EFFICIENCIES OF TRANSMISSION. 185 



128. The above table is deduced on the basis of transmitting 

 equal amounts of power over equal distances at equal differences 

 of potential between line- wires and at the same percentage line loss. 



If the voltages of transmission are so great that the question 

 of insulation becomes of importance, the basis of comparison has 

 to be altered, since now the limiting factor is the maximum 

 voltage between any two line-wires. Under these conditions, a 

 di-phase three- wire system would require 1*457 times the copper 

 of a mono-phase system, since the potential difference between the 

 two outside wires is \/2 times that between either outside 

 wire and the common return. 



PROBLEMS ON CHAPTER XVI. 



1. A tri-phase generator is used to transmit 1000 kilowatts over a distance of 

 50 miles at 10,000 volts. Calculate the weight of copper in the lines if 20 per 

 cent, of the energy is lost in transmission. 



Answer. 133 tons, nearly. 



2. If the generator in Question 1 has an efficiency of 94 per cent, and feeds a 

 transformer of 98 per cent, efficiency at the other end of the line, what is the 

 efficiency of the system, and what is the output of the transformer? 



Answers. (1) 73'7 per cent. ; (2) 784 kilowatts. 



3. What would be the weight of copper in Question 1 if it were mono- 

 phase ? 



Answer. 177 tons, nearly. 



