The paper have the purpose of investigating the influence of different parameters to enable better understanding of the transient performance of complex lightning protection systems (LPS). Lightning discharges constitute the major source of atmospheric or natural noise that can interfere with electric and electronic installations. The electromagnetic characterisation of the LPS environment plays a fundamental role in order to prevent unwanted coupling phenomena that may generate abnormal signals, electric stresses dangerous for the insulation of electric components, disruptive discharges and danger to persons. The model, developed by the authors, is based on a field-approach; the numerical solution of a modified version of the thin-wire electric field integral equation in frequency domain is carried out. Time profiles of electromagnetic interesting quantities are computed by using a discrete fast Fourier transform algorithm. In particular, the model takes into account coupling effects among aerial parts and ground electrodes in order to correctly estimate the quantities that can determine electromagnetic hazard inside the LPS. Influence of the shape of the earth-termination and the air-termination system, soil resistivity, location of feed point are investigated in order to pick out the parameters which have the greatest influence in the transient performance of LPS. Moreover, different lightning current shape are used in order to evaluate the influence of the current impulse front and, more specifically, the average steepness of the impulse.