The Lightning of Transmission LineOvervoltages on power systems are traceable to three basic causes, lightning, switching, and contact with circuits of higher voltage rating. The power system designer seeks to minimize the number of these occurrences ,to limit the magnitude of the voltages produced,and to control their effects on operating equipment. Lightning results from the presence o clouds which have become charged by the action of falling rain and vertical air currents, a condition commonly found in cumulus cloudsVoltages may be set up on overhead lines due to direct strokes and due to indirect strokes . In a direct stroke, the lightning current path is directly from the cloud to the subject equipment-an overhead line. From the llne, the current path may be over the insulators and down the pole to ground. The voltages setup on the line may be that necessary to flash over this path to ground. In the direct stroke, the lightning current path is to some nearby object, such as the tree shown In Fig. 10 lb. The voltage appearing on the line is explained as follows As the cloud comes over the line, the positive charges it carries draw negative charges from distant points and hold them bound on the line under the cloud in position as shown. The voltage on the ine is zero assuming that the line is not energized, IF the cloud is assumed to discharge on the occurrence of the stroke in zero time, the positive charges suddenly disappear, leaving the negative charges unbound. Their presence on the llne implies a negative voltage with respect to ground. On the occurrence of a stroke, lightning clouds do not discharge in zero time. Instead,the stroke current rises from zero value to maximum value (perhaps 50, 000 amperes) in a few microseconds and is completed in a few hundred microseconds.Direct lightning strokes to lines as shown in Fig. lO-la are of concern on lines of all voltage class ,as the voltage that may be set up is in most instances limited by the flashover of the path to ground, Increasing the length of insulator strings merely permits a higher voltage before flashover occurs. The most generally accepted method of protection against direct strokes is by use of the overhead ground wire For simplification only one ground wire and one power conductor are shown. The ground wire is placed above the power conductor at such a position theractically all lightning-stroke paths will be to it instead of to the power conductor. Stroke current then flows to the ground most of it passing through the tower footing ground resistance Rwhde a smaller part goes down the line and to ground through the adjacent tower footings. The tower rises in voltage due to the current I1 through the resistance R1 to a value which is Approximately this voltage appears between the tower and the power conductor (which was not struck). If this voltage is less than that required to cause insulator flashover, no trouble results. Protection by this method is improved by using two carefully placed ground wires and by making tower footing ground resistance of low value.The lightning record of lines supported on towers 80 to 90 feet tall substantiates the simple theory of line protection just presented. The poorer record of lines on towers over 100 ft in height indicates that other factors, perhaps the inductance of the path down the tower, should be considered. low-voltage lines supported on small insulators. They are of little importance on high-volt-age lines whose insulators can withstand hundreds of kilovolts without flashover. Insulation is required to keep electrical conductors separated from each other and from other nearby objects. Ideally, insulation should be totally nonconducting, for then currents are totally restricted to the intended conductors. However, insulation does conduct some current and so must be regarded as a material of very high resistivity. In many applieatlons, the current flow due to conduction through the insulation is so small that it may be entirely neglected. In some instances the conduction currents, measured by very sensitive instruments, serve as a test to determine the suitability of the insulation for use in service. Although insulating materials are very stable under ordinary circumstances, they may change radically in characteristics under extreme conditions of voltage stress or temperature or under the action of certain chemicals. Such changes may, in local regions, result in the insulating material becoming highly conductive. Unwanted current flow brings about intense heating and the rapid destruction of the insulating material. These insulation failures account for a high percentage of the equipment troubles on electric power systems. The selection of proper materials, the choice of proper shapes and dimensions and the control of destructive agencies are some of the problems of the insulation-system designer. Many different materials are used as inaulation on