EN IEC 61125:2018

Insulating liquids - Test methods for oxidation stability - Test method for evaluating the oxidation stability of insulating liquids in the delivered state

IEC 61125:2018 describes a test method for evaluating the oxidation stability of insulating liquids in the delivered state under accelerated conditions regardless of whether or not antioxidant additives are present. The duration of the test can be different depending on the insulating liquid type and is defined in the corresponding standards (e.g. in IEC 60296, IEC 61099, IEC 62770). The method can be used for measuring the induction period, the test being continued until the volatile acidity significantly exceeds 0,10 mg KOH/g in the case of mineral oils. This value can be significantly higher in the case of ester liquids. Additional test methods such as those described in IEC TR 62036 based on differential scanning calorimetry can also be used as screening tests, but are out of the scope of this document. This second edition cancels and replaces the first edition published in 1992 and Amendment 1: 2004. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) the title has been modified to include insulating liquids different from mineral insulating oils (hydrocarbon); b) the method applies for insulating liquids in the delivered state; c) former Method C is now the main normative method; d) precision data of the main normative method has been updated concerning the dissipation factor; e) former Method A has been deleted; f) former Method B has been transferred to Annex B; g) a new method evaluating the thermo-oxidative behaviour of esters is included in Annex C.

Isolierflüssigkeiten - Prüfverfahren für die Oxidationsbeständigkeit - Prüfverfahren zur Evaluierung der Oxidationsbeständigkeit von Isolierflüssigkeiten im Anlieferungszustand

Isolants liquides - Méthodes d’essai de la stabilité à l’oxydation - Méthode d’essai pour évaluer la stabilité à l’oxydation des isolants liquides tels que livrés

L'IEC 61125:2018 spécifie une méthode d’essai permettant d’évaluer dans des conditions accélérées la stabilité à l’oxydation des isolants liquides tels que livrés, que des additifs antioxydants soient présents ou non. La durée de l’essai peut différer selon les types d’isolants liquides, et elle est définie dans les normes correspondantes (par exemple, dans l’IEC 60296, l’IEC 61099, l’IEC 62770). La méthode peut être utilisée pour mesurer la période d’induction, l’essai se poursuivant jusqu’à ce que l’acidité volatile dépasse significativement 0,10 mg KOH/g dans le cas des huiles minérales. Cette valeur peut être nettement plus élevée dans le cas des esters liquides. Des méthodes d’essai complémentaires, par exemple celles basées sur l’analyse calorimétrique différentielle par balayage décrites dans l’IEC TR 62036, peuvent également être utilisées comme test de dépistage, mais elles ne font toutefois pas partie du domaine d’application du présent document. Cette deuxième édition annule et remplace la première édition parue en 1992 et l’Amendement 1: 2004. Cette édition constitue une révision technique. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente: a) modification du titre afin d’inclure les isolants liquides différents des huiles minérales isolantes (hydrocarbures); b) application de la méthode aux isolants liquides tels que livrés; c) l’ancienne méthode C est dorénavant la méthode normative principale; d) actualisation des données de fidélité de la méthode normative principale concernant le facteur de dissipation; e) suppression de l’ancienne méthode A; f) transfert de l’ancienne méthode B dans l’Annexe B; g) une nouvelle méthode d'évaluation de la stabilité à l’oxydation thermique des esters est proposée dans l’Annexe C.

Izolacijske tekočine - Metode za preskušanje oksidacijske stabilnosti - Preskusna metoda za vrednotenje oksidacijske stabilnosti dobavljenih izolacijskih tekočin

