The Siemens 3RV1901-1E is a SIRIUS transverse auxiliary switch designed to mount on the front of circuit breakers in sizes S00, S0, S2, and S3. It provides one normally-open and one normally-closed instantaneous contact, giving you a local status signal from the breaker without needing a separate contactor or relay. The screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) solid/stranded wire, which covers the typical control-circuit wiring in a panel.
The maximum switching current is 2.5 A, but the usable current drops with voltage: 1 A at 24 V, 0.3 A at 48 V, 0.15 A at 60 V, and 0.5 A at 230 V. The 24 V rating of 1 A is typical for a DC control loop feeding a small relay or indicator. If your circuit draws more than these per-voltage limits, the contact will overheat or weld closed.
The 3RV1901-1E is a phased-out product. Siemens lists the official successor as the 3RV2901-1E. If you have an existing BOM or panel that uses this switch, the 3RV2901-1E is the drop-in replacement to spec for new builds.
The switch uses plug-in fixing — it snaps onto the front of the circuit breaker without tools. The physical footprint is 45 mm wide by 12 mm tall by 17 mm deep, so it adds minimal depth to the breaker assembly. The screw terminals are sized for a Pozidriv #2 or flathead 5–6 mm driver, and the torque spec for auxiliary contacts is 0.8 to 1.2 N·m. Wire strip length should be 8 mm to avoid exposed conductor past the terminal cage.