The Sulfonic acid compounds caused
the ECT with reverse polarity. The increase extent of ECT was
smaller in alkyl benzene with di-sulfide, thiophene and sulfone
compounds than in alkyl benzene with the above compounds.
Thiophene compounds, which are the major sulfur compounds
in mineral insulating oils, have only small influences on the
increase of ECT while sulfide compounds have large influences.
Sulfide compounds and their oxides are considered to increase
the ECT.