PROTECTION AGAINST INDIRECT LIGHTNING STRIKESIt is a question here of considering the overvoltages, related to lightning, which reach a material via conductors carrying overvoltage, or by induction effect.
Induction, linked to lightning, causes overvoltages on open circuits which lead to overvoltages that cause common mode (to earth) tripping, and on closed circuits, current flows that are destructive due to thermal effect.
An ideal case of representing this strategy would be that of equipment powered autonomously, by batteries or solar panels, which would communicate with the outside world only by radio, GM or satellite. Sensors or actuators that are only local would have no reference, even indirect, to earth. All the materials containing the electronics would be insulating and the electrical conductors would not form a loop. Only differential mode overvoltages could then disturb this system. There would then be no ground or earth to connect. But in reality, such cases are extremely rare.
The protection of persons often requires the presence of an earth connection, metallic masses exist, and therefore differences in potential appear on the installations. However, the existence of earthing systems does not exclude the possibility of using the advantages offered by the isolation strategy.
Powering sensitive equipment autonomously, using batteries or solar panels, avoids the arrival of majorlightningshocks via the energy distribution network, which is one of the preferred vectors for lightning. But if the energy network is to be used, the isolation strategy can still be implemented:
- Isolation momentarily,
- Isolation permanent.
Field mills: We have previously seen the practical drawbacks of these systems (heaviness, cost...). A major disadvantage generally excludes it from the chosen solutions:
the adjustment of the field mills is delicate:
- set too low, they trigger unwanted alarms,
- set too high, they are ineffective.
the isolation transformer : For reasons of cost, the means used to isolate oneself are usually reserved for low consumption installations.
It's about:
-Isolation transformers
-Network conditioners, which combine an isolation transformer and a voltage regulator.
-Wave absorbers, which consist of alightning arrester, an inductor and an isolation transformer.
Used alone or in combination with other components, the isolation transformer is the centrepiece of insulation protection processes.
As soon as the frequencies to be protected from are higher than 100 kHz, the galvanic isolation is no longer sufficient (this is the case with lightning), it is necessary to use an isolation transformer with screen. In this case the screen is connected to the earth and the transformer ground by the shortest possible connections.
Good results can be obtained without a screen, but this requires a very low level of ability between primary and secondary school. The use of an isolation transformer leads, to ensure the protection of people, to apply the prescriptions of the NFC 15-100 standard on separate circuits.
For the telephone lines we proceed by galvanic separation using specific products such as: Mimosa housing, Isolitel or other transducers. These materials are generally expensive and the first two can only be used on analogue lines. Broadband transducers can also be used on digital lines.
The transmission of information, on a wired medium, between devices, presents a significant risk of destruction bylightning for these devices.
To introduce galvanic isolation, the following is used à the air or at the light as a transmission vector.
-Relaying transmission lines: efficiency depends on the insulation of the relay contacts, generally less than 2000V, and on parasitic capacitances.
The transformer-based galvanic isolator can be used for analogue signals. The principle is to transform the analog input signal into a frequency and to do the reverse after passing through the transformer.
-In the case of radio transmissions, the air, which is the medium of transmission, isolates the transmitter from the receiver. However, it is still necessary to protect these instruments locally and to ensure that the antennas are isolated from the ground network and from the masses. Otherwise, they act as a lightning conductor.
In both cases the process consists of transforming an electrical signal into an optical signal. It is then possible to communicate through an electrically insulated transparent medium.
-The opto-coupler is an electronic component placed at the input and/or output of electronic boards.
This solution has several drawbacks compared tolightning protection lightning: The distance between the transmitter and the light receiver is necessarily small since they are included in the same component. reduced insulation (2000V), existence of parasitic capacitance, to preserve the light separation, the power supplies of the diodes, transmitter and receiver of the same channel, must be different.
When there are several opto-couplers at the input of a device, in general the power supply of the transmitter diodes is common to all the opto-couplers. In the event of a common mode fault on one of these inputs, the power supply is coupled to the other inputs. The information received can then be distorted.
-The optical fibre has an incomparable isolation, due to the distance between the light transmitter and receiver. The fibre, used to transmit digital information, avoids the problems of common power supply points encountered with opto-couplers. For mechanical protection reasons, shielded fibres are available. If this shielding is metallic, the insulation obtained corresponds to the greatest distance existing between one of its ends and the ground of the site where it is located. The interest of the fibre could therefore be questioned. Although circuit insulation is an attractive strategy, forlightning protection , by its apparent simplicity, the limits encountered during its application make it necessary to consider the possibilities offered by the flow strategy.
