The function of this circuit is to stop the power supply at the output even the power is recovered after Main Power Failure. This is important for the valuable equipment and safety.
S1 is the Main Switch of this circuit
S2 is Press Button Switch (No lock). When start up, S2 is pressed and the contacts of the Relay RY1 will be connected. After that RY1 will keep connected even S2 is not pressed.
If Main Power Failure happens, RY1 will be released and will not re-connect before the Main Power recovers and S2 is pressed.