PC starts by itself when forced shutdown

[Hamza93]

sometimes it happens that when i forced shut down or may be i dont know but normally shut down my pc and then i switch it off from the back plug and when i turned the plug on it starts by itself without even switching the pc power button on , my friend tested something which i didnt understand with my mother board , gave it a push to start with a current and a spark inside the MB and he said that motherboard seems ok , it happens , pc shuts down , then i unplug the main switch and when i plug back in it starts itself , system is a HP Z400 with DELTA 475 PSU , my friend is linking this is a reason for my GPU failure which i suspect may not be the case , also i want to test my power supply but i dont have a power supply tester , is there any other way , can voltage metter be sufficient to check if any problem exists or not .... thank you

 

[Paperdoc]

Some mobos have options you can set in BIOS Setup for what to do when the power from the wall outlet fails and then comes back on. The settings for possible action after a power outage or failure usually include staying off, or automatically starting up when power is restored. Yours may be set to automatically re-start after power is turned on.