Abstract: This paper focuses on the secondary voltage fault of a capacitive voltage transformer (CVT) on a 220 kV transmission line. The fault phase is identified through the analysis of monitoring signals and secondary voltage data. Infrared thermography, electrical testing, and oil chromatographic analysis are combined for fault diagnosis. The conclusion is finally verified through equipment disintegration analysis. The results show that the fault originates from an interturn short circuit in the secondary winding of the electromagnetic unit inside the phase-C CVT. This defect causes the blown of grounding lead, thereby resulting in voltage distortion and protection alarms. This study systematically presents a complete diagnostic chain from phenomenon analysis and comprehensive testing to disassembly verification. By integrating the three-ratio results of oil chromatographic analysis with electrical characteristic data, it predicts internal insulation faults and provides a clear technical basis for early warning and rapid fault handling of similar equipment.