What it is and what it does
The Siemens 3RU2126-4AC0 is a SIRIUS thermal overload relay — size S0, Trip Class 10 — designed to protect motors against sustained overcurrent by monitoring the current draw and tripping the associated contactor when the thermal model exceeds the threshold. Class 10 means it must trip within 10 seconds at 7.2× the current setting, which is the standard for standard-duty induction motors that don't need the extended start time of Class 20 or 30 — common for pumps, fans, and compressors on a general-purpose line. Rated for a maximum of 690 V, the relay carries a 7.5 kW motor at 400 V and 11 kW at 690 V, so the same unit covers both 400 V and 690 V networks without a change in hardware — just the setting dial.
Mounting and wiring
Mounts directly onto the contactor via the fastening method — no DIN rail required, which saves a rail slot and keeps the motor starter assembly compact. Any mounting position is permitted, so panel layout is flexible. The main current circuit uses spring-loaded terminals, accepting 2 × 0.5 to 2.5 mm² solid or stranded wire. No screw torque to verify — just strip to 8 mm and push in. The auxiliary switch is integrated, so no separate block to order for the trip signal. Dimensions are 45 mm wide, 102 mm high, 84 mm deep — size S0 footprint.
Environmental and lifecycle
Storage and transport range is -55 to +80 °C, which covers unheated warehouses and truck trailers. MTTF at high demand rate is 2 280 years — a reliability figure that reflects the passive thermal design with no moving electronics to fail. Substance prohibition date of 10/01/2009 indicates RoHS compliance by that date.