470 



SUBMABINE CABLE LAYINa AND BEPAIRING. 



When a milarameter is available the calculations are much 

 simplified by connecting it in the line and adjusting the line 

 currents so that equal currents flow through the fault during 

 the "free " and " earth " tests. The late Mr. W. J. Murphy, 

 to whom many useful simplifications in testing formulae and 

 methods are due, has shown that the current through the fault 

 is the same when 



vc 



b' -X 



where c is the current to line in the free reading, 

 nc „ „ ,, earth „ 



The currents can be regulated by adjusting a resistance in 

 the battery circuit or varying the battery power. There is no 



High Resistance 



jl|l|l{l|l|l{l{l{l|t|l{l{l|l|i|H 

 Carbon Zinc 



Fig. 258.— Connections for taking Blavier Test. 



need for exact equality in the voltage per cell, and it is not neces- 

 sary to determine the battery resistance in order to know whether 

 it may be neglected or not. Nor is it necessary to use an 

 even bridge. 



The connections for the test are shown in Fig. 258. 



The above formula is derived as follows : — 



Let / represent the fault resistance, 



y „ line resistance beyond the fault. 



The current through the fault in the earth reading is equal to 



nc 



1_ 



y+i 



The current through the fault in the free reading is equal to 



