The relay coil is energized by the primary power ... you might not need a transfer switch at all, though. You can instead use an interlock plate which is even simpler than that.

Transfer switches transfer electrical power back and forth between two or more power systems or buses such as a utility power line and a backup generator. They are used in applications that require a ...

it relays them from the switch's input port to the output port, hence the name. Attachment to a frame relay network is made via a FRAD on the customer's premises, which may be a separate device or ...