What it is and what it does
The Siemens 3RK1304-5LS70-3AA0 is a SIMATIC-brand reversing motor starter — a self-contained unit that combines a contactor, overload relay, and reversing functionality in one IP65 enclosure. It's designed for direct-on-line reversing control of three-phase motors up to 12 A at 400 V AC, with a breaking capacity of 100 kA at that voltage. The trip class is field-adjustable between CLASS 5, 10, 20, and 30, so you can match the thermal protection to the motor's start-up profile — a pump with a long acceleration ramp gets CLASS 20 or 30, a fast-cycling conveyor gets CLASS 10 or 5. The unit communicates over PROFINET and PROFIBUS DP, giving you direct fieldbus integration without a separate gateway. It carries 4 digital inputs (M12 socket) and no digital outputs — the outputs are 2 bytes of process data on the bus. The 24 V DC control supply is rated 20.4 to 28.8 V, so it's tolerant of a sagging 24 V bus in a long daisy-chain.
Where it goes and how it mounts
This is a screw-fixed, IP65-rated enclosure (230 × 110 × 160 mm), so it mounts directly on a machine frame or wall — no DIN-rail panel required. The IP65 rating means the housing and the M12 connectors withstand washdown spray, making it suitable for food-and-bev lines, car wash tunnels, or outdoor conveyor sections. Mounting position is vertical or horizontal, which gives flexibility in tight machine compartments. Ambient operating range is -25 to +55 °C; storage and transport range is -40 to +70 °C. The main energy infeed uses a socket per ISO 23570, so the power cable connection is standardized. Vibration resistance is 2g; shock resistance is 15g at 11 ms — it holds up on a vibrating pump skid or a press feed.
What the ratings mean for fit
The headline 12 A at 400 V AC is the rated operational current for AC-3 motor duty — that's the current it can switch under full load for a standard squirrel-cage motor. The 100 kA breaking capacity at 400 V means it safely interrupts a short-circuit fault up to that level without welding contacts or cascading upstream. The adjustable trip class (5/10/20/30) sets the overload relay's thermal memory: CLASS 5 trips fastest (for submersible pumps or compressors that start unloaded), CLASS 30 allows the longest start time (for high-inertia loads like centrifuges or large fans). The 4 digital inputs are 24 V DC, M12 socket, and the 2-byte input/output process data maps directly to PROFINET or PROFIBUS cyclic telegrams. If your PLC expects a specific I/O byte layout, check the GSD file before commissioning — the input/output byte assignment is fixed at 1 (type of assignment).