What it is and what it does
The Siemens 3RU2126-4BJ0 is a SIRIUS thermal overload relay, size S0, designed to protect motors against overload and phase failure by monitoring motor current and tripping a contactor via its integrated auxiliary switch. It is a Class 10 trip device, meaning it will trip within 10 seconds at 7.2× the set current — fast enough to protect standard induction motors during a stall or locked-rotor condition. Rated for 690 V and 50/60 Hz operation, it covers the common industrial voltage range up to 690 V line-to-line.
Where it fits and how to mount it
Mounts directly onto a contactor via the contactor mounting fastening method — no DIN rail needed, saving panel space. Accepts any mounting position. The S0 size (45 mm wide, 85 mm deep, 85 mm high) is the standard footprint for SIRIUS contactors in this current range, so it swaps in without re-drilling the panel. Main circuit connections use ring cable lugs with M4 screws; the auxiliary switch is integrated, no separate block needed.
Key ratings and what they mean for your motor
The thermal overload release is temperature-compensated from -40 to +60 °C, so the trip curve stays accurate even in a hot panel or cold environment. Auxiliary contact ratings are given at multiple voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V — these are the switching capacities for the signal circuit feeding the PLC or contactor coil. Power dissipation per pole is 2.7 W, which matters for thermal management inside a sealed enclosure — three poles dissipate about 8.1 W total. MTTF of 2,280 years under high demand rate.
Lifecycle and sourcing reality
The note "for message 'Tripped'" on the relay indicates the auxiliary contact position that signals a trip condition to the control system — a standard feature, not a special marking. Substance prohibitance date of 10/01/2009 confirms RoHS compliance.