METHOD FOR DETECTING TRACE BETWEEN ACCESSIBLE TERMINAL OR EARTH FAULT AND SUPPLY OR KNOWN TERMINAL OF POWER CABLE UNDER VOLTAGE AND/OR FOR DETECTING PHASE OF CONDUCTOR GIVEN AT WANTED PLACE OF LINE SECTOR BETWEEN ACCESSIBLE TERMINAL AND SUPPLY TERMINAL, AS WELL AS LOADING DEVICE WITH A TWO-POLE OF PERIODICALLY VARIABLE IMPEDANCE SUITABLE TO THE IMPLEMENTATION OF THE METHOD