Why Lightning?

The protection of buildings and structures against direct lightning strikes is well understood. All the recognised world standards on lightning protection present techniques for the protection of buildings and structures through the use of air terminals, down conductors and earthing systems. All these techniques are largely unchanged from the days of Benjamin Franklin and still the most common air terminal is the Franklin Rod. It is only now that other non-conventional systems are gaining some acceptance although much controversy still surrounds their effectiveness.

A lightning strike is an electrical discharge resulting from the build up of electrical charge in a thundercloud. Discharge may occur from cloud to cloud, or from cloud to ground. The current which flows in a cloud to ground strike ranges from about 2,000A to 200,000A with a log normal distribution.

The current in most ground flashes is from a negatively charged cell, although positive discharges also occur. The current flow is unidirectional with a rise time of less than 10 microseconds and a decay time of 100 microseconds or less. Multiple flashes along the same ionised path are likely. These may be spaced by around 50 to 100 milliseconds.

Conventional lightning protection systems work well at protecting buildings and structures from the direct effects of a lightning strike and adequately protect the occupants of these structures.

The indirect effects of these events must also be considered. It is not only lightning that can create damage but a host of other events caused by power line disturbances and load switching transients. These are caused by inductive load switching, capacitor banks, thyristor power supplies and other electrical switching operations.