Prusa i3 USB cable fried

Recently bought a Prusa i3 kit (Ebay link). When I plugged it in to mains power and USB simultaneously, the USB cable and controller card fried. After this, the printer still worked (auto home etc) when only plugged in to mains power.
To try and fix it, I got a new controller card and USB cable. With the new card, auto home does not work, although I can heat the bed and extruder. I have not plugged in the USB yet, because it seems like the potential between negative on the computer USB and the controller card USB is 12V, and between positive and negative the potential is 17V.
I have checked the mains wiring, as well as the power supply, and it all seems to check out. (it is a SP-360 supply)
Does anyone have any idea what might be wrong?

... this is a common problem with cheap chinese PSU's - the GND potential is not proper defined to mains ground, but can float some Volts off!

So when connecting PSU's with different GND-potentials, the difference will cause high 'averaging' currents over the connected GND traces and cables

Edited 1 time(s). Last edit at 05/11/2016 08:38AM by VDX.

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Viktor
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Yeah, that's definitely what fried the first card and cable. I hooked up a different PSU, and things should hopefully work now, although I need one that gives around 30A.
Do you have any idea why I can't seem to move any of the motors (auto home etc)? It works with the old, fried card but now with the new one.
Thanks for the help