What it is and what it does
The Siemens 3RV2021-1HA25 is a SIRIUS motor-protection circuit breaker designed for motor starting and overload protection in industrial control panels. It combines a CLASS 10 trip characteristic with a 100 kA short-circuit breaking capacity at 400 V, so it clears faults fast enough to protect downstream contactors and motor starters without cascading upstream. Phase failure detection is built in, which means a single-phased motor trips before winding damage sets in.
Key ratings and what they mean for fit
Rated at 8 A at both 480 V and 600 V, this breaker covers the common motor FLA range for fractional-to-small integral-horsepower motors on 480 V three-phase lines. The 100 kA SCCR at 400 V and 240 V means it can be installed on high-fault-capacity distribution without needing a current-limiting upstream device — a real time-saver in panel coordination studies. At 690 V the breaking capacity drops to 4 kA, so verify the available fault current if the breaker lands on a 690 V system.
Mounting and wiring
Snap-on mounting onto 35 mm DIN rail per DIN EN 60715, with a screw-fastening option for vibration-prone panels. The 45 mm width takes a single 9 mm module slot on the rail. Spring-loaded terminals on the main circuit accept 2x (0.5 to 2.5 mm²) solid or stranded — no screw torque to verify, just strip and push. Mounting position is any, so it fits horizontal or vertical bus arrangements.