Lightning surge currents vary from a few amperes to 200 000 amperes. Portions of the wave change so rapidly that time intervals of the order of a microsecond need to be measured, while at the same time the duration of the complete stroke may be longer than one second, or 1000 000 microseconds.
The element of chance is also introduced in that the point at which lightning may strike is unpredictable. The probability of a given point being struck is enhanced by height so that in some cases instruments are installed on tall objects. However, the available evidence indicates that discharges to such objects differ in important aspects from those to low objects.
Stekolnikov and Valeev22 in Russia in 1936 obtained the first oscillograms of the current in direct strokes. These studies were made with a balloon attached to a metal cable and flown at altitudes between 500 and 800 meters during a thunderstorm. Resistance shunts were connected directly in the cable circuit for a cathode-ray oscillograph and a high-speed rotating klydonograph. In addition, a short horizontal antenna about 20 meters above the ground was used with a cathode-ray oscillograph and a rotating klydonograph.
During the lightning season only two storms occurred near the lightning station, but six strokes to the vertical antenna were recorded, five in one storm. The cathode-ray oscillogram of one of these is shown in Fig.
From International Standard IEC 1024-1:
"In order to disperse the lightning current into the earth without causing dangerous overvoltages, the shape and dimensions of the earth-termination system are more important than a specific value of the resistance of the earth electrode. However, in general, a low earth resistance is recommended.
From the viewpoint of lightning protection, a single integrated structure earth termination is preferable and is suitable for all purposes (i.e. lightning protection, low voltage power systems, telecommunication systems).
Earth termination systems which must be separated for other reasons should be connected to the integrated one by equipotential bonding…"
Also According to MIL-STD-188-124B, Grounding, Bonding and Shielding, Department of Defense, Washington DC, 1992:
"The resistance to earth of the earth electrode subsystem should not exceed 10 ohms at fixed permanent facilities." (p. 8)