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Hkkjrh; ekud IS 335 : 2018

Indian Standard

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New Insulating Oils —

Specification
(Fifth Revision)

ICS 29.040

© BIS 2018

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MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI-110002
www.bis.org.in www.standardsbis.in

September 2018 Price Group 6

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Fluids for Electrotechnical Applications Sectional Committee, ETD 03

FOREWORD
This Indian Standard (Fifth Revision) was adopted by the Bureau of Indian Standards, after the draft finalized by
the Fluids for Electrotechnical Applications Sectional Committee has been approved by the Electrotechnical
Division Council.
This standard, first published in 1953, was based on BS 148 : 1951 ‘Insulating oil for transformers and switchgear’,
issued by the British Standards Institution. It was revised in 1963 to bring the test methods in line with the
practices in vogue. The second revision was undertaken in 1972 mainly to include oxidation test as given in IEC
Pub 296 (1982) ‘Specification for unused mineral insulating oils for transformers and switchgear’. The third
revision was undertaken in 1983 to include an ageing test based on ASTM D 1934 : 1968 ‘Standard method of test
for oxidative ageing of electrical insulating petroleum oils by open beaker method’, issued by the American
Society for Testing and Materials. Subsequently, the fourth revision was undertaken in 1993 to include amendments
issued since the adoption of third revision and also a new method of test adopted for detection of oxidation
inhibitor.
This standard is largely based on IEC 60296 : 2012. Main changes with regard to the fifth revision include: Only
one class of oil has been replaced by three classes, namely Type I transformer oil (in line with IEC 60296), Type
II transformer oil with higher viscosity max, values at 40oC and third low temperature switchgear oil, but a new
concept, the lowest cold start energizing temperature, has been included; new properties have been added (that is
sulphur content, gassing tendency, PCA, PCB and furan); values for properties have been revised, and test methods
have been revised (that is acidity, corrosive sulphur and oxidation stability).
This standard also covers the requirements of inhibited mineral insulating oils which were earlier covered in
IS 12463 : 1988. With the publication of this standard IS 12463 is superseded and hence shall be withdrawn.
For the maintenance and supervision of insulating oils conforming to this standard and used in transformers,
switchgear and certain other similar oil immersed equipment, reference shall be made to IS 1866 : 2000 ‘Code of
practice for maintenance and supervision of mineral insulating oil in equipment (third revision )’.
For the purpose of deciding whether a particular requirement of this standard is complied with, the final value,
observed or calculated expressing the result of a test, shall be rounded off in accordance with IS 2 : 1960 ‘Rules
for rounding off numerical values ( revised)’. The number of significant places retained in the rounded off value
should be the same as that of the specified value in this standard.
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IS 335 : 2018

Indian Standard
NEW INSULATING OILS — SPECIFICATION
(Fifth Revision)
1 SCOPE NOTE — This does not include insulating liquids such as esters,
synthetic aromatics or silicone fluids.
This standard covers specifications and test methods
3.4 Additive — Suitable chemical substance which is
for unused mineral insulating oils. It applies to oil
deliberately added to a mineral insulating oil in order
delivered to the agreed point and time of delivery,
intended for use in transformers, switchgear and similar to improve certain characteristics.
electrical equipment in which oil is required as an NOTE — Examples include antioxidants, metal passivators,
insulant and for heat transfer. These oils are obtained metal deactivators, gas absorbers, pour point depressants,
electrostatic charging tendency depressants, anti-foam agents,
by distillation and refining of crude petroleum. refining process improvers, etc.
Oils with and without additives are both within the 3.5 Antioxidant Additive — Additive incorporated
scope of this standard. in insulating oil that improve oxidation stability.
This standard is applicable only to unused mineral NOTE — A large number of additives which improve oxidation
insulating oils. stability, including inhibitors, peroxide decomposers, metal
passivators and metal deactivators are available and may be
Reclaimed oils are beyond the scope of this standard. used in oils if declared (see 6.11.1 and 6.11.2)

