What this RCBO is and what it does
The Siemens 5SU9356-7KK40 is an RCBO from the SENTRON family with a 40 A rated current (at 30 °C) and a 30 mA residual-current trip in a 2-module-wide (36 mm) DIN-rail housing. Breaking capacity is 6 kA per EN 60898, adequate for domestic and commercial sub-distribution where the prospective short-circuit current stays under that threshold. The 1P+N design switches the line conductor and disconnects the neutral, but does not protect the neutral against overcurrent — only the line pole is protected. This is standard for single-phase final circuits where the neutral is not overloaded.
Temperature derating — the real-world current rating
The 40 A rating holds at 30 °C ambient. At 40 °C it drops to 37.6 A, at 45 °C to 36.4 A, at 50 °C to 34.8 A, and at 55 °C to 32.4 A. If this RCBO sits in a crowded enclosure with other heat sources, the effective continuous current is lower than the nameplate. Panel builders should apply the derating curve from the datasheet when calculating fill factor and thermal load — a 40 A circuit in a 50 °C panel is actually a 34.8 A circuit. Mounting position is unrestricted, and the operating range spans -40 °C to 75 °C, so it handles outdoor enclosures in cold climates or warm indoor switchgear without issue. The IP20 rating applies only when the distribution board is installed with conductors connected — the front face is touch-safe, but the terminals are not sealed against dust ingress.