Residual current circuit breakers (RCCB) disconnect the power supply when dangerously high fault currents occur, thus protecting people, animals, and installations. DFS 4 are two- or four-pole residual current circuit breakers for single-phase or three-phase networks. In standard design, these compact devices are four division units wide. They are available with many different fault current types and designs with rated currents up to 125 A. The large double-decker terminals can accommodate large conductor cross-sections. The triggered position of the multifunction switch knob facilitates troubleshooting. The free labeling software assists with marking. Type B switches are sensitive to all currents at frequencies from 0 Hz (smooth direct fault currents) to 150 kHz. They are also resistant to high surge currents and lightning. The SK series residual current circuit breakers provide fault current protection and high system availability. They feature lower sensitivity to higher frequencies. The SK characteristic is optimized for systems where fire protection is not required. Devices in standard design are intended for monitoring circuits with a rated voltage of 230, 400 V and a rated frequency of 50 Hz.