A network node for a substation or a local network station has a control transformer with a primary side and a secondary side, an input line connected to the primary side, and an output line connected to the secondary side. A protective switch is provided in the input line or in the output line, and a sensor detects an electrical parameter in the input line or in the output line and generate a measurement signal. A controller coupled to the control transformer, to the protective switch, and to the sensor, serves to operate the control transformer in dependence on the measurement signal such that the control transformer has a predetermined transmission ratio and open the protective switch in dependence on the measurement signal as soon as the control transformer has the predetermined transmission ratio.