This may be a silly question, but is it possible that it wouldn't start up because it detected there was something wrong with the mobo, or that there weren't enough fans???
I don't remember if I had any of the case fans molex connectors plugged in or not.
Also it's an ATX power supply, and I have installed both the 20 pin and the 4 pin connector to the motherboard (ep-8rda3i), but has those oddly shaped power connectors that go to nothing currently installed.