What it is and what it does
The Siemens 3RU2126-1GJ0 is a SIRIUS thermal overload relay, size S0, designed to protect motors against overload and phase failure by monitoring current and tripping a contactor. Its thermal bimetallic release mechanism responds to heating from the motor current, and the Trip Class 10 rating means it will trip within 10 seconds at 7.2× the set current — fast enough to protect standard induction motors during a stalled-rotor condition. Rated for 690 V, this relay mounts directly onto a contactor (contactor mounting per), so it integrates into the motor starter stack without extra wiring or a separate enclosure. The auxiliary switch is integrated, saving a slot in the panel for a separate aux block.
Key ratings and what they mean
The Trip Class 10 is the headline spec here — it defines how fast the relay opens on a locked-rotor event. For a motor driving a pump or conveyor, Class 10 gives enough time to ride through normal starting inrush without nuisance trips, but still protects the winding from overheating during a stall. If your application needs faster protection (like on a high-inertia fan), you'd step up to Class 5 or 10A, but Class 10 covers the vast majority of standard motor starts. The auxiliary contact ratings span the common control voltages you'll find in a panel: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, and 1 A at 400 V. These are the currents the built-in aux switch can break — enough to drive a PLC input or a contactor coil directly at those voltages. At 690 V the aux contact is rated 0.75 A, so if you're switching a 690 V control circuit, keep the load under that. Power dissipation per pole is 2.2 W. In a densely packed panel with several overload relays side by side, that heat adds up — factor it into your enclosure ventilation calculation, especially if ambient hits 40 °C or higher.
Where it fits in the panel
Size S0 is the mid-frame in the SIRIUS overload relay family — it mates with S0 contactors like the 3RT102 or 3RT103 series. The dimensions are 45 mm wide, 85 mm deep, 85 mm high, so it slides onto the contactor's mounting base without taking extra DIN-rail space. Mounting position is any, which helps when you're retrofitting into a cramped enclosure and can't guarantee vertical orientation. Main circuit connections use ring cable lugs with M4 screws. The screwdriver spec is Pozidriv PZ2 with a 5–6 mm shaft diameter — if your kit only has Phillips, grab a PZ2 bit before you start; Pozidriv cam-out is different and you'll strip the head with a Phillips driver.
Lifecycle and sourcing reality
Temperature compensation is rated from -40 °C to +60 °C, which means the trip curve stays accurate across that range — useful if the panel sits in an unheated warehouse or near a furnace. Storage and transport range is -55 °C to +80 °C, so it can sit in a cold van overnight without issue.