There's several things wrong with paperclip trick. First there's no load on the PSU. It can pass this and still fail when connected to the system. All you really know is that there's enough power to work a fan. You do not know if any of the main power lines are in tolerance or not. And it does not check the "PowerOK" signal on the gray wire. Without this signal, the PC will not boot.
On the other hand, if the fan does not spin up, the PSU is dead.