THE LOCALISATION OF BKEAKS AND FAULTS. 461 



B end. B's balance is then too low, and the correction to 

 apply according to the Rule on page 446, is — 



(7.230-l,7fi5)- _ (5-465)^ ^^ 

 800,000 0-8 ~^' ° 



Ic will be noticed that as resistance is added to the line the 

 ohmic distance of the NRF from B is 1,500 + 265=1,765, which 

 figure appears in the above calculation. The stations then 

 test again A balancing to 7,230 as before and B to 7,230 - 37 

 = 7,193. The added resistances are thereby altered to 

 E = 5,066, 

 r = 225. 



Therefore the corrected distance to fault from the A end 



becomes — 



8,200 + 225-5,066 , nnr, , 



iv= =1,680 ohms. 



2 



That is, 20 ohms nearer the A end than the uncorrected 

 result put it. 



Kempe's Loss of Current Test. — In overlap tests there is an 

 interval of time between successive observations, which, although 

 of very short duration, permits of variations in the resistance 

 of the fault taking place. And as the overlap formulae assume the 

 fault to remain constant during successive tests with the same 

 current, there is a certain chance of error depending upon the 

 behaviour of the fault. This is in some degree self-correc- 

 tive by manipulation and judgment in the selection of uniform 

 readings, but it will be understood that, if tests at both 

 ends of the cable can be made exactly at the same moment 

 of time, the fault resistance is constant for both readings, and 

 the above possibility of error eliminated. It is for this reason 

 that Kempe's localisation test by loss of currentln which the 

 observations are simultaneous, is such a useful method for the 

 the localisation or rapidly varying faults. 



The connections for the test are as in Fig. 257, the distant 

 end being earthed and the battery applied from the testing end 

 zinc to line. At each end current indicators, or milammeters, 

 are connected in the line by which the sent and received cur- 

 rents are observed. Milammeters are much to be preferred, as 

 their indications are in known units and directly comparable, 



