Figure 10 points out the relationship between the molecular
weight and the ECT of various sulfide compounds in
accelerated aging tests. This figure shows the ECT in the test
condition “with oxygen, with copper catalyst” at 100 h of
heating when the most outstanding increase of ECT was
detected. A convex-shaped tendency of the ECT was detected
in the sulfide compounds if their molecular weights are 100
g/mol to 400 g/mol. The increase of ECT caused by sulfide
compounds is considered to reflect the ease of transformation
of sulfide compounds and the affinity of the produced
compounds having high ECT with insulating oil (hydrocarbon
compounds).