546 ELECTRICAL EQUIPMENT 



to that of the copper temperature coil, and the other winding is in 

 series with the copper temperature coil. When the temperature 

 of the copper coil rises, the current in that branch of the circuit 

 decreases and causes a corresponding deflection toward a higher 

 temperature on the scale of the instrument. The reverse is the 

 case when the temperature falls. 



Curve-drawing Instruments. (1) They give a permanent 

 record of the running conditions of the circuits in which they are 

 connected without the loss of time and possible chance of error 

 which occur when such records are computed from the readings 

 of indicating instruments. Showing, as they do, the distribution 

 of the load for every hour of the day throughout the year, they 

 place in the hands of the management very valuable information 

 which forms the basis for future extensions or improvements of 

 service and load distribution. 



The following tables give the instrument equipment usually 

 employed for use on the circuits enumerated. In giving these, 

 each circuit is considered a complete unit in itself. A combina- 

 tion of two units does not mean that all instruments listed for each 

 separately will be used on the combination. For instance, where 

 a generator and transformer are permanently connected together 

 and operated as a unit, there is no necessity' for using an ammeter 

 in the transformer circuit, since it would simply duplicate the 

 reading of the generator ammeter. Other similar cases are numer- 

 ous, such as combined generator and feeder circuit, combined 

 transformer and feeder circuit, etc. Special operating conditions 

 and requirements will often demand different measuring apparatus 

 than that given, but the tables will at least serve as a guide in 

 choosing a suitable equipment in all cases. The small letters 

 in the tables refer to the notes following the tables. 



Current and Potential Transformers. When the voltage or 

 current of the circuit to which the instruments are to be con- 

 nected exceeds a certain limit above which primary instruments 

 are not built, potential and current transformers are employed, 

 the instrument coils being operated from the secondaries of these 

 transformers. As a matter of safety to the operator, secondary 

 instruments are recommended for all circuits in excess of 650 

 volts. 



Since the normal rating of the secondary of current transform- 

 ers is 5 amperes, secondary current coils are ordinarily wound 



