An apparatus for quickly determining a fault in an electric power system includes a current transformer (800), a current determination unit (810) and a fault determination unit (850). The current transformer detects current supplied to the electric power system and outputs a current detection voltage. The current determination unit respectively compares the current detection voltage, the first-order differential voltage of the current detection voltage and the second-order differential voltage of the current detection voltage with predetermined first, second and third reference voltages. The fault determination unit determines whether a fault occurs based on the compared result of the current determination unit and generates a trip signal when it is determined that the fault has occurred.