The Siemens SIRIUS 3RH2921-4DA11 is a first laterally mountable auxiliary switch designed for contactor relays and power contactors in sizes S00, S0, S2, and S3. It snaps onto the contactor without tools, adding auxiliary contact blocks to the side of the device. This is the standard way to add feedback or interlock contacts to a motor starter or contactor assembly on a DIN rail. Rated operational current at AC-12 reaches 10 A maximum, with a mechanical service life of 10 million operating cycles typical. It carries a mirror contact function per IEC 60947-4-1 when paired with a 3RT2 contactor, which is the safety-relevant feature: a mirror contact mechanically linked to the main contacts so a welded main contact is reflected in the auxiliary contact position. That matters for any safety circuit that needs to detect contactor weld.
Mounting and connection — panel integration
Snap-on mounting to the contactor side; the 10 mm width means it adds minimal DIN rail real estate. Height is 57.4 mm, depth 66 mm — check clearance against the enclosure door or gland plate if stacking multiple blocks. Connection is via ring terminal lug for the auxiliary and control circuits. That is a permanent, vibration-resistant termination — not a spring-cage or screw-clamp block. Plan for ring terminals with the correct stud size; no push-in wiring here. Front protection class is IP00 — no ingress protection on the front face. This is an inside-the-panel component only; not for washdown or outdoor use without an enclosure.
The AC-15 operational current at 690 V is rated 1 A — this is the inductive load rating for solenoid or contactor coils at the higher voltage. The AC-12 rating (10 A) covers resistive loads like indicator lamps or heater contactors. The buyer selecting for a 24 VDC PLC input circuit should look at the contact reliability spec (17 V, 1 mA) rather than the thermal current — this switch is designed to switch dry signals reliably, not just power. No external surge suppression needed for the auxiliary circuit.