440 SUBMAEINE CABLE LAYING AND KEPAIEING. 



the battery circuit (50 range) the working out would be set 

 down thus : — 



Balance B with the larger current to line taken first in each 

 pair. 



99 



-g m/amp. B= 4,620 4,605 4,612 



42 

 g- m/amp. A= 4,7 10 4,700 4,704 



A-B= 90 95 92 



P= 1-73 1-73 1-73 



(A-B)P- 155 165 159 



A-(A-B)P= 4,555 4,535 4,545 



Mean of three pairs = 4,545. 



The milammeter readings were 99 and 42 on the 50 range ; 

 therefore, dividing these by 3, gives the respective currents in 

 milllamperes, namely 33 and 14. But as the instrument was 

 used in the battery circuit and with even bridge ratios, the 

 currents to line were half this, namely, 16-5 and 7. It is not 

 necessary to set down the actual currents to line, but the steps 

 are given here for clearness. All that is required is the ratio 

 between the currents, which is — 



99 



42 



By the table P = 1-73 for this ratio. 



The mean result, 4,545, represents the resistance up to the 

 break plus something less than the break resistance. From 

 the rate of polarisation and comparison with other methods it 

 appeared that the proper resistance to allow in this case was 30 

 ohms, leaving 4,515 as the distance to the break from the 

 testing station. 



Correction for Natural Resultant Fault in Break and Fault 

 Localisations. — The insulation resistance of a cable is equi- 

 valent to a large number of high resistance leaks to earth, distri- 

 buted along the line uniformly, or nearly so, when the cable is new 

 or in perf ec t condition. This leakage is approximately equivalent 

 to a single high resistance fault known as the natural resultant 

 fault, situated about the centre of the line, when the insulation 



