Protection characteristics of a Faraday cage compromised by lightning burnthrough.

Abstract

A lightning flash consists of multiple, high-amplitude but short duration return strokes. Between the return strokes is a lower amplitude, continuing current which flows for longer duration. If the walls of a Faraday cage are made of thin enough metal, the continuing current can melt a hole through the metal in a process called burnthrough. A subsequent return stroke can couple energy through this newly-formed hole. This LDRD is a study of the protection provided by a Faraday cage when it has been compromised by burnthrough. We initially repeated some previous experiments and expanded on them in terms of scope and diagnostics to form a knowledge baseline of the coupling phenomena. We then used a combination of experiment, analysis and numerical modeling to study four coupling mechanisms: indirect electric field coupling, indirect magnetic field coupling, conduction through plasma and breakdown through the hole. We discovered voltages higher than those encountered in the previous set of experiments (on the order of several hundreds of volts).3

Loop-to-loop coupling.

“…2 is the analytic solution for the normalized magnetic field when the magnetic vector potential associated with the rectangular loop geometry (7) is carried out. While this approach shows excellent agreement with the numerical simulations (for both  = 0 and  = 4"), it is important to recognize that it is a much more tedious calculation than the closed-form calculation based on the constant magnetic line charge (3). Note that the results in Fig.…”

“…Assuming the rectangular loop is thin with gap 2∆   we can calculate the magnetic flux per unit length emanating from the twin stripline as Φ =  0  (4) where the stripline inductance per unit length is  0 and can be approximated as [3] …”