SIST-TP IEC TR 60725:2025

IEC/TR 60725 ®
Edition 3.0 2012-06
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Consideration of reference impedances and public supply network impedances
for use in determining the disturbance characteristics of electrical equipment
having a rated current ≤75 A per phase

Étude des impédances de référence et des impédances des réseaux publics
d’alimentation aux fins de la détermination des caractéristiques de perturbation
des équipements électriques utilisant un courant nominal ≤75 A par phase
IEC/TR 60725:2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester.
If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,
please contact the address below or your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite ni
utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les
microfilms, sans l'accord écrit de la CEI ou du Comité national de la CEI du pays du demandeur.
Si vous avez des questions sur le copyright de la CEI ou si vous désirez obtenir des droits supplémentaires sur cette
publication, utilisez les coordonnées ci-après ou contactez le Comité national de la CEI de votre pays de résidence.

IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé Fax: +41 22 919 03 00
CH-1211 Geneva 20 info@iec.ch
Switzerland www.iec.ch
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.

Useful links:
IEC publications search - www.iec.ch/searchpub Electropedia - www.electropedia.org
The advanced search enables you to find IEC publications The world's leading online dictionary of electronic and
by a variety of criteria (reference number, text, technical electrical terms containing more than 30 000 terms and
committee,…). definitions in English and French, with equivalent terms in
It also gives information on projects, replaced and additional languages. Also known as the International
withdrawn publications. Electrotechnical Vocabulary (IEV) on-line.

IEC Just Published - webstore.iec.ch/justpublished Customer Service Centre - webstore.iec.ch/csc
Stay up to date on all new IEC publications. Just Published If you wish to give us your feedback on this publication
details all new publications released. Available on-line and or need further assistance, please contact the
also once a month by email. Customer Service Centre: csc@iec.ch.

A propos de la CEI
La Commission Electrotechnique Internationale (CEI) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications CEI
Le contenu technique des publications de la CEI est constamment revu. Veuillez vous assurer que vous possédez
l’édition la plus récente, un corrigendum ou amendement peut avoir été publié.

Liens utiles:
Recherche de publications CEI - www.iec.ch/searchpub Electropedia - www.electropedia.org
La recherche avancée vous permet de trouver des Le premier dictionnaire en ligne au monde de termes
publications CEI en utilisant différents critères (numéro de électroniques et électriques. Il contient plus de 30 000
référence, texte, comité d’études,…). termes et définitions en anglais et en français, ainsi que
Elle donne aussi des informations sur les projets et les les termes équivalents dans les langues additionnelles.
publications remplacées ou retirées. Egalement appelé Vocabulaire Electrotechnique
International (VEI) en ligne.
Just Published CEI - webstore.iec.ch/justpublished
Service Clients - webstore.iec.ch/csc
Restez informé sur les nouvelles publications de la CEI.
Just Published détaille les nouvelles publications parues. Si vous désirez nous donner des commentaires sur
Disponible en ligne et aussi une fois par mois par email. cette publication ou si vous avez des questions
contactez-nous: csc@iec.ch.
IEC/TR 60725 ®
Edition 3.0 2012-06
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Consideration of reference impedances and public supply network impedances

for use in determining the disturbance characteristics of electrical equipment

having a rated current ≤75 A per phase

Étude des impédances de référence et des impédances des réseaux publics

d’alimentation aux fins de la détermination des caractéristiques de perturbation

des équipements électriques utilisant un courant nominal ≤75 A par phase

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX U
ICS 33.100.01 ISBN 978-2-83220-161-9

