Ø Relatively high fault
current to ground in relation to the area of the grounding system and its resistance
to remote earth.
Ø Soil resistivity and
distribution of ground currents such that high potential gradients may occur at
points at the earth surface.
Ø Presence of a person at
such a point, time, and position that the body is bridging two points of high
potential difference.
Ø Absence of sufficient
contact resistance or other series resistance to limit current through the body
to a safe value under the above circumstances.
Ø Duration of the fault and
body contact and, hence, of the flow of current through a human body for a sufficient time to cause harm at the
given current intensity.
Relative infrequency of
accidents is largely due to the low probability of coincidence of the above un favorable
conditions. To provide a safe condition for personnel within and around the
substation area, the grounding system design limits the potential difference a
person can come in contact with to safe levels.
