ETS-Lindgren HI-3638 ELV/VLF Electric Field Meter - Manual (Page 28)

28 Power Frequency Fields www.ets-lindgren.com
This inherent shielding effect of the body, unless the body is sufficiently distant
from the instrument, can lead to inaccurate measures of the electric field
strength. Depending on the proximity and orientation of the body, the
perturbation effect of the body can lead to either enhanced electric field strength
readings or reduced readings when compared to the true unperturbed field
strength. While in some cases it may be desirable to determine the enhanced
fields near objects, in general, most field measurements should be directed
toward assessing the unperturbed values. For example, unperturbed field
strengths or free space values, are more easily related to internal induced
currents in the body. Induced currents represent one potential dosimetric
measure of electric field exposure.
The illustration on page 25 also suggests that the electric field lines which
terminate on the earth are essentially purely vertically oriented directly beneath
the conductors, but at extended lateral distances from the line there can be some
horizontal component to the field. Thus, in measurements of electric fields near
power lines, it may be important to explore different polarization components of
the field to assess the resultant electric fields at points above the earth.
Measurement of electric field strength under a power line or near any other
source of electric fields may be accomplished by supporting the HI-3638
ELF/VLF Electric Field Meter on a non-conductive tripod. Because of the physical
asymmetry in the displacement current sensor it is imperative that the front side
of the sensor be directed toward the electric field source. In this position, the
electric field lines which are directed downward toward the earth will strike the
correct side of the displacement current sensor resulting in an accurate
measurement of the field strength. In addition, the presence of the operator will
tend to perturb the electric field that is being measured. The operator should
remain approximately one to two times their height away from the HI-3638
sensor.