508 



ELECTRICAL EQUIPMENT 



ments and meters are connected to a current transformer their 

 accuracies are naturally affected by the total load imposed on the 

 transformer secondary, decreasing rapidly as the load rises above 

 a certain point. Some oil circuit breaker trip coils have a high 

 impedance, and meter combinations requiring considerable accu- 

 racy consequently should not be used in series with them. By inter- 

 posing a relay, which cuts out the trip coils except at the moment 

 of trouble, the total load can be very materially reduced. The 

 relay therefore simply serves to control the tripping circuit and 

 may be either circuit-closing or circuit-opening. In the former 

 case (Fig. 312), the relay contacts are normally open and the trip 

 coils dead, but at the moment of operation contact is made, thus 



Relay 



_*_ \ ' Ground 

 tSV/z. Current Transformer 

 *'r* Generator 



FIG. 312. Connections of Circuit- 

 closing Relay. 



FIG. 313. Connections of Circuit- 

 opening Relay. 



completing the circuit and energizing the trip coil, which in turn 

 causes the switch to be released. In the latter case (Fig. 313) 

 the relay contacts are normally closed and the trip coils de-ener- 

 gized, because the current will then take the path of least resist- 

 ance through the contact blocks and not through the comparatively 

 high impedance path through the trip coil winding. When a short- 

 circuit occurs on the main circuit, the contacts open, and force the 

 current through the trip coils, which then operate and open the 

 switch. As noted from the diagrams, circuit-closing relays require 

 a separate source of power, preferably direct current, for operat- 

 ing the trip coil, while for the open-circuit type the tripping cur- 

 rent is obtained from the secondary of the current transformer. 

 Circuit-closing relays, are, however, almost exclusively employed 

 in connection with the circuit breakers used on large power systems 



