I sometimes do a live panel change in domestic premises here to avoid a call-out from the local utility. That's 240V to ground, rather than 277V though. I occasionally work live in a 3-phase 240/415V panel to add/change circuits.
I certainly wouldn't try a big 3-phase panel change live though, or a 3-ph. outlet replacement - Too many "hot" wires with 415V between them to keep an eye on.
