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. It carries a 10 kA short-circuit current rating (SCCR) per IEC/EN 61008, meaning it can safely interrupt fault currents up to that level without upstream coordination failure. 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.
Thermal derating and ambient limits
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. The minimum operating ambient is -5 °C; storage range extends from -40 °C to 75 °C. If the panel ambient exceeds 45 °C, size the upstream protective device for the derated value, not the 40 A nameplate.
Mounting and panel integration
Snap-on DIN rail (REG) mount, 72 mm wide (4 width units at 18 mm each), 77 mm deep, 90 mm tall. Installation depth is 70 mm behind the panel surface. Mounting position is unrestricted — any orientation works. IP20 protection applies only when installed in a distribution board with conductors connected. The supply can enter from top or bottom. Overvoltage category III, pollution degree 2 typical for fixed-installation switchgear.