IEC 61203:2025

Synthetic organic esters - Guidelines for maintenance and use in electrical equipment

IEC 61203:2025 This document provides procedures and supervision for the use and maintenance of synthetic esters in transformers and other electrical equipment. This document includes recommendations on tests and evaluation procedures and outlines methods for reconditioning and reclaiming the liquid, when necessary

Esters organiques synthétiques - Lignes directrices pour la maintenance et l'utilisation dans les matériels électriques

IEC 61203:2025 Le présent document décrit les procédures et la surveillance pour l'utilisation et la maintenance des esters synthétiques dans les transformateurs et d'autres matériels électriques. Le présent document inclut des recommandations sur les essais et les procédures d'évaluation et indique des procédés de retraitement et de régénération du liquide, si nécessaire.

General Information

• Status: Published
• Publication Date: 20-May-2025
• ICS: 29.040.10 - Insulating oils
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: TC 10 - Fluids for electrotechnical applications
• Drafting Committee: MT 36 - TC 10/MT 36
• Current Stage: CFDIS - Draft circulated as FDIS
• Start Date: 31-Jan-2025
• Completion Date: 26-Nov-2024

Overview

IEC 61203:2025 is the International Electrotechnical Commission (IEC) standard providing comprehensive guidelines for the maintenance and use of synthetic organic esters in electrical equipment, specifically transformers and other related apparatus. The standard covers diagnostic testing, evaluation procedures, and methods for reconditioning and reclaiming synthetic esters used as insulating and cooling fluids.

As synthetic esters are increasingly adopted in electrical power generation, transmission, and distribution systems, this standard is essential for ensuring reliable, safe, and efficient operation of electrical equipment utilizing synthetic esters. It replaces the original 1992 edition, reflecting technical advancements and expanded industry experience with synthetic ester fluids.

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Key Topics

• Scope and Application
  IEC 61203:2025 applies to synthetic organic esters used in transformers and electrical equipment. It sets guidelines for use, in-service monitoring, maintenance, testing, and corrective actions to ensure optimal performance and longevity.

• Diagnostic Testing and Evaluation
  The standard categorizes equipment types and recommends specific field and laboratory tests such as:

  • Water content measurement
  • Breakdown voltage (BDV)
  • Viscosity and acidity testing
  • Dielectric dissipation factor (DDF)
  • Fire point and interfacial tension (IFT)
  • Particle analysis and oxidation stability

• Testing Frequency and Limits
  IEC 61203 provides tables with recommended testing intervals and acceptable property limits for synthetic esters in new and in-service electrical equipment. This helps maintenance teams monitor ester fluid condition proactively and decide when corrective actions are necessary.

• Maintenance and Reconditioning
  The document outlines procedures for retrofilling transformers with synthetic esters and methods for reconditioning and reclaiming ester fluids to extend service life and reduce environmental impact.

• Materials Compatibility and Special Considerations
  Annexes include guidance on compatibility of synthetic esters with materials and tap-changers (OLTCs), addressing practical operational concerns in diverse electrical setups.

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Applications

IEC 61203:2025 serves a vital role for engineers, maintenance professionals, and manufacturers working with transformers and other electrical equipment that use synthetic organic esters as insulating liquids. Key applications include:

• Transformer Maintenance Programs
  Implementing standardized testing protocols to monitor ester fluid aging, contamination, and dielectric integrity.

• Equipment Reliability and Safety Assurance
  Minimizing failures by ensuring synthetic ester fluids meet prescribed diagnostic test criteria during equipment lifecycle.

• Fluid Reconditioning and Sustainability
  Sustainable management of synthetic esters through approved reconditioning and reclaiming methods to reduce waste and operational costs.

• Engineering Design and Quality Control
  Guiding new equipment design and filling processes by adhering to recommended ester properties before energization.

This standard supports power generation plants, electricity transmission companies, and industrial facilities in maintaining high-performance electrical assets with enhanced environmental and safety compliance.

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Related Standards

• IEC 60966 - Synthetic esters for transformers - Specifications and test methods
• IEC 60296 - Mineral insulating oils for electrical equipment - Specifications
• IEC 60422 - Maintenance of insulating oils in electrical equipment - Guidelines
• IEC 61099 - Hepta- and octabromodiphenyl ethers - Specifications for flame retardants in electrical equipment

Utilizing IEC 61203:2025 alongside these complementary standards ensures holistic management of insulating liquids and electrical equipment maintenance throughout the asset's operational life cycle.

