converter 30 to the current comparator 90 which may be made of a metal communication line a coaxial cable an optical cable radio or power line carrier using a power line in fig1 the load current in each of the sections of the high voltage distribution is transmitted to the current comparator 90 and then compared with the current sent by the sending end of the distribution to detect only the grounding fault at the high voltage distribution due to the residual current zero phase current based on the load unbalance that always occurs fig2 is a circuit diagram showing the details of fig1 in fig2 references 30 a 30 b and 30 c denote over current relays of the respective phases of the distribution which have a function of converting currents that pass through the over current relays into small currents converting analog currents into digital values as occasion demands references 50 a 50 b and 50 c denote distribution transformers references 51 a 51 b and 51 c are cts current transformers which derive the secondary side currents of the distribution transformers 50 a 50 b and 50 c and references 52 a 52 b and 52 c are current converters that convert the secondary side currents into small currents convert.