IEC 61558-2-12:2011

Safety of transformers, reactors, power supply units and combination thereof - Part 2-12: Particular requirements and tests for constant voltage transformers and power supply units for constant voltage

IEC 61558-2-12:2011 deals with the safety of constant voltage transformers for general applications and power supply units for constant voltage for general applications. Constant voltage transformers incorporating electronic circuits are also covered by this standard. This second edition cancels and replaces the first edition published in 2001. It constitutes a technical revision. The main changes consist of updating this part in accordance with IEC 61558-1:2009. This part has the status of a group safety publication in accordance with IEC Guide 104: The preparation of safety publications and the use of basic safety publications and group safety publications. The attention of National Committees is drawn to the fact that equipment manufacturers and testing organizations may need a transitional period following publication of a new, amended or revised IEC publication in which to make products in accordance with the new requirements and to equip themselves for conducting new or revised tests. It is the recommendation of the committee that the content of this publication be adopted for implementation nationally not earlier than 12 months from the date of publication. In the meantime, the previous edition can still be ordered by contacting your local IEC member National Committee or the IEC Central Office.
This publication is to be read in conjunction with IEC 61558-1:2005.

Sécurité des transformateurs, bobines d'inductance, blocs d'alimentation et des combinaisons de ces éléments - Partie 2-12: Exigences particulières et essais pour les transformateurs à tension constante et les blocs d'alimentation pour tension constante

La CEI 61558-2-12:2011 traite de la sécurité des transformateurs à tension constante pour applications d'ordre général et des blocs d'alimentation pour tension constante pour applications d'ordre général. Les transformateurs à tension constante incorporant des circuits électroniques sont également couverts par la présente norme. Cette deuxième édition annule et remplace la première édition parue en 2001, dont elle constitue une révision technique. Les principales modifications consistent en la mise à jour de la présente partie, conformément à la CEI 61558-1:2009. La présente partie a le statut de publication groupée de sécurité, conformément au Guide CEI 104: Élaboration des publications de sécurité et utilisation des publications fondamentales de sécurité et publications groupées de sécurité. L'attention des Comités nationaux est attirée sur le fait que les constructeurs des appareils et les organismes d'essai peuvent avoir besoin d'une période transitoire, à la parution d'une publication CEI qu'elle soit nouvelle, modifiée ou révisée, pour mettre les produits en conformité avec les nouvelles exigences et pour s'équiper eux-mêmes afin de réaliser de nouveaux essais ou des essais révisés. Le comité recommande que le contenu de la présente publication soit adopté pour mise en uvre au niveau national au plus tôt 12 mois après la date de publication. Pendant ce temps, l'édition précédente peut toujours être commandée auprès du Comité National CEI de votre pays ou auprès du bureau Central de la CEI.
Cette publication doit être lue conjointement avec la CEI 61558-1:2005.

General Information

• Status: Published
• Publication Date: 26-Jan-2011
• ICS: 29.180 - Transformers. Reactors
• ICS: 31.080.01 - Semiconductor devices in general
• Technical Committee: TC 96 - Transformers and reactors
• Drafting Committee: MT 1 - TC 96/MT 1
• Current Stage: DELPUB - Deleted Publication
• Start Date: 10-Apr-2024
• Completion Date: 29-Jul-2022

Relations

• Revised: IEC 61558-2-12:2024 - Safety of transformers, reactors, power supply units and combinations thereof - Part 2-12: Particular requirements and tests for constant voltage transformers and power supply units for constant voltage
• Effective Date: 05-Sep-2023
• Revises: IEC 61558-2-12:2001 - Safety of power transformers, power supply units and similar devices - Part 2-12: Particular requirements for constant voltage transformers
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61558-2-12 ®
Edition 2.0 2011-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
GROUP SAFETY PUBLICATION
PUBLICATION GROUPÉE DE SÉCURITÉ

Safety of transformers, reactors, power supply units and combination thereof –
Part 2-12: Particular requirements and tests for constant voltage transformers
and power supply units for constant voltage

Sécurité des transformateurs, bobines d’inductance, blocs d’alimentation et des
combinaisons de ces éléments –
Partie 2-12: Exigences particulières et essais pour les transformateurs à tension
constante et les blocs d'alimentation pour tension constante

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IEC 61558-2-12 ®
Edition 2.0 2011-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
GROUP SAFETY PUBLICATION
PUBLICATION GROUPÉE DE SÉCURITÉ

