Cool video...thanks. Seen that happen when station battery was dead or disabled. Usually also a huge fireball. Not saying that's what happened, but an arc that large for that long tells me something didn't trip that should have. No fire means the fault was probably ahead of the transformers, so there may not have been any high side relay protection at the sub. Most distribution substation relays or primary fuses are for transformer protection. Since work was being done during a shutdown, I'm voting for "Oops...forgot to hook that wire back up" or "I thought YOU took the grounds off!"
Let us know what happened....might save the rest of us relay techs a black eye. Pics also would be great.