Gossen MetraWatt GEOHM 5, Руководство пользователя, Страница: 19 / 40

GEOHM 5 Instrument description
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2.7 Specific Earth Resistance
To ensure accurate calculations, it is advisable to measure the Earth Resistivity, when
defining the parameters of the earthing system (required length and surface of earth
electrodes, most appropriate depth for installing the earthing system etc.).
The four test probes are driven in at equal distances “a” to a depth of 5 % of “a”.
The specific Earth Resistance ρ is calculated by the following formula
ρ = 2 π a R
a distance between two probes
R ground resistance between the middle two probes
Different units are used for Earth Resistivity parameter namely:

m (European countries)

ft (The United States)
This is why the Earth Tester offers both units, as well as allowing the use of both units
for the selected di
stance “a” (m / ft) between the test rods. See the procedure on how to
select appropriate unit in chapter 3.4 General settings.
Step 1

Connect test leads to the instrument and to test rods, in accordance with the figure below.
Figure 13. Connection of standard 20 m long test leads
The following table shows typical soil resistivity for several types of soil.
Type of Soil Soil Resistivity

E
[

m]
Wet bog soil 8 ... 60
Arable fields, loam and clay, moist gravel 20 ... 300
Moist, sandy soil 200 ... 600
Dry sandy soil, dry gravel 200 ... 2000
Rocky ground 300 ... 8000
Bedrock 10
4
... 10
10
Table 2. Soil Resistivity

E
for Various Types of Soil