– 2 – TR 60725 © IEC:2012
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Systems of low-voltage supply . 6
3.1 Three-phase supply systems . 6
3.2 Single-phase two-wire supply systems . 7
3.3 Single-phase three-wire supply systems . 7
4 Supply impedances . 7
4.1 Typical residential premises . 7
4.2 Large residential, commercial and light industrial premises . 10
4.2.1 General . 10
4.2.2 Supply impedance relevant to the connection of three-phase
equipment . 11
4.2.3 Supply impedances relevant to the connection of single-phase
equipment . 11
5 Reference impedances . 12
5.1 General . 12
5.2 Reference impedances for equipment with current ratings ≤16 A . 12
5.2.1 Overview . 12
5.2.2 50 Hz and 60 Hz low-voltage supply systems . 13
5.3 Reference impedance for 50 Hz and 60 Hz equipment with current ratings
>16 A and ≤75 A per phase . 15
6 Impedance at frequencies above the supply frequency . 15
Annex A (informative) Methods for determining the maximum modulus values of public
electricity supply low-voltage network impedances relevant to three-phase services of
more than 100 A per phase at 50 Hz . 16
Annex B (informative) Methods for determining the maximum modulus values of public
electricity supply low-voltage network impedances relevant to three-phase services of
more than 100 A per phase at 60 Hz . 24
Annex C (informative) Measurement of supply impedance and survey method . 26
Bibliography . 29

Figure 1 – Representation of a single-phase three-wire supply system . 7
Figure A.1 – Model used for determining the impedance of a network line conductor
from a transformer to a three-phase service cut-out . 18
Figure A.2 – Three-phase impedance diagram of a typical 500 kVA transformer and
mains cable . 19
Figure B.1 – Model used for determining the impedance of a network line conductor
from a transformer to a three-phase service cut-out . 24
Figure C.1 – Measurement of impedance at a customer’s premises . 27

Table 1 – Residential consumers' complex supply impedances at 50 Hz . 8
Table 2 – Single-phase device capacities <100 A per phase . 9
Table 3 – Three-phase service capacities <100 A per phase . 9

TR 60725 © IEC:2012 – 3 –
Table 4 – Single or two-phase service capacities ≥100 A per phase. 10
Table 5 – Three-phase service capacities ≥100 A per phase . 10
Table 6 – Modulus values of supply impedance, in ohms at 50 Hz, relevant to the
connection of three-phase equipment and having a 95 % probability of not being
exceeded . 11
Table 7 – Modulus values of supply impedance, in ohms at 50 Hz, relevant to the
connection of single-phase equipment and having a 95 % probability of not being
exceeded . 12
Table 8 – Reference impedances for testing purposes . 13
Table 9 – Reference impedances for 100 V/200 V and 120 V/240 V supply systems
<100 A . 14
Table 10 – Reference impedances for 200 V to 240 V supply systems <100 A . 14
Table 11 – Reference impedances for 200 V to 240 V supply systems, ≥100 A per
phase . 15
Table 12 – Reference impedances for testing purposes, for 200 V to 240 V supply
systems, ≥100 A . 15
Table A.1 – Modulus values of the maximum supply impedance, in ohms, of the line-
conductors of 230 V/400 V, 50 Hz, public electricity supply networks, relevant to
three-phase services having service capacities of 200 A per phase . 21
Table A.2 – Modulus values of the maximum supply impedance, in ohms, of the line
and neutral conductors of 230 V/400 V, 50 Hz, public electricity supply networks,
relevant to three-phase services having service capacities of 200 A per phase . 22
Table C.1 – Impedance values for copper conductor installation wiring . 28

– 4 – TR 60725 © IEC:2012
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CONSIDERATION OF REFERENCE IMPEDANCES AND PUBLIC SUPPLY
NETWORK IMPEDANCES FOR USE IN DETERMINING THE DISTURBANCE
CHARACTERISTICS OF ELECTRICAL EQUIPMENT HAVING A RATED
CURRENT ≤75 A PER PHASE
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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60725, which is a technical report, has been prepared by subcommittee 77A: EMC – Low
frequency phenomena, of IEC technical committee 77: Electromagnetic compatibility. This
third edition cancels and replaces the second edition, published in 2005, and constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
• new survey and other data from countries having public supply networks operating at
60 Hz have been included;
• recommendations that were applicable to 50 Hz systems are now mirrored by new
recommendations that are relevant to 60 Hz systems.