This standard does not apply to mineral oils used as 3.5.1 Inhibitor — Antioxidant additives of the phenolic–
impregnants in cables or capacitors. or amine-type, such DBPC and DBP described
in IS 13631.
NOTE — Mineral insulating oils complying with the
requirements of this standard, of the same class and containing Note — DBPC = 2,6 di-tert-butyl-para-cresol, DBP = 2,6
no additives ( see 3.4), are considered to be compatible with di-tert-butyl-phenol.
one another and can be mixed in any proportion. This does not
apply to oils containing additives. Where the user wishes to 3.5.2 Other Antioxidant Additive — Antioxidant
mix such oils, a check is recommended to be made to ensure additive of the sulphur or phosphorous type.
that the mixture meets the requirements of this standard.
3.5.3 Passivator — Metal passivator additive used
2 REFERENCES primarily as electrostatic charging depressant, but which
may also improve oxidation stability.
The standards listed in Annex A contain provisions
which, through reference in this text, constitute NOTE — Metal passivators are sometimes described as metal
deactivators or corrosion inhibitors.
provisions of this standard. At the time of publication,
the editions indicated were valid. All standards are 3.6 Uninhibited Oil — Mineral insulating oil
subject to revision, and parties to agreements based on containing no inhibitor.
this standard are encouraged to investigate the NOTE — No inhibitor means that the total inhibitor content is
possibility of applying the most recent editions of the below the detection limit of 0.01 percent (see IS 13631).
standards listed in Annex A.
3.7 Trace Inhibited Oil — Mineral insulating oil
3 TERMINOLOGY containing less than 0.08 percent of total inhibitor
content as measured as per IS 13631.
For the purposes of this standard, the following
definitions shall apply: 3.8 Inhibited Oil — Mineral insulating oil containing
a minimum of 0.08 percent and maximum of 0.4 percent
3.1 Transformer Oil — Mineral insulating oil for of total inhibitor content as measured as per IS 13631.
transformers and similar electrical equipment where
normal oxidation resistance is required. 3.9 Unused Mineral Insulating Oil — Mineral
insulating oil not recycled as delivered by the supplier.
3.2 Low Temperature Switchgear Oil — Mineral
NOTES
insulating oil for oil-filled switchgear for outdoor
1 Such an oil has not been used in, nor been in contact with
application in very cold climatic conditions. electrical equipment or other equipment not required for
manufacture, storage or transport. The manufacturer and
3.3 Mineral Insulating Oil — Insulating oil obtained supplier of unused oil will have taken all reasonable precautions
by refining, modifying and/or blending of petroleum to ensure that there is no contamination with polychlorinated
products and other hydrocarbons. biphenyls or terphenyls (PCB, PCT), used, reclaimed or

1
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IS 335 : 2018

dechlorinated oil or other contaminants. 5 CLASSIFICATION, IDENTIFICATION,

2 A blend of unused and recycled oil in any proportion is GENERAL DELIVERY REQUIREMENTS AND
regarded as being recycled.
SAMPLING
3.10 Recycled Oil — General term to encompass
mineral insulating oils previously used in electrical 5.1 Classification
equipment that has been subjected to re-refining or 5.1.1 Classes
reclaiming (regeneration) off-site.
For the purpose of this standard, mineral insulating oils
NOTES are classified into three classes:
1 The characteristics of recycled oil are heavily dependent on
the crude from which it was refined, the original refining a) Type I Transformer oils,
technique, the service history and the type of recycling process. b) Type II Transformer oils, and
Natural antioxidants originally present in the oil might have
c) Low temperature switchgear oil.
been depleted in service or removed by the recycling process.
2 Recycled oil is not appropriate for use in some applications. 5.1.2 Antioxidant Additive (Inhibitor) Content
3 Such recycled oils are often produced from mixtures of mineral Transformer oils are classified into three groups,
insulating oils of different origins.
according to their content of antioxidant additive:
4 A blend of unused and recycled oils in any proportion is
regarded as being recycled oil. a) uninhibited transformer oils: marked with U,
5 Due to local legislation, in some countries, it is advisable to b) trace inhibited transformer oils: marked with
categorize used electrical oil as waste oil and complete T, and
separation of handling of unused and used oils is required
c) inhibited transformer oils: marked with I.
4 PROPERTIES OF OIL 5.1.3 Lowest Cold Start Energizing Temperature
Characteristics are given in Table 1 and Table 2 . The (LCSET)
list of IEC and/or IS for specific properties is given in After the inhibitor marking, the LCSET shall be
Table 3 (informative). indicated.

4.1 Functional Properties Standard LCSET in this standard is –30°C; optionally,

other LCSET can be selected according to Table 1.
Properties of oil which have impact on its function as
5.2 Requirements
an insulating and cooling liquid.
General requirements of this standard are given in
NOTE — Functional properties include viscosity, density, pour
point, water content, breakdown voltage and dielectric
Table 2.
dissipation factor. Specific requirements are defined under 7.
4.2 Refining and Stability 5.3 Miscibility
Properties of oil that are influenced by quality and type Unused insulating oils of the same class (see 5.1.1),
of refining and additives. the same group (see 5.1.2) and the same LCSET
(see 5.1.3) and containing the same types of additives
NOTE — These can include appearance, interfacial tension, are considered to be miscible and compatible with each
sulfur content, acidity, corrosive sulphur, 2 -furfural and related
compounds contents and stray gassing.
other (see IS 1866).
5.4 Identification and General Delivery
4.3 Performance
Requirements
Properties that are related to the long-term behaviour Identification and general delivery requirements are as
of oil in service and/or its reaction to high electric stress follows:
and temperature.
a) Oil is normally delivered in bulk, rail tank cars,
NOTE — Examples include oxidation stability, gassing tank containers or packed in drums or IBC
tendency and electrostatic charging tendency (ECT). (intermediate bulk containers). These shall be
clean and suitable for this purpose to avoid
4.4 Health, Safety and Environment (HSE)
any contamination.
Properties
b) Oil drums and sample containers shall carry
Oil properties related to safe handling and environment at least the following markings:
protection. 1) supplier’s designation,
NOTE — Examples can include flash point, density, PCA 2) classification (see 5.1), and
(polycyclic aromatics), PCB / PCT (polychlorinated biphenyls 3) oil quantity.
and terphenyls).
c) As agreed to between the supplier and the

