Abstract: This paper presents a case study on insulation failure of a 40.5 kV bus bridge with a phase-to-phase spacing of 300 mm. Analysis reveals that contamination adhering to the surface of insulating components, combined with high humidity inside the switchgear, are the primary factors accelerating insulation degradation. By using Ansys finite element simulation, the electric field distribution of insulators under clean and contaminated wet conditions is analyzed and compared, further confirming that wet contamination significantly increases the risk of insulation failure. Finally, practical anti-contamination operation strategies and improvement measures are proposed, providing theoretical and practical guidance for field maintenance and insulation fault prevention.