Minimum Clearances in Air

Because at the tip of the rod, the voltage gradient through the air is higher than it is for a cylindrical conductor, owing to their different shapes.

Analogy: lightning conductor.

The shape of an electrode has a great deal to do with the stress of the electrical field. 2 round balls can have a very low stress while 2 sharpened points will high stress. I learned this on my own as a youngster, playing arounnd, it was reinforced by high school and college physics, many EE courses, but mainly during years of design engineering and field engineering. If you attempt to draw the equipotential and stress lines for the two configurations, you will see that the standard is based on reality.

I'm facing some Dificulty to understand what is the rod structure design ?

in general we use corona rings, special clamps and conductor and tubes with larger diameter to avoid having any sharp edges .

so would anybody please advise a sample design for the rod structure