Polytechnic Association. 955 



Testing for Location of Faults.* 



28. The principle upon which the methods of distance testing are 

 founded, is that of finding the resistance of the line wire between the 

 testing station and the fault, by the methods above described. 



It is very essential that the resistance of each circuit should be fre- 

 quently measured and recorded, so that when a fault occurs the actual 

 resistance of the line per mile may be known. 



If the broken line gives a full ground, its resistance, divided by the 

 resistance per mile, at once gives the distance of the break from the 

 testing station ; and if the distant station obtains a like result, the 

 confirmation is complete. 



Thus, in a line of 100 miles, if the tests from the two extremities 

 indicate distances of forty-five and fifty-five miles respectively, the 

 locality of the interruption is clearly indicated. 



29. As the fault, however, usually gives a very considerable resist- 

 ance at the point where the line is in contact with the earth, and the 

 sum of the two resistances, measured from stations at the opposite 

 ends of the lines, greatly exceed the resistance of the line itself when 

 perfect, it is usual in such cases to estimate the fault midway between 

 the two points indicated. Thus, when the respective distances indi- 

 cate eighty-six and twenty-six miles, the sum of these exceed 100 

 miles by twelve, and, therefore, half this excess, or six, is deducted 

 from each of the measures, the resistance of the fault having been 

 included in each measurement. 



30. "When the line is unbroken, but shows a, heavy escape or partial 

 ground, sufficient to weaken signals, two methods are available for 

 determining its locality. The first is that of direct measurement, 

 alternately from each end ; the distant end at the same time being 

 insulated, or, in other words, left open, as before explained (24). 



In this case the resistance of the fault is measured twice over, and is 

 roughly allowed for by the method of calculation above given (29). 



The Loop Test. 



31. A second and more accurate method, which gives a measure 

 entirely independent of the resistance of the fault, is known as the 

 loop test. ' It is only available, however, in cases where there are 

 two or more parallel wires on the same route. In making this test, 

 let the operator proceed as follows : 



32. Make the length to be tested as short as possible, and have all 

 the instruments in circuit taken out ; select a good wire, similar, if 



* Pope's Modern Practice of the Electric Telegraph, pp, 80-82. 



