|Holding down the power button reverses the process that occurs when you press it to start the computer, it breaks the connection between the green and earth from the power supply. The green must be earthed for the power supply to run. When the power button is pressed and held, this connection is reversed and the power supply stops and the computer goes of. It is this process that allows a computer to boot up normally after a power failure when power is restored without any user intervention. This is controlled by a setting in the BIOS but not all BIOS implement it. The green and black is connected as soon as power is applied. A useful facility for an unattended server. |
During a normal shutdown the Operation System goes through its shutdown procedures, closing files, stopping services, flushing buffers, etc. When the OS is finished it tells the BIOS which then opens the green cable and the computer turns off. So pressing the start button for four second, the computer ends up doing the same as a normal shutdown except that it by-passes the software part and goes straight to the hardware via the BIOS and Motherboard. The only software involved is in the BIOS.
When the power supply is off this green and black wire is open circuit. When the power button is pressed, an electronic switch in the main chips-set closes the circuit and the power supply starts up. They Grey power-good signal then comes into play. This is controlled by the power supply and is at Ov at start up. While the power good signal is at Ov minimal power is sent to the motherboard and an NMI to the CPU which effectively stops the CPU from doing anything.
When the power supply has stabilized the power good signal is raised to 5v, full power is applied, the NMI is removed and the boot up proceeds. This process usually takes about half a second but for a CPU the can execute millions if not billions of instruction in half a second, it is a very long time. If during normal operation the power supply detects any anomalies in the power, it lowers the power-good and stops the computer. If this anomaly is short lived, less the second, a spontaneous reset occurs. This what happens when a power supply is overloaded and the voltages begins to drop. As soon as the CPU is stopped, the overload is removed and the computer continues, until the next time.
However is the anomaly lasts longer, such as a gross over voltage, The power supply shuts down via the green cable, all under the control of the chip-set.
Some cheep power supplies to don't properly implement this power good signal and holds it permanently at 5v. This is why a cheap power supply can destroy the motherboard in the event of a power supply fault. There are other safety devices built into power supplies as well that prevent bad power getting to the motherboard. Cheap power supplies don't implement them all.
Good quality power supplies have an electronic fuze that will trip if anything untoward happens. It acts a bit like a RCB. The only way to reset this fuze is to remove all power which means pulling the plug, waiting a few seconds and starting all over again, but with an undamaged motherboard.
It is for this reason that I don't believe and spikes caused the arcing from pulling the plug will cause any damage, except in a cheap power supply where non of the safety devices are implemented. Probably why I have never experienced the problem as I have always used good quality power supplies.