Power failure transfer

In telecommunication, the term power failure transfer has the following meanings:

1. The switching of primary utilities to their secondary backup whenever the primary source operates outside its design parameters. This is accomplished via a transfer switch.
2. In telephony, a function which, when activated in the event of a commercial power failure or a low-voltage battery condition at a subscriber location, supplies power to predesignated subscriber equipment via the central office trunk.

Power-failure transfer is an emergency mode of operation in which one and only one instrument may be powered from each trunk line from the subscriber location to the central office.