Relation between surface flashover voltages (FOV) and numbers (N) of voltage application was investigated under repeated impulse voltages by using a GIS spacer model of φ15/φ60 in diameter. Applied impulse voltages were standard lightning and switching impulse voltages. FOV for negative polarity was expressed as V∞N^-/n, where n was 124 for lightning and 134 for switching impulse. Polarity effect existed and FOV's were higher for positive polarity than for negative polarity. Moreover, polarity reversal tests were carried out, and voltage-number (V-N) characteristics were clarified to coincide with the relation for negative polarity described above.
Flashover characteristics were statistically analized and it was clarified that initial deffects caused the flashover. Finally, a mechanism was discussed and it was suggested that initial electrons played an important role of V-N characteristics.