To recall, EATS 41-24 [2] states that ‘if the potential rise of the earth electrode exceeds
the appropriate CCITT level (430 V or 650 V),. . .measures shall be taken to counter
the risk of danger’. This standard also specifies limits for touch and step voltages
corresponding to probabilities of heart fibrillation based on IEC 60479-1 and certain
assumed values of resistance in the accidental circuit. However, experience has shown
that, in many situations, it is not reasonably practicable to satisfy these requirements,
particularly in respect of third parties. As already mentioned in section 8.5.3, the
minimum impedance of a substation earthing system is often limited by the available
land area and ground resistivity, and the minimum earth fault current is limited by
the electrical system configuration. In many cases, the minimum achievable rise of
earth potential may exceed several thousands of volts, and high potential contours
may extend considerably beyond the substation. Consequently, transferred potentials
affecting third parties in the vicinity of a substation may exceed the levels specified
in the standards, and the costs involved in mitigating these potentials could be very
substantial.