Circuitry for detection of ground fault currents in alternating current circuits utilizable with grounded or ungrounded systems and for protectively interrupting the circuits, comprises means for deriving monitoring signals from representative of the fault currents and applying the signals to first and second relays. The first relay is responsive at a low level of fault current to pass control current from a source of control power to warning such as a lamp and horn. The second relay is responsive at a higher level of fault current to pass control current to an interrupter device for the circuits being monitored. Adjustment means in the circuitry sets the two threshold levels of fault current at which the relays respectively respond. Seal-in circuit means keep the first relay actuated to activate the alarms continuously after their operation is initiated. A time delay network prevents relay response to instantaneous transient currents exceeding the lower threshold. The circuitry is unitized for use at any standard voltage, current and frequency and to isolate all current carrying components. The circuits being monitored can serve as a source of control power or an independent source can be provided.