The Siemens SIRIUS 3RU6126-4DB1 is a thermal overload relay for motor protection, adjustable from 20 to 25 A and rated for a 3-pole main circuit at up to 690 V, 50-60 Hz. Sized for the S0 contactor footprint, it mounts standalone or directly onto a SIRIUS S0 contactor, sharing the same DIN-rail or screw-mount pattern.
The adjustable current range of 20-25 A sets the motor full-load current (FLC) it protects; you dial in the exact nameplate FLC. Class 10 trip characteristic means the relay clears a fault before the motor winding insulation degrades — critical for pumps, fans, and compressors that see repetitive starts. The S0 size (45 mm wide, 97 mm tall, 95 mm deep) fits standard 45 mm DIN-rail spacing; the 6 mm side clearance to grounded parts is a minimum for air circulation and arc flash mitigation, not a suggestion. Screw terminals on both main and auxiliary circuits accept 2x (0.5-1.5 mm²) solid/stranded or 2x (0.75-2.5 mm²) with ferrules, so it plays nice with existing panel wiring without adapters. Surge voltage rating 6 kV.
Substance prohibitance per RoHS and REACH is declared as of May 1, 2012, meaning it meets EU substance restrictions for ongoing compliance documentation.
Mounts in any position — horizontal, vertical, or flat — no derating required for orientation. Standalone installation means it doesn't need a contactor underneath; you can bolt it directly to the panel backplate or DIN rail. When combined with an S0 contactor, the relay clips onto the contactor's frame, sharing the power connections through the contactor's through-hole terminals — that saves wiring time and reduces point-to-point failure risk. Tightening torque for main screw terminals is 2-2.5 N·m; auxiliary terminals follow the same spec. Use a torque screwdriver; overtightening strips the brass threads. That's low enough for a sealed enclosure without active cooling, but if you're packing multiple relays in a small box, sum the heat and check the enclosure's thermal rise.