IEC 61558-2-6:2021

• Abstract:

  IEC 61558-2-6:2021 deals with the safety of safety isolating transformers for general applications and power supply units incorporating safety isolating transformers for general applications. Transformers incorporating electronic circuits are also covered by this document. This third edition cancels and replaces the second edition published in 2009. This edition constitutes a technical revision.; - adjustment of structure and references in accordance with IEC 61558-1:2017;; - description of constructions moved in IEC 61558-1:2017;; - new symbol for power supply unit with linearly regulated output voltage.; 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.

• Status: Published
• Publication Date: 17-May-2021
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: TC 96 - Transformers and reactors
• Drafting Committee: MT 1 - TC 96/MT 1
• Current Stage: PPUB - Publication issued
• Start Date: 18-May-2021
• Completion Date: 04-Jun-2021

• Revises: IEC 61558-2-6:2009 - Safety of transformers, reactors, power supply units and similar products for supply voltages up to 1 100 V - Part 2-6: Particular requirements and tests for safety isolating transformers and power supply units incorporating safety isolating transformers
• Effective Date: 05-Sep-2023

Overview

IEC 61558-2-6:2021 is the IEC product safety standard for safety isolating transformers and power supply units (PSUs) incorporating safety isolating transformers for general applications. This third edition (2021) is a technical revision aligned with IEC 61558-1:2017, updating structure, references and symbols (including a new symbol for linearly regulated outputs). It is a Group Safety Publication (IEC Guide 104) and is to be used in conjunction with Part 1 (IEC 61558-1:2017).

Key scope points:

• Applies to stationary or portable, single- or polyphase, air-cooled dry-type transformers (natural or forced ventilation), with encapsulated or non-encapsulated windings.
• Rated supply voltage ≤ 1 000 V AC and frequencies (supply and internal) ≤ 500 Hz.
• Rated outputs normally limited to 10 kVA (single-phase) and 16 kVA (polyphase) unless agreed between purchaser and manufacturer.
• No-load output voltage ≤ 50 V AC or 120 V ripple‑free DC.
• Covers transformers incorporating electronic circuits and linear power supply units (switch‑mode PSUs are covered by IEC 61558-2-16).

Key Topics and Requirements

The standard supplements Part 1 with particular requirements and tests focused on safety isolation and practical use. Main technical topics include:

• Protection against electric shock and SELV requirements for isolated outputs.
• Ratings, classification and marking (including symbols and output voltage ratios).
• Electrical tests: insulation resistance, dielectric strength, leakage current.
• Thermal performance: heating, temperature rise and thermal protection.
• Short‑circuit and overload protection, short‑circuit voltage characteristics.
• Mechanical and environmental robustness: mechanical strength, ingress protection, resistance to rusting and external influences.
• Construction and components: internal wiring, terminals, earthing provisions, creepage and clearance distances, resistance to heat, fire and tracking.
• Special considerations for transformers with electronic circuits and power supplies with linear regulation.

Applications and Who Uses It

IEC 61558-2-6 is used by:

• Transformer and PSU manufacturers for design, testing and product compliance.
• Test laboratories and certification bodies performing safety assessments.
• OEMs and system integrators specifying safe isolating power sources for equipment.
• Regulatory and procurement teams ensuring compliance with SELV and isolation requirements. Typical applications include control circuits, SELV supplies, medical-adjacent equipment (where isolation is required by end-product standards), industrial control systems and other equipment needing double or reinforced insulation.

Related Standards

• IEC 61558-1:2017 (General requirements) - to be used together with this part.
• IEC 61558 series (other particular parts).
• IEC 61558-2-16 (switch-mode power supplies).
• Standards referenced for SELV and safety rules: IEC 61140, IEC 60364-4-41, and product/installation standards relevant to the end application.

Keywords: IEC 61558-2-6, safety isolating transformers, power supply units, transformer safety, SELV, IEC 61558-1:2017, transformer testing, linear power supply.

IEC 61558-2-6 ®
Edition 3.0 2021-05
INTERNATIONAL
STANDARD
GROUP SAFETY PUBLICATION
Safety of transformers, reactors, power supply units and combinations thereof –
Part 2-6: Particular requirements and tests for safety isolating transformers and
power supply units incorporating safety isolating transformers for general
applications
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 Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
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.

