Insulation upsize vs. cable rating upsize

[IPS]

Hi everyone,

I was wondering if it was common practice to upsize MV cable insulation from 100% to 133% or 173% to operate slightly over the cable rating (for ex. 37kV continuously with a 35kV rating)? I read on this forum (http://forums.mikeholt.com/showthread.php?t=124588) that some cable manufacturer were recommending this and I am not sure what code or standard they use to back up such a practice. How do they define what voltage increase is acceptable and what is not?

Thanks!

 

[beanland]

Cable Insulation

Cable Insulation

For 15kV applications, 175 mil and 220 mil insulation are standard; for 25kV it is 260 mil; for 35kV it is 345 mil. The insulation stresses on 175 mil lead most utilities to use 220 mil. It is not uncommon in critical applications (substation getaway cables) to uprate the voltage to improve long-term reliability. Over time, all cable insulation fails. So, by using more insulation than the bare minimum, you get longer life. For an industrial customer with a short life expectancy, it may not matter. For utilities, where cables are often used for 50+ years, and where customers get really irked with the power goes off, the longer life is important. There is no standard that says when to uprate cable insulation.