What it is and what it fits
The Siemens 3RH1921-1DA11-ZW95 is a SIRIUS first lateral auxiliary switch block, delivering one normally-open and one normally-closed contact via screw-type terminals. It is explicitly designed for use with 3RT1 contactors — the "with 3RT1" note on the order code tells you it snaps onto the side of those power contactor frames, not the 3RT2 or 3RT5 series. Snap-on mounting onto the contactor profile means no tools or extra brackets for panel integration; the 10 mm width keeps the assembly compact on the DIN rail.
What the contact ratings mean for your circuit
The auxiliary contacts are rated 10 A maximum, with specific current-carrying capacities at different voltages: 10 A at 24 V, 6 A at 125 V, 6 A at 230 V, and 3 A at 400 V. These are the thermal continuous currents the contacts can handle — the actual switching capacity for motor or inductive loads will be lower and governed by the AC/DC utilization category of your application. Contact reliability is specified at 1 faulty switching per 100 million operations at 17 V, 1 mA — this matters for PLC-level signals where dry switching (low voltage, low current) can cause oxide film buildup on contacts. The gold-flash or silver-nickel alloy on these contacts handles that regime reliably. Mechanical life is rated at 10 million switching cycles typical, which for a frequently cycled auxiliary in a production line translates to years of service before wear-out.
Lifecycle and sourcing reality
The multi-unit packaging of 252 pieces per case (noted in the description) means it ships in bulk for OEM or panel-shop volume; single-unit orders are typically re-packed from that case quantity.
Panel integration notes
Snap-on mounting onto the 3RT1 contactor profile — no DIN rail footprint of its own, it rides on the contactor body. The 10 mm width means it adds minimal side space; a contactor with this block on each side still fits a standard 45 mm slot. Screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) with core-end processing, or 2x (20 to 16 AWG) and 2x (18 to 14 AWG) for stranded wire. IP20 protection on the front means it is finger-safe inside an enclosure but not for wet or dusty environments.