Wenrick — A Study of the Time Interval, eta. 269 



Art. XXXII. — A Study of the Time Interval in a Protective 

 or Circuit Breaking Device;* by Calvin Naftzinger 



W ENRICH. 



In the ordinary plunger circuit breaker, the time between 

 the starting of the iron core and the releasing of the spring is 

 very short. This time is not appreciably affected by a change 

 of current after the starting of the core. Such an interval in 

 a breaker is called the " time element." 



The fuse, when used as a breaking device, acts on the prin- 

 ciple of an "inverse time element," that is, the greater the 

 excess of current above the maximum amount which the fuse 

 will permanently carry, the shorter is the interval before the 

 circuit is broken. This is a desirable feature. The fuse, 

 however, is non-adjustable in its time feature and very danger- 

 ous from the standpoint of ignition at its blowing temperature. 

 These features make it undesirable as a circuit breaking 

 device. 



If a circuit breaker is to serve as a heat protector, it ought 

 to operate on the principle of an "inverse time element." An 

 automatic device should act quickly on a large overload and 

 very slowly on a small overload, defining " overload" as the 

 excess of current above the value at which the device would 

 not operate during any length of time. From the standpoint 

 of the definitions of " time element," "inverse time element," 

 and " overload," a few of the most popular breakers will be 

 examined. 



The mounting of a copper or aluminium disc on a horizontal 

 axis in a rotating field has been resorted to frequently for an 

 inverse time feature. A weight is supported by a fine cord on 

 the pivoted horizontal axis. The driving torque of the rotating 

 field just balances the weight under normal conditions. In 

 the case of an overload, the weight is raised by the driving 

 torque and in its motion engages two contact points in the 

 tripping circuit. The length of the cord finishes an adjustment 

 for the time element, while the resultant driving torque gives 

 the inverse time element. The magnitude of the weight lends 

 the adjustment for the overload. 



This method — the winding of a cord on a spindle, where 

 the weight is the resisting force — has been used in a variety of 

 ways. 



The "clock type" of relay consists of a clock mechanism 

 which is released by an armature suspended above the poles 

 of a magnet. The magnet is energized by the time current. 



* Thesis presented to the Faculty of the Graduate School of the University 

 of Pennsylvania in partial fulfilment of the requirements for the degree 

 of Ph.D. 



Am. Jour. Sci.— Fourth Series, Vol. XXXII, No. 190.— October, 1911. 

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