The process for a fault of loosen bushing bottom pedestal in the transformer is described. Abnormal chromatographic data is discovered

then characteristic gas method

three-ratio method and David triangle method are used to analyze the fault type

and electrical test methods can be used to judge the position of fault. Thus the combined relation is verified effectively. The methods

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

some useful experiences for diagnosis and treatment of the inner fault in large transform are provided.