There are certain principle
design considerations of a conventional lighting system as per the
BS 6651 norms, these are-
Air Termination Network
- Air Termination Network
- Down Conductors
- Earth Termination Network
This phenomenon is applicable on high risk structures like,
explosives factories, nuclear power plants wherein no part of the
roof should be more than 2.5m from from an air termination
conductor. This can be done by applying a 5m x 10m mesh to the roof.
10m x 20m Mesh 60 m.
Generally for most structures a mesh measuring 10m x 20m is
actually sufficient. The range of air termination protection is
considered to be 45 for heights up to 20m. Above this height the
production zone is determined by the "rolling sphere method"
The location of the down conductor is majorly influenced by the
architectural design. This is like, there has to be one down
conductor after every 20m. These are evenly spaced at not more than
a distance of 20m. These must be rooted as directly as is possible
from the air termination to earth termination networks and the loop
should not exceed eight times the width of it's open side.
Earth Termination Network
Every Down conductor should be provided with a separate Earth
Termination. For testing purpose the provision should be made in
each down conductor.
The Bs6651norms mentions that the resistance to earth for complete
lightning protection at any point should not exceed beyond 10 ohms.
This norm also mentions that the combined length of the earth rod
system should be no less then 9m, while each individual earth rod
should be not less than 1.5m in length.
As soon as lightning protection system is struck, its electrical
protection with respect to earth is considerably raised and if
resistance provided by the down conductor is more than the other
metal work on the structure, the discharge looks for alternative
path to Earth by way of side flashing.
Thus, it is important that all the exposed metal work is bonded
into the lightning protection installation.