This invention relates to the protection against over-voltage situations in a telecommunications network. Especially, the invention relates to fault situations that are generated by a power distribution network. The idea of the invention is to protect the telecommunication equipment against any overvoltage caused by an external source. The invention uses a detector core around cables through which a common mode fault current goes. The subscriber line cables or the power & ground cables form a primary coil in a transformer. The transformer's secondary coil forms an output current, preferably for converting equipment that converts the fault current information into a suitable form for operational devices that perform a desired protection action.