IEC 61558-2-23:2024

IEC 61558-2-23 ®
Edition 3.0 2024-06
EXTENDED VERSION
INTERNATIONAL
STANDARD
colour
inside
This extended version of IEC 61558-2-23:2024 includes the content of the references made to
IEC 61558-1:2017
GROUP ENERGY EFFICIENCY PUBLICATION

Safety of transformers, reactors, power supply units and combinations thereof –
Part 2-23: Particular requirements and tests for transformers and power supply
units for construction sites
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.

IEC Secretariat Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
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 corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC Products & Services Portal - products.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews, graphical symbols and the glossary.
committee, …). It also gives information on projects, replaced With a subscription you will always have access to up to date
and withdrawn publications. content tailored to your needs.

IEC Just Published - webstore.iec.ch/justpublished
Electropedia - www.electropedia.org
Stay up to date on all new IEC publications. Just Published
The world's leading online dictionary on electrotechnology,
details all new publications released. Available online and once
containing more than 22 500 terminological entries in English
a month by email.
and French, with equivalent terms in 25 additional languages.

Also known as the International Electrotechnical Vocabulary
IEC Customer Service Centre - webstore.iec.ch/csc
(IEV) online.
If you wish to give us your feedback on this publication or need

further assistance, please contact the Customer Service
Centre: sales@iec.ch.
IEC 61558-2-23 ®
Edition 3.0 2024-06
EXTENDED VERSION
INTERNATIONAL
STANDARD
colour
inside
This extended version of IEC 61558-2-23:2024 includes the content of the references made to

IEC 61558-1:2017
GROUP ENERGY EFFICIENCY PUBLICATION

Safety of transformers, reactors, power supply units and combinations thereof –

Part 2-23: Particular requirements and tests for transformers and power supply

units for construction sites
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-9151-1

– 2 – IEC 61558-2-23:2024 EXV © IEC 2024
CONTENTS
FOREWORD . 9
INTRODUCTION to IEC 61558-1:2017 . 12
INTRODUCTION to IEC 61558-2-23:2024 . 14
1 Scope . 15
2 Normative references . 16
3 Terms and definitions . 19
3.1 Transformers . 20
3.2 General terms . 23
3.3 Operations and protections . 25
3.4 Circuits and windings . 26
3.5 Ratings . 27
3.6 No-load values . 29
3.7 Insulation . 29
3.8 Touch current and protective earthing conductor current . 33
4 General requirements . 33
5 General notes on tests . 34
6 Ratings . 36
7 Classification . 37
8 Marking and other information . 38
9 Protection against electric shock . 46
9.1 General . 46
9.2 Protection against contact with hazardous-live-parts . 47
9.2.1 Determination of hazardous-live-parts . 47
9.2.2 Accessibility to hazardous-live-parts . 47
9.2.3 Accessibility to non hazardous-live-part . 50
9.3 Protection against hazardous electrical discharge . 50
10 Change of input voltage setting . 50
11 Output voltage and output current under load . 51
12 No-load output voltage . 52
13 Short-circuit voltage . 54
14 Heating . 54
14.1 General requirements . 54
14.1.1 Temperature-rise test . 54
14.1.2 Alternative temperature-rise test . 56
14.1.3 Determination of steady-state conditions . 59
14.2 Application of 14.1 or 14.3 according to the insulation system. 60
14.3 Accelerated ageing test for undeclared class of insulation system . 61
14.3.1 General . 61
14.3.2 Heat run . 61
14.3.3 Vibration . 62
14.3.4 Moisture treatment . 62
14.3.5 Measurements . 62
15 Short circuit and overload protection. 62
15.1 General requirements . 62
15.1.1 Short circuit and overload test method . 62

15.1.2 Alternative short circuit and overload test method . 65
15.2 Inherently short-circuit proof transformers . 65
15.3 Non-inherently short-circuit proof transformers. 65
15.4 Non-short-circuit proof transformers . 66
15.5 Fail-safe transformers . 66
16 Mechanical strength . 67
16.1 General . 67
16.2 Impact test and shock test . 67
16.2.1 Impact test . 67
16.2.2 Shock test . 68
16.3 Portable transformers (except portable transformers with integral pins for
introduction in socket-outlet in the fixed wiring) . 68
16.4 Portable transformers provided with integral pins for introduction in socket-
outlets of the fixed wiring . 69
16.4.1 General requirements . 69
16.4.2 Portable transformers provided with integral pins according to EN
50075 (IEC plug type C) for introduction in socket-outlets of the fixed
wiring . 70
16.5 Additional requirements for transformers to be used in vehicles and railway
applications . 71
16.5.1 Transformers to be used in vehicles and railway applications . 71
16.5.2 Test requirements for the transportation of transformers . 72
17 Protection against harmful ingress of dust, solid objects and moisture . 72
17.1 Degrees of protection provided by enclosures (IP code) . 72
17.1.1 General requirements . 72
17.1.2 Tests on transformers with enclosure . 74
17.2 Humidity treatment . 76
18 Insulation resistance, dielectric strength and leakage current . 76
18.1 General . 76
18.2 Insulation resistance . 77
18.3 Dielectric strength test . 77
18.4 Insulation between and within windings . 79
18.5 Touch current and protective earthing conductor current . 79
18.5.1 General . 79
18.5.2 Touch current . 80
18.5.3 Protective earthing conductor current . 81
19 Construction . 82
19.1 General construction . 82
19.2 Flammability of materials . 83
19.3 Short-circuit characteristics of portable transformers . 83
19.4 Class II transformer contact prevention of accessible conductive parts . 84
19.5 Class II transformer insulation reassembling after service . 84
19.6 Loosening of wires, screws or similar parts . 84
19.7 Resistor or capacitor connection with accessible conductive parts . 85
19.8 Bridging of separated conductive parts by resistors or capacitors . 85
19.9 Insulating material separating input and output windings . 85
19.10 Accidental contact protection against hazardous-live-parts provided by
isolating coating . 86
19.11 Insulating material of handles, operating levers, knobs and similar parts. 87
19.12 Winding construction . 87

