Relative Importance of Electromagnetic Shield Violations

Abstract : In the topological approach to hardening systems against lightning, a barrier that is effectively impervious to electromagnetic energy is placed between the source of interference and the system to be protected. To be impervious, the barrier must form a topologically closed surface. The ideal barrier can never be achieved in practice, but it is not necessary. The three categories of compromise are, in order to importance, penetrations, apertures, and diffusion. Diffusion is almost always negligible (unless the metal walls consist of foils), and wire penetrations are almost always by far the worst compromise. Thus, an otherwise excellent barrier may be compromised by one untreated penetration. The compromise may go undetected until a major portion of the lightning current flows on the barrier near the compromise. At that time, it will become quite evident that the barrier is not sufficiently impervious to electromagnetic energy. Lightning, from an interference point of view, is a broadband electromagnetic phenomenon, and therefore practices that are adequate at low frequencies or dc will not be satisfactory for lightning protection. Examples of such practices include cable shields terminated in pigtails, grounding conductor penetrations, and apertures with a wire penetration. We have performed several simple laboratory experiments to demonstrate quantitatively the effects of those practices.