Direct Transfer Trip

When your transmission line protection scheme requires that you accelerate the tripping of the relays at the opposite end of the line from where the DTT is initiated. Your local relays will trigger the DTT depending on the conditions you specify, by operating a contact that causes your DTT equipment to send a signal via some communications medium (telephone line, fiber optic, microwave radio, etc.) to the other end.

I under stand the logic, but i want to know the conditions which trigger DT through PLCC.