446 SUBMARINE CABLE LAYING AND KEPAIRINe. 



Rule to apply the N.R.F. Correction to Break and Fault 

 Localisations. — Square the difference between the observed 

 CE. and the ohmic resistance to the N.RF. from the testing 

 end, and divide by the resistance of the N.R.F. The result is 

 the correction to add to the observed CR to obtain the true 

 distance to break or fault. 



The expression "observed CR " means also the apparent CR 

 to the break or fault as determined by localisation tests, and in 

 loop and overlap tests it includes resistance added to line. The 

 ohmic resistance to the N.R.F. means the line resistance plus 

 any resistance added to line as in loop and overlap tests, 



PARTIAL EARTH FAULTS. 



In partial earth faults the advantage of recorded vs^eekly tests 

 can hardly be over estimated. A fall in the insulation or unsteady 

 electrification may mean the presence of a small fault in the cable 

 in its incipient stage. When this happens the land line should 

 be freed at the cable house and tested to make sure that 

 the defect is not in this line or the lightning guards, and the 

 other end requested to do the same. If tests are still low the 

 beach length must be examined, and if a pipe system, discon- 

 nected at low water and retested ; in fact, every means taken 

 to lead to an absolutely certain conclusion as to whether the 

 fall in insulation is within the shore side of the water or 

 seawards. And until precise localisations are necessary the 

 fault must not be opened up by strong testing currents. 



The most useful methods of localisation applicable to faults 

 making partial earth are the Loop tests, Mance, Earth overlap, 

 Blavier, Kempe and Clark tests, the latter with Rymer-Jones' 

 simplification. Loop tests and tests from both ends of a single 

 cable when practicable, are most reliable but all observations 

 require careful correction for the total leakage through the 

 dielectric and for earth current. Sometimes a fault, although 

 not due to a complete fracture has a sufficient area of exposed 

 conductor to be dealt with by the break methods already 

 described. This can be noticed by its slow polarisation and the 

 fall in the observed CR when the testing current is increased. 



Varley Loop Test. — Where duplicate cables exist, a fault in 

 oither one of them can be localised with considerable accuracy 



