What it is and what it does
The Siemens 3RU2136-4KB0 is a SIRIUS thermal overload relay — size S2, CLASS 10A trip curve, designed to protect AC motors against overload and phase failure. It mounts directly to a contactor (contactor mounting, any position) and uses a bimetallic thermal element to track motor heating. The integrated auxiliary switch provides a 'tripped' signal contact for the PLC or annunciator. Rated for main circuits up to 690 V, it carries 73 A at both 480 V and 600 V, and supports motor loads up to 55 kW at 690 V (37 kW at 400 V, 45 kW at 500 V). That 73 A continuous rating at the higher voltages means it handles the full motor FLA for a 55 kW 690 V machine without derating — useful for 690 V drives in European or mining installations.
Trip class and temperature compensation — why they matter up-tower
CLASS 10A means the relay trips within 10 seconds at 7.2× the set current — fast enough to protect a motor winding during a stall or jam, but not so fast it nuisance-trips on normal starting inrush for a pump or fan. In a wind turbine nacelle where a pitch motor might see brief overloads during gust response, CLASS 10A gives the motor time to ride through without dropping the blade control circuit. Temperature compensation is active from -40 to +60 °C, and the relay itself operates from -40 to +70 °C. That full compensation range means the trip point stays accurate whether the relay is in a heated control cabinet or mounted in an unheated nacelle at hub height — no false trips from a cold-soaked bimetal, no delayed trip in a hot panel.
Mounting and wiring — panel reality
Mounts directly to the contactor — no separate DIN-rail footprint needed, which saves a slot in a crowded panel. Screw terminals for the main circuit, sized for 2× (0.5 to 2.5 mm²) solid or stranded. The screwdriver spec is Pozidriv PZ2, shaft diameter 5–6 mm — standard electrician's kit. Fastening torque for the M6 main contact bolts is not listed but follows Siemens standard values for the S2 frame. Width is 55 mm, depth 105 mm, height 90 mm. That's the S2 envelope — it mates with SIRIUS contactors in the same frame size.
Lifecycle and compliance — current production
Substance prohibitance date is October 15, 2014 — that's the RoHS compliance cutoff for the EU directive. The relay is RoHS-compliant and REACH-registered through Siemens' standard SIRIUS declaration. Storage temperature range -55 to +80 °C covers cold-chain shipping and warehouse storage; transport range is the same.