2
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IS 335 : 2018

purchaser each oil delivery shall be The viscosity at the lowest cold start energizing
accompanied by a document from the supplier temperature (LCSET) shall not exceed 1 800 mm2/s
specifying the supplier’s designation, oil for LCSET temperatures higher than –40 o C
classification and compliance certificate. (respectively 2 500 mm2/s at –40oC, see Table 1). This
NOTE — This standard may be traceable to a specific lowest cold start energizing temperature (LCSET) for
batch of oil processed. transformer oils is defined in this standard as being
d) The supplier shall declare the generic type of –30oC but other LCSET (see Table 1) can be agreed to
all additives, and their concentration in the between the supplier and the purchaser.
cases of inhibitors and passivators. Low temperature switchgear oil should have a lower
5.4.1 BIS Certification Marking viscosity at LCSET: 400 mm2/s, Max. Standard LCSET
The product may also be marked with the Standard of low temperature switchgear oil is defined with –40oC
Mark. but other LCSET may be agreed to between the supplier
and the purchaser.
The use of the Standard Mark is governed by the
provisions of the Bureau of Indian Standards Act, Viscosity shall be measured according to IS 1448
2016 and the Rules and Regulations made thereunder. (Part 25), viscosity at very low temperatures according
The details of conditions under which the License for to IS 16084.
the use of the Standard Mark may be granted to
6.2 Pour Point
manufacturers or producers may be obtained from the
Bureau of Indian Standards. Pour point of mineral insulating oil is the lowest
temperature at which the oil will just flow. It is
5.5 Sampling
recommended that the pour point should be minimum
Sampling shall be carried out in accordance with the 10°C below the lowest cold start energizing
procedure described in IS 6855. temperature (LCSET). If a pour point depressant
additive is used, this should be mentioned by the
6 PROPERTIES, THEIR SIGNIFICANCE AND
supplier to the user. Pour point shall be measured in
TEST METHODS
accordance with IS 1448 (Part 10/Sec 2).
6.0 General
6.3 Water Content
The properties, test methods and limits shall be in
accordance with those given in Table 2 unless otherwise A low water content of mineral insulating oil is
specified in the respective clause. necessary to achieve adequate break down voltage and
low dissipation losses. To avoid separation of free
6.1 Viscosity water, unused insulating oil should have limited water
Viscosity influences heat transfer and therefore the content. Before filling the electrical equipment, the oil
temperature rise of the equipment. The lower the should be treated to meet the requirements of IS 1866.
viscosity, the easier the oil circulates leading to Water content shall be measured in accordance with
improved heat transfer. At low temperatures the IEC 60814.
resulting higher viscosity of oil is a critical factor for
the cold start of transformers with poor or no circulation 6.4 Breakdown Voltage
of oil and therefore possible overheating at the hot Breakdown voltage of transformer oil indicates its
spots, and negatively influence the speed of moving ability to resist electrical stress in electrical equipment.
parts such as in power circuit breakers, switchgear, on Breakdown voltage shall be measured in accordance
load tap changer mechanisms, pumps and regulators. with IS 6792. The supplier shall demonstrate that after

Table 1 Maximum Viscosity and Pour Point of Transformer Oil at Lowest Cold Start
Energizing Temperature (LCSET)
(Clauses 4, 5.1.3 and 6.1)
Sl LCSET Maximum Viscosity Maximum Pour Point
No. °C mm2/s °C
(1) (2) (3) (4)
i) 0 1 800 –10
ii) –10 1 800 –20
iii) –20 1 800 –30
iv) –30 1 800 –40
v) –40 2 500 –50

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IS 335 : 2018

treatment to reduce particles, water and dissolved air NOTE — By agreement between parties, DDF may be measured
at temperatures other than 90 °C. In such cases the temperature
by vacuum procedure (see note), the oil shall have a
of measurement should be stated in the report.
high dielectric strength (breakdown voltage >70 kV).
NOTE — This treatment referred to filtration of the oil at 60 oC 6.6 Appearance
by vacuum (pressure below 2.5 kPa) through a sintered glass
filter (with a maximum pore size at 2.5 µm). A visual inspection of insulating oil (oil sample in
transmitted light under a thickness of approximately
6.5 Dielectric Dissipation Factor (DDF) 10 cm and at ambient temperature) indicates the
presence of visible contaminants, free water or
Dielectric Dissipation Factor is measure for dielectric
suspended matter.
losses caused within the oil. DDF values above
requirements of Table 2 can indicate contamination of 6.7 Acidity
the oil by polar contaminants or poor refining quality.
DDF shall be measured in accordance with IS 16086 Unused mineral insulating oil should be neutral and
at 90°C. In case of dispute, IEC 60247 at 90°C should free from any acidic compound. Acidity shall be
be used. measured following IEC 62021-1.
Table 2 General Specifications
(Clauses 4, 5.2, 6)

