SWITCHING EQUIPMENT 



517 



having a current coil and a potential coil used for both quarter- 

 and three-phase circuits. The third element is required for delta 

 or ungrounded Y circuits in order that each phase may be properly 

 represented in every short-circuit. If two elements were used 

 many single-phase troubles 

 would involve only one of these 

 elements and the benefit of poly- 

 phase action would be lost. 

 Although only one element may 

 be involved in case of a ground 

 on a grounded Y circuit, the 

 voltage triangle will not have 

 become so badly distorted as 

 when a single-phase line to line 

 short exists. For delta or un- 

 grounded Y circuits two cur- 

 rent and two potential trans- 

 formers are sufficient. The 

 third current coil carries the re- 

 sultant current of the two cur- 

 rent transformers and the third 

 potential coil is connected across 

 the open delta of the two poten- 

 tial transformers. These elements all operate through one shaft 

 to control one set of contacts. In this three-element relay, two 

 discs are used, the upper one of which is driven by one element 

 and the lower by two elements, one in front and one in back. 



The polyphase construction makes the action of the relay more 

 reliable than could be obtained by means of three single-phase 

 relays because of the fact that any incorrect tendency on the part 

 of one phase is balanced by a similar but opposite incorrect ten- 

 dency on some other phase. The incorrect tendencies being 

 balanced out, the true net power direction will not be over- 

 powered. 



The polyphase relay should not be used on systems having the 

 neutral grounded, except after proper investigation, unless two 

 or more parallel lines are involved and the relays are inter- 

 connected in a balanced group. In such case the power currents 

 are balanced out and the fault current controls the operation of the 

 relay. 



FIG. 322. Polyphase Induction-type 

 Reverse-power Relay. Cover and 

 Register Removed. 



