Abstract: Taking a 66 kV transmission line fault caused by cross-connection transposition as an example, through theoretical analysis and model listing before power failure, causes of hidden trouble were failure of cross-connect due to earthing core phase error. Theoretical analysis was confirmed when opening the grounding box after the power failure.It was pointed out that analysis of the measured value of circulating current in protective layer with infrared temperature measurement could greatly reduce the power failure time and quickly locate the defects. Results showed it was necessary to carry out differential inspection of cable lines according to their service life, to strengthen the training of construction personnel and project acceptance personnel, to summarize and establish case base for similar potential defects, and to strengthen and perfect the life cycle management of high voltage transmission cables, which could reduce the difficulty of fault detection, shorten the power outage time and improve the reliability of high-voltage cable transmission.