The Siemens 3RN2012-2BW30 is a SIRIUS 3RN2 thermistor motor protection relay — a standard evaluation unit with ATEX approval, designed to monitor PTC thermistor sensors embedded in motor windings. It detects both open-circuit and short-circuit faults in the sensor circuit, and stores the last fault in non-volatile error memory. The supply is rated 24…240 V AC/DC, covering most control voltages without a separate transformer.
The response threshold is set at 0.85 of the full-scale value, with a full-scale value of 1.1 — this means the relay trips when the thermistor resistance crosses the PTC threshold (typically around 3.3 kΩ for cold-state PTCs). Auto-reset is enabled by default, but an external reset input is also available for manual restart after a trip. IP20 protection means it is intended for use inside a control cabinet, not in wet or dusty environments. The MTBF is listed at 97 years, MTTFd at 303 years, reflecting the solid-state design and low component count.