The Siemens SENTRON 5SM3344-0 is a 4-pole residual current circuit breaker (RCCB) rated for 40 A at 400 V AC, 50 Hz, with a 30 mA trip threshold for AC fault currents. The 30 mA sensitivity is the common threshold for personal protection against indirect contact.
Snaps onto DIN rail (REG profile) and occupies 4 width units at 72 mm wide. Depth is 77 mm, installation depth 70 mm — check your gland plate clearance if the enclosure is shallow. IP20 rating applies once the distribution board is installed with conductors connected. That means finger-safe from the front but not protected against dripping water — keep it inside a cabinet, not in a wet area.
Thermal derating and breaking capacity
Full 40 A rating holds up to 45 °C ambient. 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. That's adequate for the downstream side of a distribution board where the prospective fault current is limited by the upstream MCB or fuse. The permissible let-through energy (I²t) is 70 000 A²·s.