Lightning protection of tall structures

The protection of tall structures against direct lightning strikes is usually provided by the installation of a lightning rod of the determined height on the top of the structure. The tip of the rod instead of the top of the structure becomes the point of the initiation of the upward lightning or intercepts the downward lightning. The installed lightning rod does not decrease, even slightly increases the number of lightning incidents to the protected structure. However, there is another alternative lightning protected system, which is being installed instead of the lightning rod, reduces significantly the number of lightning strikes to the structure. This system consists typically of a hemisphere of a large radius with an array of many thin metal splines with sharpened tips distributed evenly over the hemisphere's surface (LPAS). Such LPAS is usually installed on the top of the structure to be protected. It is obvious that without the upward-moving leader the number of lightning strikes into the tall structures will be reduced significantly. Theoretical study of non-stationary corona in free air established that the two most important parameters of the influence on the initiation of the lightning leader from the structure were the corona current from the individual point and the voltage drop along the first meters of the air gap between the structure and thunderstorm cloud. The corona current from the individual points of the LPAS must exceed the critical value required for transition of the glow corona to the corona with streamers. The intensification of streamers under the growing electric field eventually creates the plasma channel - lightning leader when voltage drop on the first meter of the streamer length from the structure exceeds the critical value. The LPAS installed on the top of the tall structure injects the space charge developed under the influence of the electric field originated by charges in the thunderstorm cloud into the gap thundercloud cell - the structure. The total corona current from the properly designed LPAS is distributed evenly among the individual splines (points). As a result, a higher value of the electrical field will be required in order to initiate the streamer-free corona transition to the corona with streamer flashes for the LPAS in comparison with the lightning rod. Propagation of the corona space charge toward the thunderstorm cloud smoothes the potential distribution across the air gap between the cloud and the structure. Again, the larger value of the electrical field will be required to develop the voltage across initial meters of the already developed streamers to initiate the lightning leader. Total delay in developing the upward-moving leader in most cases results in the lightning discharge which occurred into some other place but not into the protected structure.