Abstract: Abnormalities in current circuits are one of the primary causes of false tripping in UHV substation protection systems. This paper systematically analyzes an incident involving an abnormal current circuit in the first phase-separated current differential protection (PCS-931GMM) unit for the Anlan II Line at the 1000 kV Lanjiang Substation. In response to abnormal phenomena such as frequent protection tripping, low amplitude of B and C phase sampling currents, and phase shifts, the analysis employed methods including multi-device data comparison, infrared temperature measurement, waveform vector analysis, and on-site current calibration to precisely locate a high-resistance short-circuit fault between the B and C phases of the 9SD high-current switching terminal within the T032 switch control cabinet. The root cause was identified as residual copper wire and accumulated dirt within the terminal screw holes, resulting in high-resistance discharge between Phases B and C. Key factors contributing to the residual copper wire—including excessive operation frequency, copper material properties, and manufacturing defects—were analyzed, and targeted operational and maintenance improvement measures as well as accident prevention strategies were proposed. These research findings provide important references for identifying potential hazards in secondary circuits and managing the operation and maintenance of high-current terminals at ultra-high-voltage substations.