A discussion of `Faraday cage’ lightning protection and application to real building structures

Abstract

Lightning protection has been practised for almost 250 years but we still lack an exact expression of the protective effect produced by the air termination. In this paper, protection primarily refers to the `Faraday cage’ (meshwork) type of external protection. The interior of a completely enclosed metal shell is free from the effects of any external changes of electric field – but apart from special constructions – a genuine Faraday cage hardly constitutes a practical solution to the lightning problem. In most cases, we practise this type of protection with an air termination network consisted of square meshes of horizontal conductors. The mesh size results from the `rule of thumb’ rather than from a thorough analysis. Protective mechanism of the Faraday cage (meshwork) scheme for a structure (roof) constructed of good conductor, poor conductor and good non-conductor are analysed and discussed in the paper. The protective schemes were analysed based on the `critical’ distance theory and the `ratio of chord’ theorem. Differences among these three types of protection mechanisms are considered. It appears, that the meshwork only seldom works as air termination but rather as a collector for the current flowing from another part of the roof. Some important still unsolved problems are also presented.