The 3RV1901-0H: This is a Siemens SIRIUS cover cap for the transverse auxiliary switch on their 3RV1 series circuit breakers — the ones you find in motor starter combos and feeder panels. It's a small plastic part, 10 mm wide and 35 mm deep, that snaps into place over the auxiliary switch terminals to protect the wiring and maintain a clean finger-safe profile inside the enclosure. It covers breaker frame sizes S00, S0, S2, and S3, so one part number covers the whole compact-to-midsize range of the 3RV1 family. The plug-in fixing means no tools for installation — just press it onto the switch block after you've landed the wires.
The 3RV1901-0H is a phased-out product. The manufacturer's official successor is the SIRIUS 3RV2 series equivalent, order code 3RV2901-0H.
Wiring and fit
The cap accommodates two conductors per terminal: 2x (0.5 to 1.5 mm²) solid or 2x (0.75 to 2.5 mm²) solid, which covers the typical auxiliary wiring you'd run for a contactor coil or PLC input. The AWG equivalent for the larger range is 2x (18 to 14).