THE LOCALISATION OF BREAKS AND FAULTS. 



437 



Mance Break Test to True Zero. — Sir Henry Mance, to 

 whom this test is due, first introduced it to notice in his Paper 

 " On a Method of Eliminating the Effects of Polarisation and 

 Earth Currents from Fault Tests " read before the then Society 

 of Telegraph Engineers and Electricians in May, 1884. The 

 test is taken on the bridge to true zero with two different 

 strengths of current to line, the connections being arranged as 

 in Fig. 248. 



The earth current effect is eliminated assuming that no 

 material change takes place during the taking of a pair 

 of balances. It can usually be seen by the variations in the 

 readings whether the earth current is changing much, and the 

 most regular pairs of a set should be selected to work from. 



Left hand key 

 zinc to line 



Carbon Zinc 



Fig. 248.— Connections for Mance Test. 



The second reading of a pair should always be taken as soon 

 as possible after the first. The same direction of current is 

 used in all observations, namely zinc to line. 



Sir Henry Mance in his original description of the method 

 altered the strength of current to line by changing the bridge 

 ratio arms, keeping the battery the same. Dr. Kennelly in 

 working out the three-current method as a development of the 

 Mance test to true zero, {The Electrician, October 14, 1887), 

 showed that the calculations were much simplified by keeping 

 the bridge arms constant, and varying the strength of current 

 by ad j usting a resistance in the battery circuit. This is now 

 usually done in conjunction with a milammeter by which 

 means the calculations become exceedingly simple. 



