-21:2026

SLOVENSKI STANDARD
01-marec-2026
Nadomešča:
SIST EN 61643-21:2002
SIST EN 61643-21:2002/A1:2009
SIST EN 61643-21:2002/A2:2014
Nizkonapetostne naprave za zaščito pred prenapetostnimi udari - 21. del: Naprave,
priključene na telekomunikacijska in signalna omrežja - Zahteve in preskusne
metode
Low voltage surge protective devices - Part 21: Surge protective devices connected to
telecommunications and signalling networks - Requirements and test methods
Überspannungsschutzgeräte für Niederspannung - Teil 21: Überspannungsschutzgeräte
für den Einsatz in Telekommunikations- und signalverarbeitenden Netzwerken -
Leistungsanforderungen und Prüfverfahren
Parafoudres basse tension - Partie 21: Parafoudres connectés aux réseaux de signaux
et de télécommunications - Exigences et méthodes d’essai
Ta slovenski standard je istoveten z: EN IEC 61643-21:2026
ICS:
29.120.50 Varovalke in druga Fuses and other overcurrent
nadtokovna zaščita protection devices
29.240.10 Transformatorske postaje. Substations. Surge arresters
Prenapetostni odvodniki
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61643-21

NORME EUROPÉENNE
EUROPÄISCHE NORM January 2026
ICS 29.240.10 Supersedes EN 61643-21:2001; EN 61643-
21:2001/A1:2009; EN 61643-21:2001/A2:2013
English Version
Low voltage surge protective devices - Part 21: Surge protective
devices connected to telecommunications and signalling
networks - Requirements and test methods
(IEC 61643-21:2025)
Parafoudres basse tension - Partie 21: Parafoudres Überspannungsschutzgeräte für Niederspannung - Teil 21:
connectés aux réseaux de signaux et de Überspannungsschutzgeräte für den Einsatz in
télécommunications - Exigences et méthodes d'essai Telekommunikations- und signalverarbeitenden Netzwerken
(IEC 61643-21:2025) - Leistungsanforderungen und Prüfverfahren
(IEC 61643-21:2025)
This European Standard was approved by CENELEC on 2025-11-21. 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
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61643-21:2026 E

European foreword
The text of document 37A/432/FDIS, future edition 2 of IEC 61643-21, prepared by SC 37A "Low-
voltage surge protective devices" of IEC/TC 37 "Surge arresters" was submitted to the IEC-CENELEC
parallel vote and approved by CENELEC as EN IEC 61643-21:2026.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-01-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-01-31
document have to be withdrawn
This document supersedes EN 61643-21:2001 and all of its amendments and corrigenda (if any).
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.
This document is read in conjunction with EN IEC 61643-01:2025 .
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 61643-21:2025 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 NOTE Approved as HD 60364-5-51
IEC 60721-3-3:2019 NOTE Approved as EN IEC 60721-3-3:2019 (not modified)
IEC 61643-22 NOTE Approved as CLC/TS 61643-22
IEC 61643-311:2013 NOTE Approved as EN 61643-311:2013 (not modified)
IEC 61643-321:2001 NOTE Approved as EN 61643-321:2002 (not modified)
IEC 61643-331:2020 NOTE Approved as EN IEC 61643-331:2020 (not modified)
IEC 61643-341:2020 NOTE Approved as EN IEC 61643-341:2020 (not modified)
IEC 62368-1 NOTE Approved as EN IEC 62368-1
IEC 62475:2010 NOTE Approved as EN 62475:2010 (not modified)

As impacted by EN IEC 61643-01:2025/A11 2025.
