EN IEC 61203:2025

SLOVENSKI STANDARD
01-september-2025
Nadomešča:
SIST EN 61203:1997
Sintetični estri - Smernice za vzdrževanje in uporabo v električni opremi
Synthetic esters - Guidelines for maintenance and use in electrical equipment
Synthetische organische Ester für elektrotechnische Zwecke - Leitlinie zur Wartung von
Transformator-Estern in Betriebsmitteln
Esters organiques de synthèse à usages
électriques - Guide de maintenance des esters pour transformateurs dans
les matériels
Ta slovenski standard je istoveten z: EN IEC 61203:2025
ICS:
29.035.20 Plastični in gumeni izolacijski Plastics and rubber insulating
materiali materials
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61203

NORME EUROPÉENNE
EUROPÄISCHE NORM July 2025
ICS 29.040.10; 29.180 Supersedes EN 61203:1994
English Version
Synthetic organic esters - Guidelines for maintenance and use in
electrical equipment
(IEC 61203:2025)
Esters organiques synthétiques - Lignes directrices pour la Synthetische organische Ester - Leitlinien für
maintenance et l'utilisation dans les matériels électriques Instandhaltung und Betrieb in elektrischen Betriebsmitteln
(IEC 61203:2025) (IEC 61203:2025)
This European Standard was approved by CENELEC on 2025-06-25. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61203:2025 E
European foreword
The text of document 10/1259/FDIS, future edition 2 of IEC 61203, prepared by TC 10 "Fluids for
electrotechnical applications" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 61203:2025.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2026-07-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2028-07-31
document have to be withdrawn
This document supersedes EN 61203:1994 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61203:2025 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60422 NOTE Approved as EN IEC 60422
IEC 61619 NOTE Approved as EN 61619
IEC 61039 NOTE Approved as EN IEC 61039
1 2
IEC 63177 NOTE Approved as EN IEC 63177

