What it is and what it does
The Siemens SIMATIC 3RK1308-0BA00-0CP0 is a reversing motor starter — a compact, networked motor control unit that handles forward/reverse operation of a three-phase motor directly, without external contactors or reversing interlocks. It integrates a 4 via 3DI/LC module, meaning it carries four digital inputs and a load-circuit interface in one package, so the panel builder gets the reversing logic and the fieldbus connection in a single 30 mm wide DIN-rail footprint. Rated at 0.4 A continuously at both 40 °C and 50 °C ambient, this unit is sized for small motor loads — fractional-horsepower pumps, small conveyors, valve actuators. The 55 kA short-circuit breaking capacity at both 400 V and 500 V means it can safely interrupt a fault up to that level without upstream fuses needing to clear first, which simplifies coordination in a panel with high available fault current. Trip Class is adjustable to OFF, 5, or 10, so you match the thermal protection curve to the motor's starting profile — Class 10 for standard induction motors, Class 5 for fast-starting servo-type loads, or OFF if you're using an external overload relay.
Network integration and I/O
Dual-protocol fieldbus onboard: PROFINET and PROFIBUS DP. The I/O map allocates 4 bytes for inputs and 2 bytes for outputs. Mounts vertically or horizontally (horizontal requires derating per the manual). Pluggable into a BaseUnit, so replacement is a swap without touching the wiring — the base stays on the DIN rail, the electronics module lifts off. Dimensions: 30 mm wide, 142 mm tall, 150 mm deep. Keep 50 mm clearance above and below for airflow and cable bend radius.
Environmental and electrical ratings
Operates from -25 °C to +60 °C (derate above 40 °C per manual), stores and transports at -40 °C to +70 °C. Shock rated to 6g at 11 ms; vibration resistance is 15 mm at 6 Hz and 2g up to 500 Hz. Overvoltage category III, pollution degree 2 — standard for industrial control panels connected to the mains supply. Supply voltage range is 20.4 V to 28.8 V — nominally 24 VDC with tolerance for a sagging bus. Safe state is load circuit open, meaning on loss of communication or fault, the motor contacts drop out and the load is disconnected. ON-delay is fixed at 20 ms; OFF-delay is adjustable from 35 to 50 ms, useful for sequencing multiple starters without a PLC timer.