Abstract: This paper analyzes a fault in which the operation of a local manual closing switch caused a control power circuit breaker to trip during the commissioning of smart terminal retrofits in a 220kV bus coupler bay of a 500kV smart substation. Through on-site investigation and drawing verification, a comprehensive analysis was conducted on the design and implementation issues of the coordination circuit between the primary and secondary sets of smart terminals. It was confirmed that the fault was caused by a short circuit resulting from the incorrect connection of a newly added input circuit to the control power supply. The fault was resolved after modifying the circuit and replacing the relevant board. This paper systematically details the analysis and resolution process of this case, which can serve as a technical reference for circuit design and on-site commissioning of smart terminals during smart substation retrofits.