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 61643-01 2024 Low-voltage surge protective devices - Part EN IEC 61643-01 2025
01: General Requirements and test
methods
IEC 61643-21 ®
Edition 2.0 2025-10
INTERNATIONAL
STANDARD
Low voltage surge protective devices -
Part 21: Surge protective devices connected to telecommunications and
signalling networks - Requirements and test methods
ICS 29.240.10  ISBN 978-2-8327-0741-8

IEC 61643-21:2025-10(en)
IEC 61643-21:2025 © IEC 2025
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
3.1 Terms and definitions . 8
4 Classification . 11
4.1 General . 11
4.5 Type of SPD. 11
4.8 Mounting method . 11
4.13 End of life mode of the SPDA . 11
4.14 Short-Circuit protection function for OCM end of life mode . 11
4.100 SPD configurations . 12
4.101 SPD Variants . 13
4.102 Overstressed fault mode according to 3.1.115 . 13
4.102.1 Mode 1 . 13
4.102.2 Mode 2 . 13
4.102.3 Mode 3 . 13
4.103 Additional information about transmission and special applications . 13
5 Void . 13
6 Marking and other product information . 13
6.1 General . 13
6.2 List of items . 14
6.2.100 Information to be provided by the manufacturer . 15
6.2.101 Information which shall be provided by the manufacturer for type
testing, as applicable. 16
6.3 Information about status indicator . 16
7 Service conditions . 16
8 Requirements . 16
8.1 General requirements. 16
8.3 Electrical requirements . 16
8.3.1 Protection against electric shock . 16
8.3.2 Continuous current I . 17
C
8.3.3 Protective conductor current I . 17
PE
8.3.4 Measured limiting voltage . 17
8.3.5 Operating duty . 17
8.3.6 Safety performance of overstressed SPDs . 17
8.3.7 Insulation resistance . 18
8.3.8 Dielectric withstand . 19
8.3.9 Behaviour under temporary overvoltages . 19
8.3.100 Impulse durability and voltage protection level . 19
8.3.101 Impulse reset. 19
8.3.102 AC durability, if declared by the manufacturer . 19
8.3.103 Blind spot . 19
8.3.104 Series resistance . 20
8.3.105 Current-limiting requirements . 20
IEC 61643-21:2025 © IEC 2025
8.3.106 Transmission requirements . 21
8.4 Mechanical requirements . 21
8.4.1 General. 21
8.4.5 Mechanical strength . 21
8.5 Environmental and material requirements . 22
8.5.2 Heat resistance. 22
8.5.4 Tracking resistance . 22
8.5.5 Ageing behaviour under damp heat . 22
8.6 Additional requirements for specific SPD designs . 22
8.6.1 Two port SPDs and one port SPDs with separate input/output
connections . 22
8.6.3 SPDs with electrically separated circuits . 22
8.6.4 Total discharge current I , if declared by the manufacturer . 22
Total
8.6.5 Two port SPDs . 23
8.6.6 Short-circuiting SPDs . 23
9 Tests . 23
9.1 Type testing procedures . 23
9.1.1 General. 23
9.1.2 Impulse discharge current . 30
9.1.3 8/20 Current impulse . 30
9.1.4 1,2/50 voltage impulse. 30
9.1.5 Combination wave . 30
9.1.100 Waveform tolerances of impulse categories C3 and D2 . 30
9.3 Electrical tests . 31
9.3.1 Protection against direct contact . 31
9.3.2 Continuous current I . 31
C
9.3.3 Protective conductor current I . 31
PE
9.3.4 Measured limiting voltage . 31
9.3.5 Operating duty test. 31
9.3.6 Safety performance of overstressed SPDs . 32
9.3.8 Dielectric withstand . 35
9.3.9 Behaviour under temporary overvoltages (TOV) . 38
9.3.100 Impulse durability and voltage protection level . 38
9.3.101 Impulse reset. 41
9.3.102 AC durability. 44
9.3.103 Blind spot test . 45
9.3.104 Series resistance . 46
9.3.105 Current-limiting tests . 47
9.3.106 Transmission tests . 51
9.4 Mechanical tests . 56
9.4.3 Tests for external connections for copper conductors . 56
9.4.4 Verification of air clearances and creepage distances . 57
9.5 Environmental and material tests. 59
9.5.2 Heat resistance. 59
9.5.4 Tracking resistance . 59
9.5.5 Life test under damp heat . 59
9.6 Additional tests for specific SPD designs. 60
9.6.1 Tests for two-port SPDs and one-port SPDs with separate input/output
connections . 60
IEC 61643-21:2025 © IEC 2025
9.6.4 Total discharge current test for multimode SPDs . 62