V tem dokumentu je opisan način preskušanja za namen vrednotenja oksidacijske stabilnosti izolacijskih tekočin v dostavljenem stanju pod stopnjevanimi pogoji, ne glede na to, ali so prisotni antioksidativni dodatki ali ne. Trajanja preskusa se lahko razlikuje glede na vrsto izolacijske tekočine in je opredeljeno v ustreznih standardih (npr. v standardih IEC 60296, IEC 61099, IEC 62770). Metodo je mogoče uporabljati za merjenje indukcijskega časa, pri čemer se preskus izvaja, dokler v primeru mineralnih olj hlapna kislina ne preseže bistveno 0,10 mg KOH/g. Ta vrednost je lahko bistveno višja v primeru tekočih estrov.
Vzorec izolacijske tekočine se hrani pri 120 °C ob prisotnosti bakrovega katalizatorja v trdnem stanju med prepihovanjem pri stalnem pretoku. Stopnja oksidacijske stabilnosti je ocenjena z merjenjem hlapne kisline, topne kisline, usedlin, faktorja dielektričnega razsipanja ali od časa, potrebnega za nastanek dane količine hlapne kisline (indukcijski čas z zrakom).
V informativnem Dodatku B je opisana metoda preskusa za vrednotenje oksidacijske stabilnosti inhibiranih mineralnih izolacijskih olj v dostavljenem stanju na podlagi merjenja indukcijskega časa s kisikom. Ta metoda je predvidena le za preverjanje kakovosti. Ni nujno, da rezultati zagotovijo informacije o vedenju med delovanjem. Vzorec olja se hrani pri 120 °C ob prisotnosti bakrovega katalizatorja v trdnem stanju med prepihovanjem pri stalnem toku kisika. Stopnja oksidacijske stabilnosti je ocenjena na podlagi časa, ki ga olje porabi za tvorjenje določene količine hlapne kisline (indukcijski čas s kisikom). Dodatne kriterije, kot so topne in hlapne kisline, usedline in faktor dielektričnega razsipanja, je tudi mogoče ugotoviti po določenem trajanju.
V informativnem Dodatku C je opisana metoda preskusa, predvidena za simulacijo termo-oksidativnega vedenja estrskih izolacijskih tekočin (plinska faza zraka pri 150 °C za 164 h).
Dodatne preskusne metode, kot so metode, opisane v standardu IEC TR 62036, ki temeljijo na diferenčni dinamični kalorimetriji, je tudi mogoče uporabiti kot presejalne preskuse, a so zunaj obsega tega dokumenta.

General Information

• Status: Published
• Publication Date: 22-Mar-2018
• Withdrawal Date: 15-Feb-2021
• ICS: 29.040.10 - Insulating oils
• Technical Committee: CLC/SR 10 - Fluids for electrotechnical applications
• Drafting Committee: IEC/TC 10 - IEC_TC_10
• Current Stage: 6060 - Document made available - Publishing
• Start Date: 23-Mar-2018
• Completion Date: 23-Mar-2018
• Ref Project: SIST EN IEC 61125:2018 - Insulating liquids - Test methods for oxidation stability - Test method for evaluating the oxidation stability of insulating liquids in the delivered state

Relations

• Replaces: EN 61125:1993/A1:2004 - Unused hydrocarbon-based insulating liquids - Test methods for evaluating the oxidation stability
• Effective Date: 26-Jan-2023
• Replaces: EN 61125:1993 - Unused hydrocarbon-based insulating liquids - Test methods for evaluating the oxidation stability
• Effective Date: 21-Mar-2017

Overview - EN IEC 61125:2018 (Oxidation stability of insulating liquids)

EN IEC 61125:2018 specifies a laboratory test method for evaluating the oxidation stability of insulating liquids in the delivered state under accelerated conditions. The method applies regardless of the presence of antioxidant additives and is intended to determine how resistant an insulating liquid (mineral oils and other dielectric fluids) is to oxidative degradation. The edition is a technical revision that supersedes earlier editions and amendments and extends scope beyond hydrocarbon oils to other insulating liquids.

