The impact of lightning has always been something special, a mixture of fright and respect. Lightning has been one of the most outstanding symbols of the power of nature, the divine sceptre. Lightning is a fascinating phenomenon, yet at the same time it is dangerous and worrying. The width of a bolt of lightning is only a couple of centimetres but it generates a tension of a billion volts that discharges in a fraction of a second, this is equivalent to 1.000.000 100-Watt light bulbs.
Today we understand most of the phenomenon of lightning. The flash starts in a group of clouds that can be many kilometres thick. Important ascending streams of air lead to a polarisation of these clouds. An ionisation takes place in the lower part of this mass of clouds with a negative charge, the ground reacts having a positive charge. The electrical difference of this tension between the clouds and the ground is called voltage. Once this tension reaches a certain magnitude an electrical discharge occurs between the clouds and the ground: this is the flash of lightning, a huge electrical discharge.
It would be an error to believe that the bolt of lightning is a single phenomenon, rather seldom or rare. 50 to 100 flashes strike every second somewhere around the world with an average intensity of 20.000 amps, some strike with up to 200.000 amps. Some 2.000 flashes hit the Spanish peninsula every year. Taking all this into account it is easy to imagine that the resulting damages in people, animals or material belongings are quite high: the total amount sums up to hundreds of millions.
As far as scientific knowledge goes today, it is impossible to avoid the discharge of a flash. But one can protect oneself from it. In 1750 Benjamin Franklin
Discovered the main characteristics of the lightning conductor. His invention has been applied to protect buildings. The lightning conductor is a metal strip connected to the ground. It is as if at this point the ground is located at the height of the lightning conductor. The discharges take, without failing, this shortcut.
Knowing the magnitude of damage caused by a flash of lightning, it is only logical that laws have been passed to protect people and material belongings. At the beginning of 2006 a new technical code for buildings came into law. In its section 8 “safety in case of strikes of lightning” it states clearly that it is obligatory to install devices to give overall protection. Therefore it is obligatory for the following buildings:
- Buildings where toxic, radioactive, highly flammable or explosive substances are manipulated
- In buildings higher than 43 m
- An expectedly higher frequency of strikes
To calculate the expected frequency of lightning strikes several parameters are taken into account. Geographical location (number of strikes on the ground in km2 per year), the surface of a structure to be protected in m2, the location of the building in relation to its surroundings (standing alone, close to other buildings or trees of the same height or even higher, surrounded by lower buildings or standing isolated on a hill, …). The admissible risk is calculated taking into account the type of construction (structure or cover in metal, concrete or wood), the content of the building (inflammable or not), its use (commercial, sanitary, teaching or a highly frequented place) as well as the necessity to continue the ongoing activities (hospital, fire station, police station, …) or the possibility of a strong environmental impact.
Once it has been established that a protection against a possible strike by lightning has to be installed, at least two principal elements have to be considered to assure an efficient and optimal protection.
One is an exterior element that will protect the structure from a direct strike. This is what is known as a lightning conductor. It consists in a rooftop lightning rod connected to the ground by multiple conductors paths. This way a possible discharge will be carried without any danger into the ground, hereby minimizing the risk of fire or explosion.
The second one acts at the same time as an interior installation that intends to protect our electrical installation. It avoids the penetration of discharges and residual currents caused by an indirect strike of lightning in the surrounding area in a radius of 1.5 km. It also protects the electrical installation from overload coming from a flash of lightning, affecting the electrical line or the telephone line.
This lightning protection system is necessary if we want to have an efficient protection of the people and the belongings in our homes or in our businesses.