What it is and what it does
The Siemens 3RU7136-4DB0 is a SIRIUS thermal overload relay in the S2 frame size, rated for a continuous operational current of 25 A and a trip class of Class 10. It mounts directly onto a contactor of the same S2 size, protecting a three-phase motor circuit against sustained overloads by sensing the heat generated in the bimetallic strip.
Key ratings and what they mean for fit
The 25 A rated current sets the maximum continuous load the relay can carry without nuisance tripping; for a motor circuit, you size the relay to the motor's full-load amps, typically 1.0 to 1.05× FLA. The Class 10 trip characteristic means it will open within 10 seconds at 7.2× the current setting — standard for standard-duty induction motors starting unloaded or with low-inertia loads. The S2 frame is the mechanical envelope; it physically matches the S2 contactor footprint, so the relay clips onto the contactor without extra brackets. The relay accepts screw-type terminals for both the main and auxiliary circuits. Wire range is 2× (0.5 to 1.5 mm²) solid or stranded, or 2× (0.75 to 2.5 mm²). Terminal torque for the main circuit is 3 to 4.5 N·m; for auxiliary contacts, 0.8 to 1.2 N·m. The front face carries IP20 protection; the terminal area is IP00, so the panel environment must keep conductive debris away from the live terminals.
Where it goes and how it mounts
This relay is designed for contactor mounting — it clips onto the load side of a SIRIUS S2 contactor, forming a compact motor starter assembly on a DIN rail or mounting plate. The mounting position allows a vertical mounting surface with ±135° rotation and ±45° tilt forward/backward, giving flexibility in panel layout. Side spacing to grounded parts must be at least 6 mm. Operating temperature range is -20 to +55 °C; storage/transport range is -25 to +70 °C.