Handbook of Electrical Installation Practice - BeKnowledge

200 Handbook of Electrical Installation Practice
Fig. 9.3 Striking distance (last step).
resistance conducting path to earth, and is strong enough at the point of strike to
withstand the impact of the expanding air, the lightning is discharged without
damage to the structure.
Those structures that are non-conductive will require the installation of lightning
conductors. A lightning conductor is incapable of discharging a thundercloud
without a lightning flash. Its function is to divert to itself a lightning discharge which
might otherwise strike a vulnerable part of the structure to be protected and to
convey the current safely to earth. The British Standards Institution has published
a code of practice, BS 6651: 1999, Protection of structures against lightning. This code
gives sound practical advice on how to protect a structure from lightning.
The need for protection
Before proceeding to design a detailed lightning protection system, first carefully
consider if the structure needs protection.
In many cases, it is obvious that some form of protection is required. High-risk
structures, i.e. explosives factories, oil refineries, etc., will require the highest possible
class of lightning protection to be provided. In many other cases the need for
protection is not so evident.