Safety of transformers, reactors, power supply units and combination thereof –
Part 2-12: Particular requirements and tests for constant voltage transformers
and power supply units for constant voltage

Sécurité des transformateurs, bobines d’inductance, blocs d’alimentation et des
combinaisons de ces éléments –
Partie 2-12: Exigences particulières et essais pour les transformateurs à tension
constante et les blocs d'alimentation pour tension constante
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX S
ICS 29.180 ISBN 978-2-88912-341-4

– 2 – 61558-2-12  IEC:2011
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 7
3 Terms and definitions . 7
4 General requirements . 7
5 General notes on tests . 7
6 Ratings . 7
7 Classification . 8
8 Marking and other information . 8
9 Protection against electric shock . 9
10 Change of input voltage setting . 9
11 Output voltage and output current under load . 9
12 No-load output voltage . 10
13 Short-circuit voltage . 11
14 Heating . 11
15 Short-circuit and overload protection . 11
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 . 17
21 Internal wiring . 17
22 Supply connection and other external flexible cables or cords . 17
23 Terminals for external conductors . 17
24 Provision for protective earthing . 17
25 Screws and connections . 17
26 Creepage distances, clearances and distances through insulation . 17
27 Resistance to heat, fire and tracking . 18
28 Resistance to rusting . 18
Annexes . 19
Annex C (normative) . 19
Annex D (normative) . 19
Annex L (normative) . 19
Annex R (normative) . 19
Bibliography . 20

61558-2-12  IEC:2011 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SAFETY OF TRANSFORMERS, REACTORS,
POWER SUPPLY UNITS AND COMBINATION THEREOF –

Part 2-12: Particular requirements and tests
for constant voltage transformers
and power supply units for constant voltage

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
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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 61558-2-12 has been prepared by IEC technical committee 96:
Transformers, reactors, power supply units, and combination thereof.
This second edition cancels and replaces the first edition published in 2001. It constitutes a
technical revision. The main changes consist of updating this part in accordance with
IEC 61558-1:2009.
This part has the status of a group safety publication in accordance with IEC Guide 104: The
preparation of safety publications and the use of basic safety publications and group safety
publications.
– 4 – 61558-2-12  IEC:2011
The text of this standard is based upon the following documents:
FDIS Report on voting
96/253/FDIS 96/266/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
This part is intended to be used in conjunction with the latest edition of IEC 61558-1 and its
amendments. It is based on the second edition (2005) of that standard.
This part supplements or modifies the corresponding clauses in IEC 61558-1, so as to convert
that publication into the IEC standard: Particular requirements and tests for constant voltage
transformers and power supply units for constant voltage.
A list of all parts of the IEC 61558 series, 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 a particular subclause of Part 1 is not mentioned in this part, that subclause applies as
far as is reasonable. Where this part states "addition", "modification" or "replacement", the
relevant text of Part 1 has to be adopted accordingly.
In this part, 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 part the words in bold are defined in Clause 3.
Subclauses, notes, figures and tables are additional to those in Part 1 are numbered starting
from 101; supplementary annexes are entitled AA, BB, etc.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
NOTE The attention of National Committees is drawn to the fact that equipment manufacturers and testing
organizations may need a transitional period following publication of a new, amended or revised IEC publication in
which to make products in accordance with the new requirements and to equip themselves for conducting new or
revised tests.
It is the recommendation of the committee that the content of this publication be adopted for implementation
nationally not earlier than 12 months from the date of publication.

61558-2-12  IEC:2011 – 5 –
SAFETY OF TRANSFORMERS, REACTORS,
POWER SUPPLY UNITS AND COMBINATION THEREOF –

Part 2-12: Particular requirements and tests
for constant voltage transformers
and power supply units for constant voltage

