452 



SUBMARINE CABLE LAYING AND REPAIRING. 



Loops on Short Lengths. — After a repair there are a number 

 of shore lengths to be spliced up and tested. The piece con- 

 taining the fault is coupled up to the test room, and the posi- 

 tion of fault located. The faulty piece is then cut out and if 

 exhibiting any special symptoms of attack or decay, is labelled 

 and kept for reference. The measurements on these short 

 lengths are very low, and consequently low resistances have to 

 be used in the bridge arms for the various free and earth tests 

 described above. The loop test is specially applicable to the 

 location of faults in these short pieces, and the following two 

 methods have been devised to give conditions of maximum 

 sensitiveness. 



4- 



rM 



M-. 



1,000 10 



--0 o- 



hIiIiIiI- 



f III! 



Fig. 254. — Allen's Loop. 



Mr. Kingsford has proposed a second observation of the 

 Tarley loop, with cable ends reversed (The Electrician, Vol. IX.). 

 The connections are the same as in Fig. 251, using a ratio of 

 10/1,000. The first observation gives 



10 _. o: 

 1,000 



10 



and the second 



A + 2/ 



1,000 B + :r/ 



where x is the distance in ohms to the fault from the low end, 

 y that from the other, and A and B the resistances unplugged 

 to balance in each observation. 



From the above expressions we have 

 .._ lOOA-hB 

 '^"10,000-1 



