new pc went dead

i just finished building a new pc with a powmax 500w PS. it fired up fine and ran for 1 day [left on], then went dead. mobo light was out..everything. i replaced the power supply with an identical one and it fired right up, then it too went dead over night [left it running]. same thing again no power at all ..mobo light out.

can 2 identical PS be defective? seems PS didnt affect the mobo or other components , because they all worked again after installing replacemant PS.