SWITCHING EQUIPMENT 555 



have been added, and mounted on the front of the panel. It is 

 becoming very general practice, however, to mount the live part 

 back of the switchboard and operate it by a handle from the front 

 of the board. This type of field switch is regarded as a " safety 

 first " device of great importance and is to be recommended in all 

 cases. The switchboard attendant cannot come in contact with 

 the live parts or arc when operating, and instruments and other 

 adjacent equipment are safe from damage by burning which 

 occasionally happens with the front-of-board type. 



With benchboard equipments and with large capacity vertical 

 switchboards where remote control is desirable, solenoid-operated 

 field switches are often employed. While controlled from the 

 main board, they may be located at the most convenient point, 

 for example, near the generators or on the exciter board. They 

 are similar in construction to the non-automatic, self-contained, 

 solenoid-operated, air circuit breaker with the addition of a dis- 

 charge switch (Fig. 346). 



Solenoid-operated field switches for A.C. generators and for 

 synchronous motors started, as is usual with motors of 250-volt 

 excitation, with the field short-circuited, should be double-pole 

 with common closing and common opening coil. No provision 

 is made for automatically interrupting the discharge circuit 

 after the switch opens, although the discharge blade can be ope- 

 rated by hand. Where economy is of importance, it is sometimes 

 customary with A.C. generators to provide one single pole solenoid- 

 operated field switch for one pole and ordinary knife switch for 

 the other, the former being remote-controlled from the main board 

 while the latter is hand-operated. 



With synchronous motors started from the A.C. side with 

 field open as is usual with motors of 125-volt excitation, solenoid- 

 operated field switches are made ordinarily of two single-pole 

 elements with independent opening and independent closing 

 coils. Both poles close simultaneously and connect the discharge 

 resistance across the field; but one pole precedes the other a short 

 time in opening. When the other pole opens, the discharge cir- 

 cuit is interrupted. 



Occasionally the field switch has been used to cut the voltage 

 off a machine in case of trouble and this is becoming more and- 

 more a general practice. The switch is then equipped with a shunt 

 trip and an overload relay is installed in the main circuit, in which- 



