Abstract: A fault detector sensor includes a current transformer, with two multi-turn windings each formed around a portion of the core, with one winding adjacent to each of the hot and neutral wires of the power line being protected. Both windings are connected in series in a way which reinforces arc fault noise generated by arc faults involving the line and neutral, but which causes signal reduction for noise signals from the line and neutral, or either, to ground. The windings and core are selected to self resonate at a frequency that excludes power line carrier frequencies but which includes arc fault frequencies. The core optionally has a third winding, forming a grounded neutral transformer, or ground fault detector. Instead of a third winding, one of the arc fault sensing windings can act as a dual function sensor.