SIST EN IEC 60422:2025

Mineral insulating oils in electrical equipment - Supervision and maintenance guidance

IEC 60422:2024 provides monitoring guidance and procedures that are required for the use and maintenance of mineral insulating oils and other hydrocarbon-based liquids in transformers and other electrical equipment, including strategic spares and tanks for holding spare parts and components.
This document is applicable to mineral insulating oils, originally supplied conforming to IEC 60296, in transformers, switchgear and other electrical apparatus where oil sampling is reasonably practicable, and where the normal operating conditions specified in the equipment specifications apply.
This document is also intended to assist the power equipment operator to evaluate the condition of the oil and maintain it in a serviceable condition. It also provides a common basis for the preparation of more specific and complete local codes of practice.
The document includes recommendations on tests and evaluation procedures, and outlines methods for reconditioning and reclaiming oil, and the decontamination of oil contaminated with PCBs.
NOTE The condition monitoring of electrical equipment, for example by analysis of dissolved gases, furanic compounds or other means, is outside the scope of this document.

Isolieröle auf Mineralölbasis in elektrischen Betriebsmitteln - Leitlinie zur Überwachung und Wartung

Huiles minérales isolantes dans les matériels électriques - Lignes directrices pour la maintenance et la surveillance

L'IEC 60422:2024 fournit des recommandations et des procédures de surveillance nécessaires pour l'utilisation et la maintenance des huiles minérales isolantes et d'autres liquides à base d'hydrocarbures dans les transformateurs et autres matériels électriques, y compris les pièces de rechange stratégiques et les cuves qui contiennent les pièces et composants de rechange.
Le présent document s'applique aux huiles minérales isolantes qui satisfont à l'origine aux exigences de l'IEC 60296 et sont présentes dans les transformateurs, appareillages de connexion et autres appareils électriques pour lesquels le prélèvement d'échantillons d'huile est raisonnablement possible et auxquels les conditions normales de fonctionnement prévues dans les spécifications du matériel s'appliquent.
Le présent document est également destiné à aider l'opérateur du matériel électrique à évaluer l'état de l'huile et à la maintenir en état d'être utilisée. Il fournit également une base commune à l'élaboration de codes de pratique locaux plus complets et plus spécifiques.
Le document inclut des recommandations sur les essais et les procédures d'évaluation et indique des procédés de retraitement et de régénération de l'huile ainsi que de décontamination de l'huile contaminée par les PCB.
NOTE La surveillance de l'état des matériels électriques, par exemple par l'analyse des gaz dissous et des dérivés furaniques, ou par tout autre moyen, ne fait pas partie du domaine d'application du présent document.

Mineralna izolacijska olja v električni opremi - Napotki za nadzorovanje in vzdrževanje

Standard IEC 60422:2024 zagotavlja napotke in postopke za nadzor, ki so potrebni za uporabo in vzdrževanje mineralnih izolacijskih olj in drugih tekočin na osnovi ogljikovodikov v transformatorjih in drugi električni opremi, vključno s strateškimi rezervnimi deli in cisternami za rezervne dele in komponente.
Ta dokument se uporablja za mineralna izolacijska olja, originalno dobavljena v skladu s standardom IEC 60296, v transformatorjih, stikalnih napravah in drugih električnih napravah, kjer je vzorčenje olj smiselno in kjer veljajo običajni pogoji delovanja, ki so navedeni v specifikaciji opreme.
Namen tega dokumenta je tudi, da upravljavcu električne opreme omogoča, da oceni stanje olja in ga vzdržuje v uporabnem stanju. Podaja tudi skupno osnovo za pripravo bolj specifičnih in popolnih lokalnih kodeksov ravnanja.
Ta dokument vključuje priporočila o postopkih preskušanja in ocenjevanja ter podaja metode za obnavljanje in ponovno uporabo olj. Podaja tudi metode za dekontaminacijo olj, ki so onesnažena s polikloriranimi bifenili (PCB).
OPOMBA: Nadzor stanja električne opreme (npr. z analizo raztopljenih plinov, furanskih spojin ali z drugimi sredstvi) ne spada na področje uporabe tega dokumenta.

