We had a 3-phase system here, though nobody knew it was 3-phase. When they turned it off, the whole room powered off. Turns out it, the room was very unbalanced, and one leg had about 125% on it, and one leg only about 25%, and the other was about 75%, so this single 3-phase device was back feeding and balancing everything out. They had to re-wire a lot of stuff to re-balance just to turn off the one 3-phase device.
I don't know if it was Y or delta connected, most likely Y.