216 BELL SYSTEM TECHNICAL JOURNAL 



ditions. The solution should afford the necessary protection without 

 hampering the development or operation of either system. Where 

 there are two or more alternative solutions, the one which is best from 

 the engineering standpoint, including both the technical and economic 

 aspects, should of course be applied. 



Cooperative planning in advance of construction is especially impor- 

 tant in situations involving low-frequency induction, because of the 

 wide ranges in magnitude both of coupling factors and of residual 

 currents. By advance notifications of construction it is possible to 

 bring up for analysis the low-frequency effects which the proposed 

 construction would bring about and, if necessary, to agree upon changes 

 in the plans to prevent or reduce these effects. 



As to the physical dimensions and relations of power and telephone 

 lines which constitute an exposure there are no blanket rules for guid- 

 ance; each case requires specific consideration. Due to differences in 

 geological conditions and other variable factors, a given length of 

 parallelism at a given separation might give satisfactory results in one 

 location, whereas an exactly similar physical relationship of lines in 

 another location might result in the communication system being 

 rendered inoperative at times of power system fault. This fact 

 emphasizes the necessity of advance planning and cooperative study of 

 situations as they arise. Such cooperation may easily lead to a satis- 

 actory solution of situations which at first seem very difficult. On the 

 other hand situations which at first appear devoid of any possibilities of 

 trouble may on careful study be found to require protective measures. 



Protective Measures for Poiver Systems. — It will be evident from the 

 foregoing discussion that protective measures to reduce the inductive 

 influence of power systems should be directed to limiting the magni- 

 tudes of unbalanced currents and voltages, particularly under abnor- 

 mal conditions, and to reducing the duration and frequency of occur- 

 rence of abnormal conditions. Of such protective measures some are 

 concerned with fundamental questions of line and system design and 

 must be incorporated in the construction plans, while other measures 

 are of such a character that they may either be incorporated in the 

 original construction or added later if found necessary as a result of 

 subsequent experience or developments in either the power or tele- 

 phone system. 



Fault-Resistive Design and Construction. — As mentioned in the paper 

 by Messrs. Harrison and Silver the methods employed in reducing 

 the frequency of occurrence of faults are primarily involved in the 

 design and construction of the power line, i.e., adequate insulation, 

 clearances, and spacings, and so arranging the component parts of the 