Under preparation. Stage at the time of publication: IEC FDIS 63177:2024.
Under preparation. Stage at the time of publication: FprEN IEC 63177:2024.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60156 - Insulating liquids - Determination of the breakdown EN IEC 60156 -
voltage at power frequency - Test method
IEC 60247 - Insulating liquids - Measurement of relative EN 60247 -
permittivity, dielectric dissipation factor (tan d) and
d.c. resistivity
IEC 60475 - Method of sampling insulating liquids EN IEC 60475 -
IEC 60666 - Detection and determination of specified additives in EN 60666 -
mineral insulating oils
IEC 60814 - Insulating liquids - Oil-impregnated paper and EN 60814 -
pressboard - Determination of water by automatic
coulometric Karl Fischer titration
IEC 60970 - Insulating liquids - Methods for counting and sizing EN 60970 -
particles
IEC 61099 - Insulating liquids - Specifications for unused EN 61099 -
synthetic organic esters for electrical purposes
IEC 61125 - Insulating liquids - Test methods for oxidation EN IEC 61125 -
stability - Test method for evaluating the oxidation
stability of insulating liquids in the delivered state
IEC 62021-3 - Insulating liquids - Determination of acidity - Part 3: EN 62021-3 -
Test methods for non-mineral insulating oils
IEC 62535 - Insulating liquids - Test method for detection of EN 62535 -
potentially corrosive sulphur in used and unused
insulating oil
IEC 62961 - Insulating liquids - Test methods for the EN IEC 62961 -
determination of interfacial tension of insulating
liquids - Determination with the ring method
ISO 2049 - Petroleum products_- Determination of colour - -
(ASTM_scale)
ISO 2211 - Liquid chemical products - Measurement of colour in - -
Hazen units (platinum-cobalt scale)
Publication Year Title EN/HD Year
ISO 2592 - Petroleum products - Determination of flash and fire - -
points - Cleveland open cup method
ISO 3016 - Petroleum and related products from natural or EN ISO 3016 -
synthetic sources - Determination of pour point
ISO 3104 - Petroleum products - Transparent and opaque EN ISO 3104 -
liquids - Determination of kinematic viscosity and
calculation of dynamic viscosity
ISO 3675 - Crude petroleum and liquid petroleum products - EN ISO 3675 -
Laboratory determination of density - Hydrometer
method
ISO 4406 - Hydraulic fluid power - Fluids_- Method for coding - -
the level of contamination by solid particles
ISO 5661 - Petroleum products - Hydrocarbon liquids - - -
Determination of refractive index
ISO 12185 - Crude petroleum, petroleum products and related EN ISO 12185 -
products - Determination of density - Laboratory
density meter with an oscillating U-tube sensor
ASTM D 92 - Standard Test Method for Flash and Fire Points by - -
Cleveland Open Cup Tester
ASTM D 97 - Standard Test Method for Pour Point of Petroleum - -
Products
ASTM D1275 - Standard test method for corrosive sulfur in electrical - -
insulating liquids
ASTM D1500 - Standard Test Method for ASTM Color of Petroleum - -
Products (ASTM Colour Scale)
ASTM D 5950 - Standard Test Method for Pour Point of Petroleum - -
Products (Automatic Tilt Method)
ASTM D 6922 - Standard Test Method for Determination of - -
Homogeneity and Miscibility in Automotive Engine
Oils
ASTM D 7042 - Standard Test Method for Dynamic Viscosity and - -
Density of Liquids by Stabinger Viscometer (and the
Calculation of Kinematic Viscosity)
ASTM D 7155 - Standard Practice for Evaluating Compatibility of - -
Mixtures of Turbine Lubricating Oils
ASTM D 7752 - Standard Practice for Evaluating Compatibility of - -
Mixtures of Hydraulic Liquids
DIN 51353 - Testing of insulating oils; detection of corrosive - -
sulfur; silver strip test
DIN 51423-1 - Testing of mineral oils - Part 1: Measurement of the - -
relative refractive index with the precision
refractometer
IEC 61203 ®
Edition 2.0 2025-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Synthetic organic esters – Guidelines for maintenance and use in electrical
equipment
Esters organiques synthétiques – Lignes directrices pour la maintenance et
l'utilisation dans les matériels électriques

ICS 29.040.10, 29.180 ISBN 978-2-8327-0396-0

IEC 61203:2025-05(en-fr)
– 2 – IEC 61203:2025 © IEC 2025
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 9
4 Categories of equipment . 9
5 In-service synthetic ester diagnostic tests . 10
6 Evaluation of synthetic esters in new equipment . 11
7 Evaluation of synthetic esters in equipment in service . 12
7.1 General . 12
7.2 Frequency of examination . 12
7.3 Testing procedures . 13
7.3.1 Field tests . 13
7.3.2 Laboratory tests . 14
7.4 Classification of in-service synthetic esters . 14
8 General requirements for corrective actions . 19
9 Interpretation of results. 19
9.1 Water content . 19
9.2 Colour and appearance . 19
9.3 Breakdown voltage (BDV) . 20
9.4 Viscosity . 20
9.5 Acidity. 20
9.6 Dielectric dissipation factor (DDF) and resistivity . 21
9.7 Fire point . 21
9.8 Interfacial tension (IFT) . 21
9.9 Density . 21
9.10 Pour point . 22
9.11 Additives . 22
9.12 Particle (identification, counting and sizing) . 22
9.13 Compatibility and miscibility of synthetic esters . 23
9.14 Oxidation stability . 24
9.15 Refractive index . 24
9.16 Potentially corrosive sulfur . 24
10 Sampling of synthetic esters from equipment . 24
Annex A (informative) Water and synthetic esters . 25
A.1 General . 25
A.2 Water content . 25
A.2.1 General . 25
A.2.2 Water in synthetic esters . 25
A.3 Moisture equilibrium between liquid and solid insulation . 27
Annex B (informative) Replacement and treatments of synthetic esters in transformers . 30
B.1 Transformer retrofilling with synthetic esters . 30
B.2 Reconditioning and reclaiming . 31
B.2.1 General . 31
B.2.2 Reconditioning . 31

