how blackout and islanding are inter related

If a system supplied by several separate power source, we used to use islanding to prevent total black out if due to a fault in the system, one or more power source trip.The system should be divided into several island supllied by individual power source The load in an island should not be more than the capacity of the power source.Most of the time we prefer to use dF/dt relay to perform this islanding and load sheading, although some older system is still using UVR.If the islanding fail or a little bit slow, than we might go into a total black out or we can have a bigger outage area compare to if the islanding is operating properly.