– 4 – IEC 61558-2-23:2024 EXV © IEC 2024
19.13 Fixing of handles, operating levers and similar parts . 91
19.14 Fixing of covers providing protection against electric shock . 91
19.15 Strain on fixed socket-outlets caused by pin-transformers connection . 91
19.16 Portable transformers for use in irregular or harsh conditions . 92
19.17 Drain hole of transformers protected against ingress of water . 92
19.18 Plug connected transformers protected against ingress of water . 92
19.19 Flexible cable or flexible cord connection for class I portable transformers . 92
19.20 SELV- and PELV-circuit separation of live parts . 92
19.21 Protection against contact for FELV-circuit. 93
19.22 Protective earthing regarding class II transformers . 93
19.23 Protective earthing regarding class III transformers . 93
20 Components . 94
21 Internal wiring . 99
22 Supply connection and other external flexible cables or cords . 100
23 Terminals for external conductors . 106
24 Provisions for protective earthing. 108
25 Screws and connections . 109
26 Creepage distances, clearances and distances through insulation . 112
26.1 General . 112
26.2 Creepage distances and clearances . 113
26.2.1 General . 113
26.2.2 Windings covered with adhesive tape . 113
26.2.3 Uncemented insulationing parts . 113
26.2.4 Cemented insulating parts . 113
26.2.5 Enclosed parts (e.g. by impregnation or potting) . 114
26.3 Distance through insulation . 115
27 Resistance to heat, fire and tracking . 126
27.1 General . 126
27.2 Resistance to heat . 126
27.2.1 General . 126
27.2.2 External accessible parts . 126
27.2.3 Internal parts . 127
27.3 Resistance to abnormal heat under fault conditions . 127
27.4 Resistance to fire . 128
27.4.1 General . 128
27.4.2 External accessible parts . 129
27.4.3 Internal parts . 129
27.5 Resistance to tracking . 130
28 Resistance to rusting . 130
Annex A (normative) Measurement of creepage distances and clearances. 132
Annex B (normative) Testing a series of transformers . 136
B.1 General . 136
B.2 Requirements . 136
B.3 Constructional inspection . 137
Annex C (void) . 138
Annex D (void) . 139
Annex E (normative) Glow-wire test . 140

E.1 General . 140
E.2 Severity . 140
E.3 Conditioning . 140
E.4 Test procedure . 140
Annex F (normative) Requirements for manually operated switches which are parts of
transformers assembly . 141
F.1 General . 141
F.2 Switches tested as a separate component . 141
F.3 Switches tested as part of the transformer . 141
Annex G (normative) Tracking test . 144
G.1 General . 144
G.2 Test specimen . 144
G.3 Test apparatus . 144
G.4 Procedure . 144
Annex H (normative) Electronic circuits . 145
H.1 General . 145
H.2 General notes on tests (addition to Clause 5). 145
H.3 Short circuit and overload protection (addition to Clause 15) . 145
H.4 Creepage distances, clearances and distances through insulation (addition
to Clause 26) . 147
Annex I (informative) Dimensions for rectangular cross-section connectors of
transformers, basic dimensions and coordination . 149
Annex J (normative) Measuring network for touch-currents . 151
Annex K (normative) Insulated winding wires . 152
K.1 General . 152
K.2 Type tests . 152
K.2.1 General . 152
K.2.2 Dielectric strength test . 152
K.2.3 Flexibility and adherence . 152
K.2.4 Heat shock . 153
K.2.5 Retention of dielectric strength after bending . 153
K.3 Testing during manufacturing . 154
K.3.1 General . 154
K.3.2 Routine test . 154
K.3.3 Sampling test . 154
Annex L (normative) Routine tests (production tests) . 155
L.1 General . 155
L.2 Protective earthing continuity test . 155
L.3 Checking of no-load output voltage . 155
L.4 Dielectric strength test . 156
L.5 Checking of protective devices mounting . 156
L.6 Visual inspection . 156
L.7 Repetition test after routine dielectric strength test . 156
Annex M (informative) Examples to be used as a guide for 19.1 . 157
M.1 General . 157
M.2 Coil-former . 157
M.2.1 Concentric type . 157
M.2.2 Side-by-side type . 158

– 6 – IEC 61558-2-23:2024 EXV © IEC 2024
M.3 Windings . 158
M.3.1 Without screen . 158
M.3.2 With screen . 159
Annex N (informative) Examples for checking points of dielectric strength test voltages . 160
Annex O (void) . 162
Annex P (informative) Examples for measurement points of creepage distances and
clearances . 163
Annex Q (informative) Explanation of IP numbers for degrees of protection . 166
Q.1 General . 166
Q.2 Degrees of protection against access to hazardous parts and against solid
foreign objects . 166
Q.3 Degrees of protection against ingress of water . 168
Annex R (normative) Explanations of the application of 6.1.2.2.1 of IEC 60664-1:2007 . 169
R.1 Impulse dielectric test . 169
R.2 Example. 169
Annex S (void) . 171
Annex T (void) . 172
Annex U (void) . 173
Annex V (informative) Symbols to be used for thermal cut-outs . 174
V.1 General . 174
V.2 Non-self-resetting thermal cut-out (see 3.3.4) . 174
V.3 Self-resetting thermal cut-out (see 3.3.3) . 174
Annex W (normative) Coated printed circuit boards . 175
W.1 Preamble . 175
W.2 General . 175
W.3 Cold . 175
W.4 Rapid change of temperature . 175
W.5 Additional tests . 175
Bibliography . 176
Index of defined terms . 178

