Last edit: 16/05/2023
Overload relays typically operate on an inverse time curve where the tripping time becomes less as the current increases. They are rated by trip class. Trip class specifies the length of time it will take for the relay to open in an overload condition.
In Europe the shape of the overload relays tripping curves are defined by ANSI or IEEE standards. A few examples are: ANSI Extremely Inverse, Very Inverse, Inverse, Short Time Inverse; or IEEE Moderately Inverse, Very Inverse and Extremely Inverse.
In North America the NEMA Standard MG-1 defines 4 types of Classes as the most common: 5, 10, 20 & 30. Class 5, 10, 20 & 30 overload relays will trip within 5, 10, 20 & 30 seconds respectively at 600% of motor full load amps.
Class 5 is usually used for motors requiring extremely fast tripping.
Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity.
Class 20 is usually sufficient for general purpose applications.
Class 30 is usually required for high inertial loads to help prevent nuisance tripping.