FIELD: electrical engineering, in particular, electric power supply systems for cities. SUBSTANCE: method involves insertion of variable inductance coil between neutral point of three-phase mains supply and ground in order to provide circuit of inductance of three-phase mains supply with respect to earth and inductance; tuning inductance coil in resonance with capacitor to achieve parametric resonance; application of test electric signals to three-phase mains supply with insulated neutral wire, measuring of current and voltage of these signals by ammeter and voltmeter and judging insulation of three-phase main supply line with respect to ground by these readings. EFFECT: increased duration of operations of three-phase supply line one of which phases is connected to earth. 2 dwg
