Method for monitoring a multiphase electrical power supply system, wherein - a first protection device is connected to a first line of the power supply system for detection of measured values characterizing the operating state of the first line, wherein the first protection device is connected via a communications link to an adjacent second protection device, and wherein - the second protection device is connected to a second line of the power supply system, said second line running parallel to the first line, for detection of measured values characterizing the operating state of the second line, - the two protection devices (13a, 13b) transmit the measured values detected thereby with respect to their respective line (11a, 11b) and/or signals derived from these measured values to the respective other protection device (13b, 13a), - each protection device (13a, 13b) is set up to carry out a protection function, in particular a distance protection function, for its respective line (11a, 11b) whilst implementing a primary protection algorithm, - the first protection device utilizes the measured values detected at the first line and the measured values and/or signals received from the second protection device to implement its primary protection algorithm, and - the second protection device utilizes the measured values detected at the second line and the measured values and/or signals received from the first protection device to implement its primary protection algorithm, characterized in that - the protection function includes a distance protection function, wherein, in order to carry out the distance protection function by means of the primary protection algorithm, the protection devices (13a, 13b) calculate complex impedance values from the measured values detected at their respective line (11a, 11b) and generate a fault signal if the impedance values lie within a trigger field, - in the event of a single-pole fault over its respective line (11a, 11b), each protection device (13a, 13b) transfers a fault status signal characterizing the fault to the respective other protection device (13a, 13b) via the communications link, and - the protection device (13a, 13b) which receives the fault status signal reduces the trigger field for assessment of the impedance values. - a first protection device is connected to a first line of the power supply system for detection of measured values characterizing the operating state of the first line, wherein the first protection device is connected via a communications link to an adjacent second protection device, and wherein - the second protection device is connected to a second line of the power supply system, said second line running parallel to the first line, for detection of measured values characterizing the operating state of the second line, - the two protection devices (13a, 13b) transmit the measured values detected thereby with respect to their respective line (11a, 11b) and/or signals derived from these measured values to the respective other protection device (13b, 13a), - each protection device (13a, 13b) is set up to carry out a protection function, in particular a distance protection function, for its respective line (11a, 11b) whilst implementing a primary protection algorithm, - the first protection device utilizes the measured values detected at the first line and the measured values and/or signals received from the second protection device to implement its primary protection algorithm, and - the second protection device utilizes the measured values detected at the second line and the measured values and/or signals received from the first protection device to implement its primary protection algorithm, characterized in that - the protection function includes a distance protection function, wherein, in order to carry out the distance protection function by means of the primary protection algorithm, the protection devices (13a, 13b) calculate complex impedance values from the measured values detected at their respective line (11a, 11b) and generate a fault signal if the impedance values lie within a trigger field, - in the event of a single-pole fault over its respective line (11a, 11b), each protection device (13a, 13b) transfers a fault status signal characterizing the fault to the respective other protection device (13a, 13b) via the communications link, and - the protection device (13a, 13b) which receives the fault status signal reduces the trigger field for assessment of the impedance values.