Method of detecting line to ground fault in the distribution line of multiple grounding system

A method of detecting a voltage sag of a multiple grounding system is disclosed. The method of detecting a voltage sag of a multiple grounding system includes: a step of obtaining a load current in each phase of a neutral line current and a power line by a terminal device provided on a load side; a step of calculating the vector sum of load currents in each phase; a step of calculating a vector sum combining two phases with the load current of each phase if a phase difference between the neutral line current and the vector sum is greater than or equal to a predetermined first threshold value; and a step of determining that a voltage sag is detected when there is a combination in which a phase difference between the vector sum of the neutral line current and the two phases is within a predetermined second threshold value. So, it is possible to detect a high resistance line to ground fault which cannot be detected by a conventional method.