EN IEC 61643-21:2026

SLOVENSKI STANDARD
oSIST prEN IEC 61643-21:2024
01-september-2024
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 testing 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
télécommunications - Prescriptions de fonctionnement et méthodes d\'essais
Ta slovenski standard je istoveten z: prEN IEC 61643-21:2024
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
oSIST prEN IEC 61643-21:2024 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

oSIST prEN IEC 61643-21:2024
oSIST prEN IEC 61643-21:2024
37A/414/CDV
COMMITTEE DRAFT FOR VOTE (CDV)

PROJECT NUMBER:
IEC 61643-21 ED2
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2024-07-19 2024-10-11
SUPERSEDES DOCUMENTS:
37A/399/CD, 37A/411/CC
IEC SC 37A : LOW-VOLTAGE SURGE PROTECTIVE DEVICES
SECRETARIAT: SECRETARY:
United States of America Mr David Richmond
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:

TC 37
Other TC/SCs are requested to indicate their interest, if
any, in this CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some
Countries” clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is the
final stage for submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Low voltage surge protective devices - Part 21: Surge protective devices connected to
telecommunications and signalling networks - Requirements and testing methods

PROPOSED STABILITY DATE: 2029
NOTE FROM TC/SC OFFICERS:
download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National
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for any other purpose without permission in writing from IEC.

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1 CONTENTS
3 FOREWORD . 8
4 INTRODUCTION . 10
5 1 Scope . 11
6 2 Normative references . 11
7 3 Terms, definitions and abbreviated terms . 11
8 3.1 Terms and definitions. 11
9 4 Classification . 14
10 4.1 General . 14
11 4.5 Type of SPD . 14
12 4.8 Mounting method . 14
13 4.13 End of life mode of the SPDA . 14
14 4.14 Short-Circuit protection function for OCM end of life mode . 14
15 4.100 SPD configurations . 14
16 4.101 Overstressed fault mode acc. 3.1.115 . 15
17 4.101.1 Mode 1 . 15
18 4.101.2 Mode 2 . 15
19 4.101.3 Mode 3 . 15
20 4.102 Additional information about transmission and special applications . 15
21 5 Void . 15
22 6 Marking and other product information . 15
23 6.1 General . 15
24 6.2 List of items . 16
25 6.2.100 Information to be provided by the manufacturer . 17
26 6.2.101 Information which shall be provided by the manufacturer for type
27 testing, as applicable . 17
28 6.3 Information about status indicator . 17
29 7 Service conditions . 17
30 8 Requirements . 18
31 8.1 General requirements . 18
32 8.3 Electrical requirements . 18
33 8.3.1 Protection against electric shock . 18
34 8.3.2 Continuous current I . 18
C
35 8.3.3 Protective conductor current I . 18
PE
36 8.3.4 Measured limiting voltage . 18
37 8.3.5 Operating duty . 19
38 8.3.6 Safety performance of overstressed SPDs . 19
39 8.3.7 Insulation resistance . 19
40 8.3.8 Dielectric withstand . 20
41 8.3.9 Behaviour under temporary overvoltages . 20
42 8.3.100 Impulse durability and voltage protection level . 20
43 8.3.101 Impulse reset . 20
44 8.3.102 AC durability, if declared by the manufacturer . 20
45 8.3.103 Blind spot . 21

