Depending on the application, the motor starting time can vary from a few seconds (for no-load starting, low resistive torque, etc.) to several tens of seconds (for a high resistive torque, high inertia of the driven load, etc.). It is therefore necessary to fit relays appropriate to the starting time. To meet this requirement, IEC Standard 60947-4-1 defines several classes of overload relays, each characterized by its tripping curve.

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.

The relay rating is to be chosen according to the nominal motor current and the calculated starting time.

• Trip class 10A, 10 are adapted to normal duty motors.
• Trip class 20 is recommended for heavy duty motors
• Trip class 30 is necessary for very long motor starting.

Hence, for example, if the trip class is specified as Class 20 by the customer, you should not use any other type of relay. You must use Class 20 only.

In BlokSeT Mw2 co-ordination table, all the relays are of Class 10. Hence, if customer needs Class 20, please follow the device co-ordination as per the standard table and only replace the relay with Class 20 relay. In Schneider product range, Tesys U and LRD can provided for class 20.

Similarly, for a Class 30 requirement, Tesys T can be used.

Publié pour: Schneider Electric Belgium