The Siemens 3RH2921-2DA02 is a lateral auxiliary switch block from the SIRIUS family, designed to snap onto the left side of 3RT2 contactors and 3RH2 contactor relays. It provides two normally-closed (NC) auxiliary contacts — one current path with two NC contacts — wired to terminals 51/52 and 61/62 on the left side, and 31/32 and 41/42 on the right side. 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 AWG 20 to 14, matching the standard panel wiring practice for control circuits.
The maximum thermal current is 10 A, but the real-world switching capacity depends on voltage and load type. At AC-15 (inductive control-circuit duty) at 690 V, it's rated 1 A — that's the figure that governs fit for a 690 V control transformer or PLC output driving a contactor coil. At 24 V DC, the rated operational current is 6 A; at 60 V DC it drops to 2 A; at 110 V DC it's 1 A; and at 220 V DC it's 0.3 A. These DC ratings are the ones that matter when switching DC solenoids or brake coils — the AC thermal rating alone doesn't tell you whether the contacts will handle the DC arc. The insulation voltage is 690 V at pollution degree 3, and surge voltage resistance is 6 kV, so the block is suitable for 400 V and 480 V panels with proper clearance.
Snap-on mounting to the contactor body — no DIN rail slot consumed, no extra panel space needed beyond the contactor's own footprint. The block is 10 mm wide, 64.8 mm high, and 66 mm deep, matching the contactor profile.
Contact Reliability and Mechanical Life
Mechanical service life is 10 million switching cycles typical. For a machine running at one cycle per second, that's about 115 days continuous; in practice, with intermittent use, it's years of service before mechanical wear-out.