General Information

• Status: Published
• Public Enquiry End Date: 26-Jun-2023
• Publication Date: 08-Jan-2025
• ICS: 29.040.10 - Insulating oils
• Technical Committee: TPD - Fluids for electrotechnical applications
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 19-Sep-2024
• Due Date: 24-Nov-2024
• Completion Date: 09-Jan-2025
• Ref Project: EN IEC 60422:2024 - Mineral insulating oils in electrical equipment - Supervision and maintenance guidance

Relations

• Revises: SIST EN 60422:2013 - Mineral insulating oils in electrical equipment - Supervision and maintenance guidance
• Effective Date: 01-Feb-2025

Overview

EN IEC 60422:2024 - "Mineral insulating oils in electrical equipment - Supervision and maintenance guidance" provides practical, consensus-based guidance for monitoring, testing and maintaining mineral insulating oils (typically supplied to IEC 60296) used in transformers, switchgear and other oil-filled electrical apparatus. The standard applies where oil sampling is reasonably practicable under normal equipment operating conditions. It helps power equipment operators evaluate oil condition, keep oil serviceable, and form the basis for local codes of practice. Note: condition monitoring by dissolved gas analysis (DGA), furanic compounds or similar techniques is outside the scope.

Key topics and technical requirements

EN IEC 60422:2024 covers recommended tests, evaluation criteria and remedial actions, including:

• Sampling procedures and frequency guidance for oils in service and strategic spares. (See IEC 60475 references.)
• In-service diagnostic tests: breakdown voltage (BDV), water content (oil and paper), acidity, dielectric dissipation factor (DDF), resistivity, colour, appearance, interfacial tension (IFT), particles, sediment, sludge, flash point, viscosity, density and pour point.
• Additives and inhibitors: monitoring inhibitor content and oxidation stability for inhibited and uninhibited oils.
• Contaminants and hazards: guidance on corrosive sulphur, metal passivators, polychlorinated biphenyls (PCBs) - including decontamination and removal processes where contamination exists.
• Oil treatment methods: reconditioning (degassing, drying, filtration), reclaiming processes (single-use and reactivating sorbents), replacement procedures and safe handling/storage.
• Classification and corrective action: evaluating oil condition, recommended remedial actions and when to replace or treat oil.
• Limitations: excludes detailed DGA-based equipment condition monitoring.

Practical applications and users

This standard is essential for:

• Power utility asset managers and transformer maintenance teams
• Substation and switchgear maintenance engineers
• Testing and oil analysis laboratories
• OEMs, specifiers and procurement teams creating maintenance schedules and local codes of practice
• Contractors performing oil reclamation, filtration and PCB decontamination

Use cases include routine oil condition monitoring, deciding when to recondition or replace oil, accepting returned or stored spare transformers, and developing maintenance protocols tied to oil-test results.

Related standards

Key referenced standards (normative or informative) include:

• IEC 60296 (mineral insulating oils specification)
• IEC 60475 (oil sampling)
• IEC 60156, IEC 60567, IEC 60599, IEC 61198, IEC 61620 (test methods and analytical techniques)

EN IEC 60422:2024 is a practical resource for implementing robust transformer oil maintenance, improving equipment reliability and complying with harmonized European/IEC practices.

