The paper presents the results of experimental tests performed with impulse voltage with short tail (STLI: 1.2/4 μs) on composite insulators in use in the MV distribution lines and electric traction lines. The voltage-time to breakdown characteristics under 1.2/4 μs waves are presented and compared with the characteristics under standard lightning impulse (LI: 1.2/50 μs). Models based on the disruptive effect, as proposed by Kind [9], and modified by Chowdhuri [13], are also considered and applied in order to predict the Volt-time characteristics under standard and non standard lightning voltages. The viability of the application of these models to the tested insulators is considered and discussed.