450 SUBMARINE CABLE LAYING AND BEPAIRING. 



Thus, with a metallic circuit of, say, 6,300 ohms (measured as 

 in Fig. 250), the arm a 100 ohms, and the unplugged resis- 

 tance say 1,400 ohms, we have ;i== 14 and distance to fault — 



^^ = 420 ohms, 

 lo 



Correction for Natural Resultant Fault on Looped Lines. — 

 Duplicate cables, when in good repair, should be periodically 

 tested by the Varley loop, in order to keep a record of the 

 position of the natural resultant fault (N.R.F.) of the loop. 

 The correction for dielectric leakage can then be applied to 

 subsequent localisation tests by loop method on the same cables. 

 This is a very important correction to loop tests, especially 

 where the resistance of the fault is high and comparable to the 

 insulation resistance. In looped cables of perfect or uniform 



■n d . 



Obseirved True 



position position 



N.R.F. of fault of fault 



Fig. 253. — N.R.F. Correction in Looped Cables. 



insulation throughout, the N.R.F. is situated in the centre,- 

 where the cables are joined; but whena fault is present it is 

 shifted from thecentre towards the faulty end. 



The relative positions of the N.R.F. and the observed and' 

 true positions of fault are as in Fig. 253. The letters represent — 



R = insulation resistance of cable previous to fault coming in. 



7'= insulation resistance of cable after fault has come in. 



■p 

 n = the ratio of the two insulations — namely, — . 



/=: resistance of the fault. 



D = distance in ohms between the N.R.F. and the observed 



position of fault. 

 rf= distance in ohms between the observed and true positions- 

 of fault. 

 The last-named quantity is the correction to add to or subtract 

 from the observed distance of fault to obtain the true distance.- 



