SWITCHING EQUIPMENT 



521 



Main Oil S 



ing to relay 26, it will be seen that the opening of oil switch 2a 

 will have resulted in the reversal of the current in line No. 1. If 

 the relay 26 has not operated previously, it cannot fail to do so 

 now. Had the short-circuit occurred at some other point, the 

 energy intensities and directions, and, consequently, the order of 

 the operations, would have been somewhat changed from those 

 outlined above, but, in any event, the final outcome would have 

 been the isolation of the injured line 

 without affecting its companion. 



It may be observed that with line 

 No. 2 cut off, the counteracting in- 

 fluence in the relays of line No. 1 is 

 removed. Under these conditions a 

 short-circuit outside of the tie line 

 might result in the opening of the one 

 remaining circuit. This difficulty 

 may be overcome by the use of auxili- 

 ary switches connected so as to render 

 the second line nonautomatic follow- 

 ing the opening ot the oil switch in 

 the faulty line, or better still, to 

 automatically insert instead, time 

 limit overload relays. 



Balanced Relays. These are in- 

 tended for protecting parallel cir- 

 cuits against faults which would 

 materially unbalance the currents in 

 these parallel lines. In the case of 

 parallel outgoing lines, when a short- 

 circuit occurs on one line, the current 

 in that line will become greater than 

 in the others, and by reason of this difference the circuit breaker 

 of that line will be opened. So long as no fault exists on any 

 line, no relay will tend to trip, therefore, no amount of balanced 

 overload on the lines would open any circuit breaker. Balanced 

 relays operate on current alone, and should be used on the 

 power end of the circuits only. 



Split-conductor Relays. This system consists in splitting each 

 conductor into two parts and using a relay which operates when- 

 ever the current in the two halves becomes unbalanced. The 



Tripping 

 Battery 



Main Oil S 



FIG. 325A. Split-conductor 

 Method of Relay Protection. 



