Page 76 - Electrical Safety of Low Voltage Systems

The Earth 59

FIGURE 4.16 Variation of the surface earth potential as a function of distance
x from the spherical electrode’s center.

In correspondence with the vertical straight line passing through
the center of the spherical electrode (i.e., x = 0), the ground potential
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assumes finite value:
2 I
V x=0 = (4.21)
2 D

4.6 Voltage Exposure Upon Ground Faults
If the basic insulation of a grounded ECP fails, the metal enclosure may
be energized and persons are exposed to the risk of electrocution. 10
Persons touching the enclosure, in fact, close the loop between the
faulty metal frame and the earth, which, due to its conductive nature,
will carry the fault current toward the source.
If the current impressed to ground is I and the earth resistance is
R G , the potential assumed by the ECP is V G = R G I (Fig. 4.17).
Once again, V G is the potential difference between any point on the
electrode’s surface and infinity; its magnitude is generally less than
the phase voltage.
If the faulty ECP is disconnected from the grounding system (e.g.,
because of the accidental interruption of the protective conductor, PE),
in case of fault-to-ground, the enclosure will attain phase-to-ground
potential. This is a particularly hazardous situation, as in the absence
of a clear path to ground the fault cannot be cleared.

4.6.1 Touch Voltage
The amount of current possibly flowing through a person depends
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on, and is limited by, the series of the body resistance R B and the
resistance of person-to-ground R BG .