What it is and what it does
The Siemens 3RU2126-1CC0 is a SIRIUS thermal overload relay in Size S0, designed to protect motor circuits against prolonged overloads and phase loss. It mounts directly to a contactor, making it a matched pair in a motor starter assembly. The Trip Class 10 rating means 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 event without nuisance tripping during normal start-up.
Key ratings and what they mean for fit
Rated for 690 V at 50/60 Hz, this relay covers 400 V, 480 V, 500 V, and 690 V motor systems. The power ratings at each voltage tell you the maximum motor it can protect: 0.75 kW at 400 V, 1.1 kW at 500 V, and 1.5 kW at 690 V. The auxiliary contact is integrated and rated 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, and 1 A at 400 V — these are the switching capacities for the trip-signal circuit, not the motor current. The thermal release design means ambient temperature affects the trip curve; temperature compensation is active from -40 to +60 °C, so the relay holds its calibration across the panel's operating range. Power dissipation is 1.9 W per pole, which adds about 5.7 W total heat inside the enclosure. In a dense panel with multiple starters, that heat load matters for the thermal design — account for it in the ventilation or derating calculation.
Mounting and wiring
Fastens directly to the contactor — no DIN rail required for the relay itself, though the contactor typically snaps onto one. The main circuit uses spring-loaded terminals accepting 2× (0.5 to 2.5 mm²) solid or stranded wire. A 3 mm diameter screwdriver shaft (3.0 × 0.5 mm tip) releases the spring clamp. Mounting position is any, which simplifies layout in tight cabinets. Dimensions: 45 mm wide, 102 mm high, 84 mm deep — Size S0 footprint.