Uo is the AC rated voltage(effective value)of metal screen,metal armor and metal sheath of conductor used in cable design.The insulation structure design of cable and electric test all adopt Uo as the standard.It reflects the insulation level of cable.

U is the AC rated voltage(effective value)of phase conductor in cable design.It is not only related to in sulation level of cable,but also to system earthing style and cable category in using cable.

Um is the maximum value of the “highest system voltage”to be borne by equipment.