SWITCHING EQUIPMENT 



509 



and circuit opening relays only in those cases where direct current 

 is not available. On account of the heavy secondary currents 

 which are liable to flow on severe short-circuits and due to the 

 comparatively high impedance of the trip coil, which may tend 

 to hold up the voltage, a considerable arc is liable to be set up when 

 the contacts are opened, and there is therefore a limit above which 

 it is not safe to use circuit-opening relays. As a rule they should 

 not be used when the short-circuit current exceeds ten times the 

 normal rating of the current transformer. 



There are a large number of different types of relays, but only 

 a few of those in ordinary use on power transmission systems will 

 be considered. Neither will any detailed description of their con- 

 struction be given as changes and improvements are made so fre- 

 quently that this would soon be obsolete. It will therefore be 

 the aim in the following to merely deal with their fundamental 

 principles and characteristics. 



Overload Relays. These may be instantaneous, definite time 

 limit and inverse time limit. With instantaneous relays, the 



Cover 



Movable Contact 

 Block 



Insulating Base 

 Supportin 



Contacts 



'Calibrating Tuba 



l^k ^^ 



.Adjustment Devtea 



FIG. 314. Instantaneous Overload Plunger-type Relay. 



contact device will operate immediately and close the tripping 

 circuit of the breaker when the abnormal conditions which the 