Sl Property Test Method Limits

No.
Type I Type II Low Temperature
Transformer Oil Transformer Oil Switchgear Oil
(1) (2) (3) (4) (5) (6)
A Function
i) Viscosity at 40°C IS 1448 (Part 25) 12 mm2/s, Max 15mm2/s, Max 3.5 mm2/s, Max

ii) Viscosity at 0°C1) IS 1448 (Part 25) — 1 800 mm2/s, Max —

1) 2
iii) Viscosity at –30°C IS 1448 (Part 25) 1 800 mm /s, Max — —
iv) Viscosity at –40°C2) IS 16084 — — 400 mm2/s, Max
v) Pour point IS 1448 (Part 10/Sec 2) –40°C, Max –10°C, Max –60°C1) , Max
To be based on To be based on
LCSET refer LCSET refer
Table 11) Table 11)
vi) Water content IEC 60814 30 mg/kg 3)/40 mg/kg 4), Max
vii) Breakdown voltage IS 6792 30 kV/70 kV 5) , Min
viii) Density at 20°C IS 1448 (Part 16) 0.895 g/ml, Max
ix) DDF at 90°C IS 16086 0.005, Max
x) Particle content IS 13236 No general requirement10)
B Refining/stability
xi) Appearance — Clear, free from sediment and suspended matter
xii) Acidity IEC 62021-1 0.01 mg KOH/g, Max
xiii) Interfacial tension ASTM D971 No general requirement6)
xiv) Total sulphur content ISO 14596 or ASTM D429412) No general requirement

xv) Corrosive sulphur DIN 51353 Not corrosive

xvi) Potentially IS 16310 Not corrosive
corrosive sulphur
xvii) DBDS IS 16497 (Part 1) Not detectable (< 5 mg/kg)
xviii) Inhibitors according to IS 13631 (U) Uninhibited oil: not detectable (< 0.01%)
IS 13631/IEC 60666 (T) Trace inhibited oil: < 0.08%
(I) Inhibited oils : 0.08% –0.40%
(see 3.6 to 3.8)
xix) Metal passivator IS 13631 Not detectable (< 5mg/kg), or as agreed upon with the purchaser
additives according to
IS 13631/IEC 60666

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IS 335 : 2018

Table 2 — (Concluded)

Sl Property Test Method Limits

No.
xx) Other additives See7)
xxi) 2-Furfural and related IS 15668 Not detectable ( < 0.05 mg/kg) for each individual compound
compounds content
C Performance
xxii) Oxidation stability IS 12422 (Method C) For oils with other antioxidant additives and mental passivator
Test duration11) Additives (see 6.12)
(U) Uninhibited oil: 164 h
(T) Trace inhibited oil: 332 h
(I) Inhibited oil: 500 h
a) Total acidity9) 1.9.4 of IS 12422 1.2 mg KOH/g, Max
9)
b) Sludge 1.9.1 of IS 12422 0.8%, Max
c) DDF at 90°C9) 1.9.6 of IS 12422 0.500 9), Max

xxiii) Gassing tendency IEC 60628, Method A No general requirement 8)

xxiv) ECT See 6.14 No general requirement 8)
D Health, Safety and Environment (HSE)
xxv) Flash point IS 1448 (Part 21) 135°C, Min 135°C, Min 100°C, Min
xxvi) PCA content IP 346 3%, Max
xxvii) PCB content IS 16082 Not detectable ( < 2 mg/kg)

1)
This is the standard LCSET for a transformer oil (see 6.1) and can be modified depending on the climatic condition of each region.
Pour point should be minimum of 10°C below LCSET.
2)
Standard LCSET for low temperature switchgear oil.
3)
For bulk supply
4)
For delivery in drums and IBC.
5)
After laboratory treatment (see 6.4)
6)
Where it is used as general requirement, a limit of minimum 40 mN/m is recommended.
7)
The supplier shall declare the generic type of all additives and their concentrations in the case of antioxidant additives.
8)
To be agreed upon between supplier and purchaser.
9)
At the end of oxidation stability tests.
10)
Particle content in drums at the delivery of the oil can be agreed between supplier and customer based on a statistical reference at
delivery.
11)
A DDF of maximum 0.020 after 2 h of oxidation (see IS 12422 or IEC 61125) can be used for application in EHV instrument
transformers and bushings.
12)
In case of any dispute, ISO 14596 should be used.

6.8 Interfacial Tension (IFT) 6.10 Corrosive Sulphur and Potentially Corrosive
Low IFT sometimes indicates the presence of polar Sulphur
compounds. IFT shall be measured in accordance with Some sulphur compounds, for example mercaptans, are
ASTM D971. very corrosive to metal surfaces, that is steel, copper
and silver (switchgear contacts) and shall not be present
6.9 Sulphur Content in new oil. Corrosive sulphur shall be measured
Different organo-sulphur compounds are present in following DIN 51353.
mineral oils, dependent on the crude oil origin and the Some other sulphur compounds, for example dibenzyl
degree and type of refining. Refining reduces the disulphide (DBDS), may result in the deposition of
content of sulphur and aromatic hydrocarbons. As some copper sulphide (Cu2S) in paper insulation, reducing
sulphur compounds have an affinity to metals, they may its electrical insulation properties (see Annex B). This
act as natural oxidation inhibitors but they may promote has resulted in several equipment failures in service.
corrosion on metallic components in the system.
IS 16310 provides the best currently available method
Sulphur content is a specific requirement of 7.1. to detect potentially corrosive sulphur compounds in
Sulphur content shall be measured following ISO 14596 oil. It applies only to oils that do not contain a metal
or ASTM D4294. In case of any dispute, ISO 14596 passivator additive (declared or undeclared).
should be used. For passivator – containing oils (see B-3).