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(IEV) online.
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need further assistance, please contact the Customer Service
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IEC 61558-2-6 ®
Edition 3.0 2021-05
INTERNATIONAL
STANDARD
GROUP SAFETY PUBLICATION
Safety of transformers, reactors, power supply units and combinations thereof –

Part 2-6: Particular requirements and tests for safety isolating transformers and

power supply units incorporating safety isolating transformers for general

applications
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-9791-9

– 2 – IEC 61558-2-6:2021 © IEC 2021
CONTENTS
FOREWORD . 3
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 8
3 Terms and definitions . 8
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 . 10
10 Change of input voltage setting . 10
11 Output voltage and output current under load . 10
12 No-load output voltage . 10
13 Short-circuit voltage . 11
14 Heating . 11
15 Short-circuit and overload protection . 12
16 Mechanical strength . 12
17 Protection against harmful ingress of dust, solid objects and moisture . 12
18 Insulation resistance, dielectric strength and leakage current . 12
19 Construction . 12
20 Components . 12
21 Internal wiring . 12
22 Supply connection and other external flexible cable or cords . 12
23 Terminals for external conductors . 12
24 Provisions for protective earthing. 12
25 Screws and connections . 12
26 Creepage distances, clearances and distances through insulation . 12
27 Resistance to heat, fire and tracking . 13
28 Resistance to rusting . 13
Annexes . 14
Bibliography . 15

Table 101 – Symbols indicating the kind of transformer . 10
Table 102 – Output voltage ratio . 11

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

Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety
isolating transformers for general applications

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.
International standard IEC 61558-2-6 has been prepared by IEC technical committee 96:
Transformers, reactors, power supply units and combinations thereof.
This third edition cancels and replaces the second edition published in 2009. 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) description of constructions moved in IEC 61558-1:2017;
c) new symbol for power supply unit with linearly regulated output voltage.

– 4 – IEC 61558-2-6:2021 © IEC 2021
The text of this International Standard is based on the following documents:
FDIS Report on voting
96/506/FDIS 96/512/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.
NOTE When “Part 1” is mentioned in this standard, it refers to 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 safety
isolating transformers and powe
...

IEC 61558-2-6 ®
Edition 3.0 2021-05
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
GROUP SAFETY PUBLICATION
Safety of transformers, reactors, power supply units and similar products for
supply voltages up to 1 100 V combinations thereof –
Part 2-6: Particular requirements and tests for safety isolating transformers and
power supply units incorporating safety isolating transformers for general
applications
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 Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
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 online collection - oc.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. With a subscription you will always
committee, …). It also gives information on projects, replaced have access to up to date content tailored to your needs.
and withdrawn publications.
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The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and
and French, with equivalent terms in 18 additional languages.
once a month by email.
Also known as the International Electrotechnical Vocabulary

(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc

If you wish to give us your feedback on this publication or
need further assistance, please contact the Customer Service
Centre: [e-mail].
IEC 61558-2-6 ®
Edition 3.0 2021-05
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
GROUP SAFETY PUBLICATION
Safety of transformers, reactors, power supply units and similar products for

supply voltages up to 1 100 V combinations thereof –

Part 2-6: Particular requirements and tests for safety isolating transformers and

power supply units incorporating safety isolating transformers for general

applications
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-9831-2

– 2 – IEC 61558-2-6:2021 RLV © IEC 2021
CONTENTS
FOREWORD . 3
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 8
3 Terms and definitions . 8
4 General requirements . 8
5 General notes on tests . 8
6 Ratings . 9
7 Classification . 9
8 Marking and other information . 9
9 Protection against electric shock . 10
10 Change of input voltage setting . 10
11 Output voltage and output current under load . 10
12 No-load output voltage . 10
13 Short-circuit voltage . 12
14 Heating . 12
15 Short-circuit and overload protection . 12
16 Mechanical strength . 12
17 Protection against harmful ingress of dust, solid objects and moisture . 12
18 Insulation resistance, dielectric strength and leakage current . 12
19 Construction . 13
20 Components . 15
21 Internal wiring . 15
22 Supply connection and other external flexible cable or cords . 15
23 Terminals for external conductors . 15
24 Provisions for protective earthing. 15
25 Screws and connections . 15
26 Creepage distances, clearances and distances through insulation . 15
27 Resistance to heat, fire and tracking . 15
28 Resistance to rusting . 16
Annexes . 17
Annex C Creepage distances (cr), clearances (cl) and distances through insulation
(dti) Material group II (400 ≤CTI <600) .
Annex D Creepage distances (cr), clearances (cl) and distances through insulation
(dti) Material group I (CTI ≥600) .
Annex R Explanations of the application of 4.2 of IEC 60664-1:2007 (see IEC 61558-1
Subclause 26.2) .
Bibliography . 18

