The Siemens 3RV1901-1FA00-0FJ0 is a transverse auxiliary switch from the SIRIUS family, built for mounting onto S00 through S3-sized contactors and motor starters. It brings two normally open (NO) contacts into the assembly, wired via screw terminals.
What It Fits and What That Means
This switch is sized for the S00 through S3 frame range — that covers the majority of SIRIUS 3RV2 motor starter protectors and 3RT2 contactors up to around 40 A or so, depending on the duty. The transverse mounting clips onto the side of the device, so it adds width, not depth, inside the enclosure. Plan for that extra DIN-rail space when you're laying out the panel; it's a clean addition but it does eat one slot width per switch.
Screw Terminal Reality
Screw connection means a standard Pozidriv driver and a torque spec you can hit with a common screwdriver — no special crimp tool or cage-plunger required. For the MRO planner restocking a panel spares kit, that's one less specialty tool to keep on the truck. The trade-off is a slightly slower wire termination compared to spring-cage, but in a retrofit or field-repair scenario, screw terminals are dead reliable and easy to re-terminate after a swap.