The Siemens 3RH2911-4NF20 is a SIRIUS auxiliary switch designed to snap onto the front of 3RT2 contactors and 3RH2 contactor relays in sizes S00, S0, S2, and S3. It provides two normally open (2 NO) instantaneous contacts that are rated as solid-state compatible, meaning they handle the low-voltage, low-current signals typical of PLC inputs and electronic control modules without the contact oxidation problems that plague standard auxiliary contacts.
The solid-state compatible contacts are rated at 0.3 A at 24 V, 48 V, and 60 V — that's the switching capacity, not a continuous carry rating. For a PLC input or a safety relay feedback loop drawing 10-20 mA, these contacts will outlast the machine. The contact reliability spec backs this up: one incorrect switching operation per 100 million operations at 5 V, 1 mA, which is well into the dry-switching territory where standard contacts start to fret. The switch carries a mirror contact designation per IEC 60947-4-1 when used with 3RT2.2/3/4 contactors. That means the NO and NC positions are mechanically linked so they can't both be closed at the same time — a requirement for safety circuits where you need positive-guided feedback of the main contactor state. That's standard for industrial control panels running 400/480 V lines — the switch can live in the same enclosure as the contactor without tracking issues, even in dirty environments.
Snaps onto the front of the contactor — no DIN rail space consumed, no extra screws. The 36 mm width and 43.7 mm depth keep it within the contactor's own footprint. Wiring is via ring cable lug connections, which means you crimp a ring terminal onto each wire and bolt it down. This is a field-serviceable connection; no special tools beyond a crimper and a screwdriver. The switch has 2 instantaneous NO contacts, 0 CO contacts, and 0 lagging/leading contacts. The contact path is labelled.3/.4 for both NO pairs. The IP00 protection class on the front means it's not sealed — it relies on the enclosure's own IP rating.