I am involved in much ditch work this week in a small town that had gone underground with utilities in the early 70's. As such, i have called dig-safe, who in turn call the utilities. The ph & cable co's came and marked out thier underground runs. The power co. came by with an outdated print and claimed they had no responsibility as to any secondary.
During the dig, i found old triplex secondary in black flexible sewer pipe about 18" down. We came close to frying the digger a few times. The 70's installation was apparently placed in the same dig as other utilities and streelighting, so there has been no 'straight shot' to the x-former pad(s) that can be counted on.
The town is being dug up for something constantly, so the potential for disaster compounds itself, which i have informed the customer of.
I don't recall an 18" burial depth ever being acceptable by any utility around here.