The Siemens 5SM3644-6KK12 is a SENTRON 5SM3-series residual current circuit breaker (RCCB) — a 4-pole, Type A device rated at 40 A at 400 V AC, 50 Hz, with a 10 kA short-circuit current rating. Type A detection means it trips on sinusoidal AC residual currents plus pulsed DC (rectified half-wave) up to 1 kHz, which covers most modern electronic loads like VFDs, switched-mode supplies, and single-phase rectifiers. The 10 kA SCCR at 400 V tells you it can safely interrupt a fault up to that level without upstream fuses needing to clear first — important for selectivity in a distribution board.
Ratings and what they mean for fit
The 40 A rating holds at 40 °C and 45 °C ambient; at 50 °C it derates to 38.4 A, dropping to 32 A at 70 °C (–). If your panel runs hot — say near a transformer or in a roof space — check the thermal curve. The rated surge current resistance is 1 kA, and the I²t value is 70 000 A²·s, which helps coordinate with upstream MCBs or fuses for selective tripping. Conductor termination accepts solid or stranded wire from 1.5 to 25 mm². The 77 mm depth and 72 mm width — 4 width units on a DIN rail — mean it occupies four 18 mm module spaces. Mounting position is any, and the installation depth behind the panel face is 70 mm. IP20 with conductors connected; finger and back-of-hand safe.