Transformer Protection Circuit Breaker — SIRIUS 3RV2431-4BA10
The Siemens SIRIUS 3RV2431-4BA10 is a circuit breaker specifically designed for transformer protection, not general motor or feeder duty. That design choice governs the trip curve and the application scope — it's sized to handle the inrush of a transformer without nuisance tripping, while still providing short-circuit and overload protection for the transformer primary. Rated for a voltage range of 20 to 690 V, it covers low-voltage transformer primaries from small control transformers up to 690 V industrial supplies. No ground fault detection on this variant — that's a separate add-on or a different catalog number if you need GF protection.
The interrupting ratings are substantial for a 55 mm wide device: 100 kA at 240 V AC, 65 kA at 400 V AC, and still 30 kA at 480 V. Motor horsepower equivalents are listed per voltage: 3 hp at 230 V, 7.5 hp at 200/208 V, 15 hp at 460/480 V, and 20 hp at 575/600 V. These are useful for cross-referencing when a transformer feeds a motor branch circuit, but the primary application remains transformer protection — the trip curve is not a standard motor-protective curve.
The 55 mm width is a standard modular width — it occupies three 18 mm module spaces on the rail. That 10 mm side clearance is tight — adjacent breakers or terminal blocks can sit nearly flush. Main circuit terminals accept 2×(1–25 mm²) or 1×(1–35 mm²) solid or stranded copper, terminated with M6 screws.
If your application cycles the transformer load more frequently than that, consider a contactor in series and use this breaker as the backup protective device only.