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IEC 62793-2020 pdf free download
IEC 62793-2020 pdf free download.Thunderstorm warning systems — Protection against lightning Systémes d’alerte aux orages — Protection contre Ia foudre.
Lightning is produced by electrified clouds (see , ), but not all clouds are electrified. Thunderclouds produce lightning and are usually characterized by substantial vertical (deep) development and the simultaneous presence of supercooled cloud drops, ice crystals, and graupel (soft hail) particles. Thunderclouds can also produce high winds and severe weather on the ground (hail, tornados).
Thunderstorms are produced by buoyancy forces that are set up initially when sunlight heats the earth’s surface and the air in the planetary boundary layer. The thermodynamic basis for the formation of convective clouds is a conditional temperature instability and an initial trigger to start this process. The trigger can be produced by a variety of mechanisms: boundary layer thermals, atmospheric frontal surfaces (interaction of cold and warm fronts) and gust-front boundaries, orographic lifting (typical in mountains) and frontal surfaces. When a buoyant air parcel ascends and enters a lower pressure environment, the parcel expands and cools until the temperature reaches the dew point. After that, the condensation of water vapour produces a cloud, and the latent heat released by the condensing vapour enhances the parcel buoyancy. If the parcel reaches subfreezing temperatures, the conditions for the formation of ice crystals and graupel that are fundamental for cloud electrification and lightning will be present.
Typical thunderstorm cells in cumulonimbus clouds are characterized by diameters of 10 km, cloud top altitudes of 12 km, and a life cycle of less than 30 mm. But other types of thunderstorms such as multi-cell lines, cluster, super-cells, and mesoscale convective systems (MCS) have larger dimensions and durations that can reach several hours. Different types of thunderstorms tend to occur in different geographic regions.
A model of the charge distribution of a simple cumulonimbus thundercloud consists of three charge regions, a concentrated negative layer in the middle of the cloud with a more disperse positive layer above that and a small pocket of positive charge below the negative region. Lightning tends to begin at or near the edge of the negative region, and if it begins near the top of the layer, it usually develops into an intra-cloud (IC) discharge involving the main negative and positive regions. If a discharge begins at or near the lower edge of the negative layer, it can produce a downward-propagating, negative leader and a cloud-to-ground (CG) discharge.IEC 62793-2020 pdf free download.
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