I am trying to use an 12V OEM type power supply to supply 12V to stepper motors through an L293 h bridge in an Arduino powered circuit but my current setup starts burning all the negative wires and almost killed my pc when I plugged the Arduino in the USB port. I am not sure what could be causing this. I have connected my Live, Neutral and Ground wires correctly and I am then using the "adj. +V" pins as my motor voltage supply and I have connected the "-V" to the common rail/ground of my circuit. The power supply does seem to work fine and if I put an LED with a resistor between "adj. +V" and "-V" nothing strange happens and the LED goes on just as normal. I should also mentioned that the circuit where the ground wires fried has been used hundreds of times with a PC power supply and gave no trouble at all.
I am guessing that the OEM setup is probably the problem here. I have found this page http://reprap.org/wiki/RepRapPro_Mendel_power_supply indicating that I should connect the ground on mains to the -V as to earth my circuit. This could possibly explain my trouble as it means that once I plug in the Arduino in it seeks earth through my PC, or is that just absurd? Even if that is the case, following the above tutorial I have come across an issue, in the testing segment they state that there should be no resistance between ground and -V and this is indeed the case once I have done as stated in the beginning of the tutorial and connected ground and -V. The troubling thing is though that there is also almost no resistance between +V and the ground pin, even without me making the aforementioned connection. I know they don't state this as something to test or watch out for but my head tells me this means that +V and -V are then connected and making a short and that that is something very dangerous, I am therefore to afraid to test the power supply after connecting ground and -V.
Do any of you have experience with these power supplies and know if this is normal or have had the issue before and might know of a possible solution?
Here is photo of the terminals without a connection between GND and -V:
EDIT: I have now removed all the wires and there is still a connection between +V and earth.
EDIT: The circuit basically entails that firstly all the ground pins meaning the ground on the Arduino and the -V on the power supply and the ground pins of the h bridge are all connected on a common rail. The VCC on the h bridges are then connected to the 5V on the Arduino and the +V on the PSU is only connected to the motor voltage pin on the h bridge. But as I previously said, this circuit works fine with a pc power supply. Also, for some reason only the black wires connecting the ground pins were burned, the 12V and 5V wires are all fine.