In daily industrial control operation, equipment maintenance and power distribution installation, there is a common improper operation: many operators twist the current adjustment knob of the thermal relay at will. They simply think turning up the current avoids tripping and turning down makes protection more sensitive, treating the setting current as an adjustable switch with random flexibility.
Especially when adopting RDJ2 series thermal relay — a widely applicable and high-precision industrial-grade motor protection device — this experience-based random adjustment will lead to minor issues like frequent false tripping and production line shutdown. In severe cases, it will burn out motor windings, cause equipment breakdowns, and even bring potential electrical safety hazards.
Today, taking People Electric RDJ2 series thermal relay as the core, we will thoroughly explain why the setting current of a thermal relay cannot be adjusted arbitrarily, analyze the professional setting logic of motor protection, and guide you to accurately configure the overload threshold for safe and stable equipment operation.
1. Break The Misconception: 3 Hidden Risks of Randomly Adjusting Thermal Relay Setting Current
Most users hold a fundamental misunderstanding of RDJ2 thermal relay: it is not a simple current limiting switch, but a temperature monitor for industrial motors. The setting current (Ir) is the critical protection threshold, and each scale is calibrated strictly in accordance with international electrical standards and motor endurance characteristics. Blind adjustment will trigger the following three major risks:
⭕️ Excessively High Setting Current → Complete Failure of Motor Protection
If you adjust the knob to an over-high current value, it is equivalent to removing the safety protection cover for the motor.When the motor encounters overload, locked rotor or phase loss, the circuit current rises sharply, but the RDJ2 thermal relay fails to trip in time. The internal bimetallic strip cannot trigger the tripping mechanism, resulting in continuous overheating of motor windings, aging of insulation layers, and permanent motor burnout.
⭕️ Excessively Low Setting Current → FFalse Tripping & Production Disruption
If the setting current is adjusted too low, the thermal relay will trip in advance even during normal motor startup and temporary load fluctuation.Frequent start-stop of fans, water pumps, conveyors and other equipment will not only disrupt the production schedule but also accelerate the wear and aging of contactors, circuit breakers and other electrical components.
⭕️ Ignoring Phase Loss Protection → Minor Faults Evolve Into Major Accidents
Phase loss operation of three-phase motors features extremely high current imbalance, which is the top cause of motor burnout.If the setting current does not match the motor rated current, the built-in phase loss protection function of the RDJ2 thermal relay will fail to work normally. It cannot act quickly when one phase is disconnected, eventually leading to two-phase overload and motor damage.
In short: The setting current of the RDJ2 thermal relay is a professionally calibrated safety threshold matched with motor rated current, load characteristics and ambient conditions, and must never be adjusted randomly by experience.
2. Working Principle of RDJ2 Thermal Relay: Why Strict Setting Is Mandatory
To understand the importance of standard setting current, we first need to master the core working principle of the RDJ2 series thermal relay.
The RDJ2 (LR2 type) series is a bimetallic strip thermal overload relay, suitable for AC 50Hz/60Hz, rated working voltage up to 660V and rated current range from 0.10A to 630A. It is specially designed for motor overload protection and phase loss protection, fully compliant with IEC60947-4-1 international standard.
Its professional protection logic follows strict industrial design:
1. Bimetallic Strip Tripping Principle: Circuit current flows through the heating element, bending the bimetallic strip by heat, pushing the mechanical tripping structure to cut off the control circuit and stop the motor via the contactor.
2. Ambient Temperature Compensation: Maintain consistent protection accuracy under ambient temperature from -5℃ to +40℃, free from temperature fluctuation interference.
3. Precision Phase Loss Protection: Quickly identify three-phase current imbalance and trigger reliable protection.
4. Dual Reset Mode: Supports switching between automatic reset and manual reset to adapt to various on-site maintenance scenarios.
5. Test & Stop Button: Real-time inspection of tripping flexibility, equipped with anti-electricity protection cover and anti-misoperation locking device for safer operation.
The core of the entire protection system lies in the setting current — it determines the current threshold and action time of bimetallic strip deformation. Any deviation from the standard setting will disable the whole motor protection system.
3. RDJ2 Thermal Relay Setting Current Breakdown: Model, Adjustment Range & Matching Guide
The RDJ2 series covers five mainstream models: RDJ2-25, RDJ2-36, RDJ2-93, RDJ2-200, RDJ2-630, with a setting current range of 0.1A to 630A. It perfectly matches small-power micro motors to high-power industrial motors, and each model features a precisely calibrated adjustment range.
⭕️ Core Model & Standard Setting Current Range
| Product Model | Rated Current | Setting Current Adjustment Range | Matched AC Contactor |
| RDJ2-25 | 25A | 0.1~0.16A / 0.16~0.25A / ... / 17~25A | CJX2-09~32 |
| RDJ2-36 | 36A | 23~32A / 28~36A | CJX2-32 |
| RDJ2-93 | 93A | 23~32A / 30~40A / ... / 80~93A | CJX2-40~95 |
| RDJ2-200 | 200A | 80~125A / 100~160A / 125~200A | CJX2-115~225 |
| RDJ2-630 | 630A | 160~250A / ... / 400~630A | CJX2-185~630 |
⭕️ Tripping Class & Overload Threshold Characteristics
The RDJ2 thermal relay strictly follows national and IEC standards with fixed tripping classes: Class 10A for RDJ2-25/36 and Class 10 for RDJ2-93/200/630. The overload threshold and action time are factory-calibrated and cannot be altered arbitrarily:
● 1.05 times setting current: No action within more than 2 hours under cold state to ensure long-term normal operation under rated load.
