Abstract: Due to poor manufacturing process of fully insulated tubular bus and uneven distribution of field strength along the first and last edges of the metal shielding layer, intermittent discharge was caused after partial breakdown during long-term outdoor operation, resulting in abnormal operation of the main transformer in a 220 kV substation. In response to this incident, the fully insulated tube type bus is disassembled for inspection and analyses is made with consideration of the fully insulated tubular bus defect statistics of other several substations,based on which the corresponding improvement measures are proposed.