IEC 61203:2025 © IEC 2025 – 3 –
B.2.3 Reclaiming . 31
Annex C (informative) Use of synthetic esters with tap-changers (OLTCs) . 32
Annex D (informative) Materials compatibility . 33
Bibliography . 34

Figure A.1 – Saturation curves for various liquids vs temperature . 27
Figure A.2 – Moisture equilibrium values between liquids and solid insulation at

50 °C [7] . 28
Figure A.3 – Comparison of moisture equilibrium curves for new cellulose paper
impregnated with different electroinsulating liquids at various temperatures [7] . 29

Table 1 – Categories of equipment . 10
Table 2 – Diagnostic tests for in-service synthetic esters . 10
Table 3 – Recommended limits for synthetic esters properties after filling in new
electrical equipment prior to first energization . 12
a
Table 4 – Recommended frequency of testing . 13
Table 5 – Recommended limits for in-service synthetic esters in transformers . 15
Table 6 – Miscibility of synthetic esters . 24
Table A.1 – Typical values for A and B for different insulating liquids [7] . 26

– 4 – IEC 61203:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SYNTHETIC ORGANIC ESTERS – GUIDELINES FOR MAINTENANCE
AND USE IN ELECTRICAL EQUIPMENT –

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61203 has been prepared by IEC technical committee 10: Fluids for electrotechnical
applications. It is an International Standard.
This second edition cancels and replaces the first edition published in 1992. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Expanded list of Normative references.
b) Fluid tests split into routine tests, complimentary tests and special investigative tests.
c) Equipment which can be affected by this document are now grouped in categories for easier
application of the monitoring Table 5.
d) Table 2 (new) lists the in-service fluid tests and methods, which will be mentioned in this
document.
IEC 61203:2025 © IEC 2025 – 5 –
e) Table 3 (new) lists the suggested test requirements for synthetic esters after filling in new
equipment.
f) Table 4 (new) gives advice on the recommended frequency of testing.
g) Table 5 (new) is much more detailed as to the recommended action limits for each type of
equipment and advice for the type of follow-up actions required.
h) Clause 9 (new) goes into detail describing each of the recommended tests.
i) Annex A (new) give information about the interaction of moisture and synthetic esters.
j) Annex B (new) gives information about retrofilling mineral oil transformers, reconditioning
and reclaiming of synthetic esters.
k) Annex C (new) gives information about the use of synthetic esters in tap-changers.
l) Annex D (new) gives information on materials compatibility with synthetic esters.
m) Expanded Bibliography.
The text of this International Standard is based on the following documents:
Draft Report on voting
10/1259/FDIS 10/1265/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/publications.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
– 6 – IEC 61203:2025 © IEC 2025
INTRODUCTION
Synthetic esters are increasingly being used in transformers and electrical equipment employed
in electrical power generation, transmission, distribution and industrial applications.
Synthetic esters are used in transformers over a wide range of voltage classes. These can be
sealed or free breathing. Synthetic esters should only be used in transformers where the internal
insulation system has been designed to run with these liquids, given the different electrical
performance of synthetic ester as compared to mineral oil. Retrofilling mixtures are not covered
in the normative part of this document. See Annex B for more information. When in doubt,
contact the transformer or liquid manufacturer for more information.
Monitoring and maintaining liquid quality is essential to ensure the reliable operation of
synthetic ester filled electrical equipment. Codes of practice for this purpose have been
established by electrical power authorities, power companies and industries in many countries.
A review of current experience reveals a wide variation of procedures and criteria. It is possible,
however, to compare the value and significance of standardized liquid tests and to recommend
uniform criteria for the evaluation of test data.
If a certain amount of liquid deterioration (by degradation or contamination) is exceeded, there
is inevitably some erosion of safety margin and the question of the risk of premature failure
should be considered. While the quantification of the risk can be very difficult, a first step
involves the identification of potential effects of increased deterioration. The underlying
philosophy of this document is to provide users with as broad a base of understanding of liquid
quality deterioration as is available, so that they can make informed decisions on inspection
and maintenance practices.
Synthetic esters are, by most regulations, deemed to be regulated or controlled (or both) waste.
If spills occur, these can be subject to regulatory requirements with regard to their specific
location.
This document, while technically sound, is mainly intended to serve as a common basis for the
preparation of more specific and complete codes of practice by users in the light of local
circumstances. Sound engineering judgement needs to be exerted in seeking the best
compromise between technical requirements and economic factors.
Although there is significant experience going back more than 40 years, that experience has
been mostly limited to the use of synthetic esters at distribution voltages, typically up to 72,5 kV.
Experience in large power transformers is increasing but is currently limited to a smaller number
of recently installed units. While the collection of operating data allows for the development of
this document, care is important when applying the recommended values in particular at
voltages at or above 72,5 kV.
WARNING – Health and safety
This document does not purport to address all the safety problems associated with its use. It is
the responsibility of the user of this document to establish appropriate health and safety
practices and determine the applicability of regulatory limitations prior to use.
The synthetic esters which are the subject of this document can be subject to regulatory
requirements and requirements in supplier’s safety data sheets.
WARNING – Environment
This document is applicable to synthetic esters, chemicals and used sample containers. The
disposal of these items can be subject to regulatory requirements with regard to their impact on
the environment.
IEC 61203:2025 © IEC 2025 – 7 –
SYNTHETIC ORGANIC ESTERS – GUIDELINES FOR MAINTENANCE
AND USE IN ELECTRICAL EQUIPMENT –