TR 60725 © IEC:2012 – 5 –
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
77A/784/DTR 77A/789/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – TR 60725 © IEC:2012
CONSIDERATION OF REFERENCE IMPEDANCES AND PUBLIC SUPPLY
NETWORK IMPEDANCES FOR USE IN DETERMINING THE DISTURBANCE
CHARACTERISTICS OF ELECTRICAL EQUIPMENT HAVING A RATED
CURRENT ≤75 A PER PHASE
1 Scope
This Technical Report records the information that was available and the factors that were
taken into account in arriving at the reference impedances that were incorporated in
IEC 60555 and which are now incorporated in some parts of IEC 61000-3.
In addition, information is given on the impedances of public supply networks associated with
service current capacities ≥100 A per phase.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61000-3-3, Electromagnetic compatibility (EMC) – Part 3-3: Limits – Limitation of
voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for
equipment with rated current ≤16 A per phase and not subject to conditional connection
IEC 61000-3-11, Electromagnetic compatibility (EMC) – Part 3-11: Limits – Limitation of
voltage changes, voltage fluctuations and flicker in public low-voltage supply systems –
Equipment with rated current ≤75 A and subject to conditional connection
IEC 61000-3-12, Electromagnetic compatibility (EMC) – Part 3-12: Limits – Limits for
harmonic currents produced by equipment connected to public low-voltage systems with input
current >16 A and ≤75 A per phase
3 Systems of low-voltage supply
3.1 Three-phase supply systems
Three-phase, four-wire, distribution systems are used worldwide to supply low-voltage
consumers with nominal voltages in the region of 230 V/400 V.
To conform with IEC standard voltages, these systems are described as 230 V/400 V
throughout this report.
There is considerable variation in the way in which the supplies to individual consumers are
connected to three-phase systems.
In some countries, all four wires are taken into the consumer’s premises, allowing the use of
three-phase 400 V for large loads, with small appliances and lighting circuits connected
between one line and neutral at 230 V.

TR 60725 © IEC:2012 – 7 –
In other countries, three wires are taken into the consumer’s premises, allowing the use of
400 V across two phases for large loads, with small appliances and lighting circuits connected
between one line and neutral at 230 V.
In other countries, of which the United Kingdom is an example, it is unusual to take more than
one phase into a residential consumer’s premises. Therefore, both large loads that are less
than 15 kVA and lighting circuits are supplied between line and neutral at 230 V.
3.2 Single-phase two-wire supply systems
In the rural areas of most countries, it is common to connect the winding of distribution
transformers across two phases of medium voltage systems and afford supplies to low-
voltage consumers via a phase and return conductor. A wide range of voltage is associated
with this type of supply system.
In Korea, there are extensive networks supplying single-phase two-wire connections at 220 V.
3.3 Single-phase three-wire supply systems
In some countries, of which the United States of America is an example, a single-phase,
three-wire distribution is used. Large loads are connected across the outer wires at 240 V
whilst small appliances and lighting circuits are connected between one outer and the centre
wire at 120 V, as shown in Figure 1.

Phase conductor
100 V or
120 V
200 V or
Neutral
240 V
100 V or
120 V
Phase conductor
IEC  1178/12
Figure 1 – Representation of a single-phase three-wire supply system
In North America, distribution systems use smaller size transformers, each supplying 4 to
8 customers with shorter secondary (LV) feeder lengths. In Japan the nominal supply voltages
are 100 V and 200 V.
These supply systems have quite different supply impedances from those of three-phase
distribution systems and might require a different reference impedance for testing equipment
having a rated voltage within the range 100 V to 125 V.
4 Supply impedances
4.1 Typical residential premises
The supply system impedance associated with the supply to the premises of a typical
residential consumer, is determined by the average value of maximum power demand of all
the consumers connected to a typical network and the steady state voltage drop at maximum
load used to design the system.