9.6.5 Tests for two port SPDs only . 64
Annex AA (normative) Application of annexes from IEC 61643-01 . 65
Annex BB (informative) Example configurations for measuring U . 66
P
Annex CC (informative) Special resistibility in telecommunication systems . 68
Annex DD (informative) Bit error ratio (BER) . 69
Annex EE (informative) Example of transmission test methods for SPDs connected to
LAN . 71
EE.1 General . 71
EE.2 Abbreviated terms. 71
EE.3 Requirements . 71
EE.4 Test methods . 72
EE.4.1 Measurement circuit . 72
EE.4.2 Insertion loss . 72
EE.4.3 NEXT, power sum NEXT . 72
EE.4.4 Return loss . 73
Annex FF (informative) Typical threat parameters for SPDs . 74
Annex GG (informative) Examples of source voltages and currents for impulse reset
test . 75
Annex HH (informative) Test circuit examples for testing more than one mode of
protection simultaneously . 76
HH.1 General . 76
HH.2 Impulse durability and voltage protection level . 76
HH.3 Impulse reset time . 77
HH.4 AC durability . 78
HH.5 AC durability for current-limiting function . 79
HH.6 Impulse durability for current-limiting function . 81
Annex II (normative) Reduced test procedures . 82
Bibliography . 84

Figure 100 – SPD configurations . 12
Figure 101 – Example of U-I diagram providing three value pairs of voltage and current
for impulse reset test (U /I , U /I and U /I ) with U = U and e.g. I = I . 14
1 1 2 2 3 3 1 C 3 L
Figure 102 – Procedure for sample preparation for dedicated overstress test . 34
Figure 103 – Test circuits for impulse durability and voltage- protection level . 40
Figure 104 – Test circuits for impulse reset time . 43
Figure 105 – Test circuits for AC durability . 45
Figure 106 – Test circuits for series resistance, current response time, current reset
time, maximum interrupting voltage and operating duty test . 47
Figure 107 – Test circuits for AC durability of the current-limiting component
(e.g. X1-Y1) . 50
Figure 108 – Test circuits for impulse durability of the current-limiting component
(e.g. X1-Y1) . 51
Figure 109 – Test circuits for insertion loss . 52
Figure 110 – Test circuit for return loss . 53
Figure 111 – Test circuits for longitudinal balance. 54
Figure 112 – Test circuit for near-end crosstalk . 56
IEC 61643-21:2025 © IEC 2025
Figure 113 – Examples for a test arrangement of the rated load current test I . 61
L
Figure 114 – Test circuits for total discharge current . 63
Figure 115 – Examples of multimode SPDs with a common voltage-limiting component . 64
Figure BB.1 – Differential Mode U measurement of Figure 100 SPDs . 66
p
Figure BB.2 – ITU-T test setup for SPD Common Mode U measurement to C
p
connection . 67
Figure DD.1 – Test circuit for bit error ratio test . 70
Figure EE.1 – Measurement circuit of a channel link with up to two SPDs . 72
Figure HH.1 – Test circuits for impulse durability and voltage-limiting tests . 77
Figure HH.2 – Test circuit for impulse reset time testing two modes of protection
(common mode, X1 – C and X2 – C) simultaneously of SPDs with voltage-limiting
component or voltage-limiting and current-limiting component . 78
Figure HH.3 – Test circuit for AC durability testing two modes of protection (common
mode, X1 – C and X2 – C) simultaneously of SPDs with voltage-limiting component or
voltage-limiting and current-limiting component . 79
Figure HH.4 – Test circuits for AC durability of the current-limiting component . 80
Figure HH.5 – Test circuits for impulse durability of the current-limiting component . 81

Table 100 – Type test requirements . 25
Table 3 – Pass criteria for type tests . 29
Table 101 – Waveform tolerances combination wave . 30
Table 102 – Waveform tolerances impulse categories C3 and D2 . 31
Table 7 – Dielectric impulse withstand test voltages for SPD main circuits. 35
Table 9 – Dielectric AC test voltages for SPD main circuits . 36
Table 10 – Dielectric impulse withstand test voltages for separated circuits . 37
Table 12 – Dielectric AC test voltages for separated circuits . 38
