What it is and where it lands
The Siemens 3RK1903-2AC10 is a 65 mm terminal block PEN designed specifically for the ET200S High Feature starter — the interface that brings load connections into the motor starter slice on a distributed I/O rail. It's not a standalone contactor or breaker; it's the wiring termination point that mates the motor cable to the starter's power electronics inside a SIMATIC ET200S station.
Panel integration — what fits where
This terminal block snaps onto the DIN rail alongside the ET200S motor starter slices. At 65 mm wide, it occupies one standard slot in the ET200S profile rail layout. The PEN designation means it carries the protective earth and neutral conductor for the load side — critical for TN-S or TN-C-S systems where the PEN conductor must be terminated separately before the motor starter's output.