The Lightning - SpeedyLook encyclopedia

The the lightning is a natural phenomenon of electrostatic discharge disruptive.

The load

The Nuage S of Orage (Cumulonimbus) create the weather conditions favorable to the accumulation of electric charges and consequently to the creation of a condensing géant :

a difference in important temperature between the bottom and the top of the cloud, inducing violent one displacements of Air;
the presence of various particles like Ice and dust which by effect triboelectric will facilitate the wrenching or the addition of electron S, according to the sign;
air (and all that it contains) being electrically charged, it creates for itself in the cloud of the zones with electric potential différents : negative at its base and positive at its top. It follows a very important Electric field.

The electrification of the cloud of storm is based on two phenomena: the Gravitation and the Convection.

Gravitation

The drops of Rain, let us hail them and the particles of grésil (of small grains of ice) fall by gravity to the bottom of the Nuage, below the water drops and the crystals of ice of lower size which remain in suspension. When the large particles enter in collision with the crystals of ice at a temperature lower than a limit (around -15° C), the grains of grésil take care negatively, and positively if this temperature is higher than the aforementioned limit. As the grains fall more quickly than the crystals, they transport from the higher zones of the cloud, where the temperatures are lower than -15° C, of the negative charges downwards. The threshold of the -15° exceeded C, those become positive. One then obtains a tripolar structure of the cloud with a median layer charged negatively surrounded by two positive layers. However the shocks between particles are not alone at the origin of the electrification of the cloud.

Convection

The theory of the convection wants that the free Ion S in the atmosphere are collected by the droplets in the Nuage and then transported by the currents convectifs in the cloud, thus producing the areas chargées.
Indeed, on the one hand the cosmic rays strike the Molécule S of Air located above the cloud and ionize them: these Ion S negative is fixed at the crystals and the droplets of the cloud and forms a layer called “screen layer” in top of the cloud. In addition, the intense electric field in the vicinity of the pointed objects on the surface of the Earth produces a “Corona discharge” of positive ions: when the potential of the pointed object is sufficient, a intense Electric field produces the excitation of the electron S neighbouring. Those enter then in collision with atoms neutral, which releases then from new electrons which go, in their turn, to create other electrons and so on, causing a Chain reaction. It is the “electron avalanche” or ionization collision. The positive ions created are then pulled by the hot air rising by convection and thus take part in the electrification of the cloud.

the cloud is thus electrified thanks to a combination of the Gravitation and Convection
the positive sub-base of the cloud being rather fine, it is the negative layer which will have an influence on the Earth. Indeed, at the time of a Orage this one takes care positively by influence.

Discharge

When this electrostatic field exceeds the dielectric limits of the air (variables according to the conditions of moisture and pressure), it follows the discharge of the lightning aiming to one rebalances électrostatique :

the tracer or precursory , transporting a weak electric charge, advances towards a zone of load opposed at a speed of about 200 km/s, thus creating an ionized channel. In the case of a negative discharge, this precursor progresses by jumps lengths proportional to the amplitude of the discharge. It is this phenomenon which the Paratonnerre S. try to exploit
the arc-backs start then successively; they use the channel of the precursor to release the electric charges accumulated at a speed then being able to exceed 100.000 km/s.

Color

Along the traversed way, the gases are overheated and ionized (the temperature can reach 30 there; 000 °C) and forms a conducting plasma thus. What explains the emission of light that one observes. This luminous phenomenon is called flash . The color of this flash depends on several factors: the Density of current, the distance from the observer to the flash and various particles present in the atmosphere. However, in general, the color of the flash is white in an air dry, yellow in the presence of a great quantity of dust, red in the event of Pluie and blue in the presence of Grêle.

Frequency

the lightning is the object of studies Statistique S because there are many differences in characteristics (amplitude, duration, many arc-backs) according to the love at first sight (will intra cloud, cloud-ground, positive, negative).
50% of the loves at first sight has an intensity lower than 50.000 has (amp S) and 99% lower than 200.000 A. Three claps of thunder out of four are done between clouds, but one estimates at 32 million the number of flashes striking the ground each Année in the world.
the frequency of the loves at first sight is defined starting from the level keraunic (many times where the thunder was heard in the year) and especially of the density of foudroiment (many loves at first sight to the km ² per annum). This last mode of quantification can be supplied with means of measurement (sensors of fields).

