Mains supply fault detection circuit and mains supply fault detection method and protective power supply emergency power supply system

The invention provides a mains supply fault detection circuit. The mains supply fault detection circuit comprises a capacitor, a first resistor, a first comparator, a second comparator, a signal generator, two diodes and a second resistor, wherein one end of the capacitor is connected with a mains supply, the other end of the capacitor is connected with the ground through the first resistor, a positive electrode input end of the first comparator and a negative electrode input end of the second comparator are connected between wires of the capacitor and the first resistor, a negative electrode input end of the first comparator and a positive electrode input end of the second comparator are connected with the signal generator, an output end of the first comparator is connected with the positive electrode of one diode, an output end of the second comparator is connected with the positive electrode of the other diode, and the negative electrodes of the two diodes are commonly connected with the ground through the second resistor and output fault signals. The invention further provides a mains supply fault detection method and a protective power supply emergency power supply system. After the mains supply breaks down, zero switching time is used for switching to an emergency power supply, ceaseless power supply is provided for users, and quality of power supply is improved.