470 Report S.A.A. Advancement of Science. 



(c) The Power Distributor must incur heavy capital outlay 

 in dynamos and distributing mains, which the power- 

 user, employing his own direct power plant, avoids. 

 The author's object is to show, firstly, why it is possible under 

 certain conditions to supply fairly large power-users by electrical 

 distribution from a distance with advantage to both supplier and 

 supplied, and, secondly, to point in general terms to the limitations 

 of electric Power Distribution. 



The advantage of the Power Distributor over the ordinary 

 power-user employing his own plant, may be briefly described under 

 the following headings : — 



(i) Large output. 



(2) Low Diversity factor. 



(3) Large Station load factor. 



(4) Large Plant load factor. 



I. Large Output. 



It will be readily understood that as there are many charges 

 which do not increase pro rata with the output — such, for instance, 

 as management, attendance, and even stores and repairs to 

 a lesser extent — the larger the service from one generating station 

 (within limits) the lower will be the cost per unit generated. Larger 

 generating units are, moreover, more economical in fuel consumption 

 than smaller ones, and require less attendance in proportion to the 

 output. 



2. Diversity Factor. 



The diversity factor, however, plays a still more important part 

 in establishing the advantage of electrical distribution of power. 

 This may be best explained by a reference to a specific case : — 



The Natal Government Railways have an electric generatin':^ 

 station supplying power to their railway workshops at Durban. There 

 are no less than 406 motors in use, arranged to drive a corresponding 

 number of tools of various kinds. No motor is any larger than is 

 necessary to drive the particular machine to which it is attached. 

 The total power required to serve the whole of these motors at once 

 on full load would be 2000 K.W., but it is found in practice that 

 the maximum power required to supply their aggregate requirements 

 never exceeds 500 K.W. This is due to the fact that without design 

 a number of motors are always at rest for one cause or another, 

 whilst other motors are for the time being required to give less than 

 the maximum power which the tools they drive may at times 

 demand. 



It follows thnt in such a case the capacity of the generating 

 station can be reduced to about 25 per cent, of that which would be 

 required to work every motor at full load at one time. This per- 

 centage is called the diversity factor. Although in this case the 

 motors are all in one factory, the principle wouM be the same if 

 every motor represented a separate factorv. It is clear that the 

 aggregate capital expended bv the many proprietors in order to 