– 8 – TR 60725 © IEC:2012
Information on the supply system impedance was collected from as many countries as
possible and is presented in Tables 1 to 5. The impedance to be considered was the
impedance up to the point of common coupling with other consumers. However, in many
systems, particularly where there were several apartments in the same building, the point of
common coupling was close to the metering point. Hence, the impedance figures obtained
usually include both the supply system impedance and the service connection impedance.
The phase-to-neutral impedance characteristics of three-phase supply systems, in which each
consumer is supplied at 230 V, 50 Hz, differ widely between countries. An international survey
of residential consumers' complex supply impedances for single-phase connections at 50 Hz
is shown in Table 1.
Table 1 – Residential consumers' complex supply impedances at 50 Hz
Percentage of consumers having supply impedances
Year in which
equal to or less than the listed complex values in Ω
Country data was
provided to IEC
98 % 95 % 90 % 85 %
Australia 2011 0,42 + j0,38 0,30 + j0,27 0,25 + j0,23 0,22 + j0,20
Belgium 1980 – 0,63 + j0,33 0,32 + j0,17 0,28 + j0,15
France 1980 – 0,55 + j0,34 0,45 + j0,25 0,34 + j0,21
Germany 1980 0,45 + j0,25 0,36 + j0,21 0,31 + j0,17
a
Ireland 1980 1,47 + j0,64 1,26 + j0,60 1,03 + j0,55 0,94 + j0,43
Italy 1980 – 0,59 + j0,32 0,48 + j0,26 0,44 + j0,24
Netherlands 1980 – 0,70 + j0,25 0,41 + j0,21 0,32 + j0,17
Switzerland 1980 – 0,60 + j0,36 0,42 + j0,25 0,30 + j0,18
United Kingdom 1980 0,46 + j0,45 – 0,25 + j0,23 –
USSR 1980 – 0,63 + j0,30 0,50 + j0,26 –
NOTE This table shows the phase-to-neutral impedance for single-phase systems.
a
System impedances for residential consumers in Poland are similar to those in Ireland.

Since 1981, when the impedance survey was published as Table 1, there has been natural
development and reinforcement of public supply networks and the 1980 values in the 90 %
column, on which the reference impedances for residential supplies were based, are now
more relevant to the 95 % column because supply impedances have been reduced overall.
Information on the measurement of supply impedances is given in Annex C.
The impedance data for residential supply systems, based on study data from the year 2000
and surveys from countries with systems other than 230 V/400 V, are summarized in
Tables 2 to 5.
TR 60725 © IEC:2012 – 9 –
Table 2 – Single-phase device capacities <100 A per phase
Percentage of consumers having supply impedances
Connections
equal to or less than the listed complex values in Ω
Country Remarks
V
98 % 95 % 90 % 85 % Others
100 to 120 0,20+j0,06  –
Canada Survey/Calculation
200 to 240 0,20+j0,08  –
100 to 120  0,09+j0,05  Calculation
(10 % of customers
USA
have higher
200 to 240  0,10+j0,06
impedance)
100 to 120 0,10+j0,07  –
a
Mexico Calculation
127 0,16+j0,08  –
Korea 220 0,40+j0,18 0,34+j0,15 0,31+j0,11 0,28+j0,10 – Survey
100 0,35+j0,13  –
Japan Survey/Calculation
200 0,42+j0,21  –
NOTE 1 The figures for the USA are 90th percentile.
NOTE 2 All references to data from Korea relate to data from South Korea.
NOTE 3 The data from Korea has been taken from rural and urban networks.
NOTE 4 The wide difference in network topographies in the 60 Hz countries mean that it is not possible to
provide a single reference impedance 60 Hz countries.

NOTE 5 TBD (to be derived).
a
The values for Mexico are listed under the 95 percentile but Mexico is working towards 100 % of the network
impedance values to be at or below the specified values.

