The slow overcurrent develops from overloading
devices or from malfunctions in motors and other loads. For instance,
if a motor-driven conveyor is overloaded, the motor will be required
to draw extra current to try and move the load. If the motor is not protected,
it will draw the extra current and begin to overheat. If the transformer
that supplies voltage to the motor is also fully loaded, it will become
overloaded when the motor draws the extra current. If the overload condition
continues for 10 to 20 minutes, enough heat will be built up in the motor and transformer to cause the insulation in both of these devices to break
down and deteriorate. The same problem will occur if the bearings in
a motor become dry and begin to wear. After the bearing has operated
without any lubrication, it will begin to heat up and seize on the motor
shaft, which will in turn cause the motor to draw excessive current.
This condition will cause the motor and transformer to overheat to a
point where they are completely damaged because the overcurrent will
continue as long as the motor is running.