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 (13a, 13b), - 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 devices (13a, 13b) carry out the protection function for their respective line (11a, 11b), wherein, by means of the primary protection algorithm, the measured values detected at the inherent line (11a, 11b) are inspected to establish whether they indicate a fault over the inherent line (11a, 11b), and a fault signal is generated if the measured values indicate a fault over the inherent line (11a, 11b), - the protection devices (13a, 13b) exchange circuit breaker signals with one another, said signals indicating a state of a circuit breaker (12a, 12b) connected to the respective protection device (13a, 13b) and/or a switch command for the circuit breaker (12a, 12b) and, - if a circuit breaker signal is present, which indicates an opening or closing of the circuit breaker (12a, 12b) at the respective other line (11a, 11b), block the protection devices (13a, 13b) from outputting a fault signal. - 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 (13a, 13b), - 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, - the protection devices (13a, 13b) carry out the protection function for their respective line (11a, 11b), wherein, by means of the primary protection algorithm, the measured values detected at the inherent line (11a, 11b) are inspected to establish whether they indicate a fault over the inherent line (11a, 11b), and a fault signal is generated if the measured values indicate a fault over the inherent line (11a, 11b), - the protection devices (13a, 13b) exchange circuit breaker signals with one another, said signals indicating a state of a circuit breaker (12a, 12b) connected to the respective protection device (13a, 13b) and/or a switch command for the circuit breaker (12a, 12b) and, - if a circuit breaker signal is present, which indicates an opening or closing of the circuit breaker (12a, 12b) at the respective other line (11a, 11b), block the protection devices (13a, 13b) from outputting a fault signal.