Table 3 – Three-phase service capacities <100 A per phase
Percentage of consumers having supply
Connections impedances equal to or less than the listed
Country Remarks
complex values in Ω
V
98 % 95 % 90 % 85 %
Survey/CalculationCIRED
Canada 120/208 TBD 0,07+j0,04 TBD TBD
paper
USA 277/480 No data 0,10+j0,06 Estimate/survey
a
Mexico 277/480 0,11+j0,09
Korea 220/380 0,30+j0,20 0,29+j0,18 0,26+j0,16 0,22+j0,15 Survey
Survey/ Calculation
Japan 200 No data 0,38+j0,18  estimate based on JIS-C
IEC 61000-3-2
NOTE 1 The figures for the USA are 90th percentile.
NOTE 2 All references to data from Korea relate to data from South Korea.
NOTE 3 The data from Korea has been taken from rural and urban networks.
NOTE 4 The wide difference in network topographies in the 60 Hz countries mean that it is not possible to
provide a single reference impedance 60 Hz countries.
NOTE 5 TBD (to be derived).
a
The values for Mexico are listed under the 95 percentile but Mexico is working towards 100 % of the network
impedance values to be at or below the specified values.

– 10 – TR 60725 © IEC:2012
4.2 Large residential, commercial and light industrial premises
4.2.1 General
The premises considered in this subclause have service current capacities equal to or in
excess of 100 A per phase.
It is anticipated that the number of requests from consumers and their agents to distribution
network operators for information relating to the system impedance at their supply terminals
will increase as a consequence of the publication of IEC 61000-3-11 and the procedure for the
conditional connection of equipment that it promulgates.
In order to assist distribution network operating companies worldwide in determining a
practical value of actual supply impedance at a particular consumers’ premises and to assist
manufacturers in assessing the marketability of their products in particular countries
worldwide, a basic approach to the determination of maximum supply impedance has been
developed and is given in Annex A.
The following values of supply impedance have been obtained by application of the method
given in Annex A, on the assumptions that
a) the distribution transformer has a rating of 500 kVA, a 3 % voltage regulation or a 2,68 %
reactance,
b) there is 95 % probability of occurrence, i.e. 5 % of consumers, are likely to have a supply
system impedance greater than the tabled values.
If necessary, these supply impedances, or the maximum supply impedances listed in Annex A,
Tables A.1 and A.2, may be amended to represent national or particular public supply
networks in accordance with Clause A.5.
The impedance data for residential supply systems, based on recent studies and surveys from
countries with systems other than 230 V/400 V, is summarized in Tables 4 and 5.
Table 4 – Single or two-phase service capacities ≥100 A per phase
Percentage of consumers having supply impedances
Connections
equal to or less than the listed complex values in Ω
Country Remarks
V
98 % 95 % 90 % 85 %
Canada 347 TBD 0,58+j0,11 TBD TBD Survey/Calculation
USA 480 No data No data 0,10+j0,06 Estimate/survey
Korea 220 0,32+j0,14 0,29+j0,12 0,27+j0,11 0,22+j0,09 Survey
Japan No data No data No data No data
TBD = To be derived.
Table 5 – Three-phase service capacities ≥100 A per phase
Percentage of consumers having supply impedances
Connections
equal to or less than the listed complex values in Ω
Country Remarks
V
98 % 95 % 90 % 85 %
Survey/Calculation
Canada 600 TBD 0,39+j0,07 TBD TBD
CIRED paper
USA 480 No data No data No data No data
Korea 380 0,27+j0,21 0,24+j0,19 0,21+j0,17 0,20+j0,17 Survey
Japan – – – – – Not applicable
TBD = To be derived.
TR 60725 © IEC:2012 – 11 –
4.2.2 Supply impedance relevant to the connection of three-phase equipment
Table 6 contains, under the assumptions stated in 4.2, the values of the modulus in ohms, of
the supply impedance of the line-conductors of 230 V/400 V, 50 Hz public electricity supply
networks relevant to three-phase services, the various statutory voltage ranges declared to
consumers and service capacities in common use.
Table 6 – Modulus values of supply impedance, in ohms at 50 Hz,
relevant to the connection of three-phase equipment
and having a 95 % probability of not being exceeded
Service capacity in amperes per phase
Declared voltage range
%
150 A 200 A 300 A 400 A 600 A
8 0,09 0,06 0,04 0,03 0,02
9 0,10 0,07 0,05 0,04 0,03
10 0,11 0,08 0,05 0,04 0,03
11 0,12 0,09 0,06 0,05 0,03
12 0,14 0,10 0,07 0,05 0,03
13 0,15 0,11 0,08 0,06 0,04
14 0,17 0,13 0,08 0,07 0,04
15 0,18 0,14 0,09 0,07 0,05
16 0,20 0,15 0,10 0,08 0,05
17 0,21 0,16 0,10 0,08 0,05
18 0,22 0,17 0,11 0,09 0,06
19 0,24 0,18 0,12 0,09 0,06
20 0,25 0,19 0,13 0,10 0,06
4.2.3 Supply impedances relevant to the connection of single-phase equipment
Table 7 contains, under the assumptions stated in 4.2, the values of the modulus, in ohms, of
the supply impedance of the line-to-neutral conductors of 230 V/400 V, 50 Hz public electricity
supply networks relevant to the connection of single-phase equipment to three-phase 4-wire
services.
– 12 – TR 60725 © IEC:2012
