Counter charge from a lightning rod on the roof

2. Counter charge from a lightning rod on the roof

The second part of the article "Lightning rods on the roof (lightning protection of the roof)"

It is clear, that the roof won't influence the electric field at the top of the lightning rod, if its dimensional size is compatible with the radius of the rod electrode. In this extreme case, the protective effect of a rod lightning rod on a small roof should be equivalent to the lightning rod of the summary height (height of the building + lightning rod), mounted on the ground. In another extreme situation, it is necessary to study rod lightning rod, located far from the edge of a very large roof, the size of which exceeds the height of the rod. Now a lightning rod mounted on the ground must have an equivalent protective effect, but this time the height of the roof won't matter. We should only care about the height of the rod itself.

In order to see the influence of certain roof sizes, it is useful to remember the images about the orientation of the lightning channel. According to the widely-known hypothesis, the counter leader plays the main role in the process. It starts from the top of the lightning rod in the electric field of a thunder cloud and approaching to the object of the lightning channel. A corona preceeds the start of the counter leader. It is formed from the top of the lightning rod, filling the space above it with its spacial charge. For the counter leader to originate, the corona should transfer to a streamer form. Following the characteristics of corona from a lightning rod on the roof, we can get quantitative idea about the influence of its sizes. The task of such a kind was numerically solved for an axisymmetric system - cylindrical building with the rod lightning rod in the center. The results of typical calculations are presented in Fig. 3, for buildings 50 m high and a 5-meter rod lightning rod.

Fig. 3. The estimation of roof radius influence on the development of a counter discharge from a lightning rod

Ток короны - corona current

Стержень - rod

Радиус цилиндра – cylinder radius

Время - time

Red curves on the figure characterize formation of corona in the field of a thunder clout for two extreme cases, when the rods are fixed directly on the ground and are 5 and 55 m high (the summary height of the building and lightning rod). Please pay attention to the logarithmic scale of the charts. The dependence of the corona current on the height of the electrode is very storng here. The current changes in the calculations given for about 2 orders of magnitude. Black calculated curves demonstrate the values of corona current at the installation of a 5-meter rod on the roof of a cylindrical building 50 m high of different radius. The radius of the rod in the calculated remained unchanged and was equal to 1 cm. We can see, that the real weighty influence of the roof reveals when the distance from its edge to the lightning rod is commensurate with its height. With the increase of this distance, the screen effect of the roof plane increases, and that is why, corona current reduces. But even at a tenfold increase of the roof radius over the height of a lightning rod, the characteristics of corona are still different from the bottom curve, which characterizes a 5-meter rod on the ground surface. The doubtful attitude towards to the recommendations of EIC standard is natural. The prescription of the national instruction on lightning protection IS 153-34.21.122-2003 doesn't cause any trust, where the height of the lightning rod is prescribed to be counted from the ground level. Real roof sizes are not taken into account there. The absurdity of the prescription can be easily seen from an elementary calculation of rod lightning rod protection zone by emperical formula from this instructions.

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

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