How to use the thermal overload relay in the distribution box
The **thermal overload relay (Thermal Overload Relay)** in the distribution box is a core component used to protect inductive loads
such as motors from overload and phase loss damage. It is usually used in conjunction with contactors and circuit breakers. The
following are its **usage methods, key points for selection, and precautions**:
I. Core Functions
1. **Overload Protection**: When the load current exceeds the set current, the bimetallic strip bends due to heating, triggering the tripping
mechanism to disconnect the circuit.
2. **Phase Loss Protection**: When any one phase of the three-phase load is open-circuited, the current of the remaining two phases increases,
triggering the protection.
3. **Difference from Circuit Breakers**:
- Circuit Breaker: Mainly used for short-circuit protection (instantaneous action), with relatively low sensitivity for overload protection.
- Thermal Relay: Focuses on **delayed overload protection** and is more sensitive to long-term small-current overloads.
II. Selection Steps
1. Determine the Rated Current
- **Motor Rated Current**: The value indicated on the nameplate (for example, if the motor power is 10kW, the rated current is approximately 20A).
- **Thermal Relay Set Current Range**: It needs to cover **0.95 to 1.05 times** the motor rated current (for example, for a 20A motor, select a
thermal relay with a range of 16 to 25A).
2. Match the Load Type
- **Ordinary Motor**: Select a single-metal thermal relay without phase-loss protection (such as the JR36 series).
- **Three-phase Motor**: It is necessary to select a bimetallic or electronic thermal relay with **phase-loss protection** (such as the LR2-D series).
3. Voltage and Installation Method
- **Control Voltage**: Commonly used are AC 220V/380V, which needs to be consistent with the voltage of the contactor coil.
- **Installation Method**:
- **Independent Installation**: Fix it on the guide rail of the distribution box and connect it to the contactor through wires.
- **Contactor Integration**: Integrated with the contactor (such as the Schneider LC1D series), saving space.
YouTube：https://youtube.com/shorts/BM-bvahGQk4
III. Wiring and Installation
1. Typical Wiring Diagram
Power Supply → Circuit Breaker → Thermal Relay (Main Circuit) → Contactor → Motor
↳ Thermal Relay (Control Circuit) → Contactor Coil
- **Main Circuit**: The three-phase power supply is connected to the input terminals (L1/L2/L3) of the thermal relay, and the output terminals
(T1/T2/T3) are connected to the main contacts of the contactor.
- **Control Circuit**: The normally closed contacts (NO/NC) of the thermal relay are connected in series in the circuit of the contactor coil (as
shown in the figure).
2. Installation Precautions
- **Vertical Installation**: Avoid tilting to prevent errors in the action of the bimetallic strip.
- **Heat Dissipation Environment**: The ambient temperature ≤ 40°C, and keep away from heating elements (such as resistors).
- **Tight Wiring**: The screws of the main circuit terminals need to be tightened with a torque wrench (for example, the torque for a 10mm²
wire is 8N·m).
IV. Setting and Testing
1. Setting Current Adjustment
1. **Rough Adjustment**: Select the scale range of the thermal relay according to the motor rated current (for example, turn the knob to "20A").
2. **Fine Adjustment**:
- Start the motor without load and measure the actual current with a clamp ammeter.
- Adjust the setting knob so that the current value of the thermal relay = **Motor Rated Current × (1 to 1.15)** (select 1.15 for heavy load and 1
for light load).
2. Function Testing
- **Manual Testing**:
1. Disconnect the main power supply and press the **reset button** (RESET) of the thermal relay.
2. Use a screwdriver to move the **tripping mechanism** to simulate an overload and observe whether the control circuit is disconnected.
- **Loaded Testing**:
Artificially increase the load to 1.2 times the rated current, and the thermal relay should trip within **20 minutes** (under standard test conditions).
V. Daily Maintenance
| Maintenance Item | Operation Method | Cycle |
| Contact Inspection | Measure the resistance of the normally closed contacts with a multimeter (should be < 0.1Ω) | Quarterly |
| Bimetallic Strip Reset | After tripping, it is necessary to cool for 3 minutes and then manually reset | After each trip |
| Dust Cleaning | Blow out the internal dust with compressed air | Annually |
| Parameter Review | Re-calibrate the setting current after the motor overhaul | When necessary |
VI. Common Problems and Solutions
| Problem Phenomenon | Possible Cause | Solution |
| Frequent Tripping | The setting current is too small | Re-calibrate to 1.05 times the rated current |
| No Tripping | The bimetallic strip is aged | Replace the thermal relay |
| Motor Running with Phase Loss | The phase-loss protection function fails | Check whether the three-phase wiring is loose |
| False Disconnection of the Control Circuit | Oxidation of the auxiliary contacts | Polish the contacts or replace the auxiliary contact module |
VII. Selection Reference (Taking a Three-phase Motor as an Example)
| Motor Power | Rated Current | Recommended Model | Setting Current Range |
| 2.2kW | 4.8A | Chint JR36-63 | 4~6.3A |
| 7.5kW | 15A | Schneider LR2-D13 | 12~18A |
| 15kW | 30A | Delixi JRS1-63 | 23~32A |
**Conclusion**: The key to the thermal overload relay lies in **correctly setting the current** and **regular maintenance**. In combination
with the circuit breaker, it can form a dual protection of "short circuit + overload". For motors driven by frequency converters, it is necessary
to select an **electronic thermal relay** or directly use the built-in overload protection function of the frequency converter.
