Engineer's Notes: substation earthing
The grid is a safety device
An earth grid has two jobs: give fault current a low-impedance path so protection operates, and keep the voltages people can touch or step across below what the body can tolerate for the fault duration. The second job drives most of the design.
Fault current and time come first
The earth fault current, how much of it actually flows into the soil (split factor) and how fast protection clears it decide everything else. Faster clearing raises the tolerable voltages; a realistic split factor can shrink the grid.
Soil is the unknown
Resistivity can vary from under 10 Ω·m in wet saline ground to thousands in dry sand or rock. Always test the site, in more than one direction, and design for the driest season.
Watch the edges
The highest touch voltages are in the corner meshes and just outside a bonded fence. Extra conductors near the perimeter, rods at the corners and a crushed rock layer do most of the work.
Frequently asked questions
What is the difference between touch voltage and step voltage?
Touch voltage is the difference between earthed metalwork a person touches and the ground under their feet. Step voltage is the difference between two points on the ground 1 m apart. Both appear when earth fault current flows into the soil, and both must stay below tolerable limits.
What is ground potential rise (GPR)?
GPR is the voltage the earth grid and everything bonded to it rise to during an earth fault, relative to remote earth. It equals the grid current multiplied by the grid resistance. If GPR is below the tolerable touch voltage, the design is safe without further checks.
Why is crushed rock used in substations?
A layer of high-resistivity crushed rock adds resistance in series with a person's feet, so the body current for a given touch or step voltage is lower. This raises the tolerable voltages, often by a factor of two or more.
How is soil resistivity measured?
The Wenner method uses four equally spaced pins in a line. Current flows between the outer pins and voltage is measured between the inner pins. Apparent resistivity is 2 x pi x a x R, and repeating the test at increasing spacings shows how resistivity changes with depth.
What is a hot site?
A site where the earth potential rise exceeds 430 V, or 650 V for faults cleared within 0.2 s. Telecom and LV services entering the hot zone need isolation or special earthing to avoid transferring dangerous voltages.
Topics: substation earthing · earth grid · IEEE 80 · touch voltage · step voltage · mesh voltage · GPR · EPR · grid resistance · split factor · decrement factor · crushed rock · Wenner test · soil resistivity · earth conductor sizing · hot zone · transferred potential · combined HV/LV earthing
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