230 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



and circuit designer for deciding how much his designs may be penalized 

 by varying the quantities used. It consists of using the costs determined 

 from equation (12), and modified by an amount indicated by (13), or 

 in ordinary cases by arbitrarily increasing (12) by the factor 1.25. 



1 .3 THE EQUIVALENT MANUFACTURING COST OF POWER CONSUMED BY A 

 RELAY 



The largest part of the power plant used in telephone central offices 

 is the 50- volt equipment needed for the switching apparatus. Because 

 of its special voltage range and the requirements on its stability and 

 reliability, it must be provided as a part of every central office to convert 

 the normal power company voltages into the desired telephone values. 

 This equipment involves generators, banks of storage batteries, and 

 switchboard, bus-bar and control equipment; it is an expensive portion 

 of every central office which, of course, it is desirable to minimize as far 

 as possible. For every relay required in the central office, one must 

 associate a small "chunk" of this power plant; so that each relay needed 

 implies an associated investment in the power plant. In a later section 

 the means for minimizing the power and relay costs are described, and 

 since they will involve comparisons on the same basis, it is first necessary 

 to state the value of the power plant in terms related to the amount of 

 power consumed by any individual unit. The method of evaluating the 

 power plant is given in this section. 



The problem may be broken into two parts: (a) the equivalent first 

 costs assignable to the power plant equipment, and (b) the equivalent 

 first costs of the charges per kilowatt-hour paid to the power company. 

 Then the figures are combined. 



1.31 The Power Plant 



Power plants furnished for central offices vary over a wide range of 

 size and cost, depending on the size of the office and its estimated ac- 

 tivity. This plant must of course be planned so as to carry the load during 

 the period of peak activity, even though this occurs for only a small 

 part of the time, so that in the long run the cost of the power plant is 

 decided by the share of busy-hour power taken by each relay. To get 

 such costs one must first find the cost of the plant as a function of total 

 power requirements. 



Present practice in power plant planning results in the purchase of 

 basic power units which may be combined to cover certain ranges of 

 power supplied over roughly a two-to-one range. Within this range, by 



