Triton Opal, fixed PRD but now its blown the isolation switch?

I spent 10 minutes replacing the PRD on this unit, thinking that this was a nice easy fix. When I have come to pull the cord on the isolation switch for the first time a lovely bright blue flash appeared from the switch.

I have checked that I have not shorted any internal wiring, all appears sound, and the unit has been working fine until I twisted the outlet pipe which caused the PRD to blow.

Erm, any ideas appreciated? Could the heater unit have become damaged with the pressure build up before the valve blew?