The Schneider Electric MGN61821 is a Multi9 C60H-DC miniature circuit breaker, a 2-pole DC-rated thermal-magnetic breaker built for distribution panels that need reliable overcurrent and short-circuit protection on DC circuits. Rated 40 A at 50 °C per EN/IEC 60947-2, it breaks up to 20 kA Icu at 250 V DC — meaning it clears high-fault DC arcs cleanly, which is the main thing a DC breaker has to do that an AC breaker can't.
The B curve (magnetic trip at 3.5 to 5.5 x In) is the standard for resistive and general-purpose DC loads — think control transformers, lighting, or heater circuits — where inrush is low and you want fast trip on a hard short. The breaking capacity steps down with voltage: 20 kA at 220/250 V DC, 10 kA at 440 V DC, 6 kA at 500 V DC — the higher the bus voltage, the lower the fault current it can interrupt, so match the Icu to your available fault current at the panel.