What it is and what it does
The Siemens 5SU1656-6KK40 is a SENTRON RCBO — a combined residual-current circuit breaker with overcurrent protection — rated 40 A at 30 °C, with a B-curve tripping characteristic and Type A residual-current sensitivity. It occupies 2 modular-width units (36 mm) on a DIN rail, with a depth of 77 mm and an installation depth of 70 mm. The 6 kA breaking capacity per EN 60898 (10 kA per IEC 60947-2) covers most residential and light-commercial fault levels.
Ratings that matter for panel fit
The 40 A rating derates with ambient temperature: 37.6 A at 40 °C, 36.4 A at 45 °C, 34.8 A at 50 °C, 32.4 A at 55–60 °C, 30.8 A at 65 °C, and 29.2 A at 70 °C. If your panel runs warm, size the upstream protection for the derated value, not the 30 °C headline. The 2-MW width (36 mm) and 70 mm installation depth mean it fits standard 18 mm-pitch DIN-rail enclosures without crowding adjacent breakers. Mounting position is any orientation; supply can enter from top or bottom. Halogen-free and silicon-free construction matters for environments where outgassing during a fault could damage sensitive electronics or violate clean-room specs. IP20 applies only when the distribution board is installed with connected conductors — the RCBO itself is not sealed against dust ingress in open-panel service.