What it is and what it does
The Siemens 3RV6321-4NC10 is a SIRIUS circuit breaker specifically designed for starter combinations — meaning it combines motor protection (overload and short-circuit) in a single device sized to coordinate with a contactor downstream. It's a 3-pole, size S0 frame breaker rated 28 A in AC-3 duty at 400 V, so it switches motor loads up to that current under the standard motor-switching category (AC-3 covers starting and plugging of squirrel-cage motors). The magnetic short-circuit trip responds at 400 A, which is the instantaneous threshold; below that, the thermal bimetal handles overloads. Rated insulation voltage is 690 V at pollution degree 3, and surge voltage resistance is 6 kV — both tell you this breaker is designed for 400 V line-to-line systems with adequate clearance for industrial environments.
Mounting and integration
Mounts via screw and snap-on onto a 35 mm standard mounting rail per DIN EN 60715. The 45 mm width (one S0 module) keeps panel density reasonable. Fastening torque for the main screw terminals is 2 to 2.5 N·m. Wire range: accepts 2x (1 to 2.5 mm²) solid or stranded, or 2x (2.5 to 10 mm²) — also 2x (16 to 12) and 2x (14 to 8) AWG. Finely stranded with ferrule: 2x (1 to 2.5 mm²), 2x (2.5 to 6 mm²), or 1x 10 mm². Connectors are top and bottom entry. Mounting position is any, but clearance distances must be observed: 50 mm upwards and downwards, 30 mm at the sides, 0 mm forwards and backwards — this matters for heat dissipation and arc flash clearance in a packed enclosure.
Ratings and operating conditions
Rated operational current is 28 A at AC-3, 400 V. Operating frequency at AC-3: maximum 15 operations per hour — that's a thermal limit, not a switching-speed spec; don't cycle a motor faster than this without checking the contactor and load duty. Power loss total typical is 11 W at rated current; factor that into enclosure thermal calculations. Ambient temperature during operation: -20 to +60 °C. Storage/transport: -50 to +80 °C. Relative humidity: 10 to 95%. Installation altitude maximum: 2000 m. Shock resistance: 25 g / 11 ms per IEC 60068-2-27. The 690 V rated value and 6 kV surge resistance confirm it's suitable for 400 V systems with good margin, but verify coordination with upstream protection.
Compliance and documentation
Substance prohibitance date (RoHS) is 05/01/2012 — the part complies with RoHS as of that date. The SIRIUS range typically carries UL, CSA, and CE certifications; the insulation voltage rating of 690 V at pollution degree 3 is a key parameter for IEC 60947-2 compliance. For UL 508A panel builders, verify the SCCR coordination with the downstream contactor — the 400 A magnetic trip sets the let-through energy for the combination.