SWITCHING EQUIPMENT 



513 



Fig. 317 shows a number of time-current characteristic curves 

 of this relay and the constantly decreasing time as the current 

 increases should be noted. The curves consist of an inverse time 

 portion up to approximately 20 times the minimum current t set- 

 ting, blended into a definite time portion instead of converging. 



The above type of relay may also be used where a definite 

 time action is required. Otherwise a bellows type relay may 

 be used in which the moving part starts immediately when the 

 tripping value is reached and compresses a spring, and this in turn 

 actuates the diaphragm and the contact device, the time required 

 by the spring for this operation being entirely independent of the 

 magnitude of the disturbance, but dependent only on the stored- 

 up energy of the spring and the setting of the air-escape hole. To 



Oil Switch 



Three-Phase 



Inductive Load ID 



Power stat ion 



Three-Phase 



Ungrounded 



Three-Phase 

 Grounded Neutral 



FIG. 318. Circuit-closing Overload Relay Connections Showing Use of 

 Single-, Two- or Three-pole Units. 



obviate inaccuracies due to slow closing it is advisable to combine 

 this relay with an instantaneous one. No mechanical action would 

 then be exerted on the spring until the disturbance had risen to a 

 value sufficiently large to operate the instaneous relay and to 

 throw the definite time limit relay into circuit. Where direct 

 current is available, the coil of the instantaneous relay should be 

 connected to the main A.C. circuit and the definite time limit 

 relay, having a D.C. potential coil, connected to the contact device 

 of the instantaneous relay, and tripping in turn the circuit-dis- 

 connecting device. Where no direct current is available, a cir- 

 cuit-opening instantaneous relay in combination with a definite 

 time limit relay with A.C. coil is required, so that the definite time- 

 limit relay is not connected in until the disturbance has reached 

 a value sufficiently high to operate the instantaneous relay. 