Key topics and technical requirements

• Test principle: Accelerated oxidation of a delivered-state sample using controlled heating and oxygen/air flow; monitoring of chemical and electrical deterioration.
• Measured parameters:
  • Induction period (optional or as defined by specific product standards)
  • Volatile acidity (VA) and soluble acidity (SA) (used to track oxidation products)
  • Total acidity (TA)
  • Dielectric dissipation factor (DDF / tan δ) to assess electrical property degradation
  • Sludge formation and oxidation rate
• Pass/fail criteria: Test duration and acceptance limits depend on insulating liquid type and are defined in corresponding product standards (e.g., IEC 60296, IEC 61099, IEC 62770). For mineral oils the method continues until VA significantly exceeds 0,10 mg KOH/g; thresholds for ester liquids may be higher.
• Apparatus and reagents: Heating blocks or baths, oxidation/absorption tubes, catalysts (where applicable), analytical balances, titration reagents and indicators (referenced IEC methods for acidity measurement).
• Precision and reproducibility: The standard includes updated precision data (including dissipation factor) and example conditions (e.g., datasets for 164 h at 120 °C are referenced) to support interlaboratory comparability.
• Annexes:
  • Annex B - method for induction period measurement with oxygen (inhibited liquids)
  • Annex C - thermo-oxidative behaviour evaluation for ester insulating liquids
  • Former methods reorganized; Method C is now the main normative method.

Applications and who uses this standard

• Transformer and electrical equipment manufacturers for new fluid qualification and quality control.
• Insulating liquid producers (mineral oils, esters, synthetic fluids) for product development and batch release testing.
• Independent test laboratories performing oxidation stability testing and interlaboratory comparisons.
• Utilities and asset owners assessing fluid suitability, lifetime expectancy and maintenance planning.
• Regulatory and certification bodies referencing standardized test methods for compliance and procurement.

Related standards (for implementation)

• IEC 60296, IEC 61099, IEC 62770 - specification limits and test durations for specific insulating liquids
• IEC 62021-2 / -3 - acidity determination methods
• IEC 60247 - dielectric dissipation factor measurement
• IEC TR 62036 - DSC-based screening methods (not in scope but referenced as alternative screening)

EN IEC 61125:2018 is essential when you need a reproducible, industry‑accepted method to quantify oxidation resistance and predict in-service stability of insulating liquids.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2018
1DGRPHãþD
SIST EN 61125:1997
SIST EN 61125:1997/A1:2005
,]RODFLMVNHWHNRþLQH0HWRGH]DSUHVNXãDQMHRNVLGDFLMVNHVWDELOQRVWL3UHVNXVQD
PHWRGD]DYUHGQRWHQMHRNVLGDFLMVNHVWDELOQRVWLGREDYOMHQLKL]RODFLMVNLKWHNRþLQ
Insulating liquids - Test methods for oxidation stability - Test method for evaluating the
oxidation stability of insulating liquids in the delivered state
Ta slovenski standard je istoveten z: EN IEC 61125:2018
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 61125

NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2018
ICS 29.040.10 Supersedes EN 61125:1993
English Version
Insulating liquids - Test methods for oxidation stability - Test
method for evaluating the oxidation stability of insulating liquids
in the delivered state
(IEC 61125:2018)
Isolants liquides - Méthodes d'essai de la stabilité à Isolierflüssigkeiten - Prüfverfahren für die
l'oxydation - Méthode d'essai pour évaluer la stabilité à Oxidationsbeständigkeit - Prüfverfahren zur Evaluierung der
l'oxydation des isolants liquides tels que livrés Oxidationsbeständigkeit von neuen Isolierflüssigkeiten
(IEC 61125:2018) (IEC 61125:2018)
This European Standard was approved by CENELEC on 2018-02-16. 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden,
Switzerland, Turkey 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
© 2018 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61125:2018 E
European foreword
The text of document 10/1047/FDIS, future edition 2 of IEC 61125, prepared by IEC/TC 10 "Fluids for
electrotechnical applications" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 61125:2018.
The following dates are fixed:
(dop) 2018-11-16
• latest date by which the document has to be
implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2021-02-16
standards conflicting with the
document have to be withdrawn
This document supersedes EN 61125:1993 and EN 61125:1993/A1:2004.

