What this is and where it fits
The Siemens 3RH1921-1FA40 is a 4-pole auxiliary switch block from the SIRIUS family, designed to snap onto a 3RT1 contactor or contactor relay. It adds four normally-open (4 NO) instantaneous contacts for signaling or interlocking in a motor control panel. The screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) with core-end processing, so it wires up with standard ferrule sizes.
Key ratings and what they mean for your circuit
The block carries a maximum thermal current of 10 A. The voltage-specific ratings tell you the switching capability at different control voltages: 6 A at 24 V, 6 A at 125 V, 6 A at 230 V, 3 A at 400 V, and 0.3 A at 220 V. For DC-13 duty (solenoid loads) the rated values are 10 A at 24 V, 4.7 A at 60 V, 3 A at 110 V, 1.2 A at 220 V, 0.5 A at 440 V, and 0.26 A at 600 V. The AC-15 rating at 690 V is 1 A. These numbers govern what inductive load you can switch reliably — match the voltage column to your control circuit. Contact reliability is specified at 1 faulty switching per 100 million operations at 17 V, 1 mA — that is the low-energy end where oxide films can cause trouble. The mechanical life is 10 million operating cycles typical. Surge voltage resistance is 6 kV, and the insulation voltage at pollution degree 3 is 690 V AC. IP20 on the front means finger-safe when mounted in an enclosure.
Panel integration
Snap-on mounting onto a 35 mm DIN rail or directly onto the 3RT1 contactor. Dimensions are 44 mm wide, 38 mm tall, 51 mm deep — it adds minimal depth to the contactor stack. Operating temperature range is -25 to +60 °C; storage range is -55 to +80 °C. The note "with 3RT1" in the catalog means this block is specifically keyed for that contactor series, not the 3RT2.