Abstract: An automatic domestic power line breaker comprising a current-to-voltage converter to which is applied an input signal obtained from the secondary of a differential transformer coupled to the power line. The converter output is coupled through a low pass filter to a rectifier, the output of which is connected to a function generator, which provides an output signal comprising a voltage having a slope proportional to the peak amplitude of the rectifier output signal. The function generator output is applied to a first input of a comparator having a second input to which is applied a d.c. threshold voltage. The comparator output signal is applied through an amplifier to control the state of a set of normally open relay contacts series connected in the power line conductors. A feed-back diode shunts the first input and output of the comparator to lock the relay contacts in the open state if the breaker has controlled opening of the contacts.