Abstract: In order to improve reliability of power equipments, the fault case of fuse melting at higher voltage side of voltage transformer in 10kV substation is investigated and analyzed in this paper. Then the polarity problem of 10kV zero sequence voltage circuit is analyzed, and zero sequence voltage space vectorgraph is given. The abnormal conditions of zero sequence secondary voltage that generally occurs in power system are summarized, and the cause of fuse melting at voltage transformer high\|voltage side is found to be the wrong polarity for zero sequence secondary voltage circuit. A new method which can verify correctness of polarity for the zero sequence voltage circuit is proposed, and secondary circuits with problems are examined one by one. The results show that the proposed method is simple, practical, safe and effective, which can be applied to inspection process of second protection.