The invention relates to a master breaker device for back-up protection against electric shock in earthed user units (TT-System according to IEC 60364), comprising a housing (1) with connector clampsfor the mains lines, preferably made up of two thermally-separated functional chambers in which a contactor (2), for interruption of the external lines L1, L2 and L3 and (4) for interruption of the neutral line N with a corresponding switch escapement (3), a summation current transformer (5), an electronic storage and/or amplifier circuit (6) which is preferably independent of mains-voltage and anelectromechanical relay or thyristor are housed. The electronic circuit is connected to the secondary winding of the summation current transformer (5) and, when the residual current exceeds a given value, operates a working current breaker (8) by means of the electromechanical relay or the thyristor independent of the mains voltage, which releases the switch escapement and hence brings about theopening of the contact gear. The electronic storage and/or amplifier circuit is so designed that, independent of the size of the residual current the trigger time for the master breaker device is always larger than 0.2 s and, for residual currents greater than ten times the value of the specified residual current, the total switch-off time is always less than 0.5 s.