496 ELECTRICAL EQUIPMENT 



ring principle. In a case of this nature the current is liable to be 

 fed in either direction at any time, and the protection would best 

 be accomplished by equipping all the circuit breakers where 

 parallel connection is made by balanced or interconnected reverse 

 power relays. Where transformers are involved this would be on 

 the low-tension side of these, the interconnection being between 

 similar phases of the two parallel lines. 



Where balanced current conditions may be assured, the 

 relays may be set for instantaneous action, otherwise it might be 

 necessary to impose a slight time delay. In case one line should 

 become disabled it will then immediately be disconnected and 

 arrangements can be made whereby the circuit breakers of the 

 other line would be automatically provided with time-limit fea- 

 tures by the opening of the circuit breakers of the disabled line. 



Oil Circuit Breakers. Oil circuit breakers are nearly always 

 used for rupturing alternating-current circuits, due to the fact 

 that they do not cause any abnormal disturbances in the circuit, 

 and because they confine the destructive effects of the arc to 

 a small volume. One of the distinctive features of the oil circuit 

 breaker is that the current is interrupted when the current which 

 is maintaining the arc passes through zero, at which point the 

 electro-magnetic energy is minimum. It remains so until the 

 voltage between the contacts rises to a sufficient value to punc- 

 ture the oil insulation. When this takes place the flow of cur- 

 rent is reestablished and flows for another half cycle and so on 

 until sufficient insulation is interposed between the contacts to 

 resist the maximum voltage. This feature is taken advantage of, 

 and modern oil circuit breakers are designed with a view of utiliz- 

 ing the pressure developed by the arc to introduce a large amount 

 of oil between the contacts. 



Owing to the great range and the amount of current, voltage 

 and power to be handled by oil circuit breakers for such cir- 

 cuits, various types have been designed to suit different conditions. 

 For moderate amounts of power, where the size and cost of the 

 breaker is to be kept to a minimum, it is often possible to locate 

 all of the poles of the breaker in one oil tank. For slightly larger 

 amounts of power, each pole is placed in a separate oil tank, but 

 all poles are mounted on the same frame; for still greater amounts 

 of power, at moderate voltages, each pole is in a separate tank, 

 and each tank is in a separate masonry compartment, while for 



