What is Thermal Protection?
Thermal protection is a protective mechanism that interrupts an electrical circuit when there is overheating due to overcurrent. It protects, among other things, electric motors, transformers and cabling against damage from prolonged overload.
Thermal protection devices are often used in combination with Contactors, motor starters, soft starters or variable frequency drives in industrial installations.
🧠 How does thermal protection work?
- With a bimetallic element
- Under overcurrent, a bimetallic strip heats up
- It deforms as the temperature rises and opens the circuit
- Time-delay effect
- The protection does not respond immediately, but with a delay → suitable for short-term inrush peaks
- Ideal for applications with starting currents (such as motors)
- Manual or automatic reset
- After cooling, the protection usually has to be reset manually
- Some models support automatic reset (only for safe applications)
🏭 Application in industrial environments
- Motor protection for pumps, fans, conveyors
- MCCs and control cabinets
- Protection of transformers, relay circuits, power supplies
- As part of a motor starter with Contactor
Correctly setting the thermal protection is essential to prevent both damage and unnecessary interruption.
🔍 Different types of thermal protection
| Type | Application |
|---|---|
| Thermal relay (bimetallic) | Most common for motors and pumps |
| PTC/NTC sensors | Built into motor windings, readable via PLC |
| Thermal fuses | Single use, suitable for electronics |
| Digital motor protection | Combined with measurement modules or PLC |
Bimetallic relays are standard in many MCCs and can easily be set to the motor rating.
🔐 Security and safety aspects
- Correct setting of the current rating (In) is crucial
- Verify that motor current draw and protection are properly matched
- The reset procedure must be included in the Lock-out Tag-out or work permit
- Contacts can wear with frequent switching → test periodically
- In ATEX zones, explosion-proof variants are required
An incorrectly set protection device may lead to motor damage or unplanned downtime.
📌 In summary
Thermal protection prevents damage to electrical installations from overcurrent and overheating. It is essential for the reliable and safe operation of motors and power circuits in industry.