412 ELECTRICAL MACHINERY 



on the line. A fuse is used to protect against fire, overheating 

 of machinery, etc. and when the fuse fails to work dangerous 

 results may follow. If a fuse is replaced by a much larger 

 one, or copper wire, etc., the circuit is no longer protected. 

 An operator who replaces fuses by pieces of wire, etc., is 

 just as foolish as a fireman who sits on the safety valve of his 

 boiler and waits there for the boiler to blow up. 



133. Circuit Breakers. In many kinds of service over- 

 loads occur quite frequently; this is especially true in railway 

 work. If several cars start up at the same time or some 

 cars start up just when two or three others are on an up- 

 grade, the feeder supplying power for that section of the 

 road is sure to be overloaded and, if it were fused, the 

 fuses would be continually blowing, causing much work 

 and annoyance to the operator. Also, during the time 

 spent in replacing the fuses, the feeder would be dead and 

 soon the cars would all be off their schedule time. 



For this kind of service where overloads occur frequently 

 and an operator is present, fuses are not used for protection; 

 a switch that opens automatically when an overload occurs, 

 called a circuit breaker, is used instead. The circuit 

 breaker may open the circuit by an air break or by a break 

 under oil. The oil-break circuit breaker resembles in appear- 

 ance an ordinary oil-break switch, except that it has, of 

 course, the automatic tripping and opening mechanism. 

 These oil-break circuit breakers, when properly designed 

 will successfully open high-voltage a-c. circuits through 

 which thousands of kilowatts of power are being carried. 



The air-break circuit breaker does not resemble a knife 

 switch very closely as may be seen by reference to Figs. 

 270 and 271. This illustrates a double-pole breaker for 

 a low voltage circuit of perhaps 100 amperes capacity. 



Use of Multiple Contacts on an Air-break Circuit Breaker. 

 The breaker shown in Fig. 270 is an air-break circuit breaker 

 and it has three sets of contact surfaces; when the break is 

 closed practically all of the current is carried through a 



