The Siemens 3RH2131-2KV40 is a coupling contactor relay in the SIRIUS 3RH2 family, designed to bridge PLC outputs to higher-current contactor coils or other loads in a control panel. It carries 3 normally-open and 1 normally-closed contact set, with a 36 V DC coil rated for 0.7 to 1.25 times the rated control supply voltage. The integrated suppressor diode clips the inductive kickback when the coil de-energizes, which means you don't need an external flyback diode across the coil terminals — a common point of failure on panels where that diode gets omitted or reversed. Frame size S00 keeps the footprint compact for DIN-rail mounting inside a standard control cabinet.
RoHS compliance dates from October 2009, so it meets the EU restriction on hazardous substances for current builds.
The 36 V DC coil with a 0.7–1.25× Us operating range means the relay pulls in reliably between about 25 V and 45 V DC. That tolerance covers a sagging 24 V bus (common on long cable runs to remote I/O) and still holds in during a moderate overvoltage. The 3 NO + 1 NC contact arrangement is the standard coupling relay complement: three poles to switch a three-phase contactor coil, one NC for feedback or status interlock. The integrated suppressor diode absorbs the inductive flyback spike internally, protecting the PLC output. Frame size S00 and the not-expandable-with-auxiliary-switch note mean this relay is a fixed-contact-count device. If you need additional auxiliary contacts beyond the 3 NO + 1 NC, you must select a different 3RH2 variant or add an external auxiliary contact block. For a simple PLC-to-contactor coupling where four contacts are enough, this is the cleanest option — no extra parts to spec.