Figure 1 – IEC 61558 principle . 13
Figure 2 – Mounting box for flush-type transformer . 36
Figure 3 – Test pin (see IEC 61032, test probe 13) . 48
Figure 4 – Standard test finger (see IEC 61032, test probe B) . 49
Figure 5 – Example of back-to-back method – Single phase . 58
Figure 6 – Example of back-to-back method – Three phase . 58
Figure 7 – Amplitude spectrum density for random testing . 71
Figure 8 – Normalised spectrum of shock . 72
Figure 9 – Test voltage sequence . 79
Figure 10 – Test configuration: single-phase equipment on star TN or TT system . 81
Figure 11 – Abrasion resistance test for insulating coated layers . 87
Figure 12 – Flexing test apparatus . 104
Figure 13 – Test arrangement for checking mechanical withstanding of insulating
materials in thin sheet layers . 118
Figure 14 – Ball-pressure apparatus . 126

Figure 101 – Impact test for horizontal surface . 131
Figure 102 – Impact test for vertical surface . 131
Figure A.1 – Example 1. 132
Figure A.2 – Example 2. 133
Figure A.3 – Example 3. 133
Figure A.4 – Example 4. 133
Figure A.5 – Example 5. 134
Figure A.6 – Example 6. 134
Figure A.7 – Example 7. 135
Figure A.8 – Example 8. 135
Figure H.1 – Example of an electronic circuit with low-power points . 148
Figure J.1 – Measuring network for touch-current . 151
Figure M.1 – Examples for concentric type constructions . 157
Figure M.2 – Examples for side-by-side type constructions . 158
Figure M.3 – Examples for winding constructions without screen . 158
Figure M.4 – Examples for wrapped winding constructions . 159
Figure M.5 – Examples for winding constructions with screen . 159
Figure N.1 – Transformer of class I construction with metal enclosure . 160
Figure N.2 – Transformer of class II construction with metal enclosure . 161
Figure N.3 – Transformer of class II construction with enclosure of insulating material . 161
Figure P.1 – Transformer of class I construction . 163
Figure P.2 – Transformer of class I construction with earthed metal screen . 164
Figure P.3 – Transformer of class II construction with metal enclosure . 164
Figure P.4 – Transformer of class II construction with enclosure of insulating material . 165
Figure V.1 – Restored by manual operation . 174
Figure V.2 – Restored by disconnection of the supply . 174
Figure V.3 – Thermal link (see 3.3.5) . 174
Figure V.4 – Self-resetting thermal cut-out . 174

Table 1 – Symbols used on equipment or in instructions . 42
Table 101 – Symbols indicating the kind of transformer . 44
Table 102 – Output voltage ratio for safety isolating transformers . 53
Table 103 – Output voltage ratio for isolating transformers . 53
Table 2 – Values of maximum temperatures in normal use . 59
Table 3 – Explanation of the maximum winding temperatures required in Table 2 . 60
Table 4 – Test temperature and testing time (in days) per cycle . 61
Table 5 – Maximum values of temperatures under short-circuit or overload conditions . 64
Table 6 – Values of T and k for fuses . 65
Table 7 – Pull force on pins . 70
Table 8 – Conditions for vibration testing (random) . 71
Table 9 – Amplitude spectrum density ASD values for accelerated life testing . 71
Table 10 – Frequency values depending on the weight of the specimen . 71
Table 11 – Excitation values for vibration testing . 72

– 8 – IEC 61558-2-23:2024 EXV © IEC 2024
Table 12 – Solid-object-proof transformer test . 74
Table 13 – Values of insulation resistance . 77
Table 14 – Table of dielectric strength test voltages. 78
Table 15 – Limits for currents . 81
Table 16 – Nominal cross-sectional areas of external flexible cables or cords . 101
Table 17 – Pull and torque to be applied to external flexible cables or cords fixed to
stationary and portable transformers . 105
Table 18 – Torque to be applied to screws and connections . 110
Table 19 – Torque test on glands . 112
Table 20 – Clearances in mm . 119
Table 21 – Creepage distances in mm . 120
Table 22 – Distance through insulation in mm . 121
Table 23 – Creepage distances and clearance between terminals for external
connection . 122
Table 24 – Values of FIW wires with minimum overall diameter and minimum test
voltages according to the total enamel increase . 124
Table A.1 – Width of groove values depending on the pollution degree . 132
Table F.1 – Peak surge current of additional loads. 142
Table I.1 – Dimensions of rectangular copper connectors . 149
Table K.1 – Mandrel diameter . 153
Table K.2 – Oven temperature . 153
Table Q.1 – Degrees of protection against access to hazardous parts indicated by the
first characteristic numeral . 167
Table Q.2 – Degrees of protection against solid foreign objects indicated by the first
characteristic numeral . 167
Table Q.3 – Degrees of protection indicated by the second characteristic numeral . 168
Table R.1 – Impulse test voltage according to 6.1.2.2.1 of IEC 60664-1:2007 . 169

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SAFETY OF TRANSFORMERS, REACTORS,
POWER SUPPLY UNITS AND COMBINATIONS THEREOF –

Part 2-23: Particular requirements and tests for transformers and
power supply units for construction sites

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.
This extended version (EXV) of the official IEC Standard provides the user with the
comprehensive content of the Standard.
made to IEC 61558-1:2017.
The specific content of IEC 61558-2-23:2024 is displayed on a blue background.

