Assessment of protection system positioning and models using observations of lightning strikes to structures

This paper presents the results of an analysis of the observed incidence of lightning strikes to a wide range of earthed structures in Hong Kong. The data were obtained for 161 structures and span the period 1988–1996. These structures were protected with lightning air terminals, placed in the optimum locations on each structure using a new positioning method which is an improvement of the simple ‘electrogeometric model’. The aims of the paper are fourfold, namely to assess(i) the effectiveness of this new lightning protection positioning method,(ii) the validity of various attractive radius models, especially since many of the structures are located in high–density city blocks,(iii) the validity of a Poisson model for describing the strike frequency, and(iv) the effect of any assumed height dependence of the median peak current on the attractive radius models.The results show a significant positive correlation between expected and actual strike frequency and confirm that the interception efficiency of the lightning protection system is in accord with the specified protection levels. In general, attractive radius and Poisson strike models for ‘free–standing’ structures are found to be valid except for structures of height less than 100 m in dense city blocks. The results also show an influence due to positive flashes and a decrease in attractive radius if the peak lightning stroke current is assumed to be dependent on the structure height.