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

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

Um is the maximum value of the highest system voltageto be borne by equipment.

2. Category of Power Cable

According to the sustained time of earthing breakdown, power cable is divided into two categories:

Category 1: The sustained time of earthing breakdown does not exceed 1 minute, that is, it requires a system when any phase conductor contacts the earth or earthing conductor, can separate the phase conductor from the system in 1 minute.

Category 11: The sustained time of each earthing breakdown usually should not exceed 1 hour, the longest not exceeding 8 hours and the accumulated time in a year should not exceed 125 hours.