The mains tester I carry just tells me that there is an actual earth (not waving its leg in the air) and that active and neutral are the correct way round.
Its just got three bulbs on it. There's a placard with expected "correct" display (two ambers) and the various faults. None of them say "you are across two phases of a three phase".
I wonder what that would display. Especially if, as well as being across the phases, it was miswired as well - anybody who can get it that wrong can't even be trusted to get the earth right.
Maybe it indicates this condition by emitting smoke?
Were they running through surge protectors? Be useful to know if they were and they failed to give protection.