The Siemens 3RV2021-4FA15 is a SIRIUS motor protection circuit breaker designed to protect three-phase induction motors against overload and short-circuit. It's a Class 10 device, meaning it trips within 10 seconds at 7.2× the thermal setting — fast enough to protect standard squirrel-cage rotors during a stall or locked-rotor condition. The 10 kA breaking capacity at 400 V tells you it can safely interrupt a bolted fault up to that level without welding contacts or cascading upstream, which is the figure that governs panel SCCR coordination.
Mounting and panel fit
Snap-on mounting onto 35 mm DIN rail per DIN EN 60715, with screw fastening as a backup if vibration is a concern. The 45 mm width is a standard 2-module footprint — fits alongside a 3RT2 contactor in a motor starter combination without stealing extra rail space. Depth of 97 mm means it projects about the same as a typical contactor, so the gland plate clearance is the same for the whole starter row.
Protection features and ratings
Phase failure detection is built in — if one line drops, the breaker trips on the remaining single-phase current before the motor cooks. The auxiliary contact ratings are worth noting for the control circuit: 1 A at 24 V, 0.5 A at 120/125/230 V, and only 0.15 A at 60 V. That last figure catches people out if they're switching a 60 VDC PLC input — the contact is rated for it, but the current limit is tight.