Islanding

Islanding is the intentional or unintentional division of an interconnected power grid into individual disconnected regions with their own power generation.

Intentional islanding is often performed as a defence in depth to mitigate a cascading blackout. If one island collapses, it will not take neighboring islands with it. For example, nuclear power plants have safety-critical cooling systems that are typically powered from the general grid. The coolant loops typically lie on a separate circuit that can also operate off reactor power or emergency diesel generators if the grid collapses.