Lightning stroke simulation by means of the leader progression model. I. Description of the model and evaluation of exposure of free-standing structures

A model of lightning-channel progression towards the Earth is given based on the latest knowledge on the physics of discharge in long air gaps. The physical assessment of the electric field is done with the charge simulation method, which is adopted to input into a computer program all parameters involved in the progression of the negative downward channels towards the Earth and those involved in the inception and propagation of upward positive channels from earthed structures. Starting from the geometrical configuration of the earthed object and taking into account the orographic conditions, the lateral distance may be evaluated as a function of the severity of the lightning stroke in terms of impulse charge and thus of current amplitude. Provided the statistical distribution of the currents is given, the model therefore allows the number of lightning strokes to an object to be estimated, yielding an assumed ground flash density. Reference is made to the critical radius concept acquired with laboratory tests with long front surges. A check of the model with field data relevant to free-standing structures is presented and a reasonable agreement is found