SWITCHING EQUIPMENT 525 



energy flows in the same direction in the two ends of the line, all 

 the contacts of the relays connected to the tie line will take a 

 uniform position. If the direction of energy should change 

 over the entire line, both contacts would simultaneously reverse, 

 bringing them once more to a uniform position. Under these 

 circumstances, the circuit of the low-voltage trip (see Fig. 328), 

 will be unbroken and the tripping circuit will consequently be 

 kept open. A slight time delay is provided for the overload 

 relays simply to insure sufficient delay to allow all relay contacts 

 to swing to their proper position on the occurrence of a normal 

 reversal of energy in the tie line. If, however, trouble should 

 occur between the stations, power would be fed into the line from 

 each end, and, as a consequence, the relay contacts on one end of 

 the line will remain at one side while the relay contacts at the 

 other end of the circuit will be thrown to the opposite side. This 

 will result in opening the circuit of the time-limit, low-voltage 

 relays, and the falling of the low-voltage relay plungers will close 

 the oil switch tripping circuits at each end of the line and isolate 

 the circuit. 



High-tension Series Relays. These are, in general, of the same 

 principle as the ordinary plunger type relay. They are chiefly 

 used with high-tension oil circuit breakers for overload protection 

 where current transformers are not installed or warranted, and 

 may be either of the instantaneous or inverse time-limit type. 

 The coil is connected directly in series with the line and mounted 

 on a post-type insulator, the size of which depends on the voltage. 

 The plunger of the relay is by means of a long wooden rod con- 

 nected to a circuit-closing switch which can be mounted on any 

 vertical flat surface below the location of the relay coil. 



Over-voltage Relays. These may be either instantaneous or 

 time limit and are similar in construction to overload plunger- 

 type relays, differing only in that potential windings are sub- 

 stituted for the current coils. They may be used to protect gen- 

 erators, transformers or other power apparatus against damage due 

 to abnormal voltages. For this purpose the relay should be con- 

 nected so as either to open up the field circuit of the alternators 

 or introduce into each field circuit a sufficient resistance to insure a 

 reasonably low potential on the system. 



The conditions most frequently responsible for a dangerous 

 rise in potential is the loss of load on a power-station while the 



