SWITCHING EQUIPMENT 489 



imately one-quarter second for a large breaker to open after the 

 tripping coils have been energized. The time interval between the 

 moment at which a short-circuit takes place and the moment at 

 which the tripping circuit of the breaker is closed may be varied 

 at will by selecting relays of different time settings. 



By means of such overload relays in connection with reverse 

 power, balanced, differential and pilot wire relays, the character- 

 istics and uses of which are described in the section on " Relays," 

 it is possible to obtain a selective automatic switch action, which 

 will only disconnect the faulty section of the system without 

 interrupting the remainder thereof. The types of relays and their 

 arrangement to accomplish such a result depend entirely on the 

 system of connection and the conditions to be met. 



The generators should preferably be paralleled on a low- 

 tension bus and this should be arranged so that it can be inspected 

 and cleaned from time to time without shutting down the sta- 

 tion. With smaller stations a single bus may be sufficient and 

 by sectionalizing the same the operation may be so arranged that 

 one section can be cleaned when the units belonging thereto are 

 cut out during light load. As a rule, however, important stations 

 should be provided with double generator buses (Fig. 303) and 

 the generators connected thereto either by means of double oil 

 circuit breakers or by means of one common oil circuit breaker and 

 two sets of disconnecting switches, one for each bus. Double 

 oil circuit breakers are preferable as they permit the transfer to 

 be done entirely from the main switchboard and thus insures a 

 speedier operation. This also applies to the transfer on the high- 

 tension side, and in this case it is even more important, due to 

 the greater difficulty of manipulating the large high-tension dis- 

 connecting switches. Double oil circuit breakers further permit 

 of inspection and repair of one breaker, while the other is in service. 



The low-tension buses should, furthermore, be sectionalized 

 if the capacity of the station is large, so as to limit the short-cir- 

 cuit current to a value which can safely be ruptured by the oil- 

 circuit breakers, as fully described in the section on " Current- 

 limiting Reactors." 



The transformers should preferably be grouped so as to 

 form units with the lines and with such an arrangement the 

 double low-tension bus is preferable in order to obtain the most 

 flexible method of transfer. 



