Lightning ;The results of research conducted in France in recent years by Franklin France and Meteorage have led to a, significant breakthrough in the knowledge and understanding og the lightning phenomenon . This knowhow has halped to develop beter protection and prevention devices;but lightning retains many of its mysteries and will remain, both for researchers and manufacturers , a focus of fascination and awe. The ideal answer to the question of what lightning really is can provided by quoting P.G. Lairent , who wrote in 1950.

• ‘’Lightning is an important event which no one can avoid from its danger , but also it can be observed easily . It prossesses milions of volts, capable of shattering insulating obstacles, but flows through small diameter conductors. Since it is corious during its descent to explore neigbouring metal structures and conductors, where, rightly or wrongly, it hopes to find an easier outlet , it is wiser to pyave its way with appropriate interconnections than to bar its way tith obstacles that are ineffective.’’

• ’Lightning explores the surronding space supplies to asort of sensitivity that it procuses from the electrical field that it propagates and this same field means of breaking through inslator by applying its electrical force. The essential idea behind every protection system is to avoid situations in which these forces can be brought into play.’’

This figurative language characterized the physical reality of lightning.

The Storm Cloud ; Lightning is produced by cumulo-nimbus storm clouds.The upper part of the cloud is made up of positively charged ice crystals, while the base contains negatively charged water droplets.This separation of charges is caused by atmospheric turbulance.

A highly intense electrical field is created between the charges and when this field reaches its breakdown point, an electrical discharge occurs, either as ligtning flashes between clouds or cloud zones or as lightning between the cloud and the ground. During fine weather, the electrical field on the ground is around 100V/m. As the storm (negavite storm) approaches, this becomes a positive value possibly reaching 10 to 25 Kv/ m. The potantial difference between the cloud and the ground is then of some several tens of megavolts (see )diagram.

Discharge Phenomenon;From The cloud base, generally charged , a low-luminosity discharge known as the tracer, is released. This makes its way to the ground in leaps of some tens of meters. I is in fact series of discharges each taking the route ionized by the previous discharge an inteeuption of 40 to 100 us (avarage speed of progression of 0.5 to 1 m/us).As it approaches the ground, the highly charged tip of the tracer causes the electric field vertically below it to increase considerably eaching values of 400 to 500 Kv/ m. Whwn the ionization threshold of the atmospheric air is reached (30Kv/cm)at points highly prone to lightning (tree tops, chimneys , lightning conductors , etc.) jets are released. Where the field values are the highest, this jet with the best triggering characteristics, or which travels most quickly, will reach the leader. This forms an ionized channel providing the perfect electrical junction between the storm cloud and the ground.There is a return stroke from ground to the cloud by which it neutralizes its own charge. Within the space of 0.2 to 1 seconds, several lightning strokes may be exchanged in continuous progression and a very high propagation speed.

The main effects of lightning are as follows:

Thermal effects: These effects are linked to the quantity of charges involved whwn lightning strikes. For materials with high resistivity , they cause various melting points at large amount of energy is released the form of heat.The moisture they contain causes a sudden overpressure that may result in explosion.

Effects Due To Arching : The resistivity of the soil makes earthing resistant and therefore unables to prevent a sudden rise in the facility’s potential whwn lightning current passes through it . This creates differences in potantiel between the various metal parts . Earthings and connections between the metal parts must therefore be carefully designed to down conductors.

Electrodynamic Effects : These effects are produced if part of the path along which the lightning current travels is within the magnetic field of another part. This may produce repulsion and attraction forces when lightning travels through conductors close to each other.

Electrochemical Effects : These are negligible and have no effect on the earthing (compared with stray current in the soil).

Acoustic Effects – Thunder : Thunder is due to the sudden pressure rise (2 to 3 atmoshernes) in the discharge channel subject to electrodynamic forces during the lightning strike. The duration of the thunder depends on the length of the ionized channel.The propagation of the spectral components produced by the shock wave is at right angles to the channel fort he higher frequencies but omni directional for lower frequancies. The results is a series of rumbling and crackling sounds that vary according to the distance of the observer from the lightning chamels and the direction taken by the channels.

Induction Effects : Induction effects are often the biggest challange to protection systems. When lightning approaches a site and flows through its conductors , it creates anmagnetic flux that produces high , and sometimes destructive induced vltages. Electromagnetic loops may be formed between lightning conductor down leads and electrical circuit. This is why protection systems must be very carefully designed and must include any necessary additional protection devices.

Luminous Effects : A lightning strike creates an image on the observer’s retina which may leave him dazzled for several seconds before regaining sight.

Indirect Effects : Ofset potential or pace voltage. Dispersion of lightning currents in the soil depends on the nature of the terrain. A heterogeneous soil may create dangerous differences of potential between two neigboring points.

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