514 ELECTRICAL EQUIPMENT 



Bellows-type relays are very rugged and are extensively used 

 for ordinary service, while the induction relay is desirable where 

 extreme accuracy is required such as to insure selective switch ac- 

 tions on complicated networks. 



Overload relays are usually made single-pole, but one, two or 

 three relays may be combined as the conditions may demand, 

 the usual practice being shown by the connection diagrams i 

 Fig. 318. Single-pole relays may be used on single-phase and 

 balanced three-phase circuits; double-pole relays on ungrounded 

 three-phase circuits and two-phase circuits which are not inter- 

 connected; triple-pole relays on three-phase grounded neutral 

 and interconnected two-phase circuits. 



Reverse Power Relays. These operate on a reversal of the 

 energy in the circuit to which they are connected. They may 

 be either of the dynamometer type or the induction type. 



The dynamometer type (Fig. 319) consists of a potential coil 

 pivoted in the center of a current coil in such a manner as to 

 obtain dynamometer action, the two coils being mounted in a 

 magnet frame. The pivot which supports the potential coil also 

 supports the movable contact, and when the flow of power is in 

 normal direction or at no load, the contact lever is held against 

 a stop by a spring. Upon reversal of power the potential coil 

 tends to turn and throws the contact lever against a station- 

 ary contact, completing the tripping circuit of the oil circuit 

 breaker. 



The dynamometer type of relay is generally built in single- 

 pole units which may be combined in the same manner as overload 

 relays, for the protection of polyphase circuits, as previously 

 described. Figs 320 and 321 show the connections for a relay 

 of this type as used on three-phase nongrounded and grounded 

 circuits. Three potential transformers are shown for the latter 

 case, but two may be used if the volt-ampere load permits. 



Reverse power relays are in themselves always instantaneous 

 and for time action they must be combined with overload relays 

 with such features. An overload relay is always recommended 

 for the induction type reverse power relay, even for instantaneous 

 action due to its sensitiveness. This overload relay, although not 

 necessary, is nevertheless also recommended with the dynamom- 

 eter type. When used in connection with overload relays the con- 

 tacts of both relays are connected in series so that both must 