Table 101 – Output voltage difference .
Table 101 – Symbols indicating the kind of transformer . 10
Table 102 – Output voltage ratio . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SAFETY OF TRANSFORMERS, REACTORS, POWER SUPPLY
UNITS AND SIMILAR PRODUCTS FOR SUPPLY VOLTAGES UP TO 1 100 V
COMBINATIONS THEREOF –
Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety
isolating transformers for general applications

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.
This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition IEC 61558-2-6:2009. A vertical bar appears in the margin
wherever a change has been made. Additions are in green text, deletions are in
strikethrough red text.
– 4 – IEC 61558-2-6:2021 RLV © IEC 2021
International standard IEC 61558-2-6 has been prepared by IEC technical committee 96:
Transformers, reactors, power supply units and combinations thereof.
This third edition cancels and replaces the second edition published in 2009. 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) description of constructions moved in IEC 61558-1:2017;
c) new symbol for power supply unit with linearly regulated output voltage.
The text of this International Standard is based on the following documents:
FDIS Report on voting
96/506/FDIS 96/512/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.
NOTE When “Part 1” is mentioned in this standard, it refers to 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 safety
isolating transformers and power supply units incorporating safety isolating transformers for
general applications.
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,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

– 6 – IEC 61558-2-6:2021 RLV © IEC 2021
INTRODUCTION
IEC/TC 96 has 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, but in certain cases including limitation of voltage and horizontal safety function for
SELV in accordance with IEC 60364-4-41.
The group safety function (GSF) is necessary because of responsibility e.g. for safety extra-low
voltage (SELV) in accordance with IEC 61140:2016, 5.2.6 and IEC 60364-4-41:2017, 414.3.1
or control circuits in accordance with IEC 60204-1:2016, 7.2.4.
The group safety function is needed 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.

SAFETY OF TRANSFORMERS, REACTORS, POWER SUPPLY
UNITS AND SIMILAR PRODUCTS FOR SUPPLY VOLTAGES UP TO 1 100 V
COMBINATIONS THEREOF –
Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety
isolating transformers for general applications

1 Scope
Replacement
This part of IEC 61558 deals with the safety of safety isolating transformers for general
applications and power supply units incorporating safety isolating transformers for general
applications. 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 safety isolating
transformers for general applications and power supply units incorporating safety isolating
transformers for general applications.
NOTE 2 For power supply units (linear) this document is applicable. For switch mode power
supply units IEC 61558-2-16 is applicable together with this part.
This document is applicable to stationary or portable, single-phase or polyphase, air-cooled
(natural or forced) independent or associated dry- type transformers. The windings may can
be encapsulated or non-encapsulated.
The rated supply voltage does not exceed 1 100 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:
• 10 kVA for single-phase transformers;
• 16 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 32 Transformers intended to supply distribution networks are not included in the scope.
The no-load output voltage or the rated output voltage does not exceed 50 V AC or 120 V
ripple-free DC.
This document is not applicable to external circuits and their components intended to be
connected to the input terminals and output terminals of the transformers.
NOTE 3 Transformers covered by this document are used in applications where double or reinforced insulation
between circuits is required by the installation rules or by the end product standard.
NOTE 4 Attention is drawn to the following:

– 8 – IEC 61558-2-6:2021 RLV © IEC 2021
• additional requirements for transformers intended to be used in vehicles, on board ships,
and aircraft (from other applicable standards, national rules, etc.) may be necessary;
• measures to protect the enclosure and the components inside the enclosure against
external influences such as fungus, vermin, termites, solar-radiation and icing should also
be considered;
• the different conditions for transportation, storage, and operation of the transformers
should also be considered;
• additional requirements in accordance with other appropriate standards and national rules
may can be applicable to transformers intended for use in special environments.
NOTE 5 Future technological development of transformers may can necessitate a need to
increase 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 TCs 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 TC is, wherever applicable, to make use of BSPs and/or GSPs
in the preparation of its publications.
2 Normative references
This clause of Part 1 is applicable except as follows:
Addition
IEC 61558-1:20052017, Safety of power transformers, power supplies, reactors and similar
products Safety of transformers, reactors, power supply units and combinations thereof – Part
1: General requirements and tests
3 Terms and definitions
This clause of Part 1 is applicable.
For the purposes of this document, the terms and definitions given in Part 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
4 General requirements
This clause of Part 1 is applicable.
5 General notes on tests
This clause of Part 1 is applicable.