Table 103 – Categories and minimum requirements for impulse durability . 41
Table 104 – Test currents for response time . 48
Table 105 – Typical parameters for Figure 110 . 52
Table 106 – Typical impedance values for longitudinal balance test . 55
Table 16 – Cross-sections of copper conductors for screw-type or screwless terminals . 57
Table 107 – Altitude correction factors for calculating the air clearances . 58
Table 108 – Rated impulse withstand voltages for air clearances for SPDs . 58
Table 21 – Air clearances for SPD main circuit(s) . 59
Table 109 – I current distribution . 63
Total
Table AA.1 – Application of annexes from IEC 61643-01 . 65
Table DD.1 – Test times for BER test . 69
Table EE.1 – List of abbreviated terms . 71
Table EE.2 – Typical Tests for an SPD . 71
Table FF.1 – SPDs parameter for typical threats . 74
Table GG.1 – Source voltages and currents for impulse reset test . 75
Table II.1 – Reduced test procedure for SPDs complying with IEC 61643-21:2012 . 82

IEC 61643-21:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Low-voltage surge protective devices -
Part 21: Surge protective devices connected to telecommunications
and signalling networks - Requirements and test methods

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
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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 61643-21 has been prepared by subcommittee 37A: Low-voltage surge protective devices,
of IEC technical committee 37: Surge arresters. It is an International Standard.
This second edition cancels and replaces the first edition published in 2000, Amendment1:2008
and Amendment 2:2012. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) New structure of IEC 61643-21 based on IEC 61643-01:2024;
b) Several safety requirements based on IEC 61643-01:2024 have been added.
IEC 61643-21:2025 © IEC 2025
This International Standard is to be used in conjunction with IEC 61643-01:2024.
The requirements of this part of IEC 61643 supplement, modify or replace certain of the general
requirements contained in IEC 61643-01 and are to be read and applied together with
IEC 61643-01:2024.
Numbering of clauses follows the numbering of IEC 61643-01, but, dependent on the application
of clauses from part 01, does not necessarily follow sequentially.
If a clause in IEC 61643-01 is not explicitly called up or referred to in this document, then this
clause does not apply to SPDs covered by this document. Any instructions in this document
calling up clauses from IEC 61643-01 are written in Italic type.
NOTE In other words, if e.g. Clause 4 is called up in this document all subclauses of Clause 4 of IEC 61643-01 are
applied without modification. But, if e.g. some modifications are required on subclauses of Clause 9 of IEC 61643-
01, then the relevant second level subclauses of IEC 61643-01 (e.g. 9.3, 9.5 etc.) are called up separately and it is
indicated how they are applied.
The numbering of additional subclauses, figures and tables to IEC 61643-01 in this document
starts with the number 100 in the last section of the subclauses added (e.g. 4.100 or 6.2.100).
The numbering of additional tables and figures to IEC 61643-01 in this document starts with the
number 100.
The text of this International Standard is based on the following documents:
Draft Report on voting
37A/432/FDIS 37A/445/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.
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 Low-voltage surge
protective devices, 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.
IEC 61643-21:2025 © IEC 2025
INTRODUCTION
It has been assumed in the drafting of this International Standard that the execution of its
provisions is entrusted to appropriately qualified and experienced persons.
This document recognizes the internationally accepted level of protection against hazards such
as electrical, mechanical, thermal, fire and radiation of SPDs when operated as in normal use
taking into account the manufacturer's instructions.
