What this RCBO is and where it fits
The Siemens 5SU1874-8BK81 is a 4-pole RCBO from the SENTRON 5SU1 series, combining overcurrent protection (MCB function) with residual current protection (RCD function) in a single DIN-rail mounted unit. It carries a rated current of 100 A at 30 °C and a rated voltage of 230/400 V AC, with a Type B fault current characteristic — meaning it detects AC, pulsating DC, and smooth DC residual currents, which is essential for circuits with variable-speed drives, UPS systems, or photovoltaic inverters that can generate DC fault currents. The D tripping characteristic (10–20× In magnetic trip) suits loads with high inrush, like motors, transformers, or welding equipment. Breaking capacity is 10 kA per EN 60898 and IEC 60947-2, so it handles fault levels up to that threshold without upstream coordination issues in most commercial and light industrial distribution boards.
Selective design — what that means for coordination
This RCBO is explicitly a selective (Type S) device per the product design. That means it has a deliberate short time delay before tripping on residual current, allowing downstream RCDs to clear a fault first. In a panel with multiple sub-circuits, you can place this as the main incomer RCBO and feed standard (non-delayed) RCDs or RCBOs downstream — the selective unit lets the branch device trip without taking the whole board down. The 100 A rating and 4-pole configuration make it a natural fit for a main switch or feeder position in a three-phase distribution board.
Current derating and thermal limits
The 100 A rating is at 30 °C ambient. At 40 °C it derates to 92.5 A, at 45 °C to 89 A, at 50 °C to 85 A, at 55 °C to 81 A, and at 60 °C to 77 A. If the panel internal temperature runs hot — common in densely packed enclosures or near heat sources — size the upstream conductor and load accordingly. The device is rated for any mounting position, which gives flexibility in tight cabinets, but the thermal derating curve is the binding constraint. IP20 protection applies once installed in a distribution board with connected conductors; the live terminals are not touch-safe outside the enclosure.
Physical fit and panel integration
The unit occupies 11 width units (198 mm) on a DIN rail, with a depth of 77 mm and an installation depth of 70 mm (the extra 7 mm accounts for the housing overhang). Height is 90 mm. Supply can enter from either the top or bottom, which simplifies busbar and cable routing in retrofit panels. It is halogen-free and silicon-free, meeting clean-room and fire-safety requirements in data centers or critical infrastructure.