Fault duration directly influence the intensity of maximum
allowable dangerous voltages in the affected part of the
system. This is the main criterion for optimization of
grounding resistances. By optimization mean minimum cost
of the grounding system which is still safe. It is very
important to understand that “semi safe” grounding system
actually does not exist. One grounding system which is not
safe could work years without any accident but it is still just a
matter of time to become actual hazard