– 10 – IEC 61558-2-23:2024 EXV © IEC 2024
IEC 61558-2-23 has been prepared by IEC technical committee 96: Transformers, reactors,
power supply units and combinations thereof. It is an International Standard.
This third edition cancels and replaces the second edition published in 2010. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) adjustment of structure and references in acco
...

IEC 61558-2-23 ®
Edition 3.0 2024-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
GROUP ENERGY EFFICIENCY PUBLICATION
PUBLICATION GROUPÉE SUR L'EFFICACITÉ ÉNERGÉTIQUE
Safety of transformers, reactors, power supply units and combinations thereof –
Part 2-23: Particular requirements and tests for transformers and power supply
units for construction sites
Sécurité des transformateurs, bobines d'inductance, blocs d'alimentation et des
combinaisons de ces élements –
Partie 2-23: Règles particulières et essais pour les transformateurs et les blocs
d'alimentation pour chantiers
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 l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.

IEC Secretariat Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
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 corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC Products & Services Portal - products.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews, graphical symbols and the glossary.
committee, …). It also gives information on projects, replaced With a subscription you will always have access to up to date
and withdrawn publications. content tailored to your needs.

IEC Just Published - webstore.iec.ch/justpublished
Electropedia - www.electropedia.org
Stay up to date on all new IEC publications. Just Published
The world's leading online dictionary on electrotechnology,
details all new publications released. Available online and once
containing more than 22 500 terminological entries in English
a month by email.
and French, with equivalent terms in 25 additional languages.

Also known as the International Electrotechnical Vocabulary
IEC Customer Service Centre - webstore.iec.ch/csc
(IEV) online.
If you wish to give us your feedback on this publication or need

further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) 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 IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.

Recherche de publications IEC -  IEC Products & Services Portal - products.iec.ch
webstore.iec.ch/advsearchform Découvrez notre puissant moteur de recherche et consultez
La recherche avancée permet de trouver des publications IEC gratuitement tous les aperçus des publications, symboles
en utilisant différents critères (numéro de référence, texte, graphiques et le glossaire. Avec un abonnement, vous aurez
comité d’études, …). Elle donne aussi des informations sur les toujours accès à un contenu à jour adapté à vos besoins.
projets et les publications remplacées ou retirées.

Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
Le premier dictionnaire d'électrotechnologie en ligne au monde,
Restez informé sur les nouvelles publications IEC. Just
avec plus de 22 500 articles terminologiques en anglais et en
Published détaille les nouvelles publications parues.
français, ainsi que les termes équivalents dans 25 langues
Disponible en ligne et une fois par mois par email.
additionnelles. Egalement appelé Vocabulaire

Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc

Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC 61558-2-23 ®
Edition 3.0 2024-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
GROUP ENERGY EFFICIENCY PUBLICATION

PUBLICATION GROUPÉE SUR L'EFFICACITÉ ÉNERGÉTIQUE

Safety of transformers, reactors, power supply units and combinations thereof –

Part 2-23: Particular requirements and tests for transformers and power supply

units for construction sites
Sécurité des transformateurs, bobines d'inductance, blocs d'alimentation et des

combinaisons de ces élements –

Partie 2-23: Règles particulières et essais pour les transformateurs et les blocs

d'alimentation pour chantiers
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.180  ISBN 978-2-8322-8866-5

– 2 – IEC 61558-2-23:2024 © IEC 2024
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 7
3 Terms and definitions . 7
4 General requirements . 8
5 General notes on tests . 8
6 Ratings . 8
7 Classification . 9
8 Marking and other information . 9
9 Protection against electric shock . 11
10 Change of input voltage setting . 11
11 Output voltage and output current under load . 11
12 No-load output voltage . 11
13 Short-circuit voltage . 13
14 Heating . 13
15 Short-circuit and overload protection . 13
16 Mechanical strength . 13
17 Protection against harmful ingress of dust, solid objects and moisture . 15
18 Insulation resistance, dielectric strength and leakage current . 15
19 Construction . 15
20 Components . 18
21 Internal wiring . 18
22 Supply connection and other external flexible cable or cords . 18
23 Terminals for external conductors . 18
24 Provisions for protective earthing. 18
25 Screws and connections . 18
26 Creepage distances, clearances and distances through insulation . 18
27 Resistance to heat, fire and tracking . 18
28 Resistance to rusting . 18
Annexes . 20
Annex L (normative) Routine tests (production tests) . 21
Bibliography . 22

Figure 101 – Impact test for horizontal surface . 19
Figure 102 – Impact test for vertical surface . 19

Table 101 – Symbols indicating the kind of transformer . 10
Table 102 – Output voltage ratio for safety isolating transformers . 12
Table 103 – Output voltage ratio for isolating transformers . 12

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SAFETY OF TRANSFORMERS, REACTORS,
POWER SUPPLY UNITS AND COMBINATIONS THEREOF –

Part 2-23: Particular requirements and tests for transformers and
power supply units for construction sites

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 61558-2-23 has been prepared by IEC technical committee 96: Transformers, reactors,
power supply units and combinations thereof. It is an International Standard.
This third edition cancels and replaces the second edition published in 2010. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) adjustment of structure and references in accordance with IEC 61558-1:2017;
b) new symbol for power supply units with linearly regulated output voltage.

