Abstract: This paper conducts a systematic fault diagnosis and mechanism analysis regarding a current abnormality observed in a three-phase three-wire energy meter for a 10 kV high-voltage dedicated transformer user. The anomaly was characterized by a significant persistent difference in the current amplitudes between phases A and C, while their fluctuation trends remained highly synchronized. Through monitoring via the electricity consumption information acquisition system and on-site step-by-step troubleshooting, the root cause was identified as a loose screw fastening at the wiring terminal of the secondary circuit for phase C current transformer. This led to a significant increase in contact resistance, reducing the sampled secondary current to approximately one-third of its actual value. By analyzing the terminal structure of the current transformer and its equivalent circuit model, this paper theoretically examines the impact of increased contact resistance on the secondary current and clarifies that insufficient tightening torque is the key factor causing the degradation of contact reliability. Furthermore, preventive measures and standardized recommendations are proposed from four aspects: construction and installation, acceptance and commissioning, operation monitoring, and management safeguards. This case provides a reference for the diagnosis and rectification of similar current abnormalities in high-voltage metering devices.