AbstractThis paper studies the possibility of an aerial flashover between a coil conductor and the casing in a rotating machine at voltages below Paschen's voltage even when sufficient insulation distance has been provided. It might be considered that partial surface flashover generated from microscopic pores of a solid insulator generates the aerial flashover. In this study, the flashover voltage was measured using a cable which had simulated microscopic pores to confirm the above reasoning. When a potential gradient existed on the surface of the insulating solid, the flashover voltage decreased from that in the absence of a potential gradient. It also became clear that the aerial flashover probability depends mainly on the energy of the surface flashover rather than the energy of the main aerial flashover. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(4): 16–23, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21221