(isr, ELECTRICAL MEASUREMENTS 



lead. If this resistance is very low, the resistance across the open 

 fault has practically been measured in No. 1. If it is not low, it 

 should be determined by measuring the insulation of the open 

 phase when the far side is grounded through one of the unfaulted 

 conductors. 



(B) In order to locate the ground, it is necessary that the fault 

 resistance be low, so that sufficient current for the tests may be 

 obtained with low voltage. This is convenient in the loop tests 

 and is necessary in the subsequent exact localization by means 

 of exploring coils. For, if considerable voltage is used, the 

 charging current going to the parts of the cable beyond the 

 fault produces confusion and renders the localization a matter of 

 difficulty. 



After having determined the nature of the fault and gained 

 an idea of its resistance, it will probably be found necessary to 

 reduce the fault resistance. The fundamental idea is to carbon- 

 ize a sufficient amount of the paper insulation at the fault, so 

 that a current of 1 or 2 amperes may be carried for several hours. 

 Practice is required to accomplish this in the shortest possible 

 time and without any approach to an explosive short-circuit at 

 the fault, which would destroy the continuity of the path via 

 the carbonized paper. When the fault is not submerged in 

 water, for in water the paper cannot be carbonized, the procedure 

 is to send a current of from 3 to 5 amperes through the fault for 10 

 minutes, in order to dry it out, and to follow this by a current of 

 about 1 ampere, for 5 minutes, to carbonize the paper. High- 

 resistance faults at a considerable distance from the testing station 

 give trouble on account of the charging current. In such cases, 

 the current through the fault may be determined by aid of a 

 wattmeter. 



(C) After having reduced the fault resistance, a Murray 01 a 

 Varley loop test is used to determine the approximate location 

 of the trouble. 



In making this test, great care must be exercised in applying 

 the jumpers at the far end and in joining the bridge to the cable, 

 in order that extraneous resistances may not be introduced. 

 Also allowance must be made for the leads connecting the bridge 

 to the cable or else the bridge must be so constructed that these 

 resistances are eliminated. 



