THE LOCALISATION OF BREAKS AND FAULTS. 365 



ends, as in a CE test, the earth current due to the diflference of 

 potential in the earth at the cable ends causes a current to flow 

 along the cable from one point in the earth to the other. 

 When one end of the cable is free the currents in the cable 

 are produced inductively from the actual currents flowing in 

 the earth. They are sometimes so unsteady that it is impos- 

 sible to test, but if fairly steady their effect can be satisfactorily 

 eliminated by (1) balancing to "false zero," (2) using reversals 

 of current, or (3) using two different strengths of current and 

 reading to true zero, as in the Mance test. 



Where a fault exists there is generally a current in the 

 cable due to the difference of potential set up by the exposed 

 copper at the fault forming, with the iron sheath and sea 

 water, a voltaic couple. This current is called the natural 

 current from the fault and must be carefully distinguished 

 from that due to polarisation, which is set up by the testing 

 current. When a fault is present in the cable the term earth 

 current is assumed to include the natural current from the 

 fault, as the latter cannot, on a cable, be separately observed. 



(2) Electrostatic Charge and Discharge Currents. — These 

 momentary and violent rushes of current take place on the 

 application or disconnection of the testing battery. Imme- 

 diately the battery contact is made, a strong, but momentary 

 and gradually decreasing, current rushes into the cable. The 

 greater the battery power and capacity of cable the stronger 

 will the charge be. 



On releasing the contact key (so putting cable to earth) the 

 accumulated charge is dissipated and produces a similar rush 

 of current out of the cable, at first very strong and gradually 

 dying away. These currents are not permitted to pass through 

 the galvanometer, but are bye-passed through a small key 

 (Fig. 217). When the key is at rest, as shown, the galvano- 

 meter is protected, and currents passing between A and B 

 are shunted through the key. After a few seconds, when 

 the rush of charge or discharge has subsided, the key is 

 opened (at first by tapping until the bridge is balanced) and 

 then held down by a catch provided for the purpose in order 

 to watch the behaviour of a fault by the spot of light. 



When used with a dead-beat galvanometer, such as Sullivan's, 

 It is not desirable to connect the key as a short-circuit on the 



