SWITCHING EQUIPMENT 541 



insulation, or to damage apparatus for which the machines fur- 

 nish power. 



(2) They are required when paralleling machines, which must 

 be of the correct polarity and at very nearly the same voltage 

 in order to enable throwing them together with the least disturb- 

 ance. 



(3) They can be used as ground-detecting devices by making 

 proper connections to the system. 



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. 



For Alternating-current Installations. Alternating-current 

 Ammeters. (1) They give an indication of the heating of the 

 armature of the machine. This is a thing which the indicating 

 wattmeters will not do because of the fact that it measures only 

 the energy component while the ammeter measures the reactive 

 as well as the energy component of the current, both of which 

 produce heating. 



(2) In case machines in multiple are running at the same power- 

 factor ammeters show the division of load. 



(3) On feeder circuits, ammeters indicate which feeders are 

 overloading the machine. 



(4) On overhead- transmission lines the use of three ammeters, 

 one in each phase gives an indication of trouble on the lines, such 

 as grounding. 



Alternating-current Voltmeters. (1) They show that machines 

 are being operated at a voltage not too high to damage the in- 

 sulation, or to damage apparatus for which the machines furnish 

 power. 



(2) They are valuable when paralleling machines which must 

 be at very nearly the same voltage in order to enable throwing 

 them together with the least disturbance to the system. 



(3) They can be used as ground-detecting devices by making 

 proper connections to the system. 



