LIGHTNING PEEK 



183 



double in value when it strikes the end of the line where it starts 

 back. If the voltage is high enough it may break down insulators 

 or discharge over an arrester. If no break occurs it is eventually dis- 

 sipated by losses. The change in shape and voltage of lightning 

 waves traveling along transmission lines at 186,000 miles per second 

 has been measured in the laboratory. 



The strength of the insulation of lines operating at various 

 voltages was determined by measurements with artificial lightning. 

 By comparing the insulation strength witli the maximum lightning 

 voltage the ability to withstand lightning was determined. 



THE GROUND WIRE 



When a grounded wire is placed near and parallel to the trans- 

 mission wires it has a marked effect on the induced voltages. The 

 effect of the ground wire was studied on models to scale. The dia- 

 grammatic arrangement is shown in Figure 17, while the actual 

 models are shown in Figures 18, 19, and 20. 



Transformer 



C'^Gap 



T 



\C' Condenser 



■ R -Resistance 

 ' of Lightning 



■ Path 



Plate Representing 

 Over-head Cloud 



Model Line 



Conneciions Used in Studying Induced Voltages 

 on Transmission Lines 

 C=I.3I X IO''mf L-2.88 X IO"mh 



R=5000 Ohms 



Fig. 17 



The method of making the test was to measure the induced 

 voltages on the model line with and without a ground wire. In 

 general the single ground wire reduces the induced lightning voltage 

 to one-half, two to one-third, and three to one-fourth. Very good 

 connections must be made to ground. The ground wire thus offers 

 an important practicable means of reducing lightning voltages. 

 It is also important in case of a direct stroke, when the ground wire 

 is struck 98 per cent of the time. 



INSULATING A TRANSMISSION LINE TO WITHSTAND LIGHTNING 



The investigation has shown the maximum lightning voltages 

 that can occur on transmission lines with and without a ground 

 wire. It is of interest to compare these voltages wnth the lightning 

 insulator arc-over voltages for modern transmission lines. 



The lightning spark-over voltage for insulators and the light- 

 ning breakdown voltage for insulation is always higher than for 



