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Required insulation level of 5kV cable on a HRG syst

Question asked by janice.wilson on Jul 16, 2019
Latest reply on Jul 19, 2019 by chiefns830

We have a 4160V wye system that is grounded via a high resistance ground system and we are trying to determine the insulation level of the phase conductors.


We’ve referred to Table 310.104(E) but it doesn’t give any mention to 5kV cable above 100% insulation level.  


Note 1 (100 percent insulation level) says: “Cables in this category shall be permitted to be applied where the system is provided with relay protection such that ground faults will be cleared as rapidly as possible, but, in any case, with in 1 minute.”  


Although every effort will be made to find and clear the fault, it won’t happen in one minute.  And with an industrial system the size we have, finding and clearing the fault within one hour (133 percent insulation level) cannot be assured.


The last comment on Note 3 (173 percent insulation level) says: “Also, cables with this insulation thickness shall be permitted to be used in 100 or 133 percent insulation level applications where additional insulation strength is desirable. “ 


Since we don’t have any assurance the fault will be cleared in 1 minute or 1 hour, are we required to use 173% insulation level?


  1. If so, what is 175% insulation level on 5kV cable?


  1. Would this be a code requirement or under Best Practices?