This standard applies to three-phase ac systems above 1 kV. It specifies the procedure for selection of the withstand voltages for equipment phase-to-earth and phase-to-insulation systems. It also identifies a list of standard insulation levels. This standard recommends that the selected insulation level should be based on the voltage stress to which the equipment is being exposed. Insulation coordination studies usually identify overvoltages in per unit of the crest nominal or maximum system voltage to ground. Although the principles of this standard also apply to transmission line insultation systems, the insulation levels may be different from those identified as standard insulation levels.