Gossen MetraWatt GEOHM C Mode D'emploi page 55


Note
Measurement cables must be well insulated in order to avoid
shunting. In order to reduce coupling influence to a minimum,
measurement cables should neither cross one another, nor should
they run parallel to each other over long distances.
Measurement is performed as described in chapter 5.4 on page 16.
4-Wire Connection
E ES S
≥ 20 m ≥ 20 m
S
E
Figure 3 Measuring Earthing Resistance with a 4-Wire
Test Setup
4-wire connection is used in the event of excessive cable resistance
between the earth electrode and the test instrument.
Ð
Position the probe at least 20 meters, and the auxiliary earth electrode at
least 40 meters from the earth electrode (see also Figure 3 on page 19).
Ð
Make sure that contact resistance between the probe and the soil is
not too great.
Ð
The earth electrode is connected to jacks "E" and "ES" with separate
cables for 4-wire connection, the probe is connected to jack "S" and
the auxiliary earth electrode is connected to jack "H".
Ð
Press the R key in order to select 4-wire connection.
E4-P
Cable resistance from the earth electrode to jack "E" at the instrument is
not included in the measurement with this test setup.
GMC-I Messtechnik GmbH
H
H

Note
Measurement cables must be well insulated in order to avoid
shunting. In order to reduce coupling influence to a minimum,
measurement cables should neither cross one another, nor should
they run parallel to each other over long distances.
Measurement is performed as described in chapter 5.4 on page 16.
Potential Gradient
Suitable positioning of the probe and the auxiliary earth electrode can
be determined by observing voltage characteristics and dissipation resis-
tance in the ground.
The test current which is generated by the test instrument and conducted
through the earth electrode and the auxiliary earth electrode causes volt-
age distribution around the earth electrode and the auxiliary earth elec-
trode in the form of a potential gradient (see also Figure 5 on page 20).
Resistance distribution is analog to voltage distribution.
Earth electrode and auxiliary earth electrode dissipation resistance are
generally different. For this reason, the two potential gradients are not
symmetrical.
Dissipation Resistance of Earth Electrodes with Minimal Spread
Positioning of the probe and the auxiliary earth electrode is extremely
important for correct determination of earth electrode dissipation
resistance. The probe must be positioned between the earth electrode
and the auxiliary earth electrode in the so-called ground reference plane
(see also Figure 4 on page 20).
The characteristic resistance curve is thus practically horizontal within the
ground reference plane.
Proceed as follows in order to select suitable probe and auxiliary earth
electrode resistances:
Ð
Position the auxiliary earth electrode approximately 40 meters from
the earth electrode.
Ð
Position the probe halfway between the earth electrode and the
auxiliary earth electrode and measure earthing resistance.
Ð
Move the probe from its original position 2 to 3 meters closer to the
earth electrode, and then 2 to 3 meters closer to the auxiliary earth
electrode, and measure earthing resistance at each position.
19