6 Ratings
This clause of Part 1 is applicable except as follows:
Addition
6.101 The rated output voltage shall not exceed 50 V AC or 120 V ripple-free DC.
6.102 The rated output shall not exceed:
– 10 kVA for single-phase transformers;
– 16 kVA for polyphase transformers.
Transformers without limitation of the rated output shall be subject to agreement between the
purchaser and the manufacturer.
6.103 The rated supply frequency and the internal operating frequencies shall not
exceed 500 Hz.
6.104 The rated supply voltage shall not exceed 1 100 1 000 V AC.
Compliance with the requirements of 6.101 to 6.104 is checked by inspection of the marking.
7 Classification
This clause of Part 1 is applicable.
8 Marking and other information
This clause of Part 1 is applicable except as follows:
8.1 h)
Replacement
Replace the content up to the first sentence semi-colon by 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 61558-2-6:2021 RLV © IEC 2021
Table 101 – Symbols indicating the kind of transformer
Symbol or
Explanation or title Identification
graphical symbol
F
Fail-safe safety isolating transformer IEC 60417-5222:2002-10

Non-short-circuit-proof safety isolating transformer IEC 60417-5946:2002-10

Short-circuit-proof safety isolating transformer
IEC 60417-5947:2002-10
(inherently or non-inherently)

L
Power supply unit, linear IEC 60417-6210:2013-10

9 Protection against electric shock
This clause of Part 1 is applicable.
10 Change of input voltage setting
This clause of Part 1 is applicable.
11 Output voltage and output current under load
This clause of Part 1 is applicable.
12 No-load output voltage
This clause of Part 1 is applicable except as follows:
Addition
The no-load output voltage is measured when the transformer is connected to the rated
supply voltage at the rated supply frequency at ambient temperature.
12.101 The no-load output voltage shall not exceed 50 V AC or 120 V ripple-free DC.
For independent transformers, this output voltage limitation applies even when output
windings, not intended for interconnection, are connected in series.
NOTE The requirement for series connection does not apply to associated or IP 00
transformers.
12.102 The difference between the no-load output voltage and the output voltage under
load shall not be excessive.
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.
The difference is expressed as a percentage of the latter voltage calculated according to the
following formula:
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
Type of transformer Ratio between no-load output voltage and output
voltage under load
Rated output
VA %
Inherently short-circuit-proof transformers:
– up to and including 63 100
– over 63 up to and including 630 50
– over 630 20
Other transformers:
– up to and including 10 100
– over 10 up to and including 25 50
– over 25 up to and including 63 20
– over 63 up to and including 250 15
– over 250 up to and including 630 10
– over 630 5
– 12 – IEC 61558-2-6:2021 RLV © IEC 2021
Compliance with the requirements of 12.101 and 12.102 is checked by measuring the no-load
output voltage at the ambient temperature when the transformer, is connected to the rated
supply voltage at the rated supply frequency.
The difference shall not exceed the values shown in Table 101.
Table 101 – Output voltage difference
Type of transformer Difference between no-load output voltage and
Rated output 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 20
Other transformers:
– up to and including 10 100
– over 10 up to and including 25 50
– over 25 up to and including 63 20
– over 63 up to and including 250 15
– over 250 up to and including 630 10
– over 630 5
The ratio shall not exceed the values shown in Table 102.
13 Short-circuit voltage
This clause of Part 1 is applicable.
14 Heating
This clause of Part 1 is applicable.
15 Short-circuit and overload protection
This clause of Part 1 is applicable.
16 Mechanical strength
This clause of Part 1 is applicable.
17 Protection against harmful ingress of dust, solid objects and moisture
This clause of Part 1 is applicable.
18 Insulation resistance, dielectric strength and leakage current
This clause of Part 1 is applicable.

