THE LOCALISATION OF BREAKS AND FAULTS. 441 



is uniformly good. If one portion of the cable is lower in 

 insulation than the other, the N.R.F. will be situated nearer 

 the low end. 



When the CR balances taken from both ends of a cable to 

 false zero are not the same it is an indication that the insula- 

 tion towards the low reading end is lower than the rest of the 

 cable. The N.R.F. is then situated towards the low end at a 

 distance from the centre of the line proportionate to the fall 

 in insulation on that side. 



In fault and break localisations if the N.R.F. is between the 

 testing station and the fault or break, the observed CR will be 

 too low and the fault appear closer than it really is, or it will 

 appear too far away if the N.R.F. is beyond the fault. To 

 make the necessary correction we must know the resistance 

 and position on the line of the N.R.F. Its resistance is the 

 insulation resistance of the cable or section of the cable in 

 circuit, and is generally taken as at the first minute. 



Fig. 249. 



The late Mr. W. J. Murphy treated this subject fully in an 

 article on " The Interpretation and Correction for Leakage of 

 Conductor-Resistance Tests on Submarine Cables " {The Elec- 

 trician, August 12, 1898), in which he gave the following useful 

 formulae for finding the position of the N.R.F. and applying a 

 correction to CR. observations. Referring to diagram Fig. 249, 

 Let A = the observed CR from A end, 



M -0= ,, ,, ,, iJ I, 



„ L = the true CR at sea-bottom temperature, 

 ,, ^ = line resistance between N.R.F. and A end, 

 5. 1= i> jj ), » » Bend 



,, R= „ resistance of the N.R.F. ( = insulation resistance 

 at first min.) 

 Then we have L =p + q 



24-R 



