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thundercloudpylondischargeinduced lightning surgeinduced lightning surgeinduced positive chargepylonthundercloudthunderclouddischargeinduced lightning surgeinduced lightning surgemagneticfieldpylonlightning surgelightninglightning rodincreased ground potentialequipment No. 2equipment No. 1chargedelectric chargedissipateddischargebetween cloudsthundercloudthunderclouddischargedincreasedground potentialnegative chargesurgecurrentgroundresistancerelativepotentialdifference5Figure 2-1Electrostatic inductionWhen thunderclouds located above a power cable or communication cable contain negative charges at their bottom parts, high level positive charges are in-duced electrostatically within the cable and high volt-age is developed by electrostatic induction from thun-derclouds (Figure 2-1).At that instance, the negative charge at the bottom of the thundercloud disappears by discharging between the clouds or between the clouds and the ground. Then, the positive charge which is trapped by the ca-ble are freed and led to both directions on the cable as a surge voltage (Figure 2-2).Electromagnetic inductionA discharge between the clouds and the ground oc-curring near from a power cable or communication cable generates a magnetic field due to its surge cur-rent. When the magnetic waves propagated within the field reach the cable, a lighting surge is induced (Figure 3-1).Increased ground potentialWhen lightning strikes a building or a lightning rod, high current flows to the ground and the ground po-tential rises. This causes a potential difference be-tween the equipment No. 1 located close to the light-ning rod (high potential) and the equipment No. 2 lo-cated far away from it (low potential), and as a result the lightning surge is directed from the ground to the cable between these locations (Figure 4-1). In addition, if a building is in charged state by thun-derstormʼs electrostatic induction, an atmospheric discharge dissipating the electric charge at the bot-tom of the cloud causes the electric charge on the building to flow toward the ground. This also leads the ground potential to rise and the lightning surge di-rected to the cable from the ground (Figure 4-2).Figure 2-2Figure 3-1Figure 4-1Figure 4-2

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