19 Construction
This clause of Part 1 is applicable except as follows:
Replace 19.1 of Part 1 by the following:
19.1 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, taking Clauses 18 and 26 into
consideration.
19.1.1 The insulation between input and output winding(s) shall consist of double or
reinforced insulation (rated for the working voltage) unless the requirements in 19.1.3 are
complied with.
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 earth shall
consist of at least basic insulation rated for the input voltage. The insulation between
the output windings and the body connected to earth, 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);
– 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), for SELV circuits basic insulation only is
required.
19.1.2 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 following requirements
are applicable:
19.1.2.1 for class I and class II transformers, 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);
– 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 SELV circuits basic insulation only is required;
– for transformer different from independent (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).
19.1.2.2 as alternative to 19.1.2.1 for class I transformer not intended to be
connected by means of a plug and for transformer different from independent (IP00),
if the construction assure that all laminated plates of the iron core are connected to
earth (e.g by soldering / welding) and if in the data sheet or instruction sheet clearly
state that the safety of the transformer depends on the earth connection and t
...

IEC 61558-2-6 ®
Edition 3.0 2021-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
GROUP SAFETY PUBLICATION
PUBLICATION GROUPÉE DE SÉCURITÉ
Safety of transformers, reactors, power supply units and combinations thereof –
Part 2-6: Particular requirements and tests for safety isolating transformers and
power supply units incorporating safety isolating transformers for general
applications
Sécurité des transformateurs, bobines d'inductance, blocs d'alimentation et des
combinaisons de ces éléments –
Partie 2-6: Exigences particulières et essais pour les transformateurs de sécurité
et les blocs d'alimentation incorporant des transformateurs de sécurité pour
applications générales
ICS 29.180  ISBN 978-2-8327-0445-5

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– 2 – IEC 61558-2-6:2021 © IEC 2021
CONTENTS
FOREWORD . 3
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 8
3 Terms and definitions . 8
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 . 10
10 Change of input voltage setting . 10
11 Output voltage and output current under load . 10
12 No-load output voltage . 10
13 Short-circuit voltage . 11
14 Heating . 11
15 Short-circuit and overload protection . 12
16 Mechanical strength . 12
17 Protection against harmful ingress of dust, solid objects and moisture. 12
18 Insulation resistance, dielectric strength and leakage current . 12
19 Construction. 12
20 Components . 12
21 Internal wiring . 12
22 Supply connection and other external flexible cable or cords . 12
23 Terminals for external conductors . 12
24 Provisions for protective earthing . 12
25 Screws and connections . 12
26 Creepage distances, clearances and distances through insulation . 12
27 Resistance to heat, fire and tracking . 13
28 Resistance to rusting . 13
Annexes . 14
Bibliography . 15

Table 101 – Symbols indicating the kind of transformer . 10
Table 102 – Output voltage ratio . 11

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

Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety
isolating transformers for general applications

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,
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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
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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
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Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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
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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
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) 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.
International standard IEC 61558-2-6 has been prepared by IEC technical committee 96:
Transformers, reactors, power supply units and combinations thereof.
This third edition cancels and replaces the second edition published in 2009. 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) description of constructions moved in IEC 61558-1:2017;
c) new symbol for power supply unit with linearly regulated output voltage.

– 4 – IEC 61558-2-6:2021 © IEC 2021
The text of this International Standard is based on the following documents:
FDIS Report on voting
96/506/FDIS 96/512/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.
NOTE When “Part 1” is mentioned in this standard, it refers to 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 safety
isolating transformers and power supply units incorporating safety isolating transformers for
general applications.
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,
• replaced by a revised edition, or
• amended.
– 6 – IEC 61558-2-6:2021 © IEC 2021
INTRODUCTION
IEC/TC 96 has 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, but in certain cases including limitation of voltage and horizontal safety function for
SELV in accordance with IEC 60364-4-41.
The group safety function (GSF) is necessary because of responsibility e.g. for safety extra-low
voltage (SELV) in accordance with IEC 61140:2016, 5.2.6 and IEC 60364-4-41:2017, 414.3.1
or control circuits in accordance with IEC 60204-1:2016, 7.2.4.
The group safety function is needed 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.

SAFETY OF TRANSFORMERS, REACTORS, POWER SUPPLY
UNITS AND COMBINATIONS THEREOF –

Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety
isolating transformers for general applications

