Least that's what I think had happened. Think the poco feeds us 277VAC in a Y configuration, and that directly powers the ceiling florescent lights. About a third of them went dead. And about 2/3 the 120V outlets looked to be delivering very low voltage, maybe about 60V. A few incandescent desk lamps glowed dimly, and many computer power supplies barely kept alive. I suppose that in an electrical closet there's a transformer to convert the 480V delta to 120V/208v Y, and a dead phase (open circuit?) would cause 2 of the 3 phase 120V to go to around 60V. If B is dead, primary of A to B and B to C would end up in series across A and C, both those primaries seeing half the usual voltage, and then their secondaries will be at half their usual voltage. But that mid point, B, would end up at around 120V above ground? But not enough to drive the 277V florescent lights on the B phase. The building elevators shut down, but I think they have a relay that cuts all its power when a phase is lost. But apparently not the 480V delta to 120V Y transformer.
This seem reasonable?
