578 BELL SYSTEM TECHNICAL JOURNAL 



lines. The load current may be substantial in magnitude but is 

 normally confined to the line wires and is commonly called "balanced 

 current." Under this condition, the induced voltage is small. If, 

 now, one of the power conductors is grounded, current will flow to 

 ground at that point. This current is the "residual current" men- 

 tioned previously. When this is done in the demonstration in such a 

 way that the residual current flows through the exposure, the voltage 

 induced along the telephone circuit rises very materially. Also, since 

 the induced voltage acts along the telephone circuit, opening the ground 

 connection at the far end of the telephone line reduces the voltage 

 shown by the meter to a very small amount. 



That the power current causing the induction is unaffected by trans- 

 positions in the power circuit, can be shown by transposing the power 

 circuit in the set-up. No appreciable change in the induced voltage 

 occurs when this is done. Likewise the induced voltage, since it is 

 induced along the telephone wires in parallel, is unaff'ected by telephone 

 circuit transposition, as can be shown by transposing the telephone 

 circuit. Consequently, the matter of power or telephone circuit 

 transpositions can be neglected in the further analysis. 



Coupling Factors 



Using the demonstration arrangements, some of the basic factors in 

 coupling can be observed. For example, since the voltage is due to 

 magnetic induction and accumulates along the telephone circuit, the 

 coupling should be proportional to the length of the (uniform) exposure 

 through which the fault current flows. This can be observed by noting 

 the reduction in induced voltage as the fault on the power line is moved 

 from the end of the exposure toward the supply end. (In the demon- 

 stration, the fault current is the same regardless of the location of 

 the fault.) 



Likewise, if the voltage accumulates along the telephone circuit, the 

 longitudinal voltage measured should be proportional to the length of 

 telephone circuit exposed. This can be observed by again placing 

 the fault on the power line at the end of the exposure and moving the 

 measuring point along the telephone line. As this point is moved 

 toward the grounded end, the indicated voltage goes down. 



Another basic factor in coupling is its relationship to the separation 

 between the lines. Generally speaking, the greater the separation, 

 the smaller the coupling. How much coupling will exist for a given 

 separation depends on a number of factors, one of which is the structure 

 of the earth. This effect will be discussed first. 



