Preventing sparks between external LPS and structure conductive parts

This paper presents a study on the overvoltages that appear between the conductive parts of a structure and its Lightning Protection System (LPS) when it is struck by lightning. The overvoltage calculation is carried out in frequency-domain using the Method of Moments and the results are translated to time-domain by Fourier transform. The structure is represented by an interconnected steelwork and the voltages are computed for standard waveforms corresponding to negative first and subsequent strokes. The likelihood of insulation breakdown is assessed by the disruptive effect method, with conservative criteria that take into account oscillatory voltage waveforms. It is shown that sparks between LPS and structure steelwork are likely to occur, both for first and subsequent strokes, even if the separation distance as prescribed by the international standard is observed. The results also show that bonding the LPS to the steelwork at the top of the structure prevents such sparks for the considered conditions: structure height up to 60 m and Lightning Protection Level (LPL) III/IV. The paper also discusses how this subject is addressed in the international standard on protection against lightning.