THE LOCALISATION OF BREAKS AISTD FAULTS. 



447 



by one of the loop tests. The faulty cable is joined or looped 

 to the good cable at one end, thus forming one continuous cir- 

 cuit. There are, therefore, two cable ends at the testing 

 station to put on to the bridge as In Fig. 250. 



In Varley's loop test the end of the faulty cable is joined to 

 the earth terminal of the bridge, and the end of the good 

 cable to the line side. The metallic circuit is first measured — 

 that is, the copper resistance of the looped cables — irrespective 

 of the presence of a fault. For this test the earth side of the 

 key is connected to the bridge (as in Fig. 250) instead of to 

 earth, so that the fault does not form part of the circuit tested. 



The metallic circuit test should be perfectly steady, as it is 

 unaffected by variations in the fault. If it is unsteady it indi- 



FiG. 250. — Varley Loop Metallic Circuit. 



cates an intermittent contact in the conductor. The battery 

 must be very well insulated in this test, and in all loop tests, 

 especially on long cables. 



The cables are again looped at the distant station and a set 

 of balances taken with the earth side of the key disconnected 

 from the bridge and put to earth (Fig. 251). 



Readings may be taken with both currents as the fault is in 

 the battery circuit and variations in its resistance do not affect 

 the correctness of the balance. 



The resistance unplugged, added to that of the short length 

 of cable up to fault, on an even bridge, is equal to the resis- 

 tance of the rest of the cable. That is, the fault is electrically 

 in the centre of the line, and therefore 

 a; + R = L - x, 



L-R 



whence «^ = — o — » 



