IEC 62535:2008

Insulating liquids - Test method for detection of potentially corrosive sulphur in used and unused insulating oil

IEC 62535:2008 specifies a test method for detection of potentially corrosive sulphur in used and unused mineral insulating oil. Most recent failures due to corrosive sulphur are related to the formation of copper sulphide deposits in and on the surface of winding cellulosic paper. The test method uses a copper conductor, wrapped with one layer of paper, immersed in the oil and heated to evaluate the capability of the oil to yield copper sulphide and transfer it to paper layers. The growth of copper sulphide on bare copper may cause the presence of conductive particulates in the oil, which can act as nuclei for electrical discharge and may lead to a fault. Other test methods exist using a bare copper strip immersed in oil and heated to detect the corrosive behaviour of oil against copper. ASTM D1275 Method B is also used for this test and a modified procedure using low oil volumes is included in Annex A. Tests with and without paper are considered as complementary and may lead to different results.

Liquides isolants - Méthode d'essai pour la détection du soufre potentiellement corrosif dans les huiles usagées et neuves

La CEI 62535 :2008 spécifie une méthode d'essai relative à la détection du soufre potentiellement corrosif dans les huiles minérales isolantes usagées et neuves. La plupart des défaillances récentes du fait du soufre corrosif sont liées à la formation de dépôts de sulfure de cuivre dans et à la surface du papier cellulosique de l'enroulement. La méthode d'essai utilise un conducteur en cuivre, recouvert d'une couche de papier, immergé dans l'huile et chauffé pour évaluer la capacité de l'huile à produire du sulfure de cuivre et le transférer sur les couches de papier. La croissance du sulfure de cuivre sur le cuivre nu peut provoquer la présence de particules conductrices dans l'huile, qui peuvent agir comme des noyaux pour la décharge électrique et peuvent conduire à un défaut. D'autres méthodes d'essai qui utilisent une bande de cuivre nu immergée dans l'huile et chauffée existent pour détecter le comportement corrosif de l'huile par rapport au cuivre. La norme ASTM D1275, méthode B est également utilisée pour cet essai et une procédure modifiée utilisant de faibles volumes d'huile est incluse dans l'Annexe A. Des essais avec et sans papier sont considérés comme complémentaires et peuvent conduire à des résultats différents.

General Information

• Status: Published
• Publication Date: 07-Oct-2008
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 29.040.10 - Insulating oils
• Technical Committee: TC 10 - Fluids for electrotechnical applications
• Current Stage: PPUB - Publication issued
• Start Date: 08-Oct-2008
• Completion Date: 08-Oct-2008

Standards Content (sample)

IEC 62535
Edition 1.0 2008-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE

Insulating liquids – Test method for detection of potentially corrosive sulphur in

used and unused insulating oil
Liquides isolants – Méthode d’essai pour la détection du soufre potentiellement
corrosif dans les huiles usagées et neuves
IEC 62535:2008
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62535
Edition 1.0 2008-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE

Insulating liquids – Test method for detection of potentially corrosive sulphur in

used and unused insulating oil
Liquides isolants – Méthode d’essai pour la détection du soufre potentiellement
corrosif dans les huiles usagées et neuves
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX
ICS 29.040.10 ISBN 978-2-88910-756-8
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – 62535 © IEC:2008
CONTENTS

FOREWORD...........................................................................................................................3

INTRODUCTION.....................................................................................................................5

1 Scope...............................................................................................................................6

2 Normative references .......................................................................................................6

3 Terms and definitions .......................................................................................................6

4 Sampling ..........................................................................................................................7

5 Procedure ........................................................................................................................7

5.1 Principle..................................................................................................................7

5.2 Apparatus and materials..........................................................................................7

5.3 Method....................................................................................................................8

6 Inspection and interpretation ............................................................................................8

6.1 General ...................................................................................................................8

6.2 Copper ....................................................................................................................9

6.3 Paper ......................................................................................................................9

6.4 Result .....................................................................................................................9

7 Repeatability and reproducibility.......................................................................................9

8 Report ............................................................................................................................10

Annex A (informative) Copper strip method to detect corrosive and potentially

corrosive sulphur in oil..........................................................................................................11

Annex B (informative) Analysis for copper sulphide on insulating paper by scanning

electron microscope-energy dispersive X-ray spectrometry (SEM/EDX) ................................14

Bibliography..........................................................................................................................16

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62535 © IEC:2008 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATING LIQUIDS –
TEST METHOD FOR DETECTION OF POTENTIALLY CORROSIVE
SULPHUR IN USED AND UNUSED INSULATING OIL
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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any

equipment declared to be in conformity with an IEC Publication.