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.
Endorsement notice
The text of the International Standard IEC 61125:2018 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 standards indicated:

IEC 60296 NOTE Harmonized as EN 60296.
IEC 60666 NOTE Harmonized as EN 60666.
IEC 61099 NOTE Harmonized as EN 61099.
IEC 62021-1 NOTE Harmonized as EN 62021-1.
IEC 62770 NOTE Harmonized as EN 62770.
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.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60247 -  Insulating liquids - Measurement of relative EN 60247 -
permittivity, dielectric dissipation factor
(tan δ) and d.c. resistivity
IEC 62021-2 -  Insulating liquids - Determination of acidity EN 62021-2 -
- Part 2: Colourimetric titration
IEC 62021-3 -  Insulating liquids - Determination of acidity EN 62021-3 -
- Part 3: Test methods for non-mineral
insulating oils
IEC 60422 2013 Mineral insulating oils in electrical EN 60422 2013
equipment - Supervision and maintenance
guidance
ISO 383 -  Laboratory glassware; Interchangeable - -
conical ground joints
ISO 4793 -  Laboratory sintered (fritted) filters; Porosity - -
grading, classification and designation
ISO 6344-1 -  Coated abrasives - Grain size analysis - - -
Part 1: Grain size distribution test
ISO 3104 -  Petroleum products - Transparent and EN ISO 3104 -
opaque liquids - Determination of kinematic
viscosity and calculation of dynamic
viscosity
ASTM E287 -  Standard specification for laboratory glass - -
graduated burets
IEC 61125 ®
Edition 2.0 2018-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Insulating liquids – Test methods for oxidation stability

Test method for evaluating the oxidation stability of insulating liquids in the

delivered state
Isolants liquides – Méthodes d’essai de la stabilité à l’oxydation

Méthode d’essai pour évaluer la stabilité à l’oxydation des isolants liquides tels

que livrés
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040.10 ISBN 978-2-8322-5210-9

– 2 – IEC 61125:2018 © IEC 2018
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Apparatus . 9
4.1 General principle of the method . 9
4.2 Equipment . 9
4.2.1 Heating arrangement . 9
4.2.2 Test vessels . 10
4.2.3 Absorption tubes. 10
4.2.4 Filtering crucibles . 10
4.2.5 Porcelain vessels. 11
4.2.6 Flowmeter. 11
4.2.7 Timer . 11
4.2.8 Gas supply . 11
4.2.9 Analytical balance . 11
4.2.10 Burette . 11
4.2.11 Volumetric pipette . 11
4.2.12 Volumetric flask . 12
4.2.13 Graduated measuring cylinder . 12
4.2.14 Thermometer . 12
4.2.15 Erlenmeyer flask . 12
4.3 Reagents . 12
4.3.1 Normal heptane . 12
4.3.2 Alkali blue 6B indicator according to IEC 62021-2 . 12
4.3.3 Phenolphthalein indicator . 12
4.3.4 Potassium hydroxide according to IEC 62021-2 . 12
4.3.5 Oxidant gas . 12
4.3.6 Acetone . 12
4.4 Cleaning of test vessels . 12
4.5 Catalyst . 13
4.6 Insulating liquid sample conditioning . 13
4.7 Preparation of the test . 13
4.8 Determinations on the oxidized insulating liquid . 13
4.8.1 Sludge formation . 13
4.8.2 Soluble acidity (SA) . 14
4.8.3 Volatile acidity (VA) . 14
4.8.4 Total acidity (TA) . 15
4.8.5 Dielectric dissipation factor (DDF) . 15
4.8.6 Oxidation rate with air . 15
4.8.7 Induction period with air (IP with air) (optional) . 15
4.9 Report. 15
4.10 Precision . 16
4.10.1 General . 16
4.10.2 Repeatability (r) (95 % confidence) . 16
4.10.3 Reproducibility (R) (95 % confidence) . 16

IEC 61125:2018 © IEC 2018 – 3 –
Annex A (normative) Thermometer specifications . 20
Annex B (informative) Method for evaluating the oxidation stability of inhibited
insulating liquids in the delivery state by measurement of the induction period with
oxygen . 21
B.1 Outline of the method . 21
B.2 Reagents and test conditions . 21
B.3 Procedure . 21
B.3.1 General . 21
B.3.2 Preparation of the test . 21
B.3.3 Oxidation . 22
B.3.4 Determination of the induction period with oxygen . 22
B.3.5 Determinations on the oxidized oil (optional). 22
B.4 Report. 23
B.5 Precision . 23
B.5.1 General . 23
B.5.2 Relative repeatability (r) (95 % confidence) . 23
B.5.3 Relative reproducibility (R) (95 % confidence) . 23
Annex C (informative) Method for evaluation of thermo-oxidative behaviour of unused
ester insulating liquids . 24
C.1 Outline of the method . 24
C.2 Equipment . 24
C.2.1 Heating arrangement . 24
C.2.2 Test vessels . 24
C.2.3 Reagents . 24
C.3 Test procedure . 24
C.3.1 Sample conditioning and preparation . 24
C.3.2 Ageing procedure . 25
C.4 Determination of the oxidized insulating liquid . 25
C.4.1 Soluble acidity . 25
C.4.2 Dielectric dissipation factor (DDF) at 90 °C . 25
C.4.3 Appearance . 25
C.4.4 Kinematic viscosity . 25
C.5 Report. 25
C.6 Precision . 26
Bibliography . 27

