The Siemens 3RU2126-4FB0 is a premium thermal overload relay from the trusted SIRIUS 3RU2 Series, engineered to provide dependable protection for three-phase motors against overload conditions, phase failure, and prolonged overcurrent operation. Designed for industrial motor control applications, this relay features an adjustable current setting range of 34–40 A, Class 10 tripping characteristics, and compatibility with Size S0 Siemens contactors, making it ideal for compact motor starter assemblies and industrial automation systems.
The relay operates using Siemens' proven thermal overload protection technology, continuously monitoring motor current and disconnecting the motor when overload conditions persist. Its inverse-time delayed tripping characteristic helps prevent motor overheating, insulation damage, and premature motor failure while allowing normal motor starting currents. The relay also provides phase-failure sensitivity for enhanced motor protection and operational reliability.
The Variant FB0 offers an adjustable overload release setting of 34–40 A, making it suitable for protecting motors used in pumps, compressors, conveyor systems, fans, HVAC equipment, machine tools, packaging machinery, and industrial automation installations. Its compact S0 frame size enables efficient integration into motor control panels, MCCs, and motor starter combinations.
As part of the Siemens SIRIUS motor protection portfolio, the 3RU2126-4FB0 supports both manual and automatic reset modes and includes 1 Normally Open (NO) and 1 Normally Closed (NC) auxiliary contact for trip indication, signaling, and control circuit integration. The relay utilizes screw-type terminals for secure wiring and dependable electrical connections.
With a rated insulation voltage of 690 V, surge voltage resistance of 6 kV, operating temperature range of -40°C to +70°C, and compact industrial-grade construction, the relay delivers long-term reliability in demanding environments.
| Specification | Details |
|---|---|
| Product Name | Thermal Overload Relay |
| Part Number | 3RU2126-4FB0 |
| SKU | 3RU2126-4FB0 |
| Brand | Siemens |
| Product Family | SIRIUS 3RU2 |
| Product Type | Motor Protection Relay |
| Variant | FB0 |
| Current Setting Range | 34–40 A |
| Relay Size | S0 |
| Trip Class | Class 10 |
| Operating Principle | Thermal |
| Mounting Type | Direct Contactor Mounting |
| Main Circuit Connection | Screw Terminal |
| Auxiliary Circuit Connection | Screw Terminal |
| Auxiliary Contacts | 1 NO + 1 NC |
| Reset Type | Manual / Automatic |
| Rated Insulation Voltage | 690 V |
| Surge Voltage Resistance | 6 kV |
| Rated Operational Voltage | Up to 690 V AC |
| Operating Frequency | 50–60 Hz |
| Number of Main Poles | 3 |
| Operating Temperature | -40°C to +70°C |
| Protection Features | Overload & Phase Failure Protection |
| Application | Industrial Motor Protection |
| Manufacturer | Siemens |
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