692 ELECTRICAL MEASUREMENTS 



Where there are several pairs of wires in the cable, a pair 

 may sometimes be used in place of the standard condenser, as 

 indicated in Fig. 434. 



-Length * Ht Length 



Similar 

 Pair* 



-Total Length = L 



Generator 



FIG. 434. Bridge measurement of capacity to total disconnection; using 

 the capacity of an unfaulted pair as a standard. 



In this case, from the construction of the cable, the capacity 

 per unit length for both pairs is nominally the same, and as 



M C, 



N '''' C.' 



distance to the fault = total length of cable X (^ j . (16) 



These methods of location by capacity measurements are con- 

 venient when dealing with telephone cables. The difficulty in 

 applying them to power cables is that the disconnection, due to 

 a burn-out, may not be total and that the apparent capacity 

 per unit length may not be uniform. 



Breakdown Tests of High-voltage Cables. In addition to 

 the tests which are made during the process of manufacture, 

 every high-voltage cable must be subjected to a breakdown test 

 after it has been installed and before it is accepted, the object 

 being to search out the weak spots due to defects of manufacture 

 and of installation, especially defects at the joints. 



In this connection it may be pointed out that laboratory tests 

 on short samples show higher breakdown voltages than are 

 realized in practice and are without value as indicating what 

 may be expected of long lengths of cable. 



The idea of the breakdown test is simple. A specified voltage 

 is applied between conductors, or between conductors and sheath, 

 for a specified time. If a breakdown occurs, the fault is located 

 by one of the methods already described, the cable is patched and 



