The Siemens SIRIUS 3RU6126-1CB0 is a thermal overload relay for motor protection, adjustable from 1.8 A to 2.5 A, Trip Class 10, in the size S0 footprint. It mounts directly onto a contactor (contactor mounting) and protects the motor by thermally sensing overcurrent — the bimetallic strip follows the motor heating curve, so a sustained overload above the set point trips the auxiliary contact and drops out the contactor coil.
Adjustable current range 1.8…2.5 A sets the trip threshold — set it to the motor nameplate FLA. Class 10 means the relay will trip within 10 seconds at 600 % of the set current, which is the standard for standard-duty induction motors (pumps, fans, compressors) that don't need the longer start-time allowance of Class 20 or 30. Rated insulation voltage 690 V AC and surge voltage resistance 6 kV mean it's rated for 400 V and 480 V line-to-line systems with margin, and it will withstand transient spikes common in industrial panels without flashover. Operating temperature range –40 to +70 °C covers unheated enclosures and hot panel interiors; the relay derates thermally above 40 °C per the curve, but the –40 °C floor means it starts reliably in cold warehouses or outdoor cabinets.
Size S0 is the standard Siemens contactor frame for currents up to roughly 40 A AC-3. The 3RU6126-1CB0 snaps onto the contactor (contactor mounting) — no DIN rail needed, though the contactor itself may be DIN-rail or screw-mounted. Accepts 2 x (0.5…1.5 mm²) or 2 x (0.75…2.5 mm²) solid/stranded, and AWG equivalents. Any mounting position is allowed.