Selective RCBO for feeder protection
The Siemens 5SU1874-7BK82 is a 4-pole selective RCBO from the SENTRON 5SU1 series, combining overcurrent protection and residual current detection in a single 11-module-wide unit. Rated 125 A at 230/400 V AC with a C tripping characteristic, it delivers a 10 kA breaking capacity certified to both EN 60898 and IEC 60947-2 — the dual standard covering domestic and industrial installations. The Type B fault-current detection handles smooth DC, pulsed DC, and AC residual currents, which matters for variable-speed drives, UPS systems, and EV charging infrastructure where pure AC-sensing RCDs would blind. Because it is selective (time-delayed), the 5SU1874-7BK82 coordinates with downstream RCDs so that a fault on a sub-circuit trips the local device first — the main stays in, keeping the rest of the distribution board live. That selectivity is the difference between one machine dropping out and a whole panel going dark.
Thermal derating and mounting flexibility
The 125 A rating holds at 30 °C ambient; at 40 °C it derates to 115.62 A, at 50 °C to 106.25 A, and at 60 °C to 96.25 A. For a panel running warm — say a packed enclosure near a drive — that 60 °C figure is the practical continuous current limit. The device accepts any mounting position and the supply can enter from top or bottom, which simplifies busbar routing in a crowded distribution board. Installation depth is 70 mm (overall depth 77 mm), width 198 mm (11 width units), height 90 mm. IP20 protection applies once the distribution board is installed with conductors connected — standard for enclosed panel use. Pollution degree 2 and overvoltage category III cover fixed-installation environments.