LIGHTNING PROTECTION
Protection against direct impact of lightning strikes on buildings and people
The Phenomenon of Lightning
How lightning is formed
Thunderstorms develop when ground heat creates a hot rising current of air. This air gradually cools until it condenses into small cumulus clouds. The cumulus continues to grow vertically until it eventually becomes a storm cloud or cumulonimbus.
These atmospheric conditions lead to the creation of electric charges resulting from collisions between water, hail and ice particles of different sizes. There are charges separation inside the cloud, with a negative charge at the bottom of the cloud and a positive charge at the top.
The centre of the negative charges is usually at the base of the cloud due to the movement of electrons through the heavier droplets and hailstones, whilst the centre of the positive charges moves up to the top of the cloud carried by convection currents, which can easily lift the light positively charged particles.
The effect produces a similar change at the earth's surface due to the charge repulsion, of about the same magnitude but opposite polarity.
The potential inside the cloud is usually of the order of several million volts and electric field may exceed 5 kV/m at ground level, which gives the creation of the upwards leaders rising from surface irregularities or metallic structures. (Fig.2 a).
Electrical field is so strong that small discharges are produced from the cloud, known as step leaders. As these step leaders get closer to ground level, the upward leaders rise up to meet them. When the step leader and the upward leader meet, the circuit is completed creating a short circuit, the lightning strike, with discharges current from 10 to 200 kA (Fig. 2c).
The energy carried by a lightning strike can easily reach 20 GW.
The most common lightning strikes are from the cloud to ground (in 80% of cases), and are named negative discharges, but when the discharge is positive in the downward direction, the intensity is extremely high.
Statistics
Weather changes due to natural phenomena and to human activities are more and more important, affecting for example the higher frequency and the intensity of thunderstorms worldwide. Known statistic information indicates that there are around 5,000 thunderstorms at any time around the world, with the associated threat to people and property, buildings, houses and various industrial structures. The worldwide average discharge intensity of lightning strikes is reckoned to be 5 kA.
Lightning density may vary in different parts of the world and periods of the year. In the Iberian Peninsula, about two million lightning strikes fall every year, killing people and animals. The faults and damages caused to industry by lightning every year can be estimate in many millions of euros. Of course the orography of each country determines the number and intensity of thunderstorms produced, which can vary within an own country. Knowledge of the danger zones its an important information in order to determine the most appropriate lightning protection system and its technical features.
The Effects of Lightning
The effects of lightning may be classified as direct when is the result of an impact of a lightning strike or indirect when is due to the phenomenon of electromagnetic field generated during its discharge to the ground. (Fig. 1). Through a direct effects may have catastrophic consequences for people and animals, buildings, industrial structures and telecom antennas whilst the indirect effects tends to be more common, and causes huge financial losses.
Lightning strikes can affect directly (b) and indirectly (a).