The gravel in a substation yard limits the current that flows through a person's body, and its resistivity is almost always taken in the wet condition. IEEE 81 recognizes seven published methods for measuring it, does not set a single procedure, and recommends measuring before the material is placed so the design can still be adjusted.
Short answer. The gravel layer in a substation yard increases the resistance between a person’s feet and the ground, and so limits the current that flows through them during a fault. That is why its resistivity enters the calculation of tolerable voltages. IEEE Std 81-2012 (Annex D) explains how to measure it: it recognizes seven published methods, with no industry standard for measuring wet resistivity, and recommends measuring a sample of the material before it is placed.
Why the wet value matters
According to Annex D.1, safe step and touch voltage values are based on the wet resistivity of the surface material. The reason is that the dry resistivity of almost all materials is very high and does not represent the worst design value. Local conditions, moisture, and the size and type of stone all change it. That is why it matters to measure wet samples of the stone that will actually be used.
The guide includes a table of typical values taken from IEEE Std 80-2000, with samples from different regions of the United States. In that table, crushed or washed granite and limestone, in the wet condition, range from about 1,200 Ω·m to 10,000 Ω·m. Dry, the values reach millions of Ω·m. The guide itself states that those values do not hold for all types and sizes of stone in a region and that IEEE 80 suggests testing the stone that the utility habitually buys.
There is no standardized method
The guide is explicit: there is no industry standard on how to measure the wet resistivity of a surface material or on how wet the sample should be. In general, the methods described can measure either dry or wet. For the wet case, a procedure is defined that soaks the sample thoroughly and then drains the water until an arbitrary moisture content is reached. Today it is up to whoever runs the test to define the method and that moisture content.
Before or after placing the material
The resistivity of a material can be measured after it has been placed, using soil resistivity methods. But the methods in clause 7 are not applicable to thicknesses of 100 mm to 400 mm, which are the usual ones in substations. In addition, a measurement taken after the material is placed reflects only one set of weather and soil conditions. It may not show the resistivity under all operating conditions, and adjusting the grid or the surface layer afterward is impractical.
Measuring a sample under controlled conditions before placing the material allows a wider range of practical design modifications. That is the underlying recommendation of Annex D.
The seven published methods
The literature includes seven methods for measuring the resistivity of a soil or a surface material.
Three are soil methods and are in the guide. The two-point method, the depth variation method and the four-point method can be applied to surface rock. With a layer of 150 mm or less, all of them have problems. The two-point method would have to deal with the contact resistance between the probes and the rock. The depth variation method requires a large sample depth, on the order of 2 m to 3 m, and is not practical for a thin layer that is already installed. The four-point method assumes that the probes are small compared with their spacing, and with a layer of about 200 mm that assumption no longer holds.
A fourth soil method is in ASTM G57-2012 and can also be used with rock.
Three methods were published specifically for rock:
| Method | Device | What it shows |
|---|---|---|
| Carmen and Woodhouse | 150 mm PVC cylinder with copper mesh electrodes at the top and bottom | Developed to evaluate the effect of water; dry rock of several sizes was measured |
| Laird (Abledu and Laird) | Plastic cylinder about 154 mm high and 298 mm in diameter on an aluminum base, covered with aluminum and a 10 kg weight | Effect of the water resistivity and the drainage time on the wet resistivity |
| Rectangular volume (Patel) | Fiberglass boxes of about 900 mm × 600 mm × 150 mm exposed to the weather, with copper electrodes on two faces | Seasonal variation of resistivity with the weather |
What the published results teach
From the Laird method, the guide cites a result that shows why the wet value depends on the procedure. The same sample of washed granite, wetted with 500 Ω·m water, gave 10,410 Ω·m after 10 minutes of drainage and 16,562 Ω·m after 130 minutes. With 5.7 Ω·m water it gave 2,180 Ω·m and 4,136 Ω·m, respectively. Because of that spread, the authors recommend testing with 100 Ω·m water and measuring after 10 minutes of drainage. Water of another resistivity can be used with a correction to 100 Ω·m.
From the rectangular volume method, the contact stands out: without aluminum balls between the gravel and the electrodes, the resistance of the sample came out up to 600 % of the value measured with them. There were also significant differences between washed gravel and crushed rock with fines, so choosing the sample material well is part of the measurement. From the Carmen and Woodhouse method, a typical dry value was 14,000 Ω·m for small particles and 1,000,000 Ω·m for large particles. The guide notes that the studies do not fully define the categories or the number of measurements.
In the sample methods, resistivity comes from the measured resistance, the area of the sample and its thickness: ρ = R × A / L (equation D.1). When dry rock is measured at high resistances, a capacitive charging effect also appears that prolongs the reading for hours.
What to do with the result
The measured value for wet gravel feeds the calculation of the tolerable step and touch voltages. The surface layer factor of IEEE 80 is what brings that data into the design. Over time the stone in the yard deteriorates, because clay from the subsoil invades it, weeds grow, or it becomes contaminated with fines. That is why IEEE 81 states that a resistivity test of the surface material helps establish the tolerable limits when step and touch voltages are measured in the field.
To see how the safety of an installed grid is verified, read the guide on step and touch voltage and the one on high step and touch voltages in the grid. To compare the result with what happens in the field, see Measuring step and touch voltages in a substation and Potential map and integrity of the installed grid.
We measure your grounding system
At Tiza Group we measure soil, surface layer and grid resistivity in substations and plants, in Colombia, or elsewhere on request, and we deliver the result interpreted against IEEE 80 and the local code and utility requirements. See the testing and commissioning service or write to us through contact. In the quotation we recommend the test and the equipment your case needs.
Related guides
- Gravel layer in substations: Cs factor and safety
- Potential map and integrity of the installed grid
- Measuring step and touch voltages in a substation
- Soil resistivity: Schlumberger and other methods
- Step and touch voltage: difference and IEEE 80
References
- IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: Annex D (D.1 to D.4, Table D.1) and clause 9.3 i) (surface material resistivity).
- IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding.
- ASTM G57-2012, four-electrode method for soil resistivity.
Frequently asked questions
Why is the wet resistivity of gravel used instead of the dry value?
Because the dry resistivity of most surface materials is very high and does not represent the worst design case. Tolerable step and touch voltages are calculated with the wet value, which is lower.
Can I use a typical table value for the gravel in my substation?
Only as an order of magnitude. IEEE 81 states that the values in its table do not hold for all types and sizes of stone in a region. The recommendation of IEEE 80 that it cites is to test the stone that is normally purchased.
Can the resistivity of gravel already in place be measured with the Wenner method?
Not directly. The guide states that soil resistivity methods are not applicable to layers 100 mm to 400 mm thick, which are the usual thickness in substations. A thin layer compromises the result of any of them.