What it is and what it does
The Siemens 3RV2021-4PA15-ZX95 is a SIRIUS motor protection circuit breaker designed to protect three-phase motors against overload and short-circuit. It carries a Trip Class 10 rating, meaning it will trip within 10 seconds at 7.2× the thermal setting — the standard for standard-duty motor starting where the load is a pump, fan, or conveyor that doesn't hang on the start curve. The 10 kA breaking capacity at 400 V is the number that governs its short-circuit withstand: it can safely interrupt a fault up to that level without upstream devices needing to coordinate. Phase failure detection is built in, so a lost leg on the supply side will drop the motor before single-phasing damages the winding.
Panel fit and mounting
Width is 45 mm, depth 97 mm, height 97 mm — a standard 45 mm module that fits a 35 mm DIN rail per DIN EN 60715. The mounting method is screw and snap-on, so it clips onto the rail and can be screwed down for vibration environments. Clearances: 70 mm upward and downward, 30 mm at the side. That side clearance is tight enough that adjacent breakers can sit without forced spacing, but the 70 mm vertical is the minimum for cable bend radius and heat dissipation. Screw-type terminals for the main circuit, accepting 2x (0.5...1.5 mm²) or 2x (0.75...2.5 mm²) solid or stranded.