What it is and what it does
The Siemens SENTRON 5SM3446-0 is a 4-pole residual current circuit breaker (RCCB) rated 63 A at 45 °C on a 400 V AC, 50 Hz supply. It trips instantaneously on AC fault currents — no intentional delay, so it's the first device to open on a ground fault in a distribution board. The 10 kA short-circuit current rating (SCCR) means it can safely interrupt a fault up to that level without welding contacts or cascading damage upstream.
Mounting and panel fit
Snaps onto a DIN rail (REG profile) and occupies 4 width units (72 mm wide). Installation depth is 70 mm; overall depth is 77 mm. Mounting position is unrestricted — any orientation works. Supply can enter from top or bottom. IP20 with the distribution board installed and conductors connected — standard for enclosed panel use, not for wet or outdoor locations.
Thermal derating and real-world current
At 40 °C and 45 °C the device holds its full 63 A rating. Above that it derates linearly: 59.22 A at 50 °C, 55.44 A at 55 °C, 51.66 A at 60 °C, 47.88 A at 65 °C, and 44.1 A at 70 °C. If the panel ambient runs hot — say a crowded enclosure near a drive — size the load to the derated figure, not the nameplate 63 A. Maximum power loss is 9 W.