1 Scope
This document provides procedures and supervision for the use and maintenance of synthetic
esters in transformers and other electrical equipment.
This document is applicable to synthetic esters, originally supplied conforming to IEC 61099
and other applicable standards in transformers, switchgear and electrical apparatus where
liquid sampling is practical and where the normal operating conditions specified in the
equipment specifications apply.
This document is also intended to assist the power equipment operator in evaluating the
condition of the synthetic ester and in maintaining it in a serviceable condition. It also provides
a common basis for the preparation of more specific and complete local codes of practice.
This document includes recommendations on tests and evaluation procedures and outlines
methods for reconditioning and reclaiming the liquid, when necessary.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60156, Insulating liquids – Determination of the breakdown voltage at power frequency –
Test method
IEC 60247, Insulating liquids – Measurement of relative permittivity, dielectric dissipation factor
(tan δ) and d.c. resistivity
IEC 60475, Method of sampling insulating liquids
IEC 60666, Detection and determination of specified additives in mineral insulating oils
IEC 60814, Insulating liquids – Oil-impregnated paper and pressboard – Determination of water
by automatic coulometric Karl Fischer titration
IEC 60970, Insulating liquids – Methods for counting and sizing particles
IEC 61099, Insulating liquids – Specifications for unused synthetic organic esters for electrical
purposes
IEC 61125, Insulating liquids – Test methods for oxidation stability – Test method for evaluating
the oxidation stability of insulating liquids in the delivered state
IEC 62021-3, Insulating liquids – Determination of acidity – Part 3: Test methods for non-
mineral insulating oils
– 8 – IEC 61203:2025 © IEC 2025
IEC 62535, Insulating liquids – Test method for detection of potentially corrosive sulphur in
used and unused insulating oil
IEC 62961, Insulating liquids – Test methods for the determination of interfacial tension of
insulating liquids – Determination with the ring method
ISO 2049, Petroleum products – Determination of colour (ASTM scale)
ISO 2211, Liquid chemical products – Measurement of colour in Hazen units (platinum-cobalt
scale)
ISO 2592, Petroleum and related products – Determination of flash and fire points – Cleveland
open cup method
ISO 3016, Petroleum and related products from natural or synthetic sources – Determination of
pour point
ISO 3104, Petroleum products – Transparent and opaque liquids – Determination of kinematic
viscosity and calculation of dynamic viscosity
ISO 3675, Crude petroleum and liquid petroleum products – Laboratory determination of density
– Hydrometer method
ISO 4406, Hydraulic liquid power – Liquids – Method for coding the level of contamination by
solid particles
ISO 5661, Petroleum products – Hydrocarbon liquids – Determination of refractive index
ISO 12185, Crude petroleum, petroleum products and related products – Determination of
density – Laboratory density meter with an oscillating U-tube sensor
ASTM D92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester
ASTM D97, Standard Test Method for Pour Point of Petroleum Products
ASTM D1275, Standard Test Method for Corrosive Sulfur in Electrical Insulating Liquids
ASTM D1500, Standard Test Method for ASTM Color of Petroleum Products (ASTM Color
Scale)
ASTM D5950, Standard Test Method for Pour Point of Petroleum Products (Automatic Tilt
Method)
ASTM D6922, Standard Test Method for Determination of Homogeneity and Miscibility in
Automotive Engine Oils
ASTM D7042, Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger
Viscometer (and the Calculation of Kinematic Viscosity)
ASTM D7155, Standard Practice for Evaluating Compatibility of Mixtures of Turbine Lubricating
Oils
ASTM D7752, Standard Practice for Evaluating Compatibility of Mixtures of Hydraulic Liquids
DIN 51353, Testing of insulating oils – Detection of corrosive sulphur – Silver strip test