1 Scope
Replacement:
This part of IEC 61558 deals with the safety of constant voltage transformers for general
applications and power supply units for constant voltage for general applications.
Constant voltage transformers incorporating electronic circuits are also covered by this
standard.
NOTE 1 Safety includes electrical, thermal and mechanical aspects.
Unless otherwise specified, from here onward, the term transformer covers constant
voltage transformers for general applications and power supply units for constant voltage
for general applications.
This part is applicable to stationary or portable, single-phase or polyphase, air-cooled
(natural or forced), independent or associated dry-type:
– constant voltage auto-transformers;
– constant voltage separating transformers;
– constant voltage isolating transformers;
– constant voltage safety isolating transformers.
The windings may be encapsulated or non-encapsulated.
This standard is applicable to transformers and power supply (linear) with internal operating
frequencies not exceeding 500 Hz.
This standard used in combination with Part 2-16 for switch mode power supply (SMPS)
units is also applicable to power supplies with internal operating frequencies higher than
500 Hz. Where the two requirements are in conflict, the most severe take precedence.
The rated supply voltage does not exceed 1 000 V a.c., the rated supply frequency does
not exceed 500 Hz, the internal operating resonant frequency does not exceed 30 kHz and
the internal operating frequency does not exceed 100 MHz.
The rated output does not exceed:
– 40 kVA for single-phase constant voltage auto-transformers;
– 200 kVA for poly-phase constant voltage auto-transformers;
– 25 kVA for single-phase constant voltage separating transformers and constant
voltage isolating transformers;
– 40 kVA for poly-phase constant voltage separating transformers and constant
voltage isolating transformers;
– 10 kVA for single-phase constant voltage safety isolating transformers;

– 6 – 61558-2-12  IEC:2011
– 16 kVA for poly-phase constant voltage safety isolating transformers.
This part 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.
Where applicable to constant voltage auto-transformers
– the no-load output voltage or the rated output voltage does not exceed 1 000 V a.c. or
1 415 V ripple-free d.c., and for independent constant voltage auto-transformers, the
no-load output voltage and the rated output voltage exceed 50 V a.c. or 120 V ripple-
free d.c.;
– constant voltage auto-transformers covered by this part are used only in applications
where no insulation between circuits is required by the installation rules or by the end
product standard.
Where applicable to constant voltage separating transformers
– the no-load output voltage or the rated output voltage does not exceed 1 000 V a.c. or
1 415 V ripple-free d.c., and for independent constant voltage separating
transformers, the no-load output voltage and the rated output voltage exceed 50 V
a.c. or 120 V ripple-free d.c.;
– constant voltage separating transformers covered by this part are used only in
applications where double or reinforced insulation between circuits is not required by
the installation rules or by the end product standard.
Where applicable to constant voltage isolating transformers
– the no-load output voltage or the rated output voltage does exceed 50 V a.c. or 120 V
ripple-free d.c. and where applicable, does not exceed 500 V a.c. or 708 V ripple-free d.c.
The no-load output voltage and the rated output voltage may be up to 1 000 V a.c. or
1 415 V ripple-free d.c. for special applications;
– constant voltage isolating transformers covered by this part are used only in
applications where double or reinforced insulation between circuits is required by the
installation rules or by the end product standard.
Where applicable to constant voltage safety isolating transformers
– the no-load output voltage or the rated output voltage does not exceed 50 V a.c. or
120 V ripple-free d.c.;
– constant voltage safety isolating transformers covered by this part are used only in
applications where double or reinforced insulation between circuits is required by the
installation rules or by the end product standard.
This part 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 Attention is drawn to the following:
– for transformers intended to be used in vehicles, on board ships, and aircraft, additional requirements (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 be applicable
to transformers intended for use in special environments.
NOTE 4 Future technological development of transformers may necessitate a need to increase the upper limit
of the frequencies, until then this part may be used as a guidance document.

61558-2-12  IEC:2011 – 7 –
2 Normative references
This clause of Part 1 is applicable.
3 Terms and definitions
This clause of Part 1 is applicable except as follows:
Addition:
3.101
constant voltage transformer
transformer intended to limit the influence of the input voltage variations
NOTE This type of transformer may also limit the influence of transients.
3.102
regulation tolerance
deviation in per cent of the rated output voltage when the constant voltage transformer is
supplied within the rated supply voltage variation
3.103
internal operating resonant frequency
frequency produced within a constant voltage transformer
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:
– 1 000 V a.c. or 1 415 V ripple-free d.c. for constant voltage auto-transformers and
constant voltage separating transformers;
– 250 V a.c. for single-phase portable constant voltage isolating transformers;
– 400 V a.c. for polyphase portable constant voltage isolating transformers, and
– 500 V a.c. or 708 V ripple-free d.c. for constant voltage isolating transformers. For
constant voltage isolating transformers, the rated output voltage may be up to 1 000
V a.c. or 1 415 V ripple-free d.c. to be in accordance with the national wiring rules or for a
special purpose;
– 50 V a.c. or 120 V ripple-free d.c. for constant voltage safety isolating transformers.

