186 ANNUAL BEPOET SMITHSONIAN INSTITUTION, 1925 



must have a low resistance. It must also have a low time lag, other- 

 wise the transient voltage may rise to high values before the dis- 

 charge occurs. 



Figure 22 shows that the need of an arrester becomes less as the 

 operating voltage increases. 



Corona and other losses also rapidly reduce lightning voltages. 



OTHER CAUSES OF TROUHLK ON TRANSMISSION LINES 



Most troubles on transmission lines are caused either by lightning 

 voltages in excess of the insulation breakdown voltage or to some 

 form of dirt that lowers the normal breakdown voltage to such 

 an extent that failure occurs under normal conditions. 



Certain transient voltages are produced in lines during switch- 

 ing, etc., but these are usually quite harmless. 



CORONA AND SPARK OVER OF LIGHTNING VOLTAGES 



The corona and spark-over voltages due to lightning are usually 

 quite different from the continuously applied 6{)-cycle or direct- 

 current voltages. This also applies to liquid and solid insulation. 

 The lightning voltage is a transient or impulse and in effect is ap- 

 plied at an extremely rapid rate. The insulation does not have 

 time to break down when its usual breakdown voltage is reached 

 but the break is completed at a later time when the voltage has 

 risen to a higher value. The time lag will be discussed. in more 

 detail later. 



CORONA 



Lightning voltages cause corona in the same manner as continu- 

 ously applied voltages. Corona produced by lightning voltages 

 of less than a microsecond duration (millionth of a second) can 

 be readily seen. It is also easy to tell whether the impulse is posi- 

 tive or negative. In general, corona produced by impulse voltages 

 follows the same law as corona produced by continuously applied 

 voltages. It is of interest that the eye can readily see coi-ona pro- 

 duced by voltage of a microsecond duration. 



SPARK-OVER OF NEEDIJ: GAPS 



A typical discharge from the lightning generator at 1,500,000 

 volts between points is shown in Figure 10. The discharge has a 

 zigzag path and characteristic side flashes of lightning. The light- 

 ning spark-over curve for needle gap is given in Figure 23. This 

 curve shows that for the particular impulse used (fig. 24) a voltage 

 is required approximately 2.25 times the 60-cycle voltage to spark 

 over a given gap. The factor .2.25 is called the impulse ratio. 



