The Siemens 5SJ4120-8HG42 is a 1-pole SENTRON miniature circuit breaker with a D tripping characteristic, rated 10 kA breaking capacity per EN 60898 and 15 kA per IEC 60947-2 at 400 V AC.
Rated 400 V AC (pollution degree 3, overvoltage category 3) and 60 V DC maximum — the DC rating is for control-circuit or signal isolation, not for high-current DC loads. Breaking capacity: 10 kA under EN 60898 (the household/commercial standard) and 15 kA under IEC 60947-2 (the industrial standard). That 15 kA figure gives headroom for industrial panels with higher fault current potential. The D-curve trips between 10× and 20× rated current, so it's sized for loads with a high starting surge — a small motor or a bank of HID lamps.