In this paper

a rare non equipotential fault of end shield of capacitive current transformer causing overproof acetylene in the oil chromatography test results is introduced and diagnosed.This fault can’t be effectively found through dielectric loss factor test in the condition maintenance and routine test.In the oil chromatography test

the overproof acetylene directly occurs in great deal when hydrogen and total hydrocarbon haven’t grown apparently（namely

in case of not exceeding standard）

which violates the law of development of general insulation fault.When disassembled

it is found that the cause of the failure is on winding screens in the copper braid with design flaws making the end shiled is not in equipotential body.If in the color spectrum test results

the content of hydrogen and methane did not yet have significantly increased

it appears directly acetylene content

this indicating the capacitive current transformers may have occurred such failure.