Standards Content (Sample)

SLOVENSKI STANDARD
01-februar-2025
Mineralna izolacijska olja v električni opremi - Napotki za nadzorovanje in
vzdrževanje
Mineral insulating oils in electrical equipment - Supervision and maintenance guidance
Isolieröle auf Mineralölbasis in elektrischen Betriebsmitteln - Leitlinie zur Überwachung
und Wartung
Huiles minérales isolantes dans les matériels électriques - Lignes directrices pour la
maintenance et la surveillance
Ta slovenski standard je istoveten z: EN IEC 60422:2024
ICS:
29.040.10 Izolacijska olja Insulating oils
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60422

NORME EUROPÉENNE
EUROPÄISCHE NORM August 2024
ICS 29.040.10 Supersedes EN 60422:2013
English Version
Mineral insulating oils in electrical equipment - Supervision and
maintenance guidance
(IEC 60422:2024)
Huiles minérales isolantes dans les matériels électriques - Isolieröle auf Mineralölbasis in elektrischen Betriebsmitteln -
Lignes directrices pour la maintenance et la surveillance Leitlinie zur Überwachung und Wartung
(IEC 60422:2024) (IEC 60422:2024)
This European Standard was approved by CENELEC on 2024-08-22. 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
© 2024 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60422:2024 E
European foreword
The text of document 10/1233/FDIS, future edition 5 of IEC 60422, prepared by TC 10 "Fluids for
electrotechnical applications" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 60422:2024.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2025-05-22
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2027-08-22
document have to be withdrawn
This document supersedes EN 60422:2013 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 60422:2024 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 60567 NOTE Approved as EN IEC 60567
IEC 60599 NOTE Approved as EN IEC 60599
IEC 61198 NOTE Approved as EN 61198
IEC 61620 NOTE Approved as EN 61620
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 EN 60156 -
breakdown 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 60296 2020 Fluids for electrotechnical applications - EN IEC 60296 2020
Mineral insulating oils for electrical
equipment
IEC 60475 - Method of sampling insulating liquids EN IEC 60475 -
IEC 60666 2010 Detection and determination of specified EN 60666 2010
additives in mineral insulating oils
IEC 60814 - Insulating liquids - Oil-impregnated paper EN 60814 -
and pressboard - Determination of water
by automatic coulometric Karl Fischer
titration
IEC 60970 - Insulating liquids - Methods for counting EN 60970 -
and sizing particles
IEC 61125 2018 Insulating liquids - Test methods for EN IEC 61125 2018
oxidation stability - Test method for
evaluating the oxidation stability of
insulating liquids in the delivered state
IEC 61619 - Insulating liquids - Contamination by EN 61619 -
polychlorinated biphenyls (PCBs) - Method
of determination by capillary column gas
chromatography
IEC 62021-1 - Insulating liquids - Determination of acidity EN 62021-1 -
- Part 1: Automatic potentiometric titration
IEC 62021-2 - Insulating liquids - Determination of acidity EN 62021-2 -
- Part 2: Colourimetric titration
Publication Year Title EN/HD Year
IEC 62535 2008 Insulating liquids - Test method for EN 62535 2009
detection of potentially corrosive sulphur in
used and unused insulating oil
IEC 62697-1 - Test methods for quantitative EN 62697-1 -
determination of corrosive sulfur
compounds in unused and used insulating
liquids - Part 1: Test method for
quantitative determination of
dibenzyldisulfide (DBDS)
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 2719 - Determination of flash point - Pensky- EN ISO 2719 -
Martens closed cup method
ISO 3016 - Petroleum and related products from EN ISO 3016 -
natural or synthetic sources -
Determination of pour point
ISO 3104 - Petroleum products – Transparent and EN ISO 3104 -
opaque liquids – Determination of
kinematic viscosity and calculation of
dynamic viscosity
ISO 3675 - Crude petroleum and liquid petroleum EN ISO 3675 -
products - Laboratory determination of
density - Hydrometer method
ISO 6247 - Petroleum products - Determination of - -
foaming characteristics of lubricating oils
ISO 9120 - Petroleum and related products - - -
Determination of air-release properties of
steam turbine and other oils - Impinger
method
ISO 12185 - Crude petroleum and petroleum products - EN ISO 12185 -
Determination of density - Oscillating U-
tube method
ASTM D7042 - Standard Test Method for Dynamic - -
Viscosity and Density of Liquids by
Stabinger Viscometer (and the Calculation
of Kinematic Viscosity)
ASTM D971 - Standard Test Method for Interfacial - -
Tension of Oil Against Water by the Ring
Method
DIN 51353 - Testing of insulating oils; detection of - -
corrosive sulfur; silver strip test

