Siemens SIRIUS 3RB2 Electronic Overload Relays

Siemens SIRIUS 3RB2 Electronic Overload Relays Suppliers, Exporters in India

Siemens SIRIUS 3RB2 Electronic Overload Relays protect large three-phase motors against overload, phase failure and phase imbalance. They are generally used in high-current motor starters, MCC panels and industrial equipment where the smaller 3RB3 range does not cover the required motor current.

The 3RB2 range covers motor currents from 50 A to 630 A. Size S6 models are available for 50–200 A, while S10/S12 versions cover 55–250 A and 160–630 A. They can be mounted directly with a compatible Siemens SIRIUS contactor or installed separately inside the panel.

The relay measures the current flowing through all three phases. If the motor remains overloaded, loses one phase or operates with excessive current imbalance, the relay opens the contactor control circuit. A separate signalling contact can send the trip condition to a PLC, alarm or indication lamp.

Siemens offers the 3RB2 range in two main versions. 3RB20 relays come with a fixed Class 10E or Class 20E trip characteristic. 3RB21 relays allow the trip class to be selected from Class 5E, 10E, 20E or 30E. The adjustable versions also provide internal ground-fault detection for suitable applications.

ARP IMPEX supplies and exports Siemens SIRIUS 3RB2 Electronic Overload Relays for large motor starters, process plants, industrial machinery and replacement requirements. Share the complete Siemens part number, motor current, contactor model and required quantity for price and availability.

Technical Specifications

• Brand: Siemens
• Product range: SIRIUS
• Series: 3RB2
• Product type: Electronic overload relay
• Application: Protection of large three-phase AC motors
• Protection functions: Overload, phase failure and phase imbalance
• Main versions: 3RB20 and 3RB21
• Available frame sizes: S6 and S10/S12
• Current ranges: 50–200 A, 55–250 A and 160–630 A
• Trip class – 3RB20: Fixed Class 10E or Class 20E
• Trip class – 3RB21: Adjustable Class 5E, 10E, 20E or 30E
• Ground-fault monitoring: Available on 3RB21 versions
• Rated operating frequency: 50/60 Hz
• Rated operating voltage: Up to 1000 V on applicable models
• Current setting: Adjustable according to the motor nameplate current
• Reset options: Manual or automatic reset
• Remote reset: 24 V DC electrical remote reset on applicable 3RB21 models
• Auxiliary contacts: One NC contact for contactor disconnection and one NO contact for trip indication
• Power supply: Internally powered through the motor circuit
• Main circuit connection: Busbar connection or straight-through current transformer, depending on the model
• Auxiliary terminal options: Screw or spring-loaded terminals
• Mounting: Contactor mounting or separate panel installation
• Operating temperature: Generally -25°C to +60°C
• Compatible equipment: Siemens SIRIUS contactors and motor starter assemblies

The exact current range, trip class, terminal arrangement and mounting method depend on the selected Siemens article number.

Siemens 3RB20 Electronic Overload Relays

Siemens 3RB20 relays are suitable when the motor starting condition is already known, and a fixed trip class is sufficient.

Class 10E versions are generally selected for motors with normal starting times. Class 20E versions allow more time for motors connected to loads that take longer to accelerate.

Common 3RB20 ranges include:

• 50–200 A in size S6
• 55–250 A in size S10/S12
• 160–630 A in size S10/S12
• Fixed Class 10E versions
• Fixed Class 20E versions
• Busbar and straight-through transformer models
• Screw and spring-type auxiliary terminals

These versions work well for pumps, fans, compressors and other large motors where the starting time remains reasonably consistent.

Siemens 3RB21 Electronic Overload Relays

Siemens 3RB21 relays provide more flexibility for motors with different starting conditions. The trip class can be set to Class 5E, 10E, 20E or 30E from the front of the relay.

They also include internal ground-fault detection. This function can help identify leakage or insulation problems before they develop into a more serious fault. Ground-fault monitoring must be applied according to the starter arrangement and should not be activated in unsuitable circuits, including certain star-delta configurations.

