THE LOCALISATION OF BREAKS AND FAULTS. 



511 



the testing current it acts like a resistance added to the cable, 

 and the balance obtained on the bridge is higher than the true 

 resistance. The equivalent resistance of the earth current is 

 the active potential of the earth current divided by the testing 

 current to line. 



Carbon Zinc 



Fig. 284.— Connections for Manee Test. 



Cable 



Consider a good cible of 2,000 ohms CR, in which an 

 earth current of 2,778 millivolts is present (Fig. 285), and 

 opposite in direction to the testing current zinc to line. This 

 will be negative in sign because equivalent to a negative current 

 to line applied at the distant end. The effect will be like an 



Cable 2000' 



I 1^ 



1. 



Testing Current 

 S^ Zinc to Line 



E.C. Negative 

 i 2778 m' volts 



Fig. 285. — Good Cable with Negative Earth Current. 



■extra resistance to the cable. Let the testing current be 

 •5 milliamperes to line. The balance A will be 



L + i 



that is 



2,000 + ^'^ = 2,555-5 ohms. 

 5 



Similarly, the balance B, with 12 milliamperes to line, will be 



The ratio 



2,000 + ^^ = 2,231-5 ohms. 



12 

 ^lz=i;'=.2•40, 

 5 



