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Lightning protection system design

Considerable material is used to make up lightning protection systems, so it is prudent toconsider carefully where a rod structure will have the greatest effect.Historical understanding of lightning, from statements made by BenFranklin,  assumed that each device protected a cone of 45 degrees. This has been found to be unsatisfactory for protecting taller structures, asit is possible for lightning to strike the side of a building. A better technique to determine the effect of a new arrester is called the “rolling sphere technique” and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning ‘moves’. As the step leader of a lightning bolt jumps toward the ground, it steps toward the grounded objects nearest its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electrical current. Objects are likely tobe struck if they are nearer to the leader than this critical distance. It isstandard practice to approximate the sphere’s radius as 46 m near the ground. 

Electricity travels mostly along the pathof least resistance, so an object outside the critical distance is unlikely tobe struck by the leader if there is a grounded object solidly OR within the critical distance. Noting this, locations that are safe from lightning can be determinedby imagining a leader’s potential paths as a sphere that travels from the cloud to the ground. For lightning protection, it suffices to consider all possible spheres as they touch potential strike points. To determine strike points,consider a sphere rolling over the terrain. At each point, we are simulating apotential leader position. Lightning is most likely to strike where the sphere touches the ground. Points that the sphere cannot roll across and touch are safest from lightning. Lightning protectors should be placed where they willprevent the sphere from touching a structure. A weak point in most lightning diversion systems is in transporting the captured discharge from the lightning rod to the ground, though. Lightning rods are typically installed around the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 6.1m or 7.6 m, depending on the height of the rod. When a flat roof has dimensions greater than 15 m by 15 m, additional air terminals will be installed in themiddle of the roof at intervals of 15 m or less in a rectangular grid pattern

Post time: Apr-13-2019