re: If contractor digs and breaks work another contractor has done, who is responsible?

Likely this is too small of an amount to have a real legal proceeding so this is a man-to-man dispute. I would suggest you call your electrician and tell him that he should eat the bill and provide examples why. Did you give him permission to work in that area at that time? Did you give him a drawing? Was he digging using proper methods? I would've used one of those T-shape pipe locators first, I don't know where everything is in my land.

Eventually you will have to either work out a split, pay all of it, or tell him to see you in court, but either extreme is not desireable. So work out a split.