– 4 – IEC 61558-2-23:2024 © IEC 2024
The text of this International Standard is based on the following documents:
Draft Report on voting
96/590/FDIS 96/596/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/standardsdev/publications.
It has the status of a group safety publication in accordance with IEC Guide 104.
This International Standard is to be used in conjunction with IEC 61558-1:2017.
This document supplements or modifies the corresponding clauses in IEC 61558-1:2017, so as
to convert that publication into the IEC standard: Particular requirements and tests for
transformers and power supply units for construction sites.
A list of all parts in the IEC 61558 series published under the general title Safety of transformers,
reactors, power supply units and combinations thereof, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
Where this document states "addition", "modification" or "replacement", the relevant text of
IEC 61558-1:2017 is to be adapted accordingly.
In this document, the following print types are used:
– requirements proper: in roman type;
– test specifications: in italic type;
– explanatory matter: in smaller roman type.
In the text of this document, the words in bold are defined in Clause 3.
Subclauses, notes, figures and tables additional to those in IEC 61558-1:2017 are numbered
starting from 101; supplementary annexes are entitled AA, BB, etc.
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.
INTRODUCTION
IEC/TC 96 has a group safety function in accordance with IEC Guide 104 for transformers other
than those intended to supply distribution networks, in particular transformers and power supply
units intended to allow the application of protective measures against electric shock as defined
by TC 64, which is about electrical installations and protection against electric shock, but in
certain cases including the limitation of voltage and horizontal safety function for SELV, in
accordance with IEC 60364-4-41.
The group safety function (GSF) is used because of responsibility for safety extra-low voltage
(SELV) in accordance with IEC 61140:2016, 5.2.6 and IEC 60364-4-41:2005, 414.3.1 or control
circuits in accordance with IEC 60204-1:2016, 7.2.4.
The group safety function is used for each part of IEC 61558-2 because different standards of
the IEC 61558 series can be combined in one construction but in certain cases with no limitation
of rated output power.
For example an auto-transformer in accordance with IEC 61558-2-13 can be designed with a
separate SELV-circuit in accordance with the particular requirements for IEC 61558-2-6
relating to the general requirements of IEC 61558-1.

– 6 – IEC 61558-2-23:2024 © IEC 2024
SAFETY OF TRANSFORMERS, REACTORS,
POWER SUPPLY UNITS AND COMBINATIONS THEREOF –

Part 2-23: Particular requirements and tests for transformers and
power supply units for construction sites

1 Scope
Replacement:
This part of IEC 61558 deals with the safety of transformers for construction sites and power
supply units incorporating transformers for construction sites. Transformers incorporating
electronic circuits are also covered by this document.
NOTE 1 Safety includes electrical, thermal and mechanical aspects.
Unless otherwise specified, from here onward, the term transformer covers transformers for
construction sites and power supply units incorporating transformers for construction sites.
This document is applicable to stationary or portable, single-phase or polyphase, air-cooled
(natural or forced) independent or associated transformers, being isolating or safety
isolating dry-type transformers for the use on construction sites. The windings can be
encapsulated or non-encapsulated.
For power supply units (linear) this document is applicable. For switch mode power supply
units, IEC 61558-2-16 is applicable together with this document. Where two requirements are
in conflict, the most severe takes precedence.
The rated supply voltage does not exceed 1 000 V AC, and the rated supply frequency and
the internal operating frequencies do not exceed 500 Hz.
The rated output does not exceed:
– 25 kVA for single-phase transformers;
– 40 kVA for polyphase transformers.
This document is applicable to transformers without limitation of the rated output subject to
an agreement between the purchaser and the manufacturer.
NOTE 2 Transformers intended to supply distribution networks are not included in the scope.
Isolating transformers for construction sites have a no-load output voltage and a rated
output voltage exceeding 50 V AC and not exceeding 250 V AC.
Safety isolating transformers for construction sites have a no-load output voltage and a
rated output voltage not exceeding 50 V AC.
NOTE 3 This document is applicable to transformers for the supply of electricity in locations as specified in
IEC 60364-7-704. The latter also specifies the protection by using an earthed midpoint or starpoint of the output
winding.
NOTE 4 Transformers covered by this document are used in applications where it is required by the installation
rules or by the appliance specification for protection purposes.

When the transformers are incorporated into low voltage switchgear and controlgear
assemblies for construction sites as specified in IEC 61439-4, the additional requirements
of IEC 61439-4 apply to the assembly.
NOTE 5 For transformers filled with liquid dielectric or pulverised material, such as sand, additional requirements
are under consideration.
Attention is drawn to the following if necessary:
– for transformers intended to be used in vehicles, on board ships, and aircraft, additional
requirements (from other applicable standards, national rules, etc.);
– measures to protect the enclosure and the components inside the enclosure against
external influences such as fungus, vermin, termites, solar-radiation, and icing;
– the different conditions for transportation, storage, and operation of the transformers;
– additional requirements in accordance with other appropriate standards and national
rules can be applicable to transformers intended for use in special environments.
It is possible that the future technological development of transformers will require an increase
in the upper limit of the frequencies. Until then this document may be used as a guidance
document.
This group safety publication focusing on safety guidance is primarily intended to be used as a
product safety standard for the products mentioned in the scope, but is also intended to be used
by technical committees in the preparation of publications for products similar to those
mentioned in the scope of this group safety publication, in accordance with the principles laid
down in IEC Guide 104 and lSO/lEC Guide 51.
One of the responsibilities of a technical committee is, wherever applicable, to make use of
basic safety publications and/or group safety publications in the preparation of its publications.
2 Normative references
IEC 61558-1:2017, Clause 2 is applicable, except as follows:
Addition:
IEC 60068-2-27:2008, Environmental testing – Part 2-27: Tests – Test Ea and guidance: Shock
IEC 60245-4:2011, Rubber insulated cables – Rated voltages up to and including 450/750 V –
Part 4: Cords and flexible cables
IEC 61558-1:2017, Safety of transformers, reactors, power supply units and combinations
thereof – Part 1: General requirements and tests
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61558-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp

