The selection of the rated insulation level consists of the selection of the most economical set of standard rated withstand voltages (Uw) of the insulation sufficient to prove that all the required withstand voltages are met.
The highest voltage for equipment is then chosen as the next standard value of Um equal to or higher than the highest voltage of the system where the equipment will be installed.
For equipment to be installed under normal environmental conditions relevant to insulation, Um shall be at least equal to Us.
For equipment to be installed outside of the normal environmental conditions relevant to insulation, Um may be selected higher than the next standard value of Um equal to or higher than Us according to the special needs involved.