ETS-Lindgren HI-3604 ELF Survey Meter - Manual (Page 28)

Page -- 20 HI-3604 Manual
Electric and magnetic fields produced by the power line originate
because of the voltages impressed on the conductors and the
magnitude of current (electricity) flowing through the
conductors. Figure 6 depicts the approximate spatial orientation
of these fields; electric field lines are shown to be directed such
that they terminate at perpendicular angles to the earth's
surface and magnetic field lines are shown as lines encircling the
conductors. At any particular point in space, the field can be
determined by the superposition of the fields associated with
each conductor; because the voltage and current of each
conductor is out of phase with that in any of the others, and the
conductors have some finite spacing between them, the
resulting electric and magnetic fields are calculated on the basis
of the vector sum of fields caused by each of the three
conductors. At some points the fields can constructively add
together causing a relatively elevated field strength. At other
points the fields from the conductors may destructively add
leading to minima in the fields. Thus, power line fields can have
rather complex spatial distributions about the line. Figure 4-2
illustrates this field distribution for a typical 345 kV transmission
line carrying 1000 A. In this figure the field strengths have been
computed for a height of one meter above the ground from one
side of the line to the other.
In addition to the normal variation in field strength which is
observed along a line transverse to the power line, electric fields
beneath power lines are perturbed by the local surroundings.
Figure 4-1 illustrates the phenomenon of electric field
concentration which occurs above the head of a person standing
under the line. Because electric field lines have a tendency to
terminate on grounded objects, and because the human body is
conductive and is electrically near ground potential, there tends
to be a concentration of field lines at the top of the head. This
same phenomenon occurs with virtually any grounded object
immersed in the electric field environment of a power line and
can be confirmed via field measurements. A similar perturbation
of the magnetic field does not occur because the body is non-
magnetic. Figure 4-1 also suggests that the electric field lines
which terminate on the earth are essentially purely vertically