Thunder

The lightning is accompanied by a Onde Acoustique, the Tonnerre . This wave is generated by brutal the Dilatation of the air overheated by the Electric arc. It can consist of a sharp snap or a deaf bearing according to the distance separating the listener from the lightning.

Other effects

The lightning can be accompanied, in the cases of strong discharges, secondary luminous phenomena in high-altitude. The brevity of these flashes, like their altitude (Mésosphère and Ionosphere), pushed back their discovery by the scientists at these last decades.

Outdistance

Respective speeds of the Light and the its allow a good approximation of the Distance in Kilomètre S of the storm while dividing by three the number of seconds which separate the vision from the luminous flash and the noise of the thunder. For example, if you count 10 seconds before hearing the thunder, the flash fell roughly to 3 km from you (without taking into account altitude and the wind who modify the propagation of the sound).

Let us note that one always perceives the Tonnerre after having seen the flash, the light propagation (~ 300.000.000 m/s) being much faster than that of the sound (~ 340 m/s), so much so that time that it puts to reach us becomes negligible in front of that of the thunder…

Also let us note that this distance is overestimated because initially, the speed of the sound waves is higher than 340 m/s. Indeed, around the electric arc the air is ionized, thus changing the speed of the sound wave.

Various types of the lightning

Intra-cloudy and inter-cloudy flashes

The provision of the electric charges in the storm, as explained before, creates potential differences between the top, the center and the base of the storm. When the potential is sufficiently large, the air between these various levels is not rather any more insulating and a Claquage occurs. The lightning then generated can occur between the various parts of the cloud or nearby clouds.

As these layers are more close in general between them than they are to it ground, this kind of flashes will be the first to be occurred. As the storm takes vertical extension and that the potential increases, the lightning cloud-ground will take the top without never being only. The change of proportion between the type inter/intra-cloudy and cloud-ground is thus an indication of the developmental stage of the cumulonimbus.

Flashes cloud-ground

There exist two types of the lightning cloud-ground: either positive (top of the cloud towards the ground) or or negative (ground towards base of the cloud). The negative type is most frequent but a swing of this type towards the positive type is often indicative violent time because the cloud is then particularly developed. The positive type also often occurs in front of the cloud itself, because it leaves the anvil what can surprise people who think themselves safe from seeing the storm in the distance. The lightning forms on average 3 angles of surroundings 80° before touching its point of impact .

The lightning in ball

Dangers

There are some 2000 storms in the whole world at every moment. These storms produce between 30 and 100 flashes cloud-ground a second or approximately 5 million flashes per day.

The dangers of the lightning are defined by:

direct effects (Thermoelectric S): the circulation of a very strong electric current overheats the matter and causes mechanical damage often very important, even spectacular. Each year, in France, between one and two million loves at first sight are noted, which destroys 250 bell-towers and causes 15.000 fires.

indirect effects (electromagnetic): the induced current of the lightning on the one hand a tension of common mode ( U = R I + L dI/dt) and a electromagnetic Field of an exceptional intensity. It follows the generation of very powerful parasitic electric impulses, which are statistically mainly causes some in the damage. These parasites are indeed enough to degrade sensitive electronic materials (television sets, computers, etc) even if the flash is distant. If the flash is closer, the parasite can also destroy resistant materials more (lamps, engines, furnaces…).

the Conduction: Why our cows fear they it the lightning? The direct blasting of animals (or people) is very rare. However when the lightning strikes the ground, the electric charges are dissipated in the ground and create a more or less important electric tension according to the nature of this one (its Résistivité) and of the distance to the impact, a current then which can pass in the lower extremities. This phenomenon is called " tension of pas".

Detection

Protection

See also: Lightning conductor, Surge protector

The lightning is like exit of a perfect Générateur of current. One of the methods of protection is thus to facilitate the circulation of the electric charges towards the ground by means of nonfunctional drivers.

The lightning conductor will facilitate the way of the channel the lightning by effect of point, on the condition of being in the presence of a love at first sight going down whose precursor only advances by successive jumps and sensitive to the effect of point (90% of the loves at first sight in our areas). It is, then, very important to ensure an electric continuity of great capacity until the Ground. It does not guarantee the interception of an electric arc (a shock of the lightning can fall right in the vicinity). So the significant industrial premises are equipped with many points and conducting ropes. Moreover, it is good to carry out the interconnection of all the conducting parts present at the accesses (for example water pipelines) with this Circuit of descent of the lightning.

