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

One of the characteristics of a Thermal Overload Relay (e.g. our LRD** range) is the class setting. There are multiple class settings available, depending on the range.

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. Note that this table is taken from the IEC 60947-4-1 standard. Please refer there for full information regarding tripping classes.

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 10 or 10A will be the first choice.

Tripping Class Table