The purpose of this document is to identify the requirements for Surge Protective Devices
(SPDs) used in protecting telecommunications and signalling networks, for example, low-
voltage data, voice, and alarm circuits.
If the intended applications of an SPD are covered by different parts of the IEC 61643-X1 series,
all relevant parts are applied.
This part of the IEC 61643 series addresses safety and performance tests for surge protective
devices (SPDs) for telecommunications and signalling networks in conjunction with
IEC 61643-01:2024.
This document addresses SPDs containing voltage-limiting components only, or a combination
of voltage-limiting components and current-limiting components. Protection devices containing
current-limiting components only are not within the coverage of this document.
This document covers a wide range of testing conditions and requirements; the use of some of
these is at the discretion of the user. How the requirements of this document relate to the
different types of SPD is described in Table 100.
Selection and application principles are covered in IEC 61643-22.

IEC 61643-21:2025 © IEC 2025
1 Scope
This document, together with IEC 61643-01:2024, is applicable to devices for surge protection
against indirect and direct effects of lightning or other transient overvoltages.
These devices are intended to be connected to telecommunications and signalling networks,
and equipment rated up to 1 000 V RMS and 1 500 V DC.
These telecommunications and signalling networks can also provide power on the same line,
e.g. Power over Ethernet (PoE).
Performance and safety requirements, tests and ratings are specified in this document. These
devices contain at least one voltage-limiting component (clamping or switching) and are
intended to limit surge voltages and divert surge currents.
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 61643-01:2024, Low-voltage surge protective devices - Part 01: General requirements and
test methods
3 Terms and definitions
Clause 3 of IEC 61643-01:2024 applies with the following additions, modifications and
replacements:
3.1 Terms and definitions
Subclause 3.1 of IEC 61643-01:2024 applies with the following additions, modifications and
replacements:
3.1.8
continuous current
I
C
Subclause 3.1.8 of IEC 61643-01:2024 applies with the following replacement:
Replace U by U .
test C
3.1.25
means for short- circuiting the SPD
SC-means
Subclause 3.1.25 of IEC 61643-01:2024 applies with the following replacement:
Replace short-circuit current rating I by n times I .
SCCR L
3.1.28
mode of protection
Subclause 3.1.28 of IEC 61643-01:2024 applies with the following addition:
Add in Note 2 the examples "line to earth and shield to earth".
IEC 61643-21:2025 © IEC 2025
3.1.64
test voltage
U
test
Subclause 3.1.64 of IEC 61643-01:2024 does not apply.
3.1.100
AC durability
characteristic of an SPD which allows it to conduct alternating current of a specific magnitude
and duration for a specified number of times
3.1.101
blind spot
situation where voltages above the maximum continuous operating voltage U cause
C
incomplete operation of the multi-stage SPD
Note 1 to entry: Incomplete operation of the SPD means not all current paths in a multi-stage SPD have operated
during the impulse test.
3.1.102
Current-limiting
action of an SPD, containing at least one non-linear current-limiting component, that causes
currents exceeding a predetermined value to be restricted
3.1.103
current reset time
time required for a self-resetting current-limiting component to revert to its normal or quiescent
state
3.1.104
current response time
time required for a current-limiting component to operate at a specified current and a specified
temperature
3.1.105
impulse durability
characteristic of an SPD which allows it to conduct impulse current of a specified waveform and
peak value for a specified number of times
3.1.106
insertion loss
loss resulting from the insertion of an SPD into a transmission system
Note 1 to entry: It is the ratio of the power delivered to that part of the system following the SPD, before insertion
of the SPD, to the power delivered to that same part after insertion of the SPD. The insertion loss is generally
expressed in decibels.
