Quick disconnecting is needed, because during a line-to-ground short circuit the grounding pin potential on the power outlet can rise relative to the local earth (concrete floor, water pipe etc.)
If the short-circuit current is lower than this figure, special precautions need to be taken to make sure that the system is safe; those usually include using a residual-current device (a.k.a.
The application of a low voltage allows a small current to pass from the socket back through earth to the supply transformer and distribution board.
Where rotating machines (generators and motors) are present in the system, the time-varying effect of their contribution to a short circuit may be evaluated.
An isolated generator may be specially designed to ensure that it can source enough current on a short circuit to allow subordinate overcurrent protection devices to operate properly.
In polyphase electrical systems, generally phase-to-phase, phase-to-ground (earth), and phase-to-neutral faults are examined, as well as a case where all three phases are short-circuited.