Siemens SIRIUS 3RB3 Electronic Overload Relays

Siemens SIRIUS 3RB3 Electronic Overload Relays protect three-phase motors against overload, phase failure and phase imbalance. They are used with Siemens SIRIUS contactors in motor starters, MCC panels, machine control panels and other industrial motor circuits.

Unlike a thermal overload relay, the 3RB3 measures motor current electronically. This gives a wider current-setting range and allows better selection for motors with normal as well as heavy starting conditions. The series covers motor currents from 0.1 A to 115 A in frame sizes S00, S0, S2 and S3.

The relay is set according to the rated current shown on the motor nameplate. If the motor remains overloaded, loses a phase or operates with excessive phase imbalance, the relay opens the contactor control circuit and stops the motor. A separate signalling contact can send the trip status to a PLC, alarm or indication lamp.

Standard versions are available with fixed Class 10E or Class 20E tripping. Adjustable models allow the trip class to be selected from Class 5E, 10E, 20E or 30E. This makes the range suitable for ordinary pumps and fans as well as motors driving crushers, conveyors, mixers and other equipment with a longer starting time. Selected models also include internal ground-fault detection.

The 3RB3 takes its operating power from the motor circuit, so it does not need a separate auxiliary supply for its basic protection functions. It can be mounted directly below a compatible Siemens 3RT contactor or installed separately inside the panel.

ARP IMPEX supplies and exports Siemens SIRIUS 3RB3 Electronic Overload Relays for new motor starters, industrial projects and replacement requirements. Share the full Siemens part number, motor current, contactor model and required quantity for pricing and availability.

Technical Specifications

• Brand: Siemens
• Product range: SIRIUS
• Series: 3RB3
• Product type: Electronic overload relay
• Application: Protection of three-phase AC motors
• Current-setting range: 0.1 A to 115 A across the series
• Available frame sizes: S00, S0, S2 and S3
• Protection functions: Overload, phase failure and phase imbalance
• Ground-fault protection: Internal ground-fault detection on selected models
• Trip classes: Fixed Class 10E or Class 20E; adjustable Class 5E, 10E, 20E and 30E on selected models
• Operating frequency: 50/60 Hz
• Rated operating voltage: Up to 690 V, depending on the model
• Power supply: Internally powered through the motor circuit
• Current adjustment: Front rotary setting matched to the motor’s rated current
• Reset options: Manual or automatic reset
• Remote reset: Electrical remote reset on selected versions
• Auxiliary contacts: One NC contact for contactor disconnection and one NO contact for trip indication
• Mounting options: Direct contactor mounting or separate installation
• Main circuit connections: Screw terminals, spring-loaded terminals or straight-through transformer, depending on the model
• Auxiliary connections: Screw or spring-loaded terminals, depending on the article number
• Operating temperature: Generally -25°C to +60°C
• Temperature compensation: -25°C to +60°C
• Front protection: IP20
• Compatible equipment: Siemens SIRIUS 3RT contactors and motor starter assemblies

The exact current range, trip class, terminal type and mounting arrangement depend on the selected Siemens part number. For example, available models include 6–25 A relays in size S0, 20–80 A relays in size S2 and 32–115 A relays in size S3.

Features

• Electronic current measurement provides consistent motor overload protection.
• A broad current range reduces the number of relay models required for different motor ratings.
• Phase-failure protection helps prevent motor damage during single-phasing.
• Phase-imbalance monitoring protects motors against uneven current across the three phases.
• Fixed and adjustable trip-class options suit both normal and heavy motor starting.
• Selected models provide internal ground-fault detection.
• Manual reset prevents the motor from restarting automatically after a trip.
• Automatic reset is available where the machine control arrangement safely allows it.
• Selected versions support 24 V DC electrical remote reset.
• One NC contact disconnects the motor contactor after a fault.
• One NO contact provides a trip signal for a lamp, buzzer or PLC.
• The front current-setting dial allows the relay to be matched to the motor nameplate current.
• A visible trip indicator shows whether the relay has operated.
• The TEST function allows the protection and control wiring to be checked.
• Built-in self-monitoring checks important internal relay functions.
• Direct contactor mounting keeps the motor starter compact.
• Stand-alone versions suit panels where the overload relay cannot be fitted directly below the contactor.
• The relay requires no separate supply voltage for normal overload protection.
• Low internal power loss helps reduce heat inside the control panel.

Siemens provides fixed Class 10E and Class 20E models as well as adjustable Class 5E to Class 30E versions. Adjustable versions are useful when motor starting time varies considerably between applications.

Applications

• Siemens SIRIUS 3RB3 Electronic Overload Relays are commonly used in:
• Direct-on-line motor starters
• Reversing motor starters
• Star-delta starter panels
• Motor control centres
• Pump control panels
• Air compressor panels
• Industrial fans and blowers
• Conveyor systems
• Crushers and screening equipment
• Mixers and agitators
• Machine tools
• Packaging machinery
• Material-handling equipment
• HVAC and refrigeration plants
• Water and wastewater treatment systems
• Process plant machinery
• Production and assembly lines
• Hoists and lifting equipment
• Motors with long starting times
• Industrial maintenance and replacement work

Fixed and Adjustable Trip-Class Versions

Fixed Trip-Class Relays

Fixed Class 10E models suit motors with standard starting times, including many pumps, fans, machine tools and general industrial drives.

Class 20E versions allow a longer starting period before tripping. They are used where the motor takes more time to accelerate because of the connected load.

Adjustable Trip-Class Relays

Selected 3RB3 models allow the panel builder to choose Class 5E, 10E, 20E or 30E. These versions are useful where the starting condition is not the same across every machine or where a motor drives a load with high starting inertia.

The trip class should match the motor starting time and the equipment manufacturer’s recommendation. Selecting a higher trip class only to prevent nuisance tripping can leave the motor exposed to excessive heating.

Selecting the Correct Siemens 3RB3 Relay

The adjustable range of the overload relay must include the rated current shown on the motor nameplate. A motor drawing 22 A, for example, needs a relay whose setting range covers 22 A.

Motor power alone is not enough for accurate selection. Motors with the same kilowatt rating can have different full-load currents because of voltage, efficiency, speed and construction.

The following details should be checked before placing an order:

• Motor rated current
• Motor power and voltage
• Starting method
• Approximate starting time
• Required trip class
• Compatible Siemens contactor model
• Direct or stand-alone mounting
• Main and auxiliary terminal type
• Ground-fault detection requirement
• Manual, automatic or remote reset requirement

When a 3RB3 relay is used in a star-delta starter, its setting depends on where it is installed in the circuit. The current setting should follow the starter design rather than automatically being set to the full motor current.

Important Protection Note

The Siemens 3RB3 protects the motor against overload and phase-related faults, but it does not provide short-circuit protection. The circuit must also include a correctly selected fuse, motor protection circuit breaker, or other short-circuit protective device.

For motors operating through a variable-frequency drive, the installation position also matters. Siemens permits these overload relays on the incoming side of a frequency converter. Motor protection on the drive output side may require a different protection arrangement, such as direct winding-temperature monitoring.

Siemens 3RB3 Electronic Overload Relay Supplier and Exporter

ARP IMPEX supplies Siemens SIRIUS 3RB3 Electronic Overload Relays for panel builders, machine manufacturers, EPC contractors, factories and industrial maintenance companies.

We can quote standard Class 10E and Class 20E relays, adjustable Class 5E–30E models, contactor-mounted versions and relays for separate installation. Buyers replacing an older Siemens overload relay should share the complete existing part number so that the current equivalent can be checked properly.