What it is and what it does
The Siemens 3RV1021-4DA10-0FJ0-ZX95 is a SIRIUS motor protection circuit breaker — the thing that sits between your contactor and the motor, combining overload protection with short-circuit interruption in one 45 mm wide package. It's designed for motor protection, meaning it tracks current and temperature to catch an overload before the winding insulation gives up, and it clears a fault fast enough that the contactor downstream doesn't weld shut. Rated 25 A continuous current, it handles an 11 kW motor at 400 V AC-3 duty — that's your standard three-phase squirrel-cage induction motor on a conveyor, pump, or fan. The CLASS 10 trip class means it disconnects within 10 seconds at 7.2× the thermal setting, which is the right speed for protecting standard motors against locked-rotor conditions without nuisance tripping on normal starting inrush. Breaking capacity is 50 kA at 400 V AC, so it can safely interrupt a fault current up to that level without the arc flashing over to adjacent gear. That's enough for most industrial distribution panels fed by a transformer in the 500–1000 kVA range — you're not going to outrun this breaker on a typical motor branch circuit.
What the ratings mean for fit
The 25 A continuous rating is the thermal setting range — you set the overload dial to match the motor's full-load amps, and the bimetal element tracks the heating curve. At 400 V, the 11 kW motor rating is the sweet spot; if your motor is 15 kW at 400 V, you step up to the next frame. The 50 kA SCCR at 400 V means this breaker coordinates with upstream fuses or a molded-case breaker rated at least that high — if your available fault current at the panel is 65 kA, this breaker alone won't hold, and you need a current-limiting device ahead of it. Phase failure detection is built in. No ground-fault detection here, so if you need that, you add an external ground-fault relay. Mounts on a 35 mm DIN rail per EN 50022, screw and snap-on. Any mounting position works, which simplifies panel layout. The IP20 finger-safe front cover means it's protected against accidental contact when the panel door is open, but it's not sealed — keep it inside a cabinet, not in a washdown zone.
Integration and wiring
Screw-type terminals on the main current circuit. Accepts solid or stranded conductors: 2× (1…2.5 mm²) or 2× (2.5…6 mm²) per terminal, or AWG 14–10 for the main contacts. That covers most motor branch wiring up to 6 mm² — if your motor feeder is 10 mm², you'll need a terminal adapter or a larger frame breaker. Depth is 96 mm, height 97 mm, width 45 mm. The 45 mm width is three 15 mm module spaces on the DIN rail — standard for a motor-protective device in this current class. No clearance needed at the sides or behind the unit (0 mm backwards clearance), so you can pack it tight against adjacent devices. Operating temperature range is -20 to +60 °C, storage/transport -50 to +80 °C. Shock rated at 25g for 11 ms — survives the ride in a panel on a vibrating conveyor line. Switching frequency max 15 operations per hour at AC-3, which is fine for a motor start/stop cycle but not for a rapid-pulse application like a welding gun.