Shower keeps tripping when turned to hot?

Power is fine when the water is on cold or warm setting. But trips at main RCD breaker on the trip board when set to hot. Wiring inside shower appears to be OK with no leaks. Any advice much appreciated.

Tech details

Shower: Aquatronic 2 Plus
Voltage: 7.8-8.5 volts
Power isolation switch outside bathroom
and individual trip switch on board (Series E 6006 - 3, B40) But this does not trip, only the main RCD breaker