Explain center-off relays. When and where should they be used ?

A center-off relay such as a HC or ZC contactor (relay) has its contacts in a null position, when deenergized. This means that the moving contact is not making contact with either stationary contact. The relay contactor has two coils, and each coil operates the moving contact to one or the other stationary contact position.

Center-off relays/contactors can do the following :

• • – Transfer a power source to two different positions
  • – Transfer one circuit to another circuit