Page 79 - Electrical Safety of Low Voltage Systems
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62 Chapter Four
FIGURE 4.20 Person
standing in an area
at nonzero potential,
while touching a
faulted enclosure.
The person is subject to the potential difference V T as imposed by
the voltage divider (Fig. 4.21). In general, we can say that V T ≤ V ST ≤
V G . In low-voltage systems, international standards conservatively
impose permissible limits to the prospective touch voltage, instead of
the touch voltage. As a result, protective devices must automatically
disconnect the supply to faulty circuits, so that the prospective touch
voltage does not persist long enough to cause harm to people. Given
the truth of the previous inequality, by limiting the source touch volt-
age in case of a fault, we also limit the touch voltage, which is the true
voltage exposure to people.
FIGURE 4.21
Equivalent fault-loop
for a person
standing in an area
at nonzero potential.
