442 SUBMARINE CABLE LAYING AND REPAIRING. 



By subtraction we obtain 



L-A = -J- (1) 



and similarly L - B = ^'^ (2) 



If R (the insulation resistance of cable) is large compared 

 with j; and €[, we may consider ^ + R and (/ + R as R simply. 

 Then approximately, 



L-A = ^ (3) 



and ^~^ = R ('^) 



"Whence the distance of the N.R.F. from the A end is approxi- 

 mately , 



^ ^=x/(L-B)R (5) 



and the distance from the B end is approximately 



q= V(L-A)R (6) 



The above formulae are useful to find the position of the 

 N.R.F. when the observed CR's (A and B) at both ends, the 

 insulation resistance (R) at the first minute, and the true CR 

 are known. If the true CR is not known the approximate CR 

 can be used. In fault testing L may be taken as the observed 

 CR or the apparent CR as worked out by a localisation test. 



By subtraction and substitution of the values of p = L - (/ and 

 2 = L-p in (3) and (4) we obtain the distance of the N.R.F. 

 from A end in another form approximately 



i' = *(L-'f) (7) 



and the distance from B end approximately 



-i(L-^f), (8) 



where d is the difference between the observed CR's at both 

 ends, the B reading being greater than A. If A is greater 

 than B the sign within the bracket in each case must be 

 changed. 



These two formulae are useful to find the position of the 

 N.R.F. when we know the difference between the observed CR's 



