tailieunhanh - A Guide to BS EN 62305:2006 Protection Against Lightning Part 4

Sét cũng được tạo ra bởi những cột tro trong những vụ phun trào núi lửa hoặc trong những trận cháy rừng dữ dội tạo ra một làn khói đặc đủ để dẫn điện. | BS EN 62305-3 Physical damage to structures and life hazard Rolling sphere method Given the lightning process already described in Theory of lightning starting on page 4 it is logical to assume that a lightning strike terminates on the ground or on structures at the point where the upward streamer was originally launched. These streamers are launched at points of greatest electric field intensity see Figure and can move in any direction towards the approaching downward leader. It is for this reason that lightning can strike the side of tall structures rather than at their highest J Downward leader Figure Development of downward leader striking distance This hypothesis can be expanded to explain why corners of structures are vulnerable to lightning strikes. Figure illustrates a sphere rolling over the surface of the building. The dotted line represents the path of the centre of the sphere as it is rolled over the building. The radius of the sphere is the striking distance or last step of the lightning discharge. Thus it can be clearly seen that the corners are exposed to a quarter of the circular path of the sphere. This means that if the last step falls within this part of the circular path it will terminate on the corner of the building. Figure Development of downward leader striking distance The position of the greatest field intensity on the ground and on structures will be at those points nearest to the end of the downward leader prior to the last step. The distance of the last step is termed the striking distance and is determined by the amplitude of the lightning current. For example points on a structure equidistant from the last step of the downward leader are equally likely to receive a lightning strike whereas points further away are less likely to be struck see Figure . This striking distance can be represented by a sphere with a radius equal to the striking distance. Figure Striking distance last step 38 BS EN 62305-3 Rolling