● 1.2 times setting current: Must trip within 2 hours under hot state for critical overload protection.
● 1.5 times setting current: Trip within 2 minutes (Class 10A) / 4 minutes (Class 10) for rapid protection against moderate overload.
● 7.2 times setting current: Trip within 2~10 seconds under cold state to protect motor from locked rotor and pre-short circuit risks.
For three-phase phase loss fault (current imbalance), RDJ2 is set with a dedicated threshold: it will trip within 2 hours when two phases carry rated current and one phase is open-circuited, completely avoiding motor burnout caused by phase loss.
All parameters are designed to match the motor thermal endurance curve. Improper setting current will invalidate the action time and lose protection performance.
4. Motor Protection Setting Logic: Standard Matching Rules for RDJ2 Thermal Relay
Since random adjustment is not allowed, how to set the optimal setting current for RDJ2 correctly? The core rule is: Take motor rated current as the benchmark, and make fine adjustment according to load type and ambient conditions, complying with GB50055-2011 Code for Electrical Design of General Electric Equipment.
⭕️ Universal Setting Principles for All Application Scenarios
● Standard load (fans, water pumps, conveyors): Setting Current = Motor Rated Current × 0.95~1.05
● Heavy-duty startup load (crushers, ball mills, stamping machines): Setting Current = Motor Rated Current × 1.1~1.2 to avoid false tripping caused by startup inrush current
● Frequent start-stop equipment (cranes, elevators): Set setting current to 1.1 times rated current with RDJ2 temperature compensation to prevent heat accumulation mis-tripping
⭕️ Step-by-Step Setting Guide for RDJ2 Thermal Relay
1.Check the motor nameplate and record the motor rated current (In) as the only setting benchmark.
2.Select the correct RDJ2 model to ensure the relay’s adjustment range fully covers the motor rated current.
3.Adjust the knob and align the arrow with the calculated setting current value, then lock the anti-misoperation device.
4.Press the TEST button on RDJ2 panel to verify tripping flexibility.
5.Select automatic reset or manual reset according to actual operation and maintenance needs.
⭕️ Ambient Temperature Compensation Adjustment (Core Advantage of RDJ2)
Equipped with built-in temperature compensation, RDJ2 adapts to -5℃~+40℃ working environment:
● High temperature above 35℃: Adjust setting current down to 0.9 times motor rated current
● Low temperature below 0℃: Adjust setting current up to 1.1 times motor rated current
● Altitude below 2000m with non-corrosive and dust-free environment: Set according to standard rated value directly
5. Why Choose RDJ2 Series Thermal Relay for Accurate Motor Protection?
With numerous thermal relay models on the market, RDJ2 series stands out as the preferred choice for industrial motor protection, thanks to its high-precision setting and reliable performance:
✔️ Anti-misadjustment Lock Design: Prevent accidental twist of the adjustment knob and keep the setting current stable for long-term operation.
✔️ Clear Trip Status Indication: Intuitive status display for fast on-site fault troubleshooting.
✔️ Isolated NO/NC Auxiliary Contacts: Higher safety for control circuit connection, compatible with alarm system and PLC signal access.
✔️ Flexible Installation & Compatibility: Compact size with multiple mounting methods, perfectly matched with full range of CJX2 AC contactors and easy wiring.
✔️ IP20 Protection Grade: Built-in electric shock protection cover to avoid contact with live parts and ensure operational safety.
In conclusion: The adjustment knob of RDJ2 thermal relay is a precision calibration tool, not an optional switch for random adjustment. Each scale marks the safety baseline of industrial motors.
6. Common Setting Mistakes & Troubleshooting Tips
We sorted out frequent setting errors in on-site application for quick self-inspection:
✖️ Mistake 1: Only check relay rated current instead of adjustment range
Solution: The shell rated current is the upper limit, while the setting adjustment range is the key for model selection, which must cover the motor rated current
✖️ Mistake 2: Set setting current according to motor startup current
Solution: Motor startup current is 5~7 times the rated current. The thermal relay only needs to avoid startup duration and shall be set based on rated current.
✖️ Mistake 3: Ignore phase loss protection configuration
Solution: RDJ2 phase loss threshold is factory fixed. As long as the setting current matches the motor rated current, phase loss protection will work automatically.
✖️ Mistake 4: Mismatched wire and fuse specification
Solution: Select matching wire cross-section and fuse according to RDJ2 parameter specifications to realize coordinated circuit protection.
7. Final Conclusion: Scientific Setting Is The Key To Motor Safety
Back to the core question: Can the setting current of RDJ2 series thermal relay be adjusted freely?The answer is absolutely No.
The setting current of RDJ2 thermal relay is scientifically configured by combining motor thermal characteristics, working load conditions, ambient temperature and international electrical standards. Random adjustment means abandoning the last safety line of defense for industrial motors.
Whether for factory production lines, building power distribution, complete equipment supporting or small industrial control scenarios, People Electric RDJ2 series thermal relay delivers precise setting and reliable overload & phase loss protection. With standardized parameters, humanized design and strict quality control, it safeguards the stable operation of your motor equipment.
For full model parameters, installation dimensions and complete matching solutions of RDJ2 series, please visit our official product page:RDJ2 Series Thermal Relay Product Page
Post time: May-11-2026
