Adopting a simple model in which the discharge is represented by an equivalent electrical network, we determine the whole positive lightning discharge instantaneous parameters such as the current, the velocity, the electromagnetic fields,... The effects induced by a very close lightning on a 500 m power distribution line are also characterised. It is shown that for the distances that we considered, the leader electric fields change contribute significantly to the amplitude and wave shape of the induced voltages and could operate some lightning protection devices before the return stroke.