IEC 60422 ®
Edition 5.0 2024-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Mineral insulating oils in electrical equipment – Supervision and maintenance

guidance
Huiles minérales isolantes dans les matériels électriques – Lignes directrices

pour la maintenance et la surveillance

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040.10  ISBN 978-2-8322-9202-0

– 2 – IEC 60422:2024 © IEC 2024
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 11
4 Properties and deterioration or degradation of oil . 12
5 Categories of equipment . 12
6 Sampling of oil from equipment. 13
7 In-service oil diagnostic tests . 14
7.1 General . 14
7.2 Colour . 15
7.3 Appearance . 15
7.4 Breakdown voltage (BDV) . 15
7.5 Water content . 16
7.5.1 Water content in the oil and paper system . 16
7.5.2 Influence of water on the solid and liquid dielectric system . 16
7.5.3 Water in oil . 17
7.5.4 Water in the solid insulation . 18
7.5.5 Interpretation of results . 19
7.6 Acidity . 20
7.7 Dielectric dissipation factor (DDF) and resistivity . 21
7.8 Inhibitor content . 22
7.8.1 Oxidation stability . 22
7.8.2 Monitoring of uninhibited and inhibited oils . 22
7.9 Sediment . 22
7.10 Sludge . 23
7.11 Interfacial tension (IFT) . 23
7.12 Particles . 23
7.13 Flash point . 24
7.14 Compatibility . 24
7.14.1 General . 24
7.14.2 Compatibility between different insulating mineral oils (miscibility) . 24
7.15 Pour point . 25
7.16 Density . 25
7.17 Viscosity . 26
7.18 Polychlorinated biphenyls (PCBs) . 26
7.19 Corrosive sulphur in mineral insulating oil . 26
7.19.1 General . 26
7.19.2 Corrosive sulphur . 27
7.19.3 Potentially corrosive sulphur . 27
7.19.4 Dibenzyl disulphide (DBDS) . 27
7.20 Metal passivator. 28
7.21 Air release and foaming . 28
8 Evaluation of mineral insulating oil in new equipment . 28
9 Evaluation of oil in service . 29
9.1 General . 29

IEC 60422:2024 © IEC 2024 – 3 –
9.2 Frequency of examination of oils in service . 30
9.3 Testing procedures . 31
9.3.1 General . 31
9.3.2 Field tests . 31
9.3.3 Laboratory tests . 32
9.4 Classification of the condition of oils in service . 32
9.4.1 General . 32
9.4.2 Transformers . 32
9.4.3 Tap-changers . 32
9.4.4 Instrument and protection transformers. 33
9.4.5 Circuit breakers and switchgear . 33
9.4.6 Oil filled and OIP bushings . 33
9.5 Corrective action . 34
10 Interpretation of results . 35
11 Handling and storage. 45
12 Replacement of oil in electrical equipment . 46
12.1 Replacement of oil in transformers with an U below 72,5 kV and in
m
switchgear and associated equipment . 46
12.2 Replacement of oil in transformers with an U of 72,5 kV and above. 46
m
12.3 Replacement of oil in electrical equipment contaminated with PCBs . 47
13 Addition of metal passivators to in-service oil . 47
14 Treatment . 47
14.1 Warning . 47
14.2 Classification of treatment processes . 48
14.2.1 General . 48
14.2.2 Off-line (de-energized) vs on-line (energized) treatment . 48
14.2.3 Batch vs continuous mode treatment . 49
14.3 Reconditioning (degassing, drying and filtration) . 49
14.3.1 General . 49
14.3.2 Reconditioning equipment. 50
14.3.3 Application to electrical equipment . 52
14.4 Reclaiming . 53
14.4.1 General . 53
14.4.2 Reclaiming by percolation with single use sorbents . 54
14.4.3 Reclaiming process using reactivating sorbents . 54
14.4.4 Renewal of additives . 56
14.5 Removal of PCBs using dehalogenation and related processes . 56
14.5.1 General . 56
14.5.2 Dehalogenation processes using sodium and lithium derivatives . 56
14.5.3 Dehalogenation processes using polyethylene glycol and potassium
hydroxide (KPEG) . 56
14.5.4 Dehalogenation in continuous mode by closed circuit process . 56
14.5.5 Corrosive sulphur removal using KPEG . 57
14.6 Criteria for oil reclamation and treatments . 57
Annex A (normative) Criteria for reclamation and treatments . 58
A.1 General . 58
A.2 Key parameters to measure before and after reclamation . 58
A.3 Addition of inhibitor after reclamation . 59

