What it is and where it fits
The Siemens 3RH2911-2HA12-ZW98 is a SIRIUS auxiliary switch designed to snap onto the front of contactor relays and power contactors in sizes S00, S0, S2, and S3. It adds one normally-open instantaneous contact to the base device, giving you a local feedback or interlock signal without extra wiring runs back to the panel. Rated at 6 A at 24 V (AC or DC — the spec doesn't distinguish, so treat it as thermal current), it handles typical PLC input or relay coil switching. The contact reliability spec — one faulty switching per 100 million operations at 17 V, 1 mA — means it's fine for low-energy signals like a PLC digital input, not just power loads. The switch carries a mirror contact classification per IEC 60947-4-1 when paired with a 3RT2 contactor, which is critical for safety circuits: a mirror contact ensures the N/C pole cannot close if the N/O pole welds, so your safety relay sees a true state.
Mounting and panel integration
Snap-on mounting to the contactor front — no DIN rail needed, no screws. The 36 mm width and 47.7 mm depth keep it within the contactor's own footprint; you don't lose panel space. IP20 on the front means it's protected against finger contact but not washdown — keep it inside the enclosure. Terminals accept 2 x (0.5 to 2.5 mm²) solid or stranded, or 2 x (20 to 14 AWG). Fine-stranded with ferrule goes to 1.5 mm². Strip length is standard; spring-cage or screw terminals aren't specified, but the cross-section range covers most control wiring.