The Siemens SIRIUS 3RV2431-4EA10-0BA0 is a circuit breaker designed specifically for transformer protection — that means its thermal-magnetic curve is shaped to handle the inrush of a transformer without nuisance tripping, while still clearing a hard fault. It carries a CLASS 10 trip class, so it will hold through the magnetizing inrush of a transformer and still trip fast enough to protect the winding on a short. Phase failure detection is built in, which is a real advantage on a transformer feeding a three-phase panel: lose a phase on the primary and the breaker opens before the secondary loads see undervoltage.
Mounts on 35 mm DIN rail per DIN EN 60715 with a screw and snap-on fastening. The size S2 footprint is 55 mm wide, 149 mm deep, 140 mm high. Clearance: 50 mm above and below, 10 mm to the side, zero forward or backward (–,). That 10 mm side gap is tight — if you're packing breakers side-by-side in a multi-row panel, you can run them flush. Screw-type terminals on the main circuit accept up to 2x 25 mm² or 1x 35 mm² solid/stranded. The screwdriver spec is Pozidriv size 2, shaft diameter 5–6 mm.