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Keywords: IEC 61203, synthetic organic esters, electrical equipment, transformer maintenance, insulating fluids, diagnostic testing, reconditioning synthetic esters, dielectric properties, electrical transformer standards, synthetic ester guidelines, electrical power equipment maintenance.

Standards Content (Sample)

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

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– 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

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
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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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.
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.
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

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

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.
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 owners, this approach can indicate
different testing frequencies from those indicated in Table 4.
a
Table 4 – Recommended frequency of testing
Property Equipment category
b
A B D E F
C
c
1 to 2 1 to 4 1 to 12 1 to 2 2 to 6 2 to 6
Group 1 (Routine tests) – years
These tests can be done periodically but less frequently than routine
Group 2 (Complementary tests)
tests. The frequency will depend upon the condition of liquid, age and
type. First (baseline) measurements should be done in new or
refurbished equipment prior to energization.
Group 3 (Special investigative tests) These are very special tests that need be done only under special
circumstances.
a
These proposed periods refer to a normal routine maintenance programme. Should one or more of the
measured properties indicate that the liquid is in a fair or poor condition or if an abnormal ageing trend is
observed, these periods should be shortened according to the importance of the equipment. For traction
transformers, sampling can be distance based rather than time based.
b
See 7.2 d).
c
Group 1 tests shall be performed after filling or refilling the transformer, prior to energizing.

7.3 Testing procedures
7.3.1 Field tests
In some circumstances there is a need to perform tests closer to the point of sampling rather
than in the laboratory. Field tests can help the operator to achieve prompt estimation of the
synthetic ester condition. Field testing shall be managed by experienced operators.

– 14 – IEC 61203:2025 © IEC 2025
7.3.2 Laboratory tests
A complete examination scheme includes all the tests listed in Table 2. These tests are
subdivided into the three groups identified as routine, complementary and investigative.
7.4 Classification of in-service synthetic esters
The condition of in-service synthetic ester shall be classified according to the results of the
tests listed in Table 2. The trend of these results over a period of time will add useful information
for arriving at the classification of the synthetic ester.
The condition of synthetic esters can be classified as "good", "fair" or "poor" based on the
evaluation of properties listed in Table 2. Table 5 provides guidance to assist in this
classification process.
• Good
Synthetic ester is in normal condition.
• Fair
Synthetic ester has some detectable degradation.
• Poor
Synthetic ester has significant degradation.
Table 5 provides guidance on follow-up actions based on condition.
No action shall be taken on the basis of one result and one property. Repeat samples are
recommended where the result appears abnormal compared to the trend of the results
previously obtained.
For Category F (tap-changers), only breakdown voltage and water content are specified. Even
if it is preferred for vacuum type models to be used in combination with synthetic esters (see
Annex C), the values specified are valid for both vacuum and non-vacuum type tap-changers.
Vacuum type tap-changers do not generally require regular time or condition-based oil
replacement, and oil ageing should be similar to the oil in the transformer main tank. Therefore,
monitoring of acidity is appropriate.
All OLTCs will present a certain amount of sediment in their oil compartment, due to normal
abrasive wear. Non-vacuum models can show significant amounts of sediment and sludge, due
to contact erosion and oil deterioration which are produced during their normal service life,
therefore limit values for colour, appearance and dielectric dissipation factor cannot be
provided. They can present high amounts of particles due to normal contact and mechanical
wear; additionally, normal arc breaking activity can produce high levels of carbon within the
liquid and can also produce volatile toxic compounds. Sufficient venting is required when
opening the tap-changer oil compartment for maintenance.
Vacuum type models should maintain a high level of oil quality during their whole service life,
only particles from mechanical wear should be present.

Table 5 – Recommended limits for in-service synthetic esters in transformers
a b c
Property Recommended action limits Notes
Category Recommended action
Good Fair Poor
Good: No further action.
Fair and poor – Typically no
Darkening (compared to the
action to be based on colour
Colour A-B-C < 2 > 2 and < 3,5 > 3,5 original colour) is a symptom of
alone. Action dictated by other
chemical contamination or ageing.
tests. Rapid change in colour
could indicate degradation.
Good: No further action.
Clear, free from Turbidity is a symptom of high-
Appearance A-B-C sediment and Lightly turbid Turbid water content or material
Fair and poor – As dictated by
suspended matter incompatibility.
other tests.
A > 60
...