IEC 61039:2025

IEC 61039 ®
Edition 3.0 2025-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of insulating liquids
Classification des liquides isolants
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IEC 61039 ®
Edition 3.0 2025-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of insulating liquids
Classification des liquides isolants
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040.10 ISBN 978-2-8327-0162-1
– 2 – IEC 61039:2025 © IEC 2025
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 ISO classification system . 6
5 Classification of electrical insulating liquids . 7
5.1 General . 7
5.2 Class classification . 7
5.3 Category classification . 8
5.4 Identifying code . 8
6 Summarizing outline . 11
Bibliography . 12

Figure 1 – Meaning of all the letters and digits present in the classification of
insulating liquids . 11

Table 1 – Class classification of petroleum products or related products . 7
Table 2 – Examples of classification for different insulating liquids . 10

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CLASSIFICATION OF INSULATING LIQUIDS

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
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consensus of opinion on the relevant subjects since each technical committee has representation from all
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
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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 61039 has been prepared by IEC technical committee 10: Fluids for electrotechnical
applications. It is an International Standard.
This third edition cancels and replaces the second edition published in 2008. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) updating of the classification of insulating liquids, taking into account the largest number
possible of substances that have, or may have, a possible application in electrical
components.
– 4 – IEC 61039:2025 © IEC 2025
The text of this International Standard is based on the following documents:
Draft Report on voting
10/1249/FDIS 10/1258/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.
INTRODUCTION
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 insulating liquids which are the subject of this document should be handled with due regard
to personal hygiene. Direct contact with eyes can cause slight irritation. In the case of eye
contact, irrigation with copious quantities of clean running water should be carried out and
medical advice sought.
Some of the tests specified in this document involve the use of processes that could lead to a
hazardous situation. Attention is drawn to the relevant standard for guidance.
WARNING – Environment
This document involves insulating liquids, chemicals and used sample containers. The disposal
of these items can be subject to regulatory requirements with regard to their impact on the
environment.
All insulating liquids that float on water are generally a water hazard, as they reduce oxygen
ingress into the water. No liquid, regardless of its classification, can be freely spilled in the
environment. The handling of insulating liquids can be subject to regulatory requirements with
regard to their impact on the environment. Every precaution should be taken to prevent the
release of insulating liquids into the environment.

– 6 – IEC 61039:2025 © IEC 2025
CLASSIFICATION OF INSULATING LIQUIDS

1 Scope
This document establishes the detailed classification of the N family (insulating liquids) that
belongs to class L (lubricants, industrial oils and related products) in accordance with ISO 8681
and ISO 6743-99, affecting product categories that include products derived from petroleum
processing, synthetic chemical products and synthetic and natural esters.
This document applies to unused liquids. For liquids in service, additional testing can be
required to ensure compliance with 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 60296, Fluids for electrotechnical applications – Mineral insulating oils for electrical
equipment
ISO 2592, Petroleum and related products – Determination of flash and fire points – Cleveland
open cup method
ISO 8681, Petroleum products and lubricants – Method of classification – Definition of classes
OECD 301:1992, OECD guidelines for testing of chemicals – Ready biodegradability
ASTM D240, Standard test method for heat of combustion of liquid hydrocarbon fuels by bomb
calorimeter
3 Terms and definitions
No terms and definitions are listed in this document.
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
4 ISO classification system
ISO 8681 sets out the main rules of the classification system that applies to petroleum products,
lubricants and related products. This document sets out the classification method which can be
used for electrical insulating liquids.
ISO 8681 suggests, as far as possible, to choose the application field as the main principle for
the classification of petroleum products, lubricants, and related products. It also suggests
classifying on the basis of the product typology, for example fuels are classified first of all on
the basis of typology and secondly on the basis of end use.

The ISO classification principle is based on the allocation of a code consisting of letters and
numbers for the main classes and categories of petroleum products.
The complete nomination consists of:
– the initials "ISO";
– the class of the petroleum product or related product, indicated by a letter (see Table 1),
which shall be clearly separated from the other symbols;
– the category, indicated by a group of four letters, the first one always identifying the family
to which it belongs and the others assuming a meaning, appropriately explained in the
reference standard, which depends on the particular category of concerned products;
– (optional) some numbers, which can be added, to complete the nomination and that have a
meaning appropriately explained in the reference standard for that particular category of
products.
In compliance with ISO 8681, the code should have the following general form:
ISO – CLASS – CATEGORY –NUMBERS (if applicable)
or the short form:
CLASS – CATEGORY –NUMBERS (if applicable)
5 Classification of electrical insulating liquids
5.1 General
In accordance with ISO 8681, the classification system indicates the products with a nomination
that includes:
– the abbreviation "ISO";
– the class of the petroleum products or related products indicated by a letter that in this
document has the meaning defined in Table 1;
– the category indicated by four letters whose meaning is explained in 5.3;
– a seven-digit number that makes up the identification code (described in 5.4).
5.2 Class classification
The class of petroleum products or related products is indicated by a letter having the meaning
reported in Table 1.
Table 1 – Class classification of petroleum products or related products
Class Indication
F Fuels
S Solvents and raw materials for chemical industry
L Lubricants, industrial oils and related products
W Waxes
B Bitumen
In accordance with ISO 6743-99:2002, the electrical insulating liquids belong to class L
"lubricants, industrial oils and related products".

