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What are the differences between the various tripping classes of thermal overload?

There are four classes of thermal overload relay:

10A
10
20
30
These class numbers indicate the trip characteristics from cold state, and should be selected according to the starting characteristics of the motor and its application. Below is a table giving the characteristics for each class

Ir = Current setting of overload relay. This should be the Full Load Current (FLC or FLA) shown on motor rating plate. For the vast majority of applications 10A will be the first choice.

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