– 4 – IEC 60422:2024 © IEC 2024
A.4 Time and location of sampling . 60
Annex B (informative) General guideline for moisture %RS for continuous online
monitoring . 61
Annex C (informative) Normalizing water in oil for comparison and trending (historical
practice) . 62
C.1 General . 62
C.2 Sampling temperature at or above 35 °C . 62
Annex D (informative) Material compatibility . 64
Annex E (informative) Test method for determination of sediment and sludge . 66
E.1 Sediment determination . 66
E.2 Sludge determination . 66
Annex F (informative) Contamination of oils with silicone . 67
Bibliography . 68

Figure 1 – Water solubility curves for unused mineral oil as adapted from literature [5]

(according to Formula (3)) . 18
Figure 2 – Hysteresis loops of %RS vs. oil temperature in transformers with 3 different
water content levels [5] . 20
Figure 3 – Correlation between resistivity and dissipation factor [6] . 21
Figure C.1 – Normalization factors according to Formula (C.2) . 63

Table 1 – Categories of equipment . 13
Table 2 – Tests for in-service mineral insulating oils . 14
Table 3 – Recommended limits for mineral insulating oils after filling in new electrical
equipment prior to first energization at site . 29
Table 4 – Recommended frequency of testing . 31
Table 5 – Transformers and reactors – Application and interpretation of tests . 35
Table 6 – Tap-changers – Application and interpretation of tests . 41
Table 7 – Instrument and protection transformers – Application and interpretation of

tests . 42
Table 8 – Circuit breakers and switchgear – Application and interpretation of tests . 43
Table 9 – Oil filled and OIP bushings – Application and interpretation of tests . 44
Table 10 – Summary of typical actions . 45
Table 11 – Conditions for processing mineral insulating oils (both inhibited and
uninhibited) . 50
Table 12 – Beta ratio related to filter efficiency . 51
Table A.1 – Parameters where limits should be agreed upon . 58
Table A.2 – Parameters where limits should not necessarily be set but should be
measured for baseline and trending . 59
Table B.1 – Moisture %RS, continuous online monitoring, general guideline . 61

IEC 60422:2024 © IEC 2024 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MINERAL INSULATING OILS IN ELECTRICAL EQUIPMENT –
SUPERVISION AND MAINTENANCE GUIDANCE

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
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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 60422 has been prepared by IEC technical committee 10: Fluids for electrotechnical
applications. It is an International Standard.
This fifth edition cancels and replaces the fourth edition published in 2013. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) This new edition represents a major revision of the fourth edition, bringing this document in
line with the latest developments in oil condition monitoring. New interpretation tables have
been created, containing limits for oil parameters specific to plant type with suggested
corrective actions in the tables and new test methods.
b) The action limits for all oil tests have been revised and changes made where necessary to
enable users to use current methodology and comply with requirements and regulations
affecting safety and environmental aspects.