6) All users should ensure that they have the latest edition of this publication.

7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and

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 62535 has been prepared by IEC technical committee 10: Fluids

for electrotechnical applications.
The text of this standard is based on the following documents:
FDIS Report on voting
10/746/FDIS 10/749/RVD

Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

---------------------- Page: 5 ----------------------
– 4 – 62535 © IEC:2008

The committee has decided that the contents of this publication will remain unchanged until

the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in

the data related to the specific publication. At this date, the publication will be

• reconfirmed;
• withdrawn;
• replaced by a revised edition; or
• amended.
---------------------- Page: 6 ----------------------
62535 © IEC:2008 – 5 –
INTRODUCTION

In recent years, several failures of transformers and reactors due to copper sulphide

formation in/on the cellulose insulation have been reported worldwide. The tendency of

transformer oils to form copper sulphide in the presence of copper is seen as one of the major

contributing factors.

The most common reason for such failures is arcing between adjacent disks or conductors of

a winding, due to the formation of deposits of copper sulphide on the cellulosic insulating

paper.

It has been demonstrated that existing test methods for corrosive sulphur, ASTM D1275

method A and DIN 51353, are unable to detect oils having potentially corrosive behaviour.

For this reason, IEC technical committee 10 has prepared this International Standard for the

detection of potentially corrosive sulphur in mineral insulating oils. The wrapped conductor

test method is suitable for used and unused mineral oils.

This test method is based on a study performed by Conseil International des Grands Réseaux

Electriques (CIGRE) working group A2.32 [1] .
Health and safety

This International Standard does not purport to address all the safety problems associated

with its use. It is the responsibility of the user of the standard to establish appropriate health

and safety practices and determine the applicability of regulatory limitations prior to use.

The mineral oils which are the subject of this standard should be handled with due regard to

personal hygiene. Direct contact with eyes may 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 standard involve the use of processes that could lead to a

hazardous situation. Attention is drawn to the relevant standard for guidance.
Environment

This standard involves mineral oils, chemicals and used sample containers. The disposal of

these items should be carried out in accordance with current national legislation with regard

to the impact on the environment. Every precaution should be taken to prevent the release

into the environment of mineral oil.
___________
Figures in square brackets refer to the bibliography.
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– 6 – 62535 © IEC:2008
INSULATING LIQUIDS –
TEST METHOD FOR DETECTION OF POTENTIALLY CORROSIVE
SULPHUR IN USED AND UNUSED INSULATING OIL
1 Scope

This International Standard specifies a test method for detection of potentially corrosive

sulphur in used and unused mineral insulating oil.

Most recent failures due to corrosive sulphur are related to the formation of copper sulphide

deposits in and on the surface of winding cellulosic paper.

The test method uses a copper conductor, wrapped with one layer of paper, immersed in the

oil and heated to evaluate the capability of the oil to yield copper sulphide and transfer it to

paper layers.

The growth of copper sulphide on bare copper may cause the presence of conductive

particulates in the oil, which can act as nuclei for electrical discharge and may lead to a fault.

Other test methods exist using a bare copper strip immersed in oil and heated to detect the

corrosive behaviour of oil against copper. ASTM D1275 Method B is also used for this test

and a modified procedure using low oil volumes is included in Annex A.

