AUXILIARY APPARATUS 415 



being closed it cannot snap open. If, however, the switch 

 in series with it is opened first, then the circuit breaker 

 closed, and then the switch closed, the circuit breaker is 

 free to operate and protection against overloads is always 

 obtained. 



There is always an adjustment on a circuit breaker 

 which fixes the current at which it opens the circuit. A 

 certain breaker for example may be set to trip at any cur- 

 rent between 45 amperes and 90 amperes. 



134. Overload, Time-limit Relays. Any electric 

 machine will stand an overload for a short time without 

 suffering injury. A manufacturing company will generally 

 guarantee that their machine will carry a 25% overload for 

 two hours and a 50% overload for one minute. 



Heavy Overloads for a Short Time Not Dangerous. Now 

 in certain kinds of work the load on an electric machine 

 is intermittent and for short periods of time there may be 

 quite a heavy overload on the machine. But if the dura- 

 tion of this overload is short, the machine will carry it 

 ssafely and it is not desirable to have the circuit opened 

 by a breaker or fuse. But if this overload should continue 

 too long, the machine would be injured, and it is thus 

 evident that a fuse or circuit breaker could not properly 

 take care of this kind of a load. 



An overload, time-limit relay is designed to fit such a 

 service. It is essentially a circuit breaker, the tripping coil 

 of which is operated through a local circuit controlled by 

 a relay. When an overload occurs the relay armature 

 begins to move but the damping is such that a considerable 

 time is required for the armature to move far enough to 

 close the local circuit and thus trip the breaker. The time 

 elapsing from the moment the overload occurs to the tripping 

 of the breaker is adjustable by a valve in a dash-pot, or 

 similar device. It is evident from this description that 

 such a piece of apparatus as the time-limit relay just suits 

 the needs of motors operating punch-presses, rolls, hoists, etc. 



