IEC 61203:2025

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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