CABLE TESTING 



(B) Reduction of the resistance of the fault (if necessary) 

 so that current sufficient to make the location and verification 

 tests will flow through the fault with a moderate voltage 



(C) Preliminary location of the fault by a loop test If the 

 conductor is burned off the loop test is not applicable In 

 this case the ground is located by use of an exploring coil, see 

 D below. 



(D) Verification of the location by use of an exploring coil. 

 (A) Taking a three-phase cable: 



1. All three conductors are insulated at both ends and the 

 resistance to ground of each conductor determined by the volt- 

 meter method, using a direct-current potential of about 110 

 volts. This shows whether the fault is a ground or not and if it 

 is, gives an idea of the fault resistance. 



2. If a low-resistance ground is found, verify No. 1 by using 

 a test lamp, that is, an incandescent lamp with one side of the 

 socket attached to the 110-volt direct-current supply, the other 

 being provided with a flexible cord so that it may be attached to 

 any of the conductors. If the lamp glows, the resistance of the 

 fault is to be measured by a bridge, two readings, with reversed 

 currents, being taken. This measurement of the fault resist- 

 ance shows whether it is necessary to further reduce it before 

 making the loop and verification tests. 



3. Conductors which show the same insulation to ground should 

 now be tested to determine whether this is due to the wires being 

 crossed. The test is made by grounding one of the wires and 

 remeasuring the resistance to ground of the others. If the volt- 

 meter method shows that this resistance is low, it should, for the 

 reason stated in 2, be measured by the bridge. 



4. The far ends of all three conductors should be carefully 

 connected together and the resistances of all the uncrossed loops 

 measured by the bridge. A comparison of these results with the 

 resistances computed from the known size and length of the line 

 will show if there are open faults; that is, places where the wires 

 are broken or burned off. If an open fault exists, an idea of its 

 resistance should be obtained. The resistance to ground of the 

 near side of the open fault has been obtained in No. 1. The 

 resistance to ground of the far side of the open fault is obtained 

 by measuring it via an unfaulted conductor, which is used as a 



