THE LOCALISATION OF BREAKS AND FAULTS. , 463 



where L = true CR of line. 



71 = the ratio of the sent to the received currents, 



E, = the resistance through which the cable deflection is 



reproduced, 

 X = distance in ohms to fault. 



Note. — The milammeter at station B i3 considered of negli- 

 gible resistance compared with the whole line resistance L. 



The Application of Break Methods to the Localisation of 

 Partial Earth Faults : Water Column Eesistance. — If a 

 partial earth fault runs down with increase of current and the 

 polarisation with positive current is not excessively rapid, it is 

 evidence of a sufficient exposure of copper present for break 

 methods to be applicable to its localisation. When these 

 methods are applied the distant end of the cable must be freed, 

 so that the whole of the testing current passes through the fault 

 exactly as when localising a break by any of the single-ended 

 tests. 



Also the testa must be taken from both ends of the cable in 

 order to eliminate any part of the fault's resistance not taken 

 account of in the particular break method employed. For 

 instance, a Mance does not eliminate the break or fault resis- 

 tance, and when used as a single-ended test on a break the 

 resistance of the break has to be allowed for as experience 

 directs. But when employed on a fault localisation, tests can 

 be taken from both ends, and all the elements making up the 

 resistance of the fault thereby eliminated. 



There is one element present in the resistance of a partial 

 earth fault which does not exist in a break — namely, what is 

 known as the water-column resistance. In a total break the 

 conductor is exposed unrestrictedly to the sea, and therefore 

 the resistance in the path of the current from the con- 

 ductor to the sheath is quite insignificant. But a partial 

 earth fault is in the nature of a puncture or pin-hole in the 

 percha into which sea water enters, and it is by way of this 

 pin-hole of water that connection is established between the 

 conductor and earth. The path of the testing current, there- 

 fore, at the fault is through a very fine water column, which 

 has a material resistance and must be taken account of. This 

 can be completely eliminated by tests from both ends, after the 



