BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 401 



Rq at which no deflection of the galvanometer occurs when the battery 

 is reversed. The two short-circuited pairs are then connected to- 

 gether at the distant end, the reversing switch is left in one position, 

 and the rheostat is adjusted to a value Re to balance the bridge. The 

 location of the unbalance from the distant end is: 



D = T 



Ro 

 Re 



As will be clear from the following discussion, both the formula for 

 the short cable method and that for the long cable method are based 

 on the assumption that the wires under test are of short electrical 

 length. Theoretically, either method could be used with cables of 

 any physical length provided the testing frequency were chosen 

 properl3\ The specific measuring schemes described here are well 

 adapted to practical application, however. 



Short Cable Method ^ 



When the bridge measurement is made with the distant ends of 

 wires 1,2,3 and 4 open, as shown in Fig, 13, the impedance of wire 1 

 to 3-4 is compared to the impedance of wire 2 to 3-4. Assume a 



r-x r-x X X 



2 2 F 2 2 



=rCi 



4=C: 



=rC, 



-03-4 



4=C; 



p t I LzJL LiL JL J_ 



2 2 2 2 



-o2 



Fig. 15 — Equivalent circuit — .short cable method for locating a series resistance 



unbalance. 



testing current of sufficiently low frequency that the wires are elec- 

 trically short. Calling the capacitance and the conductor resistance 

 of the length {T — D) oi each wire, Ci and (r — x), respectively, and of 

 the length D of each wire, C2 and x, respectively, the bridge circuit of 

 Fig. 13 is practically equivalent to that of Fig. 15. 



The impedance presented to the bridge terminals by the network 



*The short cable method is described briefly in the paper, "Cable Testing," by 

 E. S. Ritter, loc. cit. 