– 8 – 61558-2-12  IEC:2011
The rated output voltage shall exceed:
– 50 V a.c. or 120 V ripple-free d.c. for independent constant voltage auto-transformers,
constant voltage separating transformers and constant voltage isolating
transformers.
6.102 The rated output shall not exceed:
– 40 kVA for single-phase constant voltage auto-transformers,
– 200 kVA for polyphase constant voltage auto-transformers,
– 25 kVA for single-phase constant voltage separating and isolating transformers,
– 40 kVA for polyphase constant voltage separating and isolating transformers,
– 10 kVA for single-phase constant voltage safety isolating transformers,
– 16 kVA for polyphase constant voltage safety isolating transformers,
except for constant voltage transformers subject to an agreement between the purchaser
and the manufacturer.
6.103 The rated supply frequency shall not exceed 500 Hz.
6.104 The rated value of the output regulation tolerance shall be given at the rated supply
voltage range, the rated output, and the power factor = 1.
6.105 For constant voltage independent transformers, the input voltage variation shall
not be less than 10 %.
6.106 The internal operating resonant frequency shall not exceed 30 kHz.
6.107 The internal operating frequency shall not exceed 100 MHz.
Compliance with 6.101 to 6.107 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 a) Replacement of the first sentence by the following:
rated supply voltage(s) and the input voltage variation in %;
8.1 b) Replacement of the first sentence by the following:
Replace the first sentence by the following:
rated output voltage(s) and the regulation tolerance of this voltage(s) in %;
8.1 h) Replacement of the first sentence by the following:
relevant graphical symbols shown in 8.11 indicating the kind of transformer;

61558-2-12  IEC:2011 – 9 –
8.11 Addition:
Symbol or graphical symbol Explanation or title Identification
Fail-safe constant voltage separating transformer IEC 60417-6011

Non-short-circuit-proof constant voltage
IEC 60417-6011
separating transformer
Short-circuit-proof constant voltage separating
IEC 60417-6011
transformer
Fail-safe constant voltage isolating transformer IEC 60417-6012

Non-short-circuit-proof constant voltage isolating
IEC 60417-6012
transformer
Short-circuit-proof constant voltage isolating
IEC 60417-6012
transformer
Fail-safe constant voltage safety isolating
IEC 60417-6013
transformer
Non-short-circuit-proof constant voltage safety
IEC 60417-6013
isolating transformer
Short-circuit-proof constant voltage safety
IEC 60417-6013
isolating transformer
Fail-safe constant voltage auto-transformer IEC 60417-6017