Figure 1 – Typical 8 hole (4 x 2) aluminium heating block . 17
Figure 2 – Aluminium alloy temperature measuring block . 17
Figure 3 – Position of the tube in the oil bath . 18
Figure 4 – Oxidation tube or absorption tube . 18
Figure 5 – Oxidation tube and absorption tube assembly . 19
Figure C.1 – Headspace vial with copper catalyst . 25

Table 1 – Repeatability and reproducibility of the oxidation stability test of uninhibited
mineral oil in the delivered state for 164 h at 120 °C . 16
Table A.1 – Thermometer specifications . 20
Table B.1 – Precision data for induction time with oxygen for the oxidation test for
mineral oil according to Annex B . 23

– 4 – IEC 61125:2018 © IEC 2018
Table C.1 – Precision data for headspace procedure according to Annex C . 26

IEC 61125:2018 © IEC 2018 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATING LIQUIDS – TEST METHODS FOR OXIDATION STABILITY

Test method for evaluating the oxidation stability of insulating
liquids in the delivered state

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.
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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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61125 has been prepared by IEC technical committee 10: Fluids
for electrotechnical applications.
This second edition cancels and replaces the first edition published in 1992 and
Amendment 1:2004. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) the title has been modified to include insulating liquids different from mineral insulating
oils (hydrocarbon);
b) the method applies for insulating liquids in the delivered state;
c) former Method C is now the main normative method;
d) precision data of the main normative method has been updated concerning the dissipation
factor;
– 6 – IEC 61125:2018 © IEC 2018
e) former Method A has been deleted;
f) former Method B has been transferred to Annex B;
g) a new method evaluating the thermo-oxidative behaviour of esters is included in Annex C.
The text of this standard is based on the following documents:
FDIS Report on voting
10/1047/FDIS 10/1052/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication 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 61125:2018 © IEC 2018 – 7 –
INSULATING LIQUIDS – TEST METHODS FOR OXIDATION STABILITY

Test method for evaluating the oxidation stability of insulating
liquids in the delivered state