Applicable 3RB21 models also support electrical remote reset using a 24 V DC signal. This is useful where the MCC panel is controlled from a PLC, control room or remote operating station.

Features

• Covers large motor currents up to 630 A
• Electronic current measurement provides stable overload protection
• Protects against prolonged overload, phase loss and phase imbalance
• Wide current-setting ranges reduce the number of models required
• Fixed and adjustable trip-class versions are available
• Class 5E to Class 30E options suit different starting times
• Internal ground-fault detection is available on 3RB21 models
• Manual reset helps prevent an unexpected motor restart
• Automatic reset can be selected where the control system safely permits it
• Remote electrical reset is available on applicable versions
• One NC contact disconnects the motor contactor after a trip
• One NO contact provides a fault signal to a lamp, alarm or PLC
• Front trip indication helps maintenance teams identify the relay operation
• TEST function allows the relay and control wiring to be checked
• No separate auxiliary supply is required for basic overload protection
• Busbar and straight-through transformer versions suit different panel arrangements
• Direct and separate mounting options are available
• Low internal power loss helps limit heat inside the MCC panel

The 3RB2 auxiliary contacts have sufficient switching capacity to operate a contactor coil directly and can also provide a trip signal to a PLC input.

Applications

Siemens SIRIUS 3RB2 Electronic Overload Relays are commonly used in:

• Large pump motors
• Water and wastewater pumping stations
• Industrial air compressors
• Cooling tower fans
• Large ventilation and exhaust fans
• Crushers and screening equipment
• Belt conveyors
• Bucket elevators
• Mixers and agitators
• Mills and process machinery
• Material-handling systems
• Mining and quarry equipment
• Cement plant machinery
• Steel plant equipment
• Oil and gas facilities
• Chemical and petrochemical plants
• Marine and port equipment
• Large HVAC and refrigeration systems
• Industrial motor control centres
• Motors with long starting times
• High-current starter replacement work

Selecting the Correct Siemens 3RB2 Relay

The relay setting range must cover the rated current shown on the motor nameplate. Motor kilowatt rating alone is not sufficient because the full-load current changes with operating voltage, efficiency, speed and motor design.

The trip class must also match the actual starting time. A pump or fan with a light starting load may work with Class 10E, while a crusher, conveyor or high-inertia machine may require Class 20E or a selectable Class 5E–30E relay.

For accurate selection, check:

• Motor rated current
• Motor power and operating voltage
• Starting method
• Actual motor starting time
• Required trip class
• Existing Siemens contactor part number
• Size S6 or S10/S12 requirement
• Busbar or straight-through transformer connection
• Fixed or adjustable trip class
• Ground-fault monitoring requirement
• Manual, automatic or remote reset
• Screw or spring-type auxiliary terminals

In a star-delta starter, the relay setting depends on where the overload relay is fitted. When it is installed in the delta circuit, the setting normally follows the current at that position rather than the full motor line current. Siemens notes that an overload relay mounted with the line contactor is set differently, and internal ground-fault detection must not be used in certain star-delta assemblies.

Important Protection Note

The Siemens 3RB2 protects the motor against overload and phase-related faults. It does not replace the fuse, circuit breaker, or other short-circuit protective device required in the motor circuit.

The 3RB2 range can be used on the incoming side of a frequency converter. Protection on the motor side of a drive may require a different arrangement, such as direct winding-temperature monitoring with a thermistor protection relay.

Siemens 3RB2 Electronic Overload Relay Supplier and Exporter

ARP IMPEX supplies Siemens SIRIUS 3RB2 Electronic Overload Relays for MCC manufacturers, panel builders, EPC contractors, industrial plants and maintenance companies.

The range includes 3RB20 fixed Class 10E and Class 20E relays as well as 3RB21 adjustable Class 5E–30E versions. Siemens Industry Mall continues to list active 3RB2 products in S6 and S10/S12 sizes for currents up to 630 A.