Abstract: A circuit arrangement for determining the reactance of a power transmission line in the event of a short circuit from first and second measurement valves proportional to the short circuit current and voltage respectively, in which, in order to perform this determination, in a very short time, the circuit arrangement contains a phase shifter in which, from the first measurement variable an auxiliary measurement variable which lead the first measurement variable approximately by a phase angle corresponding to the phase difference between the current and voltage in the case of a metallic short circuit is formed and switches used to couple the auxiliary measurement variable and the second measurement variable to an evaluation arrangement for determining the phase angle and to an arrangement for storing the maximum value of the measurement variables which two arrangements are followed by a computer which computes the reactance from the phase angle and stored maximum values.