Table 7 – Modulus values of supply impedance, in ohms at 50 Hz,
relevant to the connection of single-phase equipment
and having a 95 % probability of not being exceeded
Service capacity in amperes per phase
Declared voltage range
%
150 A 200 A 300 A 400 A 600 A
8 0,13 0,10 0,06 0,05 0,03
9 0,15 0,12 0,08 0,06 0,04
10 0,18 0,13 0,09 0,07 0,04
11 0,20 0,15 0,10 0,08 0,05
12 0,23 0,17 0,11 0,08 0,06
13 0,25 0,19 0,12 0,09 0,06
14 0,27 0,20 0,14 0,10 0,07
15 0,30 0,22 0,15 0,11 0,07
16 0,32 0,24 0,16 0,12 0,08
17 0,34 0,26 0,17 0,13 0,09
18 0,37 0,28 0,18 0,14 0,09
19 0,39 0,29 0,20 0,15 0,10
20 0,42 0,31 0,21 0,16 0,10
5 Reference impedances
5.1 General
Values of reference impedances appropriate to low-voltage public supply systems are given in
the following subclauses; some are values already established in IEC 61000-3-3 and
IEC 61000-3-11, whilst others, pertaining to 60 Hz supply systems, are recommended values.
It should be clearly understood that it is not possible to define a single reference impedance
that applies in all regions of the world, because of different supply voltages and different
distribution systems.
5.2 Reference impedances for equipment with current ratings ≤16 A
5.2.1 Overview
Equipment having current ratings ≤16 A is mainly connected in premises having service
current capacities less than 100 A per phase. Such premises are predominantly in residential
supply areas, which were initially surveyed in Europe in 1980, while other surveys have been
made at more recent dates. Reference impedances relevant to the connection of equipment
having current ratings ≤16 A have therefore been derived from the values given in Table 1.
It was planned that the reference impedances should represent existing system impedances
and have values that can be used to assess the emissions of equipment against voltage limits
with a view to ensure that connection of equipment to a public supply network would not
cause undue voltage disturbance and distortion.
It has not proved possible to find an automatic and logical way of relating the reference
impedance to the range of system impedances. It was recognised that to say that 10 % of
consumers had supply impedances greater than a given value did not imply that 10 % of
consumers would be disturbed. A consumer at the far end of a line causes less disturbance
(due to voltage fluctuations or harmonic distortion) to consumers nearer to the source than to
his immediate neighbour.
TR 60725 © IEC:2012 – 13 –
Divergence of views about the use of a single reference impedance may be summarized as
follows:
a) some countries with high impedance networks do not consider it economically possible to
reinforce their networks;
b) some countries with high impedance networks have no need to reinforce their networks
because they have readily available alternative fuels for cooking and heating appliances;
c) some countries are not concerned with the switching of significant loads at 230 V because
they connect large appliances to two or three phases at 400 V.
The values in the following subclauses were chosen as reference impedances and take
account of experience with the use of existing appliances on existing systems as well as the
survey values of system impedance presented in Tables 1, 2, 3, 4 and 5.
5.2.2 50 Hz and 60 Hz low-voltage supply systems
5.2.2.1 Three-phase, four-wire, 230 V/400 V supply systems with service capacities
<100 A
Adoption of the following reference impedances, Z , for testing purposes is recommended,
ref
see Table 8.
Table 8 – Reference impedances for testing purposes
Impedances
Conductor
Ω
Phase conductor 0,24 + j0,15
Neutral conductor 0,16 + j0,10
Total 0,40 + j0,25
NOTE In Korea, there are three-phase four-wire 220 V/380 V low-voltage supply networks.
5.2.2.2 Single-phase, two-wire 230 V systems with service capacities <100 A
In this category of supply systems, Ireland has a network in which a high percentage of
consumers have supply impedances greater than (0,4 + j0,25) Ω. Italy and Poland also have a
large proportion of rural networks with relatively high supply impedances. In the United
Kingdom, supplies to only about 2 % of consumers exceed (0,4 + j0,25) Ω.
A single value of reference impedance of (0,4 + j0,25) Ω (phase to neutral) has been adopted
with the advantages that
• this value gives the same limit conditions for appliances manufactured for use in all
countries;
• it complies with the decision that there should be a single reference impedance used for
the assessment of emissions from equipment rated ≤16 A per phase;
• it simplifies the test house procedure;
• experience shows that most appliances already connected to public supply systems
comply with limits based on this impedance (but there are exceptions);
• it simplifies the setting of limits.
The choice of a single impedance also has disadvantages, namely:
• although conditions on networks with relatively high impedance are normally acceptable at
present, this may not be so if equipment intended for simultaneous use in large numbers
were designed to produce the maximum values of voltage change foreseen;

