512 ELECTRICAL MEASUREMENTS 



maximum demand which the consumers make for the com- 

 pany's product, and in certain systems of charging, maximum- 

 demand indicators are used in conjunction with the watt-hour 

 meters as an aid in apportioning the fixed costs among the 

 consumers. 



Demand indicators record the greatest sustained amount of 

 current, or power, which the consumer uses. They are not 

 supposed to indicate demands which are of such short duration 

 that no serious burden is placed thereby on the generating 

 machinery. The length of time during which the demand must 

 be sustained in order that the indicator may register depends 

 upon the character of the service; for power work some com- 

 panies use a J^-hr. period, but a 5-min. period is not uncommon, 

 especially with badly fluctuating loads. 



Strictly speaking, to furnish adequate data for use in deter- 

 mining rates, a demand indicator should give not only the demand 

 but the hour at which it occurs; for a consumer who takes a 

 large demand at a time when the generating machinery and dis- 

 tribution system would otherwise be idle necessitates no addi- 

 tional investment and can be given a better rate than a con- 

 sumer who makes the same demand at the time of peak load. 

 It is only in the case of large consumers that a supply company 

 is justified in installing an expensive form of demand indicator 

 which will show the time at which the maximum demand occurs 

 as well as its magnitude. 



With small consumers, instruments such as the Wright or 

 the General Electric Co. M-2 demand meters are used and the 

 allowance for the fact that all the demands do not occur simul- 

 taneously is made, by use of the diversity factor, when the rates 

 are originally determined. The diversity factor is defined as the 

 ratio of the sum of the maximum power demands of the subdi- 

 visions of any system or part of a system to the maximum de- 

 mand of the whole system or of the part of the system under 

 consideration, measured at the point of supply. 



Diversity factors can be determined only by actual observa- 

 tion of the consumers' maximum demands and the corresponding 

 maximum demand on the station. They will be different for 

 different classes of service. 



In consequence of the detailed study of electrical rates now 



