The Siemens 5SM3644-0 is a 4-pole SENTRON residual current circuit breaker (RCCB) rated for 40 A at 400 V AC, 50 Hz only. The instantaneous AC-type trip responds to sinusoidal AC residual currents — no time delay, so it's suited for general-purpose protection where nuisance tripping from transient surges isn't a concern.
Rated 40 A continuous at ambient up to 45 °C. Above that, current must be derated 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.
Snap-on DIN rail (REG) mount, 72 mm wide (4 width units at 18 mm each), 77 mm deep, 90 mm tall.