What it is and where it fits
The Siemens 3RV2021-4DA20-ZX95 is a SIRIUS motor protection circuit breaker — the device that sits between your contactor and the motor, combining overload protection and short-circuit interruption in one 45 mm-wide package. It snaps onto 35 mm DIN rail per DIN EN 60715 and terminates with spring-loaded terminals, so no screwdriver torque check after installation. The 97 mm depth means it clears standard 120 mm-deep enclosures with room for wiring behind the rail.
The ratings that matter for motor duty
Trip Class 10 means it will trip within 10 seconds at 7.2× rated current — fast enough to protect standard induction motors from locked-rotor heating, but not so fast it nuisance-trips on normal starting inrush. Phase failure detection is built in: if one phase drops, the breaker opens before single-phasing cooks the winding. At 240 V it interrupts 100 kA; at 400 V that drops to 25 kA — still enough for most industrial panel SCCR requirements. The gL/gG fuse coordination at 400 V is 63 A, meaning upstream fuse sizing is straightforward for selectivity.