Unitest 8986 Mode D'emploi page 26

Earth Resistance Measurement four-wire method in line with the Wenner principle
6.3 Earth Resistance Measurement four-wire
method in line with the Wenner principle and
only possible with the accessory set, order
number:1048!)
Measurements in dangerous proximity to elec-
trical systems should only be carried out in con-
junction with clear instructions from a responsible
electrical specialist and must not be carried out
alone.
For a preliminary calculation of the diffusion resis-
tance of a ground, the specific earth resistance must
be known. The resistance may not be read directly.
The value displayed must be included in a formula
and then calculated.
4 Insert the four earth rods in aline ensuring each
rod is the same distance apart . The earth rods
may be inserted by maximum 1/20 of distance"
a" into the earth.
4 Clamp the four clamps H, S, ES, E to the probes,
as described in figure 6.3.
4 Turn the rotary switch (13) to the 4 pole posi-
tion.
H S ES
a a
4 Set contact voltage (11).
4 Set automatic (16) or manual (12) frequency se-
lection.
4 Press key "Start" (14).
4 Read the resistance value.
26
E
6.3
a
The specific earth resistance is calculated as fol-
lows:
re = 2p x a x RE
The specific earth resistance is subject to large fluc-
tuations depending on a number of factors: humid-
ity and temperature are of extreme importance. For
observations during one year, the fluctuations may
be compared to a sinus curve (see figure 6.3.1).
Also refer to display indications in chapter 7.0