The use of ‘Field Intensification Factors’ in calculations for lightning protection of structures

The spatial distribution of the electric field in the vicinity of a prospective strike point on a structure is an important factor in the determination of the likelihood of a lightning strike to that point. Hence, the assessment of the effectiveness of various positions of lightning rods for the protection of structures ideally requires three-dimensional calculations of the electric field, not only for the various trial positions of the lightning rods, but also for various positions of the downward lightning leader. This paper shows that such calculations can be greatly simplified by the use of “field intensification factors”. These factors are calculated for likely lightning attachment points such as corners, edges and lightning rods, but using an imposed electric field which is uniform in the absence of the perturbing structures. It is shown that these factors can be applied in the general case of any lightning downward leader at any arbitrary position, with errors of less than 20% compared with exact calculations made without the use of the approximation. Examples are given of calculated field intensification factors for various shapes and dimensions of typical structures, including composite structures, and protective lightning rods placed on structures. Quantitative relations for the spatial values of the field intensification factor as a function of structure and rod dimensions are obtained, leading to a simple method for the estimation of conditions for corona and upward leader inception. The results are discussed in terms of practical aspects of lightning protection, such as the use of such calculations for the placement of lightning rods in the most appropriate locations on structures.