– 8 – IEC 61558-2-23:2024 © IEC 2024
3.2 General terms
Addition:
3.2.101
low voltage switchgear and controlgear assembly for construction sites
combination of one or several transforming or switching devices with associated control,
measuring, signalling, protective and regulating equipment complete with all their internal
electrical and mechanical connections and structural parts, designed and built for use on all
construction sites, indoors or outdoors
4 General requirements
IEC 61558-1:2017, Clause 4 is applicable.
5 General notes on tests
IEC 61558-1:2017, Clause 5 is applicable.
6 Ratings
IEC 61558-1:2017, Clause 6 is applicable except as follows:
Addition:
6.101 The rated output voltage shall not exceed:
– 250 V AC for isolating transformers with a non-earthed mid-point (single-phase) or a
non-earthed star-point (three-phase) or delta connection (three-phase);
– 115 V AC for isolating transformers with a mid-point (single-phase) earthed in the
construction or a star-point (three-phase) earthed in the construction;
– 50 V AC for safety isolating transformers.
The rated output voltage shall exceed:
– 50 V AC for isolating transformers
Preferred values for the rated output voltage are
– 115 V and 230 V for portable, single-phase isolating transformers;
– 72 V, 115 V and 230 V for other isolating transformers;
– 6 V, 12 V, 24 V, 42 V and 48 V for safety isolating transformers.
6.102 The rated output shall not exceed:
– 25 kVA for single-phase isolating and safety isolating transformers;
– 40 kVA for polyphase isolating and safety isolating transformers;
Preferred values for the rated output are
– 25 VA, 40 VA, 63 VA, 100 VA, 160 VA, 250 VA, 400 VA, 630 VA, 1 000 VA, 1 600 VA,
2 500 VA, 4 000 VA, 6 300 VA, 10 kVA, 16 kVA and 25 kVA for single-phase
transformers;
– 630 VA, 1 000 VA, 1 600 VA, 2 500 VA, 4 000 VA, 6 300 VA, 10 kVA, 16 kVA, 25 kVA
and 40 kVA for polyphase transformers.

Intermittent duty cycle can be assigned only to portable transformers having a rated output
not exceeding 6,3 kVA.
Transformers without limitation of the rated output shall be subject to agreement between the
purchaser and the manufacturer.
6.103 The rated supply frequency shall not exceed 500 Hz.
6.104 The rated supply voltage shall not exceed 1 000 V AC.
6.105 Transformers with intermittent duty cycle shall be intended for a rated operating time
of 5 min "on" and a resting time of 15 min "off".
6.106 The supply current is limited to a maximum of 125 A, and in the case of flexible cable or
socket outlet, to 63 A.
Compliance with 6.101 to 6.106 is checked by inspection of the marking.
7 Classification
IEC 61558-1:2017, Clause 7 is applicable, except as follows:
Replacement:
7.5 Transformers are classified in accordance with their duty type:
– continuous duty;
– intermittent duty cycle.
8 Marking and other information
IEC 61558-1:2017, Clause 8 is applicable, except as follows:
8.1 h)
Replacement of the content up to the first semi-colon with the following:
relevant graphical symbols shown in Table 101 that indicate the kind of transformer
8.11
Addition:
The symbol for linear power supply units shall be used in conjunction with the symbol
indicating the kind of transformer.

– 10 – IEC 61558-2-23:2024 © IEC 2024
Table 101 – Symbols indicating the kind of transformer
Symbol or
graphical Explanation or title Identification
symbol
Isolating transformer for construction sites, fail-safe IEC 60417-6010-1:2007-02

Safety isolating transformer for construction sites, fail-safe IEC 60417-6010-2:2007-02

Isolating transformer for construction sites, fail-safe, IEC 60417-6010-3:2007-02
mid-point or star-point earthed

Isolating transformer for construction sites, IEC 60417-6010-4:2007-02
non-short-circuit proof
Safety isolating transformer for construction sites, IEC 60417-6010-5:2007-02
non-short-circuit proof
Isolating transformer for construction sites, IEC 60417-6010-6:2007-02
non-short-circuit proof, mid-point or star-point earthed

Isolating transformer for construction sites, IEC 60417-6010-7:2007-02
short-circuit proof (inherently or non-inherently)

Safety isolating transformer for construction sites, IEC 60417-6010-8:2007-02
short-circuit proof (inherently or non-inherently)

Symbol or
graphical Explanation or title Identification
symbol
Isolating transformer for construction sites, IEC 60417-6010-9:2007-02
short-circuit proof (inherently or non-inherently), mid-point or star-
point earthed
Power supply unit, linear IEC 60417-6210:2013-10
To identify the electronic device incorporating transformer(s) and
L
electronic circuitry(ies), that converts electrical power into single
or multiple power outputs, the output voltage is linearly regulated.

The internal operating frequency does not exceed 500 Hz.

