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.
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. Shock-rated to 25g / 11 ms, so it survives transport and moderate panel vibration.