A protective device is disclosed for combination with an electrical switching device including a main current line or for integration in such a switching device. In at least one embodiment, the protective device includes a switching element and a switching apparatus, which is provided for the actuation of the switching element and is influenced by a current flow through the main current line. In at least one embodiment, in order to influence the switching apparatus, a current transformer is integrated in the main current line and an electrical resistance of a secondary winding surrounded by the current transformer can be measured. A method for operating a protective device is also disclosed wherein the switching apparatus is triggered so as to actuate the switching element if the measured resistance reaches or exceeds a predetermined or predeterminable threshold value.