The Siemens 3RV5901-1D is a SIRIUS transverse auxiliary switch that mounts on the front of 3RV circuit breakers in frame sizes S00, S0, S2, and S3. It adds one normally-open instantaneous contact to the breaker's auxiliary circuit, letting you pick up a status signal or interlock without a separate contactor block. Rated for 10 A maximum, with AC-15 switching capacity at 690 V of 0.5 A. The 1 NO contact handles 1 A at 24 V and 3 A at 230 V, so it's sized for PLC inputs, relay coils, and indicator lamps — not motor loads directly. The front face carries IP20 finger-safe touch protection per IEC 60529, meaning you can reach into the panel without exposing live terminals during troubleshooting. Screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) solid or ferruled stranded.
Snaps directly onto the breaker front — no DIN rail footprint, no extra wiring space needed beyond the breaker's own envelope. The 45 mm width matches the S0/S2/S3 breaker profile; depth adds only 17 mm to the assembly, so it stays inside a shallow enclosure. Strip length of 8 mm on the screw terminals avoids whiskers under the clamp. The plug-in fixing means no mounting screws; it locks into the breaker's accessory slot and releases with a screwdriver push.