Measurement of soil resistivity

For vertical grounding bodies, the grounding resistance is

ρ=2πLR/1n4L/d

Where ρ - soil resistivity, Ω · m;

L——the depth of the steel pipe or round steel buried in the soil, m;

D——the outer diameter of the steel pipe or round steel, m;

R——Measured grounding resistance of the grounding body, Ω.

Soil resistivity when using flat iron as a horizontal grounding body

ρ=2πLR/1n4L2/bh

B——the width of the flat iron, m;

H——the depth of the flat iron centerline from the ground (approx. burial depth), m;

R——Measured grounding resistance of the grounding body, Ω.

When measured by the three-pole method, the soil near the grounding body plays a decisive role. The soil resistivity measured by this method mainly reflects the soil condition near the grounding body. If necessary, several points should be selected in the grounding grid area.

Second, quadrupole method, soil resistivity

The wiring for measuring soil resistivity by the quadrupole method is shown in Figure 14-6. It can be measured by current-voltage method or by using a grounding resistance meter with four terminals (see Figure 14-7). Four uniform round steels having a diameter of 1.5 cm and a length of 0.5 m were used as electrodes, and the buried depth b was 0.1-0.15 m, and the electrode distance a was 2-3 m.

During the measurement, the current I is applied from the outer electrodes C1 and C2, and the potentials appear on the electrodes P1 and P2, respectively.

UP2=ρI/2π[1/(2a)-1/a]

The potential difference between the two poles is Up1-Up2=ρI/2πa

Where I - ammeter reading, A;

A——the distance between electrodes, m;

Ρ——soil resistivity, Ω·m,

So ρ=2πaU/I (current, voltmeter method)

Or ρ=2πaR (grounding resistance meter method)

Considering that the soil resistivity is greatly affected by the season. Therefore, the measurement results should be corrected, ie

Maxmax=Φ ρ

Where Φ is the soil drying coefficient (see Table 14-1). If the measurement is dry season, take the smaller value in the table, and vice versa.

It can be seen from Table 14-1 that the horizontal grounding body with a depth of 0.8 m has a variation of soil resistivity of nearly 50% in the dry season and the wet season.

In summary, when measuring the soil resistivity by the three-pole method, the soil near the grounding body plays a decisive role, and only reflects the situation near the grounding body to a large extent. If necessary, several measurements should be selected in the proposed grounding grid area. When the soil resistivity is measured by the quadrupole method, the range reflected by the electrode is related to the distance a between the electrodes, and the reflected depth increases with the increase of a. When the a is small, the soil resistivity is only the resistivity of the earth's surface layer. When measuring, select 3-4 points for measurement, and take the average of multiple measurements as the measured value.

For large-area grounding grids, it is recommended to use the four-pole method in the electrical measurement method for measurement. For the specific measurement methods, the reader can refer to the relevant part of the "Guidelines for the Measurement of Power Frequency Characteristics of Grounding Devices".