– 8 – IEC 61039:2025 © IEC 2025
5.3 Category classification
In the case where the specific classification of insulating liquids is described in the
corresponding standard, it shall have priority over the classification given in this document. For
example, for mineral oils, IEC 60296 shall be applied.
The four letters identify the category, with the following meaning:
• First letter
The first letter, which identifies the insulating liquid family, will be N: Electrical insulation
(ISO 6743-99:2002, Table 1).
• Second letter
The second letter identifies the main application field as follows:
– C capacitors;
– T transformers and switching equipment;
– S switching equipment operating at temperature lower than −10 °C;
– Y cables.
NOTE 1 In order to provide an indication of fire behaviour of insulating liquids, and also wishing to benefit from the
experience gained by CT 14 of CENELEC, the following parameters have been added as well as the classifications
"fire point" and "low heat value".
• Third letter
The third letter identifies the presence of antioxidant additives, if applicable. Liquids may
contain different antioxidants and different levels of antioxidants. Check with corresponding
liquid standards. The third letter is defined as:
– U if no antioxidant additives are present;
– I if antioxidant additives are present.
NOTE 2 In this document Classification I also encompasses category T according to IEC 60296.
• Fourth letter
The fourth letter identifies the fire point as determined in accordance with ISO 2592. At the time
of writing of this document, liquids falling into category L are being phased out from use. There
is no IEC International Standard covering such liquids.
– O if the fire point is < 300 °C;
– K if the fire point is ≥ 300 °C;
– L if the fire point of the liquid is not detectable.
5.4 Identifying code
To complete the nomination, a seven-digit number is added, with the following meaning:
• First three digits
The first three digits correspond to the last three digits of the IEC reference standard identifier,
if applicable, using number 000 in the case where the IEC reference standard is missing.
• Fourth digit
The fourth digit identifies the IEC sub-classification, if applicable, using number 0 if there is no
sub-classification. Examples of sub-classifications occur with IEC 60867 (aromatic
hydrocarbons, Table 2 of this document, rows 5, 6, 7) and IEC 63012 (modified and blended
esters, Table 2 of this document, row 12). See Table 2 for reference.
• Fifth digit
The fifth digit identifies the net calorific value as determined in accordance with ASTM D240
(also known as the low heat value), as follows:
– 1 if the low heat value is ≥ 42 MJ/kg;
– 2 if the low heat value is < 42 MJ/kg and ≥ 32 MJ/Kg;
– 3 if the low heat value is < 32 MJ/kg.
• Sixth digit
The sixth digit identifies the "lowest cold start energizing temperature" (LCSET) defined as
follows.
The information below is based largely on IEC 60296:
– 0 if LCSET is not known;
– 1 if LCSET is ≥ 0 °C;
– 2 if 0 °C > LCSET ≥ −10 °C;
– 3 if −10 °C > LCSET ≥ −30 °C;
– 4 if −30 °C > LCSET ≥ −40 °C.
• Seventh digit
The seventh digit identifies the biodegradability of the insulating liquid, according to OECD 301:
methods B, C or F, defined as follows.
Note different methods may give different results.
– 0 if liquid is not biodegradable
– 1 if liquid is inherently biodegradable;
– 2 if liquid is readily biodegradable.
Note that the disposal of these items can be subject to regulatory requirements with regard to
their impact on the environment. Refer to the environmental warning in the introduction.

– 10 – IEC 61039:2025 © IEC 2025
Table 2 – Examples of classification for different insulating liquids
Type of the insulating liquid
Comments
Mineral insulating oil for transformers with
1 L NTUO 296 – 43 MJ/kg −7 Inherent
fire point equal to 200 °C, low heat value (net
calorific value) equal to 43 MJ/kg,
non-inhibited, with LCSET = −7 °C
L-NTUO-2960121
Mineral insulating oil for transformers with
2 L NTIK 296 – 43 MJ/kg −7 Inherent
fire point equal to < 300 °C, low heat value
equal to 43 MJ/kg, inhibited in trace, with
LCSET = −7 °C
L-NTIO-2960121
Mineral insulating oil for transformers with
3 L NTIO 296 – 43 MJ/kg −7 Inherent
fire point equal to 200 °C, low heat value
equal to 43 MJ/kg, inhibited, with
LCSET = −7 °C
L-NTIO-2960121
Mineral insulating oil for switching equipment
4 L NSIO 296 – 43 MJ/kg −30 Inherent
operating at low temperature with fire point
equal to 200 °C low heat value equal to
43 MJ/kg, inhibited, with LCSET = −30 °C
L-NSIO-2960131
5 L NYUO 867 1 43 MJ/kg – Inherent IEC sheet 1 of IEC 60867
alkyl benzene: for cables, uninhibited, fire
point < 300 °C
L-NYUO-8671101
IEC sheet 2, of IEC 60867,
6 L NCUO 867 2 43 MJ/kg – Inherent
alkyldiphenylethanes: for capacitors,
uninhibited, fire point < 300 °C, LCSET not
defined
L-NCUO-8672101
IEC sheet 3, of IEC 60867,
7 L NCUO 867 3 43 MJ/kg – Inherent
methylpolyarylmethanes uninhibited, for
capacitors, fire point < 300 °C, LCSET not
defined
L-NCUO-8673101
IEC 60836 – unused silicone liquids for
8 L NTUK 836 – < 32 MJ/kg ≤ −40 Not
transformers, uninhibited, fire point > 300 °C
biodegradable
L-NTUK-8360340
Synthetic ester with fire point > 300 °C for
9 L NTIK 099 – > 42 MJ/Kg – Readily
transforme
...