A layer of gravel or another high-resistivity material, of about 0.08 m to 0.15 m, increases the resistance between the soil and the feet and reduces the current through the body. IEEE 80 quantifies it with the Cs factor, which corrects the surface resistivity according to its thickness and that of the soil below, and which raises the tolerable step and touch voltages.
Short answer. The gravel in a substation yard is not decorative: it is a safety measure. By increasing the resistance between the ground and a person’s feet, it reduces the current that goes through the body during a fault. IEEE Std 80-2013 accounts for it with the Cs factor, a number less than or equal to 1 when the layer is more resistive than the soil, which is applied to the surface resistivity when calculating the tolerable step and touch voltages.
What the surface layer does
A layer of high-resistivity material, such as gravel, is usually spread over the grid with a thickness of 0.08 m to 0.15 m. Its function is to increase the contact resistance between the soil and the feet of the people in the substation. When the soil below the gravel has layers of different resistivity, the two-layer soil model is applied.
The guide clarifies a point that invites confusion (clause 14.1). A high-resistivity layer, much thinner than the depth of the grid, might seem to worsen the step and touch voltages. It does not. The surface material only increases the resistance between the foot and the soil, so, for the same maximum current allowed through the body, considerably higher step and touch voltages can be accepted. The circuit formed between the feet and the soil is covered in body resistance and the accidental ground circuit.
In addition, gravel slows the evaporation of moisture and limits the drying of the upper soil layers during long dry periods.
How it is considered in the calculation
Without a surface layer, the foot is modeled as a disk on a uniform soil. With a thin layer, that assumption no longer holds, because the thickness is small compared with the equivalent radius of the foot. But for a person inside the substation it can be assumed that the layer extends without limit to the sides.
The guide represents the effect with the derating factor Cs, which corrects the resistance of the foot. It depends on three data: the resistivity of the surface material (ρs), that of the soil below (ρ) and the thickness of the layer (hs). It is calculated with a reflection factor between both materials:
K = (ρ − ρs) / (ρ + ρs) (equation 21)
If the layer is more resistive than the soil, K is negative and Cs is less than 1. The standard offers a series to calculate Cs and graphs for a 0.08 m foot. It also gives an empirical expression, whose values are within 5 % of the analytical ones:
Cs = 1 − 0.09 · (1 − ρ/ρs) / (2 hs + 0.09) (equation 27)
with hs in meters. With the Cs factor, the tolerable voltages of clause 8.4 use the corrected resistivity Cs · ρs instead of that of the soil. For a 70 kg person, for example, the tolerable step voltage is (1000 + 6 · Cs · ρs) · 0.157/√t and the touch voltage (1000 + 1.5 · Cs · ρs) · 0.157/√t.
An example from the standard itself
In the Annex B example, the soil is 400 Ω·m, the gravel 2500 Ω·m, the thickness 0.102 m and the shock time 0.5 s, for a 70 kg person. The result is Cs = 0.74. The tolerable voltages are about 838 V for touch and 2687 V for step.
| Condition | Tolerable touch | Tolerable step |
|---|---|---|
| With 2500 Ω·m gravel, 0.102 m | 838 V | 2687 V |
| Without a surface layer (same other data) | about 355 V | about 755 V |
The second row results from applying the same equations with Cs = 1 and the soil resistivity. The difference shows why the gravel layer is a relevant design decision: with it the grid has much more margin.
What resistivity gravel has
Clause 12.5 notes that the value of the layer in reducing shock currents is not always appreciated. In a yard in France, a wet river gravel of 5000 Ω·m in a layer of 0.10 m to 0.15 m reduced by 10 times the ratio between the body current and the short-circuit current, compared with wet natural ground. In Germany, the current of a person on dry grass was about 20 times that of a person on wet coarse gravel of 6000 Ω·m.
The real resistivity depends on the type and size of the stone, on whether it is clean or has fines, on moisture and on atmospheric contamination. The guide’s table of typical resistivities shows that the water used to wet it also matters: washed granite gravel wetted with rainwater gives 5000 Ω·m to 10,000 Ω·m, whereas crushed granite with fines wetted with groundwater drops to 1300 Ω·m. That is why the standard itself asks for samples of the stone actually purchased to be measured, and not for table values to be taken.
What to look after so the layer keeps working
- Contamination of the layer. The voids between stones can fill by compression of the lower layers against the soil, by material from later excavations that was not properly removed and by dust from the air. This lowers the real resistivity.
- Step voltage outside the fence. If the safety inside the grid depends on a layer that does not extend beyond the fence, the step voltage outside can be dangerous and must be reviewed.
- Layer less resistive than the soil. If the soil below is more resistive than the top layer (for example, a wet concrete slab over resistive ground), the guide’s Cs factor does not apply and a multilayer soil analysis must be used.
- Environment. A layer exposed to sea water spray will have a considerably lower resistivity than one used in arid climates.
We measure your grounding system
The safety the layer provides depends on its real resistivity and thickness being the design ones. We measure the resistivity of your soil and of the gravel samples and verify the voltages in your yard, in Colombia or elsewhere on request. With our free grounding resistance tool you can compare the tolerable voltages with and without a surface layer in the “Tolerable” tab. See the testing and commissioning service or write to us through contact. If you need to define or review the design of the layer, see the engineering design and studies service.
Related guides
- IEEE 80: Safety in Substation Grounding
- Accidental Ground Circuit and Body Resistance
- Soil resistivity: factors and chemical treatment
- Two-layer soil model from measurements
- Step and touch voltage: difference and IEEE 80
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 7.4 (effect of a thin layer of surface material), 8.4 (step and touch voltage criteria), 12.5 (use of a layer of surface material) and 14.1; equations (21), (27), (29) to (33); table 7 (typical resistivities of surface materials); Annex B, example 1.
- Bodier and Langer, tests with wet gravel, cited in IEEE Std 80-2013, clause 12.5.
Frequently asked questions
Does gravel reduce the voltage on the ground during a fault?
Not appreciably. What it does is increase the resistance between the soil and the feet, so that, with the same voltage, less current passes through the body. That is why it allows higher step and touch voltages to be accepted, and not because it lowers the voltage at the surface.
What thickness of gravel should be placed?
IEEE 80 mentions layers of about 0.08 m to 0.15 m (3 to 6 inches) and shows that Cs depends on the thickness and the resistivities. The thickness and material for your substation are defined in the design with your measured resistivity, not with a generic value.
Can concrete or asphalt be used as a surface layer?
It depends on the resistivity of the soil underneath. The guide's Cs expression does not apply when the top layer is less resistive than the soil below, such as a wet concrete floor over a more resistive ground; that case is analyzed with a multilayer soil model.