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Monday, 24 February 2020

Thermal Over Current Relay- Working Principle, Construction.

Thermal Over Load Relay



  • Thermal over load relay works on the principle of heat and temperature rise. When an electric current flows through a conductor heat is produces which causes the conductor to have an increased temperature rise. 
  • When a temperature rise reaches a predetermined set point, the relay act to open the protected circuit.
Thermal Over Load Relay
Thermal Over Load Relay

Construction And Working Of Thermal Relay

There is two bimetallic strips that is two metal soldered together. One has lower coefficient of expansion and other is higher coefficient of expansion.

When over current flows through the coil then it heats the bimetallic strips. Both the metal expands due to the heat generation in coil. 

But the expansion of higher coefficient strip is high than the lower coefficient strip. So because of this dissimilar expansion of both the strips it starts to bend.

The mechanical bending movement is used to open the protected circuit. After the unit cools it will self reset and the load can be started again. 


Thermal Over Current Relay construction
Thermal Over Current Relay  construction

  • This types of relays relatively slow to respond. So this type of relays used for the overloads and not for the short circuit faults which requires high speed clearance.


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