Today's electric power systems are complex systems, but every interconnected system can split up into several subsystems due to faults, maloperation or intended islanding due to targeted switching. The questions then arise whether these subsystems are "survivable", and whether the size of these subsystems are random or predefined, and under which conditions can these isolated subsystems be operated in a stable manner. Due to the energy transition a lot more renewable generation units are installed in the distribution grid. This facilitates local intended isolated grids, but also increases the probability of unintended isolated grids. These must be controlled and dealt with.