JOINT DEVELOPMENT AND LOW-FREQUENCY INDUCTION 227 



consideration in congested areas. Measures which affect only the 

 frequency of occurrence of faults, or their duration, while very helfpul, 

 are not as effective from a protection standpoint as measures which 

 limit the magnitude of residual currents and voltages. 



As to measures which would allow telephone circuits to operate 

 through a strong inductive field, the use of lead-sheathed cable sur- 

 rounded by magnetic material seems to offer the physical possibility 

 of affording the most effective protection. Precautions would be re- 

 quired, however, to prevent the shielding structure itself from rising to 

 a dangerous potential with respect to earth. On open-wire lines where 

 the occurrence of high induced voltages cannot be prevented, some form 

 of protector for limiting the magnitude of voltage to ground seems to be 

 a logical line of development. 



Devices such as acoustic shock reducers, which protect only against 

 a single effect of induced voltages, do not afford a solution of most 

 specific situations, but have to be used in combination with other 

 protective measures. In many situations, no single protective measure 

 is adequate and if the exposure is severe several may be required. 



In considering the effects which a new exposure may produce, all 

 the relevant factors are capable of advance determination except 

 frequency of occurrence of induced voltages, which has to be estimated 

 on the basis of experience or judgment and a statistical analysis of line 

 failures. 



Selection of measures to be employed in specific cases should be 

 made with the above considerations in mind to the end that the best 

 engineering solution may be obtained irrespective of whether the pro- 

 tective measures are applied to the telephone system, to the power 

 system, or to both. 



Reaction of Physical Exposures and Lightning on Loio- Frequency 

 Induction Problem. — As telephone circuits which are exposed to in- 

 duced voltages may also be exposed to possible contact with power 

 circuits and to lightning, any comprehensive scheme of protection 

 must take into consideration the high currents resulting from contact 

 and the high voltage due to lightning. In this connection there are 

 some points of difference in the reactions on the protection scheme of 

 induction, contact, and lightning. 



Contacts between power and telephone wires may occur at crossings 

 or conflicts or they may occur on joint pole construction as described 

 in the paper by Messrs. Huber and Martin. In any event such con- 

 tacts can occur only where the two lines are in close proximity, whereas 

 in cases of inductive exposure, a fault outside as well as inside the ex- 

 posure, may produce disturbances in the telephone circuits. Moreover, 



