What it is and what it does
The Siemens 3RU1126-1JD1 is a SIRIUS thermal overload relay — the part that sits between a contactor and the motor, watching current and tripping if the motor draws too much for too long. Class 10 trip curve means it will open within 10 seconds at 7.2× the set current, which is the standard for standard-start induction motors (pumps, fans, compressors). Size S0 frame, so it mates with S0 contactors in the SIRIUS family. Rated for main circuit voltages up to 690 V AC-3, with screw-type terminals accepting 1x (16…12) or 1x (14…8) AWG for the power connections. The auxiliary contact block is separate — this relay ships with zero auxiliary contacts built in, so you add a block if you need a signal back to the PLC.
What the ratings mean for fit
Trip class 10 is the key selection parameter: it means the relay will not nuisance-trip on normal motor inrush (which typically decays in a few seconds) but will protect the motor winding from sustained overload. If your motor starts against a heavy load or has a long acceleration time (centrifuge, crusher), you may need Class 20 or 30 — this is not that relay. The S0 frame size dictates the physical envelope: 45 mm wide, 97 mm tall, 96 mm deep. That 96 mm depth is the dimension you need to clear inside the enclosure — it projects forward from the mounting surface. Stand-alone installation, mountable on a vertical surface with ±135° rotation and ±45° tilt front/back, so panel layout is flexible. IP20 on the front means finger-safe from the operator side, but the terminals are not sealed — this is a panel-inside component, not for washdown zones. Operating temperature range of -20 to +70 °C covers most indoor industrial environments; storage and transport range extends to -55 to +80 °C. Power dissipation is 6.6 W total at hot operating state (2.2 W per pole). That matters for enclosure thermal calculations — if you pack multiple relays in a small cabinet, you need to account for the heat. Surge voltage resistance rated at 6 kV, which aligns with IEC 60947-1 overvoltage category III for industrial control panels.
Where it is used
Thermal overload relays of this class are the standard motor protection component in motor control centers (MCCs) and individual starter enclosures. They protect three-phase induction motors in conveyors, pumps, fans, compressors, and machine tools. The ATEX approval (DMT 98 ATEX G 001) means it can be used in potentially explosive atmospheres — typically Zone 2 gas or Zone 22 dust, when paired with an appropriate contactor and enclosure. The 100 % relative humidity rating means it can operate in condensing environments (e.g., unheated warehouses, washdown areas if the enclosure is rated), though the IP20 front limits where it can be mounted exposed.
Sourcing and lifecycle reality
For a BOM freeze or a panel build, this is a straightforward line item: one Siemens order code, no cross-reference needed. If you are sourcing for a new design, the S0 frame and Class 10 curve are widely used, so supply depth is good. Submit an RFQ for current pricing and lead time.