What it is and what it does
The Siemens 3RU2146-4KB0 is a SIRIUS Size S3 thermal overload relay — the part that sits between the contactor and the motor, watching current and tripping on a sustained overload before the winding gets cooked. CLASS 10 trip characteristic means it will open within 10 seconds at 7.2× the set current, which is the standard match for standard squirrel-cage motors starting across the line. If you are protecting a motor that starts hard (high-inertia load, long acceleration), CLASS 10 may be too fast — but for pumps, fans, and compressors on a typical 400 V line, this is the correct class.
Motor power coverage and what the ratings mean
At 400 V it is rated for 37 kW motor load; at 500 V that climbs to 45 kW, and at 690 V to 55 kW. The corresponding current ratings are 65 A at 480 V and 62 A at 600 V — so the relay is sized for the motor full-load current, not the supply voltage alone. If your motor nameplate says 37 kW at 400 V, this relay covers it. The auxiliary contact ratings cover 2 A at 24 V and 230 V, 3 A at 120 V, and 0.75 A at 690 V — enough to signal a trip to the PLC or annunciator without an interposing relay.
Mounting and wiring
Mounts directly onto the contactor — no DIN rail required, no extra bracket. The fastening method is contactor mounting, using M8 bolts for the main circuit. Screw-type terminals on the main circuit accept solid conductors from 2.5 to 16 mm² (two per clamp) and stranded up to 70 mm² on the larger clamp. The auxiliary circuit uses smaller terminals: 0.5 to 2.5 mm² solid or stranded. The screwdriver drive is a 4 mm hexagon socket — keep that bit in your panel-building kit.
Environmental and compliance
Operates from -40 to +70 °C, stores and ships from -55 to +80 °C. Temperature compensation is active from -40 to +60 °C — so the trip curve stays accurate across the working range. The relay can be mounted in any position, which helps when the panel layout is tight. RoHS compliance date is 03/01/2017, meaning it meets the substance restrictions in effect from that date.