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IS 335 : 2018

6.11 Additives (see 3.4) (TTA). Detection and measurement of these additives
shall be according to IS 13631.
6.11.1 General
Several other compounds can be used as metal
The generic type of all additives shall be declared in
passivator additives, such as N,N -bis(2-Ethylhexyl)-
product data sheets and certificates of compliance. For
1H-1,2,4-triazole-1 methanamine (TAA), diamino-
antioxidant additives and passivators, their
diphenyldisulphide, nicotinic acid, hydroquinoline and
concentrations shall also be stated.
other sulphur-based compounds, for which no test
6.11.2 Antioxidant Additives (see 3.5) methods are available2.
Antioxidants slow down the oxidation of oil and [2. Examples of commercially available TTAA and TAA are
therefore the formation of degradation products such Irgamet 39© and Irgamet 30© respectively. This information
is given for the convenience of users of this standard and does
as oil sludge and acidity. It is useful to know whether not constitute any endorsement of these products.]
and in what proportion antioxidant additive has been
added in order to monitor additive depletion during 6.11.4 Pour Point Depressants
service. These additives are used to improve the viscosity and
Additives that slow down the oxidation of mineral pour point of oils at very low temperatures. Detection
insulating oils include: and measurement of the two main types of pour point
depressant additives used (polynapthalenes and
a) Inhibitors such as phenols and amines (see polymethacrylates) shall be according to IS 13631.
3.5.1). The most widely used inhibitors are
DBPC and DBP (see 3.5.1). Detection and 6.12 Oxidation Stability
measurement of DBPC and DBP shall be
carried out in accordance with IS 13631. Test Oxidation of oil gives rise to acidity and sludge
methods are not available for other types of formation. This can be reduced by using oils with a
inhibitors. high oxidation stability leading to longer service life
time by minimizing sludge deposition and maximizing
b) Other antioxidant additives such as sulphur
insulation life. Oxidation stability is measured in
and phosphor – containing compounds, for
accordance with IS 12422. There is an option for stricter
example organic polysulphides and di-thio-
limits for special applications . More stringent limits
phosphates (see 3.5.2). An antioxidant
and/or additional requirements and tests may be
additive of this type is DBDS (see 6.10), but
specified, if required.
it is not accepted as it is known to be corrosive
to copper and will likely result in the oil failing Test durations for oils containing inhibitors shall be as
the potentially corrosive sulphur test of indicated in Table 2. Test duration for oils containing
IS 16310. Test methods are in preparation only other antioxidant additives and metal passivators shall
for DBDS (see 6.21) and not for other be 500 h.
antioxidant additives of this type.
Passivator-containing oils shall be tested for oxidation
c) Metal passivators (see 6.11.3).
stability before the passivator additive has been added
6.11.3 Metal Passivators to the oil (when possible), using the test durations of
Table 2.
Some of these additives form thin films on copper,
preventing the catalytic effect of copper in oil and the 6.13 Gassing Tendency
formation of harmful copper sulphide deposits in paper
by reaction with corrosive sulphur compounds Gassing tendency of mineral insulating oil, that is the
contained in the oil. Some of them protect the oil from gas absorbing property of oil when subjected to corona
the catalytic action of metals and slow down the rate of partial discharges, is only necessary and important for
oxidation of oil. Passivators therefore slow down the special equipment like HV (high voltage) instrument
oxidation process according to IS 12422 as they transformers and bushings. It is a measure of the rate
passivate the surface of the catalyzing copper wire, thus of absorption or evolution of gas into oil under
leading to an optimistic result of the oxidation stability prescribed laboratory conditions. Gas absorption
test. Some of them are also used to reduce the properties are related to oil aromatic content. Gassing
electrostatic charging tendency of oils (see 6.14). tendency is measured using IEC 60628 (Method A).

Three main types of benzotriazole derivatives are Gassing tendency is a specific requirement of 7.3.
typically used as metal passivator additives: N - bis (2- NOTE — Additives such as 1, 2, 3, 4-tetrahydronapthelene
Ethylhexyl)-aminomethyl-tolutriazole (TTAA), (tetralin), mono or dibenzyl toluene and others have been
benzotriazole (BTA) and 5-methyl-1H-benzotriazole proposed to reduce the gassing tendency of some oils, but are

