|The classic models|
The classic models are based on strike distance (or last permeable distance) S (I). If head of leader gets inside that distance, then that object will attract the lightning. The strike distance depends on quantity of leader's charge, which is proportional to the peak value of strike current:
In above-said formula the peak current of lightning is inserted in kiloampers and the last permeable distance is in meters. It is very important relation because it is connecting the peak current of lightning and last permeable distance. According to classic models that distance is same for the earth and for objects. Essentially, an assumption is that when a leader comes close to some object on permeable distance, lightning will strike into that object. Different authors have proposed the different values of coefficients in above said formula, and in the next table we can see some of these values.
It is important to say that although this is the oldest model, it is still in use, especially in protection of objects where we can be satisfied with lower safety conditions, or in designing smaller lightning rod installations.