JOINT DEVELOPMENT AND LOW-FREQUENCY INDUCTION 219 



When double faults occur on either grounded or nongrounded sys- 

 tems, severe magnetic induction is liable to result and under these 

 conditions it is difficult to limit the residual current. 



Shielding. Ground wires on a power line, while tending to increase 

 the total residual current, serve the purpose of shielding by reducing 

 the strength of the external electric and magnetic fields set up by the 

 residual voltages and currents. The net effect of ground wires from 

 the low-frequency standpoint is to reduce the voltages induced in 

 paralleling communication circuits under abnormal power circuit con- 

 ditions. The effectiveness of such shielding depends on the impedance 

 of the shielding conductor and its ground connections. Under favor- 

 able conditions the induced voltage at 60 cycles in paralleling commun- 

 ication circuits may be reduced about 40 per cent by this method. 

 Such ground wires, if used on wood pole lines, have a disadvantage in 

 that they impair to some extent the insulating property of the poles. 



High-Speed Circuit Breakers and Relays. —Very sensitive high-speed 

 relay systems have been developed which, together with high-speed 

 types of circuit breakers reduce the time duration of a power line fault 

 to approximately 1/10 second, as compared with one half second to 

 three seconds required by the older forms of relays and circuit breakers, 

 thus tending to minimize the effects of induction. On the other hand 

 inadequate relaying, or the omission of automatic circuit breakers, 

 may extend the duration of faults to a point where the hazards to 

 power apparatus are serious. High-speed breakers and relays are 

 expensive and it is difficult to justify them solely as a remedial measure 

 for induction, particularly as the speeds of operation now available 

 for relays and breakers on power systems, have not reached values 

 which make them a complete solution of coordination problems. 

 However, with the increasing size and interconnection of power sys- 

 tems, high-speed relays and circuit breakers are playing and increas- 

 ingly important part in promoting power system stability. 



Periodic testing of relays and circuit breakers accompanied by com- 

 plete overhauling at regular intervals, will do much to reduce the dura- 

 tion of faults and to prevent improper functioning of the equipment. 



The subcommittee is following the developments in high-speed 

 breakers and relays with much interest. If such devices should come 

 into general use for all classes of service it is expected that they would 

 materially improve the whole inductive situation. 



Improvement in Balance. — As mentioned above, low-frequency in- 

 duction between power and communication lines is sometimes exper- 

 ienced under normal operating conditions. On grounded telegraph and 

 signal lines the trouble usually manifests itself by a chattering of tele- 



