Insulated tubular bus is a novel type of bus product with excellent performance. However

the period of its application is less two decades and its research is insufficient

which resulting in many accidents. In this paper

the full-scale model of epoxy cast insulated tubular bus was designed and fabricated

and its electrical breakdown mechanism was investigated by withstand voltage test. In the experiment

the failure mode of breakdown caused by gas expansion was found for the first time. The research showed that electrical-mechanical breakdown should be its breakdown mode

and the breakdown process could be divided into two stages: gas increase stage and breakdown threshold stage. The research also obtained the characteristics and the change criteria of the two stages. The gas in the bubble defects generated from epoxy decomposition which caused by partial discharge. When the gas pressure exceeds the critical fracture toughness of the bubble defects wall

the unstable crack extension occurs

therefore

improving the ability about resistance to partial discharge of epoxy resin or improving the fracture toughness of epoxy resin by modifying would enhance the breakdown performance of insulated tubular bus. The result also showed that it was difficult to judge the development status of breakdown channel by traditional testing method. The results of this research also have reference value for other epoxy cast insulated electrical equipment.