What it is and what it does
The Siemens 3RU2126-4CJ0 is a SIRIUS thermal overload relay, size S0, designed to protect motors against overload and phase failure. It's a CLASS 10 device, meaning it will trip within 10 seconds at 7.2x the set current — fast enough to protect standard induction motors during a stall or jam. Rated for 690 V, it handles motor loads up to 11 kW at 400 V and 18.5 kW at 690 V. The thermal bimetallic release is temperature-compensated from -40 to +60 °C, so the trip curve stays consistent whether the panel is in a cold warehouse or a hot machine room.
Where it fits
Mounts directly onto a contactor via the contactor mounting fastening method — no DIN rail required, though the S0 footprint is standard for the SIRIUS family. Dimensions are 85 mm deep, 45 mm wide, 85 mm high, so it slots into a compact motor starter combination without eating up panel real estate. Main circuit connections use ring cable lugs (M4 screw size), and the auxiliary switch is integrated. A Pozidriv PZ 2 screwdriver with a 5–6 mm shaft diameter is what you'll need for the terminals.
What the ratings mean for fit
The switching ratings across the voltage range tell you what auxiliary contacts can handle: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V, 0.75 A at 690 V. These are for the integrated auxiliary switch, not the main motor circuit — the main path is rated for the full motor kW figures above. If you're using the auxiliary contact to signal a PLC input, check the voltage and current draw against these numbers. Trip class CLASS 10 is the key selection parameter: it matches standard squirrel-cage motor starting profiles. If your application has a long acceleration time (high-inertia loads like fans or flywheels), you'd typically step up to CLASS 20 or CLASS 30 — but for most conveyors, pumps, and compressors, CLASS 10 is the right call.