What it is and where it fits
The Siemens 3RV1901-1J is a SIRIUS lateral auxiliary switch that clips onto the front of a circuit breaker in sizes S00, S0, S2, or S3 — it adds two normally-open and two normally-closed instantaneous contacts for signaling or interlocking. The screw-type terminals accept 2x (0.5…1.5 mm²) or 2x (0.75…2.5 mm²) solid or ferruled stranded, so the wiring matches standard panel-building practice. The 18 mm width and 68 mm depth keep it inside the breaker envelope; no extra DIN-rail space is needed.
What the ratings mean for the buyer
The 10 A maximum is the thermal current the contacts can carry continuously in a resistive circuit — but the real-world switching capacity depends on voltage and duty. At 230 V AC-15 (typical for solenoid or contactor coils) it's rated 4 A; at 400 V AC-15 it drops to 3 A. For 24 V DC loads it handles 2 A, and at 110 V DC it's 0.5 A. The IP20 finger-safe front means the contacts are protected against accidental touch from the front, which is the standard for enclosed panel gear — not for open washdown environments.
Lifecycle and sourcing reality
The 3RV1901-1J is a phased-out product. The official Siemens successor is the SIRIUS 3RV2 series, preferred successor type 3RV2901-1J. For a BOM line that still calls out this order code, the part is sourced through independent distribution channels — quoted to order against an RFQ, with availability and current pricing confirmed at quote time. If you're holding a panel design that references this auxiliary switch, the 3RV2901-1J is the drop-in replacement to evaluate for new builds or last-time-buy coverage.