What is Causing PC to Kill Power Supplies?

I’ve got a Dell Optiplex 745 Tower. Power supply failed so installed a new one and got the same results tried two others and finally realized that the power supply was good, tested good with testers but after placing them in the PC and hitting power button they are no longer good. Power supply tester only shows one green light +5vsb all the rest are dark. What might be causing this?