~ National Lightning Safety Institute ~

Section 5.1.2

Before, during, and after lightning strikes to ground, highly mobile charged cloud centers redistribute themselves, attempting to reach equilibrium with opposing polarity earth charges, including man-made (conductive) structures. This atmospheric charge redistribution (ACR) creates strong electromagnetic fields similar to those of lightning. ACR can deliver voltage and current surges into conductors similar to those caused by cloud-to-ground lightning.

Example: ACR charges may be imposed upon parts of a building. Differing magnitudes of charge (di/dt) then are a function of the building’s various metallic systems (e.g., buried cast iron water lines, overhead power lines, adjacent connected radio towers, roof-mounted service equipment). Charge differentials can occur due to faster buildup or reduction in one nearby conductor than in another. When the dielectric breakdown of the air between the two sources is exceeded, a localized arc can occur. Resulting damage can range from data corruption to physical equipment failure. Yet there may have been no observed lightning. Proper attention to bonding and surge protection can ameliorate this problem.