The Siemens 3RA2125-1KD24-0BB4 is a SIRIUS non-fused motor starter — a pre-assembled combination of a contactor and a thermal bimetallic overload relay in a single S0 frame. It is a direct starter, meaning it applies full line voltage to the motor on start, and is rated for AC-3 switching duty up to 690 V. The 24 V DC coil is held in by the control supply; the bimetallic overload provides adjustable trip protection with Trip Class 10 response, so it clears a stalled rotor fast enough for standard induction motors on conveyors, pumps, and fans.
This unit snaps directly onto a 60 mm busbar system (the supplied busbar adapter 8US1251-5NT10 is included). Mounting position is vertical; the S0 footprint occupies 45 mm width, 260 mm height, and 165 mm depth. Clearance requirements: 30 mm upwards, 10 mm forwards, 10 mm downwards, 9 mm at the side, and 0 mm backwards — so it can sit flush against a back panel. The supplied contactor is 3RT2024-1BB40 and the link module is 3RA2921-1BA00, meaning the assembly arrives as a single orderable unit ready for panel wiring.
The AC-3 motor ratings tell you the real-world switching capacity across common supply voltages. At 400 V it handles 5 500 W (roughly 7.5 hp); at 460/480 V it is rated for 7.5 hp; at 575/600 V it reaches 10 hp. The same unit at 230 V is rated 2 hp.