About the practice to choose lightning rods

4. About the practice to choose lightning rods

The fourth part of the article "How to choose lightning rods after all?"

The problem of electromagnetic compatibility at all the consequences forces to use lightning rods of minimally possible elevation above the protected facility in order to avoid significant increase of the number of close strikes, inciting strong electromagnetic field in the protected object. Reduction of height, as a rule, is provided by the increase of the number of rod lightning rods or use of multi-wire lightning rods. Design on protection zones for the indicated purpose is impossible, as in any situation the total volume of space, protected by the system of lightning rods, significantly increases the sum of protecton volumes of every lightning rod. Statistical method was based in order to estimate the protective effect of multi-rod or multi-wire system of lightning rods.

National and foreign regulatory documents give the values of reliability of lightning rod protection zones P_з, and consequently, the admissible probability of a lightning breakthrough to the protected constructions

formula-1

It is reasonable to ask in what extent the admissible probability of a breakthrough to estimate the real security of object's functioning in a thunderstorm environment.

Unfortunately, the probability of a breakthrough doesn't give an explicit answer, because the second significant value is the summary number of lightning strikes, captured by lightning rods. Only knowing it, it will be possible to define the expected number of lightning strikes during the fixed term of object exploitation and only then judge about the real danger of thunderstorm impacts. Here we see the advantages of statistical method, which allows to directly determine these values without estimating them on the protection zones with the error on the level of magnitude order.

You should not think that the statistical method we offer in its today's implementation can be studied as flawless. The reflection of the statistical nature of the formation of lightning is its advantage. However, the used calculation algorithms are far from being perfect, because they contain formalistic simplifications in its base. It is impossible to avoid them at the moment. The ways to improve the method are obvious. First of all, the method should include the charge, transported by the lightning channel, on which its electric field depends, and consequently, the conditions of the counter charge start defining the height of the orientation. In the result, it will bring to the dependence of the height of orientation on the lightning current. The latter comes from all the existing hypothesis of the main lightning stages, connecting the potential with the lightning leader's charge and its current. For example, basing on [17], it is possible to get the following functional relation between the potential of the downward lightning head potential ad its main stage current (after the contact of the channel with the grounded structure), which is well approximized by the polynominal (fig. 10).

Fig. 10 Dependence between the lightning chanell head potential and its current of the main stage according to the hypothesis [17]

Fig. 10
Dependence between the lightning chanell head potential and its current of the main stage according to the hypothesis [17]

formula-21 (21)

In simplistic terms, further the linear dependence on the orientation height on the potential of the lightning leader was supposed, which finally brought to the empirical expression

formula-22 (22)

Dependence, built in Fig. 11 according to this expressionm shows that in a practically significant range, the orientation height increases with the growth of the lightning current, achieving the value of H_o ≈ 12h at extremely large current with the amplitude of 200 kA.

Fig. 11 Height of the orientation for lightning discharges with different currents of the main stage on [17]

Высота ориентировки – orientation height

Ток молнии - lightning current

Dependence of H_o (IM) in the frames of the statistical method opens the way to the calculation of the expected number of lightning breakthroughs with different currents, which gives a chance of a proper evaluation of expected damages. Such evaluations will become possible, when somewhat reliable expolitation data for control checking of the main lightning stage model or according to the differentiation of the number of lightning strikes by its currents will appear.

E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow