THE LOCALISATION OF BREAKS AND FAULTS. 419 



and has to be reduced by the earth current correction term 



o 



— (w— 1)P. The last term in the equation is the earth current 

 wy 



correction for the fridge reading. Now it is convenient to com- 

 bine these two earth current corrections into one and when this 

 is done the resultant comes out an additive correction, and 

 the expression for x becomes — 



:A-(A-B)P + ^£p(7i-l)-7.]. . . (5) 



K- 



This explains why the correction is additive for a negative 

 earth current in Schaefer's test where it is normally substrac- 

 tlve, as shown by the last terms in equations (1) and (4). 

 Let P(?i - 1) — ?i = M and we have 



.. = A-(A-B)P + ^^M (6) 



which is the form used for working out the test, when the 

 earth current is negative. 



With a positive earth current acting in the same direction 

 as the testing current the apparent resistance of the cable as 

 measured on the bridge is less than the true CR, and the 

 earth current correction must be added as follows : — 



3j + F = A+~. ...... (7) 



=« + i#=S+- (8) 



\f n '"- 



From these the equation for x is derived in the same way as in 

 the previous case and is 



a; = A-(A-B)P--M (9) 



nc 



ks, B Is higher than A with a positive earth current A - B is a 

 minus quantity but becomes plus by the negative sign before 

 the bracket. Consequently, we may write the expression In 

 the more convenient form 



ir=A + (B-A)P- — M, . . . (10) 



ThG 



which Is the formula to use when the earth current is positive. 



It will be noticed that the same alteration of sign takes 



place in the correction as in the case of a negative earth cur- 



E E 2 