A good device external of protection of an installation against the lightning consists of 3 components:

a device of capture, which can take several forms: tended wire, lightning conductors with stem or lightning conductors with priming device, drivers with a grid,… These devices must be dimensioned, according to the level of protection wished, by the method of the fictitious sphere deduced from the model electrogeometric of the lightning, so that an impact the lightning occurs preferentially on the device and not on the installation to be protected.
an earth electrode, made up of a network of naked and buried drivers, in intimate contact with the ground, which must make it possible to disperse " facilement" currents in the ground. With this intention, these drivers must have a low ground resistance (typically, less than 10 Ohm), which makes it possible moreover to limit overpressures likely to appear on the external electric connections which penetrate in the installation to protect.
Of the drivers of descent, which ensure the junction between the device of capture and the earth electrode.

This whole of drivers must be inter-connected correctly and durably.

The dimensioning of a device external of protection the lightning is governed by standards NF INTO 62.305 and NF C 17-100, which propose a method of analysis of risk starting from parameters such as the sensitivity of an installation (presence of dangerous matters, risks of panic,…) and its exposure to the phenomenon the lightning, estimated starting from statistics of blasting of the geographical area in which the installation to be protected is located. This analysis leads to the determination of a level of protection to bring and from which the standard proposes an adequate dimensioning of the device external of protection of the installation. The lightning protectors with priming device are governed by standard NF C17-102.

One will note however that this dimensioning does not take into account the indirect effects of the lightning on an installation, even equipped with a device external limiting the risk of direct impacts on the installation. Indeed, the flow of the current the lightning on the drivers of the device generates an intense impulse magnetic field which can even disturb to destroy certain components of the electrical installation of the building to be protected. Several solutions can be planned in order to limit these effects:

to move away the drivers from capture and descent of the installation to be protected, since the magnetic field radiated by a driver is inversely proportional to the distance compared to this driver;
to multiply these drivers so as to divide the currents: one thus reduces the levels of fields near the drivers and if the current is well distributed autout of the installation to be protected, one also obtains an effect of compensation of the magnetic field created by each driver;
to increase the attenuation suitable for the structure of the installation, for example by an improvement of the electric continuity of reinforcement in the case of reinforced concrete constuctions (welding of the crossings and iron overlappings) so as to constitute a better electromagnetic screen;
to improve the equipotentiality of the metal masses of the installation to limit the potential differences induced, by inter-connecting the various conducting elements of the installation (metal, pipeline beams water, frame of the cupboards and electrical equipment,…) by means of bonding strips for example;
to bring a treatment particular to the wiring of the installation: to place the cables closest to the metal masses (beams for example) or on metal cable shelves connected to the mass at their two ends, which makes it possible to reduce surfaces of the loops of masses and thus the parasitic tensions induced to the inputs/outputs of electrical equipment.

In the electrical communications, one prevents that the lightning falling on the electric lines is not propagated inside station while installing with the top of the drivers electric of these lines of the earth wires, which in addition to their role of support of communication (they contain fiberoptics), play a part of lightning protection. Beyond this primary protection, the protection of electrical installations against the overpressures produced by the lightning on the active drivers of the electric connections is carried out by the use of components Parasurtenseur S (Parafoudre S, spark-gaps with gas, thermisters, diodes transil) the purpose of which are of Court-circuit er the impulses parasite S walking on on the electric connections by deriving the major part of energy from the impulse directly towards the ground. The good wiring of these components is essential with their effectiveness. The length and the position of the cables play a central role indeed. One will be able to refer to standard NF INTO 61643-11 for the choice of the characteristics of these components and to guide UTE C 15-443 for information on their integration in an electrical installation.

Lightning protection in installations BT

Energy

An old chimerical dream is to recover the energy of the lightning to feed in electricity. Such a recuperation of the energy of the flashes always appeared impossible, because not only it would require the cover of the whole of the territory by an immense number of lightning conductors, but it would be far from productive. Indeed a flash is a specific phenomenon releasing large a power but over a weak duration, produced energy is thus relatively weak, even compared to others renewable energies. To take an image: while launching very violently the water of a bucket on a fine pane, one breaks it. That does not mean that there was much water but that water went very quickly.

The order of magnitude of the energy of the lightning is of a few hundreds of kilowatt-hours by shock (approximately 280 kWh, by including the energy of the magnetically radiated wave). There is between 1 and 2 million shocks per annum in France. Thus, if this energy were recovered and shared between the 65 million inhabitants of this country, each one would obtain a share of about 6 kWh per annum, equivalent of only three hours of use of a furnace.

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