Grading rings are often mistaken for corona rings. 

They indeed look similar and they both are designed to affect the electric field in the near vicinity. 

However, unlike corona rings, reducing partial discharge is not a function of grading rings. 

The purpose of grading rings on arresters is to significantly modify the voltage distribution along the vertical axis of the arrester. 

This voltage distribution allows the arrester to operate without an increase in its temperature near the top of the arrester. 

The lower temperature level allows the arrester to absorb more energy during a switching surge event. 

The grading ring in effect increases the energy handling capability of the arrester.