[SOURCE: IEC 60050-726:1982, 726-06-07, modified – "network" replaced by "SPD", second
part of definition converted to note to entry]
3.1.107
insulation resistance based on the continuous current
resistance between designated connections of an SPD when U is applied to those connections
C
3.1.108
longitudinal balance (analogue voice frequency circuits)
electrical symmetry of the two wires comprising a pair with respect to ground
IEC 61643-21:2025 © IEC 2025
3.1.109
longitudinal balance (communication and control cables)
ratio of the disturbing common mode (longitudinal) RMS voltage (V ) to ground and the resulting
s
differential mode (metallic) RMS voltage (V ) of the SPD, expressed in decibels (dB)
m
Note 1 to entry: The longitudinal balance in dB is given by the formula: 20 log10 V /V where V and V are
s m s m
measured at the same frequency.
3.1.110
longitudinal balance (data transmission)
measure of the similarity of impedance to ground (or common) for the two or more conductors
of a balanced circuit
Note 1 to entry: This term is used to express the degree of susceptibility to common mode interference.
3.1.111
longitudinal balance (telecommunications)
ratio of the disturbing common mode (longitudinal) voltage V and the resulting differential mode
s
(metallic) voltage V of the SPD, expressed in decibels (dB)
m
3.1.112
maximum interrupting voltage
maximum voltage (DC or RMS) that can be applied to the current-limiting components of an
SPD without degradation of the SPD
3.1.113
multi-stage SPD
SPD which has more than one parallel current path of voltage-limiting component
Note 1 to entry: These voltage-limiting components may or may not be electrically separated by a series component.
The voltage-limiting components may be either switching or clamping types.
3.1.114
near-end crosstalk (NEXT)
crosstalk that is propagated in a disturbed channel in the direction opposite to the direction of
propagation of the current in the disturbing channel
Note 1 to entry: The terminal of the disturbed channel at which the near-end crosstalk is present is ordinarily near
to, or coincides with, the energized terminal of the disturbing channel.
3.1.115
overstressed fault mode
3.1.115.1
mode 1
condition wherein the voltage-limiting part of the SPD has been disconnected
Note 1 to entry: The voltage-limiting function is no longer present, but the line is still operable.
3.1.115.2
mode 2
condition wherein the voltage-limiting part of the SPD has been short-circuited by a very low
impedance within the SPD
Note 1 to entry: The line is inoperable, but the equipment is still protected by a short circuit.
IEC 61643-21:2025 © IEC 2025
3.1.115.3
mode 3
situation wherein the SPD has undergone an internal open-circuit in the line, or an external
disconnector has operated, if any
Note 1 to entry: The line is inoperable, but the equipment is still protected by an open line.
3.1.116
resettable current-limiting
action of an SPD that limits current and can be manually reset after operating
3.1.117
return loss
modulus of the reciprocal of the reflection factor, generally expressed in decibels (dB)
Note 1 to entry: When impedances can be defined, the return loss in dB is given by the formula:
20 log MOD [(Z +Z )/(Z –Z )]
10 1 2 1 2
where Z is the characteristic impedance of the transmission line ahead of the discontinuity, or the impedance of the
source, and Z is the impedance after the discontinuity or load impedance seen from the junction between the source
and the load.
[SOURCE: IEC 60050-702:1992, 702-07-25, modified – modification of formula]
3.1.118
self-resetting current-limiting
action of an SPD that limits current and will self-reset after the disturbing current is removed
4 Classification
Clause 4 of IEC 61643-01:2024 applies with the following additions and modifications:
4.1 General
Subclause 4.1 of IEC 61643-01:2024 applies with the following replacement:
In this document, the term "voltage clamping” replaces the term "voltage limiting” from
IEC 61643-01.
4.5 Type of SPD
Subclause 4.5 of IEC 61643-01:2024 does not apply.
4.8 Mounting method
Subclause 4.8 of IEC 61643-01:2024 does not apply.
4.13 End-of-life mode of the SPDA
Subclause 4.13 of IEC 61643-01:2024 does not apply.
4.14 Short-circuit protection function for OCM end-of-life mode
Subclause 4.14 of IEC 61643-01:2024 does not apply.
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