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46 8.3.104 Series resistance . 21
47 8.3.105 Current-limiting requirements . 21
48 8.3.106 Transmission requirements . 22
49 8.4 Mechanical requirements . 22
50 8.4.1 General . 22
51 8.4.5 Mechanical strength . 22
52 8.5 Environmental and material requirements . 22
53 8.5.2 Heat resistance . 22
54 8.5.4 Tracking resistance . 23
55 8.5.5 Ageing behaviour under damp heat . 23
56 8.6 Additional requirements for specific SPD designs . 23
57 8.6.1 Two port SPDs and one port SPDs with separate input/output
58 connections . 23
59 8.6.3 SPDs with electrically separated circuits . 23
60 8.6.4 Total discharge current I , if declared by the manufacturer . 23
Total
61 8.6.5 Two port SPDs . 23
62 8.6.6 Short-circuiting SPDs . 23
63 9 Tests . 23
64 9.1 Type testing procedures. 24
65 9.1.1 General . 24
66 9.1.2 Impulse discharge current. 29
67 9.1.3 8/20 Current impulse . 29
68 9.1.4 1,2/50 voltage impulse . 29
69 9.1.5 Combination wave . 29
70 9.1.100 Waveform tolerances of impulse categories C3 and D2 . 29
71 9.3 Electrical tests . 29
72 9.3.1 Protection against direct contact . 30
73 9.3.2 Continuous current I . 30
C
74 9.3.3 Protective conductor current IPE . 30
75 9.3.4 Measured limiting voltage . 30
76 9.3.5 Operating duty test . 30
77 9.3.6 Safety performance of overstressed SPDs . 30
78 9.3.8 Dielectric withstand . 33
79 9.3.9 Behaviour under temporary overvoltages (TOV) . 36
80 9.3.100 Impulse durability and voltage protection level . 36
81 9.3.101 Impulse reset . 39
82 9.3.102 AC durability . 41
83 9.3.103 Blind spot test . 42
84 9.3.104 Series resistance . 43
85 9.3.105 Current-limiting tests . 44
86 9.3.106 Transmission tests . 47
87 9.4 Mechanical tests . 51
88 9.4.3 Tests for external connections for copper conductors . 51
89 9.4.4 Verification of air clearances and creepage distances . 51
90 9.5 Environmental and material tests . 53
91 9.5.2 Heat resistance . 53
92 9.5.4 Tracking resistance . 54

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93 9.5.5 Life test under damp heat . 54
94 9.6 Additional tests for specific SPD designs . 54
95 9.6.1 Tests for two-port SPDs and one-port SPDs with separate input/output
96 connections . 54
97 9.6.3 SPDs with separated circuits . 56
98 9.6.4 Total discharge current test for multimode SPDs . 56
99 9.6.5 Tests for two port SPDs only . 58
100 Annex A (normative) Application of annexes from IEC 61643-01 . 59
101 Annex B (informative) Example configurations for measuring U . 60
P
102 Annex C (informative) Special resistibility in telecommunication systems . 62
103 Annex D (informative) Bit error ratio (BER) . 63
104 Annex E (informative)  Example of transmission test methods for SPDs connected to
105 LAN . 65
106 E.1 General . 65
107 E.2 Abbreviations . 65
108 E.3 Requirements . 65
109 E.4 Test methods . 66
110 E.4.1 Measurement circuit . 66
111 E.4.2 Insertion loss . 66
112 E.4.3 NEXT, power sum NEXT . 66
113 E.4.4 Return loss . 67
114 Annex F (normative) Variants of SPDs . 68
115 Annex G (informative) Typical threat parameters for SPDs . 69
116 Annex H (informative) Examples of source voltages and currents for impulse reset test . 70
117 Annex I (informative) Test circuit examples for testing more than one mode of
118 protection simultaneously . 71
119 I.1 General . 71
120 I.2 Impulse durability and voltage protection level . 71
121 I.3 Impulse reset time . 72
122 I.4 AC durability . 73
123 I.5 AC durability for current limiting function . 73
124 I.6 Impulse durability for current limiting function . 74
125 Annex J (normative) Reduced test procedures . 76
126 Bibliography . 79
128 Figure 1a – Two- connection SPD . 14
129 Figure 1b – Three- connection SPD Figure 1c – Three- connection SPD . 14
130 Figure 1d – Four- connection SPD Figure 1e – Five- connection SPD. 15
131 Figure 1f – Multi- connection SPD . 15
132 Figure 1 – SPD configurations . 15
133 Figure 2 – Example of U-I-diagram providing three value pairs of voltage and current
134 for impulse reset test (U /I , U /I and U /I ) with U =U and e.g. I =I . 16
1 1 2 2 3 3 1 C 3 L
135 Figure 3 – Procedure for sample preparation for dedicated overstress test . 32
136 Figure 4a: Test circuit for common mode (e.g. X1 – C) of SPDs with voltage-
137 limiting component only, having no separate output connections . 37

