LIGHTNING PEEK 



185 



line with a favorably installed ground wire is not likely to have 

 trouble from lightning under any circumstances, unless the towers 

 are very high. The next curve gives the usual highest voltage 

 for a line with one ground wire. Under usual conditions, very little 

 trouble should be expected for lines insulated for over 100-kv. opera- 

 tion. The lowest curve shows that three ground wires reduce the 

 probability of lightning to still lower operating voltages. The 

 probability of trouble with direct strokes would be reduced in 

 proportion. 



The values of voltages taken in the above curves are for the 

 most severe storms directly over exposed lines. Such conditions 

 might not occur during a year or several years. It is a well-estab- 

 lished fact that during any storm there are likely to be induced 

 many low-voltage impulses, a less number of moderate-voltage 

 impulses, and frequently none at high voltage. 



4000 



■^3000 



^2000 

 ^iOOO 



AO 



80 120 160 200 

 Operating Voltage in K\/. 



240 280 



Fig. 22. — Insulator spark-over voltage compared with 

 maxim Lim lighting voltage for severe storms directly 

 over the line 



Lightning voltages are likely to cause high local stress in induc- 

 tive apparatus such as transformers. In designing inductive appa- 

 ratus to withstand liglitning voltages, it is important to prevent 

 concentration of stress. This can be done by means of shields. 

 Like the ground wire, the action of the shield is preventative. 

 Shields have been used on both line insulations and transformers. 



LIGHTNING ARRESTERS 



Preventive methods of guarding against lightning on transmis- 

 sion lines, such as the ground wire, shields, and extra insulation, 

 have already been discussed. The remaining method is the light- 

 ning arrester. 



The object of the arrester is to permit transient or other excess 

 voltages to discharge to earth and to suppress the dynamic arc that 

 follows. Since transient currents are generally high, the arrester 



