The Siemens 3RH1911-2FA01 is an auxiliary switch block with one normally-closed contact, using spring-type terminals and sized for the S00 frame. It mounts directly onto SIRIUS contactors and contactor relays.
This part is a phased-out product with lifecycle stage listed as current. The successor is the SIRIUS 3RH2 series.
The spring-type terminal (EN 50005) accepts solid or ferruled stranded wire without screw-clamp torque — handy when you're packing multiple blocks into a tight panel and can't reach every terminal with a screwdriver. The 1 NC contact gives you one normally-closed path; if your circuit needs more than one, you stack additional blocks. The S00 size matches the smallest SIRIUS contactor frame, so it won't overhang the contactor body — keeps the DIN-rail layout clean.