Soil resistivity and Wenner readings

The resistivity of the soil, in Ω·m, enters the earth mat's resistance, its mesh and step voltages and the surface layer's derating factor (IEEE Std 80-2013 Eq. 27, 52, 80 and 92). A measured value is the first input of the earth mat's design.

1Why the soil mattersCopy link to Why the soil matters

The earth mat leaks the fault current into the soil. The soil's resistivity scales the mat's resistance and, with it, the rise of earth potential and the voltages a person meets on the surface. In the IEEE 80-2013 equations of a mat in uniform soil, the grid resistance and the mesh and step voltages are proportional to the resistivity.

Soils differ widely. IS 3043:1987 gives a range from 1 Ω·m to 10 000 Ω·m and notes that the soil at many station sites is not uniform (§36.1.1). The resistivity varies more with depth than across the site, because the earth is in layers. It also varies with the soil's moisture, so IS 3043 advises tests in the dry season (§36.1.2).

2The Wenner methodCopy link to The Wenner method

IS 3043:1987 recommends Wenner's four-electrode method (§36.3.1). Four electrodes are driven into the ground along a straight line at equal intervals $S$ (Figure 1). A current passes through the two outer electrodes, and the voltage between the two inner electrodes is proportional to the field the current sets up in the soil. An earth tester combines the source and the meter and reads a resistance $R$ directly (§36.3.1.2).

Eq. (1) holds when the electrodes are buried to a small depth compared with their spacing. The electrodes are driven of the order of 10 cm to 15 cm into the ground. Where the depth of burial is more than 1/20 of the spacing, IS 3043 gives a longer equation in place of Eq. 3 (§36.4.1). IS 3043 asks for tests in at least eight directions from the centre of the station (§36.2.1).

3Spacings and uniformityCopy link to Spacings and uniformity

A reading at a larger spacing reflects the soil deeper down. IS 3043:1987 detects the variation with depth by repeating the test at spacings from 2 m to 250 m, preferably at 2, 5, 10, 15, 25 and 50 m (§36.5.1). If the readings vary within 20 % to 30 %, the soil near the test location may be taken as uniform (§36.5.1, §36.6.1). Otherwise the readings describe layers, and a two-layer model is fitted to them (Two-layer soil).

The design service measures the variation as the difference between the largest and the smallest of the mean readings per spacing, divided by the largest. It takes the upper end of the IS 3043 range: a soil whose readings vary by 30 % or less is uniform.

4The design valueCopy link to The design value

In a uniform soil the design takes one resistivity: the value entered on the Site & soil page, or the average of the Wenner readings. It multiplies that value, ${\rho }_0$, by a season factor $k_{\mathrm{season}}$, 1.5 by default, for the change of resistivity with moisture: $\rho ={\rho }_0\times k_{\mathrm{season}}$. The IEEE 80-2013 equations of the earth mat take the result.