Applies to equipment for three phase alternating current systems with system voltage above 1kV nominal, includes only phase to ground insulation. Defines specifications for insulation of equipment in a given installation. For tests prescribed the values of the basic lightning impulse insulation levels and the basic switching impulse levels shall be chosen from preferred values of this standard. Each standards developer must specify the BILs, BSLs and test procedures for its equipment using this standard, where practical.