How many connection points on the tank? Presumably feed (with a ball
valve), outlet and an overflow. Water level is probably below 2 of
those, so either the outlet leaks, the coupling to the ball valve leaks
(which is presumably full mains pressure) or the tank is split. Must be
a substantial leak to fuse the electrics. I'd expect a pvc tank to
deform considerably before it would split, but it might cause a leak
round the tank fitting if it's been badly placed.