The Siemens SIRIUS 3RK4340-3FR51-1BA0 is an I/O-controlled reversing motor starter designed for direct-on-line reversing of three-phase motors up to 2 A AC-3 at 400 V. It integrates a contactor, bimetal overload relay (Class 10 trip), and short-circuit protection coordination into a single IP55 enclosure — one part number instead of a panel-built stack. The 340-440 V operating voltage range covers standard 400 V industrial mains; the control supply is 20.4-26.4 V DC, so it needs a 24 V DC power supply, not an AC coil circuit. Rated 2 A in AC-3 duty (motor switching), this starter handles small reversing loads: conveyors, gate drives, small hoists, or any application where forward/reverse is needed. The Class 10 bimetal overload matches standard motor thermal curves for general-purpose induction motors — it will trip before the winding overheats on a sustained overload or phase-loss condition.
The 2 A AC-3 at 400 V rating is the motor-switching current, not the resistive (AC-1) rating. For a 400 V three-phase motor, 2 A corresponds roughly to a 1.1 kW output — confirm your motor FLA stays under this figure. The operating voltage range of 340-440 V means it works on 400 V nominal systems with typical tolerance, but not on 480 V or 575 V supplies. This is the standard class for most general-purpose induction motors; if your motor has a high-inertia load requiring a longer start time (Class 20 or 30), this starter is not the right fit. No PROFIBUS DP, no PROFINET, no AS-Interface protocol support. This is an I/O-controlled unit — you wire the forward/reverse/stop commands directly to the control terminals. If your line requires fieldbus control, you need a different variant in the SIRIUS family with a communication module.
The IP55 enclosure means this starter is dust-protected and can handle water jets from any direction — suitable for washdown areas, outdoor installations under cover, or dusty industrial environments. It is not submersible (IP67 would be needed for that). This helps fitting into tight cabinets or angled machine frames. Dimensions are 145 mm deep, 182 mm wide, 220 mm high — account for cable entry clearance on the bottom and side. Short-circuit protection is designed for circuit-breakers (not fuses). The starter includes short-circuit protection coordination, so you can size the upstream breaker per the manufacturer's tables without adding a separate motor protection breaker.