The Siemens SENTRON 5SM1744-6 is a 4-pole residual current operated circuit breaker (RCCB), type A, rated 40 A at 400 V AC with a 500 mA tripping residual current. It's an instantaneous design — no intentional delay — so it clears earth faults as fast as the fault current drives it past the 500 mA threshold. That makes it a fit for general-purpose protection where you need fast disconnection on a 4-wire (3-phase + neutral) circuit.
The 40 A rating holds across the 40 °C to 45 °C band without derating — still 40 A at both temperatures. Above that it steps down: 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. If your enclosure runs hot, that's the curve you size against, not the nameplate 40 A. That's the standard choice for mixed-load panels where you can't guarantee a clean sine-wave fault.
DIN-rail fit and panel integration
Snaps onto a DIN rail (REG profile), occupies 4 width units, and the installation depth is 55 mm. Conductor range is 1.5 mm² minimum to 25 mm² maximum. IP20 rating applies with the distribution board closed and conductors connected — standard for enclosed panel gear. Finger and back-of-hand safe touch protection is built in, so an opened enclosure still offers basic shock protection as long as the conductors are terminated.