The Siemens SIRIUS 3RV1421-0CA10-ZI41 is a 3-pole circuit breaker designed specifically for transformer protection, rated at 0.25 A continuous current. It carries a 100 kA breaking capacity across the full voltage range from 240 V to 690 V AC, which means it can safely interrupt fault currents up to that level without cascading upstream — critical for maintaining selectivity in a distribution panel.
The 45 mm width and 97 mm height fit a standard modular enclosure grid; depth is 96 mm, so check your enclosure depth — it's deeper than a typical MCB. Screw-type terminals on both main and auxiliary circuits accept 2x (1 to 2.5 mm²) solid or stranded, or 2x (2.5 to 6 mm²) — the larger cross-section is useful for looping through downstream loads. Zero clearance needed at the sides or back, so you can pack units tight on the rail.
Ground fault detection is not included, so if you need that, you'll add an external module or upstream RCD. Operating temperature range is -20 to +60 °C; storage and transport range is -50 to +80 °C. Shock rated at 25g / 11 ms, so it holds up in mobile or high-vibration installations.