552 ELECTRICAL EQUIPMENT 



P.I.W. *= Poly phase indicating wattmeter. 



P.R.W. = Polyphase watthour meter. 



P.R. = Potential receptacle. 



P.P. = Potential plug. 



P.T. = Potential transformer. 



Rheo. = Rheostat. 



S. = Shunt. 



S.R. = Synchronizing receptacle. 



S.P. = Synchronizing plugs. 



T.B. = Terminal board for secondary leads from current and 



potential transformers. 

 T.C. =Trip coil on oil switch. 

 V. = Voltmeter. 



Exciter and Field Control. For the electrical control of 

 exciter circuits it is usual to omit fuses or other overload devices in 

 order to prevent any interruption in the supply of field current to 

 the alternating-current generators, thereby insuring continuous 

 operation, which, in most stations, is an essential feature and 

 is of more importance than protection of the exciters from dam- 

 age. Also as an insurance against injury to the alternating- 

 current generator field windings. When trouble occurs in the 

 exciting system and ope is the overload devices on all the exciters 

 connected, the generator field circuits are broken at points where 

 no discharge resistances are interposed and the generator field 

 windings are consequently liable to puncture by the high-induced 

 voltage to which they are subjected. If overload protection is 

 insisted upon, it is recommended that the overload devices, 

 fuses or circuit breakers, be based on double the normal capacity 

 of the exciter so as to open only in case of very serious trouble. 



For large plants having a number of exciters in parallel and 

 where the expense involved is of secondary consideration, it is 

 customary to provide reverse-current circuit breakers without 

 any overload attachment. The reverse-current device serves to 

 disconnect a defective exciter while the remaining exciters con- 

 tinue in service. 



Circuits for motors driving exciters are usually considered as 

 feeder circuits and overload protection is accordingly recom- 

 mended for the motor. A time-limit device is preferable for this 

 overload feature, and, if an instantaneous device is used, it should 



