SWITCHING EQUIPMENT 



515 



operate before the breaker will be tripped. Any type of overload 

 relay can be used, although the plunger type is recommended when 

 instantaneous action is desired. Otherwise the induction type 

 may equally well be used. 



The induction type reverse power relay is based on the prin- 



FIQ. 319. Single-pole Dynamometer-type Reverse-power Relay. 



ciple of the wattmeter, in which a disc or rotating element is actu- 

 ated by both current and voltage windings. The torque generated 

 is proportional to the instantaneous products of the current and 

 voltage, i.e., the watts. 



The relay shown in Fig. 322 is the polyphase type and the 

 arrangement of the driving elements on a common shaft has sev- 

 eral advantages. There are three separate driving elements, each 



