PURPOSE: To allow highly accurate location of fault point by determining the distance up to a fault point based on an electric quantity obtained from the voltage and current between two points on the opposite sides of the fault point and the time derivatives of voltage and current at one terminal thereby eliminating the influence of charging current produced by the distributed capacitances of a transmission line.

CONSTITUTION: An operating circuit comprises a first means (formula) for calculating an AC electrical quantity WA using the currents IA, IJ and voltages VA, VJ at two terminals A, J on the opposite sides of a fault point and the time derivatives I'J, V'J, of the current IJ and voltage VJ, and a second means 2 for calculating an AC electrical quantity UA using the line constants, i.e., the conductance G and capacitance C. In this regard, the resistance and inductance of the transmission line between the terminals A, J are R and L, respectively. A third means 3 determines an electrical quantity TA from the electrical quantities WA, UA and then a fourth means 4 determines the distance X from the terminal A to the fault point based on the electrical quantity TA. In the means (formulas) 3, 4, * represents the complex number of [] and γ is a propagation constant of the transmission line 1. This system allows accurate calculation of the distance X up to the fault point.