ETS-Lindgren HI-3638 ELV/VLF Electric Field Meter - Manual (Page 27)

www.ets-lindgren.com Power Frequency Fields 27
Spatial distribution of electric and magnetic field strength
beneath a 345 kV transmission line
This illustration shows the field distribution for a typical double circuit 345 kV
transmission line carrying 1000 A. The field strengths have been computed for a
height of one meter above the ground from one side of the line to the other and
indicate that the maximum electric field strength beneath the 345 kV line is
expected to be about 3.4 kV/m. The maximum magnetic field strength will be
dependent on the magnitude of current flowing in the line. If the line was carrying
a current of 1000 A, the magnetic fields indicate an expected maximum value of
175 mG (equivalent to 14 A/m).
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. This illustration shows the phenomenon of electric field
concentration which occurs above the head of a person standing under the line.
Localized enhancement of the electric field will lead to decreased field strength in
other nearby areas. 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
through field measurements.