What It Is and Where It Fits
The Siemens SIRIUS 3RH1921-1LA20 is an auxiliary switch block delivering 2 NC contacts via screw-type terminals, designed for snap-on mounting directly onto a 3RT1 power contactor. It occupies the 44 mm width slot on the contactor's front face and adds 51 mm depth to the assembly. The IP20 rated front means it's intended for enclosed panel use only — no exposed live parts in a dead-front cabinet.
Switching Capability Across the Voltage Range
Rated for a maximum of 10 A, the block's actual interrupting capacity depends on the circuit voltage. At 24 V it handles 6 A (AC-15/DC-13 typical), rising to 6 A at 125 V, 230 V, and 250 V. At 400 V it still manages 3 A. The ratings drop at higher voltages: 0.3 A at 220 V and 0.5 A at 440 V. For control circuits at 24 VDC, the continuous rated current is 10 A. These are the numbers that govern whether this block can interrupt your specific pilot-duty load without welding contacts.
Lifecycle Reality
This is a phased-out product. The official successor is the SIRIUS 3RH2 series. If you are maintaining an existing panel with a 3RT1 contactor and need a drop-in replacement, the 3RH1921-1LA20 is the correct form, fit, and function. For new designs, the 3RH2 block is the current-production path. Sourcing this part goes through the surplus and NOS channel — quoted to order against an RFQ.
Mechanical and Environmental Limits
Rated for 10 million mechanical operating cycles typical, with contact reliability of 1 faulty switching per 100 million operations at 17 V, 1 mA. Operating temperature range is -25 to +60 °C; storage range extends from -55 to +80 °C. The block withstands a surge voltage of 6 kV and is rated for an insulation voltage of 690 V at pollution degree 3. Wire termination accepts 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) with core end processing, or 2x (20 to 16) or 2x (18 to 14) AWG.