THE LOCALISATION OF BREAKS AND FAULTS. 465 



Let the mean result obtained at the A end be 



R^x + (f-cW) 



and the mean from B end be 



R^ = L-x+(f-dF) 



The latter is communicated to the observer at the home end, 

 who works out from the two results the distance of the fault 

 by the free overlap formula which, as previously explained, 

 (page 454), eliminates the fault resistance. The quantity 

 (f-dF) therefore cancels out and leaves the value of :>• in the 

 form : — 



^._ L + R-E , 

 2 



L being the true CR of the Une. This gives the distance to 

 the fault from the A end, all the components of the fault 

 resistance having been eliminated, 



Schaefer's Break Test applied to the Localisation of a 

 Partial Earth Fault.— As already explained a partial earth 

 fault possesses one more resistance component than a break, 

 namely the resistance of the pin-hole of sea water, known as 

 the water column. In a single-ended test on a fault this com- 

 ponent still remains in the result, but if tests are taken from 

 both ends it may be eliminated by the free overlap solution. 



Each station applies the break test in turn, the opposite sta- 

 tion, for the time, freeing the distant end. The tests are taken 

 to scale or true zero, exactly as described on page 406 for localising 

 a break, observations being taken with currents to line in the 

 ratio of approximately 2-5 to 1. To obtain the best results, 

 both stations should use approximately the same currents and 

 ratios of currents to line. Several pairs are taken and the 

 result worked out by the Schaefer formula, which in this case 

 gives — 



x-i-t« = A-(A-B)P-^^-^M 



or ^+„,^A-|-(B-A)P-:^M, 



according as the earth current is negative or positive and 

 where u- Is the resistance of the water column. 



P H 



