~ National Lightning Safety Institute ~

Section 5.1.1

When lightning strikes an Asset, Facility or Structure (AFS), the return stroke current will divide up among all parallel conductive paths between attachment point and earth. Division of current will be inversely proportional to the path impedance Z, (Z = R + XL, resistance plus inductive reactance). The resistance term will be very low, assuming effectively bonded metallic conductors. The inductance and corresponding related inductive reactance presented to the total return current will be determined by the combination of all the individual inductive paths in parallel—the more parallel paths, the lower the total impedance.

Lightning can be considered current source, i.e. output current is independent of load impedance. A given stroke will contain a certain amount of charge (coulombs = amps x seconds) that must be neutralized during the discharge process. If the return stroke is 50 kA, then that is the magnitude of current that will flow, whether it flows through one ohm or 1000 ohms. Therefore, achieving the lowest possible path impedance serves to minimize the transient voltage developed across the path through which the current is flowing [e(t) = I(t)R + L di/dt)].

A risk management approach to lightning safety must assume the AFS will be struck by lightning. Now what? By adopting a judicious combination of lightning protection subsystems, we can attempt to mitigate lightning’s consequences. Since each AFS is unique, as is each lightning flash, the lightning safety engineer must apply site-specific designs. Application of subsystem approaches for air terminals, conductors, bonding and shielding, earth electrodes, surge protection devices/transient limiters, etc. will depend on geographic location and risk to the AFS. Good guidance is found in reputable codes and standards such as IEEE 142, and 1100, Air Force 32-1064, NASA E-0012E, FAA 019c, Army 385-64, Navy OP5, German DIN 57185, South African SABS 03-1985, UK MOD ESTC No. 7, and the IEC 61024 venue.