The Siemens 3RV1421-0HA10 is a SIRIUS circuit breaker for transformer protection. Rated 0.8 A continuous, it is sized for small control transformers. Breaking capacity is a flat 100 kA across the full AC voltage band: 240 V, 400 V, 500 V, and 690 V. Trip Class 10 means the bimetal trips within 10 seconds at 7.2× rated current — fast enough to protect transformer windings from sustained overload, but slow enough to ignore the 100–200 ms inrush pulse.
The 45 mm width (3 module spaces) and 96 mm depth fit standard sub-distribution boards. Solid or stranded wire: 2× (1 … 2.5 mm²) or 2× (2.5 … 6 mm²). AWG equivalent 2× (14 … 10) for the main contacts. No special tooling needed — standard Pozidriv #2. Zero clearance needed at the back or sides (0 mm backwards, 0 mm at the side), so it can be packed tight in a row. Mounting position any orientation.