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138 Figure 4b: Test circuit for common mode (e.g. X1 – C) of SPDs with voltage-
139 limiting component or voltage-limiting and current-limiting component, having separate
140 input and output connections . 37
141 Figure 4c: Test circuit for differential mode (e.g. X1 – X2) of SPDs with voltage-
142 limiting component only, having no separate output connections . 37
143 Figure 4d: Test circuit for differential mode (e.g. X1 – X2) of SPD with voltage-
144 limiting component or voltage-limiting and current-limiting component, having separate
145 input and output connections . 38
146 Figure 4 – Test circuits for impulse durability and voltage- protection level . 38
147 Figure 5a: Test circuit for common mode (e.g. X1 – C) of SPDs with voltage-
148 limiting component or voltage-limiting and current-limiting component . 40
149 Figure 5b: Test circuit for differential mode (e.g. X1 – X2) of SPD with voltage-
150 limiting component or voltage-limiting and current-limiting component . 40
151 Figure 5 – Test circuits for impulse reset time . 40
152 Figure 6a: Test circuit for common mode (e.g. X1 – C) of SPDs with voltage-
153 limiting component or voltage-limiting and current-limiting component . 41
154 Figure 6b: Test circuit for differential mode (e.g. X1 – X2) of SPD with voltage-
155 limiting component or voltage-limiting and current-limiting component . 42
156 Figure 6 – Test circuits for AC durability . 42
157 Figure 7 – Test circuits for series resistance, current response time, current reset
158 time, maximum interrupting voltage and operating duty test . 44
159 Figure 8 – Test circuits for AC durability of the current-limiting component (e.g. X1–Y1) . 46
160 Figure 9 – Test circuits for impulse durability of the current-limiting component (e.g.
161 X1-Y1) . 46
162 Figure 10 – Test circuits for insertion loss . 47
163 Figure 11 – Test circuit for return loss . 48
164 Figure 12 – Test circuits for longitudinal balance . 49
165 Figure 13 – Test circuit for near-end crosstalk . 50
166 Figure 14a – Example for a test arrangement for three- connection SPDs of the rated
167 load current I at line to earth (asymmetrical) configuration . 54
L
168 Figure 14b – Example for a test arrangement for five and multi- connection SPDs of
169 the rated load current I at line to earth (asymmetrical) configuration . 55
L
170 Figure 14c – Example for a test arrangement for four- connection SPDs of the rated
171 load current I at line to line (symmetrical) configuration . 55
L
172 Figure 14 – Examples for a test arrangement of the rated load current test I . 55
L
173 Figure 15 – Test circuits for Total discharge current . 57
174 Figure 16 – Examples of multimode SPDs with a common protective component . 58
175 Figure B.1 – Differential Mode U measurement of Figure 1 SPDs. 60
p
176 Figure B.2 – ITU-T test setup for SPD Common Mode U measurement to C terminal . 61
p
177 Figure D.1 – Test circuit for bit error ratio test . 64
178 Figure E.1 – Measurement circuit of a channel link with up to two SPDs . 66
179 Figure I.1a: Test circuit for testing two modes of protection (common mode, X1 – C
180 and X2 – C) simultaneously of SPDs with voltage-limiting component only, having no
181 separate output connections . 71

