What this breaker does on the line
The Siemens 3RV5331-4FC10 is a SIRIUS circuit breaker sized for starter combinations — meaning it's the combined motor-protective and short-circuit device in a single 3-pole package. Rated 40 A operational current at AC-3 duty on 400 V, it handles the motor-starting inrush without nuisance tripping, and the magnetic short-circuit trip fires at 520 A (instantaneous). That 520 A threshold is the key: it clears a bolted fault fast enough to protect the downstream contactor and overload relay, so you don't lose the whole starter assembly on a phase-to-phase hit. Operating frequency is 50/60 Hz, so it's at home on either side of the pond.
The S2 frame size gives you a 55 mm wide body, 149 mm deep, 140 mm tall. Clearance needs: 50 mm above and below, 30 mm at the sides, 140 mm in front, zero behind. That zero-back clearance means you can push it flush to a backplate or gland plate — useful when you're packing a tight row. Any mounting position works, so vertical or horizontal rail orientation doesn't matter. Terminal wiring: screw-type box terminals accept 2x 0.75–25 mm² solid/stranded or 1x up to 35 mm²; AWG equivalent 18–2. Torque spec is 3–4.5 N·m. Terminals on top and bottom — standard line/load routing.
Humidity tolerance 10–95% non-condensing. Shock-rated at 25 g for 11 ms per IEC 60068-2-27. Installation altitude up to 2000 m above sea level. Typical power loss is 20 W — plan for that in your thermal budget.