What it is and what it does
The Siemens 3RU2126-4DJ0 is a SIRIUS thermal overload relay, Size S0, designed to protect motors against overload and phase failure. It mounts directly onto a contactor, saving panel space and simplifying wiring. Trip Class 10 means it will trip within 10 seconds at 7.2× the current setting — fast enough to protect standard induction motors during a locked-rotor condition, without nuisance tripping on normal start-up. Rated for 690 V, it covers 400 V and 480 V motor circuits common in European and North American industrial panels. The 50/60 Hz operating frequency matches standard mains supplies.
Key ratings and what they mean for fit
Dimensions are 85 mm deep, 45 mm wide, and 85 mm high — a compact footprint that fits standard DIN-rail contactor assemblies. The contactor-mount fastening method means no additional DIN-rail clip is required; it snaps onto the contactor's own mounting base. Auxiliary switch is integrated, saving an add-on block for status feedback. The thermal design compensates for ambient temperature from -40°C to +60°C, so the trip curve stays accurate in hot enclosures near motors.
Sourcing and lifecycle
It is a direct-mount companion to Siemens SIRIUS contactors of the same S0 frame size. When sourcing, verify the contactor's mounting interface matches the relay's base — the S0 frame is a standard Siemens platform, so fit is consistent across the SIRIUS range.
Integration notes
Mounting position is any, which simplifies panel layout — the relay can be oriented vertically, horizontally, or even upside-down without affecting the trip curve. Main circuit connections accept ring cable lugs; screw terminals with a Pozidriv PZ2 tip are used for the auxiliary circuit. The screwdriver shaft diameter is 5–6 mm, so a standard electrician's screwdriver fits. The relay includes a 'Tripped' message indicator (note on the front), giving immediate visual confirmation of an overload condition without needing to open the panel door.