The Siemens SIRIUS 3RV1421-1HA10 is a 3-pole circuit breaker designed specifically for transformer protection, rated at 8 A continuous current with a Trip Class 10 characteristic. It mounts on a 35 mm DIN rail per DIN EN 50022 via screw or snap-on, and accepts screw-type terminals for both main and auxiliary circuits — solid wire up to 6 mm², stranded up to 6 mm² with ferrule. At 400 V AC it interrupts faults up to 100 kA; at 690 V AC that drops to 6 kA, so keep it coordinated with downstream devices.
The 45 mm wide body — 97 mm tall, 96 mm deep — fits a standard 45 mm slot on the rail with zero required clearance at the back or sides. IP20 on the front means it's finger-safe for panel builders; the any-position mounting lets you orient it vertically or horizontally inside a gland plate layout. Phase failure detection is built in; ground fault detection is not — if you need that, add an external module or step up to the 3RV2 family.
Power dissipation runs 9.25 W total in hot operating state, or 3.1 W per pole — plan for that in a sealed enclosure with limited convection. The 15 operations per hour at AC-3 duty is a thermal limit, not a switching-life limit; for high-cycle applications like conveyor indexing, derate or move to a contactor-based solution. Shock-rated to 25g / 11 ms, so it survives transport and moderate panel vibration.