9 Protection against electric shock
IEC 61558-1:2017, Clause 9 is applicable.
10 Change of input voltage setting
IEC 61558-1:2017, Clause 10 is applicable.
11 Output voltage and output current under load
IEC 61558-1:2017, Clause 11 is applicable.
12 No-load output voltage
IEC 61558-1:2017, Clause 12 is applicable except as follows:
Addition:
12.101 The no-load output voltage shall not exceed:
– 250 V AC for isolating transformers with a non-earthed mid-point (single-phase) or a
non-earthed star-point (three-phase) or delta connection (three-phase);
– 115 V AC for isolating transformers with a mid-point (single-phase) earthed in the
construction or a star-point (three-phase) earthed in the construction;
– 50 V AC for safety isolating transformers.
For independent transformers, the no-load output voltage limitation applies even when
output windings, not intended for interconnection, are connected in series.
The no-load output voltage shall exceed:
– 50 V AC for isolating transformers.
12.102 The difference between the no-load output voltage and the output voltage under
load shall not be excessive.
– 12 – IEC 61558-2-23:2024 © IEC 2024
The ratio between the no-load output voltage measured in Clause 12 and the output voltage
under load measured during the test of Clause 11, expressed as a percentage of the latter
voltage, shall not exceed the values shown in Table 102 or Table 103.
The ratio is determined by Formula (1):
UU−
no-load load
×100 (%)
(1)
U
load
where
U is the no-load output voltage, expressed in V;
no-load
U is the output voltage under load, expressed in V.
load
Table 102 – Output voltage ratio for safety isolating transformers
Type of transformer Ratio between no-load output voltage and output
Rated output voltage under load
VA %
Inherently short-circuit proof transformers:
− up to and including 63 100
− over 63 up to and including 630 50
− over 630
Other transformers:
− up to and including 10 100
− over 10 up to and including 25 50
− over 25 up to and including 63
− over 63 up to and including 250
− over 250 up to and including 630 10
− over 630 5
Table 103 – Output voltage ratio for isolating transformers
Type of transformer Ratio between no-load output voltage and output
Rated output voltage under load
VA %
All type of transformers:
− up to and including 63
− over 63 up to and including 250 15
− over 250 up to and including 630 10
− over 630
For single-phase transformers with earthed midpoint, the voltage between any pole of the
output circuit and earthing shall not exceed the result of Formula (2):
U
no-load
± 2,5 %
(2)
For three-phase transformers with earthed star-point, the voltage between any pole of the
output circuit and earthing shall not exceed the result of Formula (3):
U
no-load
± 2,5 %
(3)
Compliance with the requirements of 12.101 and 12.102 shall be checked by measuring the
no-load output voltage at ambient temperature when the transformer is connected to the
rated supply voltage at the rated supply frequency.
13 Short-circuit voltage
IEC 61558-1:2017, Clause 13 is applicable.
14 Heating
IEC 61558-1:2017, Clause 14 is applicable.
15 Short-circuit and overload protection
IEC 61558-1:2017, Clause 15 is applicable.
16 Mechanical strength
IEC 61558-1:2017, Clause 16 is applicable except as follows:
16.1 General
Replacement:
Transformers for construction sites shall have adequate mechanical strength and be so
constructed as to withstand such rough handling and transport as can be expected in normal
use.
Compliance is checked by the tests of 16.2 for stationary transformers and by the tests of
16.2, 16.3 and 16.4 as appropriate, for portable transformers.
After the tests, the transformers shall show no damage within the meaning of this document.
In particular, hazardous live parts shall not become accessible, when tested as described in
9.2. Insulating barriers shall not be damaged and handles, levers, knobs and the like shall not
move on their shafts.
– 14 – IEC 61558-2-23:2024 © IEC 2024
NOTE 1 Damage to the finish, small dents which do not reduce creepage distances or clearances below the
values specified in Clause 26, and small chips which do not adversely affect the protection against electric shock or
moisture, are ignored.
NOTE 2 Cracks not visible with normal vision or corrected vision without magnification and surface cracks in fibre
reinforced mouldings and the like are ignored.
In addition, with respect to the test of 16.4, bending of the pins during the test is considered
acceptable.
16.2 Stationary transformers
Replacement:
16.2 Impact test and shock test
16.2.1 Impact test
NOTE This test simulates collisions between the transformer and mechanical handling equipment for construction
site.
The complete transformer, as applicable, shall be subjected to a series of impacts of 6 J
applied to the enclosure as follows:
The equipment to be tested shall be fixed on a support of adequate rigidity to restrict movement
of the transformers to 0,1 mm under the effect of the prescribed impact. Three successive
impacts shall be applied to the most unfavourable point of the enclosure by means of either:
a) a solid smooth steel sphere approximately 50 mm in diameter and with a mass of
500 g ± 25 g, falling freely from a rest position from a vertical height of 1,2 m onto the
enclosure surface held in a horizontal plane. The hardness of the sphere shall be not less
than 50 HR and not more than 58 HR (see Figure 101), or
b) a similar steel sphere, which shall be suspended by a cord and swung as a pendulum in
order to apply a horizontal impact, falling through a vertical distance of 1,2 m (see
Figure 102).
Sloping surfaces can be tested using the pendulum but if this is not convenient the surface will
be aligned in the horizontal plane by turning the transformers, as applicable, on the support
and the test a) is used. Before each test, an inspection of the sphere shall be made to ensure
that it is free of burrs and defects.
The test shall be so arranged that the impacts are applied at positions where the weaknesses
are most likely to be revealed. A minimum of 18 impacts shall be applied to the transformers,
as applicable.
The test is not applicable to components such as socket-outlets, operating handles, illuminating
lights, push buttons, actuators, etc., when these components are mounted in recesses with
respect to the main surfaces, so that the distance between the most exposed parts of these
components and the surfaces is at least 10 mm.
After the test, the enclosure shall continue to provide the degrees of protection specified in
19.16; any distortions or deformations of the enclosure and components shall neither be
detrimental to the proper functioning of the transformer, nor decrease creepage distances
and clearances to below the required values; actuators, handles, etc. shall still be operable.
Superficial damage, removed paint, broken cooling ribs or similar parts, small indentations,
cracks not visible with normal or corrected vision without further magnification, or surface cracks
shall not constitute test failures.

