My BIG BANG theory is that it's related to overconfidence and always happens when things seem to be going swimmingly well
In my case I was retapping the isolation transformer to go up from 60Vrms to 120Vrms and I thought in the process I would ground the available centre tap for enhanced safety! All sounds fine, isolated scope, isolated laptop, what could possible go wrong
Well I like my music but most times when I am testing it's switched off and due to the audio jack of the laptop being right next to the heavely used USB's unplugged from the sound system, but today I was happy, I knew exactly what tests I had to do so managed to get the audio jack in with the USB's. Everything ready, cannot remember the tune but when I plugged in the isolation transformer BANG!!!
loud buzzing noise from the sound system, took a few seconds before I unplugged, EVERYTHING...nasty burning smell OMG
.......HOW
Well it turned out the problem was twofold, grounding the isolation transformer centre tap and having the sound system plugged into the laptop, those were both new things I had not done during previous testing
The sound system provided a path from the laptop to mains earth via the cable screen. The scope was monitoring something on both the grid side and solar side of the inverter connecting one leg of the grid to solar negative, thats ok normally as the scope is isolated anyway but now that referance was 60Vrms above ground and the final connection in the short circuit ? The PIC debugger referanced to solar negative and connected to the laptop USB
The moral of the story, check and check again everytime a test harness is changed or something like a sound system added to it!! The sound system survived, the laptop had a track blown off by the headphone socket, the pickit3 is a bit of a mess with at least 3 burnt chips and some burnt tracks, most of the GTI survived but lots of control board chips were destroyed and sadly as I live in the sticks it will take over a week just to get the parts
I dont want to post any pics today, too embaressed!