i am using a 3 m long cu rod on my sub-station rooftop of height 7 m. plz do tell the working philosophy of the arrestor.

Ashish,

A lightning rod is not a lightning arrester, it is part of a lightning protection system. The topic is too broad to cover here, there are plenty of fine papers and educational materials available from the web using your favorite search engine. After you do your research and learn the basics then you should post specific advanced questions.

Alan

Basic Rule: using rolling sphere
Put tangent (tangent radius = 100 meter) touching top of the rod. Area under the curve will be the protected area. ( your total hight is 10+3=13 meter so the protected area comes 56.5 meter of circumference from the base of the rod.)

Protected place will be covered within cone from top of the rod by 30 Degree. But it is not Lightning arrester , it is lightning conductor.

One copper rod as an air terminal will work with limitations. Because of the leader phenomenon, you may want to consider a number of other air terminals minimum 1 terminal every 15ft. Additionally, 1 rod must be planted every 30ft in the soil with a network resistance not greater than 10 ohms.

For a better lightning protection of yard it may be proposed for placing of number of Air terminations surrounding the yard at such height with connection of Air termination tips with conductor in criss cross way so that 30 degree cones can cover a total umbrella. All termination rods to be earthed on two independent Earth Pits