The Siemens SIRIUS 3RH1921-1MA20 is an auxiliary switch block that snaps onto 3RT1 motor contactors, adding 2 normally-closed (NC) contacts for status feedback or interlock circuits. It's rated for a maximum of 10 A and carries a 690 V insulation voltage with degree of pollution 3, so it's built for industrial control panels where contactors are switching motor loads. The screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) with core-end processing, or the AWG equivalents 2x (20 to 16) and 2x (18 to 14).
Snap-on mounting onto the contactor means no tools needed for installation — just push it onto the 3RT1 base unit until it latches. The front face carries IP20 protection, so it's fine inside a closed panel but not for washdown areas. Dimensions are 44 mm wide, 38 mm tall, and 51 mm deep, which keeps the assembly compact alongside the contactor. Operating temperature range is -25 to +60 °C; storage range is -55 to +80 °C. The contact reliability spec is 1 faulty switching per 100 million operations at 17 V, 1 mA, which matters for low-voltage PLC input circuits where contact oxidation can cause intermittent faults.
The 3RH1921-1MA20 carries 2 NC contacts only. The successor 3RH2122-1WB40 carries 2 NO and 2 NC, so if your circuit needs both normally-open and normally-closed auxiliary contacts, the replacement block gives you that flexibility. The 3RH2 blocks are also slightly larger — 45 mm wide and 73 mm deep versus the 3RH1921's 44 mm by 51 mm — so verify panel clearance before swapping. Both use the same screw-terminal wiring range and snap onto a 35 mm DIN rail via the contactor base.