LIGHTNING PEEK 



181 



a transmission line within its area. The line is said to have a 

 " bound charge." If the voltage between earth and cloud becomes 

 high enough, a lightning flash will occur. Although this flash may 

 be a mile away from the line, the charge on the line is released 

 and the insulated line increases from earth potential to some value 

 above with polarity opposite to that of the cloud. An insulated 

 line can be at earth potential just previous to the discharge because 

 of the effect of the long part of the line not under the cloud, leak- 

 age over insulators, or through grounded neutral. A charge of the 

 same sign as the cloud leaks away leaving a bound charge on the 

 line with sign opposite to that of the cloud. The line remains at 



Fig. 16. — Transients for cloud discharge to earth 



earth potential until the cloud discharges. The effect is then of a 

 voltage suddenly applied between line and ground. The voltage of 

 the line becomes maximum Avhen the cloud voltage becomes zero. 

 The manner in which the voltage increases on the line with decrease 

 in voltage on the cloud is shoAvn in Figure 16. The field that ex- 

 tended between line and cloud now extends between line and ground. 

 The voltage wave travels over the line at the velocity of light. If 

 the line insulators are strong enough or have a high enough impulse 

 ratio, the impulse may travel to the power house to break down 

 apparatus or to be harmlessly discharged to ground over the ar- 

 rester, if it has low reactance and low impulse ratio. As this light- 

 ning wave travels over the line it becomes gi-adually dissipated 



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