Introduction to Industrial Automation

Figure 2.20  Operation principle of the thermal overload relay with bimetallic strip, (a) during 

normal operation (direct heating), (b) during normal operation (indirect heating), and (c) during 

fault case operation.

Reset button

Bimetallic strip

Auxiliary

contacts 

Adjustable

current setting

Terminals for

connection to

the contactor

To motor

Selector automatic

or manual reset

Supply terminals

Trip test

button

Figure 2.21  Typical external view of a thermal overload protection relay.

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  Introduction to Industrial Automation

cools and returns to its initial position either manually by a reset button or automatically by a self-

resetting mechanism, the last being selectable from a corresponding switch. A manual test button 

is also provided for testing the operation of the overload relay’s auxiliary contacts in the automa-

tion circuit. For motor rated currents over approximately 80 A, the motor current is conducted 

via current transformers, and in this case, the thermal overload relay is heated by the secondary 

current of the current transformer.