What it is and what it does
The Siemens 5SU1354-3KK40 is a SENTRON RCBO — a combined residual-current circuit breaker and overcurrent protection device in a single 2-module-width (36 mm) package. It protects a 1P+N circuit at 40 A rated current with a B tripping characteristic, and its Type F residual-current detection handles mixed-frequency waveforms from variable-speed drives and other non-sinusoidal loads. The interrupting capacity is 10 kA per EN 60898 and 15 kA per IEC 60947-2, giving it solid fault-clearing headroom for most commercial and light-industrial distribution boards.
What the ratings mean for fit
The 40 A rating holds at 30 °C ambient; at 40 °C it derates to 37.6 A, and at 50 °C to 34.8 A — so if the panel runs hot, size up or verify the actual load stays under the derated curve. The B-curve trips between 3 and 5 times rated current, which is standard for resistive and general-purpose loads; it's not suited for high-inrush motor circuits (use a C or D curve for that). Type F residual current detection covers sinusoidal AC, pulsating DC, and mixed-frequency faults up to 1 kHz — exactly what you get from a VFD-fed load or a washing machine inverter. The 10/15 kA SCCR means it safely interrupts faults up to that level without welding contacts or cascading upstream; that's enough for a typical sub-distribution board fed by a 100–200 A transformer.
Mounting and integration
Snaps onto standard 35 mm DIN rail, occupies 2 modular units (36 mm wide). The supply can enter from either top or bottom — no forced orientation. Installation depth is 70 mm behind the panel face. Rated IP20 once installed in a distribution board with connected conductors, so it's intended for enclosed panels, not exposed washdown areas. Mounting position is any orientation. Halogen-free and silicon-free construction matters for clean-room or high-reliability environments where outgassing can't be tolerated.