IEC 61203:2025 © IEC 2025 – 9 –
DIN 51423-1, Testing of mineral oils – Part 1: Measurement of the relative refractive index with
the precision refractometer
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
NOTE ASTM and IEEE terminology are available at:
– ASTM D2864: Standard Terminology Relating to Electrical Insulating Liquids and Gases [1] ;
– IEEE C57.12.80 Standard Terminology for Power and Distribution Transformers [2].
3.1
group 1 – routine tests
minimum tests required to monitor the liquid and to ensure that it is suitable for continued
service
Note 1 to entry: If the results obtained from these tests do not exceed recommended action limits, usually no further
tests are considered necessary until the next regular period for inspection but, under certain perceived conditions,
complementary tests can be deemed prudent.
3.2
group 2 – complementary tests
additional tests, which can be performed to obtain further specific information about the quality
of the synthetic ester, and which can be performed to assist in the evaluation of the synthetic
ester for continued use in service
3.3
group 3 – special investigative tests
tests performed mainly to determine the suitability of the synthetic ester for the type of
equipment in use and to ensure compliance with environmental and operational considerations
4 Categories of equipment
In order to take into account the different user requirements, equipment has been placed in
various categories as shown in Table 1 below.
___________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC 61203:2025 © IEC 2025
Table 1 – Categories of equipment
Category Type of equipment
Category A Power transformers or reactors with a nominal system voltage above 170 kV. Also, power
transformers of any rated voltage where continuity of supply is vital and similar equipment for
special applications operating under onerous conditions.
Category B Power transformers or reactors with a nominal system voltage above 72,5 kV and up to and
including 170 kV (other than those in Category A).
Category C Power transformers or reactors for MV/LV application e.g. nominal system voltages up to and
including 72,5 kV (distribution transformers) and traction transformers (other than those in
Category A).
Category D Instrument or protection transformers with a nominal system voltage above 170 kV.
Category E Instrument or protection transformers with a nominal system voltage up to and including 170 kV.
Category F Diverter tanks of On Load Tap Changers (OLTC), including combined selector/diverter tanks.
NOTE 1 Separated selector tanks of on-load tap-changers belong to the same category as the associated
transformer.
NOTE 2 Regardless of size or voltage, a risk assessment can justify condition-monitoring techniques usually
appropriate to a higher classification.
NOTE 3 For practical and economic reasons, some electrical utilities can decide that their small transformers up
to 1 MVA and 36 kV are not included in this classification. Routine monitoring programmes will possibly not be
considered economical for this type of equipment. Where a monitoring programme is required for these
transformers, the guidelines given for category C will usually be adequate.
NOTE 4 Owing to the very limited number of actual applications, recommended limits are not yet available for
Categories D and E. Therefore, these categories are here reported only for IEC 60422 [3] consistency and possibly
limits will be added at the next revision if there is feedback.
NOTE 5 For Category F, limit values are specified for breakdown voltage (BDV) and water content only. For
other parameters relevant to the tap-changer liquid, values can be adopted from the associated transformer
category and can also be applicable to vacuum type tap changers.

