Tham khảo tài liệu 'ăngten chống sét', kỹ thuật - công nghệ, kiến trúc - xây dựng phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 3 Double pitch roof Hipped roof 6 Sawtooth roof Pyramid roof PROTECTION FROM LIGHTNING Around a latitude of 50 lightning strikes the ground approximately 60 times and cloud 200-250 times per hour of storms. Within a radius of 30m from the point of strike trees masonry work etc. persons in the open air are in danger from stepped voltages and consequently should stand still with their feet together. The damage liable to be inflicted on building constructions is due to the development of heat. Ground strikes heat and vaporise the water content to such a degree that walls posts trees etc. can explode due to the overpressure generated wherever dampness has collected. Roof structures dormer windows chimneys and ventilators should receive particular attention in lightning protection systems and should be connected into the system. A lightning protection system consists of lightning rods down conductors and earthing devices. In essence a lightning protection system represents a Faraday cage except that the mesh width is enlarged. Also initial contact points or lightning rods are fitted so that the point of impact of the strike can be fixed. Thus the lightning protection system has the function of fixing the point of lightning strike by means of the air terminals and ensuring that the structure lies within a protected zone. The air terminals or lightning conductors are metal rods roof wires surfaces roof components or other bodies. No point on the roof surface should be further than 15m from an air terminal device. On thatched roofs due to the danger of ignition resulting from the corona effect metal bands 600mm wide should be laid over the ridge on wooden supports - 8 . When flowing a lightning current can reach 100000A and due to the earthing resistance a voltage drop of 500000V occurs. In the instant of the strike the entire lightning protection system and all components which are connected to it by metal parts are subjected to this high potential. Equipotential bonding