16.2.2 Shock test
NOTE This test simulates the shocks received by transformers carried loose on board vehicles on normal roads
or on railway cars for long periods of time.
The complete transformers, as applicable, in working order shall be tested in accordance with
IEC 60068-2-27 with a severity of 500 m/s peak acceleration and duration of 11 ms.
After the test, the enclosure shall continue to provide the degrees of protection specified in
19.16; any distortions or deformations of the enclosure and components shall neither be
detrimental to the proper functioning of the transformer, nor decrease creepage distances
and clearances to below the required values; actuators, handles, etc., shall still be operable.
Superficial damage, removed paint, broken cooling ribs or similar parts, small indentations,
cracks not visible with normal or corrected vision without further magnification, or surface cracks
shall not constitute test failures.
17 Protection against harmful ingress of dust, solid objects and moisture
IEC 61558-1:2017, Clause 17 is applicable.
18 Insulation resistance, dielectric strength and leakage current
IEC 61558-1:2017, Clause 18 is applicable.
19 Construction
IEC 61558-1:2017, Clause 19 is applicable except as follows:
19.1 General construction
Replacement:
19.1.1 The input and output circuits shall be electrically separated from each other, and the
construction shall be such that there is no possibility of any connection between these circuits,
either directly or indirectly, via other conductive parts, except by deliberate action.
Compliance is checked by inspection and measurements in accordance with Clause 18 and
Clause 26.
19.1.2 The insulation between input and output winding(s) shall consist of double or
reinforced insulation (rated for the working voltage).
In addition, the following applies:
– for class I transformers not intended for connection to the mains supply by means of a
plug, the insulation between the input windings and the body connected to earthing
shall consist of at least basic insulation rated for the input voltage. The insulation
between the output windings and the body connected to earthing, shall consist of at
least basic insulation (rated for the output voltage);
– for class I transformers intended for connection to the mains supply by means of a
plug, the insulation between the input windings and the body shall consist of at least
basic insulation, and the insulation between the output windings and the body shall
consist of at least supplementary insulation (both basic and supplementary
insulations rated for the working voltage);

– 16 – IEC 61558-2-23:2024 © IEC 2024
– for class II transformers, the insulation between the input windings and the body shall
consist of double or reinforced insulation (rated for the input voltage). The insulation
between the output windings and the body shall consist of double or reinforced
insulation (rated for the output voltage).
19.1.3 For transformers with intermediate conductive parts (e.g. the iron core) not
connected to the body and located between the input and output windings the insulation
between the input windings and any intermediate conductive part shall consist of at least
basic insulation, and the insulation between the output windings and any intermediate
conductive part shall consist of at least supplementary insulation (both basic and
supplementary insulations rated for the working voltage).
NOTE 1 An intermediate conductive part not separated from the input or output windings or the body by at
least basic insulation is considered to be connected to the relevant part(s).
NOTE 2 Basic insulation and supplementary insulation are interchangeable.
In addition, the following applies:
– for class I transformers, the insulation between the input and output windings via the
intermediate conductive parts (even if they are connected to earthing) shall consist of
double or reinforced insulation (rated for the working voltage);
– for class II transformers, the insulation between the input windings and the body, and
between the output windings and the body via the intermediate conductive parts
shall consist of double or reinforced insulation (rated for the input and output
voltage);
– for transformers different from independent transformers (IP00), the insulation between
the input and output windings via the intermediate conductive parts shall consist of
double or reinforced insulation (rated for the working voltage).
In this Subclause 19.1.3 the possibility to consider the intermediate conductive part
connected to earthing and consequently to require basic insulation in both circuits (primary
and secondary) is not allowed for the following reason:
– the intermediate conductive part is normally the iron core made by laminated strips
insulated from other by oxide coatings. It is not assured that all laminations are correctly
connected to earthing.
– for transformers different from independent, it is not assured that in the final applications
the iron core will be connected to earthing.
19.1.4 For class I transformers not intended for connection to the mains supply by means
of a plug, the insulation between the input and output windings can consist of basic
insulation plus protective screening instead of double or reinforced insulation, provided
the following conditions are complied with:
– the insulation between the input winding and the protective screen shall comply with
the requirements for basic insulation (rated for the input voltage);
– the insulation between the protective screen and the output winding shall comply with
the requirements for basic insulation (rated for the output voltage);
– the protective screen shall, unless otherwise specified, consist of a metal foil or of a
wire wound screen extending at least the full width of the input winding and shall have
no gaps or holes;
– where the protective screen does not cover the entire width of the input winding,
additional adhesive tapes or equivalent insulation shall be used to ensure double
insulation in that area;
– if the protective screen is made of a foil, the turns shall be insulated from each other.
In case of only one turn, it shall have an isolated overlap of at least 3 mm;

– the wire of a wire wound screen and the lead out wire of the protective screen shall
have a cross-sectional area at least corresponding to the rated current of the overload
device to ensure that if a breakdown of insulation should occur, the overload protective
device will open the circuit before the lead-out wire is destroyed;
– the lead-out wire shall be soldered to the protective screen or secured in an equally
reliable manner.
NOTE 4 For the purpose of this Subclause 19.1.4, the term "windings" does not include internal circuits.
NOTE 5 Examples of construction of windings are given in IEC 61558-1:2017, Anne
...