– 6 – IEC 60422:2024 © IEC 2024
c) Category O has been removed and is now incorporated within category A.
d) Online moisture interpretation is now incorporated.
e) More guidance on oil treatment (including reclamation criteria) is now available.
f) Guidance has been updated regarding corrosive sulphur.
g) In addition, this document incorporates changes introduced in associated standards since
the fourth edition was published.
The text of this International Standard is based on the following documents:
Draft Report on voting
10/1233/FDIS 10/1239/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.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

IEC 60422:2024 © IEC 2024 – 7 –
INTRODUCTION
Insulating mineral oils are used in electrical equipment employed in the generation,
transmission, distribution, and use of electrical energy.
Monitoring and maintaining oil quality is essential to ensure the reliable operation of oil-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 guidance. It is
possible, however, to compare the value and significance of standardized oil tests and to
recommend uniform criteria for the evaluation of test data.
If a certain amount of oil deterioration (by degradation or contamination) is exceeded, there is
inevitably some erosion of safety margins and the question of the risk of 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 philosophy underlying this
document is to furnish users with as broad a base of understanding of oil quality deterioration
as is available, so that they can make informed decisions on inspection and maintenance
practices.
Mineral oils are valuable resources and should be utilised accordingly. Used mineral oils are,
by most regulations, deemed to be controlled waste. If spills occur, this can have a negative
environmental impact especially if the oil is contaminated by persistent organic pollutants such
as polychlorinated biphenyls (PCBs).
This document, whilst 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 will have to be exerted in seeking the best
compromise between technical requirements and economic factors.
Reference should also be made to instructions from the equipment manufacturer.
General caution
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 handling of mineral oils can be subject to local regulatory requirements and suppliers’
safety datasheets.
Environment, health, and safety
This document is applicable to mineral oils, chemicals and used sample containers. The
disposal of these items can be subject to local regulatory requirements regarding their impact
on the environment.
Attention is drawn to the fact that, at the time of writing this document, some mineral oils in
service are known to be contaminated to some degree with other liquids, for example, silicone
oils and PCBs.
Because of this, safety countermeasures should be taken to avoid risks to workers, the public
and the environment during the life of the equipment, by strictly controlling spills and emissions.
The disposal or decontamination of these oils can be subject to local regulatory requirements.
Every precaution should be taken to prevent release of mineral oil into the environment.

– 8 – IEC 60422:2024 © IEC 2024
Typically, each country has specific regulations around health and safety. Safety Data Sheets
(SDS) are normally used by the industry internationally and are usually written in accordance
with an international regulation set (such as REACH [1] ). Please consult the SDS from the
suppliers of the insulating product that is used. These documents provide essential information
regarding health, safety, and environmental impacts.

___________
Numbers in square brackets refer to the Bibliography.

IEC 60422:2024 © IEC 2024 – 9 –
MINERAL INSULATING OILS IN ELECTRICAL EQUIPMENT –
SUPERVISION AND MAINTENANCE GUIDANCE

1 Scope
This document provides monitoring guidance and procedures that are required for the use and
maintenance of mineral insulating oils and other hydrocarbon-based liquids in transformers and
other electrical equipment, including strategic spares and tanks for holding spare parts and
components.
This document is applicable to mineral insulating oils, originally supplied conforming to
IEC 60296, in transformers, switchgear and other electrical apparatus where oil sampling is
reasonably practicable, and where the normal operating conditions specified in the equipment
specifications apply.
This document is also intended to assist the power equipment operator to evaluate the condition
of the oil and maintain it in a serviceable condition. It also provides a common basis for the
preparation of more specific and complete local codes of practice.
The document includes recommendations on tests and evaluation procedures, and outlines
methods for reconditioning and reclaiming oil, and the decontamination of oil contaminated with
PCBs.
NOTE The condition monitoring of electrical equipment, for example by analysis of dissolved gases, furanic
compounds or other means, is outside the scope of this document.
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 60296:2020, Fluids for electrotechnical applications – Mineral insulating oils for electrical
equipment
IEC 60475, Method of sampling insulating liquids
IEC 60666:2010, 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 61125:2018, Insulating liquids – Test methods for oxidation stability – Test method for
evaluating the oxidation stability of insulating liquids in the delivered state

