II. ELECTRICAL EQUIVALENT MODEL
The cylindrical metal structure can be considered as a grid of conductors electrically connected to each other forming a cage as shown in Fig. l(a). The cage has been simulated by means of an electric equivalent network. The equivalent model takes into account that the lightning current flowing into the conductors, as an effect of a direct lightning stroke, presents significant harmonic components not exceeding 10 MHz. This upper limit does not allow the representation of conductor elements using simple lumped parameters. The condition