In short. When earth-fault current flows into the ground, the earthing grid rises in potential. A person standing on the ground and touching a metal part, or taking a step, can receive part of that difference. The study compares calculated voltages with tolerable limits and adds mitigation where they are exceeded.
Three quantities
| Quantity | Meaning |
|---|---|
| Ground potential rise (GPR) | Grid potential relative to remote earth during a fault |
| Touch voltage | Difference between the grid and the surface where a person stands, hand to feet |
| Step voltage | Difference between two points one pace apart on the surface |
GPR = Igrid × Rgrid
Igrid is only the part of the fault current that returns through the earth, not the total fault current. Metallic returns such as cable sheaths and neutral conductors carry part of it away.
Tolerable limits
Limits depend on the time the fault lasts, body weight assumptions and the resistivity of the surface layer. Faster clearing allows higher tolerable voltage. A thin layer of high-resistivity material such as crushed stone raises the limits for people wearing footwear. Use the standard you work to, such as IEEE 80 or the national equivalent, for formulas and values.
Study steps
- Obtain soil resistivity and the grid layout. See earthing architecture.
- Obtain the maximum earth-fault current in each mode and the fault clearing time. See fault studies.
- Estimate the current split between grid and metallic returns.
- Calculate grid resistance and GPR.
- Calculate mesh (touch) and step voltages across the grid.
- Compare with limits and place mitigation: more conductors, closer spacing, rods, surface layer, bonding of fences and equipment, restricted access.
Generator and transferred potential
- Generator earth faults are usually small, but a long clearing time can keep the voltage on a surface for seconds.
- Metallic parts connected to remote locations, such as cables, pipes and fences, can carry potential outward. Insulate or bond them as the design requires.
- Where a site shares earthing with a utility substation, check the transferred potential.
Illustrative example
A switchyard grid carries 8 kA into earth for 0.5 s. With a grid resistance of 0.5 ohm, the GPR is about 4 kV. The study finds that touch voltage at the corners exceeds the tolerable limit. Adding perimeter conductors and rods, plus a crushed-stone surface layer, brings the mesh voltage below the limit.
Three voltages that harm people
| Quantity | Meaning |
|---|---|
| Ground Potential Rise (GPR) | Voltage of the earthing system relative to remote earth during a fault |
| Touch voltage | Voltage between a grounded structure and the ground beneath a person’s feet |
| Step voltage | Voltage between a person’s two feet when walking over a fault-energised area |
GPR = ground fault current flowing into the earth electrode × earth electrode resistance
Example (illustrative)
If 3,000 A flows into the earth with an electrode resistance of 0.5 ohm, GPR is 1,500 V. Touch voltage is a fraction of that, depending on grid design. Tolerable values depend on fault duration; shorter clearing times allow higher tolerable voltages.
Design factors
- Faster fault clearing increases tolerable limits.
- Surface layers such as crushed stone increase contact resistance and raise tolerable limits.
- Closer grid spacing and more conductors reduce touch voltage.
- Equipotential bonding of metalwork removes dangerous differences inside buildings.
Where it applies to generator sites
Higher-voltage generators, substations and plants with HV supply need formal GPR studies. At LV, the same principle applies to the fault path and bonding. External exposure includes metallic fences, pipes and cables that carry voltage out of the area.
Use IEEE 80, IEC 61936 or the project’s governing standard, and confirm its current edition. See earthing study.
Common mistakes
- Using the total fault current instead of the grid share.
- Ignoring clearing time.
- Not checking fence and cable bonding.
What to send us
Send soil data and fault levels. Send your application details.
Questions engineers ask
Why does clearing time matter?
Tolerable body current falls as exposure time grows.
Do small sites need this?
Large substations need full studies; small sites need proper bonding and a sound earth design.
What is a surface layer for?
It raises the resistance between the ground and a person’s feet, lifting the tolerable voltage.
Is GPR the same as touch voltage?
No. GPR is the whole grid rise. Touch voltage is the part a person may bridge.
This guide gives general principles. Use the applicable standard and qualified design.