Tests with and without paper are considered as complementary and may lead to different

results.
2 Normative references

The following referenced documents are indispensable for the application 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 60475, Method of sampling liquid dielectrics

IEC 60554-3-1, Specification for cellulosic papers for electrical purposes – Part 3:

Specifications for individual materials – Sheet 1: General purpose electrical paper

ASTM D1275, Methods A and B: Standard test method for corrosive sulfur in electrical

insulating oils

ASTM D130, Standard test method for corrosiveness to copper from petroleum products by

copper strip test

DIN 51353, Testing of insulating oils; detection of corrosive sulfur; silver strip test

EN 13601, Copper and copper alloys. Copper rod, bar and wire for general electrical

purposes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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62535 © IEC:2008 – 7 –
3.1
potentially corrosive sulphur

organo-sulphur compounds present in transformer oils that may cause copper sulphide

formation.

NOTE Some of these compounds may be initially corrosive, or become corrosive under certain operating

conditions
4 Sampling

Samples shall be taken, following the procedure given in IEC 60475. Ensure that the test

portion is representative by thoroughly mixing.
5 Procedure
5.1 Principle

A piece of copper conductor wrapped with Kraft paper is immersed in the oil and subjected to

heating for 72 h at 150 °C in a sealed glass headspace vial.

The copper is examined for indications of discolouration and the paper is examined for the

presence of copper sulphide deposits.
5.2 Apparatus and materials
The following apparatus and materials shall be used:

– Glass headspace vial as used for chromatography, capacity approximately 20 ml, with a

nominal diameter of 22,5 mm to 23 mm.

NOTE 1 Vials are commonly available from suppliers of chromatography consumables and from instrument

manufacturers.
– PTFE-faced silicone septum and aluminium cap for sealing the vial.
– Crimping tool.
– Heating chamber or oven capable of being maintained at 150 °C ± 2 °C.
– Flat, unvarnished, paper-wrapped copper conductor obtained from a transformer

manufacturer or winding supplier. Dimension of the flat copper: approximately 7,5 mm x

1,5 mm (or with a surface area corresponding to these dimensions). Wrapping paper width

10 mm to 14 mm. The most suitable have proven to be flat conductors having four layers

of wrapped paper. The paper adjacent to copper should be wound gap to gap (the gap

+1,00

should be in the tolerance mm, the negative value means overlapping). This is the

−0,2

layer used with the copper conductor for the test itself. The rest of the paper layers have

only a mechanical and protective function during transport and storage.
NOTE 2 Possible sources of supply are :
– Asta Elektrodraht GmbH & Co. (Austria)
– Essex Nexans L+K GmbH, (Germany)
– Siemens AG, Trafowerk Nuremberg (Germany).2
– Kraft paper according to IEC 60554-3-1:
– Density: 0,70 to 0,85 g/cm
– Thickness: 0,060 mm to 0,100 mm
___________

2 This information is given for the convenience of the users of this International Standard and does not constitute

an endorsement by the IEC.
---------------------- Page: 9 ----------------------
– 8 – 62535 © IEC:2008
– Air permeability M or H
– Conductivity: < 4 mS/m
– Free nitrogen content: 0 %
– Cu-ETP according to EN 13601, (old material code: E-Cu58):
– Material code: CW 004A
– Oxygen content: < 0,04 % (w/w)
– Cu: 99,90 %
– Solvent for rinsing:
– Cyclohexane or heptane, analytical grade.
5.3 Method

Pour 50 ml of oil into a beaker of 100 ml and leave in the open air for 60 min (protected from

sunlight).

Transfer 15 ml of this oil into the headspace vial. The precision given by a measuring cylinder

is adequate, as is use of a headspace vial pre-marked at 15 ml.

Cut the conductor in 30 mm ± 3 mm lengths. Unwrap the outer layers of the copper conductor

and leave only one layer in contact with the copper. Do not touch paper or copper with fingers

during this and subsequent operations and ensure that all tools are pre-cleaned with solvent.

Place the wrapped flat conductor inside the headspace vial and close it with the septum

(PTFE face towards the oil) and close the cap using a crimping tool. The crimped cap should

be sufficiently tight such that hand rotating of the cap on the vial is not possible.

