In addition to the voltages and currents that a computer needs to operate, power supplies also provide a signal called the Power Good signal, sometimes written as Power_OK or Power_Good or it can be distinguished by its gray color. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present at startup, the CPU is held in reset state. If a Power-Good signal goes down during operation, the CPU will shut down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself.
The atx specification defines the Power-Good signal as a +5 volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherbord, where it is received by the processor timer chip that controls the reset line to the processor.