Lightning Discharge

shutterstock_73347937Worldwide, some 44,000 storms occur every day, bombarding land and sea with about 9 million lightning strikes. A typical lightning strike has a peak current of between 5kA and 150kA (30kA is the average), which can cause extensive damage including risk of fire at the strike point.

The risk of a direct lightning strike to a structure can be statistically evaluated using ground flash density maps that show the number of ground strikes expected per square kilometer per year. The apparent area of the structure is calculated to assess the likely number of strikes. A taller structure is a more “attractive” target for lightning, therefore its “apparent” area is larger than a lower height building.

Metallic structures, such as silos, inherently provide a good electrical path for conducting lightning energy to ground. Concrete and other non-metallic structures (e.g. grain silos) should utilize lightning down-conductors to provide a controlled route to ground, rather than the signal circuit becoming an inadvertent path. A low impedance grounding system for the down-conductors will reduce ground potential rise problems and will be discussed later.

The large metal platforms of vehicle weighing systems are also often lightning targets. For critical systems, a part of the successful protection approach is to install a lightning protection terminal mounted on a nearby taller object to draw the lightning away. This “protection-by-isolation” may utilize an existing light pole or purpose installed mast system.