He did not state WHAT level he was high potting to, a fairly low hy-pot will not harm an older cable.
He will want to remove any PT fuses prior to high pot. Any equipment connected to that buss is going to be rated for system voltage or is already self destructed. I would suggest hy-pot to system voltage only.
I'd still do a thorough visual of the buss and terms to check for tracking. The problem may be painfully obvious.
