Residual current circuit breakers (RCCB) disconnect the power supply when dangerously high fault currents occur, thereby protecting people, animals, and equipment. 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 types of fault currents and designs with rated currents up to 125 A. The large double-decker terminals can accommodate large conductor cross-sections. The tripped position of the multifunction switch knob facilitates troubleshooting. The free labeling software assists with marking. Type B switches are sensitive to all currents. They protect against pulsating and alternating fault currents, fault currents with mixed frequencies, and smooth direct fault currents at frequencies up to 150 kHz. They are also enhanced for surge current and lightning protection. The all-current-sensitive residual current circuit breakers of the NK series provide classic fire protection.