What happens if one phase is down in a high voltage transmission line?

Usually transmission lines (above 11kV) are delta connected because of 2 factors:

1. To save copper without needing an extra conductor for neutral.
2. To reduce current per phase (as Current per Phase = 0.866 times Line Current value).

However, if one of the phases goes out, then the connection reduces to an open delta connection, thereby power reduces to 57.7% of the rated power capacity. Below link explain the open delta phenomenon in power transformers in a better way.

The line protection relay (distance relay) will activate and trips the breakers to isolate the faulted line. Those customers being served by that particular line will experience a power outage until maintenance crews complete the repair and re-energize the line.