Our company has been using dc hypot test for years. As of late they are starting have us us lower voltages for proof tests. A 13 kv test is now at 26kv for five minutes. The theory is, that resudual dc voltage remains in the cable, even after it has been grounded, go figure. Anyway when the cable is put back into service, the ac volatage creates a spike, causing treeing, and damage to the xlp.