444 SUBMARINE CABLE LAYING AJ^D REPAIRING. 



Suppose now that the true CR was not known, the foregoing 



formula and also formulae (5) and (6) would be unworkable, 



but we could get the N.E.F. position from (7) or (8) using the 



mean observed CR instead of the true, provided the insulation 



resistance was measured. Say this was found to be 0"24 megohm 



at the lat mint The mean observed CE. is 8,402, and the 



difference (d) between the two is 165 ohms. Therefore, we 



have by (7 ) 



/ 165x240,000\ 

 Distance(p)from A = if 8,402- g^^^^ j= 1,850, 



which it will be seen is very near to the first determination. 



Formulae (5) and (6) are very useful when tests can only be 

 taken from one end of the cable. 



Suppose, for instance, we could only get tests from A end, 

 the true CE and insulation resistance being known, we have 



by (6) 



Distance (q) from B = Vl80 x 240,000 = 6,580. 



Subtracting this from the line (8,500) we have — 



Distance from A = 1,902 ohms, 



practically the same as before. The above shows the applica- 

 tion of these three useful sets of formulae to special cases. The 

 example we have taken so far relates only to a cable in good 

 condition, and the localisation of the N.E.F. above determined 

 Is from the weekly CE and DE tests. 



We have now to see how the resistance and position of the 

 N.R.F. helps in arriving at a closer localisation of a fault or 

 break. The position of the N.E.F. is shifted when a fault or 

 break comes in and its new position cannot be determined, but, 

 nevertheless, the previously found position gives an approxima- 

 tion remarkably near. 



Suppose now the above cable is interrupted, and the result of 

 localisation tests place the break 5,220 ohms from the A end. 



The break is here beyond the N.E.F. position, which is 

 always the condition to which this correction applies. If the 

 break was on the near side of the N.R.F. the latter would not 

 Interfere with the usual localisations from the near end of the 

 cable, but the correction would then be applicable to localisa- 

 tions from the other end. 



