THE LOCALISATION OF BKEAKS AND FAULTS. 445 



From (3) we have approximately 



L^A + f" (12) 



showing that the observed CR is too low, due to the shunting 

 effect of the N.R.F., and the correction is in the form of a 

 resistance to add to make up for this deficiency. 



Under the altered conditions of the break we may write 

 approximately 



Putting this value in (12) and remembering that L is the 

 true CR of the cable in circuit, which in this case is the length 

 of cable up to the break, we have 



True distance to break =A-f ^=^^ (13) 



from the A end. 



It is convenient to express (A-p) in thousands and R 

 in megohms : the amount to add is then in ohms. 



The localisation of 5,220 ohms to break obtained above as 

 the mean of various tests can now be corrected for the N.R.F. 

 From tests on this cable previous to the interruption (given 

 above) the position of the N.R.F. is known to be about 1,900 

 ohms (p) from the A end and its resistance 0*24 megohm (R). 

 Hence by (13)— 



(5,220-1.900)2 



True distance to break= 5,220 + 



240,000 



=5,220 + (3'32)^ ^g^20 + 46=5,266 ohms. 

 0-24 ' 



It will be seen by the large correction to add in this case that 

 it is most important to locate the N.R.F. while the cable is in 

 good condition, as however good localisations of subsequent 

 breaks or faults may be, the true position may be considerably 

 out if this correction is not applied. 



Note.— 5,220-1,900 = 3,320. Expressed in thousands 

 = 3-32. 



The N.R.F. correction as in formula (13) may conveniently 

 be put in the form of a rule by which the correction may be 

 applied at either end of the cable when the N.R.F. is between 

 the testing end and the break or fault, as follows. 



