What it is and where it lands
The Siemens 5SU1354-0LB40 is a SENTRON RCBO — a combined residual-current circuit breaker with overcurrent protection — rated 40 A at 30 °C with a B tripping characteristic and a 10 kA breaking capacity per EN 60898. It protects a 1P+N (230 V AC) final subcircuit against overload, short-circuit, and earth leakage (AC type fault current). Two width units (36 mm) on a DIN rail; install in any mounting position, feed from top or bottom.
Current derating — the number that matters for the panel
The 40 A rating holds at 30 °C ambient. At 40 °C it's 37.6 A; at 50 °C it's 34.8 A; at 55 °C it's 32.4 A. If the panel ambient runs warm — say 45 °C inside the enclosure — the usable current drops to 36.4 A. That derating curve is the spec to check against the actual load, not the nameplate 40 A.
Panel fit and wiring
The RCBO is 36 mm wide (2 width units), 90 mm tall, 77 mm deep with an installation depth of 70 mm. Terminals accept solid or stranded conductors from 0.75 mm² up to 35 mm². Halogen-free and silicon-free construction. IP20 with the distribution board installed and conductors connected. Overvoltage category III, energy limiting class 3.