When the grounding inspection device of substation DC system selects fault branch, the existences of zero point drift, transformer remanence, and transformer stability may impact the efficiency of grounding troubleshooting, which will lead to wrong selection or missed selection. The calculation of grounding resistance based on balanced bridge and unbalanced bridge method is derived, and the reasons of wrong selection or missed selection are analyzed. According to the logical relationship between state variables such as voltage, current and calculated value such as grounding resistance before and after the fault, the characteristics of the faults are identified based on circuit schematic. Automatic inspection mode based on logic event driven is proposed to identify branch with zero drift and grounding branch, and the calculation process of grounding resistance is checked based on absurdum. The real data validate this inspection mode and check method, which can not only improve the accuracy of DC lookup, but also provide useful information for status maintenance.