Abstract: For 220kV and above substations, the on-site DC system typically adopts an operation mode configured with two chargers and two sets of battery packs, where critical loads such as protection, measurement and control devices are powered by two bus sections. With the extension of substation operation years, cables in outdoor substations are prone to aging, insulation breakage and other defects, which easily trigger cross-pole faults in the DC system. In addition, improper wiring by construction personnel in newly-built substations also contributes to the high incidence of such cross-pole faults. Currently, the performance of on-site DC insulation monitoring devices varies significantly; some insulation monitors can only detect grounding faults of the local bus section, but fail to identify cross-pole faults between the two bus sections. Meanwhile, the professional competence of construction personnel is inconsistent. When abnormal DC bus voltage occurs, they often cannot determine whether a cross-pole fault has occurred, leading to difficulties in defect handling. To address these problems, this paper deeply analyzes different forms of cross-pole faults between two DC bus sections, including single-pole cross-pole fault, double-pole cross-pole fault, direct cross-pole fault and resistive cross-pole fault between the two buses. Through theoretical formula derivation and Matlab simulation verification, the characteristics of cross-pole faults are summarized, providing practical troubleshooting ideas for on-site construction and maintenance personnel.