SWITCHING EQUIPMENT 551 



purpose of protecting the switchboard attendant and freeing the 

 instruments from the effects of electrostatic charges which might 

 otherwise collect on the cases and cause errors. 



The primary of current transformers should never be left in the 

 line with the secondary open-circuited, as this will set up a heavy 

 flux through the core, over-saturating the iron and causing it to 

 greatly overheat. If for any reason, therefore, it becomes neces- 

 sary to remove the meter or any current-carrying device from the 

 secondary circuit of a current transformer, the secondary should 

 be short-circuited by a wire or some other means. 



Potential transformers are used to insulate the meters from the 

 high potential circuit as well as to do away with a large amount 

 of resistance in series with the meters which would be necessary 

 if the meters were connected directly to the high-potential cir- 

 cuits. Except in special cases, they are generally protected by 

 fuses inserted in the primary leads. 



The connections for the multitude of instruments, meters, 

 relays, etc., with their current and potential transformers which 

 are used in the modern power station are very intricate. While 

 for individual equipments such connections may be standardized, 

 the combinations used in a large station are generally such as to 

 make the connections more or less special in order to give the best 

 results. Individual diagrams are as a rule contained in the 

 bulletins issued by the various manufacturers, and the making 

 up of the main wiring diagram for any important installation 

 should be left to the manufacturer supplying the switchboard. 

 A typical diagram of connections for an individual exciter, an 

 A.C. generator and an outgoing feeder is shown in Fig. 345 as an 

 example. 



KEY TO SYMBOLS 



A. = Ammeter. 



B.A.S. = Bell-alarm switch. 



C.T. = Current transformer. 



F. = Fuse. 



F.A. = Field ammeter. 



F.S. = Field switch. 



G.C.S. = Governor- control switch. 



K.S. = Knife switch. 



L.S. = Limit switch (included with governor motor). 



O.S. = Oil switch. 



