The Siemens SIRIUS 3RH2911-4HA31 is a front-mounted auxiliary switch block carrying three normally-open instantaneous contacts. It snaps onto the front of SIRIUS contactors in frame sizes S00, S0, S2, and S3 — that covers the 3RT2 and 3RT1 families from about 7 A up through 300 A motor loads. The ring terminal lug connection means you land ring forks on the screw terminals, not push-on spades or cage clamps; plan your wire prep accordingly.
Three NO contacts, zero NC, zero lagging contacts. If your safety circuit demands a positively-guided NC, this block cannot provide it — you need a different variant. The AC-15 rating at 690 V is 1 A; that is the inductive switching limit for solenoid and contactor coils at high line voltage. At 24 V the AC-15 rating climbs to 10 A, so it handles multiple small contactor coils in parallel without a separate interposing relay. The mirror-contact feature per IEC 60947-4-1 applies when used with 3RT2 contactors — relevant for applications requiring welded-contact detection in motor circuits. That is the dry-circuit threshold — if your signal chain runs at logic-level voltage and milliamps, this block is rated for it.
Snap-on front mounting — no tools, no DIN rail. The block clips directly onto the contactor's front face. Dimensions are 36 mm wide, 43.7 mm deep, 37.5 mm high. The IP00 front rating means no ingress protection on the front face; the contactor's own enclosure provides the panel-level IP rating.