Evaluation of Lightning Protection Systems for Explosives

The unpredictable nature of lightning requires that lightning protection systems (LPS) be described in statistical terms such as the expected efficiency of protection or the probability of failure. This implies, as has been observed, that lightning channels occasionally penetrate what has been considered to be a zone of protection provided by the LPS. This lightning penetration exposes assets, such as explosives and related fusing and test electronics, to possible direct effects of being part of the lightning current path. Depending on the current amplitude, these direct effects can cause malfunction, upset, or catastrophic damage to these assets and perhaps to personnel and structures in the immediate vicinity. Even in cases where the LPS has not failed there are indirect effects caused by inductive and capacitive coupling which transfers electromagnetic energy to the interior of the zone of protection in the proximity of down conductors and other elements connected to the LPS. These conductors and elements can carry the bulk of the lightning current or temporarily store a significant amount of charge from the strike. These indirect effects can also cause malfunction, upset or damage to assets depending on the vulnerability of the asset to electric and magnetic fields and currents. Vulnerability depends on operational configurations, such as, stored in an underground igloo in closed metal containers or exposed in a maintenance building connected to electronic test equipment. Some military and industrial LPS specifications require the LPS to be bonded to other metal objects and to other electrical grounding systems. In some geometrical configurations, this additional bonding can enhance (rather than reduce) the possibility of direct and/or indirect coupling to assets. A computer model solving the three dimensional Maxwell's Equations for various LPS environments and corroborated by data from triggered lightning tests is used to show that there are areas wi See also ADA260986, Volume III. Published in Minutes of the Twenty-Fifth Explosives Safety Seminar Held in Anaheim, CA on 18-20 August 1992, p.139-160.