Formation of lightining
The ground and the electrosphere, the conductive zone of the atmosphere (thickness in the order of 50 to 100 km), constitute a natural spherical condenser which charges by ionization, leading to an electric field directed towards the ground in the order of a few hundreds of volts/meter. The building up of the stormy clouds, masses of water in the form of aerosols, comes with the electrostatic phenomena of charges separation: the light positively charged particles are pulled by the ascending airstreams, and the heavy negatively charged particles fall because of their weight. It also happens that small islands of positive charges are located at the base of the cloud, at the place of intense rains. All in all, on a macroscopic scale, a dipole is created. When the limit gradient of discharge resistance is reached, a discharge occurs within the cloud or between clouds or cloud and ground. In this last case one speaks of lightning. A thunderbolt between cloud and ground breaks up into two phases: the development of a tracer, ionized canal, which causes the thunderbolt itself and the discharge of intense current arc.
The figure illustrates the case of the descending negative thunderbolt (development of the tracer from the clouds towards the ground, with discharge of the negative part of the cloud), only application case of the electro-geometric model. This type of thunderbolt is by far the most frequent.
The building up or the arrival of a stormy cloud causes the creation of an electric field (ambient) between the cloud and the ground. This electric field can exceed 5 KV/m at ground level, thus initiating the creation of crown discharges starting from the irregularities of the ground or the metal masses.
The thunderbolt begins with the formation within the stormy cloud of a descending tracer being propagated by jumps towards the ground. The descending tracer carries electric charges, increasing the field on the ground.
An ascending tracer grows from a structure or an object connected to the ground. It is propagated up to join the tracer going down and the lightning current flows in the channel thus created. Several ascending tracers can be emitted from various structures on the ground. The first of them which joins the descending tracer determines the impact point of the lightning.
Speeds of the ascending and descending tracers are close to 1 m/µs.
For people indoors:
- Abrupt rise in potential of elements connected to external lines (power lines, telephone, cables of the outdoors TV antennas)
- Increase of the contact voltage of metal objects
For the equipment:
- Abrupt degradation (breakdown) or premature ageing (repeated constraints), interruption
- Malfunction (e.g. the control)
The oscillogram below gives some idea of the current at play and its wave shape.
When the lightning hits a wiring, all voltage and power levels are concerned.
- Fusion or perforation of metal sheets.
- Fusion of antennas cables and wires (small section)
- Attraction, Repulsion
- Potential differences and energizing effects
- Potential rise of the earth electrode
- Induction phenomena
Thermal effects : An exceptional thunderbolt is able to pierce metal sheets with a thickness of 2 or 3 mm.
Discharges of low intensity, but of long duration easily light a fire.
The bad contacts are particularly dangerous points on the way of a lightning’s current. Even contact resistances of a couple thousandths of an ohm cause heat releases that are large enough to liquefy and squirt appreciable quantities of metal into parts.
Fusion effects are often observed in the small sections or for strong resistivities. In bad conductors, much energy is released in the form of heat by the flow of the current. That is why the water contained in wood, masonry and similar materials is overheated and vaporized. Because of the brevity of the phenomenon as a whole, and as a consequence of the overpressure resulting from it, bursting of trees, wooden masts, beams and walls can occur.
Electrodynamic effects : The most notable consequences of the electrodynamic effects are the powerful attraction efforts between parallel ways offered to the lightning’s current, when their spacing is rather small. This is that way thin tubular antennas are crushed and parallel conductors clash together.
Potential differences and energizing: Our sites have an earthing by a buried meshed grid to which all the conducting parts are connected. The earth electrode of the lightning conductor and those of the other equipments are thus connected to each other. The flow of the lightning’s current in the resistance of the earth electrode will increase the potential of the descent of the lightning conductor, but as all is inter-connected, there is no specific potential rise of a few parts, it is the whole wiring which rises in potential, which prevents breakdowns.
