IEC 62535 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.

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.