The paper reports the results of an investigation carried out in order to determine the critical flashover voltage, and the relevant standard deviation, of the rod-rod air gaps spacing 0.1 and 0.2 m stressed by standard lightning impulse (LI) (1.2/50 μs) and an impulse voltage with short tail (1.2/4 μs). The volt-time characteristics of these spark-gaps under both impulses are also determined and the experimental test results are discussed.Models based on the disruptive effect, as proposed by Kind and by Chowdhuri, are considered and applied in order to reproduce the volt-time characteristics under 1.2/50 and 1.2/4 μs impulses. The viability of the application of these models to the above-mentioned insulation is discussed. The results of the application of the two models are compared with each other and with the experimental characteristics. © 2006 Elsevier B.V. All rights reserved.