676 



ELECTRICAL MEASUREMENTS 



The value of x is 



(4) 



R, 



If an electrostatic voltmeter is used, no allowance is necessary 

 for the voltmeter current and 



V 



x = r 



Loop Tests. By a loop test is meant any method of locating 

 grounds or crosses by determining the resistances of the two sec- 

 tions of the loop formed by connecting the faulted conductor at 

 its far end to an unfaulted conductor which returns to the send- 

 ing station (see Fig. 418). 



* 



FIG. 418. Loop test for locating faults. 



The grounded conductor is represented by 06, the fault being 

 at 6'. The unfaulted conductor is shown by dc; at the far end it 

 is electrically connected with the faulted conductor by a low- 

 resistance jumper which must be insulated from ground. It is 

 essential that the contacts at b and c be perfect. 



The superiority of the loop tests is due to the fact that the 

 results are independent of the resistance of the fault itself. 



A 



FIG. 419. Two-ammeter loop test for locating faults. 



Two-ammeter Loop Test. In this method the two sections 

 of the loop are fed in parallel from the same battery and the 



ratio - is determined from the readings of two ammeters, one 