Non-short-circuit-proof constant voltage auto-
IEC 60417-6017
transformer
Short-circuit-proof constant voltage auto-
IEC 60417-6017
transformer
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 except as follows:
11.1 Replacement:
– 10 – 61558-2-12  IEC:2011
11.1 When the transformer is connected to the rated supply voltage plus or minus the
input voltage variation declared by the manufacturer, at the rated supply frequency, and
loaded with an impedance resulting in the rated output at the rated output voltage and, for
a.c., at the rated power factor, the output voltage shall not differ from the rated value by
more than the regulation tolerance.
Compliance is checked by measuring the output voltage when steady-state conditions are
established. During the test, the transformer is connected to the rated supply voltage, plus
or minus the highest input voltage variation, at the rated supply frequency, and loaded with
an impedance resulting in the rated output at the rated output voltage and the rated power
factor.
For transformers incorporating a rectifier, the output voltage shall be measured at the
terminals of the d.c. circuit by means of a voltmeter giving the arithmetical mean value, unless
the effective (r.m.s.) value is specifically stated (see 8.1).
For transformers with more than one rated supply voltage, the requirement is applicable for
each of the rated supply voltages.
For transformers with multiple output windings, the loads are applied to every multiple
section simultaneously, unless otherwise declared.
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:
– 1 000 V a.c. or 1 415 V ripple-free d.c. for constant voltage auto-transformers and
constant voltage separating transformers;
– 250 V a.c. for single-phase portable constant voltage isolating transformers;
– 400 V a.c. for polyphase portable constant voltage isolating transformers, and
– 500 V a.c. or 708 V ripple-free d.c. for other constant voltage isolating transformers.
In this case, the no-load output voltage may be up to 1 000 V a.c. or 1 415 V ripple-free
d.c. to be in accordance with the national wiring rules or for special purpose;
– 50 V a.c. or 120 V ripple-free d.c. for constant voltage safety isolating transformers.
The no-load output voltage shall exceed:
– 50 V a.c. or 120 V ripple-free d.c. for constant voltage independent auto-, separating and
isolating transformers.
For independent transformers, this output voltage limitation applies even when output
windings, not intended for interconnection, are connected in series.
12.102 The difference between the no-load output voltage and the output voltage under
load shall not be excessive.
61558-2-12  IEC:2011 – 11 –
The difference between the no-load output voltage measured in this clause and the output
voltage under load measured during the test of Clause 11, expressed as a percentage of the
latter voltage, shall not exceed 10 %.
U −U
no-load load
NOTE The ratio is defined as follows: × 100 %
U
load
where U is the no-load output voltage and U is the output voltage under load.
no-load load
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 plus the highest input voltage variation at the rated supply frequency.
13 Short-circuit voltage
This clause of Part 1 is applicable.
14 Heating
This clause of Part 1 is applicable, except as follows:
14.1 Replacement of the 10th paragraph starting with "Transformers are supplied." by the
following:
The constant voltage transformers are connected to the rated supply voltage and loaded
with an impedance resulting in the rated output, at the rated output voltage and, for a.c., at
the rated power factor. The supply voltage is then increased to the value of the highest input
voltage variation declared by the manufacturer. After this voltage increase, no change or
adjustment is made in the circuit. The test is repeated under no-load condition with the
highest or the lowest input voltage variation if this is a more unfavorable condition.
15 Short-circuit and overload protection
This clause of Part 1 is applicable, except as follows:
15.1 Replacement of the 2nd paragraph starting with " Compliance is checked." by the
following:
Compliance is checked by inspection and by the following tests, which are carried out
immediately after the test according to 14.1, at the same ambient temperature, and without
changing the position of the constant voltage transformer at the maximum value of the
rated supply voltage designed for the constant voltage transformer.
15.2 Replacement:
Inherently short-circuit-proof constant voltage transformers shall be tested as follows:
Before starting the test, it is necessary to determine the highest output current of the
constant voltage transformer at the maximum value of the supply voltage designed for the
constant voltage transformer. The constant voltage transformer is then loaded with the
maximum output current or the output winding(s) short-circuited, whichever results in
the maximum temperature.
– 12 – 61558-2-12  IEC:2011
15.3 Non-inherently short-circuit-proof constant voltage transformers are tested as
follows:
15.3.1 Replacement:
The output windings are short-circuited or overloaded as stated in 15.2. The incorporated
overload protection device shall operate before the temperature exceeds the values shown in
Table 3 for any value of the supply voltage between the marked values of rated supply
voltage.
15.5 Fail-safe transformers
15.5.1 Replacement:
Three additional specimens are required, only for the following test. Constant voltage
transformers used in other tests are not subjected to this test.
Each of the three specimens is mounted as for normal use on a 20 mm thick dull black
painted plywood surface. Each constant voltage transformer is operated at rated supply
voltage plus or minus the input voltage variation declared by the manufacturer, the output
winding which produced the highest temperature during the test of 14.1 being initially loaded
with 1,5 times the rated output current (or, if this is not possible, the maximum value of the
output current obtainable) until steady-state conditions are reached or the constant voltage
transformer fails (whichever occurs first).
If the constant voltage transformer fails, it shall comply, during and after the tests, with the
criteria given in 15.5.2.
If the constant voltage transformer does not fail, the time to reach steady-state conditions is
noted and the chosen output winding is then short-circuited. The test is continued until the
constant voltage transformer fails. Each specimen shall do so within a duration, for this part
of the test, which is no longer than that necessary to attain steady-state conditions, but in any
case it shall not exceed 5 h.
The constant voltage transformer shall fail safely. It shall comply, during and after the test,
with the criteria given in 15.5.2.
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 except as follows:
Modification for constant voltage auto-transformers:
18.2 Insulation resistance
Table 7: values between each input circuit and all other input circuits, between each
output circuits and all other output circuits are not applicable.

