Abstract: Taking the bus coupler automatic back up power supply device logic and line in automatic back up power supply device logic of 110kV and the below for instance, and associated with field engineering practice, the logical flaws of segment automatic back up power supply device WBT 821A are analyzed, and the improved automatic back up power supply device logic is proposed for software design and panel design. The special automatic back up power supply device blocking logic problems after the main transformer’s fault occurring of the line in automatic back up power supply device CSC 246 are analyzed. Some important engineering application tips are proposed.