SIRIUS 3RN2 Thermistor Motor Protection Relay
The Siemens SIRIUS 3RN2012-1BA30 is a thermistor motor protection relay designed to monitor motor winding temperature via PTC thermistors, tripping the output when the motor exceeds safe thermal limits. It is a standard evaluation unit with ATEX approval, open-circuit and short-circuit detection in the sensor circuit, and non-volatile error memory. The response threshold is set at 0.85 of the full-scale value, with a release value of 1.1, meaning the relay trips when the thermistor resistance reaches the upper threshold and resets when it drops back below the lower threshold. The output contacts are rated for 1 A at 24 V, 0.2 A at 125 V, and 0.1 A at 250 V. The relay supports auto-reset and external reset functions, and it includes galvanic isolation between the input and output circuits as well as between the outputs.
Mounting and Environmental Ratings
It can be installed in any position, with zero clearance required on all sides (upwards, downwards, forwards, backwards, and at the side). The IP20 protection class allows installation in clean, dry control cabinets.
The relay carries ATEX approval for use in potentially explosive atmospheres, and its design includes open-circuit and short-circuit detection in the sensor circuit — a critical feature for safety-instrumented functions where a failed sensor must not mask a motor over-temperature condition. With an MTBF of 97 a and MTTFd of 303 a, the relay offers high reliability for continuous-duty applications. The non-volatile error memory retains the last fault state even after power loss, aiding diagnostics in remote or unattended installations.