The Siemens 3RN1012-2BB00 is a thermistor motor protection evaluation unit from the SIRIUS 3RN1 series, designed to monitor motor winding temperature via PTC thermistor sensors and trip the control circuit on over-temperature. It runs on a 24 V AC/DC control supply and provides two changeover (CO) contacts for alarm and trip signaling.
Control supply is 24 V AC/DC, so it drops into panels with a standard 24 VDC power supply or a 24 VAC control transformer without needing a separate rectifier. The two CO contacts are rated for general-purpose auxiliary switching; there are no dedicated NC or NO contacts, meaning both outputs are form-C and can be wired for trip and pre-alarm independently. The LED display gives a quick visual on trip status, useful for troubleshooting without a meter.
Width is 22.5 mm, the standard single-module DIN-rail footprint, so it occupies one slot on a TS35 rail. Fastening method is screw and snap-on mounting — the screw provides a positive lock in high-vibration panels, while snap-on is faster for standard cabinets. Depth of 91 mm and height of 103 mm fit most standard enclosures.
The unit includes a reclosing lockout function, which prevents automatic restart after a trip until the lockout is manually reset — important for applications where an unexpected restart could be hazardous.