An exemplary method for detecting a three-phase islanding state in a three-phase electricity network includes supplying power to a three-phase electricity network via a power supply assembly, controlling an output frequency of the power supply assembly with a frequency reference signal adapted to deviate the output frequency of the power supply assembly from a grid frequency representing a frequency of a common electricity network whose portion of the three-phase electricity network is in normal operating conditions. The method also includes detecting a three-phase islanding state in the electricity network if the output frequency of the power supply assembly is outside an allowable value range. During a normal operating state of the electricity network, the frequency reference signal depends on an active output current of the power supply assembly.