The ever increasing use of sensitive electronics means that electrical equipment is becoming more and more vulnerable to transient overvoltages caused by lightning.The overvoltages are either of atmospheric origin or industrial origin. The most harmful are however atmospheric overvoltages which are the result of three main effects:

Conduction : An overvoltage that propagates along a conductor which has been in direct contact with the lightning strike. This effect is all the more destructive as the majority of the lightning energy is propagated through the entire network. This problem is resolved by fitting suitable protection to the installation that is able to support high currents

Induction : caused by the electromagnetic field radiated by the lightning strike. It generates an overvoltage on conductors within a range that is proportional to the power and the rate of change of speed of the lightning strike. Consequently, under the influence of abrupt variations in current, the cables, and even the ducts which act as aerials, may be subjected to destructive overvoltages. This is the reason that placing the network underground does not guarantee lightning protection.

Rising up from the ground : When a lightning strike hits, an overvoltage can rise up from the ground attempting to find a more favourable path to ground. This can, in part, be dealt with through a) equipotential bonding between the metal structures and ground of the entire installation of a structure. b) overvoltage protection installed on services.