Page 220 - Electrical Safety of Low Voltage Systems

Safety Against Overvoltages
203
FIGURE 12.1 Low and high sides of substation sharing the same earthing
electrode in TN systems.
To ensure safety, magnitude and duration of the touch voltage V T
must fulfill the following equation:
(12.1)
V G = R N I G ≤ V T
where I G is the earth current caused by a high-voltage fault and V T is
the maximum permissible value for the touch voltage as a function of
high-voltage fault duration as shown in the curve in Fig. 12.2, as per
IEC 60364–4-44. 1
FIGURE 12.2 Permissible touch voltage as a function of the duration of
high-voltage faults.