ECON'OMICS OF TKLKPIIOXK IHOLAV A I'I'LK'ATIONS 233 



OH a basis comparable to relay manufacturing cost, savable power was 

 worth between vSoOO and $2,500 per kilowatt, dependinp; on size, savable 

 power, etc. 



Now these figui-es must be associated with the power needed by any 

 particular relay. The total switching power during the busy part of the 

 day, which is the period determining the size of the power plant, is the 

 sum of all the individual relay power drains at any particular moment. 

 This in turn may be taken as the sum of the power required by each 

 relay times the probability that it will be energized at any particular 

 time, which is taken as the fraction of tlu^ })usy hour that the relay is 

 expected to be energized. For every relay, this ratio can be determined 

 with some certainty, by consultation with the circuit designers. 



Summarizing, the switching power capacity required by the office may 

 be stated as the sum, for all relays, of power w for each relay, times m, 

 the fraction of the busy hour that it is energized (i.e., hrs. per busy hr.). 

 The cost of the power plant capacity reciuired for each relay is then 



C = kmw dollars, (15) 



where k is the equivalent value of powder. 



\'ery often it is desirable to state this cost on the basis of the annual 

 power drain. In this way, the plant investment can be easily correlated 

 with the costs paid to the power companies. In telephone offices, the 

 annual power has been found to correspond to 3,000 times the power 

 consumed in a busy hour, and as a matter of convenience power drain in 

 the central office is often stated in terms of the energy per year based on 

 a 3,000-busy-hour year. Then the ratio of use during the busy hour may 

 also be stated as 



"^ 3000 ' 



where h = hours per year energized. 



Writing A'-'/T 000/2 for the power in kilowatts drawn by any relay, the 

 equivalent first cost of the power plant assignable to the relay is then 



-_^ V J^ 



l^power plant ^^^^j^ A ^^^^ , 



(IG) 



F/h 



For this case, in other words, tlic (Miuivalcnt lirst cost of power plant 

 has a value Cp^ dollars for each walt-hour-per-year of switching power 

 required. 