NOTE It is essential that, if a butyl rubber septum is used, the PTFE face seals correctly to the glass vial. The hot

oil must have no access to the butyl rubber, which contains sulphur.
Place in a heating chamber for 72 h ± 30 min at 150 °C ± 2 °C.

After the vial has cooled, remove the cap and take out the wrapped conductor. Tweezers

should always used to handle the conductor and the paper.
Unwrap the paper for evaluation of the copper surface.

Rinse the copper conductor with cyclohexane or heptane, and allow to dry for 2 min in air.

Prior to evaluation, degrease the paper by immersion in 50 ml of solvent in a beaker for 1 min.

Repeat the degreasing twice in fresh solvent and allow the paper to dry for at least 5 min until

all solvent is evaporated.
Carry out the test in duplicate.

Carry out a blank test, in order to ensure that all materials used are sulphur free. The blank

test should be carried out with a white oil or other oil containing less than 5 mg/kg sulphur.

Such a test will also assist in detecting changes to the copper.
6 Inspection and interpretation
6.1 General

All inspections should be carried out under very good light conditions. Bright daylight or

strong fluorescent lighting has been proven to be satisfactory. It is important to view objects

with light from different angles.
---------------------- Page: 10 ----------------------
62535 © IEC:2008 – 9 –

Both the copper and the paper shall be inspected for signs of sulphide formation. The results

from the duplicate tests must show the same discolouration.
6.2 Copper

Examine all surfaces of the copper for discolouration and note the appearance and colour.

The result is positive if the copper strip has one of the following colours: graphite grey, dark

brown or black. All other colours are considered a negative result.
6.3 Paper

Examine both the inside and outside surfaces of the paper strip. A magnifying glass (approx.

5 × magnification) can be helpful.

Copper sulphide deposition on the paper appears metallic, from clearly shiny to almost

lustreless, often with a lead- or tin-like appearance. It can also have the appearance of silver,

brass or bronze. The metal-like surface of sulphide can have an overlay of blue and/or purple

due to interference phenomena. Other discolourations (i.e. by-products of paper ageing and

oil deterioration) shall not to be taken as copper sulphide formation.

Copper sulphide may be formed both on the outside as well as the inside of the paper. Even

though sulphide may be formed anywhere, particular attention should be paid to edges and

inside bends. Note that deposits localized at some edges may come from copper

mechanically transferred to the paper while cutting the conductor. Sometimes the paper

surface is smoothed at the edges by the cutting tool. If any shiny appearance of the paper is

limited to such edges, it shall not be considered a positive result unless confirmed to be

sulphide.

Although copper sulphide formation is often clearly evident even when the paper is

discoloured, it may possibly be obscured by strong discoloration. SEM-EDX, or alternative

methods to determine total copper and sulphur content of the paper may be used to assist the

interpretation. An example of a method using SEM-EDX to evaluate the presence of copper

sulphide deposits on paper is described in Annex B.

In case of doubt concerning the composition of the precipitate, the result cannot be

considered as corrosive unless the precipitate is positively identified as copper sulphide.

X-ray diffraction or determination of copper and sulphur content of the paper may also give

guidance. It is also highly recommended in those cases to analyse for both copper and

sulphur on paper from an unused test specimen.
6.4 Result

If, for both of the duplicate samples, a positive result is found for copper, or paper, or both,

the oil shall be reported as potentially corrosive. If, for both samples, a negative result is

obtained for both copper and paper, the oil shall be reported as non-corrosive.

If the results for the duplicate sample are different, the test shall be repeated.

