What it is and what it does
The Siemens SIRIUS 3RU2126-1HJ0 is a thermal overload relay in the S0 frame size, designed for contactor mounting in motor control circuits. It provides Class 10 trip protection, meaning it will trip within 10 seconds at 7.2× the thermal current setting — fast enough to protect standard squirrel-cage induction motors during a locked-rotor start, but without nuisance tripping on normal acceleration. Rated insulation voltage is 690 V, and it operates on 50/60 Hz line frequencies, covering standard industrial supply voltages worldwide. The thermal bimetallic release is temperature-compensated from -40 to +60 °C, so the trip curve stays consistent whether the panel sits in a cold warehouse or near a hot process line.
Ratings that matter for fit
The auxiliary contact set includes a normally-closed 'tripped' signal (Note: for message 'Tripped') that gives the PLC a clean status when the relay has operated. Switching capacity is listed across multiple voltages: 2 A at 24 V and 230 V, 3 A at 120 V, 1 A at 400 V, and 0.22 A at 125 V — these cover typical 24 VDC control circuits and 120/230 VAC indication lamps. Power dissipation per pole is 2.2 W, which matters for enclosure thermal budgeting when multiple relays are ganged on a bus. The S0 frame is physically compact at 45 mm wide, 85 mm deep, and 85 mm tall — fits standard 45 mm DIN-rail spacing with contactor footprint. Terminals accept ring cable lugs on the main circuit (M4 screw) and Pozidriv PZ2 screwdriver tip for the auxiliary connections. Mounting position is any orientation, which simplifies panel layout in tight enclosures.