FI/LS combinations (RCBOs) provide personal and property protection as well as protection against electrically ignited fires in accordance with DIN VDE 0100-410 and DIN VDE 0100-530. The FI/LS combinations of the DS250N-UC series ensure protection against sinusoidal alternating currents, pulsating currents to earth, and smooth direct fault currents with various frequencies. They offer fault protection (protection against indirect contact), additional protection (with IΔn ≤ 30 mA), and protection against electrically ignited fires (with IΔn ≤ 300 mA). They meet the product standards for Type A DIN EN 61009-1 (VDE 0664-20) Annex G and Type B DIN EN 60947-2 (VDE 0660-101). The DS250N-UC series are special FI/LS combinations designed for use in the field of direct current local transport railways, particularly within the overhead line area. They have been developed in accordance with the above product standards and precisely meet the recommendations of the VDV document 509 (10/08): "Use of residual current (FI) protection circuits in electrical energy systems of direct current local transport railways." The DS250N-UC consists of an all-current-sensitive FI block (Type B) and an LS part with one or three phase conductors in AC design, as well as two oppositely connected neutral conductors in UC design – for polarity-independent disconnection of direct currents (N-UC: N pole for Universal Current). This allows for the safe disconnection of high compensating direct currents between the neutral conductor of the general network and the DC earth (the return conductor of the direct current railway network).