6
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IS 335 : 2018

not described in IS 13631. Mono and dibenzyltoluene are 6.20 Particle Content
described in IEC 60867.
Particles in unused mineral insulating oil may result
6.14 Electrostatic Charging Tendency (ECT) from manufacturing, storage or handling of oil, and may
affect its breakdown voltage (see 6.4). Measurement
Electrostatic Charging tendency of oil is an important
shall be carried out according to IS 13236.
property for certain designs of HV and EHV
transformers which have oil pumping rates that can give 6.21 DBDS Content
rise to the build-up of electrostatic charge. This can
This compound is corrosive at normal transformer
result in energy discharge causing transformer failure.
operating temperatures and can produce copper
ECT testing is a specific requirement of 7.2. sulphide. It therefore shall not be present in unused oil
(see 6.10). For the test method for measuring DBDS,
NOTE — A method to measure ECT is proposed by CIGRE
Technical Brochure 170. ECT can be reduced by using metal see IS 16497 (Part 1).
passivator additives such as BTA and TTA.
6.22 Stray Gassing of Oil
6.15 Flash Point Some oils can produce gases such as hydrogen,
The safe operation of electrical equipment requires an hydrocarbons and carbon oxides at low temperatures
adequately high flash point that is measured in (< 120 °C) without thermal or electrical faults in a
accordance with IS 1448 (Part 21) (Pensky-Martens transformer, sometimes even without operational stress.
closed cup procedure). This phenomenon could result in a high production of
gases and a misinterpretation of DGA results.
6.16 Density NOTE — Methods to measure stray gassing are described in
CIGRE Brochure 296 and ASTM D7150. Inhibited grades
In cold climates, density of oil shall be low enough to typically produce less stray gassing than uninhibited ones.
avoid the ice that results from the freezing of free water
to float to the oil surface and possibly lead to fault 7 SPECIFIC REQUIREMENTS FOR SPECIAL
conditions developing such as flashover of conductors. APPLICATIONS
Density shall be measured in accordance with IS 1448
(Part 16). 7.1 Higher Oxidation Stability and Low Sulphur
Content
6.17 Polycyclic Aromatics (PCA)
For transformers with higher operating temperatures or
Some PCAs are classified to be carcinogens and designed for extended service life, there may exist
therefore need to be controlled to an acceptable level restricted limits after oxidation test [see IS 12422
in mineral insulating oil. PCAs are defined so as to be (Method C)]. Mostly, such oil is inhibited (I).
detectable by extraction with DMSO
a) Total acidity 0.3 mg KOH/g, Max
(Dimethylsulfoxide) under the conditions of IP 346.
b) Sludge 0.05 percent, Max
NOTE — Acceptable limits of total or individual PCAs are
specified in national and local regulations.
c) DDF at 90°C 0.050, Max
d) Total sulphur content 0.05 percent, Max
6.18 Polychlorinated Biphenyls (PCB) (before oxidation test)
Unused mineral insulating oil shall be free from PCB. 7.2 Electrostatic Charging Tendency (ECT)
The reference test method is IS 16082.
For equipment with high oil circulation speed (OF -or
6.19 2-Furfural (2-FAL) and Related Compounds OD-cooled power transformers [IS 2026 (Part 2)]), as
Content for example HV/DC transformers, a limit may be agreed
2-FAL and related compounds in unused mineral between purchaser and manufacturer.
insulating oils can result either from improper re-
7.3 Gassing Tendency
distillation after solvent extraction during refining or
from contamination with used oil. For equipment with high electrical field stress or special
Unused insulating oils should have a low level of 2- design, gases formed when subjected to corona partial
FAL and related compounds; measurement shall be discharges (see 6.13) shall be absorbed by the oil.
done according to IS 15668. Therefore the gassing tendency according to IEC 60628
shall be agreed upon between the supplier and the
NOTE — “Related compounds” are: 5-hydroxy methyl-2-
furfural (5HMF) 2-furfuryl alcohol (2FOL), 2-acetyl furan
purchaser of the oil for such equipment.
(2ACF) and 5-methyl-2-furfural (5 MEF). NOTE — In Poland, a gassing tendency < 5 mm3/min is used
for equipment > 400 kV.

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IS 335 : 2018

Table 3 List of International Standard and/or Indian Standard Numbers for the Specific Properties
(Informative)
(Clause 4)
Sl Characteristics International Standard Indian Standard Identical
No.
(1) (2) (3) (4) (5)
i) Viscosity, @ 40°C, 0°C & @ –30°C, ISO 3104 IS 1448 (Part 25) No
ii) Viscosity, @ –40°C IEC 61868 IS 16084 Yes
iii) Pour point ISO 3016 IS 1448 (Part 10/Sec 2) Yes
iv) Water content IEC 60814 Not available No
v) Breakdown voltage IEC 60156 IS 6792 Yes
vi) Density @ 20°C ISO 3675 IS 1448 (Part 16) Yes
vii) Dielectric dissipation factor IEC 61620 IS 16086 Yes
@ 90°C
viii) Particle content IEC 60970 IS 13236 Yes
ix) Appearance — IS 335 No
x) Acidity IEC 62021-1 Not available No
xi) Interfacial tension ASTM D971 Not available No
xii) Total sulphur content ISO 14596 or ASTN D4294 Not available No
xiii) Corrosive sulphur DIN 51353 Not available No
xiv) Potentially corrosive sulphur IEC 62535 IS 16310 Yes
xv) Oxidation inhibitor content IEC 60666 IS 13631 Yes
xvi) Metal passivator additives IEC 60666 IS 13631 Yes
xvii) 2-Furfural content and related IEC 61198 IS 15668 Yes
xviii) Oxidation stability IEC 61125 Method C IS 12422 Yes
xix) Gassing tendency, mm3/min IEC 60628 Method A Not available No
xx) Flash point, °C ISO 2719 IS 1448 (Part 21) Yes
xxi) PCA content, percent IP 346 Not available No
xxii) PCB content, ppm IEC 61619 IS 16082 Yes
xxiii) Sampling of mineral insulating oil IEC 60475 IS 6855 Yes
xxiv) DBDS IEC 62697-1 IS 16497 (Part 1) Yes
xxv) Stray gassing CIGRE Brochure 296 or Not available No
ASTM D 7150