A lightning conductor’s going down conductor (that which connects it to the ground) forms open loops with various metal structures of a building which will be the seat of electromotive forces.
- Lightning conductor protects buildings from the lightning
- Lightning conductor is located above the buildings so that it collects the lightning and deviates it towards the ground
- Thus it avoids the risk of striking people, degradation of the equipment and fire
The lightning conductor is located above the buildings so that it collects the lightning and deviates it towards the ground, and thus avoids the risk of striking people, degradation of the equipment and fire.
- The Lightning Protector protects the electric grids against over-voltages (atmospheric or resulting from an intervention.
- The Lightning protector limits the terminal voltage of an equipment
- It includes a non-linear resistance which becomes conductive when the voltage exceeds a given value
- It is found as a component of a sub-station where it is connected between phase and ground of an air line likely to collect the lightning
The lightning protector limits the terminal voltage of an equipment due to the operations on the grid or against overvoltage of atmospheric origin. It includes a non-linear resistance which becomes conductive when the voltage exceeds a given value. It is especially found as a component of a sub-station where it is connected between phase and ground of an air line likely to collect the lightning.
Lightning Protectors and Conductors
Protection levels and effectiveness
A direct thunderbolt directly strikes the structure or its protection system, whereas an indirect thunderbolt strikes the ground near the structure or the networks (electricity, gas, communication, etc.) which penetrates into the structure.
The characteristics of the protection system against lightning depend on the characteristics of the structure and of the protection levels to complete. For example, the protection level will fix the parameters of the lightning’s current (peak current, total charge, impulse charge, specific energy, average stiffness) used in the study, the periodicity of checking, the distance, the influences on the distance of security between the descending conductor and a conducting mass connected to the ground.
Selection of protection level
- Determination of the awaited frequency of thunderbolts on the structure Nd.
- Determination of the accepted frequency of the blows on a structure Nc.
- If Nd < Nc : Optional Protection
- If Nd > Nc : Compulsory Protection
With effectiveness E > 1 – Nc/Nd choice of the protection level corresponding to E
Nd = Ng max. Ae. C1. 10th-6 (/year)
Ng is the average annual density of striking on the ground (no. of impacts / year / km²). It can be obtained by the map of striking density, the consulting of a localization network, or starting from the local keraunic level.
Ae is the equivalent capture surface of the structure alone (m²)
C1 is the environmental coefficient.
Nc = 5,5. 10th-3/C, with C = C2. C3 C4. C5 where C2 is the structural coefficient, C3 holds account of the contents of the structure (flammable products), C4 is related to the structure’s land use, C5 is evaluated according to the consequences of a striking.
Volume protected by a capture device
The volume protected by a shaft of vertical capture is considered as having the shape of a right cone, whose axis is the capture shaft and whose half angle is given by the following table according to the protection level.
The angle depends on the height of the shaft and the selected protection level.
The more the height increases, the more the angle decreases, and the higher the level of protection is (less effectiveness), the more the angle increases for the same height.
There are lightning conductors with energizing device which are shaft lightning conductors equipped with a system generating an early energizing, highlighted when it is compared under the same conditions with a simple shaft lightning conductor. It has a larger radius of protection than the simple shafts.
A capture device is a part of the external wiring intended to intercept the lightning. The impact point of the lightning is determined by the object on the ground which will be the first at the distance D of the descending tracer, even if this object is the ground itself. The distance D between the meeting point of the descending and ascending tracers is called « energizing distance » : it is also the development length of the ascending tracer.Thus, all occurs as if the tip of the descending tracer were surrounded by a fictitious sphere, of radius D, centred on it, and as if this sphere rigidly accompanied the tip of the descending tracer.
The position of the capture device is suitable if no point of the volume to be protected comes in contact with the sphere of radius D travelling on the ground, around and on the structure in all the possible directions. The sphere can only touch the ground and/or the capture device. This distance D is a function of the protection level.