a)Introduction to lightning

In a normal weather, there is an electrical field at the surface of Earth. This electrical field is about 100 to 150V/m worth. Indeed, Earth atcs like a condensator with a good insulator : the low parts of the atmosphere, the tropopause, situated between 2 conductors, the ground and the inonosphere.

The ionosphere is the higher part of the atmosphere, not very dense, in which the atmosphere’s ionization by the bombardment of particles or by the radiation( UV or X-rays coming from the sun, cosmic radiation, etc …) is the most important. Indeed there are 2 phenomenons in the atmosphere: ionizations (particularly in the higher parts of the atmosphere ) and ions losses, caused by the phenomenon of ions recombinations(the less dense the atmosphere is, the less important these recombinations are). These 2 phenomenons balanced in the ionosphere and so a large number of ions are present in that zone of the atmosphere : then this zone is conductive. The ionosphere is globally neutral where as the ground is naturally neagtivly charged, wich explains the permanent presence of the magnetic field at the surface of Earth.

The atmosphere is not a perfect insulator because there is a slight outflow current between the ground and the ionosphere. This current would end up discharging the ground if there wouldn’t have been mechanisms to reload it, like storm.

c) How a lightning protection system works

A lightning protection system is designed to control or force the lightning discharge on a specified path, thus eliminating the chance of fire or explosion within nonconductive parts of the house. This may include the phone, cable, or electrical lines, the water or gas pipes, or (in the case of a steel-framed building) the structure itself. Lightning usually will follow one or more of these paths to ground, sometimes jumping through the air via a side flash to reach a better-grounded conductor (watch animation above). As a result, lightning presents several hazards to any house or building:

Fire– Fire can start anywhere the exposed lightning channel contacts, penetrates or comes near flammable material (wood, paper, gas pipes, etc) in a building – including structural lumber or insulation inside walls and roofs. When lightning follows electrical wiring, it will often overheat or even vaporize the wires, creating a fire hazard anywhere along affected circuits.
Side flashes – Side flashes can jump across rooms, possibly injuring anyone who happens to be in the way. They can also ignite materials such as a gasoline in a garage.
Damage to building materials – The explosive shock wave created by a lightning discharge can blow out sections of walls, fragment concrete and plaster, and shatter nearby glass.
Damage to electrical appliances – Televisions, microwaves, phones, washers, lamps and just about anything plugged into an affected circuit may be damaged beyond repair. Electronic devices and computers are especially vulnerable.

Each year 32 million lightning strike truck the surface of earth. The consequences of so many lightning are thousands of homes and other properties are destroyed or damaged by lightning strikes. Unprotected homes risk to be damaged by fire, explosions or electrical surges. Personal injury can occur in an unprotected house which has been struck by the lightning. The effects of such a lighting strike can devastate a human both physically and emotionally.

A lightning protection system does not prevent a lightning strike, but provides a safe path on which the lightning current can safely be directed to the ground. The air terminals, cables and ground rods work together to prevent fire and other hazard which can damage the structure and appliances.

a) Components of a lightning protection system

Currently, the lightning conductor refers to all a protection system designed to protect people from the lightning, so the real lightning rod is only a small part of a complete lightning protection system. In fact, the rods may play the least important role in a system installation. A lightning protection system is composed of three main components:

Rods or air terminals: Rods can be found in different shapes, sizes and designs. Most are topped with a tall, pointed needle or a smooth polished sphere. The functionality of different types of lightning rods, and even the necessity of rods are subjects of many scientific debates.

Conductor cables: These heavy cables are made of copper or aluminum. They connect air terminals to the grounds cables and carry lightning current from the rods to the ground. Cables are run along the tops and around the limits of roofs, then down one or more corners of a building to the ground rod(s).

Ground rods or protection grounds: Long, heavy rods buried deep into the earth, around a protected structure. Main conductors are set at least 300 meters deep in the earth and are attached to metal grounding rods. Special requirements are sometimes necessary in sandy or rocky soil. The conductor cables are connected to these rods to complete a safe path for a lightning discharge around a structure. The energy is directed into the ground as current flows through the rods. Then the chance for injury or damage is eliminated.

The conductor cables and ground rods are the most important components of a lightning protection system, which  divert lightning current safely through the structure. A full protection setup composed of good cable coverage and good grounding, would still work sufficiently without the air terminals.

The French connection: Marly-la-Ville experiment

To improve Franklin’s thoughts about electricity, the physicists had to check thunderclouds are electrified. So three of them, Buffon, Dalibard and Delor carried out the experience and wait a thunderstorm. M. Dalibard chose for this purpose a large open area, situated at Marly-La-Ville, where he placed a pointed bar of iron, twenty-meter high. Silk ropes (g) and wine bottles (e) insulated a 13 meter iron rod (a) from ground, and covers (h) protected the ropes from rain. Then the 10th of May 1752, twenty minutes past two in the afternoon, a stormy cloud having passed over the place where the bar standing, people who were appointed to observe it, drew near and attracted from it sparks of fire, perceiving the same kind of reactions as in the common electrical experiments. The result of all the tests and observations were related in Dalibard’s mémoires and especially of the last test done at Marly-la-ville, is that the matter of thunder is incontestably the same as that of electricity. Franklin’s theory ceases to be a conjecture; it has become a reality. The sparks drawn at Marly-la-Ville proved, for the first time, that thunderclouds are electrified and that lightning is an electrical discharge.