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Until recent years, people thought the installation of a lightning rod provided a building sufficient protection against lightning. This partial protection is now supplemented by a set of compatible davices designed to : -protect against direct strikes – avoid dangerous differences of potential between neighbour points of the building , - prevent induction effects on switch gear and electrical conductors and suppress overvoltage conveyed on network lines.
Standard NF C 17-100 : This Standard sets out , to the extent of present knowledge and techniques , the arrangements required for obtain adequate protection and provides information about the means required to set up this protection.
Its deals with :
Srandard NF C 17-102: This Standard concerns systems using early streamer emission devices.
Lightning Protection by Lightning Conductors : For these systems , tapered rods are placed at the highest point of the buildings to be protected. The rods are connected to the ground via the shortest way. The protection they provide depends on their installation point and height above surrounding structures. Based on the princible of ‘’ Excitation distance ‘’, the electro – geometrical model method provides a reliable calculation of the protection level.Rods fitted with ionizing devices follow the same rules , but the ‘’ excitation distance ‘’ is slightly improved (1.5 to 3 times ) because the arcing delay is reduced. Their advantage lies in increased efficiency , especially where low-intensity lightning strikes are concerned , and they have led to a reduction in the height of the rods which .
Mesh Method : Protection by mesh method involves installing a largemesh Faraday cage on the top of a building and connected to the ground. Small rods (0.50 m)called ‘’strike points’’ are installed at all emerging points around the roof meshes (chimneys, roof top structures.etc.)
The choice of the protection system depends on: The technical possibilites offered by the various procedures , taking into account (for example theat a mashed cage can only protect what it surrounds.)Cost and Physical properties of the system.
Earthing : Special care must be given to setting up ground connections , given the importance of their role in the efficient operation of lightning conductors. Standards NF C 17-100 and NF C 17-102 stipulate that each down conductor must have a specific earthing with different dimensions for cages and rods. The electrical ground or the existing belt is connected to them to provide equipotentialitiy. Finally , it is important to keep down conductor’ s earthing well away (3 to 5 meters) from any buried metal pioe of electrical conduct and to ensure that ohmic value does not exceed 10 Ohms with a low wave impedance.
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