So it's not the shower tripping the 40A breaker because even when there's no power to it the breaker trips.
You have a short circuit in the wiring to the switch or in the switch itself - best to get a sparky in.
8.5kW shower needs 10mm cable and 40A breaker
9.5kW shower needs 10mm cable and 45A breaker
Peccavi
December 2008