.554 ELECTRICAL EQUIPMENT 



whereby the opening of the A.C.' oil circuit breaker would in turn 

 trip the D.C. breaker. 



For small and medium size installations the field switches are 

 usually of the ordinary knife switch type mounted directly on the 

 main switchboard. For large installations it is, however, com- 

 mon practice to .employ solenoid-operated carbon-break circuit 

 'breakers. These are often mounted on panels near their respective 

 exciters so as to reduce the length of connections to a minimum, 

 and controlled from the main board. 



Occasionally a separate direct -current switchboard is provided 

 and located at some convenient place near the exciters. On this 

 board is then mounted all the exciter and field switches as well as 

 other low-voltage switches and circuit breakers for the various 

 station circuits. 



Field switches for disconnecting the individual fields of the 

 A.C. generators should always be provided. These switches are 

 known as " field discharge switches " because their design is such 

 that when they are opened a discharge resistance is automatically 

 inserted in series with the field circuit. If this should be suddenly 

 broken, an excessively high potential may be induced in the field 

 winding which might puncture its insulation. By inserting a 

 resistance in the circuit, the e.m.f. induced in the field coils by the 

 dying magnetic flux produces a current through this resistance; 

 thus, the energy stored up in the magnetic field, when the cur- 

 rent was compelled to increase against the induced counter e.m.f., 

 is now discharged in this resistance where it appears as heat. 

 The construction of the switch is such that in opening the same 

 the resistance circuit is closed before the field is disconnected 

 from the exciter or field bus, while, in closing the switch the 

 resistance circuit is opened before the field is connected to the 

 .exciter. By this means all destructive arcing is also avoided, 

 for the field can never be broken without shunting it through 

 '-the discharge resistance. Certain types of switches are, on the 

 : other hand, provided with a stop so that they cannot be com- 

 pletely opened until this has been withdrawn, thus giving the 

 induced field energy time to be dissipated through the discharge 

 clip to the discharge resistance before the circuit is broken. 



Field switches may be either hand operated or solenoid oper- 

 ated, similar to the exciter switches. In the former case they may 

 be identical to ordinary knife switches, to which discharge clips 



