IEC 62535 Insulating liquids - Test method for detection of potentially corrosive sulphur in used and unused insulating oil
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.

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 x 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.