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182 Figure I.1b: Test circuit for testing two modes of protection (common mode, X1 – C
183 and X2 – C) simultaneously of SPDs with voltage-limiting component or voltage-
184 limiting and current-limiting component, having separate input and output connections . 72
185 Figure I.1: Test circuits for impulse durability and voltage-limiting tests . 72
186 Figure I.2: Test circuit for impulse reset time testing two modes of protection
187 (common mode, X1 – C and X2 – C) simultaneously of SPDs with voltage-limiting
188 component or voltage-limiting and current-limiting component . 72
189 Figure I.3: Test circuit for AC durability testing two modes of protection (common
190 mode, X1 – C and X2 – C) simultaneously of SPDs with voltage-limiting component or
191 voltage-limiting and current-limiting component . 73
192 Figure I.4a: Test circuit for testing two current paths between connections line
193 (input) and protected line (output) (X1 – Y1 and X2 – Y2) simultaneously . 73
194 Figure I.4b: Test circuit for testing two current paths between connections line
195 (input) and protected line (output) (X1 – Y1 and X2 – Y2) simultaneously . 74
196 Figure I.4c: Test circuit for testing four current paths between connections line
197 (input) and protected line (output) (X1 – Y1, X2 – Y2, X3 – Y3 and X4 – Y4)
198 simultaneously 74
199 Figure I.4: Test circuits for AC durability of the current-limiting component . 74
200 Figure I.5a: Test circuit for testing two current paths between connections line
201 (input) and protected line (output) (X1 – Y1 and X2 – Y2) simultaneously . 75
202 Figure I.5b: Test circuit for testing two current paths between connections line
203 (input) and protected line (output) (X1 – Y1 and X2 – Y2) simultaneously . 75
204 Figure I.5: Test circuits for impulse durability of the current-limiting component . 75
206 Table 1 – Type test requirements . 25
207 Table 2 – Pass criteria for type tests . 28
208 Table 3 – Waveform tolerances combination wave . 29
209 Table 4 – Waveform tolerances impulse categories C3 and D2 . 29
210 Table 5 – Dielectric impulse withstand test voltages for SPD main circuits . 33
211 Table 6 – Dielectric AC test voltages for SPD main circuits . 34
212 Table 7 – Dielectric impulse withstand test voltages for separated circuits . 34
213 Table 8 – Dielectric AC test voltages for separated circuits . 36
214 Table 9 – Categories and minimum requirements for impulse durability . 39
215 Table 10 – Test currents for response time . 44
216 Table 11 – Typical parameters for Figure 8 and Figure 9 . 47
217 Table 12 – Typical impedance values for longitudinal balance test . 50
218 Table 13 – Cross-sections of copper conductors for screw-type or screwless terminals . 51
219 Table 14 – Altitude correction factors for calculating the air clearances . 52
220 Table 15 – Rated impulse withstand voltages for air clearances for SPDs . 52
221 Table 16 – Air clearances for SPD main circuit(s) . 53
222 Table 17 – I current distribution . 57
Total
223 Table A.1 – Application of annexes from IEC 61643-01 . 59
224 Table D.1 – Test times for BER test . 63
225 Table E.1 – List of Abbreviations . 65
226 Table E.2 – Typical Tests for an SPD . 65

