Each year about two million flashes of lightning occur on the Iberian peninsula, causing death to persons and animals. In industry the damage and failures due to lightning are estimated to be in the range of thousands of millions of Spanish Pesetas.

The frequency and the intensity of thunderstorms in an area are determined by the characteristics of the area, however the risk of lightning can vary within a certain region. The knowledge about the areas with high lightning risk is an important information in order to effectivly determine the most appropriate type of lightning protection.

The effects of lightning can be produced by direct lightning strokes or by indirect causes.

While a direct stroke may generate disasterous consequences for structures, persons and animals, indirect effects produced by lightning are frequently noticed and usually they produce significant economic losses. Indirect effects of lightning are observed when the lightning flash strikes close to a structure and produces by induction surge voltages in the electrical conductors.

LIGHTNING FLASHES:

Under certain athmospheric conditions the loads within a cloud get separated, where the negative loads move to the bottom of the cloud and the positive loads to the top of it. The electrical potential within the cloud can reach the range of millions of volts.

On the surface of the earth this effect is produced in a similar way, however with opposite polarity.

The electrical field between the bottom of the cloud and the surface of the earth under the cloud can become so strong that small electrical discharges from the cloud will be produced which are called downward leaders. When these discharges reach the surface of the earth, an upgoing steam of positive load is generated. When the upgoing steam meets with the discharges, then the circuit becomes closed and a discharge current between 10 to 200 kA can be produced.

In the illustrations on the left it is shown how the small discharges from the cloud and the upgoing streams, which finally lead to the flash of lightning, are initiated.

Protection against lightning:

The decision to protect an installation against lightning by means of a Lightning Protection System (LPS) depends on parameters such as the probability of lightning in the area, its intensity and the potential consequences for people, material and the functioning of the installation.

In order to provide an appropriate protection, the installation has to be equipped with two types of protections: an external protection against a direct impact of a lightning stroke (lightning rod, wire air-termination system or mesh air-termination system), and an internal protection against surge voltages produced by lightning strokes in the proximity or on conductors of the electrical network. The external and the internal protection require a good earthing system to evacuate the lightning currents, and equal potentiality within the earthing system, both of the protection system and of the electrical circuits to be protected.