Indirect lightning strikes and flowIf sensitive equipment cannot be sufficiently isolated from the local earth to protect it from shocks indirectlightning strikes, it must be protected from lightning by placing on the wired connections of the modules lightningarresters or surge arresters.
In the event of an overvoltage, these modules ensure that the surge current flows to earth. In order to provide effective protection against lightninglightning by flow, special attention must be paid to the following three areas: -Quality of the wiring of the installation and especially of the earth network, -Immunity of the electronic equipment, -Performance of the protections lightning arrester, Quality of the wiring, earth network Wiring and earth network must be particularly careful if one wants to obtain the full effectiveness of the products used to protect an installation from lightning . With the help of the C 15-100 standard, we will first of all define the various terms used when discussing protection issues, namely :
- earth network, - mass network,
- foreign elements.
The earth network consists of a set of conductors buried in direct contact with the ground and electrically connected to each other. The term " earthing system " is used for small installations, while the term "earthing network" is more commonly used for large installations. The ground network is made up of all the passive, unburied conductors that connect the metal enclosures of the devices to each other. It is not necessarily connected to earth. It can be accidentally connected to the active parts of the circuit as a result of a fault in the insulation of the active conductors.
Extraneous elements are those that can introduce a potential, generally that of the earth, and which are not part of the electrical installation. Examples are visible or non-visible conductive parts of buildings and also water, gas and heating pipes, as well as non-electrical appliances connected to these pipes (radiators, metal sinks, etc.).
The role of the earth network is to ensure the safety of people and secondly the protection of the installations with regard to lightning. It allows currents of all origins to flow inside the ground: fault currents at 50 Hz, in TT or TN mode or currents from lightning. It sets the reference potential of the installation. In low frequency or for transient currents with slow variations, the earth can be considered as resistive, its value will then depend on the shape of the elements that make it up, the installation conditions and the resistivity of the ground. This is true up to a few tens of Hz.
From a few hundred Hz the impedance of the conductors becomes mainly inductive and it varies proportionally with frequency. This ground impedance, in front of a fast variation of the current, can be higher than its resistance in low frequency.
In the case of the flow of currents of lightning, the characteristics of the ground networks must be determined for currents of up to 100 kilo Amperes and for frequencies that can exceed the megahertz.
It generally consists of : a continuous cable that surrounds the building where the electrical installation resides. A long conductor buried in a trench. Bare metal in direct contact with the earth. The long conductor is sometimes replaced by a pole driven vertically into the ground. The best results are obtained in the surface layer of the earth, which is why a long conductor is preferred to a vertical post. The quality of the connections and the resistance to corrosion are the most important installation qualities. A low value of the earth resistance is necessary for the protection of people but for the protection of lightning shocks, this value is not crucial. Very low resistance facilitates protection, but it is far better to make efforts to obtain an equipotential network and be satisfied with earth values below 50 Ω. Measurements carried out on reallightningstrikes and with alightning generator have shown that a two-legged "star" has a lower impedance at high frequencies than a straight cable buried horizontally and of the same length. If the length of the cable used to make an earth connection has very little influence on its impedance, on the other hand, the resistivity of the soil and the frequency of the shock wave influence this impedance in proportion to their square root. These characteristics lead to the use of the so-called "crow's foot" shape for down conductor flows.
The equipotentiality of the different earth inputs of the same installation conditions the efficiency of thelightning protections . It is first necessary to define the notion of "reference earth" or "distant earth" which is placed far from the point of entry into the ground of the current of lightning and whose potential is defined as zero.
The flow of current following a blow of lightning by the soil will cause the potential of the local land to rise, as it is not perfect. This potential rise can be represented by impedances between the reference measuring point and the grounding point of the installation. 
Let's take a facility with two separate grounding systems:
-One, called A, for protection against lightning is traversed by the current of lightning Ia.
-The other, called B, concerns electronic equipment and has a residual current flowing through it Ib.
If Za and Zb are the impedances of the earthing systems in relation to the local earth, and if Zc is the impedance between the local earth and the distant earth, we can calculate the potential rise of earth B in relation to earth A for alightningstrike of lightning of intensity Ia and residual current Ib.
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