Here, breakdown tests of cap‐and‐pin glass insulators with arcing horns, conducted at two different HV labs, are presented. Since only volt–time curves are measured, a curve‐fitting approach to determine leader progression model parameters is taken, which involves modelling of the impulse generator in detail and inclusion of the Motoyama model to simulate the breakdown. Furthermore, a comparison of the breakdown simulation results with LPM parameters for a pure air gap and the measurements is accomplished. Finally, a curve‐fitting procedure is developed to fit the resulting volt–time curves from simulations to the measurements.