A monitoring device for an earth line (PE) with impedance measurement is designed to be connected with a phase conductor (PH) or a neutral conductor (N) of an electrical power system (12). The device comprises a sensor (13) of a first electric quantity arranged between the phase conductor (PH) or the neutral conductor (N) and the earth conductor (PE); a generator (14) of a second electric quantity, which is different from the electric quantity of the electrical power system and is connected with the sensor; and a processing circuit (16) which is configured to calculate electrical impedance (ZPE) of the earth conductor according to the first electric quantity and the second electric quantity selected from a voltage and a current flowing through the earth conductor.