In this paper

a fault process in transformer is discussed.The fault is originated from core grounding which leads to screw loose and produce the floating potential;then discharge and current occur.This current flows in the circuit between grounding leads and grounding copper and leads active power loss.This kind of fault can be checked through the chromatographic analysis of the oil in the transformer.The reasons

steps and attentions are suggested when the content of total hydrocarbons in the oil is increased but still in the specified range.Meanwhile

it is pointed that the loose and transfer of the external grounding lead of the transformer core is also one kind of core faults.