THE LOCALISATION OF BREAKS AND FAULTS. SO^ 



The stronger the earth current the higher must be the fixed 

 resistance (d). Its exact amount does not matter so long as it 

 is high enough to admit of some added resistance in the line to 

 balance. If, for example, balance could not be obtained with 

 a certain resistance in the line it would mean that the fixed 

 resistance (d) was too low and should be increased. Mr. Black 

 states that about 25 per cent, in excess of its estimated value 



( '^"' ) generally suffices, but it may require to be about 50 per 



cent, in excess when a very strong earth current prevails. 



The bridge is connected up so that the adjustable resistance 

 in the line comes in the variable part of the box as in Fig. 282. 

 The fixed arm (d) is next to the ratio arm (a) and the cable 

 is next to the variable resistance. 



For example, a preliminary test by false zero showed the CR 

 to be approximately 3,300. Applying this test with an even 

 bridge the fixed resistance was made 4,500 ohms. With zinc 

 to line balance was obtained wiih R=570 and with carbon to 

 line Ei= 1910. Hence 



.=4,500-^1^^^=3,260 ohms 



Note. — In this example it is obvious that the earth current 

 opposed the zinc to line testing current and its equivalent 

 resistance was 



l={d-x)-'R=B^-{d-x) = 670 

 c 



As the test depends upon the current to line being the same 

 in the two observations it may conveniently be seen that this 

 condition is fulfilled by connecting a milammeter in the line. 

 Or the test may be taken with the milammeter alone by repro- 

 duced deflection, as in Fig. 283. 



The deflection is first taken through the fixed resistance d, 

 and then the cable is plugged over and the same deflection 

 reproduced by adjustment of a resistance, R, in the line. 

 Then the current is reversed and the resistance readjusted to 

 Ej^ to the same deflection on the milammeter. We have then 

 as before R + E. 

 :c=d 2-' 



If two variable resistances are available the fixed resistance 

 {d) may be first adjusted to balance the cable without resis- 



