Calculation of interception efficiencies for air-terminations using a dynamic electro-geometrical model

The paper firstly presents the method of a dynamic electro-geometrical model. In contrast to the classic rolling-sphere it does not use fixed radii, it works with a varying radius. The method only uses existing and in international standards accepted results, fundamentals of lightning physics, and investigations; on that base a numerical method is elaborated. Using the dynamic electro-geometrical model, secondly some examples of protection with air-termination rods planned with the classic rolling-sphere according to IEC 62305-3 and for the classes of protection I–II–III–IV are investigated. It is shown, that the interception efficiencies are much higher than documented in the standard series IEC 62305. Reason is, that the method of the rolling-sphere is conservative, and that it gives the planner of lightning protection systems only the points, where lightning may strike, but without a rating with a striking probability. On the other hand this result clearly indicates, that using the classic rolling sphere method one is always on the “safe side”.