The possible influence of a metal body in a lightning protection system was determined by comparing the electrical breakdown probability of two symmetrical sphere-rod gaps, one conventional and the other with a floating electrode, i.e. a rod with a series gap introduced in its conducting path. 50 switching voltage impulses were applied to the arrangement for each series-gap length. The breakdown probability was obtained for each rod as a function of the series-gap length. The results show that the rod with a series gap shows the same breakdown probability as the conventional rod for a certain limited range of the series-gap length. Beyond this range the conventional rod has always a higher breakdown probability. The analysis of the actual breakdown voltage in different configurations has shown some differences in the physical process that lead to breakdown in different arrangements.