JOINT DEVELOPMENT AND LOW-FREQUENCY INDUCTION 213 



The variation of coupling with separation and with earth conditions 

 is of great practical importance in the coordinated location of lines. 

 Most of the subcommittee's study of coupling, therefore, involves 

 field investigations of the variation of coupling with separation under 

 different earth conditions, and is furthermore directed toward devising 

 convenient and accurate methods of predetermining coupling in practi- 

 cal cases. Also, by studying and correlating experimental data derived 

 under different conditions and from widely separated parts of the coun- 

 try, the subcommittee hopes to arrive at a better empirical basis for 

 estimating coupling. Some of the work on this subject has already 

 been presented.^ 



Factors Controlling the Frequency of Occurrence of 

 Induced Voltages 



The frequency of occurrence of induced voltages in paralleling com- 

 munication lines, while chiefly dependent on the frequency of occur- 

 rence of faults on the power line, is also somewhat affected by the loca- 

 tion of the exposure with respect to the location of neutral grounding 

 points. For example, if there is only one neutral ground, faults occur- 

 ring between it and the exposure will produce relatively little induced 

 voltage. 



The frequency with which faults occur is usually traceable to features 

 of electrical and mechanical design, the character and amount of in- 

 sulation, and the location of the lines. Specifically, the factors which 

 appear to be responsible for the majority of faults on power lines are: 

 poor configuration, inadequate spacing and clearances, inferior insula- 

 tion, lightning, fog, smoke and dirt, birds and animals, proximity of 

 lines to external objects apt to interfere with operation mechanically 

 or electrically, and certain mechanical features of design affecting the 

 strength of construction, such as ineffective anchors, guys, or conduc- 

 tor and ground wire supports, particularly at angles and dead-ends, and 

 insufiicient bearing areas of subsurface structures. 



Factors Controlling the Duration of Induced Voltages 



The length of time faults are permitted to remain on a power system 

 is controlled by the kind of protective relaying employed and by the 

 type and condition of the circuit breakers and other terminal equip- 

 ment. The type of relay system, the degree of sectionalization ob- 

 tained, the adequacy of the circuit breaker as to speed and rupturing 

 capacity, and the maintenance of the equipment are the most impor- 

 tant factors. 



' For references see bibliography. 



