There are three levels of conductor insulation for medium-voltage cables: 100, 133,and 173% levels.
The solidly grounded system permits the use of 100% insulation level. When the fault on the other system will raise the system voltage above normal during the time of thefault,133% insulation level should be speciÞed if the fault is cleared within one hour.
When the fault will remain on the system for an indefinite time, 173% voltage level insulation should be used
