PURPOSE:To enable measuring of an electric amount related to an impedance ranging to a trouble point without being influenced by a trouble point impedance, by a method wherein two measuring constant-current generators are mounted to a ground fault trouble phase in a given transmission and distribution line section. CONSTITUTION:If a ground fault trouble occurs in a transmission and distribution line 20, ground fault phase discriminators 3a and 3b actuate to close contacts, corresponding to the ground fault trouble phase, of switches 4a and 4b, and simultaneously, to enable constant-correntsI1 andI2 to be outputted from measuring constant-current gnerators 1a and 1b. The constant-currentsI1 andI2 feed a measuring current via current transformers 5a and 6a or 5b and 6b for trouble phase as shown in an arrow mark. The constant currents are set toI1=I2 thereupon, and thereby, the measurement of a voltage E1 and a reactive power Q1 of the device 1a by means of the measuring device 2a or 2b permits the computation of a distance to a ground fault point.
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