Abstract: Fault-current protector circuitry has a first totalling current transformer (7) with a primary winding formed from phase (1) and neutral (2) conductor lines and a secondary winding (13) connected to a first analyzer circuit (16). A second totalling current transformer (10) has a primary winding formed by a protective conductor line (3) and a secondary winding (17) connected to a second analyzer circuit (20). A trigger device (24) connected to the first and second analyzer circuits actuates switches (4,5,6) which are connected into each conductor (1,2,3). The fault-current protector circuitry is especially for isolation monitoring of electrical circuits and which has the advantage of reliably isolating the monitored circuit from the mains if fault currents occur in the phase or neutral conductor lines and if a current appears in the protective conductor line.