61558-2-12  IEC:2011 – 13 –
18.3 Dielectric strength test
This subclause is applicable except as follows:
Modification for constant voltage auto-transformers:
Table 8a: lines 1) and 2) are not applicable.
18.4 Insulation between and within windings
This subclause is applicable except as follows:
Addition:
18.101
Table 101 – Dielectric strength test voltages for constant voltage auto-transformers
Application of dielectrical strength test voltage Working voltage
V
>50 150 300
Over functional insulation for windings intended to be connected
Working voltage + 500 V
in series or parallel
19 Construction
Replacement of this clause:
NOTE This clause reads slightly differently for each type of constant voltage transformer:
– constant voltage auto-transformers (see 19.1);
– constant voltage separating transformers (see 19.2);
– constant voltage isolating and safety isolating transformers (see 19.3).
19.1 Constant voltage auto-transformers
19.1.1 Plug-connected constant voltage auto-transformers, where the rated input
voltage is higher than the rated output voltage, shall not have any potential to earth at the
output socket higher than the rated output voltage.
This requirement shall be fulfilled by using one of the following methods:
a) Polarized input and output plug and socket-outlet system
In this case, an instruction shall be given for not using such a transformer with a non-
polarized plug and socket-outlet system.
b) Polarity detecting device (for non polarised input and output plug and socket-outlet
system)
A polarity detecting device shall only energise the output circuit when the potential to earth at
the poles of the output socket does not exceed the rated output voltage. The contact
separation of the breaking device shall be at least of 3 mm in each pole.
NOTE A magnetic relay is an example of polarity detecting device.

– 14 – 61558-2-12  IEC:2011
Compliance is checked by the following test:
The auto-transformer is connected to the mains at 1,06 times the rated input voltage under
the most unfavourable condition of load and output voltage. The test is repeated with the
polarity of the input reversed. During the test, the measured potential to earth of each pole
shall not exceed the maximum output voltage under load (1,06 times the rated output
voltage taking into account the permissible deviations of Clause 11).
The contact separation of the device shall be 3 mm minimum in each pole.
Compliance is checked by measurement.
If the polarity detecting device uses a current flowing to the earth for the detection, this
current shall not exceed 0,75 mA and shall only be flowing for the period of measurement until
the polarity is reversed.
Compliance is checked by measurement.
All the tests are repeated under the fault conditions of H.2.3 of Part 1. In this case, the
potential to earth of each pole shall not exceed 1,1 times the maximum output voltage under
load for more than 5 s.
Compliance is checked by measurement.
19.1.2 Protection against direct contact with electrical and moving mechanical parts (contact
path and drive) shall be ensured.
Compliance is checked by inspection.
19.2 Constant voltage separating transformers
19.2.1 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 of Part 1
into consideration.
19.2.2 The insulation between the input and output winding(s) shall consist of at least
basic insulation (rated for the working voltage).
In addition, the following applies:
– for class I transformers, the insulation between the input windings and the body, and
between the output windings and the body, shall consist of at least basic insulation
(both basic insulations 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, shall consist of double or reinforced
insulation (both double or reinforced insulations rated for the working voltage).
19.2.3 For constant voltage transformers with intermediate conductive part (e.g. the iron
core) or a resonant circuit not connected to the body and located between the input and
output windings, the insulation between the intermediate conductive part (or a resonant
circuit) and the input windings and between the intermediate conductive part (or a resonant
circuit) and the output windings shall consist of at least basic insulation (rated for the
working voltage).
61558-2-12  IEC:2011 – 15 –
NOTE An intermediate conductive part (or a resonant circuit) 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).
In addition, the following applies:
– for class I transformers, the insulation between the input and output windings via the
intermediate conductive parts shall consist of at least basic 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).
19.2.4 Parts of output circuits may be connected to protective earth.
19.2.5 There shall be no connections between the output circuit and the body, unless - for
associated transformers - allowed by the relevant equipment standard.
Compliance is checked by inspection.
19.3 Constant voltage isolating and safety isolating transformers
19.3.1 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 of Part 1
into consideration.
19.3.1.1 The insulation between the input and output winding(s) shall consist of double or
reinforced insulation (rated for the working voltage) unless the requirements of 19.3.1.3
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).
19.3.1.2 For constant voltage transformers with intermediate conductive parts (e.g. the
iron core) or a resonant circuit not connected to the body and located between the input and
output windings, the following requirements are applicable:
a) 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);

– 16 – 61558-2-12  IEC:2011
– 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 indep
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