Part identity and fit
The Siemens 3RH2921-2DA20 is a lateral auxiliary switch from the SIRIUS series, designed to snap onto the left side of 3RT2 power contactors and 3RH2 contactor relays. It provides two normally-open (NO) instantaneous contacts, wired to terminals 53/54 and 63/64 on the left side, and 33/34 and 43/44 on the right side — each contact path is 1 NO. The spring-loaded terminals accept 2x (0.5 to 1.5 mm²) with ferrule or 2x (0.5 to 2.5 mm²) solid, or 2x (20 to 14 AWG) stranded.
The auxiliary switch is rated for a maximum thermal current of 10 A. For inductive DC loads the switching capability drops with voltage: 6 A at 24 V, 2 A at 60 V, 1 A at 110 V, 0.3 A at 220 V. On AC-15 duty (solenoid/contactor coils) it handles 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. At 690 V AC-15 it is rated 1 A.
Integration notes
Snap-on mounting to the left side of the contactor or relay — no tools needed. The 10 mm width and 66 mm depth keep the assembly within the standard DIN-rail envelope. IP20 on the front means the terminals are finger-safe inside an enclosure but not suitable for washdown. Rated impulse withstand voltage is 6 kV, and insulation voltage at pollution degree 3 is 690 V AC.