What this RCBO is and what it does
The Siemens SENTRON 5SU1654-6KK40 is a combined residual-current circuit breaker with overcurrent protection (RCBO) in a 1P+N, 2-module-wide (36 mm) package. It carries a 40 A rated current at 30 °C with a B tripping characteristic, meaning it protects against short-circuit and overload while also detecting earth leakage currents — Type A sensitivity covers pulsating DC fault currents from electronics like VFDs or switched-mode supplies. The 10 kA breaking capacity per EN 60898 is the standard for domestic and light commercial distribution boards; the 5SU1 design is instantaneous (no intentional time delay), so it trips within milliseconds on a fault.
Derating and thermal reality — what the numbers mean for panel loading
The 40 A rating holds only at 30 °C ambient. At 40 °C it derates to 37.6 A; at 50 °C to 34.8 A; at 55 °C to 32.4 A. Above 55 °C the operating limit is 55 °C ambient, so the 32.4 A at 55 °C is the practical ceiling for a warm enclosure. The 15.6 W maximum power dissipation matters when packing multiple RCBOs in a distribution board — that heat has to leave the panel, or you derate further. The 70 mm installation depth plus 77 mm overall depth means it fits standard 80 mm deep enclosures with a few mm to spare for wiring.
Wiring and mounting — what fits
Terminals accept solid or stranded conductors from 0.75 mm² up to 35 mm² — that covers everything from 1.5 mm² lighting circuits up to 10 mm² feeders for a 40 A load. The supply can enter from top or bottom, which simplifies panel layout when busbars run either way. Mounting position is any orientation, so vertical or horizontal DIN-rail installation is fine. It snaps onto standard 35 mm DIN rail, occupying 2 width units (36 mm). The IP20 rating applies only when the distribution board is installed with conductors connected — it is not a standalone outdoor device.