At least three master boxes, that I know of, on our 2 EC135T1s in 18 months. Symptoms as described above, although we have a separate continuing saga concerning a possible short with the cockpit vent fan. We also connect GPU after BATT ON, although we also found that taking the engines to FLIGHT, switching the BATT MASTER 'off' and then switching 'on' vent, sandfilters and aircon also helped avoid the spike which trips the busties.
Head Turner, interesting that you say there's no mention of minimum voltage for starting. I'm not at work right now, but one of our engineers said the same thing. I showed him the relevant section in the FLM stating min voltage = 24.0, but cannot give you chapter and verse 'til I have the book in front of me.
National Air Support Forum? You are joking, aren't you? I applied through our company head of training way back last year - they never had the courtesy to reply. Shame really - we do virtually nothing but training on these cabs, so we learn a great deal about the aircraft every day. So do Eurocopter - these airframes operate in difficult conditions for helis, and the 135 has its faults like any other design. It'd be a pleasure to share the knowledge - and to learn what people who do a real job get up to - but we can't do any more than ask, can we?