|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).
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.
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.