Lightning overvoltages are considered as one of the major causes of insulation failure in medium voltage overhead lines. Therefore, the evaluation of the dielectric strength of overhead line insulation is of great importance for insulation coordination studies. This paper presents the flashover probability distributions and the volt-time characteristics of different types of insulator gaps subjected to both positive and negative polarity standard impulse (SI) and short tail lightning impulse (STLI) voltages. In practice, the transient overvoltages generated by a lightning stroke are superimposed over the power frequency AC voltage. Accordingly, the flashover characteristics of the insulator gaps under combined AC and lightning-induced overvoltages are also presented in this paper. The results indicate that superimposition of the impulse voltage on the power frequency AC voltage changes the flashover characteristics of the insulation gaps. Therefore, it is recommended that the effect of power frequency AC voltage should be considered in the risk assessment of insulation for the insulation coordination studies.