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227 Table G.1 – SPDs parameter for typical threats . 69
228 Table H.1 – Source voltages and currents for impulse reset test . 70
229 Table J.1 – Reduced test procedure for SPDs complying with IEC 61643-21:2012 . 76
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231 INTERNATIONAL ELECTROTECHNICAL COMMISSION
232 ____________
234 LOW-VOLTAGE SURGE PROTECTIVE DEVICES –
236 Part 21: Surge protective devices connected to telecommunications
237 and signalling networks –
238 Requirements and test methods
242 FOREWORD
243 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
244 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
245 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
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252 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
253 consensus of opinion on the relevant subjects since each technical committee has representation from all
254 interested IEC National Committees.
255 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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258 misinterpretation by any end user.
259 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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262 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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264 services carried out by independent certification bodies.
265 6) All users should ensure that they have the latest edition of this publication.
266 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
267 members of its technical committees and IEC National Committees for any personal injury, property damage or
268 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
269 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
270 Publications.
271 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
272 indispensable for the correct application of this publication.
273 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
274 rights. IEC shall not be held responsible for identifying any or all such patent rights.
275 International Standard IEC 61643-21 has been prepared by subcommittee 37A: Low-voltage
276 surge protective devices, of IEC technical committee 37: Surge arresters.
277 This second edition cancels and replaces the first edition including the amendments published
278 in 2000, 2008 and 2012. This edition constitutes a technical revision.
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280 This edition includes the following significant technical changes with respect to the previous
281 edition:
282 a) New structure of IEC 61643-21 based on IEC 61643-01 Ed.1
283 b) Several safety requirements based on IEC 61643-01 Ed.1 has been added.
285 The text of this International Standard is based on the following documents:
FDIS Report on voting
XX/XX/FDIS XX/XX/RVD
287 Full information on the voting for the approval of this International Standard can be found in the
288 report on voting indicated in the above table.
289 This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
290 The committee has decided that the contents of this document will remain unchanged until the
291 stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
292 the specific document. At this date, the document will be
293 • reconfirmed,
294 • withdrawn,
295 • replaced by a revised edition, or
296 • amended.
298 The National Committees are requested to note that for this document the stability date is
299 THIS TEXT IS INCLUDED FOR THE INFORMATION OF THE NATIONAL COMMITTEES AND WILL BE DELETED
300 AT THE PUBLICATION STAGE.
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302 INTRODUCTION
303 It has been assumed in the drafting of this International Standard that the execution of its
304 provisions is entrusted to appropriately qualified and experienced persons.
305 This standard recognizes the internationally accepted level of protection against hazards such
306 as electrical, mechanical, thermal, fire and radiation of SPDs when operated as in normal use
307 taking into account the manufacturer's instructions.
308 The purpose of this International Standard is to identify the requirements for Surge Protective
309 Devices (SPDs) used in protecting telecommunication and signalling systems, for example, low-
310 voltage data, voice, and alarm circuits.
311 If the intended applications of an SPD are covered by different parts of the IEC 61643-X1 (X =
312 1,2,3,4, etc.) series, all relevant parts shall be applied, as far as is reasonable.
313 NOTE 1: Throughout this publication, when “part 01” is mentioned, it refers to IEC 61643-01:2024.
314 This part of the IEC 61643 series addresses safety and performance tests for surge protective
315 devices (SPDs) for telecommunication and signalling systems in conjunction with part 01.
316 The SPDs addressed in this International Standard contain surge protective components only,
317 or a combination of surge protective and current-limiting components. Protection devices
318 containing current-limiting components only are not within the coverage of this standard.
319 SPDs contain one or more protective components. All SPDs are tested on a "black box" basis,
320 i.e., the number of connections of the SPD determine the testing procedure, not the number of
321 components in the SPD. The SPD configurations are described in 4.100.
322 This standard covers a wide range of testing conditions and requirements; the use of some of
323 these is at the discretion of the user. How the requirements of this standard relate to the
324 different types of SPD is described in Table 1.
325 The requirements of this part of IEC 61643 supplement, modify or replace certain of the general
326 requirements contained in part 01 and are to be read and applied together with the latest edition
327 of IEC 61643-01, as indicated by the undated normative reference in the normative references
328 of this document.
329 Numbering of clauses follows the numbering of part 01, but, dependent on the application of
330 clauses from part 01, does not necessarily follow sequentially.
331 If a clause in part 01 is not explicitly called up or referred to in this part 21, then this clause
332 does not apply to SPDs covered by this part 21. Any instructions in this standard calling up
333 clauses from part 01 are written in Italic type.
334 NOTE 2: In other words, if e.g. clause 4 is called up in this document all subclauses of clause 4 of part 01 are applied
335 without modification. But, if e.g. some modifications are required on subclauses of clause 9 of part 01, then the
336 relevant second level subclauses of part 01 (e.g. 9.3, 9.5 etc.) are called up separately and it is indicated how they
337 are applied.
338 The numbering of additional subclauses to part 01 in this document starts with the number 100
339 in the last section of the subclauses added (e.g. 4.100 or 6.2.100).
340 Selection and application principles are covered in IEC 61643-22.
341 A list of all parts of the IEC 61643 series can be found, under the general title Low-voltage
342 surge protective devices, on the IEC website.

