THE LOCALISATION OF BREAKS AND FAULTS. 451 



The resiatance r is taken approximately as the joint resis- 

 tance of R and /. That is, 



whence /=t5^ • ■ (1) 



Also the ratio of the distances D and d is proportional to the 

 resistances R and /. That is, 



^ /■ 

 Substituting the value of / from (1) we have 



D_ R(R-r) _R_.^ 



d R?- r 



And as — = n, the correction is 

 r 



d=^^ (2) 



This correction must be added to or subtracted from the 

 observed distance, according as to whether the N.R.F. is on the 

 near side of the fault or beyond it. That is, expressing the dis- 

 tances along the cable conductor in ohms, it is obvious from 

 the diagram that to obtain the true distance of fault we must 



Add the correction to the observed distance of fault if the dis- 

 tance of the N.R.F. from the testing end Is less than the observed 

 distance of fault, or 



Subtract the correction from the observed distance of fault 

 if the distance of the N.R F. from the testing end is greater 

 than the observed distance of fault. 



For example, suppose the N.R.F. to be 3,200 ohms distant, 

 and the observed distance of the fault 2, 700 ohms (D = 500) ; 

 also that the insulation was 1"185 megohms at the time the 

 N.R.F. was determined and 01 megohm when the fault came 

 in. Then n = 11-85, and the correction is 



=46 ohms. 



11-85-1 



In this case the N.R.F. distance is greater than the observed 

 distance of the fault, and therefore the correction must be sub- 

 tracted, and we have 



True distance of fault = 2,700 - 46 = 2,654 ohms. 



gg2 



