THE LOCALISATION OP BREAKS AND FAULTS. 417 



It will be seen that at the second balance with the high 

 current a change took place. Therefore, the first pair of 

 readings were corrected for the earth current before the test, 

 and the last two pairs with that after the test. 



38*5 

 Ratio of testing currents, = 2*56 



By Table P = 1-943 andM = 0-471. 



The milammeter 50 range was used; hence the reading 



38-5 (for nc) must be divided by 3 to bring to milliamperes. 



It must further be divided by 2 to get milliamperes to line, as 



the instrument was in the battery circuit. Hence 



38*5 



nc = =6'41 m/amps. 



3x2 ' ^ 



Correction for first pair is with e= 2,343 m/volts. 



Hence 5 M = ?23«><W7I ^ j 72 „|^„, 

 nc 6-41 



Correction for two last pairs : — 



6 = 3,270 m/volts. 



TT e ,. 3,270x0-471 ^.^ , 



Hence — M = = 240 ohms. 



nc 6'41 



These corrections are Additive because the earth currents 

 are Against the direction of the standard cell current. 



It will be noticed that the testing current (nc) comes into 

 the calculation of the equivalent earth current resistance 

 in the correction term, but the potential (e) in this correction 

 is that due to earth current alone and not to polarisation 

 from testing current. Consequently time must be allowed 

 for the depolarisation discharge after testing by earthing 

 the cable for at least one minute before the earth current ia 

 observed. 



Sets of Schaefers behave differently with different ends, in 

 some cases being very steady and with appreciation to half an 

 ohm, but each successive set putting the end nearer home. 

 In that case the first set of observations is generally safe to 

 accept. 



The Earth Current Correction in Schaefer's Test. — If in a 

 conductor being measured on the bridge an earth current flows 



E B 



