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The main objective of this paper is to investigate the effect of air-gap position, length, and number on the phenomena preceding flashover along a typical 735-kV porcelain station post insulator unit under heavy ice accretion. Under these conditions, this type of insulator seems to be the most likely subject to flashover. The numerical simulations were performed using Coulomb 3D, a commercial software based on the boundary element method (BEM). Computer simulations were performed during a melting period. Special attention was paid to the presence of a conductive water film at the ice surface and to a partial arc along an air gap. The results obtained showed that the number of air gaps, with or without the presence of a partial arc along them, has a direct effect on the distribution of potential along the iced insulator. Moreover, for the same total arcing distance, the number of air gaps and their position did not affect the average electric field per arcing distance that was used to determine the partial arc presence. Based on these results, suggestions for improving station insulator geometry for icing conditions were proposed.