The Siemens 5SM3644-6 is a SENTRON 5SM3 series residual-current circuit breaker (RCCB) — 4-pole, Type A, rated 40 A at 400 V AC, instantaneous trip, built for DIN-rail mounting in distribution boards. Type A detection means it catches sinusoidal AC fault currents plus pulsating DC fault currents up to 1 kHz, which covers most modern electronic loads like variable-speed drives, switched-mode power supplies, and single-phase rectifiers that a plain AC-type RCCB would miss.
Ratings and thermal behaviour
Above that, the thermal derating curve is linear: 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. The surge-current withstand is rated at 1 kA, and the I²t let-through is 70,000 A²·s, both figures for coordination with upstream MCBs or fuses.
Width is 72 mm — that's 4 width units (18 mm per unit) — so budget 4 module spaces in the distribution board.