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| Electric Shock Hazards |
1. Fault currents
System fault currents can flow in a
protective ground if
a) The grounded circuit is accidentally
reenergized from its normal source voltage(s) (e.g., inadvertent
reclosure)
b) The grounded circuit is accidentally
energized by another circuit (e.g., by sagging into another line or
an energized line falling into the grounded circuit, or both).
2. Induction (coupling)
When a line is isolated from a source
of potential and is next to one or more energized lines, it is
subject to both capacitive and magnetic
coupling from the live line(s).
Capacitive coupling
Due to the capacitive couplings between
the live conductors and each of the isolated conductors, a
voltage is induced in the isolated
conductors. The induced voltage depends on the operating voltage
and on the relative location of the
live phases.
Magnetic coupling under normal
conditions
Due to inductance of the lines, a
loaded live line will induce a continuous circulating current in a
parallel isolated and grounded line.
The flux, which cuts the de-energized line conductors, is the vector
sum of the fluxes developed around the three-phase conductors of the
energized line.
Magnetic coupling during faults
The magnetically induced currents,
being directly proportional to the current in the adjacent live
circuit, can clearly be increased many fold if the live circuit
becomes faulted.
3. Lightning
Although work on lines is generally not
done when lightning is in the immediate area, it is not possible to
guarantee that lightning will not strike near the line. The
steep-fronted voltage surge from a lightning discharge to the line
will be attenuated as it travels down the line.
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