– 14 – TR 60725 © IEC:2012
• equipment forming part of a larger appliance, which operates for only short periods and
which is known to be acceptable, would be prohibited.
Equipment rated ≤16 A, which does not comply with the voltage limits of IEC 61000-3-3 when
tested with the reference impedance may be retested or evaluated to show conformity with
IEC 61000-3-11. IEC 61000-3-11 is applicable to equipment with rated input current ≤75 A per
phase and enables conditional connection of equipment by a public distribution network
operating company.
5.2.2.3 Single-phase, three-wire, 100 V/200 V and 120 V/240 V supply systems with
service capacities <100 A
The recommended reference impedances for 50 Hz and 60 Hz single-phase three-wire supply
systems, see Figure 1, having nominal voltages within the range of 100 V to 120 V are given
in Table 9.
Table 9 – Reference impedances for 100 V/200 V
and 120 V/240 V supply systems <100 A
Impedances
Conductor
Ω
Phase conductor 0,209 + j0,103
Neutral conductor 0,143 + j0,025
Total 0,35 + j0,13
This impedance is based on data from US, Canada and Japan.
The recommended reference impedances for 50 Hz and 60 Hz single-phase three-wire supply
systems, see Figure 1, having nominal voltages within the range of 200 V to 240 V and
service capacities <100 A are given in Table 10.
Table 10 – Reference impedances for 200 V to 240 V supply systems <100 A
Impedances
Conductor
Ω
Phase conductor 0,209 + j0,103
Return phase conductor 0,209 + j0,103
Total 0,42 + j0,21
This impedance is based on data from US, Canada and Japan.

5.2.2.4 Three-phase, four-wire supply systems having service capacities ≥100 A per
phase
For 50 Hz and 60 Hz single-phase supply systems having nominal voltages within the range
of 200 V to 240 V and with service capacities ≥100 A per phase the recommended reference
impedances are given in Table 11.

