What can be expected from laboratory tests of lightning rods

Such tests were started in the 30s of the past century. Air gaps 1-3 m long were used, then in the 60s they grew to 10 m, and several years ago the experiments were carried out with the length of air gaps of up to 30 m. As you see, it's an old story, and there is no task solution still. There is no unambiguous answer about the transfer rules of major experiments results to the real lightning. It's not surprising. A spark discharge in the gap op 10-30 m has much to do to the lightning channel kilometers long.

Not only different in length gaps were used in the laboratory tests but different voltage sources. The first generators of pulse voltages formed the so-called standard thunder pulse with the front of 1.2 microsecond and duration of 50 microsecond (1.2\50 microsecond). It is still used for standard tests of external insulation, though it has slight correlation to the real speed of electric field change from the charge transported to the ground by the downstream leader. In fact, this field grows by the ground approximately like Elide field grows in time at the leader start with the linear charge 0,5 mCl/m at the height of 3000 m.

электрическое поле кВ/м electric field kV/m
время, мс time, ms

It is an average lightning in terms of electric parameters. It is easy to make sure, that the process of intensive growth of electric field is carried out at the last 200--500 m of the way in millisecond time range and that is why the impulse of 1.2/50 microsecond is little applicable for laboratory modelling. In any case, the results of laboratory experiments on the estimation of protective action of lightning rods with the impulse voltage with a low-angle (~1 microsecond) front are essentially different. Unfortunately time parameters of voltage pulse are not the only source of uncertainty of laboratory experiments. The polarity of voltage, length of the gap and configuration of electrodes strongly influence the results. That is why real sizes of protection zones can't be obtained during laboratory tests. Computer calculation models and authentic exploitation data for their testing are necessary. It is most important that these models would base on the real picture of lightning development and would not contradict the accumulated data about the physical mechanism of its development.

It is the second part of the cycle of articles "Everything about lightning protection zones". To read the full material, you must register on the site.

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