1 Scope
This document describes a test method for evaluating the oxidation stability of insulating
liquids in the delivered state under accelerated conditions regardless of whether or not
antioxidant additives are present. The duration of the test can be different depending on the
insulating liquid type and is defined in the corresponding standards (e.g. in IEC 60296,
IEC 61099, IEC 62770). The method can be used for measuring the induction period, the test
being continued until the volatile acidity significantly exceeds 0,10 mg KOH/g in the case of
mineral oils. This value can be significantly higher in the case of ester liquids.
The insulating liquid sample is maintained at 120 °C in the presence of a solid copper catalyst
whilst bubbling air at a constant flow. The degree of oxidation stability is estimated by
measurement of volatile acidity, soluble acidity, sludge, dielectric dissipation factor, or from
the time to develop a given amount of volatile acidity (induction period with air).
In informative Annex B, a test method for evaluating the oxidation stability of inhibited mineral
insulating oils in the delivered state by measurement of the induction period with oxygen is
described. The method is only intended for quality control purposes. The results do not
necessarily provide information on the performance in service. The oil sample is maintained
at 120 °C in the presence of a solid copper catalyst whilst bubbling through a constant flow of
oxygen. The degree of oxidation stability is estimated by the time taken by the oil to develop a
determined amount of volatile acidity (induction period with oxygen). Additional criteria such
as soluble and volatile acidities, sludge and dielectric dissipation factor can also be
determined after a specified duration.
In informative Annex C, a test method intended to simulate the thermo-oxidative behaviour of
ester insulating liquids (headspace of air at 150 °C for 164 h) is described.
Additional test methods such as those described in IEC TR 62036 based on differential
scanning calorimetry can also be used as screening tests, but are out of 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 60247, Insulating liquids – Measurement of relative permittivity, dielectric dissipation
factor (tan δ) and d.c. resistivity
IEC 62021-2, Insulating liquids – Determination of acidity – Part 2: Colorimetric titration
IEC 62021-3, Insulating liquids – Determination of acidity – Part 3: Test methods for non-
mineral insulating oils
– 8 – IEC 61125:2018 © IEC 2018
IEC 60422:2013, Mineral insulating oils in electrical equipment – Supervision and
maintenance guidance
ISO 383, Laboratory glassware – Interchangeable conical ground joints
ISO 4793, Laboratory sintered (fritted) filters – Porosity grading, classification and designation
ISO 6344-1, Coated abrasives – Grain size analysis – Part 1: Grain size distribution test
ISO 3104, Petroleum products – Transparent and opaque liquids – Determination of kinematic
viscosity and calculation of dynamic viscosity
ASTM E287, Standard specification for laboratory glass graduated burets
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
unused insulating liquid
insulating liquid that has not been used in, or been in contact with electrical equipment or
other equipment not required for manufacture, storage or transport
Note 1 to entry: See also IEC 60296, IEC 61099 and IEC 62770.
3.2
recycled insulating liquid
insulating liquid previously used in electrical equipment that has been subjected to re-refining
or reclaiming (regeneration) off-site
Note 1 to entry: Any blend of unused and recycled oils is to be considered as recycled.
3.3
oxidation stability
ability of an insulating liquid to withstand oxidation under thermal stress and in the presence
of oxygen and a copper catalyst
Note 1 to entry: Oxidation stability gives general information about the stability of the insulating liquid under
service conditions in electrical equipment. The property is defined as resistance to formation of acidic compounds,
sludge and compounds influencing the dielectric dissipation factor (DDF) under given conditions. Test durations for
insulating liquids are described in the corresponding standards.
3.4
induction period with air
graphical representation of the oxidation rate over the entire period which can be obtained by
titrating volatile acidity daily (or at other suitable time interval) and plotting the cumulated
results against time
Note 1 to entry: The induction period with air is determined by reading the time corresponding to 0,10 mg KOH/g
volatile acidity in the case of mineral oil. In the case of ester liquids a higher value needs to be established.

IEC 61125:2018 © IEC 2018 – 9 –
3.5
volatile acidity
measurement of the amount of oxidation products collected in the water phase in the
absorption tube
3.6
soluble acidity
acidity (neutralization value) of oil as a measure of the acidic degradation products in the
insulating liquid
Note 1 to entry: The acidity of an oxidized oil is due to the formation of acidic oxidation products. Acids and other
oxidation products will, in conjunction with water and solid contaminants, affect the dielectric and other properties
of the oil. Acids have an impact on the degradation of cellulosic materials and may also be responsible for the
corrosion of metal parts in a transformer.
3.7
total acidity
sum of volatile and soluble acidity
3.8
sludge
polymerized degradation product of solid and liquid insulating material
Note 1 to entry: Sludge is soluble in oil up to a certain limit, depending on the oil solubility characteristics and
temperature.
3.9
dielectric dissipation factor
DDF
measure for dielectric losses within the oil
Note 1 to entry: High DDF values can indicate contamination of the oil by polar contaminants or poor refining
quality.
Note 2 to entry: DDF shall be measured at 90 °C, and in accordance with IEC 60247.
4 Apparatus
4.1 General principle of the method
The liquid sample to be tested, through which a stream of air is bubbled, is maintained for a
given period at 120 °C in the presence of solid copper. The resistance to oxidation is
evaluated from the amount of total sludge, total acidity and dielectric dissipation factor formed
or from the time to develop a given amount of volatile acidity (induction period with air).
4.2 Equipment
4.2.1 Heating arrangement
In order to achieve accurate measuremen
...