Abstract: A zero phase-sequence current transformer responds to a ground fault occurring in a distribution line to produce an output. With the ground-fault current low, the output increases gradually a charged voltage across a capacitor to a predetermined magnitude through a high resistor. At that time a thyristor is turned on by a constant voltage element to open the associated circuit breaker. With the ground-fault current high, the output from the transformer breaks down a Zener diode to immediately charge the capacitor to turn the thyristor on, resulting in the opening of the circuit interrupter.