What it is and what it does
The Siemens 3RU2126-4FC1 is a SIRIUS thermal overload relay in the S0 frame size. It protects motors against overcurrent by monitoring the phase current and tripping a built-in auxiliary contact when the thermal model reaches the threshold. The Trip Class 10 rating means it will trip within 10 seconds at 7.2× the full-load current setting — standard for general-purpose motor starting where the acceleration time is short. The relay carries a rated insulation voltage of 690 V and operates at 50...60 Hz. It is temperature-compensated from -40 to +60 °C, so the trip curve stays accurate across the panel's ambient range. The auxiliary switch is integrated — no add-on module needed for the tripped signal. The note "for message "Tripped"" on the spec line refers to the auxiliary contact state that signals an overload condition back to the PLC or annunciator. That contact is what the controls integrator wires to the digital input for the motor-fault summary.
Mounting and wiring
Mounts in any position — stand-alone installation on a DIN rail or directly to the panel plate. The S0 footprint is 45 mm wide × 114 mm high × 95 mm deep, so it fits a standard 45 mm slot in a motor-starter row. Spring-loaded terminals on the main circuit accept 2× (0.5...2.5 mm²) solid or stranded copper — no screw torque to verify, just strip and push. The auxiliary contact ratings cover most control voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V. At 60 V it's 0.3 A; at 110 V and 125 V it's 0.22 A; at 220 V it's 0.11 A. Power dissipation per pole is 3.2 W — factor that into the enclosure thermal budget if you're packing several relays side by side.
Lifecycle and sourcing
The substance prohibitance date of 10/01/2009 aligns with the original EU RoHS compliance deadline. The relay is compliant with RoHS and REACH as a standard SIRIUS product. No special documentation beyond the standard declaration is required for most industrial panels.