Khi phóng điện trong khí quyển tia sét có thể di chuyển với tốc độ km/h ( mph) vì sét là sự di chuyển của các ion nhưng hình ảnh của sét là do dòng plasma phát sáng tạo ra nên có thể thấy nó trước khi nghe tiếng động vì tiếng động chỉ di chuyển với tốc độ km/h (767 mph) trong điều kiện bình thường của không khí còn ánh sáng đi được m/s. | BS EN 62305-1 General principles BS EN 62305-1 General principles This opening part of the BS EN 62305 suite of standards introduces the reader to the other parts of the standard. It defines by its five annexes the lightning current parameters that are used to design and then select the appropriate protection measures detailed in the other parts. Damage due to lightning There is an initial focus on the damage that can be caused by lightning. This is sub-divided into Damage to a structure including all incoming electrical overhead and buried lines connected to the structure Damage to a service service in this instance being part of telecommunication data power water gas and fuel distribution networks . NOTE BS EN 62305-5 part 5 which relates to this latter type of damage will ultimately be deleted from the standard. See the explanation on page 10. Damage to a structure is further subdivided into sources of damage and types of damage. 12 BS EN 62305-1 Damage due to lightning íùrse Source of damage The possible sources of damage are identified as follows S1 Flashes to the structure S3 Flashes to the services connected to the structure Overhead service connected to the structure eg Telephone Structure S2 Flashes near to the structure S4 Flashes near to the services connected to the structure Ground level V Underground service connected to the structure eg Low voltage mains power Figure Sources of damage Type of damage Each source of damage may result in one or more of three types of damage. The possible types of damage are identified as follows D1 Injury of living beings due to step and touch voltages D2 Physical damage fire explosion mechanical destruction chemical release due to lightning current effects including sparking D3 Failure of internal systems due to Lightning Electromagnetic Impulse LEMP This wider approach of taking into account the specific services power telecom and other lines that are connected to the structure is identifying that .
