JOIXT DEVELOPMENT AND LOW-FREQUENCY INDUCTION 225 



may be a field of use for this type of cable in situations for which it is 

 best adapted. 



Coordinated Location of Lines. — Since the magnitude of induced 

 voltages for given power line conditions depends upon the inductive 

 coupling of the two classes of lines, which in turn is dependent upon 

 their relative location, particularly their separation and length of 

 parallelism, it is possible by advance cooperative planning of new 

 power and telephone line locations to minimize and in some cases to 

 forestall inductive effects in the telephone system. If the cost of 

 remedial measures which inductive exposures would render necessary 

 can be avoided, additional expense in locating lines to avoid such ex- 

 posures may be justified and where a complete solution is obtained in 

 this way both parties secure greater freedom in the construction and 

 operation of their lines. However, with the rapid expansion of both 

 services, the possibilities of complete solution by separation of lines 

 alone are becoming more and more rare, particularly for lines along 

 highways. 



Coordination of Grounding Practises. — The occurrence of a fault on a 

 power system usually results in raising the ground potential at the 

 points of grounding as well as at the point of fault, but if steps are 

 taken to coordinate the grounding of the power system and the tele- 

 phone system serving the power company, particularly at transformer 

 and generating stations, the effects in the telephone system of the earth 

 potential gradient caused by a power fault may be minimized. For 

 example, if in a switching station the same ground should be used for 

 the power system neutral and for the telephone system, a power fault 

 might cause the switching station ground to rise many volts above the 

 distant telephone exchange ground, and result in operating the tele- 

 phone protectors and possibly interrupting service. If, however, in- 

 dependent grounds sufficiently separated are used at the switching 

 station, or an insulating transformer is placed in the telephone circuit, 

 the power neutral ground may rise in potential without unduly affect- 

 ing the telephone system. 



Comprehensive consideration of the low-frequency coordination 

 problem involves a study of the reactions between the grounding 

 practises employed by power companies and those employed in tele- 

 phone and telegraph systems. There is considerable diversity in 

 practise with respect to methods of grounding. Some power trans- 

 mission lines and primary distribution lines are not provided with any 

 designed grounds, although most such lines have grounded neutrals and 

 a few lines are grounded in such a way that operating current flows 

 through the earth. In built-up communities there are underground 



