What this RCCB is and what it does
The Siemens 5SM3744-0 is a SENTRON 5SM3-series residual current circuit breaker (RCCB) — four-pole, type AC, rated 40 A at 40 °C and 400 V AC. It interrupts fault currents instantaneously (no short-time delay) and carries a 10 kA short-circuit current rating, meaning it can safely clear a bolted fault up to that level without upstream devices needing to intervene. The 4-pole design makes it a fit for three-phase plus neutral applications where you need earth-leakage protection on all live conductors.
Thermal derating and real-world current
The 40 A rating holds at 40 °C and 45 °C. Above that, it derates linearly: 38.4 A at 50 °C, 36.8 A at 55 °C, 35.2 A at 60 °C, 33.6 A at 65 °C, and 32 A at 70 °C. If your panel ambient runs above 40 °C, size the circuit for the derated figure — the 40 A label is only valid within the -5 °C to +45 °C window.
Mounting and panel integration
Snaps onto DIN rail (REG profile). Occupies 4 width units (72 mm wide) and requires 70 mm installation depth behind the panel surface. The 77 mm overall depth includes the front projection. Mounting position is unrestricted — any orientation works. IP20 rated, but only when installed in a distribution board with conductors connected; not intended for open-panel washdown environments.