1 Scope
Replacement
This part of IEC 61558 deals with the safety of safety isolating transformers for general
applications and power supply units incorporating safety isolating transformers for general
applications. 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 safety isolating
transformers for general applications and power supply units incorporating safety isolating
transformers for general applications.
For power supply units (linear) this document is applicable. For switch mode power supply
units IEC 61558-2-16 is applicable.
This document is applicable to stationary or portable, single-phase or polyphase, air-cooled
(natural or forced) independent or associated dry- type transformers. The windings can be
encapsulated or non-encapsulated.
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:
• 10 kVA for single-phase transformers;
• 16 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.
The no-load output voltage or the rated output voltage does not exceed 50 V AC or 120 V
ripple-free DC.
This document is not applicable to external circuits and their components intended to be
connected to the input terminals and output terminals of the transformers.
NOTE 3 Transformers covered by this document are used in applications where double or reinforced insulation
between circuits is required by the installation rules or by the end product standard.
Attention is drawn to the following:
• additional requirements for transformers intended to be used in vehicles, on board ships,
and aircraft (from other applicable standards, national rules, etc.);

– 8 – IEC 61558-2-6:2021 © IEC 2021
• 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.
Future technological development of transformers can necessitate a need to increase 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 TCs 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 TC is, wherever applicable, to make use of BSPs and/or GSPs
in the preparation of its publications.
2 Normative references
This clause of Part 1 is applicable except as follows:
Addition
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 Part 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
4 General requirements
This clause of Part 1 is applicable.
5 General notes on tests
This clause of Part 1 is applicable.
6 Ratings
This clause of Part 1 is applicable except as follows:
Addition
6.101 The rated output voltage shall not exceed 50 V AC or 120 V ripple-free DC.

6.102 The rated output shall not exceed:
– 10 kVA for single-phase transformers;
– 16 kVA for polyphase transformers.
Transformers without limitation of the rated output shall be subject to agreement between the
purchaser and the manufacturer.
6.103 The rated supply frequency and the internal operating frequencies shall not
exceed 500 Hz.
6.104 The rated supply voltage shall not exceed 1 000 V AC.
Compliance with the requirements of 6.101 to 6.104 is checked by inspection of the marking.
7 Classification
This clause of Part 1 is applicable.
8 Marking and other information
This clause of Part 1 is applicable except as follows:
8.1 h)
Replacement
Replace the content up to the first semi-colon by 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 61558-2-6:2021 © IEC 2021
Table 101 – Symbols indicating the kind of transformer
Symbol or
Explanation or title Identification
graphical symbol
Fail-safe safety isolating transformer IEC 60417-5222:2002-10

Non-short-circuit-proof safety isolating transformer IEC 60417-5946:2002-10

Short-circuit-proof safety isolating transformer IEC 60417-5947:2002-10
(inherently or non-inherently)

Power supply unit, linear IEC 60417-6210:2013-10
L
9 Protection against electric shock
This clause of Part 1 is applicable.
10 Change of input voltage setting
This clause of Part 1 is applicable.
11 Output voltage and output current under load
This clause of Part 1 is applicable.
12 No-load output voltage
This clause of Part 1 is applicable except as follows:
Addition
12.101 The no-load output voltage shall not exceed 50 V AC or 120 V ripple-free DC.
For independent transformers, this output voltage limitation applies even when output
windings, not intended for interconnection, are connected in series.
The requirement for series connection does not apply to associated or IP 00 transformers.

12.102 The difference between the no-load output voltage and the output voltage under
load shall not be excessive.
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.
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
Type of transformer Ratio between no-load output voltage and output
voltage under load
Rated output
VA %
Inherently short-circuit-proof transformers:
– up to and including 63 100
– over 63 up to and including 630 50
– over 630 20
Other transformers:
– up to and including 10 100
– over 10 up to and including 25 50
– over 25 up to and including 63 20
– over 63 up to and including 250 15
– over 250 up to and including 630 10
– over 630 5
Compliance with the requirements of 12.101 and 12.102 is checked by measuring the no-load
output voltage at the ambient temperature when the transformer, is connected to the rated
supply voltage at the rated supply frequency.
The ratio shall not exceed the values shown in Table 102.
13 Short-circuit voltage
This clause of Part 1 is applicable.
14 Heating
This clause of Part 1 is applicable.

– 12 – IEC 61558-2-6:2021 © IEC 2021
15 Short-circuit and overload protection
This clause of Part 1 is applicable.
16 Mechanical strength
This clause of Part 1 is applicable.
17 Protection against harmful ingress of dust, solid objects and moisture
This clause of Part 1 is applicable.
18 Insulation resistance, dielectric strength and leakage current
This clause of Part 1 is applicable.
19 Construction
This clause of Part 1 is applicable and general requirements for safety isolating transformers
are defined in 19.1.4 of Part 1.
20 Components
This clause of Part 1 is applicable.
21 Internal wiring
This clause of Part 1 is applicable.
22 Supply connection and other
...