I had the same problem when assembling a PC. I did the same test that you did and I assemble the pc on a wooden table, the problem was when assembling everything in the case. I try with another power supply, I thought I had burned the mother or the memorys (never use anti-static wrist and never burn anything) and even suspect my video card which I had taken from the previous pc which had some problems, but none of that was. so I decided to check the fans ... who would say, the problem was there. The yellow wire (sensor) front fan was broken and made ​​contact with the metal walls of the case, this was the cause of the short. After many repairs, this was the first time I found something like that. Hope this helps.