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343 LOW-VOLTAGE SURGE PROTECTIVE DEVICES –
345 Part 21: Surge protective devices connected
346 to telecommunications
347 and signalling networks –
348 Requirements and testing methods
350 1 Scope
351 This part of the IEC 61643 series is applicable to devices for surge protection against indirect
352 and direct effects of lightning or other transient overvoltages.
353 These devices are intended to be connected to telecommunications and signalling networks,
354 and equipment rated up to 1 000 V RMS and 1 500 V DC.
355 These telecommunications and signalling networks may also provide power on the same line,
356 e.g Power over Ethernet (PoE).
357 Performance and safety requirements, tests and ratings are specified in this standard. These
358 devices contain at least one voltage-limiting component (clamping or switching) and are
359 intended to limit surge voltages and divert surge currents.
360 2 Normative references
361 For the purposes of this document normative reference given in part 01 apply.
362 The following documents are referred to in the text in such a way that some or all of their content
363 constitutes requirements of this document. For dated references, only the edition cited applies.
364 For undated references, the latest edition of the referenced document (including any
365 amendments) applies.
366 IEC 61643-01:2023, Low-voltage surge protective devices – Part 01: General requirements and
367 test methods
368 3 Terms, definitions and abbreviated terms
369 Clause 3 from part 01 applies with the following additions:
370 3.1 Terms and definitions
371 Clause 3.1 from part 01 applies with the following additions:
372 3.1.25
373 means for short- circuiting the SPD (SC-means)
374 Clause 3.1.25 from part 01 applies with the following replacement:
375 Replace short-circuit current rating ISCCR by n times IL
376 3.1.28
377 mode of protection
378 Clause 3.1.28 from part 01 applies with the following addition:
379 Add in Note 2 the examples “line to earth and shield to earth ”.

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380 3.1.100
381 AC durability
382 characteristic of an SPD which allows it to conduct alternating current of a specific magnitude
383 and duration for a specified number of times
384 3.1.101
385 blind spot
386 situation where voltages above the maximum continuous operating voltage U causes
c
387 incomplete operation of the SPD
388 Note 1 to entry: Incomplete operation of the SPD means not all current paths in a multi-stage SPD have operated
389 during the impulse test. This results in overstressing of components in the SPD.
390 3.1.102
391 current limiting
392 action of an SPD, containing at least one non-linear current-limiting component, that causes
393 currents exceeding a predetermined value to be restricted
394 3.1.103
395 current reset time
396 time required for a self-resetting current limiting component to revert to its normal or quiescent
397 state
398 3.1.104
399 current response time
400 time required for a current-limiting component to operate at a specified current and a specified
401 temperature
402 3.1.105
403 impulse durability
404 characteristic of an SPD which allows it to conduct impulse current of a specified waveform and
405 peak value for a specified number of times
406 3.1.106
407 insertion loss
408 loss resulting from the insertion of an SPD into a transmission system
409 Note 1 to entry: It is the ratio of the power delivered to that part of the system following the SPD, before insertion of
410 the SPD, to the power delivered to that same part after insertion of the SPD. The insertion loss is generally expressed
411 in decibels.
412 [IEV 726-06-07, modified]
413 3.1.107
414 insulation resistance based on the continuous current
415 resistance between designated connections of an SPD when U is applied to those connections
C
416 3.1.108
417 longitudinal balance (analogue voice frequency circuits)
418 electrical symmetry of the two wires comprising a pair with respect to ground
419 3.1.109
420 longitudinal balance (communication and control cables)
421 ratio of the disturbing common mode (longitudinal) RMS voltage (Vs) to ground and the resulting
422 differential mode (metallic) RMS voltage (Vm) of the SPD, expressed in decibels (dB)
423 Note 1 to entry: The longitudinal balance in dB is given by the formula: 20 log10 Vs/Vm where Vs and Vm are
424 measured at the same frequency.

oSIST prEN IEC 61643-21:2024
IEC CDV 61643-21/Ed2  IEC2024 – 13 – 37A/414/CDV

425 3.1.110
426 longitudinal balance (data transmission)
427 measure of the similarity of impedance to ground (or common) for the two or more conductors
428 of a balanced circuit
429 Note 1 to entry: This term is used to express the degree of susceptibility to common mode interference
430 3.1.111
431 longitudinal balance (telecommunications)
432 ratio of the disturbing common mode (longitudinal) voltage Vs and the resulting differential
433 mode (metallic) voltage Vm of the SPD, expressed in decibels (dB)
434 3.1.112
435 maximum interrupting voltage
436 maximum voltage (DC or RMS) that can be applied to the current-limiting components of an
437 SPD without degradation of the SPD
438 3.1.113
439 multi-stage SPD
440 SPD which has more than one parallel current path of voltage-limit
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