Abstract: After the renovation of the Xinquan Pumping Station in the Ningxia Hongsibu Water-Lifting Project, an anomaly was observed in its 10kV incoming power switchgear: the displayed active power was lower than the sum of the downstream loads, while the reactive power registered a negative value. This paper identifies the root cause of the fault by conducting a comparative analysis of the electrical parameters between the incoming power supply and the downstream loads. The analysis reveals that while the busbar voltage was normal, the anomaly stemmed from a composite wiring error in the secondary circuit of the incoming line's current transformer (CT)—a combination of phase sequence reversal and polarity inversion. This error introduced a systematic phase shift of approximately +60° in the measured current, which subsequently led to computational inaccuracies in the active power, reactive power, and power factor. The paper also briefly discusses the potential impact of phase sequence reversal and mixed polarity errors in the protection winding on the performance of the relay protection system. After the secondary wiring was corrected on-site, all monitored data returned to normal readings. This case study provides practical guidance for the standardized installation and wiring of CTs and protective devices in pumping station distribution systems.