526 



ELECTRICAL EQUIPMENT 



generators are operating under considerable excitation. The 

 abnormal voltage is, therefore, usually accomplished by a de- 

 creased current. To guard against the possibility of opening the 



Source 



Relay 



\/ 



FIG. 329. Simplified Diagram of Over-vcltage Relay Connections. 



field circuit under any condition, other than the loss of load, a 

 circuit opening overload relay or circuit closing underload relay 

 may be connected to the line with its contacts in series with those 

 of the over-voltage relay. Fig. 329 shows the connections. 



Low-voltage Relays. These are of the circuit-opening plunger- 

 type provided with a potential winding regularly wound for use 

 on the 110- volt secondaries of potential transformers. 



In operation, so long as the potential is about normal, the 

 plunger is held up, causing the contacts to remain open. When the 

 potential falls below one-half normal, the plunger is released and 

 the circuit closed. In some cases the plunger must be pushed up 

 by hand, after potential has been applied. Usually, however, 

 coils are used which will automatically raise the plunger when 

 normal voltage is restored. 



Underload Relays. These are made with circuit-closing con- 

 tacts for instantaneous operation and are similar to low-voltage 

 relays with the difference that current coils are substituted instead 

 of potential coils. 



Trip-free Relay. This is a safety device intended for use with 

 electrically controlled circuit breakers, in that it prevents them 

 from being held closed on overloads. To accomplish this, the trip- 

 free relay is simply added to the standard control wiring. After 

 the breaker comes out on overload it cannot be thrown in again 

 until the closing contacts of the control switch have been allowed 

 to return to the- open position. The diagram in Fig. 330 illustrates 

 the connections. 



Signal Relays. These are used for indicating to the attendant 

 the automatic opening of circuit breakers. When these are closed 



