The overload relay is an electrical device used to protect motors and other equipment from damage caused by excessive current flow. It is typically installed in the motor control circuit and monitors the current flowing through the motor. 

The relay operates based on the principle of thermal heating caused by current overload. It consists of a bimetallic strip or a heater element that experiences a rise in temperature as the current increases. The heating element is connected to a set of contacts that control the power supply to the motor.

When the current exceeds a pre-set limit, the heating element heats up rapidly, causing the bimetallic strip to bend or the heater element to trip. This action separates the contacts and interrupts the power supply to the motor. The relay remains in a tripped state until it cools down or is manually reset.

The primary function of the overload relay is to prevent the motor from operating under conditions that can lead to excessive heating, motor winding damage, or other potential hazards. By detecting and reacting to overcurrent conditions, the overload relay helps in maintaining the proper functioning and prolonging the lifespan of electrical equipment.