METREL MI 3290 - Specific Earth Resistance Measurements [Ρ]; General on Specific Earth

MI 3290 Earth Analyser Tests and Measurements
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11.3 Specific Earth Resistance Measurements
[ρ]
The measurement is carried out in order to assure more accurate calculation of earthing
systems e.g. for high-voltage distribution towers, large industrial plants, lightning systems
etc. AC test voltage should be used for the measurement. DC test voltage is not suitable
because of possible electro-chemical processes in the measured ground material. Specific
Earth Resistance value is expressed in m or ft, its absolute value depends on structure of
the ground material.
Specific Earth
Resistance
Measurement
Test
Mode
Distance
Limit
Filter
Test
Voltage
ρ
Wenner Method
single
m / ft
yes
FFT
20 / 40 V
Schlumberger Method
single
m / ft
yes
FFT
20 / 40 V
Table 11.41: Available Specific Earth Resistance measurements in the MI 3290
11.3.1 General on specific earth
What is Specific Earth Resistance?
It is the resistance of ground material shaped as a cube 1 m × 1 m × 1 m, where the
measurement electrodes are placed at the opposite sides of the cube, see the figure below.
Figure 11.42: Presentation of Specific Earth Resistance
The table below represents indicative values of Specific Earth Resistances for a few typical
ground materials.
Type of ground material
Specific Earth
Resistance
in  m
Specific Earth
Resistance
in  ft
sea water
0.5
1.6
lake or river water
10 – 100
32.8 – 328
ploughed earth
90 – 150
295 – 492
concrete
150 – 500
492 – 1640
wet gravel
200 – 400
656 – 1312
fine dry sand
500
1640
lime
500 – 1000
1640 – 3280
dry gravel
1000 – 2000
3280 – 6562
stony ground
100 – 3000
328 – 9842