NOTE If there are any doubts in the interpretation of the results of inspection of paper, the composition of

precipitate should be analysed by other methods (for example by SEM-EDX). If the precipitate is identified as

copper sulphide, the oil must be reported as potentially corrosive.
7 Repeatability and reproducibility

Repeatability: according to inter-laboratory tests carried out by CIGRE WG A2.32, duplicate

results had 100 % agreement.
---------------------- Page: 11 ----------------------
– 10 – 62535 © IEC:2008

Reproducibility: according to inter-laboratory tests carried out by CIGRE WG A2.32, 2

laboratories out of 16 had different results. In these cases the oils differed from the original

delivered batches.
8 Report
The test report shall contain at least the following information:
• testing laboratory;
• the type and identification of the product tested;
• a reference to this International Standard;
• the result of the test (see 6.4);
• any deviation, by agreement or otherwise, from the procedure specified;
• the date of the test.
---------------------- Page: 12 ----------------------
62535 © IEC:2008 – 11 –
Annex A
(informative)
Copper strip method to detect corrosive and
potentially corrosive sulphur in oil
A.0 Introduction

This procedure describes a downsized corrosion test for which only a 15 ml oil sample is

needed, based on ASTM D1275 method B.
A.1 Principle

A cleaned strip of copper is immersed in a degassed portion of oil under investigation and

subjected to heating for 48 h at 150 °C in a sealed container.

The test is carried out by immersing the copper strip in 15 ml of oil into a sealed glass

headspace vial, under the same heating regime as in method B of ASTM D1275. The copper

is examined for indications of corrosion and interpretation of the results is the same as for

method B of that standard.
A.2 Reagents, apparatus and materials
A.2.1 Reagents
Acetone, analytical grade, sulphur-free.
Nitrogen gas or argon gas, oxygen free.
A.2.2 Apparatus and materials
The following apparatus and materials shall be used.
Bath
Heating chamber or oven capable of being maintained at 150 °C ± 2 °C.
Copper foil
>99,9 % pure, and 0,127 mm to 0,254 mm in thickness.
Polishing material

240-grit silicon carbide paper or cloth, and also 230-mesh silicon carbide grains and

pharmaceutical absorbent cotton.
Copper strips

Cut a strip of copper foil 6 mm by 25 mm and remove blemishes from surfaces with the 240-

grit silicon carbide paper. Strips may be stored in acetone at this point for future use.

Carry out final polishing of the strip by removing it from the acetone, holding it in the fingers

protected with ashless filter paper or nitrile gloves, and rubbing with 230-mesh silicon carbide

grains picked up from a glass plate with a pad of absorbent cotton moistened with a drop of

acetone. Wipe the strip with fresh pads of cotton and subsequently handle only with stainless

---------------------- Page: 13 ----------------------
– 12 – 62535 © IEC:2008

steel forceps (do not touch with the fingers). Rub in the direction of the long axis of the strip.

Clean all metal dust and abrasive from the strip, using successive clean cotton pads until a

fresh pad remains unsoiled.

Bend the clean strip in a V-shape at approximately a 30° angle and wash successively in

acetone, distilled water, and again in acetone. Dry in an oven for 3 min to 5 min at a

temperature of betweeen 80 °C and 100 °C and immediately immerse the copper strip in the

prepared test specimen of oil.
Do not use compressed air or an inert gas to dry the copper strip.
Glass headspace vial

A 20 ml glass headspace vial (as used for chromatography), with a nominal diameter of

22,5 mm or 23 mm and a height of 75,5 mm, fitted with a crimp cap and a butyl rubber/PTFE

septum.

NOTE 1 Vials are commonly available from suppliers of chromatography consumables and from instrument

manufacturers.

NOTE 2 A butyl rubber septum with a PTFE layer facing the oil is preferred since it ensures a better seal to

prevent ingress of oxygen. Although it contains corrosive sulphur, it has not been found to interfere with this test.

A.3 Procedure
A.3.1 General
Tests should be run in duplicate.

Carry out a blank test in order to ensure that materials have no influence on the test. The

blank test should be carried out with a white oil or other oil containing less than 5 mg/kg

sulphur.
A.3.2 Sample preparation

Place the bent copper strip into a headspace vial. Bubble nitrogen or argon through the oil in

the vial, by means of a glass tube connected to a reduction or needle valve of gas supply

(rubber tubes connection must be sulphur free) for 10 min. Take about 20 ml of this oxygen

free oil in a glass gas-tight syringe using stainless steel needle.
NOTE 1 Syringes described in IEC 60567[2] for DGA sampling are suitable.
Using a glove box or other suitable apparatus with a
...