690 



ELECTRICAL MEASUREMENTS 



With III, the corresponding spacing is 35 and 65 per cent of 

 the lay (see Fig. 431.) 



When locating high-resistance grounds, all the conductors 

 are connected in parallel so that the charging current to the por- 

 tion of the cable beyond the fault may not produce a sound in the 

 telephone. To send the characteristic signal into the cable, a 

 motor-driven commutator is used which will break the circuit 

 about 3,000 times per minute; in series with it is a make-and- 

 break switch actuated by a cam also driven by the motor. The 

 result is that one hears in the telephone a definite note which is 

 interrupted in some particular manner. 



Location of Total Disconnection. A total disconnection occurs 

 when the wire breaks inside the insulating covering and the ends 

 are pulled so far apart that the two sections of the conductor 

 are insulated from each other. 



Theoretically, the conductors in a cable occupy definite posi- 

 tions with respect to each other and to the sheath. This being 

 so, the electrostatic capacity measured between two conductors 

 or between a conductor and sheath should be proportional to 



FIG. 432. Connections for direct deflection method for measuring electro- 

 static capacity of cable. 



the length of the cable. Consequently, when the insulation 

 remains intact, it should be possible to locate a total disconnec- 

 tion by measuring the electrostatic capacity of the portion of the 

 cable from the testing station up to the break and comparing it 

 with the capacity of the whole cable. If the total capacity is not 

 known, measurements must be made from both ends of the line. 

 The capacities may be measured by the direct deflection 

 method, the connections for which are shown in Fig. 432. Some 



