LOW-FREQUENCY INDUCTION 599 



tion of the power circuit relays and circuit breakers. In some 

 cases, of course, faults may clear themselves without circuit 

 breaker operation and this introduces additional variations. 



(c) Large variations in longitudinal voltage, voltage-to-ground, and 

 current through protectors may occur with relatively small 

 variations in the locations of faults when they occur inside 

 inductive exposures. 



id) The shielding effects due to the operation of protectors on telephone 

 circuits may vary considerably between different occurrences. 



The variations in induced voltage duration, etc., between different 

 occurrences are, of course, only part of the story. Obviously, the total 

 number of faults which may occur on a power line in an exposure 

 over a given period is equally important. This will be affected by 

 numerous factors such as type of line, severity of lightning and other 

 hazards, etc. 



In addition, there are variations in the reactions on the telephone 

 circuits. For example, the protector blocks used do not all break 

 down at the same voltage and the fortuitous variations in the break- 

 down voltage may have an important bearing on the number of 

 protectors which operate and consequently on the total shielding, 

 current through protectors, etc. From the standpoint of possibilities 

 of acoustic and electric shock, there are of course many other proba- 

 bility factors involved. 



All of these factors are under investigation and our knowledge of 

 them is increasing from day to day. It is probable, however, that low- 

 frequency induction will always remain a subject in which quantitative 

 analyses can tell only a part of the story. 



