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. 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
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. 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. 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.