218 BELL SYSTEM TECHNICAL JOURNAL 



arrangements of metal braces, fittings, guys, etc., to avoid so far as 

 possible shunting out the insulation of the wood. 



To the experienced designer the protective measures to be employed 

 on lines subject to frequent faults are obvious, namely, the rearrange- 

 ment and reconstruction of the tower or pole top to obtain greater 

 spacing between conductors or greater clearance between conductors 

 and other metal parts. In some cases spacing and clearances would be 

 materially improved by utilizing a triangular configuration so that 

 the conductors are not likely to come in contact with each other or the 

 ground wire when sleet or other conditions cause whipping or dancing 

 of the conductors. In other cases, merely a relocation of the point of 

 attachment of guys would improve conditions without materially 

 decreasing the strength of the structure. 



Fault-Current Limiting Measures. — Resistors, or reactors, in the 

 neutral ground connection of a power system provide a means of 

 directly limiting the magnitude of the residual currents, except in cases 

 of double faults. In cases where the residual currents can be so far 

 reduced as not to set up induced voltages of high values in the com- 

 munication system without reacting unfavorably on power system 

 operation, this method alone may afford a satisfactory solution. In 

 such cases it has the further advantage of reducing the stresses to the 

 power system due to the fault current. Where it is impracticable to 

 clear up a situation by residual current limitation alone, this method 

 may be effectively used in combination with other protective measures. 



The reduction in residual current which will be brought about by 

 adding a given amount of impedance in a neutral ground connection 

 can be estimated with reasonable precision. It is not so much this 

 question therefore, that requires study by the subcommittee as it is 

 the question of the limitations and costs of this protective measure, 

 and its reaction upon the power system. Included in this work is a 

 study of the relative advantages of inductance as compared with resist- 

 ance for accomplishing such current limitation. The subcommittee 

 has under observation a number of installations of current-limiting 

 devices and is engaged in experimental and theoretical studies and in 

 field observations by means of recording instruments to determine the 

 possibilities of this type of protection. 



In non-grounded power systems a single fault on a phase conductor 

 results in the charging current of the system flowing to earth through 

 the fault. The other phases, rising to full line voltage above the 

 grounded phase, create a system unbalance which may manifest itself 

 by induction in paralleling communication lines. In such cases the 

 problem is one of electric induction except for the magnetic induction 

 set up by the charging current. 



