Integrator's brief — what this relay does and where it fits
The Siemens 3RU2126-4EC1 is a SIRIUS thermal overload relay, size S0, designed to protect motors against overload and phase failure. It is a CLASS 10 device, meaning it will trip within 10 seconds at 7.2× the set current — fast enough to protect standard induction motors during a stalled rotor or prolonged start. Rated for 690 V, it covers most 400 V and 690 V industrial supply systems. The relay mounts stand-alone or onto a SIRIUS contactor, and the spring-loaded terminals on the main circuit accept 2x (0.5... 2.5 mm²) solid or stranded wire, so panel wiring is tool-less for the power connections.
Trip class and thermal design — what CLASS 10 means for the motor
CLASS 10 is the standard trip class for general-purpose squirrel-cage motors. It coordinates with a contactor to clear a locked-rotor condition before the motor winding temperature exceeds its insulation class. The thermal overload release is bimetallic, temperature-compensated from -40 to +60 °C, so ambient shifts in the panel do not shift the trip point. Per-pole power dissipation is 3.2 W, which matters when grouping multiple relays in a small enclosure — derating may be needed if the internal ambient climbs above 40 °C.
Mounting and integration notes
The relay is size S0, with a width of 45 mm, height 114 mm, and depth 95 mm. It can be installed in any mounting position — no derating required for side or inverted orientation. The auxiliary switch is integrated, so no separate add-on block is needed for the trip/alarm signal. The screwdriver shaft for the terminal screws is 3 mm diameter, tip size 3.0 x 0.5 mm. For the auxiliary circuit, the relay carries ratings across common control voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V, and 0.75 A at 690 V.