610 



ELECTRICAL EQUIPMENT 



relay is to take care of make their appearance and start the moving 

 part of the relay. With definite time-limit relays there is, as the 

 name implies, a definite time delay imposed between these two 

 moments, independent of the magnitude of the disturbance, and 

 the time limit therefore becomes practically constant for any 

 given setting. With inverse time-limit relays the time delay is 

 inversely proportional to the magnitude of the disturbance, so 



that with a heavy short-circuit it 

 will be practically instantaneous 

 for any time setting, while on a 

 light overload the time may be 

 several seconds, depending on the 

 setting. 



For instantaneous overload 

 relays the plunger type (Fig. 314) 

 is considered the best. It simply 

 consists of a core or plunger 

 which is movable within a sole- 

 noid. When a sufficient amount 

 of current is passed through the 

 winding the core is pulled up and 

 causes the cone-shapecl disc at 

 the top to bridge the gap between 

 the contacts. The position of 

 the plunger with respect to the 

 coil is adjustable, the lower its 

 position the more current is re- 

 quired to pull it into closing 



FIG. 315. Double-pole BeUows Type P os | tion > and b Y adjusting its 

 . Inverse Time Limit Overload Position it may be set to take any 

 Relay. predetermined strength of cur- 



rent within the range of the coil. 



Inverse time-limit relays may be either of the bellows type 

 or the induction type. The former (Fig. 315) is similar to the 

 instantaneous type to which a compressible leather bellows has 

 been interposed between the moving part and the contact device. 

 When the relay is not operating, the bellows is fully extended and 

 the moving core presses against the same and tends to force the 

 air through an aperture. The air must be driven out of the bel- 

 lowes and the beUows compressed completely before contact can be 



