Lightning: Preamble

 

This lightning presentation is intended to provide a practical approach to the various practical aspects of lightning protection for installations and equipment commonly used in industry.

Of course, the protection of individuals remains the primary concern of all those responsible.

However, this field is a matter for specialists, and the electrical installation inspection bodies fulfil this role competently.

We will therefore confine ourselves to the material aspect of the lightning subject, even if a look at the protection of persons is sometimes sketched in.

Damage: In France: Several million lightning strikes per year (particularly during hot weather).

Lightning sometimes kills and above all causes damage, often spectacular:

- People and animals struck by lightning,

- Buildings destroyed by the dozens,

- Electronic equipment, household appliances, ... damaged,

- Malfunctions in automatic systems.

The extent of the damage reflects both the power of the phenomenon and the lack of attention paid to methods of protection.

 

 

"Nothing can be done about lightning".

 

This is an extremely widespread and yet particularly false idea. A resolutely technical approach can only convince us otherwise:

 

 

"Of course, it is always possible to protect yourself from the effects of lightning. »

 

The first step is to consider lightning as a classical physical event and no longer as a divine or supernatural intervention.

We will then escape the realm of superstition and enter the realm of rationalism.

An attempt will then be made to highlight the facts and characterise the action of the lightning.

 

 

Lightning, a physical phenomenon:

 

If lightning is a physical phenomenon, its study must be possible. Numerous talents, particularly French, have brilliantly tackled this task. From their work, the following lessons can be drawn:

- Lightningdischarge process

- Exposed areas

- Shock wave

- Propagation of the wave

- The effects of lightning

 

 

Lightning discharge process :

 

Lightning only occurs in the presence of cumulonimbus clouds. They have a particularly large mass in the vertical plane and can reach heights of between 2 and 14 km. The temperature differences between the upper and lower parts of the clouds cause intense air circulation (thermodynamic activity).

By friction the moving particles then become electrically charged and these charges accumulate at the vertical ends of the clouds. By influence of the environment, including the ground, polarizes in the opposite direction to that of the nearest cloud area.

The blows of lightning A shot of lightning is qualified as negative when the negative part of the cloud discharges, and as positive when it does not.   

A lightning strike is called downward when the precursor starts from the base of the cloud, and upward if the precursor ends at the base of the cloud.

Under the influence of these charges, electric fields appear, between clouds and earth as well as between clouds.

The values of these fields can reach a few tens of kV/m.

Impurities in the air and ambient humidity during thunderstorms favour a local increase in electric fields and a decrease in ignition voltages.

Electrostatic discharges are then produced within these different dipoles.

The discharge process begins.

The discharge process therefore begins its cycle as soon as the first break in insulation occurs between the base of the cloud and any point in its environment.

Then, in successive jumps, from 50 to 100 metres, part of the cloud's load is transferred to the point of impact. The path of these "pre-discharges" traces a highly ionised path called: tracer or precursor.

The path followed by the tracer is very irregular. As it moves along, the potential gradient (electric field) increases and gives rise to numerous branches. In the vicinity of objects, constructions or any other elements that could serve as a vector for the dissipation of lightning energy, sparks (capture discharge) fly out and meet the precursor.

The ionized path between the point of impact and the cloud is then completely traced.

The potential difference along this path reaches a few hundred million volts. A current of considerable intensity immediately flows, called: first return stroke, main discharge or return arc.

The peak value of the discharge current varies from a few tens of kA (40 kA on average) to a maximum of a hundred kA for downward negative lightning strikes.

In Europe, where the climate is temperate, they account for 80 to 90% of the shocks.

Positive upward shocks can reach intensities of several hundred kA.

Several return arcs, on average 4 per flash, follow one another within a time span of 500 ms to 1 s.

 

 

Areas exposed to lightning

 

To fix ideas on the more or less great exposure of a geographical area, it is usual to use now the notion of keratin level, moreover consecrated by the standard NF C15 100. The kerunic level which indicated, for a determined place, the number of days during which thunder was heard in one year is more and more replaced by the average number of shocks per km². The relationship between these two notions oscillates between :

Nc = Nk / 10 and Nk/20

The kerunic level in France can reach 35, which means that 3 to 4 shocks per km² affect the most exposed areas of our country every year.

To fix the spirits one can retain that Indonesia knows kerunic levels of 200! Everyone understands that these attempts to measure the storm intensity of an area do not take into account the realities specific to a given site. Some complementary aspects that we will see later will help to achieve this.

 

 

Shock wave

 

To facilitate studies, analyses, tests, comparisons, it is essential to use references common to all the actors.

In order to characterize the lightning resistance of equipment as well as the ignition voltages of lightning protection devices, the lightning shock wave has been standardized. It is commonly referred to as a wave: 1.2 / 50 8 /20

It has a very wide frequency spectrum, ranging from very low frequencies to more than 1 MHz.

The voltage wave: - Rises by 1.2µs (from 10% to 90% of U), - When the disruptive voltage (ignition voltage) is reached (100% of U), the current wave appears and falls in 50 µs (50% of U). The current wave: - Rise of 8 µs (from 10% to 90% of I) - Fall of 20 µs (50% of I). - It is responsible for the destruction of materials by thermal effect.

 

 

Propagation of the wave

 

The description of the discharge process presents the cloud/earth assembly as a dipole which discharges following successive good boots.

In order to define the circumstances that favour the discharge of this huge capacitor, we must ask ourselves why a shock occurs in a given place. Among the elements that favour this, we can mention the following:

- The distance between the charges(= between the cloud and the ground): The smaller the distance, the easier it is to initiate. The most numerous impacts are always recorded in mountainous areas.

- The nature of the soil: the more conductive it is, the more it is loaded. Mining areas, coal, iron, even in flat country are more exposed as well as large areas of water.

- The topography of places which, like valleys, favour the circulation of winds and cause the air to be ionised, here more than there creating low impedance passages.

- The presence of long antennas (overhead HV cable) or high structures with or without a spike effect greatly favours ignitions.

The tracker's route by vouchers therefore owes nothing to chance, it is always carried out by the easiest route. As this is an electrical phenomenon, the easiest path is the one with the lowest impedance. It is appropriate here to speak of impedance and not only of resistance, because the frequencies of the shock wave are, as we have seen, higher than MHz.

It should be remembered that the impedance of a circuit is directly proportional to its resistance as well as to the frequency to which it is subjected and inversely proportional to its capacity.

In the case of analysis, methods and means of protection, which we are dealing with here, the earth is always the final destination of the lightning shock, even if aerial strikes (between clouds) are the most numerous. This deliberate simplification of the propagation of the shock wave, allows in concrete terms to foresee the ways to protect oneself and also to state a simple rule, for the use of non-specialists who wish to understand the phenomenon in order to master their own protection system:

A lightning strike always goes to ground and takes the path of least impedance.

 

 

The effects of lightning

 

Two types of lightning strikes can be distinguished by their effects and therefore by the means of protection against them:

Direct hits:

They are shocks that fall directly on an object or living being to flow to the ground.

Indirect shocks:

an object or a living being that is affected by the passage of a current, although it has not been touched by the lightning strike.

This current flows, either because a conductor carries the surge from the shock point to the flow point, or because it has propagated through inductive, electrostatic or electromagnetic coupling.

If the effects of a shock that directly strikes equipment or a building do not need to be detailed. Those of indirect shocks are not always identified as being due to an atmospheric discharge.

For example, electrostatic effects on an appliance are not preceded by the characteristic sound of a lightning strike and therefore not associated with thunderstorm activity.

The energies involved may be sufficient to damage, at the moment, components that will be destroyed a little later in time.