Trip Class for Overload Relays Explained

Trip Class for Overload Relays Explained


Trip Class for Overload Relays Explained

What exactly is an Overload Relay? An Overload Relay is usually used in combination with a contactor for protection of an attached electric motor. Electric motors are used in various types of applications such as Fans, Pumps, Conveyors, and other rotating type equipment. Overload Relays are used as a protective device to prevent motor failures due to prolonged usage above the motor’s rated current and or single-phase conditions.

Overload Relays have what is called a Trip Class Designation, these Designations correspond to the amount of time it takes for the overload relay to trip.  Overloads are typically built to withstand 600% (or 6x) the FLA of the motor for that short duration, but then fall to a protection level just over the FLA in steady-state operation, example:



Table 1: Trip Classes


Class Designation

*Tripping Time

Class 10 

10 Seconds or less

Class 20 

20 Seconds or less

Class 30 

30 Seconds or less

*Marking designation for tripping time at 600% of current element rating




Now, why would we have different Trip Class times? Examples would be operating a simple fan, normally it would take less than 10 seconds to reach full speed during an across the line start up, so a Class 10 overload would be what you would choose for your application. Class 10 is the most common as it provides a high level of protection for the motor, which is the main function of a contactor with overload relay protection.  Class 20 is used in situations where you want to avoid nuisance tripping.  Let's say you have a motor with a high inrush that maintains an AMP pull that can last 10-15 seconds.  If you use a Class 10 overload relay, it would trip even though there is no potential damage to the motor.  A Class 20 overload would allow for a little extra time for that motor to level its AMP draw.

So, to sum up, Class 10 overloads are the most common as they provide a high level of protection for a motor while Class 20 overloads are used in situations where nuisance tripping is an issue. If say, the motor is operating a rock crusher, then it could possibly take up to 30 seconds for the motor to reach full speed based upon the load, so then you would choose a Class 30.