5 In-service synthetic ester diagnostic tests
Many tests can be applied to in-service synthetic esters in electrical equipment. The tests listed
in Table 2 are considered sufficient to determine whether the condition of the in-service
synthetic ester is adequate for continued operation and to suggest the type of corrective action
required, if necessary. When more than one test method is reported, the reference method is
the first one listed and it is also reported in Clause 9. In case of dispute, the reference method
shall be used.
NOTE The tests are not listed in order of priority within a grouping.
Table 2 – Diagnostic tests for in-service synthetic esters
Property Sub- Method
clause
Colour 9.2 ISO 2049
Appearance (visual assessment) Clear, free from sediment and
suspended matter
Group 1 –
Breakdown voltage (BDV) 9.3 IEC 60156
Routine tests
Water content 9.1 IEC 60814
Acidity (neutralization number) 9.5 IEC 62021-3
Dielectric dissipation factor (DDF) 9.6 IEC 60247
Fire point 9.7 ISO 2592 (reference method) or
Group 2 –
ASTM D92
Complementary
tests
Interfacial tension (IFT) 9.8 IEC 62961

IEC 61203:2025 © IEC 2025 – 11 –
Property Sub- Method
clause
Density 9.9 ISO 12185 (reference method)
ISO 3675 or ASTM D7042
Additives (antioxidant) content 9.11 IEC 60666, see Note
Pour point 9.10 ISO 3016 (reference method)
manual method ASTM D97 or
automated tilt method
ASTM D5950
ISO 3104 (reference method) or
Viscosity 9.4
ASTM D7042
Particles 9.12
– identification IEC 60970 Method B
– counting and sizing IEC 60970 Method A
Group 3 – Liquid compatibility and miscibility 9.13 ASTM D7752 or D7155 or
Special
ASTM D6922
investigative
No reference method
tests
(informative)
ISO 5661 (reference method) or
Refractive index 9.15
DIN 51423-1
Corrosive sulfur 9.16 ASTM D1275 or DIN 51353
Potentially corrosive sulfur 9.16 IEC 62535
PCB testing can be required in the case of IEC 61619 [4] is the only method
retrofilled PCB contaminated transformers suitable for insulating liquids, but
or cross contamination cannot be applicable to aged
synthetic ester. However, it is
likely that a later revision of
IEC 61619 can be proved
applicable
NOTE IEC 60666 is currently being updated to include testing for non-mineral oils.

6 Evaluation of synthetic esters in new equipment
A substantial proportion of electrical equipment is supplied to the final user already filled with
a synthetic ester. In these cases, as the synthetic ester has already come into contact with
insulating and other materials, it can no longer be considered as "unused synthetic ester" as
defined in IEC 61099. Therefore, its properties shall be regarded as those applicable to a
synthetic ester in new equipment prior to energization. Synthetic ester properties for new
equipment shall be appropriate to the category and functions of the transformers and reactors
(see Table 3).
NOTE As the characteristics of the synthetic ester in new equipment prior to energization are an integral part of
that equipment design, the user can request these characteristics to be better than the minimum standards suggested
in Table 3, which are based on the experience of many years of operating practice.

