604 ELECTRICAL EQUIPMENT 



potential with respect to the earth, which may ultimately become 

 so great as to puncture the insulators. Suppose now that there 

 is a lightning discharge between cloud and cloud or between 

 cloud and ground. This is followed immediately by a redistri- 

 bution of the electrostatic field, and a general equalization of 

 potential occurs. The static charge so set free moves along the 

 line as an impulse or traveling wave. Such waves may have a 

 potential many times greater than that caused by switching, and 

 they may have a very steep wave front and thus produce high 

 potential differences between points along the conductor, such as 

 across individual transformer coils or group of coils. 



Several forms of protective devices of more or less value have 

 been devised to guard against abnormal voltage conditions. Of 

 these the aluminum-cell electrolytic lightning arrester possesses 

 ideal characteristics against such high-voltage disturbances, 

 where the excess voltage occurs between the phase conductors or 

 between the phase conductors and ground. The films of the 

 arrester introduce a barrier to the normal potential of the system, 

 but allow the energy of an abnormal disturbance to discharge 

 readily. The arrester is generally used in connection with choke 

 coils, the function of which is to retard and reflect the incoming 

 waves sufficiently to allow the arrester to better perform its duty. 



Overhead ground wires are also very generally used to protect 

 transmission lines against excessive static charges, the cost of 

 high-voltage lightning arresters making their installation along 

 the line impractical. 



The nature of high-frequency disturbances is a comparatively 

 recent discovery, and the means and methods for preventing 

 them and protecting against them is still being studied and inves- 

 tigated. The greater damage caused by such high-frequency 

 disturbances has occurred in high-voltage transformers, as would 

 naturally be expected. The best protection against them, there- 

 fore, is to insulate heavily the individual coil groups, while 

 inductances and energy-absorbing devices may, as stated, have to 

 be relied upon for further protection. 



Lightning Arresters. Aluminum-cell electrolytic lightning 

 arresters are nowadays used almost entirely for lightning pro- 

 tection of high-voltage transmission systems. This type of 

 arrester has an enormous discharge capacity, and its general 

 characteristics are well known. The arrester, however, is not a 



