What overload relays do?

• Make it possible for the motor to handle harmless
temporary overloads without interrupting the
circuit, i.e. motor starting.
• Trip and open a motor circuit, if the current exceeds
its limits and and might damage the motor.
• Are reset either automatically or manually once the
overload situation has passed.
IEC and NEMA are responsible for setting the standards
as to trip classes and thus for overload relays.

Trip class designation

Generally, overload relays react to overload relay conditions according to the trip curve. Regardless of the product style (NEMA or IEC), trip classes specify the periode of time it takes the relay to open when overload occurs. The most common classes are 10, 20
and 30. The figure refers to the periode of time it takes the relay to trip. A class 10 overload relay trips within 10 seconds or less at 600% of full-load current, a class 20 overload relay trips within 20 seconds or less and a class 30 overload relay trips within 30 seconds or less.

The degree of inclination of the trip curve depends on the motor’s protection class. IEC motors are typically adapted to the application in which they are designed to operate. This implies that the overload relay is able to handle excess amounts of current, very close to its maximum capacity. The trip time is the time it takes for
a relay to trip during overload. The trip time is divided into different classes. The most common trip classes are 10, 20 and 30. Trip class 10 is the most common one for IEC motors because they are often adapted to the application. NEMA motors are applied with more built-in excess capacity, and therefore, the trip class 20
is most common.

Trip class 10 relays shut off the motor within 10 seconds at 600% of full-load current. Trip class 10 is normally used for pump motors because the run-up time of the motor is around 0.1 – 1 second. Many high inertia industrial loads require more time to start. Many of these loads require trip class 20.

 

The trip time is the time it takes for a relay to trip during overload. The trip time is divided into different classes