What this motor starter does that a bare contactor can't
The Siemens 3RK1308-0AC00-0CP0 is a SIMATIC direct-on-line motor starter with integrated overload protection — not just a contactor. The trip class is adjustable: CLASS OFF, 5, or 10, which means you can match the thermal model to the motor's start profile — a Class 10 for a pump that ramps quickly, or Class OFF for a solid-state soft-start bypass. The 20 ms ON-delay and 35–50 ms OFF-delay give you selectivity headroom against nuisance trips on inrush. It speaks both PROFINET and PROFIBUS DP natively, so it drops into a Siemens 1500 or ET 200SP rack without a gateway. The I/O map is 4 bytes of inputs and 2 bytes of outputs — enough for status, control, and diagnostics over the fieldbus.
The module is 30 mm wide, 150 mm deep, and 142 mm tall — it plugs into a BaseUnit on the DIN rail, not directly onto the rail itself. That means the wiring and bus connections live in the base, and the starter module can be swapped without pulling wires. Mounting position can be vertical or horizontal, but derating applies for horizontal — check the manual. The vibration spec is 2g up to 500 Hz and shock is 6g for 11 ms, which is standard for industrial cabinet mounting near conveyors or compressors. Clearance above and below the module is 50 mm each way for airflow and arc venting.
There is no official successor or cross-reference from Siemens.