What it is and what it does
The Siemens SIRIUS 3RV1021-0KA15-ZW97 is a motor protection circuit breaker rated for 1.25 A continuous current with Trip Class 10 thermal overload protection. It breaks up to 100 kA at 400 V AC — enough to handle high-fault-current panels without cascading upstream. The 0.37 kW rating at 400 V tells you the motor size it's meant to protect: small pumps, fans, or conveyors on a 400 V line. It mounts on a 35 mm DIN rail per DIN EN 50022, using either screws or the integral snap-on clip. The front face is IP20 — safe for finger contact inside an enclosed panel. Screw terminals on both main and auxiliary circuits accept solid wire from 0.5 to 1.5 mm² (two conductors) and stranded up to 6 mm², so it'll handle the typical control-wiring gauges you find in a motor starter bucket.
Key ratings and what they mean for your panel
The 100 kA breaking capacity at 400 V (and at 240, 500, and 690 V) means this breaker can interrupt a fault current up to that level without welding contacts or venting plasma — critical for meeting the SCCR rating of the panel. Trip Class 10 means it will trip within 10 seconds at 7.2× the rated current, which is standard for standard-duty motor starting (not high-inertia loads). Phase-failure detection is built in, so it protects against single-phasing on a three-phase motor. Ground-fault detection is not included — that's handled separately in the feeder or by a GFCI module. Auxiliary switch is available as an add-on (product extension auxiliary switch = Yes), transverse design, so you can wire a status signal back to the PLC or annunciator without a separate contactor block. The 7.25 W power dissipation at rated current in hot state is modest — no special ventilation needed in a standard enclosure.
Integration and mounting
Mounts in any position on a 35 mm DIN rail. Zero clearance needed at the sides or rear. Depth is 96 mm, width 45 mm, height 97 mm.