Lightning protection selection practice

The need to consider the impact of electromagnetic fields on the operation of electrical devices forces the designers to use lightning rods minimally elevating above the protected buildings in the calculation of lightning protection. Otherwise, frequent lightning strikes into the facilities located near each other cannot be avoided. In the result of which, a strong electromagnetic field is induced in the protection zone.

There are many examples of adverse effects of electromagnetic fields on the operation of the equipment. Under their influence, there happen short circuits in the switchgear substations that can lead to the complete burning of cables. At a short circuit and simultaneously damaged grounding contour, an increased potential is formed on the grounding device.

The possibility of simultaneous operation of two electric devices without causing strong intereferences, influencing their operation, is defined by electromagnetic compatibility. In addition to lightning strikes and overvoltage, transients in electrical networks, static electricity, and more are their sources. The majority of scientists came to the conclusion that it is impossible to solve this problem completely. Since it is impossible to take into account all possible sources, which may provide interference influencing operation of electrical machinery in the caculations.

The maximum height reduction can be achieved through the use of rod and catenary multi-wire lightning rods. But it is necessary to keep in mind the fact that it is impossible to hold design of lightning protection by protection zones.

In this case, the total amount of space under lightning protection system will be much greater than the sum of the protection volumes of each of the lightning rods taken separately. Proper evaluation of the protective effect by using multi-wire and multi-rod system of lightning rods has become one of the main reasons for the creation of statistical techniques. In accordance with the standards mentioned in the foreign and domestic documents, the reliavbilities of protection Рз which they provide are given for lightning rods protection zones. The probability of a lightning breakthrough in the protection zone Pпр will be equal to:

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In this case, one might ask how exactly one can judge about the reliability of any electrical device during a thunderstorm on the found value of breakthrough probability.

It's unlikely to get a definite answer in this case. You need to know the total number of lightning strikes that lightning rods will intercept. It is the second most important parameter in the calculations. Knowing it, it is possible to determine the expected number of lightning strikes during a certain time. After which it will be possible to make objective conclusions about how dangerous will be the impact of lightning discharges on the object.

In this situation, benefits of a statistical method of calculation of lightning protection gain priority. Because it makes it possible to obtain accurate values of these quantities, and not to make assessments of the protection zones, which obviously cannot be accurate because of the margin of error.

Despite the obvious benefits of statistical methods, it will be a great mistake to consider it ideal. Calculation algorithms used today can hardly be called flawless due to the inability to avoid certain simplifications. However, this method allows you to display the statistical nature of the formation of lightning.

Statistical methods can be largely improved. The value of the charge transferred by the lightning channel should be entered into it. Because the electrical field of lightning and respectively starting conditions of the counter charge, that determines the height of orientation, depend on that.

Ultimately, this will give the lightning orientation dependence on its current. This conclusion can be obtained from all the currently available assumptions about the main stage of the lightning. For which there is a relationship between the leader charge, lightning current and potential.

Thus it is possible to deduce the following functional relationship which with high accuracy can be replaced by a polynomial.

Figure 1. Dependence between the main stage lightning current and channel lightning head charge

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Given the assumption of a linear dependence of the height of the orientation on the leadership potential, we obtain

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Based on the dependence shown in Figure 2 it can be concluded that the height of orientation grows together with the lightning current increase in the range, significant for its practical use. The value Ho ≈ 12h can be achieved at very highlightning currents with the amplitude up to 200 kA.

Figure 2. The height of the orientation increases with an increase of the lightning current