xxvi) Electrostatic charging tendency CIGRE Brochure 170 Not available No

xxvii) Foaming test ISO 6247 Not available No

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IS 335 : 2018

ANNEX A
(Clause 2)
LIST OF REFERRED STANDARDS

IS/International Title IS/International Title

Standard No. Standard No.
1448 Methods of test for petroleum and 16086 : 2013 Insulating liquids — Determination
its products : of the Dielectic Dissipation factor
(Part 10/Sec 2) : Cloud point and pour point, by measurement of the conductance
2013 Section 2 Determination of pour and capacitance — Test method
point (second revision) 16310 : 2017 Insulating liquids
(Part 16) : 2014 Crude petroleum and liquid Test method for detection of
petroleum products — Laboratory potentially corrosive sulphur in
determination of density — used and unused insulating oil.
Hydrometer method (fourth 16497 (Part 1) : Test methods for quantiative
revision) 2017 determination of corrosive sulphur
(Part 21) : 2012 Determination of flash point — compounds in unused and used
Pensky-martens closed cup method insulating liquids : Part 1 Test method
(third revision) for quantitative determination of
(Part 25) : 1976 Determination of kinematic and dibenzyldisulfide (DBDS)
dynamic viscosity (first revision) IEC 60247 : 2004 Insulating liquids — Measurement
1866 : 2017 Mineral Insulating Oils in Electrical of relative permittivity, dielectric
Equipment — Supervision and dissipation factor (tan ä) and d.c.
Maintenance guidance (fourth resistivity
revision) IEC 60628 : 1985 Gassing of insulating liquids under
2026 Power Transformers : Part 2 electrical stress and ionization
(Part 2) : 2010 Temperature rise (first revision) IEC 60814 : 1997 Insulating liquids — Oil
6792 : 2017 Insulating liquids — Determination impregnated paper and pressboard
of the Breakdown Voltage at Power — Determination of water by
Frequency — Test Method (second automatic coulometric Karl Fischer
revision) titration
6855 : 2017 Method of sampling for insulating IEC 60867 : 1993 Insulating liquids — Specification
liquids (second revision) for unused liquids based on
12422 : 2017 Unused Hydrocarbon—Based synthetic aromatic hydrocarbons
Insulating Liquids—Tests methods IEC 62021-1 : Insulating liquids — Determination
for evaluating the oxidation stability 2003 of acidity — Part 1: Automatic
(first revision) potentiometric titration
13236 : 2013 Insulating liquids — Methods for ISO 6247 : 1998 Petroleum products —
counting and sizing particles (first Determination of foaming
revision) characteristics of lubricating oils
13631 : 2017 Detection and determination of ISO 14596 : 2007 Petroleum products —
specified additives in mineral Determination of sulfur content —
insulating oils (first revision) Wavelength-dispersive X-ray
15668 : 2006 Mineral insulating oils — Methods fluorescence spectrometry
for the determination of 2-furfural ASTM D971 Standard test method for interfacial
and related compounds tension of oil against water by the
16082 : 2013 Insulating liquids — Contamination Ring method
by polychlorinated biphenyls ASTM D4294 Standard Test Method for Sulfur in
(PCBs) — Method of determination Petroleum and Petroleum Products
by capillary column gas by Energy Dispersive X-ray
chromatography Fluorescence Spectrometry
16084 : 2013 Mineral insulating oils — ASTM D7150 Standard Test Method for the
Determination of kinematic Determination of Gassing
viscosity at very low temperatures Characteristics of Insulating

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IS 335 : 2018

IS/International Title IS/International Title

Standard No. Standard No.
Liquids Under Thermal Stress at sulphoxide extraction refractive
Low Temperature index method
IP 346 Determination of polycyclic DIN 51353 Testing of insulating oils: Detection
aromatics in unused lubricating of corrosive sulfur — Silver strip
base oils and asphaltene free test
petroleum fractions — Dimethyl

