FOUDRE PROTECTION: Protection strategies 1/3

LIGHTNING PROTECTION: DIRECT LIGHTNING STRIKES
Reminders of the general principles of lightning protection: lightning rods and mesh cages

 

 

 

 

 

 

 

One thing's for sure:

A shock of lightning goes always to earth and takes the path of least impedance.

Two strategies in lightning protection:

- or make the path between the object to be protected from lightning and the earth as impetuous as possible.

We then place ourselves in a strategy of isolation from lightning,

- or to facilitate the passage of the lightning shock to the ground by a very low impedance path which bypasses the object to be protected from lightning.

We then place ourselves in a lightning flow strategy.

PROTECTION AGAINST DIRECT LIGHTNING STRIKES

Direct lightning strikes and lightning insulation

Problem: what are the existing means to protect buildings against direct lightning strikes using only the so-called isolation strategy?

-Isolate the building from the ground: the idea is not obvious and yet houses on stilts do exist. The materials used to separate the body of the building from the ground must be insulating.

-We can also imagine using only insulating materials for the whole construction, wood for example, we are then in the presence of a cottage.

 Boundaries :

- Earthing by networks (electrical, telephone, etc...).

Solution: - Measurement of the electric field (field mill)

>>> if predefined threshold is reached

>>> opening of electrical circuits

>>> starting the generator set

- Availability of materials and aesthetics according to regions.

- Is the isolation obtained sufficient in the face of a lightning strike? If so, is it always the same over time?

Direct lightning strikes and lightning flow protection

It is a question here of supporting the passage of the current of lightning by a way which diverts it from the building to be protected from lightning.

Lightning conductor

A lightning rod is a device that is not used to remove the lightning but instead to attract the lightning  its presence increases the risk of the lightning on the site he has to protect.

Principle of lightning protection by lightning conductor :

arrange, vertically, on or near the building for which thelightning protection is intended, a conductive structure (metal rod) connected to the ground by a low impedance conductor in order to make the lightning fall on a chosen place and not just anywhere on the installation.

To help capture the current of lightning, the upper end of the rod is either tapered or equipped with pulse or spark gap systems.

 

The effectiveness of these lightning protection systems is due to the presence of a spark or impulse just when the downgoing lightning tracer is in the vicinity of the lightning conductor.

It is in everyone's mind, that a virtual conical volume, the so-called cover volume, provides lightning protection lightning of everything it encompasses. The height of this protection would be determined by the distance between the ground and the end of the lightning conductor's tip. Its base, in the case of a simple rod, would cover a circular surface whose radius is equal to the height. The other tips would provide even greater ground coverage. The comparison between the different lightning conductor techniques is not easy to imagine, because even in highly exposed countries, the probability of a lightning strike from lightning falling on a lightning conductor is much too low (about 1%) for comparative studies to be carried out.

According to the electromagnetic model, now widely recognized, the reality of lightning protection by lightning conductor would be quite different. The protection zone with respect to the lightning is in fact determined by a lightning pick-up sphere whose centre is the end of the tracer and whose radius is defined, in an approximate way, by the formula: R = 5 x I 2/3. An average lightning strike of lightning , 40 kA is thus captured by any conductive element located about 35 m from the end of the tracer. This model indicates that the coverage zone (protection) does not depend, neither on the nature of the lightning conductor nor on the height at which the lightning conductor is placed, but only on the intensity of the lightning strike.

The bigger the blow of lightning is, the bigger the protected area of lightning is.

 

 Faraday Cage

Faraday CageA Faraday cage is a volume made up of metal walls inside which one is protected from the direct effects of a discharge (lightning for example). One does not imagine to place, to obtain a good protection lightninga building inside a totally enclosed metal envelope. But by applying the electromagnetic model to the Faraday cage, we can see that the envelope does not need to be complete to capture and then flow the discharge current from the lightning to the ground. Strapping the construction with preferably flat conductors spaced every 10m provides protection. lightning of the building for all lightning strikes above 3 kA, which is clearly sufficient for complete protection against the lightning.

This device by the multiplicity of the descents to the ground favours the distribution of the currents and thus reduces the risks of rise in potential related to the flow in only one point.

The meshed cage principle is also used for lightning protection of installations outside buildings. These are known as guard cables. These are cables stretched in the air supported by poles spaced 10 to 15 m apart and connected to a meshed earth network.

Protection against lightning, the Ariane rocket, the solar furnace at Font Romeu, and the H.T. lines on the EDF pylons are thus achieved.

 A ground network correctly realized :

Here is the most important precaution to observe, when using, for protection lightning, the run-off strategy. The lightning conductors or the strips of the mesh cages should be as vertical and straight as possible. Care should be taken to respect the bending radii, cross-section of the down conductors and other installation rules defined by the standards. In buildings with a metal structure, it is necessary to establish a connection between the different ground planes of the floors and the down conductors of the lightning conductors or mesh cages, in order to avoid potential differences (PDD) between each floor and the earthing system.

Without this connection the PDD, at a height of 10 m, on a 100 kA shock would reach approximately: ΔU = Lω x di / dt ΔU =10 x 10-6 x 100 x103 / 10-6 ΔU = 1000 kV This result is to be divided by the number of earth drops of thelightning protection lightning.

Without interconnecting the floor ground plan directly with the downspouts, this RFP is present between the floor and the downspout lightning that passes 20 or 30 cm further outside the building. The risk of breakdown is then important.