– 12 – IEC 61203:2025 © IEC 2025
Table 3 – Recommended limits for synthetic esters properties after filling in new
electrical equipment prior to first energization
Property Highest voltage for equipment U
m
≤ 72,5 kV 72,5 < U ≤ 170 kV > 170 kV
m
Appearance Clear, free from sediment matter
Colour ISO 2049, ASTM D1500 Max. 1
(ISO 2211 Max. 300 Hazen)
Breakdown voltage (kV) Min. 55 Min. 60 Min. 60
a
Max. 200 Max. 150 Max. 100
Water content (mg/kg)
Acidity (mg KOH/g) Max. 0,08
Dielectric dissipation factor at 90 °C Max. 0,1
Fire point (°C) ≥ 300
Particles See 9.12
PCB content (mg/kg) PCB content within limits
a
The values are not corrected for temperature since not enough time can have elapsed to reach an equilibrium
between the synthetic ester and cellulose insulation.

7 Evaluation of synthetic esters in equipment in service
7.1 General
A synthetic ester in service is subjected to heat, oxygen, water and catalysts, all of which are
detrimental to the properties of the insulation system. In order to maintain the quality of the
synthetic ester in service, regular sampling and analysis shall be performed.
From an environmental point of view, visual inspection can also be used to monitor leakage and
spills of synthetic esters.
Often the first sign of synthetic ester deterioration can be obtained by direct observation of the
synthetic ester clarity and colour through the sight glass of the conservator.
The interpretation of results, in terms of the functional deterioration of the synthetic ester, shall
be performed by experienced personnel based on the following elements of risk management
and life cycle management:
• characteristic values for the type and family of synthetic ester and equipment, developed by
statistical methods;
• evaluation of trends and the rate of variation of the values for a given synthetic ester
property;
• typical values, for "good", "fair" and "poor" for the appropriate type and family of equipment
are given in Table 5.
Application of synthetic esters in transformers is suitable for both sealed and free breathing
equipment. For thermally demanding (e.g. highly loaded and high temperature) applications,
sealed systems are recommended and for free breathing systems more frequent monitoring can
be appropriate.
7.2 Frequency of examination
By way of a guide, a suggested frequency of tests suitable for different types of equipment is
given in Table 4.
IEC 61203:2025 © IEC 2025 – 13 –
The testing frequency should ideally be established by means of a cost/benefit evaluation based
on a life cycle analysis and risk assessment taking into account the condition and criticality of
the unit as well as its category. In some instances, this approach can indicate different testing
frequencies from those given in Table 4. For example, some operators with many units in good
condition can decide not to perform all of the indicated programme for the particular type of
equipment, but conversely an operator with a critical transformer or one in poor condition can
decide to perform tests at a frequency appropriate to equipment in a higher category.
The optimum frequency will depend on the type, function, voltage, power, construction and
service conditions of the equipment, as well as the condition of the synthetic esters as
determined in the previous analysis. A compromise will often have to be found between
economic factors and reliability requirements.
Generally, analysis shall be carried out on the basis of the following criteria, which apply to
synthetic esters as well as other transformer dielectric liquids:
a) Characteristics can be tested periodically, at intervals as suggested in Table 4, unless
otherwise defined.
b) The frequency of examination can be increased where any of the significant properties
indicates that the liquid is in "Fair" or "Poor" condition, or when trend analysis indicates
significant changes.
c) The degradation of the synthetic ester will accelerate with increased temperature and in the
presence of oxygen (oxidation) and water (hydrolysis). Therefore, heavily loaded
transformers can possibly need more frequent liquid-sampling and complementary testing.
d) The testing frequency should be established by means of a cost/benefit evaluation based
on life cycle analysis and risk assessment. For some owne
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