ANNEX B
(informative)
(Clause 6.10)
POTENTIALLY CORROSIVE SULPHUR

B-1 MECHANISM OF COPPER SULPHIDE forming Cu2S contain this substance. However, refining
DEPOSITION processes using severe hydrotreatment can easily
remove this reactive compound from oil. Several other
The mechanism of copper sulphide (Cu2S) deposition
substances (including disulphides, thioethers, various
is still not fully elucidated, but it may involve
oxidized sulphur compounds and elemental sulphur)
dissolution and transport of copper by sulphur
have been shown to cause Cu2S formation in IS 16310
containing species forming complexes with copper.
test, when added to originally non-corrosive oils.
These complexes can then be absorbed by cellulosic
insulation where they decompose into Cu2S. B-3 DETECTION OF CORROSIVE SULPHUR
The strong influence of temperature and oxygen on this COMPOUNDS IN PASSIVATOR-CONTAINING
process indicates that some oxidized sulphur species OILS
may be more active than those originally present in oil, When oil in a transformer contains a metal passivator
or that other oxidation products are important as co- additive, a thin protective layer of passivator is formed
complexing agents (see CIGRE Technical Brochure on copper surfaces, preventing copper from dissolving
378). Cu 2 S deposition occurs preferentially in in oil, reacting with corrosive sulphur compounds
equipment where corrosive sulphur compounds are present in oil, and depositing in paper insulation as
present in oil, unvarnished or unprotected copper is harmful copper sulphide (Cu2S). The same occurs when
used, operating temperatures are high and the amount testing passivator-containing oils according to
of oxygen in oil is limited. The optimal oxygen content IS 16310. This test method therefore cannot detect
for copper transport seems to be relatively low, corrosive sulphur compounds present in passivating oils
probably in the region of a few thousand µl/l, but and may provide “false negative” results for such oils.
deposition may occur over a wide range of oxygen Passivator-containing oils testing negative as new oils
contents. may then test positive and start depositing harmful Cu2S
after the additive has been consumed by aging in
B-2 CORROSIVE SULPHUR COMPOUNDS IN transformers service.
OIL
In order to detect corrosive sulphur compounds in oil
Although many sulphur compounds are known to be containing a metal passivator additive (declared or
corrosive for copper, few have been identified as suspected), the passivator additive has to be removed
components of insulating oil. The only compound first from the oil. The two following procedures can be
shown so far to be a potent Cu2S forming agent and to used for that purpose. Both are intended for newly
be present in significant amounts in transformer oil is available types of oils only, not for normal deliveries
dibenzyl disulfide (DBDS). Most oils found to be of oil.

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IS 335 : 2018

Procedure 1 mistakenly interpreted as Cu2S), confirm Cu2S

deposition with SEM/EDX or other
In this procedure, metal passivator additives are
techniques according to Annex B of IS 16310.
eliminated by specific adsorption from the oil:
False positives can also be avoided by carrying
a) stir 100 ml of passivator-containing oil with out a second IS 16310 test without copper strip
500 mg of Chromabond HR-XC adsorbent and with paper only, and comparing the
(a strong, mixed-mode, polymer-based cation appearance of papers after both tests with and
exchanger for basic analytes), for 1 h, then without copper.
filter out the adsorbent; or NOTES
b) extract 60 ml of oil under a slight vacuum on 1 The protective layer of passivator on copper has been
observed to remain on copper after aging tests in the
a 3 ml column containing 200 mg of the
laboratory, but there is little knowledge on whether and
adsorbent, if the initial concentration of how long it will remain on copper in transformers in
passivator was < 200 mg/kg. service.
2 As a complement to IS 16310 and procedures 1 and 2
Procedure 2 for passivator-containing oils, the quantification of
This procedure is based on the observation that metal corrosive sulphur compounds in oil [for example
dibenzyldisulphide (DBDS) and total disulphide] can
passivator additives in oil are consumed by oxidation be used to ensure that none of these potentially harmful
aging (in accelerated tests in the laboratory and in compounds are present in oil.
transformers in service):
B-4 CONTAMINATION OF OILS
a) Run the passivator-containing oil in the test
Mineral insulating oils suspected of having been
cell used in IS 12422 at 120 °C for 164 h with
accidentally contaminated with silicone oils, phthalates
an air flow of 0.15 l/h to ensure that the
or other surface-active chemicals or oils should not be
passivator has been consumed by oxidation.
introduced in transformers, since these compounds can
b) Test the aged oil for corrosive sulphur in the produce foaming in oil when trying to degas the
test cell of IS 16310 with new paper wrapped transformer, thus making it difficult or impossible to
conductor. fully degas the transformer oil. The foaming tendency
c) To avoid false positives with the aged oil (that test of ISO 6247 can be used to detect such a
is where oxidation aging compounds of oil are contamination.

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This Indian Standard has been developed from Doc No.: ETD 03 (6748).

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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AMENDMENT NO. 1 NOVEMBER 2020

TO

IS 335 : 2018 NEW INSULATING OILS — SPECIFICATION

( Fifth Revision )

(Page 4, Clause 6.5) — Substitute the last two sentences with the following:
‘DDF shall be measured in accordance with IS 16086 or IS 16840 at 90°C.
In case of dispute, IS 16840 at 90°C should be used.’

(Page 4, Table 2, Row (ix), Col (3)) — Substitute with the following:
‘IS 16086 or IS 16840’

(Page 8, Table 3, Row (vii), Col (3)) — Substitute with the following:
‘IEC 61620 or IEC 60247’

(Page 8, Table 3, Row (vii), Col (4)) — Substitute with the following:
‘IS 16086 or IS 16840’

(Page 9) — Substitute ‘IEC 60247 : 2004’ with the following:

‘IS 16840 : 2018’.

(ETD 03)
Publication Unit, BIS, New Delhi, India