The Siemens 3RA2120-1FA24-0FB4 is a SIRIUS direct (on-line) starter — a pre-assembled combination of a contactor and overload relay in one package, sized S0 for motor loads up to 5 A in AC-3 duty at 400 V. The 24 VDC coil (rated value 24... 24 V) means it drops into a standard 24 VDC control circuit without a separate power supply conversion. Trip Class 10 gives you a 10-second thermal overload response at locked-rotor current — standard for general-purpose induction motors driving pumps, fans, and conveyors, where the start time stays under that window.
Clearance requirements: 50 mm upwards, 20 mm forwards, 20 mm at the side, 10 mm downwards, and 0 mm backwards — so you can butt it against a backplane or enclosure wall.
At 400 V AC-3, the starter is rated 5 A — that's the motor full-load current it can switch under IEC utilization category AC-3 (starting and disconnecting running squirrel-cage motors). Horsepower equivalents: 1.5 hp at 220/230 V, 3 hp at 460/480 V, and 5 hp at 575/600 V, so it covers common North American motor frames up to 5 hp at 600 V. The circuit-breaker portion is size S00, which coordinates with the S0 contactor housing for short-circuit protection up to the breaker's interrupting rating.
No official successor has been announced; the 3RA2120-1FA24-0FB4 remains the current order code for this S0 direct starter configuration.