水中への雷電流拡散様相

Abstract

Lightning current distribution into water is clarified by means of finite difference time domain (FDTD) method, which is one of numerical electromagnetic analysis methods. The lightning current is concentrated on the sea surface due to skin effect. The tendency becomes notable with the increase of salinity of seawater. On the other hand, the lightning current diffuses toward the seabed with the decrease of the salinity. A ship on the sea has to release a lightning current into a surface portion of seawater, and has to have earth-plate(s) for grounding installed apparatus as deep as possible to protect from a lightning. The independent earthing is difficult to realize for the ship on the lake or river due to the skin depth of the freshwater. The area of the earth-plate for the lightning protection should be increased as much as possible to decrease its grounding resistance.