Question Why does a PC restart when there's a slight glitch in mains power for a fraction?

[ravin_29]

Hi,
I have noticed this many a times. If the Mains power is cut off, obviously PC abruptly loses power.
However if mains goes OFF and comes back ON too quickly as a if only a few cycles in its frequency are missed, then the PC restarts immediately.

If mains is lost for a very short moment, may be a fraction of a second, why does the PC restart instead of just abruptly shutting down?
What triggers a restart in that case?
Thanks.

 

[Ralston18]

Power did not actually go "off".

The mains voltage just dropped/sagged enough to cause the PSU to lose some level of functionality which in turn "reset" the motherboard. If the memory modules lose power, memory is cleared and the computer must restart itself again.

Very much like pressing an existing physical reset button.

https://en.m.wikipedia.org/wiki/Reset_button

If your power is frequently disrupted, either by blackouts (fully off) or brownouts (voltage drops/sags) you should act to protect your computer.

Consider getting a least a small UPS to protect the computer and its components.