The depth of penetration of phase transition (i.e. solid-liquid) is analysed and taken as a measure of the destruction of insulation due to a high-current arc discharge. Analytical equations are obtained by replacing the actual physical situation with an equivalent analytical model. This is done at each of the two main steps in the thermal response of insulation: (a) the temperature rise, and (b) the equalisation of temperature in an adjacent surface layer. Both steps are related by the equation of energy balance and by the temperature conditions at the boundary. Satisfactory agreement has been obtained between analytical results and existing experimental data. It is concluded that the energy density of the discharge (in J/m²) has a decisive effect upon the degree of destruction of insulation. The specific heat capacity and the specific energy associated with phase transition are also shown to be of significance in the process under study.