184 



ANNUAL BEPORT SMITHSONIAN INSTITUTION, 1925 



steady voltages. In fact, this is true for all transient voltages 

 that can occur on transmission lines. Rain lowers the arc-over 

 voltage of insulators at 60 cycles, but does not reduce the lightning 

 spark-over voltage. The curves of insulator spark-over voltages 

 for transient and operating conditions are given in Figure 21. 



The transient voltage curves cover the range of lightning, 

 switching surges, and surges due to arcing grounds. The surges 

 other than lightning are limited in value by the line voltage. Ex- 

 cept as it is affected by the height of tower, lightning is independent 

 of line voltage. It is the determinine: factor in insulation. 



1800 



/ Z 3 4 5 6 7 S 9 10 // /Z /3 /4 



Number of Unil5 



Fig. 21 



The lightning flash-over voltage of insulators is plotted with 

 operating voltage in Figure 22. On this same figure are plotted 

 curves of the probable highest lightning voltage for lines with 

 and without ground wires. A direct stroke on a line without a 

 ground wire causes voltages much higher than the insulator arc- 

 over voltage. The second curve down represents the usual highest 

 voltage on a line without a ground wire. It happens that this 

 same curve corresponds to the voltage by a direct stroke on a 

 line with one ground wire. This curve crosses the insulator spark- 

 over voltage of 220 kv. This indicates that a 220-kv. line without 

 a ground wire is not likely to have insulator trouble from lightning 

 except in case of direct stroke. It further indicates that a 220-kv. 