– 10 – IEC 60422:2024 © IEC 2024
IEC 61619, Insulating liquids – Contamination by polychlorinated biphenyls (PCBs) – Method
of determination by capillary column gas chromatography
IEC 62021-1, Insulating liquids – Determination of acidity – Part 1: Automatic potentiometric
titration
IEC 62021-2, Insulating liquids – Determination of acidity – Part 2: Colourimetric titration
IEC 62535:2008, Insulating liquids – Test method for detection of potentially corrosive sulphur
in used and unused insulating oil
IEC 62697-1, Test methods for quantitative determination of corrosive sulfur compounds in
unused and used insulating liquids – Part 1: Test method for quantitative determination of
dibenzyldisulfide (DBDS)
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 2719, Determination of flash point – Pensky-Martens closed 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 6247, Petroleum products – Determination of foaming characteristics of lubricating oils
ISO 9120, Petroleum and related products – Determination of air-release properties of steam
turbine and other oils – Impinger method
ISO 12185, Crude petroleum and petroleum products – Determination of density – Oscillating
U-tube method
ASTM D971, Standard Test Method for Interfacial Tension of Insulating Liquids Against Water
by the Ring Method
ASTM D7042, Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger
Viscometer (and the Calculation of Kinematic Viscosity)
DIN 51353, Testing of insulating oils – Detection of corrosive sulphur – Silver strip test

IEC 60422:2024 © IEC 2024 – 11 –
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 terminologies are available on:
• ASTM D2864: Standard Terminology Relating to Electrical Insulating Liquids and Gases [2]
• IEEE C57.12.80: Standard Terminology for Power and Distribution Transformers [3]
3.1
local regulations
regulations pertinent to the particular process in the country concerned
Note 1 to entry: Such regulations can include local, regional or national legislation or internal regulations set by the
owner or operator of the equipment. Such regulations can refer to operational, environmental or health and safety
issues. A detailed risk assessment, including consideration of such regulations, will usually be required.
3.2
routine tests (Group 1)
minimum tests required to monitor the oil 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.
3.3
complementary tests (Group 2)
additional tests, which can be used to obtain further specific information about the quality of the
oil, and can be used to assist in the evaluation of the oil for continued use in service
3.4
special investigative tests (Group 3)
tests used mainly to determine the suitability of the oil for the type of equipment in use and to
ensure compliance with environmental and operational considerations
3.5
reconditioning
process that eliminates or reduces gases, water, solid particles, and contaminants by physical
processing only
3.6
reclamation
process that eliminates or reduces soluble and insoluble polar contaminants from the oil by
chemical and physical processing
3.7
PCBs decontamination
process that eliminates or reduces PCBs contamination from mineral oil

– 12 – IEC 60422:2024 © IEC 2024
4 Properties and deterioration or degradation of oil
The reliable performance of mineral insulating oil in an insulation system depends upon certain
basic oil characteristics that can affect the overall performance of the electrical equipment.
Certain properties of oil are necessary to accomplish its multiple roles of dielectric insulation,
cooling, arc-quenching and lubrication. Oil needs to possess certain properties, in particular:
• high dielectric strength to withstand the electric stresses imposed in service;
• sufficiently low viscosity so that its ability to circulate and transfer heat is not impaired;
• adequate viscosity range and lubricity to ensure fault-free operation and endurance of
mechanical equipment, such as tap-changers, over the whole temperature range;
• adequate low-temperature properties down to the lowest temperature expected at the
installation site;
• resistance to oxidation and pyrolytic degradation (by switching arcs) to maximize service
life;
• non-corrosive or detrimental to electrical plant construction materials that it is in contact
with.
In service, mineral oil degrades due to the conditions of use. In many applications, insulating
oil is in contact with air and is therefore subject to oxidation. Elevated temperatures accelerate
degradation. The presence of metals, organo-metallic compounds, or both, can act as a catalyst
for oxidation. Changes in colour, the formation of acidic compounds and, at an advanced stage
of oxidation, precipitation of sludge can occur. Dielectric and, in extreme cases, thermal
properties can be impaired.
In addition to oxidation products, many other unde
...