TR 60725 © IEC:2012 – 15 –
Table 11 – Reference impedances for 200 V to 240 V
supply systems, ≥100 A per phase
Impedances
Conductor
Ω
Phase conductor 0,15 + j0,15
Neutral conductor 0,10 + j0,10
Total 0,25 + j0,25
5.3 Reference impedance for 50 Hz and 60 Hz equipment with current ratings >16 A
and ≤75 A per phase
Equipment rated ≤75 A per phase is extensively used in commercial and industrial premises,
and to a lesser extent in residential premises.
50 Hz supply system impedance values for a range of higher service capacities are given in
Tables 6 and 7.
For equipment rated ≤75 A and intended to be used only in premises having a service
capacity <100 A per phase (where the single-phase voltage is in the range 200 V to 240 V) it
is recommended that test reference impedances, Z , from 5.2.2.1 are adopted.
test
For equipment rated ≤75 A and intended to be used only in premises having a service
capacity ≥100 A per phase (where the single-phase voltage is in the range 200 V to 240 V) it
is recommended that test reference impedances, Z , shown in Table 12 are adopted.
test
Table 12 – Reference impedances for testing purposes,
for 200 V to 240 V supply systems, ≥100 A
Impedances
Conductor
Ω
Phase conductor 0,15 + j0,15
Neutral conductor 0,10 + j0,10
Total 0,25 + j0,25
NOTE The above impedance values are recommended for tests
in accordance with IEC 61000-3-11 on equipment having current
ratings >20 A r.m.s. Equipment having current ratings above 19 A
by definition exceed the d limit of 3,3 % and thus require a
c
system impedance that is lower than the Z shown in Table 8.
ref
6 Impedance at frequencies above the supply frequency
Theoretical considerations suggest that resonance between power factor correction
capacitors and the system inductance is possible at harmonic frequencies, but this
phenomenon has only been observed in a few cases. For this reason, it is recommended that
the reference impedance be regarded as purely resistive and inductive for assessing
harmonic emissions.
The supply impedance values presented in this report may be used to determine the minimum
short-circuit level at the fundamental frequency used in IEC 61000-3-12 to specify the limits of
harmonic current emissions from equipment.

– 16 – TR 60725 © IEC:2012
Annex A
(informative)
Methods for determining the maximum modulus values of public
electricity supply low-voltage network impedances relevant to
three-phase services of more than 100 A per phase at 50 Hz

A.1 Relevance of this Technical Report
The main body of this Technical Report was published in 1981 in order to facilitate the
connection of residential equipment to public supply low-voltage networks by providing test
reference impedances for use with the IEC 60555 emission standards.
However, when the IEC 60555 standards were converted to IEC 61000-3 standards in 1995,
the scopes were changed to incorporate most equipment rated less than or equal to 16 A.
This change meant that equipment for use in commercial, light industrial and industrial
premises, previously considered as professional equipment, had to comply with the emission
limits for harmonics and voltage fluctuations.
Manufacturers now design and test equipment to the new standards, as much of modern
equipment is used in all environments. Modern equipment includes for example: very large
television sets, personal computers, photocopiers, air-conditioning units and high-powered
water jet equipment.
The object of this annex is to extend the supply impedance information provided by supply
authorities to service capacities in excess of 100 A per phase, thereby facilitating the
assessment of equipment for connection to a specific supply and to form a common
knowledge base which can be used by equipment manufacturers when discussing the
marketing of their products with supply authorities at a national level.
Because there is an enormous variety throughout the world of statutory supply voltages,
permitted variations and the specifications used by supply authorities for power system plant
and equipment, a statistical survey to determine supply impedances relevant to particular
service capacities would be extremely expensive and the results would be too specific.
Consumer demand for enhanced performance equipment has had the effect of driving up the
ratings of equipment and manufacturers have had problems in meeting the voltage fluctuation
limits in IEC 61000-3-3 in particular. A solution was found to manufacturers’ problems by
publishing IEC 61000-3-11 with a scope that overlaps that of IEC 61000-3-3.
IEC 61000-3-11 is applicable to equipment rated less than or equal to 75 A and subject to
conditional connection, and it permits manufacturers of equipment that does not meet the
, to retest the
limits of IEC 61000-3-3, when tested with the reference impedance Z
ref
equipment with a variable test reference impedance. Thus they may
a) either determine the maximum permissible system impedance Z at the interface point
max
of the user’s supply, which gives compliance with the standard’s limits, declare it in the
equipment instruction manual and instruc
...