On the basis of classification of impulse wave shape, the paper shows that impulse spark-over characteristics of electrodes with nonuniform fields cannot be represented by single volt-time curves but must be represented by volt-time areas of considerable extent. This classification explains the large difference obtained by various laboratories when investigating sparkover between one and five micro- seconds. The effect of impulse generator discharge circuits in producing these differences is explained. The necessity of insulation co-ordination on the basis of volt-time areas is pointed out.